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Emerging Chalcohalide Materials for Energy Applications
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Cite this: Chem. Rev. 2023, 123, 1, 327–378
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https://doi.org/10.1021/acs.chemrev.2c00422
Published November 21, 2022

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Abstract

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Semiconductors with multiple anions currently provide a new materials platform from which improved functionality emerges, posing new challenges and opportunities in material science. This review has endeavored to emphasize the versatility of the emerging family of semiconductors consisting of mixed chalcogen and halogen anions, known as “chalcohalides”. As they are multifunctional, these materials are of general interest to the wider research community, ranging from theoretical/computational scientists to experimental materials scientists. This review provides a comprehensive overview of the development of emerging Bi- and Sb-based as well as a new Cu, Sn, Pb, Ag, and hybrid organic–inorganic perovskite-based chalcohalides. We first highlight the high-throughput computational techniques to design and develop these chalcohalide materials. We then proceed to discuss their optoelectronic properties, band structures, stability, and structural chemistry employing theoretical and experimental underpinning toward high-performance devices. Next, we present an overview of recent advancements in the synthesis and their wide range of applications in energy conversion and storage devices. Finally, we conclude the review by outlining the impediments and important aspects in this field as well as offering perspectives on future research directions to further promote the development of chalcohalide materials in practical applications in the future.

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1. Introduction

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Designing and developing synthesis techniques for new energy materials and that allow the scalable fabrication of high-performing devices is a rapidly advancing topic of research. Over the last 60 years, mainstream silicon semiconductor material technology has advanced significantly to improve device efficiency and lower fabrication cost, ensuring the future large-scale use of photovoltaics (PV). (1−3) Opportunities still exist for promising thin film energy materials such as CdTe, (4−6) Cu(In, Ga)Se2 (CIGS), (5,7) Cu2ZnSn(S, Se)4 (CZTSSe), (4,5) and organic semiconductors (4,5) that promise even lower fabrication cost and shorter energy pay-back time over the well-established silicon technology. These materials efficiently harvest major parts of the solar spectrum, allowing tandem or multijunction devices to be built with efficiencies that exceed the Shockley-Queisser limit of a single-junction device. (8,9) However, already commercialized CdTe and CIGS suffer from a scarcity of Te and In elements for terawatt PV deployment, which has led researchers to investigate their counterpart CZTSSe containing earth-abundant and non/less-toxic constituent. (10−15) However, high-temperature synthesis (>500 °C), some lattice instabilities, and secondary phase formation during synthesis limit advancements in optoelectronic characteristics and device efficiencies at the commercial level. (16−18)
In recent times, halide perovskites, ABX3 (where, A = MA+ (CH3NH3+), FA+ (CH(NH2)2+), Cs+, or mixed A-cations; B = Pb2+ or Sn2+ or mixed B-cations; X = I–, Br–, or Cl– or mixed X-anions), a relatively old family of materials, have emerged as a member of the third-generation technologies for PV devices. (19−27) Recently, solar cells based on halide perovskites brought a promise of breaking a prevailing paradigm by reaching a record efficiency of 25.8% at a low fabrication cost in merely decade, (28) further challenging the 60-year reigned silicon technology (at least on the laboratory scale). Despite several key attributes of these halide perovskites, the fact that lead (Pb) is a major constituent of high performance devices entails toxicity issues from the fabrication steps to the final disposal/recycling. (29−32) Moreover, they undergo a rapid degradation on exposure to external stimuli (such as moisture, heat, and illumination or their combinations). (33,34) Furthermore, water soluble Pb ions escaping from perovskite devices can pose a severe toxicity issue owing to the moderate solubility of PbI2 (Ksp = 4.4 × 10–9). (35) Apart from the general toxicity, the neurotoxicity of Pb (the maximum blood Pb level at 5 μg/dL for children set forth by World Health Organization (WHO)) (36) is another concern. Given this, a number of significant advances are being developed that combat degradation and perform toward its stability, which again pique scientific interest. (37−42) However, the challenges associated with instability of Pb-halide perovskites, combined with their toxicity, motivate the search for alternate, intrinsically stable perovskite-type materials containing non-/less-toxic elements. A simple approach toward the synthesis of environmentally benign halide perovskite-based materials is to replace toxic Pb with Sn or Ge since both elements fulfill the ionic size, coordination, and charge balance requirements to form the perovskite structure. (22,43−50) However, Sn2+ and Ge2+ cations in perovskites can be easily oxidized to Sn4+ and Ge4+, respectively, resulting in relatively fast degradation, low performance, and challenges in reproducibility, (50−54) further limiting their practicability over Pb-based halide perovskites. (29,32,44) The phenomenal performance of the above-mentioned halide perovskites can be linked to the outermost ns2 electrons in the metal cation, which rationally leads to the possibility of utilizing Bi and Sb (from their ability to form 3+ ions) with a similar electronic configuration as Pb2+. In this regard, vacancy-ordered Bi- and Sb-based double perovskites having the structure A3B2X9 or nonperovskite structures have been developed. (53−62) However, these materials offer poor dichotomy, (i) stable in air while not offering 3D structure or direct bandgaps, and (ii) relatively small effective masses but also strongly bound excitons, inhibiting free carrier generation. (30,63) Moreover, they also exhibit low mobilities, poor charge transport, and high resistivities from 1010–1012 Ω cm, ultimately leading to poor device performances. (63) Therefore, the pursuit for alternative highly stable and less-toxic light-harvesting materials with the feature of ns2 lone pair cations is a challenging research area.
Chalcohalides are an emerging family of inorganic semiconductors that are commonly denoted as MChX, where M stands for one or more metal cations and Ch and X stand for one or more chalcogen and halogen anions, respectively (e.g., SbSI, BiSI, SbSeI, BiSeI, and their derivatives) (Scheme 1). They were extensively studied in the 1960s due to their promising glass-forming abilities and exotic physical, chemical, optical, and ferroelectric properties. (64−71) Maintaining charge neutrality in hybrid organic–inorganic metal chalcohalides (AB(Ch, X)3) requires the use of trivalent or tetravalent B cations and thus the advancement of chalcohalide perovskite as an area of research. (72) The motive behind chalcohalide perovskite stems from the stability issue of the halide perovskite owing to the presence of the ionic B-X bond. High electronegativities of halogen atoms add the ionic character to the B–X bond making these materials unstable. To improve the properties, a nonhalide Ch (chalcogenide (S, Se) anions) is substituted, which gives the B-X bond covalent character, improving the stability but with increased enthalpy of formation. Such partial substitution of halides with chalcogenide anions is called a “split-anion approach”. (73,74)

Scheme 1

Scheme 1. Overview of the Current Status of Representative Chalcohalide Materials
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Moreover, chalcohalide materials may offer good charge transport properties because of the high-Z ns2 levels of d10s2p0 electron configuration. A highly dispersive character of both valence band (VB) and conduction band (CB), due to the peculiar electronic structures, offers a high charge carrier mobility and defect tolerance in these chalcohalides. (75) Interestingly, the presence of both divalent chalcogen (S2–/Se2–) and monovalent halide (I–/Br–/Cl–) anions in these chalcohalide materials allows facile tuning of crystal and electronic structures and optical properties. (75−77) These tunable properties broaden their use in various applications including room temperature (RT) radiation detection, transparent electronic devices, photodetectors, photocatalysis, photoelectrochemical (PEC) water splitting, light-emitting diodes (LEDs), batteries, supercapacitors, and single junction and tandem solar cell devices. Table 1 summarizes the fundamental structures, optoelectronic properties, and their application in various energy conversion and storage devices. The extensive developments in chalcohalide materials bring a necessity to review chalcohalides focusing on the materials design, their properties, film fabrication, and device fabrication while providing a perspective on future directions and potential applications in energy devices (Scheme 2). The synthesis and applications of Bi- and Sb-based chalcohalide materials have been previously discussed as a part of dedicated book chapters (78−80) and reviews (81−83) as well as a brief overview of their potential in solar cell devices. (84−86)

Scheme 2

Scheme 2. Chalcohalides Presenting Tunable Optoelectronic Properties, Indicating the Wide Range of Applications That Can Be Pursued
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Table 1. Fundamental Properties and Applications of Chalcohalide Materials
chalcohalides/propertiesbandgap (experimental) eVbandgap (theory) eVband alignmentconductivity typecrystal structureeffective mass (e, h)applicationa
BiSX0.7–2.051.0–1.78direct, indirectn-typeorthorhombic0.53, 0.95PV, DSSC, PEC, PC, PD, battery, TE
BiSeX, Bi3Se4Br1.27–1.711.0–2.2direct, indirectn- and p-typeorthorhombic∼0.53–0.57, ∼2.19–2.98 
BiSSeX1.48–1.63     PEC
Bi19S27X30.77–1.6 directp-typehexagonal PC, PD
Bi13S18I20.75–1.570.6–0.91indirect, directn-typehexagonal, trigonal battery, TE
SbSX1.77–2.151.4–2.21indirect, direct orthorhombic0.07–0.54, 0.098–0.65PEC, DSSC, PC, PD, battery, PNG,
SbSeX1.671.29–1.67indirect  0.52PENG
SbxBiySI1.51–1.83     PV
Pb3S2X2, Pb4S3X2, Pb4SeBr6, Pb5S2I6 (direct), Pb7S2Br101.6–2.51.5–2.0indirect orthorhombic, pseudocubic PV, PD
Pb3Se2X21.48 indirect orthorhombic, cubic  
Sn5S4Cl2     0.5, 0.4 
Cu2I2Se6 1.95indirect rhombohedral0.32 
Pb2SbS2I3, Pb2BiS2I3, Sn2BiS2I3, Sn2BiSI5, Sn2SbS2I3, CsSnS2Cl1.2–2.191.08 (Sn2SbS2I3), 0.98 (CsSnS2Cl)direct indirect (CsSnS2Cl)n-typeorthorhombic, monoclinic PV
Ag3SI, Ag3SBr, AgaBibIa+3b-2xSx,0.9–1.87     PV
MASbSI2, MASbSeI2, MABiSI2, MA3Bi2I9–2xS1.67–2.030.83–1.4direct cubic, hexagonal0.32, 0.4PV
a

PV, Photovoltaic; DSSC, dye-sensitized solar cells; PEC, photoelectrochemical cell; PC, photocatalysis; PD, photodetector; TE, thermoelectric; PNG, pizeoelectric nanogenerator; PENG, pyroelectric nanogenerator.

In this review, we comprehensively discuss various chalcohalide materials according to the type of metal cation(s) and assess their potential in various energy conversion and storage. It includes an up-to-date overview of the heavy pnictogen chalcohalides (M = Bi and Sb), transition and post-transition metal chalcohalides (M = Cu, Sn, Pb, and Ag), mixed-metal chalcohalides (M = Bi/Sb with other metals), and hybrid organic–inorganic metal chalcohalides (i.e., chalcohalide perovskite) not only from the synthesis point of view but also from device perspective. Following the introduction (section 1), we briefly discuss high-throughput computational techniques to design, develop, characterize, and further optimize these materials and highlight the enabling attributes including optoelectronic properties and structural chemistry of interest (sections 2 and 3). Next, we discuss recent theoretical advances in the developments of Bi- and Sb-based chalcohalide materials followed by the progress of some new Cu-, Sn-, Pb-, and Ag-based chalcohalides to motivate further research in this field (section 4). We also highlight major achievements in the synthesis of these materials and their application in various devices such as solar cells, photodetectors, photocatalysis, thermoelectrics, batteries, supercapacitors, and piezo/pyro-electric nanogenerators (section 5). Beyond summarizing the developments and applications of these chalcohalides, roadblocks and key points to fabricate high-performing devices are further discussed. Lastly, we provide a brief outlook on this field and a perspective on the future directions for chalcohalide materials.

2. Materials Properties and Research Methods Enabling Attributes toward Optoelectronics

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The requirements of emerging chalcohalide semiconductor materials have four basic criteria: (1) processability and chemical stability, (2) suitable bandgap and high absorption coefficient to ensure maximal absorption of light in the visible region, (3) high mobility and dopability to enable sufficient charge carriers, and (4) defect tolerance. In this section, we discuss both the theoretical and experimental aspects of these materials properties/chemistries that enable attributes for high-performance energy conversion devices. We mainly emphasize properties relevant to solar cells, photocatalysis, photodetectors, and nanogenerators.
Lately, much progress has been made in theoretical computation and screening using the Materials Genome Initiative (MGI) that led to the development of different computational programs such as Materials Project, (87) AGLOWLID, (88) NREL-MatDB, (89,90) and the Open Quantum Materials Database. (91) These programs compile data from density functional theory (DFT) calculations and offer a progressive pathway for screening and exploring materials candidates for desired applications. This has revolutionized the way materials are selected for synthesis in the community of experimental materials scientists. A standard pathway has been established for computational materials screening to guide accelerated discovery and designs of materials across a broad range of applications, termed as the “funnel” approach. (92) Usually, it begins with more computationally “inexpensive” steps such as queries of possible structures from the available Inorganic Crystal Structure Database (ICSD). (93) This step is followed by more in-depth steps that require more computer resources to explore the new materials not available in the database using chemical substitution or structure similarities, or via random searches or using machine learning (ML). (94) The key requirements for computational materials screenings as shown in Figure 1, left panel include (1) an initial data subset enumerating different stoichiometries or structures, (2) iterations for energetic stability, (3) essential descriptors or chemical constraints to realize the properties of fundamental interests (e. g., bandgap, effective masses, mobility of charge carriers etc.), and (4) advanced electronic structure screening to derive accurate band structure and defect tolerance in materials. These effective computational screening steps allow the down-selection on the order of tens of materials for further experimental investigations. Like the computational “funnel” approach (Figure 1, right panel), materials discovery through experiments follows an “experimental funnel” approach. This follows a standard approach of experimental synthesis of materials and their series of characterizations to determine the structure, composition, and optoelectronic properties, which could be further correlated to the computational results. (93−96) In some cases, iterative computational screening-experimental validation steps would require discovering materials with an end goal of achieving high-performance energy devices.

Figure 1

Figure 1. Funnel type prediction models comprising different parts such as input data set, ML model, ML predictions, and preliminary screening (left panel) and generic high throughput computational screening of chalcohalide compounds (right panel). Based on optimum sets from the prediction model, the selected candidates will be then further screened for their stability, defect tolerance, electron effective masses, mobility, and band structure using density functional theory (DFT). Reproduced (reprinted in part) with permission from refs (92and93). Copyright 2021 Wiley and 2019 American Chemical Society.

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Utilizing the above-mentioned combined approaches of ab initio calculations and machine learning (ML), two-dimensional (2D) chalcohalide (A2B2X2, B = S/Se/Te, X = F/Cl/Br/I) materials have been discovered. (93) Initially, an input data set was generated by performing DFT calculations on the structural and electronic properties of 300 out of 5300 2D chalcohalides, each of which was described by 62 initial descriptors. Following that, different ML models were tested to find suitable ML models with a minimum complexity of 26 descriptors. This ML model predicted the electronic properties of the remaining 5000 chalcohalides and screened out 411 chalcohalides with band gaps ranging from 0.9 to 1.6 eV. Materials with high cost and toxicity were then further excluded, resulting in 71 chalcohalide materials with desired properties as the outcome of initial screening, as demonstrated by the funnel approach in Figure 1, left panel. These 73 chalcohalides were systematically scrutinized by DFT for their stability, charge carrier effective masses, and mobility (Figure 1, right panel), and three chalcohalides (Bi2Se2Br2, Bi2Se2(BrI)2, and Bi2Se2I2) were predicted to have suitable band gaps, high absorption coefficients, and carrier mobilities, making them promising candidates for optoelectronic performances. With this context and the latest updates in mind, the following sections discuss the properties of chalcohalides as a result of the computational and experimental funnel approaches described above, as well as some advanced computational outcomes in section 4.

2.1. Band Gap, Carrier Effective Masses, and Dielectric Properties

The incorporation of transition metals and chalcogen in lead-free perovskites has been proposed as a viable solution for improving the materials properties through bandgap engineering. (84,97−101) Likewise, heterovalent ions combine the excellent optical properties of both perovskites and chalcogenides since both positively and negatively charged ions addition are feasible bandgap tuning methods and thus the lifetime of charge carriers. (84−86) The semiconductor chalcohalides (V–VI–VII) consisting of thio-, seleno-, and halogens are discussed in the present review, e.g., V = Bi, Sb, Pb, Sn, Cu, Ag; VI = S, Se; VIII = Cl, Br, I. Among them, pnictogen chalcohalides are Bi- and Sb-based post-transition metal chalcohalides that comprise a trivalent heavy pnictogen (Bi/Sb) cation, a divalent (S/Se) chalcogenide, and monovalent (I/Br/Cl) halide anions. These materials show both types of band alignment (direct and indirect), with n- and p-type depending on their synthesis methods and bonding of ions into crystal structures. As an example, BiSeI grown by chemical transport reaction shows n-type conductivity, (102) whereas when grown by the Stock-Barger method (growth from the melt) shows both n- and p-type conductivity. (103) The structural arrangement of metal cations bonded to multiple anionic ligands can improve the local degree of freedom and lower local asymmetry while achieving band gap control. On the other hand, the difference in their bonding caused by the electronegativity difference in anions causes an increase in the distortion degree of coordination polyhedron. Chalcohalide materials reviewed in the current review showed experimental and theoretical band gaps between ∼0.75 and 3.0 eV and were also tunable with varying chalcogen and halogen composition, (75,104−112) suggesting their potential application in the wide optoelectronic and solar energy conversion devices. Most of the chalcohalides, which have shown PV performance up to now, exhibit high absorption coefficient (>1 × 105 cm–1) and indirect band gaps. However, the difference between direct and indirect band gaps is very small (between 0.02 to 0.1 eV), which makes them potential candidates for solar energy conversion devices. Despite the fact that this family of materials has been synthesized decades ago, their potential application in solar energy conversion has only recently been predicted (theoretically). However, the synthesis of these chalcohalide materials with the chosen optoelectronic properties is still challenging for large-scale applications.
The theoretical and experimental data revealed that chalcohalide materials, like chalcogenides (113−116) and halide perovskites, (117−121) possess highly nonlinear dielectric properties and exhibit a strong piezoelectric effect. These types of materials are characterized by their layered structure, strong anisotropy, and relativistic effect. Most of these materials also possess the ferroelectric-PV (FE-PV) effect, which refers to the generation of a steady PV response (photocurrent/photovoltage) along the polarization direction in these ferroelectric materials without central symmetry. It is different from the standard PV effect where the photogenerated charge carriers at the semiconductor p–n junction are separated by an internal built-in electric field. Notably, the direction of photocurrent in the FE-PV device can also be reversed by changing the spontaneous polarization direction with an external electrical field. (122,123) Many well-known ferroelectric materials possess the perovskite structure including halide perovskites (124−127) and recently emerging chalcogenide perovskites. (128−131) Since the dielectric constant is a measure of the of electric potential energy in the form of induced polarization, this feature increases the promise of such materials considerably. (118) It is assumed that large dielectric constants and low effective masses would promote efficient ionization of defect states and suppress deep trap states within the bandgap. (132) The antibonding character of s orbitals causes the lower hole effective masses than the electron effective masses. (133)
Ran et al. (75) calculated formation energies, band gaps, and effective masses of electrons/holes and dielectric constants for the 36 Bi- and Sb-based materials, shown in Figure 2. As shown in Figure 2a, the negative formation energy indicates the successful synthesis of these materials. The respective materials have both direct and indirect band gaps and smaller effective masses. The effective masses of holes and electrons are shown not only as a function of the average electronegativity discrepancy between anions and cations but also as a function of the bond valence sum of the cations (Figure 2b–e). Additionally, the high dielectric constant values due to large phonon contributions show the capability of the material for energy-related applications (Figure 2f). Pb-halide perovskites have high dielectric constant, low effective masses, and VB antibonding nature that lead to defect-tolerant transport characteristics due to their ns2 electronic configuration. (134,135) Similarly, these chalcohalide materials with ns2 electronic configuration have shown higher dielectric constants and accordingly smaller charge capturing cross-section defects that cause low scattering and recombination. (132)Section 4 discusses the theoretically calculated band alignment and effective masses for various chalcohalides in detail. For easy understanding, the histograms of Heyd-Scuseria-Ernzerhof (HSE) hybrid functional + spin orbit coupling (SOC) band gaps, effective masses of electron and hole for different Bi-based chalcohalides are also presented in Figure 2g and h. It reveals that iso-structural compounds, BiSeCl, BiSeBr, and BiSeI, show similar optical band gaps of ∼1.47–1.48 eV, where BiSCl, BiSBr, BiSI, and Bi3Se4Br have bandgaps in the range of 1.5–2.2 eV. The BiSeCl, BiSeBr, and BiSeI materials have shown similar transport effective masses of 0.53–0.57 m0 and ∼2.19–2.98 m0 for electrons and holes, respectively. It illustrates the prominent role of the chalcogen in the electronic structures of the upper VB and also indicates the potential as a solar absorber material, which is consistent with the present findings. In BiSCl, BiSBr, BiSI, and Bi3Se4Br materials, the electron or hole effective masses are lower than m0, indicating that these materials are potentially useful as RT radiation detection materials. SbSI indicates an optical bandgap of ∼2 eV, and in fact, it can be varied by changing the chalcogen and halide. (109,118) Theoretically calculated electron effective masses using generalized gradient approximation (GGA) for SbSI, SbSBr, and SbSeI were 0.21, 0.26, and 0.52 m0, respectively, with bandgaps of 1.51, 1.57, and 1.29 eV, suggesting possible high mobility in good quality crystals with low defect concentrations. (74) Berry phase analysis of the polarization revealed that, despite having a smaller change in polarization (ΔP) than many perovskite structures, all three materials have significant spontaneous electric dipole moments. Besides, Sn5S4Cl2 was theoretically predicted to be stable compositions with bandgaps across the visible range, but it also evidenced low electron and hole effective masses of 0.5 me and 0.4 mh, respectively, suggesting that it could be used as a low-bandgap absorber in multijunction solar cells. (85,136)

Figure 2

Figure 2. (a) Formation energies per atom (upper panel), band gaps (middle panel), and effective masses of electron and hole (lower panel) for the 31 Bi- and Sb-based compounds. Red, green, and blue shaded areas in the middle panel indicate bandgap ranges of 1.0–1.7, 1.5–2.2, and >3 eV, respectively. Gray shaded area in the lower panel indicates effective masses lower than the rest mass of the electron (m0). (b) Band gaps and (c) effective masses of holes and electrons as a function of average electronegativity discrepancy between anions and cations for the 31 Bi- and Sb-based compounds. (d) Band gaps and (e) effective masses of holes and electrons as a function of bond valence sum (BVS) of the cations. (f) Calculated dielectric constants (ε) of the considered Bi/Sb oxyhalides and chalcohalides. Total static dielectric constant (εst) and the component from phonon contribution (εphonon) has been shown. (g) Histogram of HSE + SOC band gaps, effective masses of holes and electrons for BiSeCl, BiSeBr, BiSeI, and Bi3Se4Br. (h) Histogram of HSE + SOC band gaps, effective masses of holes and electrons for BiOI, BiSCl, BiSBr, BiSI, and Bi3Se4Br. (a–h) Reproduced with permission from ref (75). Copyright 2018 Nature Publishing Ltd.

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To improve the properties and sustainability of halide perovskite, researchers focused on the addition of chalcogen ion to ABX3 perovskite making it AB(ChX)3-type chalcohalide perovskite. Although the photoconversion efficiencies of solar cells based on Bi- and Sb-based halide perovskites are significantly lower than those of organic–inorganic Pb-halide perovskites, (138−141) the inclusion of negatively charged chalcogen ions shifts their bandgaps for favorable optoelectronic properties and forms metal chalcogenide bonds. As a result, the overall covalence significance of the compound is found to be more significant, leading to improved durability. Mao et al. (137) calculated the band gaps of selected 192 mixed-anion chalcohalide perovskites ranging from 0 to approximately 7 eV as well as the influence of total anion electronegativity on band gaps for the methylammonium (MA) and formamidinium (FA) systems (Figure 3a). Similar to halide perovskites, the bandgap values of these chalcohalide perovskites increase when X″ (F–, Cl–, Br–, or I–) changes from I to Br, Cl, and F (from top to bottom in the panel) and decrease when the X′ (O2–, S2–, Se2–, or Te2–) changes from O to S, Se, and Te (from left to right in the panel) as presented in Figure 3b. This study thus introduces a viable pathway for exploring stable, lead-free chalcohalide perovskites for PV applications by screening suitable candidates from the 192 compounds by taking into account the tolerance, octahedral factor, and stability. Figure 3c shows the band gaps for the nine materials with combinations of S, Se, or Te and Cl, Br, calculated using the HSE hybrid functional with SOC. According to the Shockley–Queisser theory, the bandgap of these materials can be tuned to provide an optimum bandgap for a single-junction solar cell. (73) Red-to-black piezochromic response for a larger bandgap (CH3NH3)2[PbI2(SCN)2] (MAPSI) material captured researchers’ attention. (142) Although the performance of MAPSI may be affected by MAPI contamination, the typical optical properties and its large pressure response may originate from (i) the short interlayer contact, (ii) the covalency of the Pb–S linkages, and (iii) the electronic effects of SCN– substitution in the Pb–I lattice. However, significant response of the material to comparatively low pressures opens up new avenues for more optical and electronic diversity from 2D structures.

Figure 3

Figure 3. (a) Band gap as a function of average electronegativity and (b) calculated band gaps of 192 mixed-anion perovskites. Each square represents a material, and the color represents the corresponding band gap. The larger letters are A ions, and the smaller letters are X″ ions on the left. The larger and smaller letters below are B and X′ ions, respectively. (a, b) Reproduced with permission from ref (137). Copyright 2019 American Chemical Society. (c) Calculated band gaps of CH3NH3BiXY2 compounds (with X = S, Se, or Te and Y = Cl, Br, or I) using the HSE functional with SOC. The dashed line marks the optimal band gap for a single-junction solar cell according to the Shockley–Queisser theory. Reproduced with permission from ref (73). Copyright 2016 The Royal Society of Chemistry.

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2.2. Defect Tolerance

The ns2 electronic configuration of cations plays a vital role in the optoelectronic properties of the materials. (143) Therefore, materials containing post-transition metals, such as Bi3+, Sb3+, Cu+, Pb2+, and Sn2+ (with ns2 electronic configurations), are considered as defect tolerant, (97,144) which leads to slow electron–hole recombination kinetics from their intrinsic point defects and could make them last longer. (97,145−147) Chalcohalide materials containing these post-transition metals with a 6s2 or 5s2 electronic configuration can share the high dielectric constant, low effective masses, and antibonding character of VB, which all contribute to the defect tolerant transport properties. (97) These partially oxidized (Bi3+, Sb3+, Sn2+, Pb2+) and polarizable cations offer high-born effective charges and therefore large dielectric constants, CB with greater bandwidth due to SOC, dispersive VB, and low effective masses due to filled 5s/6s orbitals. Wlázlak et al. (81) also emphasized multiple features of these chalcohalides such as low carrier effective masses, the ferroelectric-PV effect, and the ns2 configuration of trivalent heavy pnictogens, which are thought to be the source of defect tolerance similar to in Pb-halide perovskites.
A ns2 electron configuration is mainly encountered in defect tolerant semiconductors, indicating that the presence of lone pair electrons in metals is an important point in their appealing optoelectronic properties. Consequently, this characteristic gives a more rational basis for attributes like band gaps, carrier effective masses, and defect levels same as in perovskites. (148) The antibonding feature of the valence band maxima (VBM) (with mixed halide 3/4/5p and s character) and spin–orbit effects in the CB are what give the vacancy-related states their shallow defect character. (20) As a result, the presence of lone-pair states and antibonding coupling at VBM are critical band edge characteristics for favorable optoelectronic properties. (148) In addition to producing a higher energy VBM that lowers ionization energies and results in decreasing band gaps, the antibonding component also produces more dispersive bands from s-p hybridization, raising hole mobility and lowering recombination rates. (149) Since the occupied cation “s” states both produce lighter hole masses and increase the valence band energy by introducing cation s-anion p interaction. Furthermore, the lone electron pair involved with compounds formed from metals with a ns2 electronic configuration has been shown to be strongly dependent on the electronic states of the anion. (150) The geometric variation of these compounds, including the structural distortions linked to the lone pair of electrons, may be simply explained by the relative energy differences between the electronic states participating in the orbital interactions. (151) Dangling bond defects might be drawn quantum mechanically into the continuum bands by the existence of an antibonding higher VB and a bonding lower CB, leaving the bandgap clear and enabling the creation of shallow defects. (152)
Thus, the defect tolerance property arises from the fundamental electronic band structures, the density of states (DOS), orbital characters of the CB and VB extrema, effective masses of electrons and holes, dielectric constant of the material, etc. The presence of intrinsic point defects, particularly structural defects in the defect-tolerant materials, causes relatively slow electron–hole recombination kinetics within the bandgap. (153,154) Depth of the defect level can also be quantified. (146) Although the idea of defect tolerance is claimed to extend to this class of materials, it has not yet undergone a thorough investigation. Zhang et al. (155) developed a computational method to evaluate the term “defect tolerance” by quantitatively examining the nonradiative recombination rates caused by deep-level defects, which gives rise to another phase of further research.

2.3. Band Structure

Chalcohalide materials show a high level of band dispersion and antibonding states in the VBM. This gives the materials both ionic and covalent characteristics and hence strong lattice polarization and large static dielectric constant. (75) Hybridization of cation s and anion p orbitals makes the dispersive VB (Figure 4a, upper panel). Additionally, the cross-band gap hybridization takes place between extended cations p orbital from CB with anion p orbital from VB as shown in Figure 4a (lower panel). Such cross-gap hybridization indicates covalency in the otherwise ionic materials, as stated above. (143) Also, the presence of 5s or 6s orbital in VB causes the charge transfer from s and p state, contributing to the strong optical absorption. The presence of chalcogen along with halogen reduces the intrinsic ionic conductivity in the chalcohalide materials, which in turn helps to increase the charge collection efficiency and reduce the space charge effects. (111) SbSI material is predicted to have the same band structure as that of Pb-based halide perovskite and hence exhibits low effective masses of charge carriers, high absorption coefficient, and suitable bandgap, making them suitable for PV and ferroelectrics. (75,97,156) This ns2 configuration causes the cation-s state to move down in energy, forming the lone pair state and separating from the cation-p states due to the Mass-Darwin effect. This causes the significant charge transfer only from the cation-p to the anion-p states rather than from the cation-s to the anion-p states. (75) In such materials, therefore, VBs are mainly formed by antibonding combinations of cation-s and anion-p states and CBs are dominated by spatially extended cation-p states and also hybridize in the antibonding pattern with anion-p states. All these characteristics cause dispersive VBs and CBs making the material favorable for small effective mass, high carrier mobility and Born effective charges, and high defect tolerance (Figure 4b,c). (144) Consequently, the presence and preferences of the elements from group III to VII make the material a great candidate for optoelectronics. This is because (i) the typical combination of cation and anion, as well as the partially oxidized cation, leads to shallow inner point defects in the material (Figure 4c), (144) (ii) not only is the dispersion of the VBM dependent on the hybridization between anion p-orbitals and cation s-orbitals but 70–90% of the VBM partial DOS (pDOS) in these materials originates from anion p orbitals, and (iii) when the cations dangling bond that surround the anion vacancy have minimal interaction, they form the shallow anion vacancies. This is possible when dispersive CBM is formed from heavy cation p-orbitals. In most of the selenoiodides, in which anions have low coordination number and cation–cation distances are long, shallow anion vacancies are often formed, allowing efficient electron transport and thereby making them good electronic and optoelectronic materials. (146) The presence of a relativistic effect that increases the width of the CB by lowering the CBM stabilizes chalcohalide materials against oxidation. (82,157) The relative energy of the p and s orbitals determines the contribution of the s orbital to the VBM. Because the VB forms by combining the anion p with cation s orbitals with the antibonding VBM, it again is directed toward the stabilization since s orbitals more greatly contribute to making a stronger lone pair and hence shallow states at the band edges, rather than deep gap states that act as traps and recombination centers (Figure 4c). (133,158) All of these properties are therefore supposed to provide a high degree of defect tolerance. Bi-based materials share many similarities in the chemical properties with Pb-based halide perovskites, e.g., stable electronic configuration (d10S2p0), similar polarizability, rich structural diversity (BiX6 clusters, one-dimensional (1D) structure, layered structure), and large relativistic effects. (82,133,157)

Figure 4

Figure 4. (a) Schematic of DOS for a typical compound semiconductor (not including transition-metal and rare-earth compounds with significant d or f character in the CB) (top) and a compound that contains ns2 cations (bottom). Reproduced with permission from ref (143). Copyright 2016 American Physical Society. (b) Schematic band structure indicating how greater band dispersion (curvature) gives rise to smaller hole and electron effective masses and (c) impact of bonding structure on defect tolerance, with antibonding states at the top of the VBM giving rise to shallower defects. (b, c) Reproduced with permission from ref (144). Copyright 2016 The Royal Society of Chemistry.

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2.4. Stability

Long-term stability is one of the key factors along with the efficiency, cost-effectiveness, and nontoxicity of materials, determining the perspective of commercialization. Although hybrid Pb-halide perovskite semiconductors have become one of the most researched semiconductor materials in the last ten years, they are prone to instability in the presence of moisture, oxygen, and temperature. In comparison, the current study has been intended to look a benign alternative that is also stable under atmospheric conditions. Materials can be assessed concerning their structural and device stabilities. When we talk about structural stability, we mainly refer to the ability of materials to retain their desired phase under ambient conditions. While device stability is referred to maintaining the performance without any significant changes in defects concentration or at interfaces between different stacked layers such as chemical changes including composition, diffusion of elements or decomposition of materials. Chalcohalides have been predicted to be stable compared to the inorganic and organic–inorganic hybrid halide perovskites owing to the increased strength of metal-chalcogen bonding. (75) To be specific, the metal-chalcogen interactions in chalcogenides are more covalent, leading to overall covalent bonding in the materials and hence higher stability, whereas the metal-halogen interactions in halide perovskites are less polar covalent. (84) On the other hand, a partial substitution of halide ions with chalcogen in chalcohalides offers a greater enthalpy of formation making them structurally more robust.
The Goldschmidt tolerance factor is used for the estimation of suitable components that form a stable perovskite structure at an empirical level. For chalcohalide perovskites, the tolerance factor is estimated to be 0.99 (e.g., MASbSI2). However, detailed theoretical calculations revealed negative decomposition energies, suggesting that these chalcohalide perovskites may be difficult to synthesize experimentally under thermal equilibrium conditions. (72) The perfect crystal lattice of perovskite structure does not involve any lattice distortion arising from lattice strain as well as any defect vacancies beyond the thermodynamic equilibrium conditions. (159−162) However, lattice strain and vacancies can originate during the materials synthesis and growth process, which are unavoidable and are highly detrimental to stability. (163−167) Since studies on these chalcohalide perovskites are at earlier stage, a thorough understanding of the origins and formation of lattice strain and vacancies is necessary. Though preliminary studies reveal the notable stability of chalcohalide materials, the question of whether these chalcohalides offer a synergy of good semiconducting properties of halides and good stability of chalcogenides is not entirely answered yet. The following sections may offer more insights into addressing this question.

3. Structural Chemistry

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Unlike chalcogenide and perovskite, the difference in electronegativity in these mixed-anionic compounds invariably results in distinct bonding properties. High electronegative elements prefer to bind with high electropositive elements, forming strong ionic bonds, whereas low electronegative elements prefer to coordinate with low electropositive elements, forming stable covalent bonds, resulting in unique crystal structures and physicochemical properties. (168) Since the chalcohalide cations (Sb(III), Bi(III), etc.) exhibit a second-order Jahn–Teller instability like Pb(II) and Sn(II) cations linked with changes in coordination environment from symmetric to asymmetric, the phenomenon of phase behavior can be linked to the ns2 lone pair electrons. (150) Besides, the strength of both dangling-bond hybridization and orbital repulsion at band edges is also influenced by the crystal structure and so is a driving element in the electronic structure. (99) Moreover, defect tolerance is controlled by bonding behaviors that are an equilibrium of various competing factors such as ion coordination, bond length, hybridization, bonding character, crystal symmetry, oxidation state, Jahn–Teller distortion, etc. since it affects the energetic position of the defect charge transition level in the end. (97,146,152,169)

3.1. Heavy Pnictogen Chalcohalides

3.1.1. Bi- and Sb-Based Chalcohalides

Most of the reports on the MChX (M = Bi, Sb, Ch = S, Se, and X = Cl, Br, I) were founded on the orthorhombic crystal system with Pnma and Pnam space groups. Rod or needle-like morphology of these V–VI–VII ternary or pseudoquaternary materials is due to their structural characteristics. Figure 5a and b present the structures of BiSI and SbSI(Cl). (107,170) In BiSI or SbSI, when one chalcogen (Ch = S, Se) is replaced by two halogens, the M4S6 chain (from Sb2S3 or Bi2S3) splits into two (M2S2X2) chains. As an example, the crystal structure possesses the double chains of [(MChX)∞]2 in the form of a binary screw axis linked together by a strong M-Ch (Sb/Bi-S/Se) covalent bond and the halogen anion has an ionic bond with a covalent binding bridge. (102) Double chains of [(MChX)∞]2 grow parallel to the c axis with weak interchain van der Waals forces between the chains evolving in an anisotropic nature. As a result, the growth rate occurs more than 50 times parallel to the c-direction than that perpendicular to it, proved by Molnar et al. (171) Fong et al. (172) described in detail about the ionic–covalent bonding between cations and anions of these materials. Because M (Bi, Sb) atoms cannot provide enough charge for the chalcogen anions to form an ionic bond, Bi and Ch (Se) compromise by forming a covalent bond after electrons are transferred into the halogen anions. Therefore, the bonding between M and Ch is considered more covalent than the bonding between M and X. (172) The quasi 1D chains are composed of distorted pseudo-octahedra edge-sharing, which consists of 3 Bi–Ch and 2 Bi–I bonds with the vacant site occupying the Bilone pair. (173) The vertical orientation of these chains on the substrate material could promote carrier transport and also reduce the charge recombination due to the parallel occurrence of enforced vertically aligned grain boundaries along the covalently bonded 1D axis. (133) As we transition from BiSI to BiSeI, the strength of the lone pair decreases, distorting the pseudo-octahedral lattice, and therefore a deviation in the bond angles. Therefore, charge density isosurfaces show the different electron densities (lower at VBM) surrounding the Bi ions in BiSeI making a kind of difference (Figure 5c). SbSI has also two phases that are linked by a ferroelectric distortion, where a small displacement along with the z-axis switches from Pnam (centrosymmetric, D2h) to Pna21 (noncentrosymmetric, C2v) structures and vice versa. (74) Since the ferroelectric transition produces a single phase with spontaneous polarization, SbSI is an excellent material for exhibiting both bulk and poly crystalline photoferroic effects.

Figure 5

Figure 5. (a) BiSI structures, viewed down the b- axis at the ac plane of their orthorhombic unit cells. The black circle highlights the Bi–S 1-D chain. Reproduced with permission from ref (107). Copyright 2017 American Chemical Society. (b) Crystal structure of SbSI/Cl (SbSI0.73Cl0.27) viewed along a-axis where the double-chains arrange along the c-axis within the Sb–S and Sb–(I, Cl) chemical bonds. Reproduced with permission from ref (170). Copyright 2019 American Institute of Physics. (c) Charge density isosurfaces of the (1) upper VB and (2) lower CB of BiSeI. Regions of low and high electron density are shown in blue and red, respectively. Bi, Se, and I atoms are denoted by orange, green, and purplish–red spheres, respectively. Reproduced with permission from ref (133). Copyright 2016 The Royal Society of Chemistry. (d) Structural model for Bi19S27Br3. Reproduced with permission from ref (174). Copyright 2015 The Royal Society of Chemistry. (e) Crystal structure model of hexagonal (Bi19S27I3)2/3. Reproduced with permission from ref (175). Copyright 2018 The Royal Society of Chemistry. (f) Crystallographic packing diagram of Bi13S18I2 unit cell. Bismuth, magenta; iodine, purple; sulfur, yellow. The disordered bismuth is shown in gray/magenta with an occupancy factor of 0.25. Reproduced with permission from ref (176). Copyright 2019 The Royal Society of Chemistry. (g) Crystal structure of Bi13S18I2. Inset shows the HAADF-STEM image of an as-sintered nano Bi13S18I2, showing the hexagonal periodicity of I atoms as darker dots (green spheres) and Bi atoms as brighter dots (purple spheres). Reproduced with permission from ref (177). Copyright 2018 John Wiley and Sons. (h) Averaged cell representation (ACR) of the hexagonal P63 structure of Bi13S18I2 viewed down the c-axis. Inset: normal to the c-axis to highlight bismuth (disordered/averaged dimer) sites, in hatched red. (i) Superstructure of Bi13S18I2 in P3 (where c′ = 2c). (1) Representation viewed down the c-axis, subvalent bismuth shown in teal. (2) Representation of 4–3′ Bi24+ dimers (shown in teal) and their coordination spheres viewed perpendicular to the c-axis. (h, i) Reproduced with permission from ref (178). Copyright 2017 American Chemical Society.

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3.1.2. Derivatives of Bi-Based Chalcohalides

Another Bi-based chalcohalide studied material is Bi19S27X3 (Bi19S27Cl3, Bi19S27Br3, and Bi19S27l3) (Figure 5d). It possesses few other isomorphic structures, like Bi13S18I2, belonging to hexagonal crystal structure with P63 or P63/m space group shown in Figure 5e–i. (175−178)
The very first report from this family is of Bi19S27I3, reported by Miehe and Kupčík in 1971, which possesses a hexagonal crystal system with the space group P63. (179) Similarly, Bi-S-X based derivative (Bi19S27X3) shows the hexagonal crystal structure where three (Bi4S6) chains are connected by two I atoms and an incomplete (2/3) occupation of one Bi atom, i.e., Bi2/3(Bi4S6)3I2 and it becomes the Bi19S27I3 as per the Daltonian formula (Figure 5d,e). (174,175) Though Bi19S27I3 has been previously reported, (179,180) several conflicting reports exist on its true space group symmetry. Therefore, Groom et al. (178) reconsidered its crystal structure and newly proposed the Bi13S18I2 in trigonal lattice with the P3 space group. In this context, Bi13S18I2 is proposed as a typical complex structure corresponding to the trigonal lattice shown in Figure 5f, g, and h. The structural complexity was resolved by using single-crystal X-ray diffraction (XRD) and synchrotron powder diffraction. In Bi13S18I2, the ribbon-like subunits of (Bi4S6)∞ form six spokes around a central hexagonal channel at the corners of the unit cell and the iodine in the structure is located between the spokes in triangular channels. (177,178,181) According to the work by Xu et al., (177) Bi13S18I2 exhibits four sites for disordered Bi(X), each with a 1/4 occupancy and the disordered Bi(X) atoms at the (1/3, 2/3, z) sites coordinate with six S atoms to form a wheel-like cluster (Figure 5g). The Bi-S coordination forms in these structures are the same as those in Bi2S3, and these Bi and S atoms construct a 3D structure and create two 1D tunnels propagating in each unit cell along the [001] axis, in which I atoms reside. The inset of Figure 5g shows the HAADF-STEM image of Bi13S18I2, showing the hexagonal periodicity of Bi and I atoms. The averaged cell representation (ACR) of the hexagonal P63 structure of Bi13S18I2 viewed down the c-axis is represented in Figure 5h, and Bi (disordered/averaged dimer) sites are highlighted in the inset. After a thorough study on the Bi13S18I2 structure, Groom et al. (178) reported the charge-balance when solved in the P3 space group with Bi24+ dimers as (Bi3+)12(Bi24+)0.5(S2–)18(I–)2. The corresponding chain of dimers and their coordination spheres resulting from the superstructure and partial occupancy is shown in Figure 5i. The presence of Bi24+ dimer units in this material was further confirmed from a presence of a weak peak at 106 cm–1 in the Raman spectrum and redox chemistry from thermal decomposition. (178,181)

3.2. Transition/Post-transition and Mixed-Metals Chalcohalides

Manna and co-workers studied the crystal structures of Pb-based chalcohalides (Pb4S3Br2, Pb3S2Cl2, and Pb4S3I2) from 3D electron diffraction and electron tomography of a single nanocrystal (NC), XRD, and DFT calculations. (112) They proposed that the Pb atoms possess three different coordination environments, where two Pb sites are surrounded by 4 S + 4 Br and one Pb atom surrounded by 6 (S + 2 Br), whereas S possesses two nonequivalent crystallographic sites and Br resides in a heptacoordinated pentagonal bipyramidal site. The structural varieties in these chalcohalides could originate from the different geometries of the basic structural units; otherwise, they show a similar coordination environment (Pb = octacoordinated, Cl, S = deformed octahedral) presented in Figure 6a–c. The crystal structure of Pb3S2Cl2 NCs, as shown in Figure 6c, reported by Manna and co-workers closely resembles that of bulk Pb3Se2Br2 reported by Ni et al. (182) In Pb3Se2Br2, the Pb-centered dodecahedrons consist of eight corners and twelve faces that connect neighboring dodecahedrons through sharing 8 corners, 4 edges, and 8 faces by filling the space of the unit cell, i.e., anion-centered octahedra formed with 6 Pb atoms at its vertexes. (182) This results in a compact structure leaving no space between the polyhedra. (182) However, the Pb4SeBr6 material synthesized at ambient pressure exhibits different coordination environment. (183) Pb occupies three different Wyckoff positions (2a, 2b, and 4d) in Pb4SeBr6 resulting in different coordination polyhedron and ultimately three types of Pb-centered polyhedra. It builds up chains along an axis by sharing edges (Pb1) or faces (Pb2 and Pb3) linked with each other by shared corners, edges, and faces leaving space between polyhedra. (182,183) Unlike Pb3Se2Br2, Pb atoms in Pb4S3I2 exhibited three different types of chemical environments. (184) The one type (Pb1) of atom that accounts for half of the Pb atoms in the crystal structure occupies 8d sites and is surrounded by 4 iodine and 3 sulfur atoms in a distorted 7-fold coordination. Among two different types of S atoms, one S (S1) atom occupies intralayer vertices and another (S2) atom occupies common vertices between different sheets. There is a sawtooth type of shape formation to the chains due to the interaction between short distanced Pb1 and Pb2 atoms. This type of polyhedra yields edge-sharing double-layer sheets along the b-axis. The remaining two Pb atoms (Pb2 and Pb3) occupy 4c sites coordinated by 6 sulfur atoms, or 3 sulfur plus 2 iodine atoms, respectively, with both atoms linked by forming zigzag type chains along the same b-axis generating another sheet. Pb5S2I6 exhibits a crystal structure (Figure 6d) same as Sn2BiSI5 (explained afterward and presented in Figure 6h), where Pb5S2I6 chains are separated by one [PbI6] octahedra instead of two as is the case for Sn2BiSI5. (183)

Figure 6

Figure 6. (a) Pb4S3Br2, (b) Pb4S3I2, (c) Pb3S2Cl2. (a–c) Reproduced with permission from ref (112). Copyright 2020 American Chemical Society. (d) Pb5S2I6. Reproduced with permission from refs (109and183). Copyright 2016 American Chemical Society and 1973 John Wiley and Sons. (e) Cu2I2Se6 representing the view of the unit cell along the c-axis and ab-plane. Reproduced with permission from ref (111). Copyright 2018 American Chemical Society. Crystal structures of (f) Pb2BiS2I3, (g) Sn2BiS2I3, (h) Sn2BiSI5. (f–h) Reproduced with permission from ref (109). Copyright 2016 American Chemical Society. (i) Calculated crystal structures for Cmcm and Cmc21 polymorphs of Sn2SbS2I3, in the conventional orthorhombic unit cell. The empty circles show equivalent shifts in Sb position that correspond to the Cmcm to Cmc21 transition. Reproduced with permission from ref (185). Copyright 2021 The Royal Society of Chemistry. (j) Atomic structures of CH3NH3PbI3 and CH3NH3BiSeI2 and a illustration of the split-anion approach to replace Pb in CH3NH3PbI3. Reproduced with permission from ref (73). Copyright 2016 The Royal Society of Chemistry. (k) MASbSI2 proposed from DFT relaxation. Reproduced with permission from ref (72). Copyright 2018 American Chemical Society.

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Kanatzidis and co-workers reported the crystalline and air-stable Cu2I2Se6 as a wide indirect bandgap semiconductor material, possessing a 3D structured metal–inorganic framework consisting of [Cu2I2] nodes and Se6 linkers. (111) Cu2I2Se6 crystallizes in the rhombohedral R3m¯ space group (Figure 6e) and is composed of coordinatively unsaturated [Cu2I2] dimers linked together through molecular Se6 rings. They reported that the [Cu2I2] nodes are formed through symmetric bridging of the two Cu metals with two I ions. The remaining two more coordination sites per Cu ion are occupied by two Se atoms from two adjacent Se6 rings to complete a tetrahedral coordination geometry around the CuI ions. This material showed comparatively enhanced electron transport along the crystallographic ab-plane, which is superior to the similar structure of the rhombohedral Se. (111) The presence of ns2 electronic configurations also makes this material resistant to oxidation due to the presence of an inert electron pair. (108)
The lone-pair properties of A(II) and B(III) cations control the majority of the structural behavior in the A2BCh2X3 system. Kanatzidis and co-workers in their following works reported stable, n-type, quaternary sulfoiodides Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5 as statistical disordered materials. (109) The Pb2BiS2I3 and Sn2BiS2I3 exhibit similar crystal structures; orthorhombic phase (space group, Cmcm) as shown in Figure 6f and g, whereas Sn2BiSI5 crystallizes in monoclinic form (space group C2/m) as shown in Figure 6h. Parallel infinite chains of [(Pb0.5Bi0.5)2S2] and [(Sn1–xBix)2S2] (x ≈ 0.5) extend along the crystallographic a-axis and are interconnected by trigonal prismatic [PbI6] and [SnI6] polyhedra. The 2D nature of the crystalline structure and weak interaction between metals (Pb, Bi, Sn) and I provide the materials with a 3D view (Figure 6f–h). Unlike Pb2BiS2I3 and Sn2BiS2I3, Sn2BiSI5 chains are interconnected by a trigonal prismatic [SnI6] and two (Sn1–xBixI6) octahedral units and extend along the crystallographic c direction.
Recently, Walsh and co-workers performed geometry optimizations for Sn2SbS2I3 in both Cmcm and Cmc21 space groups stating that the centrosymmetric Cmcm crystal structure represents an average over multiple polar Cmc21 configurations. (185) Unlike the previously reported Pb2BiS2I3 and Sn2BiS2I3, the authors highlighted that the Cmcm structure consists of infinite chains of (Sn2S2I2)n along the a direction and is firmly packed along the b direction to create layers in the ab plane. The (Sn2S2I2)n chains are made up of face-sharing SnS3I2 pyramids with a parallelepid S2I2 base and an apical sulfur atom. Sb and I atoms are located between layers, yielding the overall (Sn2SbS2I3)n stoichiometry (Figure 6i, left). While the Cmc21 crystal structure consists of (Sn2SbS2I3)n chains along the a direction, as the Sb atoms attach to the (Sn2S2I2)n layers (Figure 6i, right). Both polymorphs have identical a and b cell lengths, with the major difference appearing in the c direction because the Sb lone-pair is oriented toward halide-bordered voids in both polymorphs; either along the interchain gap (b direction) for Cmcm symmetry or toward the interchain gap (c direction) for Cmc21, with more marked localization observed in the Cmc21 instance. The authors stated that the dynamic stereochemical activity of the Sb lone-pair is one of the key driving reasons behind the production of the deformed, lower-symmetry Cmc21 polymorph via a second order Jahn–Teller instability, which is common in lone-pair chalcogenides. (150,185)

3.3. Hybrid Organic–Inorganic Metal Chalcohalides

Recently, attempts have also been made to explore the crystal structures of emerging 3D mixed anion Pb-free chalcohalide perovskite materials using computational screening based on first-principle calculations. Sun and co-workers reported a high-throughput screening of the mixed anion Pb-free chalcohalide perovskites for PV applications with the formula of ABChX (where A = Cs+, CH3NH3+, and HC(NH2)2+; B = Ga3+, In3+, Sb3+, and Bi3+; Ch = O2–, S2–, Se2–, and Te2–, and X = F–, Cl–, Br–, and I–) and found that these materials exhibit crystal structures similar to Pb-based halide perovskite. (73) In another study, the crystal structures of CH3NH3BiSeI2 and MASbSI2 (Figure 6j,k) were also studied and revealed the cubic otherwise distorted structure after the addition of chalcogen. (72,73) The 3D MASbSI2 perovskite is hypothesized to be in a cubic phase with MA molecules aligned in an antiferroelectric configuration. (186) It was discovered to be more energetically stable than other configurations, with sulfur atoms holding the apical sites of the octahedra (along the z-axis); however, thorough conclusions on the crystal structures have not yet been discussed. Indeed, the atomic structures of few hybrid metal chalcohalides appeared to be more complex than that of an ideal perovskite ABX3. (187) This is because, while chalcogen atom benefits the hybrid metal chalcohalides, the two anion elements, Ch and X, may exhibit structural ordering and disordering, affecting stability. Such Ch-X anion ordering affects both bandgaps and formation enthalpies, and this ordering-induced bandgap variation may also result in large Urbach tails.

4. Theoretical Band Structures and Electronic Properties

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4.1. Heavy Pnictogen Chalcohalides

The optical properties of materials are a key factor for energy-related applications. Owing to heterogeneous composition due to mixed anions, chalcohalides are considered sensitive during theoretical calculations. The true bandgap of chalcohalides is also under discussion since the band diagrams vary depending on the approximation level used during theoretical calculations. Different methodologies, calculations, and approximations (along with DFT) have been used to calculate the bandgap and band dispersion and to give insights into the optoelectronic properties of chalcohalides. Ab initio and first-principles calculations, GGA, HSE, and quasiparticle GW methods have been used to investigate the electronic structures and different optical features to their corresponding interband transition of chalcohalides. In this section, we review the advances made in chalcohalide materials using theoretical simulations and discuss ongoing studies that serve as a bridge between the structures and properties described in section 2.

4.1.1. Bi-Based Chalcohalides

It is a general phenomenon that the substitution of Se lowers the bandgap due to a variation in the lattice parameters. Similarly, the indirect bandgap was tuned between 1.63 and 1.48 eV by incorporating Se into BiSI. (106) However, a direct bandgap of about 1.27 eV would result from the 100% Se-doping (BiSeI). Unlike CdS1–xSex or CdSexTe1–x, the VBM of BiSI is predicted to consist mainly of I 5p orbitals, rather than S 3p, which did not cause any noticeable shift and were largely similar regardless of the Se content. This is ascribed to the defect states introduced just above the VBM of BiSI after Se doping. As mentioned earlier, the SOC has a strong influence on the band extrema energies, but which have not been considered in prior research.
Brandt et al. (97) observed that Bi-based chalcohalides possess a larger contribution of SOC in the CB than Sn- and Sb-based chalcohalides. Therefore, Bi-based chalcohalides were proposed to be the most suitable for achieving similarly shallow anion vacancy levels as in MAPbI3 perovskite. However, both SbSI and BiSI show similar antibonding s-orbital contributions at the VBM and the corresponding DOS of both are shown in Figure 7a. In mixed halide ions, e.g., BiSBr1–xIx, VB is formed with a higher contribution of the I 5p orbitals, as compared with that of Br 4p, because the absorption edges were continuously shifted toward longer wavelengths (1.71 to 1.51 eV) with increasing x values. (76) On the other hand, S 3p also plays an important role in the formation of VB. Substitution of O2– to S2– anion in BiOBr1–xIx resulted in the significant shift with each corresponding spectrum of BiOBr1–xIx of the absorption edges toward longer wavelength in the range of 150–260 nm, suggesting the contribution from high-energy S 3p orbitals to VB formation. Research on single Se-based Bi chalcohalides has gained attraction following this report. Shi et al. (143) thus thoroughly studied the electronic structures, dielectric properties, optical absorption, and defect properties of BiSeBr, BiSeI, and BiSI by DFT calculations. The calculated band structures (Figure 7b) and DOSs (Figure 7c) show that BiSeBr and BiSI are n- and p-type, respectively, whereas BiSeI induces substantial VB dispersion in parallel and perpendicular directions to the atomic chains due to the strong interchain iodine binding. These inspiring results as outcomes from DFT calculations suggest their potential application in PV devices, transparent conducting materials, etc. They also reported the enhanced Born effective charges, resulting in strong lattice polarization and large static dielectric constants for all of these materials, and also proved that the subsequent strong screening of charge defects and impurities decreased the carrier scattering and trapping. The defects formed by Se and the Br vacancies in BiSeBr were both shallow donors and therefore did not effectively trap electrons, allowing efficient electron transport. These defects are attributed to the strong screening (suppresses coulomb potential) and low coordination numbers of anions that lead to a weak hybridization among Bi 6p orbitals at an anion vacancy. In-depth studies on the electronic structures using hybrid DFT with the addition of SOC were still necessary to underpin the properties of the band structure. With this motivation, Ganose et al. (133) studied the BiSI and BiSeI materials by relativistic hybrid DFT, with the addition of SOC to show the promises of these materials in a PV device. As a result, BiSI and BiSeI showed the theoretical band gaps of 1.78 and 1.50 eV, respectively (Figure 7d and e, left panel). The difference with the experimental band gap is due to the absence of thermal effects, including lattice expansion and electron–phonon coupling during the DFT calculation. In the relativistic renormalization of the BiSI and BiSeI, CBs were found to exceed 0.6 eV, underlining the importance of including SOC effects in modeling these material systems. The band structures of these materials revealed several band extrema’s, mostly located in the Brillouin zone, away from high-symmetry points. Also, the presence of large DOS (Figure 7d and e, right panel) at the band edges and larger absorption coefficients of greater than 1 × 105 cm–1 seen roughly at 0.5 eV above the absorption edge makes these materials ideal solar absorbers. The VB energies (ionization potentials (IPs)) of BiSI and BiSeI were found to be 6.4 and 6.2 eV below vacuum level, respectively. The reduction in the bandgap is caused due to decrease in the IP (an increasing electron affinity) of BiSeI, which lowers the CBM relative to the vacuum level. Crystal Orbital Hamilton Population (COHP) was also used to study the stereoactivity of the antibonding states of the cations and thus the energy band formation mechanism. (133) The scalar-relativistic contraction of the Bi6s shell causes relatively diffused and therefore weakly stereoactive lone pairs, which produces the asymmetric electron density resulting in small pseudo-octahedral distortion of the lattice. (133,150) The charge density isosurfaces also show the lower electron density surrounding the Bi ions at the top of the VB. The proof for the distortion in the pseudo-octahedral lattice and hence the deviation in the bond angles, which depends on the strength of the lone pair and decreases when moving from BiSI to BiSeI, is previously shown in Figure 5c. Also, the differences between the direct and indirect band gaps are small despite possessing indirect band gaps, indicating the indirect nature will have minimal effect on optical absorption and make these materials better alternatives in targeted application.

Figure 7

Figure 7. (a) DOS for BiSI and SbSI compounds. The pDOS are separated into atomic orbital contributions, with anions on the positive y-axis and cations on the negative. The cation s states are classified into bonding σ (lower energy) and antibonding σ* (higher energy) orbitals. Reproduced with permission from ref (97). Copyright 2015 Cambridge University Press. (b) Band structures for BiSeBr, BiSI, and BiSeI, and (c) their corresponding calculated DOSs using Perdew-Burke-Ernzerhof (PBE) functionals including the spin–orbit coupling. (b, c) Reproduced with permission from ref (143). Copyright 2016 American Physical Society. (d, e) Band structures and ion decomposed partial and total DOS of BiSI and BiSeI, respectively, along with the high symmetry directions. In all cases, the VBM is set to 0 eV. (d, e) Reproduced with permission from ref (133). Copyright 2016 The Royal Society of Chemistry. (f) Optical absorption of BiSI (blue) and BiSeI (red) calculated using hQSGW at 0 K. Fundamental band gaps (EghQSGW) are shown by dotted lines. (g) SLME of BiSI and BiSeI calculated using the 0 K-band gaps and optical absorption (dashed lines) and results extrapolated to RT (solid lines). (h, i) Shockley-Read-Hall recombination (RSHR) rate across the range of accessible chemical potentials for BiSI and BiSeI, respectively. The darker regions of red represents the greater levels of recombination. (f–i) Reproduced with permission from ref (173). Copyright 2018 American Chemical Society.

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The same group later studied the optoelectronic properties using quasiparticle self-consistent GW (QSGW) theory. (173) Small effective electron masses in the direction of the BiChI (Ch= S, Se) chains coupled with extremely strong optical absorption just above the bandgap attributed to the high DOS on the band edges. As per the Boltzmann transport theory, temperature-dependent effective masses in the 1D direction are found to be relatively small and strongly anisotropic. Both BiSI and BiSeI exhibited smaller effective electrons masses than those of holes. The band gaps for BiSI (1.76 eV) and BiSeI (1.50 eV), calculated using hybrid QSGW, are in close agreement with previous hybrid DFT results and the temperature corrected band gaps of 1.56 eV (BiSI) and 1.31 eV (BiSeI) are in good agreement with the RT experimental optical band gaps (Figure 7f). (173,188,189) A strong optical absorption in combination with ideal bandgaps results in high spectroscopically limited maximum efficiencies (SLME) of 25.0 and 22.5% for BiSeI and BiSI, respectively, at small thin film thicknesses and is comparable with the best-known PV absorbers (Figure 7g). They also identified the chemical potential space regions with reduced recombination rates by ∼107 cm–3 s–1 under accessible experimental conditions that guide researchers to produce more efficient solar-energy conversion devices based on Bi-based chalcohalides. The calculated recombination rate across the range of accessible chemical potentials for BiSI and BiSeI is shown in Figure 7h and i. The p-type BiSI exhibited the lowest rate of recombination due to the relatively high energy formation of the dominant deep defects (SBi and VS), leading to low concentrations of these defects compared to the total number of charge carriers. On the other hand, the rate of recombination increases gradually (over 107 s–1 than p-type) in the case of n-type BiSI due to a lowering in the energy of the ultradeep VS and BiS defects. As a result, it has been shown that Bipoor BiSI is essential to reduce the effect of unwanted recombination on the open-circuit voltage (Voc) in PV devices. Moreover, both the p- and n-type BiSeI show high levels of recombination due to ultradeep transition defects of both types (SeBi for p-type and BiSe and VSe for n-type). Thus, it is concluded that the QSGWp offers additional accuracy and many details in the analysis as compared to the DFT methods.
In this regard, further advances in theoretical research were achieved by applying GW@PBE0 functions to the calculation of the band structure and DOS of BiSI (Figure 8a). (190) A fundamental indirect bandgap is observed at 1.77 eV (at 0 K), while a direct bandgap transition is observed at 20 meV higher energy compared to the experimental values of indirect and direct band gaps of 1.50 and 1.62 eV, respectively. The electron–phonon coupling effects could be the possible reason behind this difference between the indirect and direct band gaps. The ionization potential (IP) of the BiSI is estimated to be 6.33 eV. Moreover, it also observed that the CB is more dispersed than the VB due to SOC effects induced by Bi3+, resulting in a smaller effective mass of electrons (me* = 0.53 m0) in comparison to holes (mh* = 0.95 m0). Doping in Bi2S3 by a series of extrinsic elements (F, Cl, Br, I, and some cations) was also investigated (Figure 8b). (191) It was discovered that these elements prefer to substitute the S anion as the energetically favorable doping site. This allows much lower formation energies of FS, ClS, BrS, and IS substitution than those of Fi, Cli, Bri, and Ii interstitials, i.e., 0.2–0.5 eV (under Birich, S-poor conditions) and 0.5–0.8 eV (under Bipoor, S-rich conditions) (EF = CBM). This implies that very high electron carrier concentrations can be produced in Bi2S3. Moreover, ClS showed the lowest formation energy and the FS and IS have almost degenerate energy levels due to the smallest size difference between Cl and S. Birich condition increased the n-type conductivity after Cl and Br doping as well suppressed the deep-level recombination centers. Given the relatively low CB and high n-type conductivity, it is predicted that Cl and Br doped Bi2S3 under the Birich condition acts as an ideal candidate for the electron acceptor or counter electrode; however, the performance of Bi2S3 as a light-absorber or sensitizer material is intrinsically limited.

Figure 8

Figure 8. (a) Band structure and DOS of paraelectric BiSI calculated using GW@PBE0 formalism. The VBM and CBM are situated at points between Γ–Y and Γ–Z paths, respectively. Reproduced with permission from ref (190). Copyright 2019 American Chemical Society. (b) Calculated formation energies of F, Cl, Br, I, Cu, and Ag dopants at different sites, as a function of Fermi levels under (1) Birich and (2) S-rich conditions. Reproduced with permission from ref (191). Copyright 2017 The Royal Society of Chemistry. Crystal structure, band structure, and partial density of states (DOS) of (c) BiSCl, (d) Bi3Se4Br, and (e) BiSeCl. (c–e) Reproduced with permission from ref (75). Copyright 2018 Nature Publishing Ltd.

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To date, major theoretical studies have been focused on Bi-based chalcoiodides, while findings on chalcochlorides have been lacking until BiChCl be a promising material for optoelectronic applications. Ran et al. (75) calculated the band structures and pDOS of BiSeCl, BiSCl, and Bi3Se4Br and found that all these materials exhibit indirect band gaps in the range of ∼1.0–1.7 eV (Figure 8c–e). In particular, CBM is found to be located at G, whereas VBM along the G-Z direction in BiSeCl. The CBs are highly dispersive both parallel (Γ-Z direction) and perpendicular (Γ-X and Γ-Y direction) to the chain direction (kz) due to the extended Bi-p states. On the other hand, VBs are dispersive in a direction parallel to the atomic chains (Γ-Z direction), but less perpendicular (Γ-X and Γ-Y direction) to the chain due to the weak Se–Se interchain coupling compared with that of intrachain coupling. The band structures of BiSCl (and hence BiSBr/I) are similar to BiSeCl (BiSeBr/I) except that the former have larger band gaps due to the deeper S-p states as compared with Se-p states. Although the VBs within this group of materials consist primarily of anion p states (S-p and halogen-p), the contributions from halogen-p states near the VBM increase as the halogen ion changes from Cl to I. The relatively flat VBs further resulted in larger hole effective masses than m0, while spatially extended Bi-p states led to low electron effective masses than m0. The calculated band structure of Bi3Se4Br suggested that the CBM is located at k point along the G-M direction and VBM along the N-Z direction and both turn out to be dispersive in nature. The pDOS plot shows that the VB is dominated by anion p states and hybridize with Bi 6s orbital to form antibonding states, whereas CB is primarily derived from Bi 6p states. These findings suggest that further investigation of these materials to determine their dopability, defect and dielectric behavior, and the possibility of obtaining high resistance (for detector) is necessary to evaluate other properties relevant to optoelectronic applications.
Other derivatives of BiIII-VI–VII compounds such as Bi19S27X3, Bi13S18I2, etc. have also been attracted a great interest ranging from catalysis to electronics due to their interesting characteristics. It has encouraged researchers to further investigate their properties using theoretical methods such as ab initio, first-principle calculations, and DFT. A notable study has been reported by Li and co-workers to deeply understand the electronic structure of Bi19S27Br3/I3 using DFT calculations performed with the Vienna ab initio simulation and a projector augmented wave method by using GGA with the spin-polarized Perdew-Burke-Ernzerhof (PBE) functional. (174) It has been revealed that the VBM of Bi19S27Br3 and Bi19S27I3 is composed of S 3p states, whereas the CBM mainly consists of Bi 6p states with slightly mixed with S 3p orbitals (Figure 9a). On the other hand, Br 4p or I 5p orbitals significantly contributed to lowering the VB region, i.e., to lower than −0.4 eV, which is why possibly replacing Br with I did not change the bandgap of the as-synthesized materials. The theoretically calculated bandgap of 0.6 eV was smaller than the experimental band gap of 0.82 eV, which was attributed to the shortcomings of the GGA method in underestimating the bandgap. They further studied the adsorption and desorption behavior of iodine species on the Bi19S27Br3 via DFT and demonstrated a reversible adsorption behavior of iodine species, showing its potential application as counter electrode in dye sensitized solar cells (DSSCs). (192)

Figure 9

Figure 9. (a) DOS for Bi19S27Br3. Electronic structure data calculated for the 2–1′ dimer supercell model of Bi13S18I2. The Fermi level is set at 0 eV. Reproduced with permission from ref (174). Copyright 2015 The Royal Society of Chemistry. (b) Band structure, (c) corresponding DOS plot with 2–1′ dimer contribution in red, (d) trivalent bismuth (Bi3+) pDOS (line colors correspond to atom colors in inset) and their contribution at Bi24+ dimer energies, and (e) sulfur pDOS (line colors correspond to atom colors in inset) and their contribution at Bi24+ dimer energies. (b–e) Reproduced with permission from ref (178). Copyright 2017 American Chemical Society. (f) Electronic band structure, (g) DOS, and (h) potential wells of Bi13S18I2. (f–h) Reproduced with permission from ref (177). Copyright 2018 John Wiley and Sons.

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Since that report, research has recently been moved to study Bi13S18I2. The band structure diagram and corresponding DOS plot with 2–1′ dimer contribution is shown in Figure 9b and c, respectively. A study on the anisotropy in the Bi13S18I2 material was proven from the VBM, which is dominated by iodine and sulfur states and the flatness of the iodine states demonstrates strongly localized bands along with certain directions. (178) Besides, the corresponding pDOS of trivalent bismuth (Bi3+) with their contribution at Bi24+ dimer energies and sulfur p-DOS with their contribution at Bi24+ dimer energies are presented in Figure 9d and e, respectively. These results show that the CBM is dominated by unfilled Bi3+6p states, 1 eV above the EF and the Bi2+ states (6p) from Bi24+ dimer (red) located between 0.3 and 1 eV above EF. These disordered, partially occupied subvalent bismuth dimers along with their adjacent bonded sulfur and bismuth sites generated a group of “states in the gap” between the VB and CB, which made the dominant contribution to the bands in the gap with much smaller contributions from the Bi4S6 units and iodide states. The electronic band structure and total DOS of Bi13S18I2 show the band gap of ∼0.70 eV, which is nearly similar to the previously reported band gap of 0.82 eV (Figure 9f,g). (177) The double band extrema at the CBM located between the high symmetry points, G-M, and G–K, in the Brillouin zone exhibited the high degeneracy and moderate effective mass of the electron. Authors attributed this to increased symmetry in Bi13S18I2 (trigonal) as compared to Bi2S3 (orthorhombic). In addition, the disordered Bi(X) and I atoms show the flattest potential wells (Figure 9h) as compared to S and other Bi atoms along the c-direction, indicating the weaker bonding of Bi(X) and I atoms to the nearby atoms than the other atoms. This study concludes that the lower bandgap value of Bi13S18I2 along with its dimer property, suitable band structures, flat potential wells, is likely to have low thermal conductivity and potentially a high Seebeck coefficient and electrical conductivity making this material suitable for thermoelectric properties.

4.1.2. Sb-Based Chalcohalides

Sb-based chalcohalides are the second family after Bi-based chalcohalides which are most studied ternary compounds, usually formed of a trivalent Sb cation with divalent (S or Se) and monovalent (I, Br) anions, for their optical, electrical, and ferroelectric properties. Early work on the band structure calculation of SbSI was reported by Nako and Balkanski in 1973 using the pseudopotential method in the para- and ferroelectric phases. (193) They found that the material exhibits an indirect band gap (∼1.4 eV) in both phases and the smallest direct band gap, which exists at the U point on the surface of the Brillouin zone and brings dichroism in the absorption edge. However, several characteristics of the band structures in the complex structure were not considered during the band structure calculations, which might have led to a lower band gap value in SbSI. Audzijionis et al. (194) further revealed that the upper part of the VB of SbSI is formed by S-3p orbitals, and the lower part of the CB bottom of Sb-5p orbitals from the total DOS using empirical pseudopotential method. Alward and Fong later shed a light on the VB structure of Sb-based chalcohalides in detail. (68) They observed that all the Sb-chalcohalide materials, namely SbSI, SbSBr, and SbSeI, exhibit indirect band gaps of ∼2.11, 2.18, and 1.67 eV, respectively. Among all the constituents of Sb-chalcohalides, Sb is the least electronegative, whereas the halides are the most electronegative. Thus, the lowest unoccupied states originate from the Sb sp states, while the occupied I or Br sp states lie the farthest below the top of the VB. This further explains that the largest portion of the occupied states at the gap must come from the sp states of the S or Se and the remaining states of Sb. Since S atoms are more electronegative than Se, the bond between S and Sb should be more ionic than the bond between Se and Sb, which further concludes the lower band gap in SbSeI than the SbSI and SbSBr. Akkus and Mamedov then reported the electronic structure and linear optical properties of the ferroelectric phase of SbSeI using the DFT in the GGA. (195) Unlike the previous band structure calculations of Sb-based chalcohalide using pseudopotential method, the DFT method allowed authors to use more extensive symmetry directions in a wider range to add more VB in the band structures of SbSeI. They found that the VBM and CBM are located at the Γ and S symmetry points of the Brillouin zone, respectively. Thus, the band gap of SbSeI is indirect with a value of ∼1.65 eV. Employing a similar approach, they also calculated the electronic band structures of SbSBr in para- and ferroelectric phases and found that the SbSBr exhibits an indirect forbidden gap of ∼2.16 and 2.21 eV in the para- and ferroelectric phases, respectively.
Most of the previous reports mainly focused on the electronic band structure and phase transitions, and no attention has been paid to the effective masses of these compounds despite their suitable band gap for energy applications. In this regard, Khan et al. (196) calculated the electronic band structures of SbSI, SbSeI, and SbSBr using ab initio calculations and also the effective masses of electrons and holes from the respective bands at the top of the VB and the bottom of the CB. They found that the SbSI has a smaller effective mass of electrons (0.07 me) and holes (0.098 mh) and great mobility than SbSeI and SbSBr that described the p-type nature of these chalcohalide materials. Walsh and co-workers also calculated the electronic band structure along with the effective masses of electrons and holes and IPs of SbSI, SbSBr, and SbSeI (Figure 10a). (197) The effectiveness of these Sb-based chalcohalides as an absorber layer was investigated by calculation of the electronic structure properties and by comparing the results of scalar-relativistic (GGA-DFT), HSE, and HSE-SOC. The electronic band structures of SbSI, SbSBr, and SbSeI showed indirect band gaps as displayed in Figure 10a. The upper VB in all cases is composed of chalcogen and halide p orbitals, while the lower CB made up of Sb 5p orbitals that are impacted by SOC. Although SbSI has an indirect gap with the top of the VBM lying close to the X point and the bottom of the CBM being at the S point, the difference between direct and indirect band gaps is 0.15 eV. The substitution of I by Br, on the other hand, results in a change in the electronic band structure, with the VBM in between Z and Γ and the CBM at Γ. Similarly, replacing S with Se resulted in a significant shift, in which the VBM is between gamma and Y in the first Brillouin zone and the CBM is between Y and T. In SbSeI, the contribution of the chalcogen in upper VBM is increased, owing to the lower IP of Se (<S), which explains the band engineering effect and leads to a narrowing of the gap. The change in band extrema is caused by changes in local environment of Sb, which can be linked to the compositional dependency of the stereochemical activity of the Sb 5s2 lone pair electrons. Because of the closely matched lattice parameters and systematic band offsets, epitaxial growth of SbSBr on SbSI with aligned polarization can make great applicants for semiconductor heterojunctions. A similar group found that the effective masses of electrons and holes are all below 0.65 me that can support high-mobility band transport. Further, the IPs for SbSI, SbSBr, and SbSeI were predicted as 5.37, 5.80, and 5.26 eV, respectively. The band energies of these materials were also compared with Cu2ZnSnS4 (CZTS) and Si materials for solar energy applications. As shown in Figure 10b, the position of band energies of SbSBr is like the CZTS and several other Cu-based absorbers (CuInS2, Cu2S), where the upper VB is composed of Cu 3d states. Since the band edge positions are closely related to the nature of the halide ion, the band edge states are highly composed of the halide electronic states. Therefore, the VB of SbSBr is much deeper in energy than the iodide counterparts. Based on the comparison of the energy band positions and low electron affinities of SbSBr, SbSI, and SbSeI compounds, alternative suitable buffer materials such as ZnTe, Cu2O, Zn2TiO4, KTaO3, AlTiO3, and MgTiO3 for CdS are suggested. Mo would also be a suitable hole collecting contact for SbSBr as it has a similar VB energy to CZTS, whereas Ni and Au would match to SbSI and SbSeI because of their well-matched work functions.

Figure 10

Figure 10. (a) Energy–momentum band structures of SbSI (upper panel), SbSBr (middle panel), SbSeI (lower panel); all calculated with GGA-DFT. VBs are colored gold and CBs are in green. The top of the VB is set to 0 eV. Reproduced with permission from ref (74). Copyright 2015 The Royal Society of Chemistry. (b) Comparison of predicated band energies of SbSI, SbSBr and SbSeI with CZTS and Si. Reproduced with permission from ref (197). Copyright 2016 American Institute of Physics. (c) Band structure and (d) DOS of Pb4S3Br2. The band structure, calculated using PBE+SOC function, reveals an indirect bandgap, whereas the DOS exhibits that the VB and CB band-edge states are contributed by S and Pb, respectively, while Br-related states fall deeper in the VB. (c, d) Reproduced with permission from ref (112). Copyright 2020 American Chemical Society. (e) Electronic band structure and (f) pDOS of Cu2I2Se6. The Fermi level (EF) is set to zero energy. (e, f) Reproduced with permission from ref (111). Copyright 2018 American Chemical Society. (g, h, i) Electronic band structure and (j, k, l) pDOS of Pb2BiS2I3, Sn2BiS2I3, Sn2BiSI5. The red highlighted parts in g–i represent the flat VB existed near the VBM. (g–l) Reproduced with permission from ref (109). Copyright 2016 American Chemical Society.

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4.2. Transition/Post-transition Metal Chalcohalides

4.2.1. Pb-Based Chalcohalides

The DOS calculations of Pb4S3Br2, Pb4S3I2, and Pb3S2Cl2 proved that the band-edge states mainly formed from the Pb and S states with negligible contribution from halide which mainly located deeper below the VB edge in all cases. (112) As an example, the band structure and DOS of Pb4S3Br2 are shown in Figure 10c and d. The halide-dependent shift was observed in the band gap energies with indirect band gap values of 1.98, 1.76, and 2.0 eV for Pb4S3Br2, Pb4S3I2, and Pb3S2Cl2, respectively. The optical properties in the materials are caused due to the localized Pb–S interactions due to the halides. (182) Therefore, the electronic band structures and DOS of the hypothetical Pb3Se4 and Pb3Br4 were calculated to understand the Pb3Se2Br2, which revealed that the anions contribute to the VBM and fully occupied Pb 6s orbitals located far below the VBM and Pb states contribute to the CBM. Unlike earlier reports, Ni et al. (182) found the lower band gap energy for Pb4S3I2, where DFT calculations predicted the indirect band gap value of 1.5 eV. (184) They noted that VBM of Pb4S3I2 is contributed by all Pb, S, and I, while major contribution was from S and CBM is dominated by Pb states. Thus, as explained in the crystal structural data (section 3.2), the Pb1 atom, which covers half of the Pb atoms in the crystal structure, has a higher DOS value than Pb2 and Pb3. In other words, the different chemical environments of Pb atoms contribute differently at the VBM and CBM, reflecting different DOS values. These unique and important characteristics made the Pb-based material a promising candidate for energy-related applications. (161,198,199)

4.2.2. Cu-Based Chalcohalides

Kanatzidis and co-workers are the first to report Cu2I2Se6 as a new Cu-based chalcohalide semiconductor. (111) This material shows a wide indirect band gap of ∼1.95 eV, a low electron effective mass of 0.32 eV and high chemical stability, which comprehends from the energy-momentum band structures (Figure 10e,f). The calculated band structure revealed that the CBM is located at the F point and the VBM at the L point. The Cu 3d, Se 4p, and I 5p orbitals mainly contribute to the VBs near the VBM with no dispersion, whereas the CB minimum consists of Se p orbitals with significant dispersion that results in low electron effective masses. The pDOS further revealed that the CBM is mainly dominated by the Se p states with some contribution from Cu d-Se p hybridized orbitals. It also shows that the VBM is mainly contributed by the Cu d, Se p, and I p orbitals, with the dominance of Cu d contribution. Authors thus concluded that the VBM is formed by the sequence of localized and weak Cu d–Se p and Cu d–I p interactions, which are held responsible for the flat VBM bands and high hole effective masses. Thus, the weak and localized interactions between Cu d–Se p and Cu d–I p are responsible for the nearly flat character of the VBM and high hole effective masses. Also, the band structure calculations indicate an indirect band gap with the preferable electron transport along the crystallographic ab-plane, superior to that of amorphous Se, making the Cu-based chalcohalide type of curious material for the solar cell.

4.3. Mixed-Metals (Pb/Sn/Sb/Bi) Chalcohalides

Like Bi/SbIII-VI–VII, the electronic band structure calculations of Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5 materials revealed that the VB of these materials is dominated by the Ip orbital, whereas the CB is attributed mainly to a combination of Bip and Ip orbitals (Figure 10g–i). (109) Detailed observation further explained the differences in their band gaps. A flat VB existed near the VBM (highlighted in Figure 10g–i), in contrast to the dispersive CB near the CBM, which led to the small electron effective masses than the hole effective masses making these materials n-type. In the case of Pb2BiS2I3, VBM builds from the hybridized Sp and Ip states with very few contributions from Pb and Bi states, whereas CBM is formed from the Bip hybridized with Ip and few contributions from Sp state. However, in the case of Sn2BiS2I3, high energetic Sn 5s states made a major contribution and force the VBM toward making the band gap value lower than that of Pb2BiS2I3. In Sn2BiS2I3, flat VBM and CBM formed from the hybridized Sns, Sp, and Ip states and Bip and Ip states and showed a larger band gap compared to the former states due to the major contribution of BiI6 octahedra in the electronic structure. The physical properties of Sn2SbS2I3 have recently been studied using ab initio approaches to understand the atomistic causes of exceptional PV performance by relating the local polarization (previously hidden to macroscopic diffraction techniques) to the high optoelectronic efficiency potential and defect-tolerant properties. (185) Electronic DOS revealed that CBM is generated via Sbp–Ip interactions, while the VBM is generated by antibonding interactions between the Sn 5s2 lone pair and both anion p states. Following molecular dynamics (MD) simulation, the optical absorption spectrum showed a weak onset at the fundamental electronic band gap of 1.08 eV. A low electronic degeneracy at the band extrema (a consequence of the low crystal symmetry) and a weak transition dipole moment between the VBM and CBM electronic states (a consequence of both symmetry restrictions and low spatial overlap) were roots stated for the prior behavior. The even (gerade) parity of both the VBM and CBM wave functions about the crystal inversion center resulted in a formally symmetry-forbidden transition at the direct gap in the centrosymmetric Cmcm structure, while for the Cmc21 ground-state polymorph, this symmetry selection rule was broken by a shift in Sb positions as well as spin–orbit splitting. Despite the fact that these effects have a modest impact on the electrical structure, i.e., ΔEgCmcm vsCmc21 = 0.03 eV. Figure 11a depicts the persistence of polar distortions at RT, but also dynamic fluctuations at 500 K for Sb–S bonds in Sn2SbS2I3 studied using MD simulation. For the RT MD runs within the simulation time scale, no appreciable swapping of the Sb–S bonds corresponding to lattice polarization switching is noted. During the T = 500 K runs, however, transient hopping of Sb atoms occurs as the material approaches a phase transition to higher-symmetry Cmcm, at which point the Sb–S bonds become equivalent and the probability densities merge. The overall study predicted a large spontaneous polarization of 37 C cm–2, which could be utilized for electron–hole separation in active solar cells, as well as an ideal electronic band gap of 1.08 eV with power-conversion efficiencies of ηmax, >30% at the radiative limit for film thicknesses t > 0.5 μm. From the perspective of PV applications, the bulk PV effect causes open-circuit voltages above the electronic band gap in such compounds, potentially allowing efficiencies greater than the standard limit for a single-junction solar cell. (185,200)

Figure 11

Figure 11. (a) Variation of Sb–S bond lengths in Sn2SbS2I3 as a function of time during MD simulations. The probability densities are shown on the right given by 4πr2g(r), where g(r) is the radial distribution function. Reproduced with permission from ref (185). Copyright 2021 The Royal Society of Chemistry. Comparison of (b) band structures, (c) DOS, and (d) imaginary part of dielectric function of CH3NH3PbI3 and CH3NH3BiSI2. The band structures were obtained using a supercell containing eight formula units of CH3NH3PbI3 and CH3NH3BiSI2. The k-points Γ, X, M, Z, and R have coordinates of (0, 0, 0), (0, 21, 0), (21, 21, 0), (0, 0, 21), and (21, 21, 21), respectively. The DOS of CBs in (c) for both CH3NH3PbI3 (upper panel) and CH3NH3BiSI2 (lower panel) is increased three times to clearly show the 6p states of Pb or Bi. (e) Band structure of CsSnS2Cl calculated using the HSE function and (f) imaginary part of dielectric function (ε2) of CsSnS2Cl, CsSnI3, and Si, all of which have calculated band gaps at about 1.0–1.1 eV using the HSE functional. (b–f) Reproduced with permission from ref (73). Copyright 2016 The Royal Society of Chemistry. (g) Decomposition energies of different reaction pathways for proposed MASbSI2 and FrSbSI2 compounds and (h) band structure and DOS of the proposed MASbSI2 using HSE06. (g, h) Reproduced with permission from ref (72). Copyright 2018 American Chemical Society.

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4.4. Hybrid Organic–Inorganic Metal Chalcohalides

In this context, Yan and co-workers first time assessed the viability of mixed halide-chalcogenide perovskites for PV applications through a combination of DFT calculations and solid-state reaction. (187) Theoretical calculations proposed that all of these CsSbSX2, CsSbS2X, CsBiSX2, and CsBiS2X chalcohalide perovskites are thermodynamically unstable, despite having negative formation enthalpies and thus decomposing into binary and ternary secondary phases or forming nonperovskite phases. Solid-state-reaction attempts supported theoretical predications, as none of the reactions tested produced the desired perovskite phases. These findings suggest that the synthesis of these mixed-anion perovskites is challenging under thermal equilibrium conditions, but these materials could be formed as metastable phases under nonequilibrium conditions or by entropy stabilization. With all of this context and motivation, Sun et al. (73) first reported the chalcohalide perovskites materials based on the split-anion approach by replacing Pb with heterovalent atoms (Bi, Sn) to keep both the charge neutrality and the 3D perovskite structure. CH3NH3BiSeI2 and CH3NH3BiSI2 materials exhibited suitable direct band gaps (1.3 to 1.4 eV) and optical absorption over CH3NH3PbI3, making fit for single-junction solar cells application. Both materials exhibited similar distributions of orbitals in the band structures like CH3NH3PbI3 perovskite where VBM MABiSI2 consisted of the antibonding states of Bi 6s and I5p/ S3p orbitals and the CBM consisted mainly form the Bi6p states (Figure 11b–d). The corresponding me of 0.32 and mh of 0.40 m0 were calculated for CH3NH3BiSeI2 and CH3NH3BiSI2, respectively, to compare with MAPbI3 perovskite (0.19 and 0.25 m0), suggesting good electron transport properties in these materials as like in MAPbI3. The comparative study indicates that the 3D MABiSI2 perovskites will be good candidates as absorbers for PV devices.
The inorganic mixed anion CsSnS2Cl perovskites have also shown promising optical absorption even higher than CsSnI3 and have an indirect band gap of approximately 1 eV based on hybrid functional calculation. Figure 11e shows the corresponding band structure of CsSnS2Cl calculated using the HSE functional and the band gap of 0.98 eV. The CBM located at the Γ point and the VBM at about one-third distance from Γ to M and yielded an indirect band gap that agrees with the GW calculation within 0.01 eV. The imaginary part of the dielectric function (ε2) of CsSnS2Cl, along with CsSnI3 and Si for comparison is also shown in Figure 11f. It shows that CsSnS2Cl must have even greater optical absorption than the direct-gap CsSnI3 below 3 eV and this is partly due to the relatively large DOS near the top of the VBs due to the presence of dual anions, similar to CH3NH3BiSI2. There is still a large family of new materials that are based on the split-anion approach, which has been largely unexplored. (73) Therefore, the exploration of alternative synthetic methods that can stabilize chalcohalide perovskite by kinetic factors is necessary since all predictions are focused on theoretical calculations. (201) In contrast to all these reported works, Mitzi and co-workers carried out the combined experimental and computational validation and found no evidence of the formation of the MASbSI2 phase, instead a mixture of the binary and ternary compounds (Sb2S3 and MA3Sb2I9) took place. (72) The corresponding decomposition energies of different reaction pathways for proposed MASbSI2 materials proved this hypothesis (Figure 11g) and only under the nonequilibrium condition can the metastable mixed chalcohalide phase be formed. This combined experimental and computational study showed a contradiction to the former report of Nie et al. (186) Later, to understand the electronic properties of MASbSI2, the authors calculated the band structure and DOS-based on the relaxed structure with HSE06 functional (Figure 11h). The band structure diagram shows a direct band gap of 0.83 eV significantly less than the reported 2 eV, (186) where the CBM formed mainly from Sb p orbitals, whereas VBM formed from I and S p orbitals. Relatively dispersive band edges in the vicinity of the Γ pointed good carrier mobilities. Therefore, the authors proposed that similar Pb-free mixed chalcohalide phases could still form under nonequilibrium conditions as metastable phases and could be stabilized in a 2D perovskite lattice referring to a potential candidate for PV applications due to the appropriate energy band gap and electronic characteristics. Very recently, Mao et al. (137) performed the first-principles calculations of the band gaps of 192 Pb-free mixed-anion perovskites materials and reported that the band gap increased with an increase in the electronegativity of the anions. They reported that the contribution of anions to the band-edge states is related to electron affinity and structure parameters, whereas cations mainly contributed to the CBM. These recent findings and relevant studies encourage the researchers to try out stable and efficient chalcohalide perovskite synthesis for future optoelectronic applications.

5. Chalcohalide Materials Synthesis and Applications in Energy Devices

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Various synthetic methods have been applied for the synthesis of chalcohalide materials, e.g., growth from melt and vapor, self-flux, the Bridgman, chemical and physical vapor transport, sonochemical, microwave, colloidal, and solvo- and hydrothermal methods, etc. (122−136) Initially the Bridgeman-Stockbarger melt growth and the vapor growth methods were commonly used by researchers. However, these methods often necessitated a relatively long time at high temperatures as well as several grinding and igniting stages. Because of the inefficiency of solid-state diffusion during the reaction, high temperatures and long durations are unavoidably required for the development of thermodynamically preferable phases. To overcome these limitations, researchers turned to solution-based crystal formation processes. In the next sections, we will review the synthesis of Bi- and Sb-based chalcohalides using different methods and their applications in energy devices such as solar cells, photocatalysis, photodetectors, batteries, supercapacitors, thermoelectrics, and piezo/pyro-electric nanogenerators. We will also briefly describe the development of some new Pb-, Sn-, Ag-, and organic–inorganic perovskites-based chalcohalides and their applications in energy devices.

5.1. Synthesis of Heavy Pnictogen Chalcohalides

5.1.1. Bi-Based Chalcohalides

Among the various methods reported earlier, the most of techniques normally require complicated equipment with high growth temperature (>600 °C) and control over the temperature gradient, making the process complex and low throughput. The first synthesis of Bi-based sulfoiodide crystals was carried out by wet method and vapor phase growth by the flux method. (213−216) However, the property-based analysis remained unresolved owing to very early work in 1950. Later, in 1960, Nitsche and Merz studied the photoconductivity in BiSI material for the first time, quantifying photoelectric response and revealing maximum photoresponse at ∼785 nm. (217) The Bridgeman–Stockbarger technique was then applied to synthesize orthorhombic BiSI crystals in a quartz ampule at 430 °C in a furnace with a temperature gradient of 60 °C, using Bi, S, and I2 powders. (202) The as-synthesized BiSI crystals showed the needle-like structure of ∼4–12 mm, n-type conductivity, and higher resistivity of ∼108–109 Ω cm. The anisotropy in the PV effect was stated by measuring the spectral distribution of internal photoconductivity for different BiSI crystals. Similarly, temperature dependence dark conductivity, photoconductivity, and relative spectral sensitivity for ferroelectric BiSI with Curie temperature of −160 °C were reported in that era. (203) An electron drift mobility of 1 cm2/s V along the c-axis was specified for BiSI single crystals. After 1885, the research transitioned toward solution-based techniques. The BiSI crystals were grown at room temperature using the gel technique and the growth conditions of the crystals as well as the effect of reactant concentration, gel density, and the introduction of the neutral gel were investigated. (204) The single and needle-like BiSI crystals of sizes ∼5 and ∼7 mm, respectively, were produced. One of the important attributes of chalcohalides is their dielectric properties. Ganesha et al. (205) have briefly described the effect of temperature and frequency on the dielectric properties of the BiSeI, where measurements were carried out perpendicular to the cleavage of (110) plane. In addition, the first optical properties of the BiSI material were studied for their applications in optical memories, optical switches, etc., but systematic evaluations remained vague. (188,206) Afterward, Zhu et al. (218) further demonstrated the synthesis of hollow crystals of BiSCl and bundle-rodlike crystals of Bi19S27Br3 and BiSI, using solvothermal synthesis in ethanol at 180 °C. Here authors least explained the mechanism behind the formation of orthorhombic BiSCl, BiSI, and hexagonal Bi19S27Br3 crystals. BiSCl formed from the reaction between Bi (formed after reduction of BiO+ (BiOCl) from ethanol) and atomic S to Bi2S3 and then to BiSCl gradually in 5 days, whereas Bi19S27Br3 and BiSI formed by reaction between (Bithiourea)3+ with BiBr3 and BiI3, respectively. The BiSI crystals were also synthesized hydrothermally from the reaction between stoichiometric BiCl3, thiourea, and I2 at 160 °C for 30 h. (208) The BiSI crystals showed the orthorhombic phase (space group Pnma) with a band gap of 1.8 eV. Yet, no detailed studies or material properties have been noted. In between, high-pressure synthesis polycrystalline BiSI films with rod-like crystals were synthesized on glass substrates at 320 °C using an asynchronous ultrasonic spray pyrolysis method. (209) Nonetheless, the development of these materials via the solvothermal method was not reported until the first attempt in 2011 by Cao et al. (210) to synthesize crystalline BiSI particles using a solvothermal method at 160 °C for 30 h. Although the reaction was performed at a lower temperature, it involved both longer reaction times and conditions of high pressure. To avoid these excessive reaction conditions, low temperature and mild solution approach was testified by Zhu et al. (211) for the formation of orthorhombic BiSeI rods from the BiCl3, Se and NaI, or Bi2Se3 and BiI3 in ethanol at 200 °C for 12 h. However, thorough research on understanding of reaction mechanism has never been carried out to date. Furthermore, the optical band gap of 1.56 eV was estimated by using reflectance ellipsometry. (219) The synthesis of orthorhombic BiSI and Bi19S27I3 NCs was further reported by a one-step solid-state mechanochemical method avoiding the use of aqueous solutions and heat treatment. The as-synthesized BiSI and Bi19S27I3 NCs showed band gaps of 1.80 and 1.14 eV, respectively. (220) UPS study revealed that the EVB and ECB of BiSI were 1.959 and 0.681 eV (vs NHE), respectively, while those of Bi19S27I3 were 1.423 and 0.777 eV (vs NHE), respectively. Unexpectedly, mechanochemically synthesized BiSI and Bi19S27I3 NCs showed PL spectra with a similar emission band ranging from 425 to 500 nm. It is seen that both materials show a similar emission band ranging from 425 to 500 nm with weaker emission band intensity of Bi19S27I3 than BiSI. Later, growth of large-sized strip-like BiSeI crystals was carried out using the physical vapor transport (PVT) method with self-synthesized BiSeI polycrystals. (105) Centimeter-sized BiSeI crystal showed the n-type conductivity with a bandgap of 1.29 eV and carrier concentration of 2.1 × 1016 cm–3 and Hall mobility of ∼238 cm2 V–1 s–1. Until recently, most of the above mentioned reports on Bi-based chalcohalides are mainly aimed at synthesis methods and their basic physiochemical characterizations. Table 2 summarizes the progress in the synthesis of these Bi-based chalcohalide materials along with their reported structural, morphological, electrical, and optical properties.
Table 2. Summarized Reports on the Synthesis and Properties of Bi-Based Chalcohalide Materials
materialsynthesisreaction conditionscrystal structuremorphologybandgap (eV)
BiSI (202)Bridgeman–StockbargerBi, S, and I powders; 430 °Corthorhombicneedle-like crystals (4–12 mm) 
BiSI (203)Bridgeman–StockbargerBi2S3 and BiI3 powders; 450 oC for 100 h needle-shaped single crystals1.3
BiSI (204)gelBiCl3, Bi2O3, BiI3, thiourea, H2S, KI and HI; single crystals (5 mm) 
RT for 7 daysneedle-shaped crystals (7 mm)
BiSeI (205)vapor sublimationBi, Se, and I powders; platelets (20 × 10 × 6 mm3) 
560 °C for 4 days
BiSI (206)vertical Bridgeman methodHigh purity Bi, S, and I powders (99.9999%)orthorhombicsingle crystals (50 mm in length and 10 mm in diameter1.59
BiSeI (206)High purity Bi, Se, and I powders (99.9999%)1.28
BiSCl (207)solvothermalBiCl3 + S; 180 °C for 5 daysorthorhombicpolygonal tubular (13–18) × (17–21) μm2 
Bi19S27Br3 (207)BiCl3 + Thiourea + NaBr; 180 °C for 1 dayhexagonalbundle-rodlike (2–8) × (6–20) μm2 
BiSI (207)BiCl3 + Tu + NaI; 180 °C for 1 dayorthorhombicbundle-rodlike; (5–10) × (20–70) μm2 
BiSI (208)hydrothermalBiCl3 + Thiourea + I2 in distilled water 160 °C for 30 horthorhombicbundle-rodlike 0.1 to 2 μm in diameter1.8
BiSI (209)ultrasonic spray pyrolysisA mixed solution BiCl3 + Thiourea in Water and I2 in ethanol; 320 °C for 15 minorthorhombicrod-like particles 2 μm in length and 100–200 nm in diameter 
BiSI (210)solvothermalBi(NO3)3 + thiourea + I2 in ethanol 160 °C for 30 horthorhombicnanowires 
BiSeI (211)solvothermalBi2Se3 + BiI3 in ethanol or BiCl3 + Se + NaI in ethanol 200 °C for 12 horthorhombicrod-like monocrystals 
BiSX (X= I, Br, Cl) (212)hot-injectionBi(Ac)3 + (Me3Si)2S) + BzX (X = I, Br, Cl) + Oleic acid + Octadecene; 180- 250 °C for 15 – 180 minorthorhombic (BiSI, BiSBr) and novel polymorph (BiSCl)ribbon-like nanocrystals 170 nm in length and 20 nm in width1.5 (indirect), 1.6 (direct) (BiSI) 1.95 (indirect), 2.25 (direct) (BiSBr), 2.05 (indirect), 2.55 (direct) (BiSCl)

5.1.2. Sb-Based Chalcohalides

The synthesis of Sb-VI–VII (where VI = S, Se, Te; VII = Cl, Br, I) materials has been demonstrated in the 18th century and their structure was further revealed by E. Donges in 1950 as the rhombic-bipyramidal using X-ray diffraction. (213,216) SbSI is the first material discovered to be ferroelectric as well as photoconductive in the family of chalcohalide materials. The photoelectric properties of SbSI were investigated by Nitsche and Mertz, (70) whereas the ferroelectricity was first revealed by Fatuzzo et al. (71) by measuring the dielectric constant-temperature dependence in 1962. The discovery of ferroelectricity in SbSI was a significant breakthrough that piqued the interest of researchers to further explore this family of materials. Harbeke (65) and Fridkin et al. (69) investigated the intrinsic absorption in SbSI and found that the ferroelectric phase transition in SbSI occurred as a result of a jump of the intrinsic absorption edge of ∼0.06 eV, and by a change in the temperature dependence coefficient of the forbidden bandwidth. Fridkin et al. (64) further found that the SbSBr, like SbSI, exhibited semiconducting ferroelectric properties, whereas SbSCl and SbSeCl compounds were nonferroelectric. (221) An interesting finding was further reported by Audzijonis et al. (222) where they found that the SbSI and SbSeI exist in various phases depending on temperatures, such as ferroelectric (T < 295 K, SbSI), antiferroelectric (T < 410 K, SbSI and SbSeI), and paraelectric (T > 410 K, SbSI and SbSeI). This behavior can be linked to the ns2 lone pair electrons associated with Sb(III). The conductivity of these materials was reported to be in the range from 10–7 to 10–10 Ω–1 cm–1 depending upon the composition and phase. (67)
Although, the first synthesis of Sb-based chalcohalide materials dates back to the 18th century, tremendous studies on the different synthesis processes and structural, optical, and electrical characterizations have been conducted thereafter. However, actual applications of this exciting class of materials in energy devices have been less explored to date. The ferroelectric transition in these materials, which results in one phase with spontaneous polarization, indicates that these materials can be an ideal candidate for exhibiting bulk and poly crystalline photoferroic effects. (223) However, this class of materials has been mostly overlooked as promising candidates for their applications in energy devices. In recent years, tremendous progress and exciting outcomes on these materials by applying advanced theoretical calculations (discussed in sections 2 and 3) have revealed these materials as promising candidates in energy devices in a recent renaissance.

5.2. Applications of Heavy Pnictogen Chalcohalides

Chalcohalide materials with tunable compositions and favorable optical and electrical properties, as well as their ease of processing, open up a new avenue for their applications in various energy devices. The applications of these chalcohalide materials in various energy devices, such as solar cells, photocatalysis, photodetectors, batteries, supercapacitors, thermoelectrics, and piezo/pyro-electric nanogenerators, are discussed in this section with the aim of encouraging potential new applications. Tables 3 and 4 summarize the performance of these chalcohalide materials for above-mentioned applications.
Table 3. Summarized Reports of Bi-Based Chalcohalide Materials in Different Energy Device Applications
applicationmaterialdepositionreaction conditionsband gap (eV)device structure /measurement conditionsperformance
solar cellsBiSI (224)spray pyrolysisBi(NO3)3 + NH4I + thiourea in ethylene glycol; sprayed at 275 °C1.57 (indirect) and 1.63 (direct)FTO/n-BiSI in NaI + acetonitrile electrolyteJsc = 5 mA/cm2 at 0.4 vs Ag/AgCl
onset potential = −0.25 V vs Ag/AgCl
IPCE = ∼38% at 500 nm at + 0.4 V vs Ag/AgCl
BiS1-xSexI (106)Bi(NO3)3 + NH4I + thiourea in ethylene glycol and SeO2+ ethanol; sprayed at 275 °C1.63 (x = 0) direct, 1.48 (x = 0.4) directFTO/n-BiSI/p-CuSCNJsc = 2.5 mA/cm2
Voc = 0.25–0.38 V
PCE = 0.25%
BiSI (76)electrophoreticAnnealing BiOI/FTO at 150 °C for 10 min. under 5% H2S/Ar atm1.58FTO/BiSI in NaI + acetonitrile electrolyteJsc: ∼3.2 mA/cm2
IPCE: 64% at 700 nm at + 0.2 V vs Ag/AgCl
BiSI (225)two-step spin coatingStep 1 Bi2S3: Bi2O3 + Thiourea in 2-mercaptoethanol:ethanolamine (1:4 v/v); drying at 200 °C for 5 min1.61FTO/TiO2/BiSI/P3HT/AuJsc = 0.1 mA/cm2
Step 2 BISI: BiI3 + N-methyl-2-pyrrolidinone (NMP) coated solution on Bi2S3; annealing at 200 °C for 30 min. in a gloveboxVoc = 0.4 mV
BiSI (190)spin coatingBi(NO3)3, + thiourea + NH4I in 2-methoxyethanol: acetylacetone (4:1 v/v); heating at 200 °C in the air for 5 min.1.57 (direct)FTO/SnO2/BiSI/ F8/AuJsc = 8.44 mA/cm2
Voc = 0.445 V
FF = 35.14%
PCE = 1.32%
Bi13S18I2 (181)solvothermalBiI3 + methylammonium iodide + thiourea in ethylene glycol; 195 °C for 12 h0.75 (indirect)Photoanode: FTO/TiO2/Bi13S18I2 and Counter electrode: FTO/PtJsc = 3.82 mA/cm2
Electrolyte: LiI + I2 + BMII + TBP + GT + acetonitrile:valeronitrile (85:15 v/v)Voc = 0.58 V
FF = 38.3%
PCE = 0.85%
photocatalysisBi19S27Br3 (226)microwave-assisted aqueous processBi(NO3)3 + thiourea + CTAB in distilled water:HNO3 (10:1 v/v); 80 °C for 30 min1.42Reagent: RhBdegradation rate = 95% after 70 min visible light irradiation
Incident light: λ ≥ 420 nm
Bi19Br3S27 (227)solvothermalBi(NO3)3 + thiourea + CTAB in ethanol: glycerol (3:1 v/v); 130 °C for 6 h1.49Reagent: 2,4-dichlorophenoldegradation rate = 85.6% after 10 h reaction of the visible light irradiation
Light source: Xenon Lamp (150 W)TOC removal efficiency: 76.2% after 3 h visible light irradiation
Incident light: λ ≥ 420 nmstability: 3% decay after 5 cycles
Bi19Cl3S27 (228)solvothermalBiCl3 + thiourea in ethanol; 180 °C for 72 h1.6Reagent: RhB and Cr(VI) solutiondegradation rate for RhB = 95% after 30 min. the reaction of the visible light irradiation
Light source: Xenon Lamp (350 W)stability = negligible decay after 4 cycles
Incident light: 420 nm
Bi19S27Cl3 (229)solvothermalBiCl3 + thiourea in ethanol; 180 °C for 72 h1.6Catalysts = MoS2/ Bi19S27Cl3H2 evolution rate = 876.6 μmol g–1 h–1
Sacrificial reagents: 30 mM Na2SO3 + 10 mM Na2S
Light source: Xenon Lamp (350 W)
BiSI (108)solvothermalBi(NO3)3 + thioacetamide + NaI2 in acetic acid; 180 °C for 10 h1.6Catalysts: BiSI/MoS2/CdS Sacrificial reagents: 0.2 m L–1 Na2SO4 + 10% vol. 2-hydroxypropaonic acidH2 evolution rate = 21 mmol g–1 h–1
Light source: Xenon Lamp (300 W)
Bi19X3S27 (X = Cl, Br) (230)solvothermalBiCl3/BiBr3 + thiourea in ethanol; 120–220 °C for 72 h1.32 (Bi19Br3S27)Catalysts: Bi19X3S27 (X = Cl, Br)H2 evolution rate = 195.1 μmol g–1 h–1 (Bi19Cl3S27) and 466.7 μmol g–1 h–1 (Bi19Br3S27)
Sacrificial reagents: 30 mM Na2SO3 + 10 mM Na2S
Light source: Xenon Lamp (350 W)
photodetectorBi19S27(Br3-x,Ix) (174)hot injectionTMS in ODE and Al(acac)3 + BiBr3 in OA + OLA + ODE; 180 °C for 30 min0.81 (Bi19S27Br3)p+-Si/SiO2/Au/Bi19S27Br3Ip@VBias= ∼140 nA at +5 V, 60 mW cm–2
Bi19S27I3 (175)solvothermalBi(NO3)3 + mercaptosuccinic acid + I2 in ethylene glycol + EDRA-2Na in distilled water; 180 °C for 20 h0.83 (direct)Si/SiO2/Bi19S27I3/AgIp@VBias = ∼3.5 μA at +10 V and 900 nm, 150 W xenon lamp
BiSI (231)two-step process: spin coating and post-thermal annealingBiI3 + THF - spin-coated on FTO, hydrolyzed in a 1:1 methanol:water + annealed in an H2S atm at 150 °C for 4 h1.57 (indirect)Si/SiO2/BiSI/AuIp@VBias = ∼11 μA at +10 V, 70 mW cm–2
1.63 (direct)responsivity = 62.1 mA W–1
detectivity = 2.0 × 1013 Jones
Table 4. Summarized Reports of Sb-Based Chalcohalide Materials in Different Energy Device Applications
applicationmaterialdepositionreaction conditionsband gap (eV)device structure /measurement conditionsperformance
photodetectorSbSI (259)hydrothermalSbCl3 + thiourea + NH4I in aqueous HCl: 160 °C for 4 h1.80 (indirect)Si/SiO2/SbSI/ITOIp@VBias = ∼2.6 nA at +5 V, 147.2 mW/cm2
noise equivalent power = 1.7 × 10–10 W/Hz1/2
SbSI (156)chemical bath + vapor annealingstep 1 Sb2S3: sodium thiosulfate in water + SbCl3 in acetone + water: reaction for 2 h at RT + annealing in Ar atm at 300 °C for 5 min1.9 (indirect)FTO/SbSI/PMMA/AuIp@VBias = ∼40 nA at 0 V, 100 mW/cm2
step 2 SbSI: thermal treatment of Sb2S3in an inert atm using SbI3 vapor at 250 °C for 5–10 minspecific detectivity = 109 Jones
SbSI (260)hydrothermalaqueous SbCl3 + thiourea + I2: 180 °C for 10 h1.9 (indirect)Si/SiO2/SbSI/PbI2/AgIp@VBias = ∼1 nA at 5 V, 0.65 mW/cm2, under 650 nm light irradiation
responsivity = 26.3 mA W–1
specific detectivity = 4.37 × 1013 Jones at 650 nm and a light intensity of 0.084 mW/cm2
SbSI (261)hydrothermalaqueous SbCl3 + S powder + I2: 180 °C for 20 h1.87 (indirect)Si/SiO2/SbSI/Ag (Rigid)Ip@VBias = ∼2.918 nA (rigid) at 0.81 mW cm–2 and ∼1.7 nA (flexible) at 0.5 mW/cm2 at 5 V and 635 nm light irradiation, respectively
PI/SbSI/Ag (flexible)responsivity = 18.50 mA W1– (rigid) and 13.74 mA W1– (flexible)
specific detectivity = 7.32 × 1010 Jones (rigid) and 5.43 × 1010 Jones (flexible)
nanogeneratorsSbSI (262)solid-state reactionSb + S + I2 powders heated in tubular furnace at 250 and 350 °C for 1 h-PET/SbSI/PMMA/Ag (PNG)V = ∼5 V
I = ∼150 nA
 SbSeI (263)sonochemicalSb + Se + I2 powders in ethanol: 50 °C for 2 h-Au/SbSeI/AuV = 0.012 V
I = 11 nA
Pd = 0.59 μW/m2
photocatalysisSbSI (246)sonochemicalSb + S + I2 powders in isopropyl alcohol: 60 °C for 6 h1.91 (indirect)electrode = PET/ITO/SbSII = ∼−0.5 μA at 0 V vs SHE (+400 nm illumination)
reagents = 0.1 M KNO3 + 10 mM KI, Ar purged
light source: xenon lamp (150 W)
SbSI (247)refluxSbCl3 + thioacetamide + KI in glacial acetic acid; 110 °C for 2 h1.84 (indirect)reagent: methyl orangedegradation efficiency = 97% after 20 min under the visible light irradiation
1.94 (direct)light source: 1 Sun, 1.5Gstability = negligible decay after 5 cycles
incident light: λ ≥ 400 nm
scavenging reagents = benzoquinone, ammonium oxalate, sodium azide, and isopropanol
 SbSI (253)hydrothermal + ball millingstep 1 SbSI crystals: SbCl3 + thiourea + NH4I in aqueous HCl: 160 °C for 24 h1.77reagent: methyl orangedegradation efficiency = 98% after 5 min under the visible light irradiation
step 2 SbSI NCs: SbSI crystals + ball mill for 5 hlight source: xenon lamp, power of ∼400 mW/cm2stability = fairly good
incident light: λ ≥ 420 nm
 SbSI (248)sonochemicalSb + S + I2 powders in methanol ultrasonication at 65 °C for 80 min, followed by drying at 90 °C for 3 h1.85catalysts: SbSI@CNTsdegradation efficiency = 90% after 90 min. under the visible light irradiation
reagent: acid blue 92 (AB92)stability = favorable
light source: xenon lamp, power of ∼400 mW/cm2
incident light: λ ≥ 420 nm
solar cellsSbSI (147)chemical bath + spin coatingstep 1 Sb2S3: sodium thiosulfate in water + SbCl3 in acetone + water: reaction at 10 °C, followed by annealing in Ar atm at 300 °C for 5 min2.15FTO/BL-TiO2/mp-TiO2/SbSI/PCPDTBT/AuJsc = 9.11 mA/cm2
step 2 SbSI: SbI3 in DMF spin coated on Sb2S3, followed by annealing at 150 °C for 5 min in an inert atmVoc = 0.58 V
FF = 57.7%
PCE = 3.05%
SbSI (232)two-step spin coatingstep 1 Sb2S3: SbCl3 + thiourea in DMF spin-coated, followed by heat treatment at 150 °C for 5 min inside glovebox1.96FTO/TiO2/SbSI/P3HT/AuJsc = 5.45 mA/cm2
step 2 SbSI: spin coating of SbI3 in NMP/DMSO solution on Sb2S3, followed by annealing at ∼200 °C for 1 hVoc = 0.548 V
FF = 31%
PCE = 0.93%
Sb0.67Bi0.33SI (234)chemical bath + spin coatingstep 1 Sb2S3: sodium thiosulfate in water + SbCl3 in acetone + water, reaction at 10 °C, followed by annealing in Ar atm at 300 °C for 5 min1.62FTO/BL-TiO2/mp-TiO2/SbSI/PCPDTBT/AuJsc = 14.54 mA/cm2
step 2 Sb0.67Bi0.33SI: BiI3 in DMF spin coated on Sb2S3, followed by annealing at 250 °C for 2 min in an inert atmVoc = 0.53 V
FF = 52.8%
PCE = 4.07%

5.2.1. Solar Cells

The idea of this timely review on chalcohalide compounds for energy device applications is induced by the tremendous and rapid progress in perovskite solar cells with an unprecedented device efficiency of over 25% in a short period. (28) As aforementioned, these chalcohalides have emerged as urge to find an alternatives to Pb-based halide perovskites solar cells. These chalcohalides, containing mixed chalcogen and halogens, exhibit ns2 electronic configuration same as Pb-halide perovskites, and are thus predicted to produce efficient solar cells with high stability. Theoretical predications have quickly led their implementation in solar cells and have already demonstrated efficiencies up to 4–5%. In this section, the advances made in solar cells based on these chalcohalide materials are discussed.
5.2.1.1. Bi-Based Chalcohalides
Mullins and co-workers were the first to demonstrate the potential of Bi-based chalcohalide materials in photoelectrochemical (PEC) cells after extensive efforts in investigating different synthesis methods and fundamental properties. In 2012, the first PEC performances of n-type BiSI and BiSSeI micro rods synthesized using spray pyrolysis technique were reported. (106,224) These spray-pyrolysis synthesized BiSI thin films showed the indirect and direct band gaps of 1.57 and 1.63 eV, respectively. Initial PEC testing of BiSI films in NaI/acetonitrile showed photocurrent density up to 5 mA/cm2 with a maximum Voc of 0.37 V with an I/I3 couple under AM 1.5 G illumination and EQE of 25 to 40% across the visible range (Figure 12a). (224) Detailed analysis further revealed that BiSI absorbers had the mean hole diffusion length of ∼5 × 10–6 cm (∼50 nm), which is lower than the most common solar cell absorber materials. (224) This lower value is most likely due to the polycrystalline nature and the high defect concentration in the BiSI absorbers. Because the effective screening of the electric field at the heterojunction may also prevents electron and hole separation leading to the recombination in the neutral bulk region. Therefore, the authors tuned the optical properties and performances of BISI absorbers by incorporating Se. (106) The optical band gaps were tuned from 1.63 eV (BiSI) to 1.48 eV (BiS1–xSexI, x = 0.6). The EQE spectra in Figure 12b confirms that the Se incorporation led to the improved conversion of NIR photons, correlating with the optical band gap values. However, the incorporation of Se did not improve the conversion of UV and visible photons (Figure 12b) as well as the PEC performance of BiS1–xSexI absorbers compared to its BiSI counterpart (a PCE of 0.25%). This was attributed to an enhanced rate of recombination, which could have been induced by the inclusion of defects following Se incorporation. Immediately after, a first working solid-state heterojunction device based on n-type BiSI absorber and p-type CuSCN window layer was demonstrated. (106) However, due to charge recombination in the BiSI layer and light scattering by the CuSCN layer, the device had a very low efficiency of 0.01%. The low performance was also related to the poor electronic band alignment of the absorber with the hole and electron contact materials, which limited Voc. Nevertheless, this first attempt at fabricating solid-state solar cell devices further promised the potential of BiSI as a light absorber in solar cells.

Figure 12

Figure 12. (a) IPCE spectra of BiSI films deposited at 275 °C. Reproduced with permission from ref (224). Copyright 2012 American Chemical Society. (b) Short-circuit IPCE values measured for films (BiS1–xSexI) with various Se/(S + Se) levels measured in a two-electrode cell containing I–/I2 in acetonitrile. Reproduced with permission from ref (106). Copyright 2012 American Chemical Society. (c) IPCE spectra of the BiSI electrode in an acetonitrile solution containing 0.1 M NaI at various applied potentials and absorption spectrums of BiSI. Reproduced with permission from ref (76). Copyright 2016 Nature Publishing Ltd. (d) Energy-level band diagram of BiSI with a conducting oxide (FTO), ETL (TiO2), HTM (P3HT), and electrode (Au). Reproduced with permission from ref (225). Copyright 2019 MDPI. (e) J–V characteristics of 16 devices with the architecture of glass/F-SnO2/SnO2/BiSI(150 nm)/F8/Au. (f) Time-resolved PL of BiSI films employing 633 nm excitation. (e, f) Reproduced with permission from ref (190). Copyright 2019 American Chemical Society. (g) J–V curves of the DSSCs using bismuth-based chalcohalide and Pt as CEs. Reproduced with permission from ref (192). Copyright 2017 The Royal Society of Chemistry. (h) Schematic illustration of charge transfer processes in BiSI-based device. The energy levels of BiSI and BiI3 are based on literature data: CB of SnO2, −4.5 eV; CB of BiSI, −4.5 eV; CB of BiI3, −4.1 eV; VB of BiI3, −5.9 eV; HOMO of spiro-OMeTAD, −5.1 eV. Reproduced with permission from ref (104). Copyright 2019 American Chemical Society. (i) J–V curves of eight devices based on Bi13S18I2-based solar cells. Reproduced with permission from ref (181). Copyright 2020 The Royal Society of Chemistry.

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Kunioku et al. (76) reported the synthesis of orthorhombic BiSI and BiSeI powders from BiOI, and BiSeBr1–xIx from BiOBr1–xIx by a low-temperature treatment (<150 °C) and within a short time (<1 h) under H2S and H2Se gas, respectively. Surprisingly, the synthesis carried out at high temperature (>200 °C) resulted in the formation of Bi12.7S18I2 impurities in BiSI due to the volatilization of I2. The absorption edge of BiSI was significantly shifted from ∼ 630 to 780 nm with a successful phase transition from BiOI to BiSI. A similar trend was observed in BiSeI, however, BiSeI exhibited longer absorption (>900 nm) compared to that of the BiSI. The indirect band gap of BiSeBr1–xIx was continuously tailored in between 1.71 to 1.51 eV by controlling Br/I ratio, which significantly contribute to VB formation. To check the validity of the synthesized BiSI as absorber materials in PEC device, comparative J–V measurements of BiSI electrode prepared from BiSI NCs and electrophoretic deposited BiOI/FTO under H2S/Ar atmosphere was investigated. The BiSI-based PEC device exhibited the highest current at all potentials as a result of the decreased resistance and/or increased number of absorbed photons by densely packed particles. The IPCE spectra as shown in Figure 12c showed an IPCE of 64% at 700 nm at 0.2 V vs Ag/AgCl, which is significantly higher than the reported value by Mullins and co-workers for the BiSI photoelectrodes (e.g., ∼38% at 500 nm at 0.4 V vs Ag/AgCl) under similar conditions. (76) This work demonstrated the effective film preparation method that can be used to achieve high photocurrent density for BiSI films. Inspired by this work, a two-step solution process was demonstrated to fabricate orthorhombic BiSI films using thiol-amine solution (step I) and then BiI3 (step II) deposition at a low temperature of 200 °C. (225) As deposited BiSI film showed the formation of nanorods (60–100 nm in diameter) on TiO2-blocking layer (BL) coated FTO substrates, rather than a continuous film. The electronic structure of n-type BiSI semiconductor film was studied from UPS, which showed EF of 4.5 eV, EV of 5.9 eV, and EC of 4.3 eV (Figure 12d). The fabricated PV device using TiO2-BL as electron transport layer (ETL) and P3HT as hole-transporting material (HMT) did not work properly, which was attributed to poor electron transfer from BiSI to TiO2 and mismatched band alignment with HTM (Figure 12d). It was concluded that materials engineering via doping/alloying to tune the electronic structure of BiSI or the use of alternative ETL and HTM can further improve the efficiency of BiSI solar cells. (225)
One of the key challenges in fabricating high-efficiency BiSI solar cells is the formation of secondary phases during the multistep synthesis process. In this regard, Tiwari et al. (190) demonstrated an interesting single-precursor solution (thermolysis) approach for the deposition of phase pure, paraelectric BiSI thin films. The film composed of flake-shaped grains stacked in highly compact manner over lengths of several micrometers. A band gap of 1.57 eV estimated from absorption and PL spectra was found to be in close agreement with the theoretical calculation. Based on the detailed thermodynamic analysis, calculated IP values, and band structures, a PV device with the architecture of glass/FTO/SnO2/BiSI/F8/Au was fabricated that showed the record power conversion efficiency (PCE) of 1.32% with Jsc of 8.44 mA/cm2, Voc of 0.445 V, and FF of 35.14% under standard AM 1.5G illumination (Figure 12e). Probing into the possible origin of efficiency losses by RT time-resolved PL (Figure 12f) further revealed the short carrier lifetime of ∼1 ns and the presence of bulk recombination centers associated with the Bis and Vs (sulfur vacancy) point defects. (190) This record device efficiency for BiSI solar cells later sparked an interest in the research community to realize the potential of Bi-based derivatives as a counter electrode in DSSCs and as an electron acceptor in halide perovskite solar cells. In this regard, Wu et al. (192) simulated the adsorption and desorption behavior of iodine species in Bi19S27Br3 nanowires (NWs) to demonstrate their potential as CE in DSSCs followed by their initial attempts on the colloidal synthesis. Interestingly, NWs were found to exhibit reversible adsorption behavior of iodine species, showing their electrocatalytic activity toward the iodine reduction reaction (IRR) in DSSCs. The adsorption (1.98 eV) and desorption (1.97 eV) of the I atoms on Bi–S–Br were more spontaneous and reversible than those on the surface of Pt. Inspired by this, DSSC devices based on Bi19S27Br3, and Bi19S27I3 NWs yielded PCEs of 8.72 and 8.71%, respectively (Figure 12g), validating of multifunctionality of Bi-based chalcohalide materials. Recently, Yoo et al. (104) demonstrated that the in situ formed BiSI layer at the interface of the ETL and BiI3 absorber layer, acts as an electron acceptor to further improve the rate of photoinduced charge separation and extraction of electrons in BiI3 solar cells. An innovative and interesting approach of annealing of In2S3/BiI3 structure at 200 °C was used for in situ formation of BiSI at the interface, however, the underlying mechanism and the role of In2S3 for in situ formation of BiSI remained unexplored. A device was fabricated with the architecture of the ITO/SnO2/BiSI/BiI3/spiro-OMeTAD/Au that showed a PCE of 1.21% with Jsc of 12.6 mA/cm2, Voc of 0.33 V and FF of 29%. Transient absorption studies were further carried out to propose the charge separation/transfer mechanism in the SnO2/BiSI/BiI3/spiro-OMeTAD, which is shown in Figure 12h. It was proposed that the presence of BiSI allowed a rapid transfer of photogenerated electrons in BiI3, intercepting the photogenerated electron in the BiI3 before they decayed back to the ground state and thus improving the rate of hole transfer from the BiI3 to spiro-OMeTAD. Most of the above reports showed the growth of Bi-based chalcohalides directly on conductive substrates, which often resulted in poor control over the microstructure, which has a significant impact on device performance. To overcome these issues, colloidal synthesis of BiSX (X = I, Br, Cl) NCs using hot-coinjection approach has been introduced recently. (212) By coinjecting both the chalcogen (S) and the halogen (X = I, Br, Cl) precursors to the solution of bicarboxylate complexes in a noncoordinating solvent, a compositionally tunable BiSX NCs with high absorption coefficients and indirect band gaps (∼1.5 to 2.05 eV) was synthesized. The photoelectrodes fabricated using these BiSX NCs-based ink showed quantum efficiency above 10% across the entire visible light spectrum. Although few attempts have been made on the demonstration of working devices based on BiSI as an absorber or as electron acceptor, and a single attempt on the use of Bi19S27Br3 and Bi19S27I3 as CEs in DSSCs, no attempts have been made to fabricate devices based on other derivatives of Bi-based chalcohalide materials until recently. Li et al. (181) reported a facile solvothermal process for the synthesis of Bi13S18I2 nanorod film on a porous TiO2 film using a mixture of BiI3, thiourea, and methylammonium iodide in ethylene glycol at 195 °C for 12 h. The S/Bi ratio was found to have a profound effect on the phase formation, where a low S/Bi mole ratio formed the BiSI, and a high S/Bi ratio led to the formation of Bi13S18I2. BiSI and Bi13S18I2 were crystallized in orthorhombic (Pnma) and hexagonal (P63) phases exhibiting an n-type indirect band gap of 0.75 and 1.57 eV, respectively. The preliminary Bi13S18I2-based solar cell showed the highest PCE of 0.85% with Jsc of 3.82 mA/cm2, Voc of 0.58 V, and FF of 38.3% with high repeatability and stability (Figure 12i). The lower device performance was due to the diminished device structure, and poor crystallinity, which caused low Jsc and FF. The authors further demonstrated a physical vapor deposition (PVD) method to prepare uniform Bi13S18X2 (X = I, Br, Cl) thin films, which is a prerequisite for fabricating high-efficiency solar cells. Solar cells were fabricated based on Bi13S18X2 thin films with device configuration of FTO/TiO2/Bi13S18X2/(I3–/I– redox couple)/Pt. Interestingly, device efficiencies increased in the order of Bi13S18I2 (0.75%) < Bi13S18Cl2(0.91%) < Bi13S18Br2 (1.12%). The higher performance of Bi13S18Br2 among other devices is attributed to its higher light-absorption ability as well as the ability of the PVD method to deposit highly dense and controlled Bi13S18X2 film. It has been demonstrated that these Bi-based chalcohalides are indirect band gap semiconductors, implying that a thicker absorber is required to maximize light absorption. However, it causes trap densities in the absorber, resulting in lower device efficiency, and thus minimizing impurities is an effective way of improving device performance in solar cell devices. Therefore, future work focusing on reducing trap densities by optimizing the impurities in Bi-based chalcohalides is considered critical for fabricating high-efficiency solar cell devices.
5.2.1.2. Sb-Based Chalcohalides
Seok and co-workers at UNIST, South Korea reported a first working solar cell device based on SbSI with PCE of 3.05% under standard illumination conditions of 100 mW/cm2. (147) A two-step approach demonstrated by Godel and Steiner (156) with slight modification was used to deposit SbSI absorber. First, Sb2S3 was deposited onto a mesoporous (ms)-TiO2 electrode as an ETL using chemical bath deposition at 90 °C, followed by a spin coating of SbI3 in DMSO solution and then subsequent annealing at 150 °C for 5 min. in an inert atmosphere (Figure 13a) and finally, the device was finished by depositing the different HTM, and evaporating Au as front contacts. Figure 13b and its inset show the cross-sectional FE-SEM image and photograph of the completed device with the FTO/BL/mp-TiO2/SbSI/HTL/Au, respectively. Initial devices fabricated using different HTL showed a highest efficiency of 2.84% for a poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) HTL, which was further improved to 3.05% by thorough optimization of annealing temperature and SbI3 solution concentration. Notably, unencapsulated SbSI solar cell device retained 93% of its initial efficiency after storing for 15 days in ambient conditions (∼60% humidity, ∼25 °C temperature). This work demonstrated the potential of SbSI as an absorber in solar cells that sparked an interest in the researchers to further demonstrate alternative synthesis methods as well as to improve the device efficiency. Choi et al. (232) demonstrated an alternative two-step approach. The Sb2S3 was deposited using molecular solution processing that was further converted to SbSI by reacting with SbI3. The motivation behind employing molecular solution processing for Sb2S3 was to overcome the issues associated with the CBD method such as the formation of impurity phases and difficulty in controlling the Sb/S ratio. (233) Controlled experiments were carried out to elucidate the effect of annealing temperatures, SbCl3/thiourea ratio, reaction times after spin coating of SbI3 solution, and different solvents for SbI3 dissolution, on the growth of SbSI films. Finally, solar cell device fabricated based on SbSI absorber using TiO2 as ETL and poly(3-hexylthiophene) (P3HT) as HTL with the configuration of FTO/mp-TiO2/SbSI/P3HT/Au showed a PCE of 0.93% and well maintained PCE after exposing to ambient air for a month. Seok and co-workers further demonstrated improved PCE of SbSI-based devices to 4.07% by partially substituting Sb by Bi (Sb0.67Bi0.33SI, ASBSI). (234) Authors focused on achieving better performance by tuning the bandgap as substituting Sb by Bi can decrease the bandgap between 1.83 eV (SbSI) to 1.51 eV (BiSI). Following their previous recipe with slight modifications, the authors prepared ASBSI absorbers showing the bandgap energy of 1.62 eV (Figure 13c, inset). The visual appearance of thin films from Figure 13c, inset also confirms the tuned band gap energy of ASBSI thin films. Thorough investigations were carried out for controlled sensitization of the absorber layer onto the pores of mp-TiO2 because sufficient infiltration of HTL is necessary for high-performance solar cell devices. Authors revealed that a higher CBD time of 3.5 h sensitized excessive ASBSI absorber layer onto the mp-TiO2, which obstructed the penetration of HTL, leaving behind more pores at the bottom of the device (Figure 13d,e). They concluded that if HTL does not uniformly infiltrate to the bottom of the electrode (Figure 13e, left), the holes generated in the ASBSI cannot be efficiently transferred to the front contact. Finally, the best performing device was fabricated using FTO/BL-TiO2/mpTiO2/ASBSI/PCPDTBT:PC60BM (10:5)/PEDOT:PSS/Au configuration that showed the highest efficiency of 4.07% (Figure 13f). Notably, the unencapsulated solar cell device exhibited outstanding stability and retained ∼93% of the initial PCE after illumination for ∼20.9 h at RT (Figure 13g), and ∼92% of its initial PCE after storing for 360 h at 85 °C in air under 40% average relative humidity in the dark (Figure 13h). SbSeI has a narrow band gap energy of ∼1.67 eV (compared to 2.1 eV for SbSI), which maximizes the light absorption and thereby the device performance. (235,236) In addition, the closer ionic radii match between Se2– (198 pm) (237) and I1– (220 pm) (237) compared to that between ionic radii of S2– (184 pm) (237) and I1– (220 pm) (237) could produce superior and more stable crystal phases and thereby improve the device stability. Considering these benefits, a first solar cell device based on SbSeI with the efficiency of ∼4.1% and stability ∼38 h was demonstrated. (238) A two-step solution approach was determined to prepare SbSeI thin films, where SbI3 solution was spin-coated on Sb2Se3 prepared by thermal decomposition of single-source precursor [((SbL2Cl2)Cl)2.(CH3)2CO, where L = N,N-dimethyl selenourea]. Although the device efficiency of these Sb-based chalcohalide materials is significantly lower than that of Pb halide-based perovskite solar cells, promising initial device efficiencies demonstrated by a group of Prof. Seok at UNIST, South Korea, and comprehensive stability of these materials, demonstrate their potential in next-generation solar cell devices. The device performance of Sb-based chalcohalides is influenced by several important parameters. It includes immature synthesis methodologies, insolubility of some necessary precursors (in solution processing), poor microstructure, phase impurities (usually halide), unoptimized device structure, and meager understanding of defect states, recombination losses, HTL, and ETL. We urge future studies on the systematic investigation of these key limiting factors that will certainly help the research community to realize the full potential of Sb-based chalcohalide materials as a Pb-free alternative solar cell technology.

Figure 13

Figure 13. (a) Schematic illustration of the preparation of SbSI via two-step process followed by thermal annealing. (b) Cross-sectional FE-SEM image of the device with the FTO/BL/mp-TiO2/SbSI/HTL/Au structure. The inset of (b) shows the photograph of the finished device. (a, b) Reproduced with permission from ref (147). Copyright 2018 John Wiley and Sons. (c) UV–visible absorption spectrum of Sb0.67Bi0.33SI (ASBSI). The insets in (c) are the Tauc plot of ASBSI and the photos of Sb2S3 and ASBSI thin films. (d) Cross-sectional FE-SEM image of the device with FTO/BL/mp-TiO2/ASBSI/HTM(L)/Au structure fabricated via CBD of Sb2S3 for 3.5 h. (e) Schematic demonstrating the influence of the ASBSI amount on the infiltration of HTM in ASBSI devices. (f) J–V curves under dark and standard illumination conditions, and stability tests showing normalized PCE (g) under standard AM 1.5G illumination at RT for 1200 min and (h) measured at 85 °C in air for 350 h under 40% average relative humidity in the dark. (c–h) Reproduced with permission from ref (234). Copyright 2019 John Wiley and Sons.

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5.2.2. Photocatalysis

Fundamental criteria of ideal photocatalysts are (1) optimal band gap, (2) suitable band alignment, and (3) stability in an electrolyte solution. (239) Few other desirable properties are good electron and hole mobilities as well as high carrier mobility to enhance charge separation and transfer, respectively. Since photocatalysis only occurs on the surfaces of the photocatalyst, the photogenerated charge carriers must diffuse toward the surface of the photocatalyst. (240,241) Therefore, crystallinity and active surface area of the photocatalyst are also of interest as high crystallinity enhances the charge transport, whereas a high active surface area offers more active sites for photocatalysis. (242) Because of their highly tunable physicochemical and optoelectronic properties, chalcohalides have received a lot of attention as photocatalytic materials for both water electrolysis and wastewater treatment. Furthermore, chalcohalides with mixed chalcogen and halogen ions have structural flexibility, including layered structures with anisotropic charge transfer properties, which have a significant impact on their photocatalytic properties. There have been a number of chalcohalide-based photocatalysts reported to date, with some demonstrating efficiency superior/comparable to their chalcogenide (243,244) and halide perovskite counterparts. (163,245) As a result, we believe that mixed anion-based photocatalyst materials hold great promise for future practical applications.
5.2.2.1. Bi-Based Chalcohalides
One of the most extensively researched applications of these Bi-based chalcohalide materials is photocatalysis. Attempts have been made to utilize the different Bi-based chalcohalide materials in different nanostructures/heterostructures as photocatalysts for photodegradation of rhodamine B (RhB)/2,4-dichlorophenol/Cr species and H2 evolution. As described in section 5.1, the synthesis of Bi-based chalcohalides was mostly limited to the growth of single crystals or nanorod crystals using common techniques such as vapor transport, hydrothermal, and solvothermal methods. The microwave-assisted aqueous approach was used for the first time in 2013 to produce 2D fabric-like hexagonal Bi19S27Br3 superstructures with a band gap energy of 1.42 eV. (226) Bi19S27Br3 superstructures showed an outstanding degradation efficiency rate of 95% toward RhB as a visible light photocatalyst, far superior to the Degussa P25 (Figure 14a). It is well-known that the 1D nanostructures often offer significant photocatalytic activity toward H2 evolution or photodegradation of dyes over their 2D and 3D nanostructures. Keeping this in mind, 1D nanorods of hexagonal Bi19Br3S27 with a band gap of 1.49 eV were prepared through a general and facile one-pot solvothermal method in a mixture of glycerol/ethanol solvent. (227) The formation of 1D Bi19Br3S27 nanorods occurred via fast nucleation and anisotropic growth process. As a result of thiourea hydrolysis, the Biglycerol complex first reacted with the S2– anions to form Bi2S3 NCs. Similarly Biglycerol complex reacted with Br– to form BiBr3. Finally, the in situ reaction of formed Bi2S3 NCs with BiBr3 led to the nucleation of Bi19Br3S27 crystals. The excess use of CTAB as a capping agent was further selectively absorbed onto certain crystal planes to accelerate the preferential growth along with the polar direction of Bi19Br3S27 crystal nuclei, leading to the formation of 1D nanorods. The 1D Bi19Br3S27 nanorod photocatalyst showed a photodegradation of 2,4-dichlorophenol with an efficiency of ∼85.6% after visible light irradiation for 3 h, which is significantly higher than the Bi2S3 (∼28.8%), demonstrating the potential of chalcohalides over their chalcogenide counterparts (Figure 14b). (227) Immediately after, single-crystalline hexagonal Bi19S27Cl3 nanorods with several micrometers in length and ∼25 nm in diameter were synthesized via a one-pot template-free solvothermal method. (228) Nanorods displayed a narrow band gap of 1.6 eV, assisted for photoreduction of hazardous Cr(VI) species (Figure 14c) and photodegradation of RhB. The 1D nanorods offer an efficient electron transfer rate because of closer contact of Cr(VI) ions or RhB and Bi19S27Cl3. In addition, nanorods can also facilitate the diffusion of reactants and products during photocatalysis. Thus, these different factors synergistically led to the excellent photocatalytic activity of Bi19S27Cl3 nanorods compared to BiOCl and commercial P25. Similar to nanostructures, heterostructures offer several advantages such as a strong coupling of different active structures, enhanced electron transfer rate at the interface and a promising platform to enable synergistic effects. In this regard, the earlier group shortly reported the in situ hydrothermal synthesis of cactus-like MoS2/Bi19S27Cl3 (MBH) biomimetic heterostructures. Bi19S27Cl3 acted as a backbone to provide a stem for MoS2, whereas glucose served as a binder to help MoS2 nanosheets grow along the Bi19S27Cl3 longitudinal axis during the heterostructure formation (Figure 14d). (229) It was further revealed that the MoS2 nanosheets in MBH showed less stacking and fewer layers. It was also confirmed that the carbon from glucose promoted the transmission of carriers and acted as a binder for MoS2 and Bi19S27Cl3 to form MBH heterostructure (Figure 14e). Consequently, the MBH photocatalyst outperformed among the different photocatalysts and exhibited a photocatalytic H2 evolution rate up to 876.6 mmol g–1 h–1 for optimal loading of 5 wt% MoS2 (Figure 14f) with great stability. Following this work, multi-interface n–p–n double heterojunctions based on BiSI/MoS2/CdS were demonstrated as photocatalyst for H2 evolution. (108) The built-in electric field along the radial direction of the BiSI nanorod and MoS2 interlayer assisted to transport carriers within the lifetime, which was optimized by the multi-interface structure using PL and UV–vis. This also explained the increased photoactivity from the perspective of optical and dynamic properties of BiSI/MoS2/CdS photocatalyst. Moreover, the favorable band alignment between BiSI, MoS2, and CdS (Figure 14g) further allowed the effective separation of photogenerated charge carriers; as a result, BiSI/MoS2/CdS photocatalyst showed an H2 evolution rate of 21 mmol g–1 h–1, which was significantly higher than its other counterparts. Recently, the influence of halogens on the photocatalytic H2 evolution for Bi19X3S27 (X = Cl, Br) nanomaterials synthesized by the solvothermal method was investigated. (230) The halogen anions had an impact not only on the morphology but also on the H2 evolution performance of Bi19X3S27 nanomaterials. It was found that Bi19Cl3S27 synthesized at 180 °C showed nanorods morphology, whereas Bi19Br3S27 synthesized at 200 °C showed nanograsses morphology (Figure 14h.i), which was attributed to the difference in properties caused by changes in halogen anions. Surprisingly, Bi19Br3S27 nanograsses prepared at 220 °C showed an adverse tendency in PL intensity. This was attributed to the presence of metallic Bi and sulfur vacancies that supplied recombination sites as schematically shown in Figure 14j. Consequently, photocatalysts based on Bi19Cl3S27 nanorods and Bi19Br3S27 nanograsses showed significantly different H2 evolution rates of 195.1 μmol g–1 h–1 and 466.7 μmol g–1 h–1, respectively (Figure 14k). The superior performance of Bi19Br3S27 nanograsses was attributed to the effective carrier separation compared to that of BiOBr and Bi19Cl3S27.

Figure 14

Figure 14. Photodegradation efficiency curves of (a) RhB using Bi19S27Br3 superstructures. Reproduced with permission from ref (226). Copyright 2013 Elsevier. (b) 2,4-Dichlorophenol samples with Bi2S3 and Bi19Br3S27 prepared over different reaction times. Reproduced with permission from ref (227). Copyright 2015 The Royal Society of Chemistry. (c) Reduction Cr(VI) solution using Bi19Cl3S27 nanorods photocatalyst. Reproduced with permission from ref (228). Copyright 2017 The Royal Society of Chemistry. (d) Schematic diagram of the MoS2/Bi19Cl3S27 (MBH) biomimetic heterostructure. (e) HR-TEM images of MBH-5. (f) Photocatalytic hydrogen evolution rate of MoS2, Bi19Cl3S27, MBH-5, MDB-5, and PDB-5. (d–f) Reproduced with permission from ref (229). Copyright 2018 The Royal Society of Chemistry. (g) Schematic of experimental energy potentials of BiSI, MoS2, and CdS. Reproduced with permission from ref (108). Copyright 2019 American Chemical Society. (h, i) SEM images of Bi19Cl3S27 nanorods and Bi19Br3S27 nanograsses prepared at 180 and 200 °C, respectively. (j) Schematic representation of charge carrier recombination in Bi19Br3S27 nanograsses prepared at 220 °C. (k) Photocatalytic hydrogen evolution rate of Bi19Cl3S27 nanorods and Bi19Br3S27 nanograsses prepared at 180 and 200 °C, respectively, and BiOBr nanosheets. (h–k) Reproduced with permission from ref (230). Copyright 2019 American Chemical Society.

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5.2.2.2. Sb-Based Chalcohalides
Early developments in Sb-based chalcohalide materials revealed that these materials have suitable properties that are prerequisites for photocatalysis and PEC water splitting. Despite the above, these compounds have only been explored for their application in photocatalysis, i.e., photodegradation of dyes and contaminants. Kwolek et al. (246) studied the photoelectrochemistry of SbSI nanowires synthesized using common but most attractive sonochemical method. The 1D structure, large static dielectric constant (ε = ∼5 × 104), (71) and outstanding stability in aqueous solution make SbSI an ideal candidate for visible light photocatalysis. To investigate the PEC properties of SbSI, photocurrent spectroscopy was used, in which a constant potential was applied to the SbSI, which was then illuminated with monochromatic light in a specific wavelength range. The photocurrent action spectra of SbSI electrodes in Figure 15a indicates the switching effect, which is caused by the appropriate polarization of the SbSI electrode. Figure 15b further explains the mechanism of the switching effect in the SbSI electrode. Anodic photocurrent was generated when the hole from the VB of SbSI oxidized the iodide in the electrolyte, whereas the cathodic photocurrent was generated when the CB edge of the SbSI higher than the reduction potential of the molecular oxygen-dissolved in the electrolyte. The investigation of photoelectrochemistry of SbSI material further attracted researchers to explore SbSI for photocatalytic applications. Tamilselvan and Bhattacharyya synthesized highly crystalline 1D microrods of SbSI and self-assembled into 3D urchin-shaped structure (Figure 15c) using a simple reflux method at a relatively low temperature of 110 °C for 2 h. (247) It is well-known that the dangling bonds at the surfaces introduce the defects, which act as a center for photogenerated charge carrier recombination in light-absorbing materials. (158) Therefore, thorough investigation on the dangling bonds present at the surface planes of SbSI was carried out using Bravais, Friedel, Donnay, and Harkar (BFDH) law. Figure 15d shows the corresponding simulated morphology of SbSI along the b- and c-axes, as well as the stacked [(SbSI)]2 ribbons along a axis. The results from this modeled structure revealed that the SbSI has a lower proportion of dangling bonds at the surface, suggesting that no/few defects exist in the SbSI urchin-shaped structure, making it the suitable candidate for photocatalysis. The photocatalytic performance of SbSI was further investigated by photodegradation of methyl orange (MO), methylene blue (MB), and RhB dyes. The corresponding photodegradation mechanism for MO dye is presented in Figure 15e, which indicates that the photogenerated holes and O2– generated by the injection of electrons from the CB to the singlet oxygen are primary reactive species in the MO degradation process.

Figure 15

Figure 15. (a) Photocurrent action spectrum recorded at different steps for SbSI electrode in 0.1 M KNO3 + 10 mM KI under Ar purging condition. (b) Schematic representation of anodic (left panel) and cathodic (right panel) photoelectrochemical current switching effect in SbSI electrodes in the electrolyte. (a, b) Reproduced with permission from ref (246). Copyright 2015 IOP Publishing Ltd. (c) Low-magnification SEM image of “urchin” shaped SbSI. Inset shows the HR-TEM image with d-spacing value corresponds to (210) plane of SbSI. (d) View of the simulated morphology of SbSI along the c-axis (left) and b-axis (center) and corresponding stacked [(SbSI)∞]2 ribbons along a-axis (right). Red dotted lines are a representation of nondangling bond planes. Where, red, black, yellow, and green spheres represent Sb, I, S, and the dangling bond atoms, respectively. (e) Schematic diagram of the photodegradation mechanism of methyl orange (MO) using SbSI photocatalyst, suggesting that the pathways (I and II) are more favorable than III. (c–e) Reproduced with permission from ref (247). Copyright 2016 The Royal Society of Chemistry. (f) Photographs of hydrothermally synthesized SbSI crystals (left) and ball milled SbSI followed by the size-selective precipitated nanocrystals dispersed in water (right). (g) HR-TEM image of a SbSI NC showing crystalline core and an amorphous shell. (h) Schematic illustration of defect state-facilitated charge carrier transfer mechanism in core/shell SbSI NC-based photocatalyst. (f–h) Reproduced with permission from ref (253). Copyright 2018, American Chemical Society.

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A similar mechanism was reported by Tasviri and Sajadi-Hezave for the visible light photocatalytic activity of SbSI nanowires encapsulated by carbon nanotubes (SbSI@CNTs) toward the degradation of Acid Blue 92 (AB92) organic dye in an aqueous solution. (248) The SbSI@CNTs hybrid 1D nanostructure showed unprecedented improvement in the catalytic activity and photostability toward the visible light photodegradation of AB92 dye when compared to bare SbSI nanowires, demonstrating the effectiveness of designing novel hybrid nanostructures for high-performance energy devices. Controlling the nano size, crystal structure, and surface engineering where charges and/or excitons interact with the reagent, are among the various prerequisites for developing highly efficient visible light photocatalysts. (249−252) Guided by these, Wang et al. (253) demonstrated an interesting two-step approach utilizing the hydrothermal method to prepare bulk SbSI crystals and then ball milling of SbSI crystals followed by the size-selective centrifugation to synthesize SbSI NCs. Figure 15f shows the photographs of hydrothermally synthesized SbSI crystals (left) and ball milled SbSI followed by the size-selective precipitated nanocrystals dispersed in water (right). Along with size control in SbSI NCs, the authors also found the in situ surface engineering of SbSI NCs where the amorphous shell (3 to 5 nm) was formed during the preparation (Figure 15f). Further quantitative analysis revealed that the shell should be composed of SbOxSyI3–2x–2y. This core/shell nanostructure was further used as photocatalyst for visible light photodegradation of MO and the possible mechanism related to the SbOxSyI3–2x–2y (x ≈ 1.14, y ≈ 0.22) shell was proposed, which is shown in Figure 15g. The shell in type-I structure may passivate the surface trap states in SbSI, leading to a longer lifetime of photogenerated charge carrier, but the wider band gap of shell confines the exciton in the core and acts as a physical barrier that hinders the access of photogenerated charges to the surface and solvent. Theoretically, this type-I structure is not suitable for photocatalysis. Another possible mechanism is related to the doping features of S and I in the amorphous shell that may introduce the additional defects within the amorphous Sb2O3 shell. This allows an easy transfer of charge carriers to the shell surface when part of these interband states lies between the band gap of the SbSI core (Figure 15h). This would result in the formation of type-II structure, which allows efficient charge separation between core and shell, and, as a result, promotes photocatalytic activity. This work established a pathway for further improving the photocatalytic activity of SbSI materials by designing a type-II structure with other compounds. Other Sb-based chalcohalides, on the other hand, have yet to be investigated for photocatalysis and PEC water splitting.
What is limiting the researchers from exploring these exciting Sb-based chalcohalide materials for photocatalysis and PEC water splitting? First and foremost, there is a lack of recognition of which Sb-based chalcohalides are suitable and which of their properties would contribute to these applications. Second, there is a lack of understanding of band alignment of CBM and VBM in Sb-based chalcohalides concerning normal hydrogen electrode (NHE) potential. On the other hand, the band alignment of the materials is highly dependent on the environment, grain orientation, and interface configuration, etc., and this can be rather complicated. (254−258) Future research should focus on theoretical simulation and experimental measurements of band alignment of Sb-based chalcohalides to assess band positions and ultimately realize their application in high-efficiency photocatalysis and water splitting devices.

5.2.3. Photodetectors

Photodetectors, which can convert incident light (ultraviolet (UV), visible, or infrared (IR)) into electrical signals, are attracting increasing attention for variety of scientific and industrial applications including image sensing, optical communications, chemical/biological detection, and environmental monitoring. (264−266) A crucial component of a photodetector is a high-quality semiconductor, which produces electrical current by absorbing light, generating electron–hole pairs, and separating them using an applied and built-in electric field. (264,265) These chalcohalides have been investigated for their potential use in light detection due to their similar optoelectronic properties to halide perovskites, but with the added benefits of high stability. Few exciting results for chalcohalide photodetectors fabricated with various device configurations have been reported in recent years, (267−270) some of which have demonstrated significant photodetecting performances. In this section, we discuss their recent advances with an emphasis on their unique properties and performance, and we highlight the most interesting works in this field.
5.2.3.1. Bi-Based Chalcohalides
Wu et al. (174) were the first to report the photodetector device based on Bi19S27(Br3–x, Ix) alloyed NWs. Notably, this is also the first attempt to demonstrate a facile, colloidal approach to synthesize well-defined Bi19S27(Br3–x, Ix) alloyed NWs using Al3+ ions to mediate the oriented growth. The presence of both Al(acac)3 and AlCl3 produced NWs; however, the reaction conditions also played an important role in producing NWs with narrow size distributions. For example, the length of as-synthesized Bi19S27Br3 NWs tuned in the range of 0.15–2 μm by varying the reaction time, whereas mean diameters tuned from ∼6.9 to ∼12 nm by varying the reaction temperature. The synthesis expanded to p-type Bi19S27(Br3–x, Ix) (0 ≤ x ≤ 3) alloyed nanowires, where diameters increased gradually from ∼9.0 to ∼13.9 nm as the I content increased (Figure 16a–d). Bi19S27Br3 NWs showed a direct band gap of 0.815 eV, whereas Bi19S27(Br3–x, Ix) NWs (0 ≤ x ≤ 3) exhibited an unusual composition-independent band gap of ∼ 0.82 eV, which can be attributed to the smaller contributions of the halogens to the VBM and CBM, confirmed from the theoretical studies. The photodetector device was fabricated using alloyed NWs directly drop casted on a preprepared Au electrode (50 nm) paired onto a SiO2 (300 nm)/P+-Si substrates as shown in Figure 16e. The photodetector demonstrated sensitivity to white light, with a pronounced photoresponse and high stability (Figure 16f), confirming the good potential of chalcohalides in light detection applications. (174) Similarly, the photodetector based on Bi19S27I3 nanorod clusters (Figure 16g,h) directly grown on glass substrates using a facile solvothermal method was also reported recently. (175) Bi19S27I3 nanorod clusters grown on glass showed a strong absorption spanning the whole visible to the near-IR region with band gap energy of 0.83 eV. Notably, the Bi19S27I3 nanorod photodetector device (Figure 16i) still exhibited a very broad photoresponse ranging from UV to near IR region. As shown in Figure 16j, the device showed a limited photoresponse in the UV region; however, it was able to harness the whole visible light region and notably larger photocurrent in the near-IR region. It was also found that the photocurrent of the Bi19S27I3 nanorod cluster film was ∼400-times higher than that of powder sample, demonstrating the effectiveness of nanorod structure as well as the great potential of this material in photodetectors. The first photodetector device based on BiSI was recently demonstrated employing an entirely new process for preparing highly pure, carbon-free BiSI thin films. (231) An established simple three-step process includes (1) spin coating of BiI3 in tetrahydrofuran (THF) solution on FTO substrate, (2) hydrolysis of BiI3 in a 1:1 methanol/water solution to form BiOI, and (3) annealing of BiOI in H2S atmosphere at 150 °C for 4 h to form BiSI thin films. The BiSI thin-film photodetector outperformed among all other chalcohalide in terms of its responsivity and detectivity owing to its controlled band gap. Given that the chalcohalides are still at an early stage of development, these results are very encouraging. Understanding the role of halide ions as well as incorporating different charge transport materials with BiSI and employing encapsulating strategies are key to further improve the performance of Bi-based chalcohalide photodetectors.

Figure 16

Figure 16. Low magnified TEM images of (a) Bi19S27Br3, (b) Bi19S27Br2I, (c) Bi19S27BrI2, and (d) Bi19S27I3 NWs. The scale bars in a, b, c, and d are all 200 nm. (e) Schematic illustration of the photodetector device-based on Bi19S27(Br3–x,Ix) NW networks (x = 0, 1, 2, 3) and (f) its transient photoresponse to pulsed incident light at a bias of +5 V. (a–f) Reproduced with permission from ref (174). Copyright 2015 The Royal Society of Chemistry. (g) SEM and (h) optical microscopy images of the Bi19S27I3 nanorod film grown on a glass slide for 20 h. The scale bar for (g) is 2000 nm. (i) Schematic illustration of the photodetector device based on the Bi19S27I3 nanorod cluster film, and (j) its photoresponsivity to light with different wavelengths in the range of 200–900 nm at a fixed DC bias voltage of 10 V. The inset in (j) is the corresponding spectral-dependent photoresponse curve. (g–j) Reproduced with permission from ref (175). Copyright 2018 The Royal Society of Chemistry.

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5.2.3.2. Sb-Based Chalcohalides
Early investigations on the photoelectric property of SbSI material encouraged researchers to further demonstrate its applications in photodetectors. Chen et al. (259) were the first to report detailed studies on the photoresponse performance of 1D SbSI crystal-based photodetector. They demonstrated an alternative, low-cost hydrothermal method to grow 1D crystalline SbSI crystals (Figure 17a). A photodetector based on SbSI microrod equipped with ITO contacts were first-ever fabricated on a SiO2/Si substrate (Figure 17b and inset), which displayed a noteworthy responsivity of 0.035 mA W1– to visible light with an on–off ratio up to 727 (Figure 17c), a detectivity of 2.3 × 108 Jones and short response/recovery times (300/190 ms) with high stability. Earlier, most of the reports demonstrated the use of common synthesis methods such as sonochemical or hydrothermal or chemical vapor deposition at high temperatures, which usually require high processing times of 4 h and more and usually result in bulk SbSI crystals. Keeping this in mind, Godel and Steiner demonstrated a novel thin-film deposition method for SbSI microcrystals using two-step methodology as shown in Figure 17d. (156) Sb2S3 was first deposited using low-temperature chemical bath deposition and then converted to crystalline SbSI using SbI3 vapor at 250 °C for 5 min. The as-deposited SbSI microcrystals (Figure 17e) thin films were further used into sandwich-type photodetector device fabrication, which showed notable improvements in the optoelectronic properties, particularly response (Figure 17f) and recovery time (Figure 17g) by one to 2 orders of magnitude compared to previously reported values in the literature. This sandwich-type SbSI photodetector outperformed the halide perovskite and a high-performance CdSe-nanowire photodetector, revolutionizing the photodetector device fabrication technique for future high efficiency and low-cost light detectors. It has been demonstrated that mixed dimensional van der Waals (vdW) heterostructures show considerable performances and tunable properties and are a multifunctional platform for next-generation optoelectronic devices. (271) Considering the flexibility of strict lattice mismatching requirements in mixed-dimensional vdW heterostructures, Sun et al. (260) reported the first fabrication of a large area (millimeter-sized) heterostructure based on an individual single-crystal SbSI whisker and individual PbI2 flake (Figure 17h) that demonstrates the vdW interaction at the interface. Photodetector device based on this mixed-dimensional heterostructure showed a photoresponsivity of up to 26.3 mA W1– and a rapid response time (rising/falling time of 12/8 ms, Figure 17i and j, respectively). Although the response time of mixed-dimensional SbSI/PbI2 vdW heterojunction-based photodetector was fast, the photoresponsivity was not remarkable. However, the heterojunction device showed stable and reproducible optoelectronic properties. The authors further claimed that the effective junction area of their device can reach up to 105 μm2, which was significantly higher than reported heterostructures. (272) Recently, a similar group reported the first fabrication of a flexible photodetector device based on SbSI micro rods prepared using a hydrothermal method with SbCl3, I2, and sulfur powder. (261) A flexible SbSI microrods-based photodetector device showed a responsivity up to 13.74 mA W1– and the faster response speed (rising/falling time of 61/128 ms), which was superior to previously reported rigid photodetector devices based on SbSI microcrystals. Raman studies on SbSI microcrystals further revealed the redshift in Sb-S atomic vibrations in SbSI microcrystals than the previously reported SbSI whisker and bulk SbSI crystals, suggesting the decreased force constant of Sb-S bond in SbSI microcrystals. This decrease in force constant of the Sb-S bond adopts a strain parallel to the ab plane or perpendicular to the c-axis, which could relate to the use of sulfur powder instead of commonly used thiourea during the hydrothermal synthesis. This also explained the improved flexibility of SbSI micro rods than that of SbSI whiskers. However, detailed studies are needed to further understand the effect of sulfur source on improving flexibility.

Figure 17

Figure 17. (a) Photograph of SbSI crystals synthesized via fast (4 h) and low-temperature (160 °C) hydrothermal method, (b) I–V curves of the device in the dark and light (the inset is the schematic of the individual SbSI microrod based photodetector, and (c) its on–off photocurrent response as a function of the time at a bias of 1–5 V with a fixed light intensity of 147.2 mW/cm2. (a–c) Reproduced with permission from ref (259). Copyright 2015 The Royal Society of Chemistry. (d) Schematic representation of SbSI photodetector fabrication process in 5 steps. (e) SEM image of as-prepared SbSI microcrystals. (f) I–V characteristics of the SbSI photodetector. (g) Detailed view of one modulation cycle of time response of photocurrent of SbSI photodetector showing the rise-time (τr) and fall-time (τf). (d–g) Reproduced with permission from ref (156). Copyright 2016 The Royal Society of Chemistry. (h) Schematic representation of the vdW heterojunction device formed from the PbI2 flake and SbSI single-crystal whisker. (i, j) Time-resolved photoresponse of the SbSI/PbI2 vdW heterojunction device at 5 V bias. The photocurrent raised with a rising time of ∼12 ms when the light was on and dropped rapidly with a falling time of ∼8 ms when the light was switched off. (h–j) Reproduced with permission from ref (260). Copyright 2019 The Royal Society of Chemistry.

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Although several reports have been focused on the fabrication of SbSI-based photodetectors, the performance of these devices is still comparatively lower than that of state-of-the-art photodetectors. (273) Most of the previous reports used bulk or microcrystal of SbSI to fabricate the devices, which seem to be rather reflective and thus affect the optical absorption of photodetectors. Moreover, the electrical conductivity of SbSI bulk crystals or microcrystals is significantly lower compared to state-of-the-art photodetectors. Future work should focus on developing facile synthesis processes for preparing SbSI NCs by colloidal means, which can improve the optical absorption losses due to their antireflective nature. Additionally, utilizing novel device architecture to fabricate SbSI-based photodetectors, such as Schottky junction with highly conducting carbon compounds, can overcome the conductivity issues and enhance the device performance in the near future. Though SbSI has been used to fabricate photodetector devices, no attempt has been made to fabricate photodetector devices based on its other counterparts such as SbSX (where X = Cl, Br) and SbSeX (where X = I, Cl, Br), which can be explored in future.

5.2.4. Battery and Supercapacitor

With the growing demand for the deployment of renewable energy conversion devices, high-performance and low-cost energy storage devices such as batteries and supercapacitors are indeed anticipated. (274−283) A unique structural properties of chalcohalides, e.g., Bi13S18I2 show ribbon-like subunits of [(Bi2S3)2]∞ that form six spokes around a central hexagonal channel at the corners of the unit cells and BiSI has a layered structural arrangement, which is suitable for ion-migration making these chalcohalides a promising candidates in energy storage devices. This section summarizes recent advances in the use of chalcohalide materials as electrodes and electrolytes in batteries and supercapacitors.
5.2.4.1. Bi-Based Chalcohalides
Bi-based chalcohalides, namely BiSI and Bi13S18I2, have been used as anode materials in lithium-ion battery (LIB) systems. (284) A half-cell retained capacity of 703 mAh g–1 was achieved with BiSI/Bi13S18I2, which was comparatively higher than of Bi13S18I2 (601 mAh g–1) anode material. The coalescence of structural features was cited as a key to the LIB’s high performance. The first chalcohalide reported for use as an electrode material for supercapacitor is a hexagonal Bi13S18I2, which was prepared by thermal decomposition of a precursor solution containing bismuth xanthate, organobismuth material, and BiI3. The Bi13S18I2-based supercapacitor showed the areal capacitance of 210.68 mF cm2 with 99.7% capacitance retention after 5000 cycles, featuring promising energy storage capabilities and better long-term cycle stability. (176) The presence of both Bi(III) and Bi(II) ions in the material resulted in higher capacitance values because the lowest unoccupied CB states are predominantly derived from the Bi2+ centers with contributions from the adjacent Bi3+ and sulfur ions and the highest occupied VB states were predominantly of iodide in character. This demonstrated the potential of the Bi center to integrate various charges and illustrated the capacitance mechanism. (285) However, the potential of this chalcohalides in energy storage devices including batteries and supercapacitors still remains largely untapped. The precise mechanism of charge storage in chalcohalides is currently unknown. Furthermore, the transport properties of chalcohalides that are affected by defects, carrier concentration, and intrinsic doping are still underexplored. Future research could perhaps focus on (1) theoretical and experimental understandings of the interaction between alkali ion transport and electrostatic forces in chalcohalides and (2) thorough investigations into topological defects and dopants in chalcohalides, both of which can significantly improve the overall performance of batteries and supercapacitors.
5.2.4.2. Sb-Based Chalcohalides
All solid-state batteries (ASSBs) hold the promise of next-generation high energy density and safe storage systems by using a solid-state electrolyte (SSE) and (ideally) a lithium metal anode. (286,287) Mixed-anion chemistry of chalcohalides is promising candidates as SSEs for LIBs, due to their high ionic conductivities and air stability. The first Sb-based argyrodites reported for use as an SSE for LIBs was a solid solute of Li6+xMxSb1–xS5I (M = Si, Ge, Sn). (288) The substitution of Si4+ onto the Sb5+, and Li+ into the lattice was significantly found to lower the activation energy barrier for Li-ion mobility, which was related to the slightly increased S2–/I– site disorder but mostly to the disordered Li+ distribution. It was also found that the additional Li+ ion populates in new high energy lattice sites for Li+ diffusion and activate concerted ion migration, leading to a low activation energy barrier. As a result, Li6.6Si0.6Sb0.4S5I exhibited a superionic conductivity, the highest reported to date, up to 24 mS cm–1 for sintered pellets that enabled ASSB prototype with promising properties. Furthermore, first-principle calculations combined with experiments confirmed that the concerted Li-ion migration leads to high Li-ion diffusivity and thereby high ionic conductivity in the Si-rich Li6.75Sb0.25Si0.75S5I SSE. (289) Similar observation was also reported in the case of Ge-substituted Sb-based argyrodites, Li6.5Sb0.5Ge0.5S5I SSE, and was further confirmed using ab initio MD. (290) The Li6.5Sb0.5Ge0.5S5I SSE showed good compatibility with ASSB using NCM523, after applying Li3YCl6 as a catholyte, demonstrating a high discharge density of 164 mAh g–1 and good cycle stability.
This highlights the potential of chalcohalides as SSE in batteries, however, work on employing chalcohalides as anodes or cathodes in batteries is still limited. Understanding the specific structural features (e.g., reduced dimensionality), in our opinion, is an effective way to improve Li storage, given that the layered structure of chalcohalides has more areas accessible to accommodate more Li-ions. We postulate that chalcohalides, which have superior ionic mobility and a large surface area for charge storage, could effectively function as an alternative electrodes or electrolytes in batteries and supercapacitors.

5.2.5. Thermoelectrics

Thermoelectric (TE) generators, which can convert waste heat generated by home heating, automotive exhausts, and industrial processes to electricity, have become an area of attention for renewable energy sources. (291−294) Chalcohalides are expected to be promising TE materials owing to their low thermal conductivity and high carrier mobility; however, there are limited reports on TE properties of only Bi-based chalcohalides. (258,295−298) This section offers a brief review of progress in chalcohalide-based TE generators.
5.2.5.1. Bi-Based Chalcohalides
Sulfur/iodine flux system was used to synthesize Bi13S18I2 and BiSI phases. (178) Bismuth was reacted in sulfur melts with varying amounts of iodine to investigate the effects of iodine concentration and reaction temperatures on phase production and purity. The authors reported the ability to control iodine concentration in the S/I2 flux that allowed for the isolation of the metastable and difficult to synthesize phase of Bi13S18I2. The experimental data showed the broad absorption edge around 0.82 eV, as well as a second slightly sharper absorption edge at ∼0.90 eV. The results were in accordance with a theory that displayed the direct transitions along Γ occur at 0.76 and 0.91 eV, stating that this chalcohalide could be used in TE. Xu et al. (177) synthesized bulk polycrystalline and nanocrystalline Bi13S18I2 by spark plasma sintering of commercial Bi2S3 and BiI3, and by solution-phase method, respectively, to study their TE properties. The maximum figure of merit (zTmax) of 1.0 was achieved at 788 K along the in-plane direction of nano Bi13S18I2 + 2% BiCl3 sample, whereas zTmax of 0.8 was obtained at 738 K along the cross-plane direction. The nanoundoped sample showed lower zTmax as compared to the doped sample in two directions. Bulk undoped Bi13S18I2 showed the highest zTmax of 0.7 at 763 k. Although theoretical predications of TE properties of chalcohalides are promising, experimental investigation of their performance has only recently begun, necessitating additional effort to validate the intriguing theoretical predications. Low electrical conductivity in Bi-based chalcohalides hinders their further progress as TE materials. Improving their conductivity via self-doping or extrinsic dopants may improve their TE properties. Given the advancements in chalcogenide and halide perovskite as TE materials, we postulate that chalcohalides based on Sn, Pb, and Sb may be promising for TE applications; however, further theoretical and experimental investigations are required.

5.2.6. Piezo/Pyro-electric Nanogenerators

Over the last two decades, nanogenerators have emerged as promising energy conversion devices for transforming mechanical and thermal energy into electric power. (299,300) Based on their energy conversion mechanism, such as mechanical to electrical energy or thermal to electrical energy, these devices are classified as piezoelectric and pyroelectric nanogenerators, respectively. (301−303) Apart from exciting ferroelectric and optoelectronic properties of chalcohalides, Sb-based chalcohalides have interesting characteristics such as a pyroelectric coefficient of ∼6 × 10–2 Cm2– K–1 and piezoelectric coefficient of ∼1 × 10–9 C/N (d33) along with the highest known curie temperature of ∼22 °C, making them promising candidates for piezo-/pyro-electric nanogenerators.
5.2.6.1. Sb-Based Chalcohalides
Purusothaman et al. (262) were the first to report the development of SbSI-based piezoelectric nanogenerators (PNGs) with different polymer matrix interfaces such as polydimethylsiloxane (PDMS), polyvinylidene fluoride (PVDF), and poly methyl methacrylate (PMMA). SbSI micro rods were synthesized using a facile and low-cost solid-state reaction at a temperature of 250 °C using commonly available raw materials. The authors first fabricated SbSI (1 wt%)/PMMA photodetector with metal–semiconductor–metal configuration to investigate the involvement of semiconducting and photoactive properties in mechanical energy harvester. The fabricated photodetector device demonstrated a responsivity of ∼4.5 μA W1– and a response time (rising/falling time of 100/10 ms). This encouraged authors to further fabricate the PNGs based on SbSI/PMMA and SbSI microrods as active layers onto a cleaned indium tin oxide (ITO) substrate, followed by deposition of Ag electrode. The experimental setup used for applying external bending force onto the device in ON/OFF states is as shown in Figure 18a. When the light was off, SbSI-based PNG delivered an output voltage of ∼2.4 V (Figure 18b), which was significantly larger than the SbSI/PMMA PNG (∼0.26 V) (Figure 18c). Upon irradiation of the LED source of 630 nm, the piezoelectric voltage dropped gradually in both PNGs (Figure 18d). Notably, the SbSI-based PNG showed a high photoactive sensitivity with 72% and 68% changes in piezoelectric voltage (ΔV) than SbSI/PMMA based PNG, which showed ΔV of 31% and 26% (Figure 18e). This high photoactive sensitivity in SbSI-based PNG is a result of a direct metal–semiconductor–metal interface in single SbSI micro rods, which was limited in SbSI/PMMA PNG by a dielectric PMMA medium. This work offers new insights into the multifunctional properties of SbSI as semiconductor-photoactive-ferroelectric, which established a pathway to further explore these properties in harvesting mechanical energy toward the development of next-generation self-powered devices. Immediately after, Misterwicz et al. (263) demonstrated a simple, scalable, and cost-effective method to fabricate pyroelectric nanogenerator (PENG) based on sonochemically synthesized SbSeI nanowires and their high-pressure compression at RT to a bulk sample. The SbSeI PENG generated an output voltage of 12 mV, short-circuit current of 11 nA, pyroelectric energy harvesting figure of merit of 2.430(6) J m–3 K–2 and the maximum output power density of 0.59(4) μW/m2. Although the output voltage and power density are significantly lower than the reported BaTiO3 and PbZrTiO3, the initial device performance is encouraging. The authors claimed that further improvements in the device performance can be achieved by decreasing the series resistance by increasing the compression pressure during the SbSeI PENG fabrication process and by aligning SbSeI nanowires perpendicular to electrodes to increase the energy harvesting efficiency. These early attempts on exploring the piezo- and pyroelectric nanogenerator applications of Sb-based chalcohalides are promising and could be further applied to other binary or ternary chalcohalide materials.

Figure 18

Figure 18. (a) Optical images of experimental set up used for applying bending force on a SbSI microrod (SMR)-based piezoelectric nanogenerator (PNG) in ON/OFF states. (b) Piezoelectric response and (c) corresponding deviation in piezoelectric voltage of SbSI/PMMA-based PNG. (d) Photoactive electrical response and (e) corresponding changes in the performance of SMR-PNG under ON/OFF conditions. (a–e) Reproduced with permission from ref (262). Copyright 2019 Elsevier.

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5.3. Synthesis and Applications of New Emerging Chalcohalides

5.3.1. Transition/Post-transition and Mixed-Metals Chalcohalides

5.3.1.1. Pb-Based Chalcohalides
The discovery of Pb-based chalcohalides began with the structural characterizations of stable Pb5S2I6 and Pb7S2Br10 chalcohalides from the PbS-PbBr2 system and metastable Pb4SCl6 and Pb4SeBr6 chalcohalides from the PbS-PbCl2 and PbSe-PbBr2 systems. (78,102,183,304) The polycrystalline Pb7S2Br10, Pb5S2I6, Pb4SeBr6, and Pb3Te5I8 materials were obtained by annealing stoichiometric amounts of the binary phases below the peritectic temperatures of 381, 418, 383, and 385 °C, respectively. (78) The band gap values of 1.6, 2, 2, and 2.5 eV were reported for Pb4SeBr6, Pb5S2I6, Pb7S2Br10, and Pb4SCl6, respectively. Although the synthesis and properties of such materials were unknown at the time, this earlier study served as a starting point for future investigations.
Ni et al. (182,184) synthesized bulk Pb4S3I2 and Pb3Se2Br2 using high-pressure solid-state reaction, and both materials indicated higher resistivity (>108 Ω cm) and indirect band gaps. Later, Pb3Se2Br2 was synthesized through a high-pressure solid-state reaction by using PbBr2, Pb powder and elemental Se shots at 700 °C for 3 h under 4 GPa pressure and then quenching the reaction at RT. (182) Noncentrosymmetric cubic crystal structure (I43d) was determined for Pb3Se2Br2 with the small amounts of PbSe and PbBr2 impurities. However, it confirmed to be a diamagnetic semiconductor with an experimentally determined indirect band gap of 1.48 eV. The Pb3Se2Br2 material showed the dipole moment (one-unit cell) to be 1.73 × 10–28 Cm, and high resistivity (>108 Ω cm) at ambient temperature and pressure. The spontaneous polarization was calculated to be around 21 μC/cm2 from DFT. Similar to Pb3Se2Br2, Pb4S3I2 was also synthesized from the reaction of a stoichiometric ratio of Pb, S and PbI2 at 4 GPa and 600 °C for 3 h. (184) The material showed an orthorhombic crystal structure (Pnma) determined by XRD and an indirect band gap of ∼1.6 eV from diffuse reflectance measurements, which was further confirmed by DFT calculations.
Further advances in the synthesis of the Pb-based chalcohalides were made with developing a colloidal synthetic approach for synthesizing Pb4S3X2 (X = Cl, Br, I) NCs by Manna and co-workers. (112) A heat-up approach was developed in which the thermal decomposition of thiocyanate (SCN–) ions (as a source of S2–) was carried out in the presence of the corresponding PbBr2, PbI2, PbCl2 precursors and OLA and OA solvents at a lower temperature (<190 °C). The Pb4S3Br2 NCs showed a narrow size distribution and size tunability over the range ∼7 to 30 nm. (112) Indirect band gaps of 1.98, 1.76 and 2.02 eV were reported for Pb4S3Br2, Pb4S3I2 and Pb3S2Cl2 NCs, respectively with the applicability of Pb4S3Br2 in photodetector and solar cell. Pb4S3I2 formed anisotropically exhibiting polydisperse and elongated rod type morphology with size ranging in hundreds of nm, whereas Pb3S2Cl2 formed roughly spherical shaped NCs of ∼7.0 ± 0.8 nm (Figure 19a). The growth kinetics was studied by quenching aliquots of the reaction batch at different temperatures and surprisingly the optical absorption was found to be independent of the size of NCs. The absorption spectra of all materials were identical; however, there was a weak red-shift from chloride to bromide and subsequently to iodide-based NCs, resulting in slight variation in band gap. The former two phases showed the orthorhombic crystal structure, whereas Pb3S2Cl2 did not show resemblance to the orthorhombic, and resembled a pseudocubic structure, yet the coordination polyhedra for all three structures were quite similar. The energy alignment (Figure 19b) after the ligand exchange on the Pb4S3Br2 NCs was studied to test its suitability to fabricate solar cell devices. The nonencapsulated solar cell device (Figure 19c, inset) with the configuration of ITO/AZO/Pb4S3Br2/MoOx/Au showed a PCE of 0.21 ± 0.02% with Jsc of 1.09 mA/cm2 and Voc of 0.58 V under standard AM1.5G illumination in the air (Figure 19c). Though the need for alternative energy materials to Pb-based compounds is of huge demand, this study demonstrates a pathway to further explore this class of chalcohalide materials that to date are untapped at the nanoscale.

Figure 19

Figure 19. (a) TEM image of Pb4S3Br2 NCs prepared at 180 °C. Inset shows the size distribution of 14.2 ± 0.7 nm. (b) Energy level alignment and (c) J–V curve of the finished device with ITO/AZO/Pb4S3Br2/MoOx/Au structure. The inset of (c) shows the digital photo of the representative device. (a–c) Reproduced with permission from ref (112). Copyright 2020 American Chemical Society. (d) Tauc plots of Pb2SbS2I3 layers prepared at various concentrations of the PbI2 solution on glass/mp-TiO2 substrates. The inset of (d) shows the digital photo of Pb2SbS2I3 thin film. (e) Schematic diagram of energy levels to describe the role of trap states and (f) J–V curves in dark and under standard illumination conditions of the FTO/mp-TiO2/Pb2SbS2I3/PCPDTBT/Au device. (d–f) Reproduced with permission from ref (305). Copyright 2018 American Chemical Society. (g) Photocurrent density and PCE of Sn2SbS2I3 device obtained while holding the solar cells at their maximum voltage point (0.336 V). Reproduced with permission from ref (306). Copyright 2020 Cell Press. (h) J–V curves of the Au/PTAA/Ag3BiI6–2xSx + m-TiO2/c-TiO2/FTO device with x = 0 to 6 at%. (i) Cross-sectional FE-SEM image of the Au/PTAA/Ag3BiI6–2xSx + m-TiO2/c-TiO2/FTO. (j) Normalized PCE of the Au/PTAA/Ag3BiI6–2xSx + m-TiO2/c-TiO2/FTO device with x = 0 and 4 at% stored in air under diffuse light. (h–j) Reproduced with permission from ref (110). Copyright 2018 John Wiley and Sons.

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The synthesis of quaternary Pb2SbS2I3 material, an isostructural to Sn2SbS2I3 was first reported by Dolgikh for their dielectric properties. (307) Later, Starosta et al. (308) synthesized ingots of Pb2SbS2I3 materials by modifying the earlier reported Bridgman method. The weighed quantities of the stoichiometric elements Sb, Pb, S in quartz glass ampule were allowed to react with resublimed iodine at 650 °C for 36 h in the furnace. The as-synthesized Pb2SbS2I3 material showed band gap of 2.0 eV. (308) As explained in section 3, Pb2BiS2I3, Sn2BiS2I3, Sn2BiSI5, and Pb5S2I6 possess the close relation in their structural properties, which form from infinite ribbon [MS2I4]m = α (M = Sn, Pb, Bi) along the crystallographic c-axis. These materials show high electrical resistivities > 1 MΩ cm and experimental optical direct band gaps ranging between 1.2 and 1.6 eV, suitable for solar absorbers. These results are well accordance with theoretically results explained in section 4. After successfully demonstrating SbSI (147) and Sb0.66Bi0.33SI (234) solar cells, Seok and co-workers reported the first working device based on Pb2SbS2I3 absorbers prepared using a two-step approach (305) based on their prior recipes. (147,234) A solution of PbI2 in DMF (different concentrations 0.05, 0.10, and 0.20 mol/L) was spin coated on a chemical bath deposited Sb2S3 layers onto m-TiO2, which was further subjected to annealing at 300 °C for 2 min. in Ar atmosphere to from Pb2SbS2I3 absorbers. The as-prepared films showed a bandgap of ∼2.19 eV (Figure 19d), correlating with its orange appearance of thin film. Solar cell devices based on Pb2SbS2I3 absorbers, TiO2 as ETL, and PCPDTBT as HTL were fabricated using standard device configuration and effect of PbI2 solution concentration on the device performance was studied. The role of trap states was investigated to elucidate the mechanisms underlying the different recombination processes, which is shown in Figure 19e. Following their previous work for the further optimization, e.g., spin coating conditions, the thickness of m-TiO2 and Sb2S3 deposition time, the PCE of ∼3.12% with Jsc of 8.79 mA/cm2, Voc of 0.61 V, and FF of 58.2% (Figure 19f) was reported. The stability studies further demonstrated that irrespective of different stability conditions, devices retained ∼90% of their initial performance, explaining the impressive stability of chalcohalide materials over Pb-halide perovskite materials. While promising, these results show a wide band gap and a low short-circuit current density from the multifaceted deposition method and yet there is still plenty of room for the thorough examination.
5.3.1.2. Sn-Based Chalcohalides
The first preparation of Sn2SbS2I3 with orthorhombic structure (Cmcm) was reported by Oliver-Fourcade et al. (309) in 1980, owing to its significant semiconducting and ferroelectric properties. (310) Strarosta et al. (308) demonstrated the fabrication of Sn2SbS2I3 single crystals using a modified Bridgman method almost a decade later, reporting exact lattice parameter values as well as band gap of 1.5 eV. Despite promising semiconducting properties and a suitable optical bandgap, the use of Sn2SbS2I3 in energy devices remained unexplored until recently. Taking this into consideration, Seok and co-workers demonstrated a single-step solution process to prepare heteroleptic Sn2SbS2I3 solar cells using SbCl3-thiourea complex and SnI2 solution with the first notable PCE of 4.04%. (306) Insufficient light absorption and charge transfer, and either too low or too high concentrations of SbCl3 precursor solution were shown to lead poor device efficiency. The poor device efficiency (at a higher concentration of SbCl3 precursor solution) is mainly caused by the higher trap density and deep level traps states. Notably, the uncapsulated devices demonstrated remarkable thermal and photostability as well as long-term stability when stored for 504 h, demonstrating high stability of Sn2SbS2I3 material compared to the Sn-based halide perovskites. Recently, Kanatzidis and co-workers revisited Sn-based sulfoiodides and demonstrated the synthesis of new quaternary Sn2BiS2I3 and Sn2BiSI5 materials using isothermal heating and chemical vapor transport methods. (109) Interestingly, these compounds are found to be stable in water and air, with band gaps of 1.2–1.3 eV, making them promising for solar energy conversion applications, albeit no actual application has been demonstrated.
5.3.1.3. Ag-Based Chalcohalides
Ag3SI and Ag3SBr were first discovered by Reuter and Hardel in 1960, (311) while Takahashi and Yamamoto researched them in 1966, particularly for use as solid electrolytes due to their superionic conductivity. (312) However, costly synthesis and deposition methods hinder the development of silver chalcohalide for PV or other energy harvesting device applications. (311−314) Yet, due to their ecologically benign nature, chemical adaptability, and suitable band gap (0.9–1.1 eV), Ag-based chalcohalides look to be the most promising, which would make them interesting for single-junction solar cells as well low-band gap bottom cells in tandem configurations in the near future. (315,316) Consequently, different cation- or anion-replacements have been reported for Ag3SBr. Silver iodobismuthates (Ag-Bi-I) materials have been emerged as potential absorber materials in PV owing to their high stability and direct band gaps of 1.85–1.90 eV that enable harvesting of a broader spectrum of solar radiation. A slightly wider band gap of these compounds than the ideal value of ∼1.6 eV makes them unsuitable for single-junction solar cells; however, they can be an exciting alternate as top cells for silicon PV. On the other hand, the electronic structures of silver bismuth iodides and sulfides suggest a mixed anion structure, such as AgaBibIa+3b–2xSx with a suitable band gap and energy position, could be an ideal candidate for high-efficiency solar cells. In this regard, Pai et al. (110) developed a new class of PV materials of silver bismuth sulfoiodide using well-established solution processing, easing the barriers of a wide band gap and a downshifted VBM. The effects of the partial anionic sulfide substitution in a range of silver bismuth iodides (AgBiI4, Ag2BiI5, Ag3BiI6, and AgBi2I7) on their structure, optoelectronic properties and performance as light harvesters in solar cells were studied thoroughly. The solar cell device fabricated with the architecture of Au/PTAA/Ag3BiI6–2xSx + m-TiO2/c-TiO2/FTO demonstrated an improved efficiency from 4.33 ± 0.05 to 5.44 ± 0.07% upon changing x from 0 to 4 at% (Figure 19h,i). Ag3BiI5.92S0.04 based devices showed long-term stability and it retained more than 90% of their initial efficiency after 45 days of storage under ambient conditions (Figure 19j). While further combined theoretical and experimental efforts for optimal iodide-to-sulfide substitution, Voc and FF improvement are needed to understand the charge carrier dynamics at the interface. (110)

5.3.2. Hybrid Organic–Inorganic Metal Chalcohalides

Theoretical findings and relevant investigations motivate researchers to attempt with a stable and efficient chalcohalide perovskite, without ruling out the potential of generating the phase in nonequilibrium environments. The efforts have been made to synthesize MASbSI2 chalcohalide perovskite by Nie et al., (186) however, the authors did not reveal much on the formation mechanism and their optoelectronic properties. The MASbSI2 was prepared via a two-step solution process that involved a spin coating followed by thermal annealing at low temperatures. The fabricated solar cell devices based on MASbSI2 with FTO/BL-TiO2/m-TiO2)/MASbSI2/HTM/Au configuration demonstrated the PCE of 3.08% (Figure 20a) and retained ∼ 91% of the initial PCE after storing for 30 days. Following to this report, MA3Bi2I9–2xSx based thin film with hexagonal structure (P63/mmc) was also fabricated by the low-pressure vapor-assisted solution process. (317) Resulting S-doped MA3Bi2I9–2xSx film showed a band gap of 1.67 eV with a PCE of 0.15%, for FTO/c-TiO2/mes-TiO2/MA3Bi2I9–2xSx/Spiro-OMeTAD/Au device configuration (Figure 20b). Further, MABiI2S (MBIS) material was synthesized by a two-step solid-state reaction process and exhibited band gap of 1.52 eV with a wide absorption range that extends to over 1000 nm. (318) The fabricated device of FTO/c-TiO2/m-TiO2/MBIS/PCPDTBT/Au configuration has shown the PCE up to 0.13% displaying high defect tolerance ability.

Figure 20

Figure 20. (a) J–V curves in forward- and reverse-scan modes of the best-performing of MASbSI2. Reproduced with permission from ref (186). Copyright 2018 American Chemical Society. (b) J–V curves in forward- and reverse-scan modes of MA3Bi2I9–2xSx devices. Reproduced with permission from ref (317). Copyright 2019 John Wiley and Sons.

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The performance of these chalcohalide materials-based solar cells is summarized in Table 5. As suggested by Yan and co-workers from combined theory and experimental approaches, these chalcohalide perovskites decomposes into secondary phases or phase segregate or form nonperovskite phases under thermal equilibrium conditions. Therefore, similar to prior studies on silver-bismuth compounds, (110) these materials are also considered as chalcogenide-doped metal halides rather than chalcohalides at this stage. We postulate that the synthesis of chalcohalide perovskites may be possible as metastable phases under nonequilibrium conditions using colloidal hot-injection or heat-up approaches; however, a careful choice of chalcogen and halogen precursor is highly necessary. Future research should focus on thorough understanding of their synthesis, reaction mechanism, phase formation with advanced characterizations, along with combined experimental and theoretical approaches.
Table 5. Summarized Reports of Other (Pb-, Ag-, Sn-, and Perovskite-Based) Chalcohalide Materials in Different Energy Device Applications
materialdepositionreaction conditionsband gaps (eV)device structure /measurement conditionsperformance
Pb2SbS2I3 (305)chemical bath + spin coatingstep 1 Sb2S3: sodium thiosulfate in water + SbCl3 in acetone + water: reaction at 10 °C, + annealing in Ar atm at 300 °C for 5 min2.19FTO/BL-TiO2/mp-TiO2/Pb2SbS2I3/PCPDTBT/AuJsc = 8.79 mA/cm2
step 2 SbSI: PbI3 in DMF spin coated at 500–2000 rpm for 60 s on Sb2S3, annealing in Ar atm at 300 °C for 2 minVoc = 0.61 V
FF = 58.2%
PCE = 3.12%
Pb3S2X2 (X = Cl, Br, I) (112)colloidal heat-upPbX2 (X = Cl, Br, I) and Pb(SCN)2 in ODE + OLA + OA, reaction at 130 to 170 °C for 1 h1.76 (Pb3S2I2)ITO/AZO/Pb3S2Br2/MoOx/AuJsc = 1.2 mA/cm2
1.98 (Pb3S2Br2)Voc = 0.57 V
2.02 (Pb3S2Cl2)PCE = 0.21%
AgaBibIa+3b-2xSx (110)spin coatingAgI + BiI3 + Bi(S2CAr)3 in DMSO:DMF:HI (3:1 v/v) spin-coated at 1000 rpm for 10 s followed by 4000 rpm for 30 s, annealing at 130 °C for 15 min. in an N2-filled glovebox1.76–1.87 (direct)FTO/c-TiO2/m-TiO2/Ag3BiI5.92S0.04/PTAA/AuJsc = 14.7 mA/cm2
Voc = ∼0.57 V
FF = 65.9%
PCE = 5.56%
Sn2SbS2I3 (306)spin coatingSbCl3, thiourea, and SnI2 in DMF spin-coated at 1000–2000 rpm for 1 min, annealing at 300 °C for 5 min in Ar atm1.41FTO/BL-TiO2/mp-TiO2/Sn2SbS2I3/PCPDTBT/PEDOT:PSS/AuJsc = 16.1 mA/cm2
Voc = 0.44 V
FF = 57%
PCE = 4.04%
MASbSI2 (186)chemical bath + spin coatingstep 1 Sb2S3: sodium thiosulfate in water + SbCl3 in acetone + water, reaction at 10 °C, annealing in Ar atm at 300 °C for 5 min2.03FTO/c-TiO2/mp-TiO2/MASbSI2/PCPDTBT/PEDOT:PSS /AuJsc = 8.13 mA/cm2
step 2 SbSI: SbI3 in DMF spin coated on Sb2S3, annealing at 150 °C for 5 min in an inert atmVoc = 0.65 V
step 3: MAI in 2-propanol; spin-coated on SbSI, annealing at 150 °C for 5 min in Ar atmFF = 58.9%
PCE = 3.11%

6. Summary and Future Perspectives

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Overall, this review has greatly advanced the understanding of the emerging chalcohalide materials from both theoretical and experimental viewpoints. There has been substantial progress made in computational approaches for understanding their structural, electrical, and optoelectronic properties. In the quest for alternative, highly stable and nontoxic materials with high defect tolerance ensued from its ns2 electronic configuration, Bi- and Sb-based chalcohalide materials are being realized for a range of practical applications. In section 1, we introduced the emerging chalcohalide materials, and compared them to halide perovskites and chalcogenides. In section 2, we briefly introduced the efforts of computational and experimental designs to predict new chalcohalide semiconductor materials and investigate their fundamental structural and optoelectronic properties. Later, we focused on the materials properties allowing attributes toward energy conversion applications covering the most promising Bi- and Sb-based chalcohalides and a few new Cu-, Pb-, Sn-, Ag-, and chalcohalide perovskite-based materials. In section 3, we focused on their structural features and commented on their correlation with the optoelectronic properties. In section 4, we thoroughly summarized the progress made in the theoretical calculations of the band structures and predicting the electronic properties of these materials. And finally, in section 5, we reviewed the advances made in the synthesis of these chalcohalide materials and their promising results in energy conversion and storage devices. Most works have been aimed at utilizing the pnictogen chalcohalide materials for solar cells, photocatalysis, and photodetectors, and recently for batteries, supercapacitors, thermoelectrics, and piezo/pyro-electric nanogenerators. While the new transition and post-transition and mixed-metals chalcohalides (Pb-, Sn-, and Ag-based) have yet gained the interest for solar cells. It is reasonable to conclude that computational studies and experimental investigations are still required to realize their full potential in energy conversion and storage applications, which are still in their early stages.
In the following, we summarize some current challenges and future perspectives on computational and theoretical studies to understand the fundamental properties of existing chalcohalide materials and to discover new ones as well as next steps that can be taken to design and engineer these materials experimentally.
1.

Fundamental properties and materials discovery. Advanced computational tools ranging from DFT calculations to machine learning can be used for investigating the structural, optical, and electronic properties of the previously synthesized chalcohalides in databases and to advance the screening process. Future research can be focused on developing novel materials based on structural analogies like halide perovskites and chalcogenides. The promising new stoichiometries and chemistries can be proposed through computational design (e.g., CsBiSI2, CsSbSI2, CsSnS2Cl, and Cu2ZnSI2). Defining the distribution and degree of order–disorder of two or more anions is a unique challenge in these mixed-anion materials. (319) A range of structure-search algorithms, such as cluster expansions, specific quasirandom structures, and genetic algorithms, which are often employed in multicomponent alloys and single-anion compounds, could be utilized to analyze phase stability and solve structures in these chalcohalide compounds. (319−322)

2.

Defects, dopants, and synthesizability. Only a few attempts have been made to understand the dopants, defects, and synthesizability of these chalcohalide materials through computational screening. Recent advances in high throughput computational screening offer new tools (e.g., PyCDT, PyLADA) to facilitate the defect calculations in the materials. More defects calculations of these materials are much needed to understand their physical properties and potential in future applications. Utilizing advanced theoretical and computational screening to understand the stability, degradation, and defect tolerance in chalcohalide materials is essential to demonstrate the material’s ability in energy conversion devices.

3.

Investigation on the band gap and optical properties. The most of the chalcohalide materials showed the indirect band gap, with few demonstrating minor differences in their direct and indirect band gaps. According to a few observations, this discrepancy can result in higher optical absorption because carriers are thermalized to valleys with varying electron velocities, resulting in a high degeneracy of bands and low electron–hole recombination. (197,323) The findings that have been revealed so far neither completely demonstrate the existence of a direct band gap nor do they completely invalidate it. In fact, indirect band gap materials can be just as effective as direct band gap materials if the minority carrier diffusion length is greater than the absorption length.

Due to the frequent consideration of many of these indirect band gap materials in optoelectronics and energy harvesting devices, the indirect to direct band gap transition may receive more attention. This would be accomplished by applying several previously reported strategies, and the significant aspect is to identify suitable approaches to greatly enhance optical transitions at the band gap without compromising materials inherent properties. (324) For example, careful nanostructuring and surface engineering could result in considerable intervalley mixing in the CBM, which positively impacted optical transitions at the band edge. (324) Studying strain engineering and the stabilization of metastable crystalline phases might even lead to favorable optical transitions. (325−328) Additionally, it might be achieved by changing from ordered to disordered material (antisite defect), such as by disordering the cations in its sublattice. (329,330)

4.

Band offset and alignments. Only a portion of these materials have been studied for their band offset and alignments, and the majority of them remain unexplored from a device design point of view; such studies are also important to facilitate their applications in solar water splitting and solar cell. This can be realized experimentally by synthesizing the heterojunctions such as SbSI/SbSBr and measuring their interfacial band offset. We postulate that designing new device architectures using these chalcohalide materials and incorporating both n-type and p-type chalcohalide materials into existing devices can lead to high-performing solar water splitting and solar cells devices.

5.

Experimental investigations. The continued research into combining Bi- and Sb-based chalcohalides with complementary properties (section 2) to achieve the desired device functionality will be beneficial. This includes the continued exploration of anion and cation alloying in these materials such as doping/substituting A site cations with MA, FA, Cs, Rb, Cu, Ge, etc. and anions such as Se, Te, I, Br, and Cl to realize their full potential at device level. As discussed previously in this review, as exemplified by the halide perovskites and chalcogenide materials, many materials breakthroughs can be achieved by tuning the properties. Anionic distribution and composition, as well as local structures, and their tuning need to be studied using advanced analytical techniques such as X-ray and neutron scattering, in situ and operando techniques, electron energy loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM), X-ray absorption near edge structure (XANES) of X-ray absorption spectroscopy, and magic angle-spinning (MAS) nuclear magnetic resonance (NMR). (319,331,332)

6.

New synthesis methods and postprocessing. Most of the previous reports have been focused on growing large crystals of Bi- and Sb-based chalcohalides using sonochemical, hydrothermal, solvothermal methods, etc., and very few are focused on direct thin film formation that has a great potential in solar energy conversion device implications. Those works mostly rely on two-step methods (except for BiSI and AgBiSxI1–x) due to competitive chemistry between chalcogen and halogen anions. These multistep processes usually result in nonuniform microstructure, affecting the charge transport properties. New processing techniques such as one-step solution processing, vapor transport method, as well as nonequilibrium synthesis methods (such as hot-injection or heating approach) and postprocessing for high-quality materials that will be used to promote charge transport and controlled doping/alloying in chalcohalide materials.

7.

Device engineering. Device engineering is a key step in fabricating high-performance energy devices. The main intention of this review is to demonstrate the potential of these promising materials in solar energy conversion devices. Most of the reported solar cell devices based on chalcohalide materials are still focused on using the dye-sensitized device architecture. A thorough understanding of their electrical properties, band structure and alignments is needed to design a suitable device architecture. We suggest adopting planer device structures with a focus on utilizing different ETM (such as SnO2, ZnO, MgZnO, NiO, etc.) and HTMs (e.g., P3HT, spiro-OMeTAD, PTAA, etc.). For example, as mentioned previously in section 5, SbSbr has good band structure alignment like CZTS materials. Therefore, solid-state thin-film solar cells device structure could also be tested for these chalcohalide materials.

8.

Perspective of future applications. (i) Light-emitting devices. With highly tunable optical properties (by alloying chalcogen and halogen anions) and defect tolerance, chalcohalide materials should also be an ideal light emitter. So far, no LED devices based on chalcohalide materials have been demonstrated, emphasizing the importance of focusing on synthesis methodologies and knowledge on the structural defects in these materials. An in-depth understanding of the origin of nonradiative recombination in PL emission must be achieved to realize the potential of these materials in LEDs. Future efforts should be devoted to the fundamental understanding of PL emission using computational screening and advanced optical characterizations. (ii) Solar fuels for energy storage. It has been proposed and experimentally demonstrated that the chalcohalide materials are air and moisture stable, which suggests their future deployment in devices producing solar fuels. We postulate that chalcohalides could effectively serve as either photoanode or photocathode taking advantage of their structural and optoelectronic properties analogues to halide perovskites with the additional benefits of high stability.

Furthermore, the dimensionality and structures of chalcohalide materials are a less explored but efficient way of studying their anisotropic properties. Since anisotropic nanostructures have been shown to have higher light-harvesting efficiency than simple, isotropic building blocks, it would involve a range of possible combinations in order to ensure chalcohalide materials. Therefore, anisotropic chalcohalides, which comprise various compositions, phases, shapes, and sizes, hold great promise for future photonics, catalysis, sensing, biomedical, and energy technologies. The chemistry of the ns2 lone pair led to the revealed properties (section 2), but the explanations provided elsewhere are unclear, making it difficult to establish a comprehensive understanding of these materials that would allow the development of effective optoelectronic devices. As a result, many of such questions warrant more research in order to comprehend the structure–property correlations and reveal their realistic possibilities.
This review motivates the continued research into investigating the fundamental properties of chalcohalide materials and their further energy applications. The insights gained in these studies will surely contribute to a greater understanding of using an ionic-covalent mixed-framework for exploratory surveys of chemical systems. We envision that the future advances into materials containing mixed chalcogen and halogen anions and devices based on the suite of these materials can be a key in realizing an alternative robust, cost-effective, and nontoxic family of materials for the imminent high-performance energy storage and conversion. Knowing the underlying science and then selecting promising technologies, these materials can be developed to better comprehend the existing impediments.

Author Information

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  • Corresponding Author
  • Authors
    • Uma V. Ghorpade - Department of Chemical Sciences and Bernal Institute, University of Limerick, Limerick V94 T9PX, Ireland;  School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia;  Orcidhttps://orcid.org/0000-0001-8631-7030
    • Martin A. Green - School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia;  Orcidhttps://orcid.org/0000-0002-8860-396X
    • Tom Wu - School of Materials Science and Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia;  Orcidhttps://orcid.org/0000-0003-0845-4827
    • Xiaojing Hao - School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia;  Orcidhttps://orcid.org/0000-0001-5903-4481
    • Kevin M. Ryan - Department of Chemical Sciences and Bernal Institute, University of Limerick, Limerick V94 T9PX, Ireland;  Orcidhttps://orcid.org/0000-0003-3670-8505
  • Author Contributions

    CRediT: Uma V. Ghorpade conceptualization, writing-original draft, writing-review & editing; Mahesh P. Suryawanshi conceptualization, funding acquisition, resources, supervision, writing-original draft, writing-review & editing; Martin A. Green supervision, writing-original draft, writing-review & editing; Tom Wu writing-original draft, writing-review & editing; Xiaojing Hao writing-original draft, writing-review & editing; Kevin M. Ryan funding acquisition, resources, supervision, writing-original draft, writing-review & editing.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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U.V.G. and K.M.R acknowledge Science Foundation Ireland (SFI) under the Principal Investigator Program under Contract No. 16/IA/4629 and under Grant No. SFI 16/M-ERA/3419. X.H. acknowledges the Australian Research Council (ARC) Future Fellowship (FT190100756). M.P.S. gratefully acknowledges the support by the Australian Research Council (ARC) under Discovery Early Career Researcher Award (DECRA) (DE210101565). The views expressed herein are those of the authors and are not necessarily those of the Australian Research Council.

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    A review. Lead halide perovskites (LHPs) in the form of nanometer-sized colloidal crystals, or nanocrystals (NCs), have attracted the attention of diverse materials scientists due to their unique optical versatility, high photoluminescence quantum yields and facile synthesis. LHP NCs have a 'soft' and predominantly ionic lattice, and their optical and electronic properties are highly tolerant to structural defects and surface states. Therefore, they cannot be approached with the same exptl. mindset and theor. framework as conventional semiconductor NCs. In this Review, we discuss LHP NCs historical and current research pursuits, challenges in applications, and the related present and future mitigation strategies explored.
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  21. 21
    Yin, W. J.; Yang, J.-H.; Kang, J.; Yan, Y.; Wei, S.-H. Halide Perovskite Materials for Solar Cells: A Theoretical Review. J. Mater. Chem. A 2015, 3, 8926– 8942,  DOI: 10.1039/C4TA05033A
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    21
    Halide perovskite materials for solar cells: a theoretical review
    Yin, Wan-Jian; Yang, Ji-Hui; Kang, Joongoo; Yan, Yanfa; Wei, Su-Huai
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (17), 8926-8942CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    A review. Halide perovskites have recently emerged as promising materials for low-cost, high-efficiency solar cells. The efficiency of perovskite-based solar cells has increased rapidly, from 3.8% in 2009 to 19.3% in 2014, by using the all-solid-state thin-film architecture and engineering cell structures with mixed-halide perovskites. The emergence of perovskite solar cells revolutionized the field not only because of their rapidly increased efficiency, but also flexibility in material growth and architecture. The superior performance of the perovskite solar cells suggested that perovskite materials possess intrinsically unique properties. In this review, we summarize recent theor. investigations into the structural, elec., and optical properties of halide perovskite materials in relation to their applications in solar cells. We also discuss some current challenges of using perovskites in solar cells, along with possible theor. solns.
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    Jena, A. K.; Kulkarni, A.; Miyasaka, T. Halide Perovskite Photovoltaics: Background, Status, and Future Prospects. Chem. Rev. 2019, 119, 3036– 3103,  DOI: 10.1021/acs.chemrev.8b00539
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    Halide Perovskite Photovoltaics: Background, Status, and Future Prospects
    Jena, Ajay Kumar; Kulkarni, Ashish; Miyasaka, Tsutomu
    Chemical Reviews (Washington, DC, United States) (2019), 119 (5), 3036-3103CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    The photovoltaics of org.-inorg. lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compds. such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite prepn. via simple and inexpensive soln. processes demonstrates the immense potential of this thin-film solar cell technol. to become a low-cost alternative to the presently com. available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regarding compositional engineering of the hybrid perovskite structure are discussed, including potentials for developing all-inorg. and lead-free perovskite materials. Looking at the latest cutting-edge research, the prospects for perovskite-based photovoltaic and optoelectronic devices, including non-photovoltaic applications such as X-ray detectors and image sensing devices in industrialization, are described. In addn. to the aforementioned major topics, we also review, as a background, our encounter with perovskite materials for the first solar cell application, which should inspire young researchers in chem. and physics to identify and work on challenging interdisciplinary research problems through exchanges between academia and industry.
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    Fu, Y.; Zhu, H.; Chen, J.; Hautzinger, M. P.; Zhu, X. Y.; Jin, S. Metal Halide Perovskite Nanostructures for Optoelectronic Applications and the Study of Physical Properties. Nat. Rev. Mater. 2019, 4, 169– 188,  DOI: 10.1038/s41578-019-0080-9
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    Metal halide perovskite nanostructures for optoelectronic applications and the study of physical properties
    Fu, Yongping; Zhu, Haiming; Chen, Jie; Hautzinger, Matthew P.; Zhu, X.-Y.; Jin, Song
    Nature Reviews Materials (2019), 4 (3), 169-188CODEN: NRMADL; ISSN:2058-8437. (Nature Research)
    A review. Nanostructures of inorg. semiconductors have revolutionized many areas of electronics, optoelectronics and photonics. The controlled synthesis of semiconductor nanostructures could lead to novel phys. properties, improved optoelectronic device performance and new areas for exploration. Lead halide perovskites have recently excited the photovoltaic research community owing to their high solar-conversion efficiencies and ease of soln. processing; they also hold great promise for optoelectronic applications, such as light-emitting diodes and lasers. In this Review, we summarize recent developments in the synthesis and characterization of metal halide perovskite nanostructures with controllable compns., dimensionality, morphologies and orientations. We examine the advantageous optical properties, improved stability and potential optoelectronic applications of these 1D and 2D single-crystal perovskite nanostructures and compare them with those of bulk perovskites and nanostructures of conventional semiconductors. Studies in which perovskite nanostructures have been used to study the fundamental phys. properties of perovskites are also highlighted. Finally, we discuss the challenges in realizing halide perovskite nanostructures for optoelectronic and photonic applications and offer our perspectives on future opportunities and research directions.
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    Stranks, S. D.; Snaith, H. J. Metal-Halide Perovskites for Photovoltaic and Light-Emitting Devices. Nat. Nanotechnol. 2015, 10, 391– 402,  DOI: 10.1038/nnano.2015.90
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    24
    Metal-halide perovskites for photovoltaic and light-emitting devices
    Stranks, Samuel D.; Snaith, Henry J.
    Nature Nanotechnology (2015), 10 (5), 391-402CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    A review. Metal-halide perovskites are cryst. materials originally developed out of scientific curiosity. Unexpectedly, solar cells incorporating these perovskites are rapidly emerging as serious contenders to rival the leading photovoltaic technologies. Power conversion efficiencies have jumped from 3% to over 20% in just four years of academic research. Here, we review the rapid progress in perovskite solar cells, as well as their promising use in light-emitting devices. In particular, we describe the broad tunability and fabrication methods of these materials, the current understanding of the operation of state-of-the-art solar cells and we highlight the properties that have delivered light-emitting diodes and lasers. We discuss key thermal and operational stability challenges facing perovskites, and give an outlook of future research avenues that might bring perovskite technol. to commercialization.
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    Snaith, H. J. Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. J. Phy. Chem. Lett. 2013, 4, 3623– 3630,  DOI: 10.1021/jz4020162
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    Perovskites: the emergence of a new era for low-cost, high-efficiency solar cells
    Snaith, Henry J.
    Journal of Physical Chemistry Letters (2013), 4 (21), 3623-3630CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Review. Over the last 12 mo, we have witnessed an unexpected breakthrough and rapid evolution in the field of emerging photovoltaics, with the realization of highly efficient solid-state hybrid solar cells based on organometal trihalide perovskite absorbers. In this Perspective, the steps that have led to this discovery are discussed, and the future of this rapidly advancing concept have been considered. It is likely that the next few years of solar research will advance this technol. to the very highest efficiencies while retaining the very lowest cost and embodied energy. Provided that the stability of the perovskite-based technol. can be proven, we will witness the emergence of a contender for ultimately low-cost solar power.
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    Snaith, H. J. Present Status and Future Prospects Of Perovskite Photovoltaics. Nat. Mater. 2018, 17, 372– 376,  DOI: 10.1038/s41563-018-0071-z
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    26
    Present status and future prospects of perovskite photovoltaics
    Snaith, Henry J.
    Nature Materials (2018), 17 (5), 372-376CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
    A review. Solar cells based on metal halide perovskites continue to approach their theor. performance limits thanks to worldwide research efforts. Mastering the materials properties and addressing stability may allow this technol. to bring profound transformations to the elec. power generation industry.
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    Correa-Baena, J.; Saliba, M.; Buonassisi, T.; Grätzel, M.; Abate, A.; Tress, W.; Hagfeldt, A. Promises and Challenges of Perovskite Solar Cells. Science 2017, 358, 739– 744,  DOI: 10.1126/science.aam6323
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    27
    Promises and challenges of perovskite solar cells
    Correa-Baena, Juan-Pablo; Saliba, Michael; Buonassisi, Tonio; Graetzel, Michael; Abate, Antonio; Tress, Wolfgang; Hagfeldt, Anders
    Science (Washington, DC, United States) (2017), 358 (6364), 739-744CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    A review is given. The efficiencies of perovskite solar cells have gone from single digits to a certified 22.1% in a few years' time. At this stage of their development, the key issues concern how to achieve further improvements in efficiency and long-term stability. We review recent developments in the quest to improve the current state of the art. Because photocurrents are near the theor. max., our focus is on efforts to increase open-circuit voltage by improving charge-selective contacts and charge carrier lifetimes in perovskites via processes such as ion tailoring. The challenges assocd. with long-term perovskite solar cell device stability include the role of testing protocols, ionic movement affecting performance metrics over extended periods of time, and detn. of the best ways to counteract degrdn. mechanisms.
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    Min, H.; Lee, D. Y.; Kim, J.; Kim, G.; Lee, K. S.; Kim, J.; Paik, M. J.; Kim, Y. K.; Kim, K. S.; Kim, M. G. Perovskite Solar Cells with Atomically Coherent Interlayers on SnO2 Electrodes. Nature 2021, 598, 444– 450,  DOI: 10.1038/s41586-021-03964-8
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    Perovskite solar cells with atomically coherent interlayers on SnO2 electrodes
    Min, Hanul; Lee, Do Yoon; Kim, Junu; Kim, Gwisu; Lee, Kyoung Su; Kim, Jongbeom; Paik, Min Jae; Kim, Young Ki; Kim, Kwang S.; Kim, Min Gyu; Shin, Tae Joo; Il Seok, Sang
    Nature (London, United Kingdom) (2021), 598 (7881), 444-450CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
    In perovskite solar cells, the interfaces between the perovskite and charge-transporting layers contain high concns. of defects (about 100 times that within the perovskite layer), specifically, deep-level defects, which substantially reduce the power conversion efficiency of the devices1-3. Recent efforts to reduce these interfacial defects have focused mainly on surface passivation4-6. However, passivating the perovskite surface that interfaces with the electron-transporting layer is difficult, because the surface-treatment agents on the electron-transporting layer may dissolve while coating the perovskite thin film. Alternatively, interfacial defects may not be a concern if a coherent interface could be formed between the electron-transporting and perovskite layers. Here we report the formation of an interlayer between a SnO2 electron-transporting layer and a halide perovskite light-absorbing layer, achieved by coupling Cl-bonded SnO2 with a Cl-contg. perovskite precursor. This interlayer has atomically coherent features, which enhance charge extn. and transport from the perovskite layer, and fewer interfacial defects. The existence of such a coherent interlayer allowed us to fabricate perovskite solar cells with a power conversion efficiency of 25.8 per cent (certified 25.5 per cent)under std. illumination. Furthermore, unencapsulated devices maintained about 90 per cent of their initial efficiency even after continuous light exposure for 500 h. Our findings provide guidelines for designing defect-minimizing interfaces between metal halide perovskites and electron-transporting layers.
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    Babayigit, A.; Duy Thanh, D.; Ethirajan, A.; Manca, J.; Muller, M.; Boyen, H. G.; Conings, B. Assessing the Toxicity of Pb- and Sn-Based Perovskite Solar Cells in Model Organism Danio Rerio. Sci. Rep. 2016, 6, 18721,  DOI: 10.1038/srep18721
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    Assessing the toxicity of Pb- and Sn-based perovskite solar cells in model organism Danio rerio
    Babayigit, Aslihan; Duy Thanh, Dinh; Ethirajan, Anitha; Manca, Jean; Muller, Marc; Boyen, Hans-Gerd; Conings, Bert
    Scientific Reports (2016), 6 (), 18721CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    Intensive development of organometal halide perovskite solar cells has lead to a dramatic surge in power conversion efficiency up to 20%. Unfortunately, the most efficient perovskite solar cells all contain lead (Pb), which is an unsettling flaw that leads to severe environmental concerns and is therefore a stumbling block envisioning their large-scale application. Aiming for the retention of favorable electro-optical properties, tin (Sn) has been considered the most likely substitute. Preliminary studies have however shown that Sn-based perovskites are highly unstable and, moreover, Sn is also enlisted as a harmful chem., with similar concerns regarding environment and health. To bring more clarity into the appropriateness of both metals in perovskite solar cells, we provide a case study with systematic comparison regarding the environmental impact of Pb- and Sn-based perovskites, using zebrafish (Danio Rerio) as model organism. Uncovering an unexpected route of intoxication in the form of acidification, it is shown that Sn based perovskite may not be the ideal Pb surrogate.
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    Savory, C. N.; Walsh, A.; Scanlon, D. O. Can Pb-Free Halide Double Perovskites Support High-Efficiency Solar Cells?. ACS Energy Lett. 2016, 1, 949– 955,  DOI: 10.1021/acsenergylett.6b00471
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    Can Pb-Free Halide Double Perovskites Support High-Efficiency Solar Cells?
    Savory, Christopher N.; Walsh, Aron; Scanlon, David O.
    ACS Energy Letters (2016), 1 (5), 949-955CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    The methylammonium lead halides have become champion photoactive semiconductors for solar cell applications; however, issues still remain with respect to chem. instability and potential toxicity. Recently, the Cs2AgBiX6 (X = Cl, Br) double perovskite family has been synthesized and investigated as stable nontoxic replacements. We probe the chem. bonding, phys. properties, and cation anti-site disorder of Cs2AgBiX6 and related compds. from first-principles. We demonstrate that the combination of Ag(I) and Bi(III) leads to the wide indirect band gaps with large carrier effective masses owing to a mismatch in angular momentum of the frontier AOs. The spectroscopically limited photovoltaic conversion efficiency is less than 10% for X = Cl or Br. This limitation can be overcome by replacing Ag with In or Tl; however, the resulting compds. are predicted to be unstable thermodynamically. The search for nontoxic bismuth perovskites must expand beyond the Cs2AgBiX6 motif.
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    Xiang, W.; Tress, W. Review on Recent Progress of All-Inorganic Metal Halide Perovskites and Solar Cells. Adv. Mater. 2019, 31, e1902851  DOI: 10.1002/adma.201902851
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    Review on Recent Progress of All-Inorganic Metal Halide Perovskites and Solar Cells
    Xiang, Wanchun; Tress, Wolfgang
    Advanced Materials (Weinheim, Germany) (2019), 31 (44), 1902851CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. All-inorg. perovskites are considered to be one of the most appealing research hotspots in the field of perovskite photovoltaics in the past 3 years due to their superior thermal stability compared to their org.-inorg. hybrid counterparts. The power-conversion efficiency has reached 17.06% and the no. of important publications is ever increasing. Here, the progress of inorg. perovskites is systematically highlighted, covering materials design, prepn. of high-quality perovskite films, and avoidance of phase instabilities. Inorg. perovskites, nanocrystals, quantum dots, and lead-free compds. are discussed and the corresponding device performances are reviewed, which have been realized on both rigid and flexible substrates. Methods for stabilization of the cubic phase of low-bandgap inorg. perovskites are emphasized, which is a prerequisite for highly efficient and stable solar cells. In addn., energy loss mechanisms both in the bulk of the perovskite and at the interfaces of perovskite and charge selective layers are unraveled. Reported approaches to reduce these charge-carrier recombination losses are summarized and complemented by methods proposed from our side. Finally, the potential of inorg. perovskites as stable absorbers is assessed, which opens up new perspectives toward the commercialization of inorg. perovskite solar cells.
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    Babayigit, A.; Ethirajan, A.; Muller, M.; Conings, B. Toxicity of Organometal Halide Perovskite Solar Cells. Nat. Mater. 2016, 15, 247– 251,  DOI: 10.1038/nmat4572
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    Toxicity of organometal halide perovskite solar cells
    Babayigit, Aslihan; Ethirajan, Anitha; Muller, Marc; Conings, Bert
    Nature Materials (2016), 15 (3), 247-251CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    The recent advent of metal halide perovskite solar cells revolutionized prospects for next-generation photovoltaics. As this technol. is maturing at an exceptional rate, research on its environmental impact is becoming increasingly relevant.
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    Rong, Y. G.; Hu, Y.; Mei, A. Y.; Tan, H. R.; Saidaminov, M. I.; Seok, S. I.; McGehee, M. D.; Sargent, E. H.; Han, H. W. Challenges for Commercializing Perovskite Solar Cells. Science 2018, 361, eaat8235  DOI: 10.1126/science.aat8235
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    Correa-Baena, J. P.; Saliba, M.; Buonassisi, T.; Gratzel, M.; Abate, A.; Tress, W.; Hagfeldt, A. Promises and Challenges of Perovskite Solar Cells. Science 2017, 358, 739– 744,  DOI: 10.1126/science.aam6323
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    34
    Promises and challenges of perovskite solar cells
    Correa-Baena, Juan-Pablo; Saliba, Michael; Buonassisi, Tonio; Graetzel, Michael; Abate, Antonio; Tress, Wolfgang; Hagfeldt, Anders
    Science (Washington, DC, United States) (2017), 358 (6364), 739-744CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    A review is given. The efficiencies of perovskite solar cells have gone from single digits to a certified 22.1% in a few years' time. At this stage of their development, the key issues concern how to achieve further improvements in efficiency and long-term stability. We review recent developments in the quest to improve the current state of the art. Because photocurrents are near the theor. max., our focus is on efforts to increase open-circuit voltage by improving charge-selective contacts and charge carrier lifetimes in perovskites via processes such as ion tailoring. The challenges assocd. with long-term perovskite solar cell device stability include the role of testing protocols, ionic movement affecting performance metrics over extended periods of time, and detn. of the best ways to counteract degrdn. mechanisms.
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    Turkevych, I.; Kazaoui, S.; Ito, E.; Urano, T.; Yamada, K.; Tomiyasu, H.; Yamagishi, H.; Kondo, M.; Aramaki, S. Photovoltaic Rudorffites: Lead-Free Silver Bismuth Halides Alternative to Hybrid Lead Halide Perovskites. ChemSusChem 2017, 10, 3754– 3759,  DOI: 10.1002/cssc.201700980
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    Photovoltaic Rudorffites: Lead-Free Silver Bismuth Halides Alternative to Hybrid Lead Halide Perovskites
    Turkevych, Ivan; Kazaoui, Said; Ito, Eisuke; Urano, Toshiyuki; Yamada, Koji; Tomiyasu, Hiroshi; Yamagishi, Hideo; Kondo, Michio; Aramaki, Shinji
    ChemSusChem (2017), 10 (19), 3754-3759CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Hybrid CPbX3 (C: Cs, CH3NH3; X: Br, I) perovskites possess excellent photovoltaic properties but are highly toxic, which hinders their practical application. Unfortunately, all Pb-free alternatives based on Sn and Ge are extremely unstable. Although stable and non-toxic C2ABX6 double perovskites based on alternating corner-shared AX6 and BX6 octahedra (A=Ag, Cu; B=Bi, Sb) are possible, they have indirect and wide band gaps of over 2 eV. However, is it necessary to keep the corner-shared perovskite structure to retain good photovoltaic properties. We demonstrate another family of photovoltaic halides based on edge-shared AX6 and BX6 octahedra with the general formula AaBbXx (x =a+3b) such as Ag3BiI6, Ag2BiI5, AgBiI4, AgBi2I7. As perovskites were named after their prototype oxide CaTiO3 discovered by Lev Perovski, we propose to name these new ABX halides as rudorffites after Walter Ruedorff, who discovered their prototype oxide NaVO2. We studied structural and optoelectronic properties of several highly stable and promising Ag-Bi-I photovoltaic rudorffites that feature direct band gaps in the range of 1.79-1.83 eV and demonstrated a proof-of-concept FTO/c-m-TiO2/Ag3BiI6/PTAA/Au (FTO: F--doped Sn oxide, PTAA: poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], c: compact, m: mesoporous) solar cell with photoconversion efficiency of 4.3%.
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    Zhang, D.; Li, D.; Hu, Y.; Mei, A.; Han, H. Degradation Pathways In Perovskite Solar Cells and How to Meet International Standards. Commun. Mater. 2022, 3, 1– 14,  DOI: 10.1038/s43246-022-00281-z
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    Zhao, Y.; Ma, F.; Qu, Z.; Yu, S; Shen, T.; Deng, H.; Chu, X.; Peng, X.; Yuan, Y.; Zhang, X.; You, J. Inactive (PbI2)2RbCl Stabilizes Perovskite Films for Efficient Solar Cell. Science 2022, 377, 531– 534,  DOI: 10.1126/science.abp8873
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    Inactive (PbI2)2RbCl stabilizes perovskite films for efficient solar cells
    Zhao, Yang; Ma, Fei; Qu, Zihan; Yu, Shiqi; Shen, Tao; Deng, Hui-Xiong; Chu, Xinbo; Peng, Xinxin; Yuan, Yongbo; Zhang, Xingwang; You, Jingbi
    Science (Washington, DC, United States) (2022), 377 (6605), 531-534CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
    In halide perovskite solar cells the formation of secondary-phase excess lead iodide (PbI2) has some pos. effects on power conversion efficiency (PCE) but can be detrimental to device stability and lead to large hysteresis effects in voltage sweeps. We converted PbI2 into an inactive (PbI2)2RbCl compd. by RbCl doping, which effectively stabilizes the perovskite phase. We obtained a certified PCE of 25.6% for FAPbI3 (FA, formamidinium) perovskite solar cells on the basis of this strategy. Devices retained 96% of their original PCE values after 1000 h of shelf storage and 80% after 500 h of thermal stability testing at 85°C.
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    Luther, J. M.; Schelhas, L. T. Perovskite Solar Cells Can Take The Heat. Science 2022, 376, 28– 29,  DOI: 10.1126/science.abo3368
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    Perovskite solar cells can take the heat: layered surface structure adds durability to packaged perovskite cells
    Luther, Joseph M.; Schelhas, Laura T.
    Science (Washington, DC, United States) (2022), 376 (6588), 28-29CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
    There is no expanded citation for this reference.
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    Chen, S. S.; Dai, X. Z.; Xu, S.; Jiao, H. Y.; Zhao, L.; Huang, J. S. Stabilizing Perovskite-Substrate Interfaces for High-Performance Perovskite Modules. Science 2021, 373, 902– 907,  DOI: 10.1126/science.abi6323
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    Stabilizing perovskite-substrate interfaces for high-performance perovskite modules
    Chen, Shangshang; Dai, Xuezeng; Xu, Shuang; Jiao, Haoyang; Zhao, Liang; Huang, Jinsong
    Science (Washington, DC, United States) (2021), 373 (6557), 902-907CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
    The interfaces of perovskite solar cells (PSCs) are important in detg. their efficiency and stability, but the morphol. and stability of imbedded perovskite-substrate interfaces have received less attention than have top interfaces. We found that DMSO (DMSO), which is a liq. additive broadly applied to enhance perovskite film morphol., was trapped during film formation and led to voids at perovskite-substrate interfaces that accelerated the film degrdn. under illumination. Partial replacement of DMSO with solid-state carbohydrazide reduces interfacial voids. A max. stabilized power conversion efficiency (PCE) of 23.6% was realized for blade-coated p-type/intrinsic/n-type (p-i-n) structure PSCs with no efficiency loss after 550-h operational stability tests at 60[C. The perovskite mini-modules showed certified PCEs of 19.3 and 19.2%, with aperture areas of 18.1 and 50.0 square centimeters, resp.
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    Jiang, Q.; Tong, J.; Xian, Y.; Kerner, R. A.; Dunfield, S. P.; Xiao, C.; Scheidt, R. A.; Kuciauskas, D.; Wang, X.; Hautzinger, M. P. Surface Reaction For Efficient and Stable Inverted Perovskite Solar Cells. Nature 2022,  DOI: 10.1038/s41586-022-05268-x
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    Boyd, C. C.; Cheacharoen, R.; Leijtens, T.; McGehee, M. D. Understanding Degradation Mechanisms and Improving Stability of Perovskite Photovoltaics. Chem. Rev. 2019, 119, 3418– 3451,  DOI: 10.1021/acs.chemrev.8b00336
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    Understanding Degradation Mechanisms and Improving Stability of Perovskite Photovoltaics
    Boyd, Caleb C.; Cheacharoen, Rongrong; Leijtens, Tomas; McGehee, Michael D.
    Chemical Reviews (Washington, DC, United States) (2019), 119 (5), 3418-3451CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    This review article examines the current state of understanding in how metal halide perovskite solar cells can degrade when exposed to moisture, oxygen, heat, light, mech. stress, and reverse bias. It also highlights strategies for improving stability, such as tuning the compn. of the perovskite, introducing hydrophobic coatings, replacing metal electrodes with carbon or transparent conducting oxides, and packaging. The article concludes with recommendations on how accelerated testing should be performed to rapidly develop solar cells that are both extraordinarily efficient and stable.
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    Saparov, B.; Sun, J.-P.; Meng, W.; Xiao, Z.; Duan, H.-S.; Gunawan, O.; Shin, D.; Hill, I. G.; Yan, Y.; Mitzi, D. B. Thin-Film Deposition and Characterization of a Sn-Deficient Perovskite Derivative Cs2SnI6. Chem. Mater. 2016, 28, 2315– 2322,  DOI: 10.1021/acs.chemmater.6b00433
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    Thin-Film Deposition and Characterization of a Sn-Deficient Perovskite Derivative Cs2SnI6
    Saparov, Bayrammurad; Sun, Jon-Paul; Meng, Weiwei; Xiao, Zewen; Duan, Hsin-Sheng; Gunawan, Oki; Shin, Donghyeop; Hill, Ian G.; Yan, Yanfa; Mitzi, David B.
    Chemistry of Materials (2016), 28 (7), 2315-2322CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    In this work, we describe details of a two-step deposition approach that enables the prepn. of continuous and well-structured thin films of Cs2SnI6, which is a one-half Sn-deficient 0-D perovskite deriv. (i.e., the compd. can also be written as CsSn0.5I3, with a structure consisting of isolated SnI64- octahedra). The films were characterized using powder X-ray diffraction (PXRD), SEM (SEM), thermogravimetric anal. (TGA), UV-vis spectroscopy, photoluminescence (PL), photoelectron spectroscopy (UPS, IPES, XPS), and Hall effect measurements. UV-vis and PL measurements indicate that the obtained Cs2SnI6 film is a semiconductor with a band gap of 1.6 eV. This band gap was further confirmed by the UPS and IPES spectra, which were well reproduced by the calcd. d. of states with the HSE hybrid functional. The Cs2SnI6 films exhibited n-type conduction with a carrier d. of 6(1) × 1016 cm-3 and mobility of 2.9(3) cm2/V·s. While the computationally derived band structure for Cs2SnI6 shows significant dispersion along several directions in the Brillouin zone near the band edges, the valence band is relatively flat along the Γ-X direction, indicative of a more limited hole minority carrier mobility compared to analogous values for the electrons. The ionization potential (IP) and electron affinity (EA) were detd. to be 6.4 and 4.8 eV, resp. The Cs2SnI6 films show some enhanced stability under ambient air, compared to methylammonium lead(II) iodide perovskite films stored under similar conditions; however, the films do decomp. slowly, yielding a CsI impurity. These findings are discussed in the context of suitability of Cs2SnI6 for photovoltaic and related optoelectronic applications.
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    Konstantakou, M.; Stergiopoulos, T. A Critical Review on Tin Halide Perovskite Solar Cells. J. Mater. Chem. A 2017, 5, 11518– 11549,  DOI: 10.1039/C7TA00929A
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    A critical review on tin halide perovskite solar cells
    Konstantakou, Maria; Stergiopoulos, Thomas
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2017), 5 (23), 11518-11549CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    APbI3-xBrx perovskite solar cells with the bandgap of ∼1.5-1.6 eV, where A represents caesium, methylammonium, formamidinium and mixts. thereof, currently present certified efficiencies very close to those of established thin film technologies (such as CIGS and CdTe) and are thus one of the most important optoelectronics. To restrict the use of lead, as well as to tune the band gap of the material close to the optimum according to the Shockley-Queisser limit (being 1.34 eV), substitution (total or partial) of Pb2+ by Sn2+ should take place. In this review, we present results on single junction solar cells, utilizing CsSnI3-xBrx, CH3NH3SnI3-xBrx or NH2CH=NH2SnI3-xBrx perovskites as absorbers, as well as a mixt. of Sn2+/Pb2+ being adopted as the metal binary cation, reducing the bandgap to 1.2-1.4 eV. We also highlight very recently recorded efficiencies of perovskite-on-perovskite tandem solar cells, produced by the combination of the above low band gap materials with typical highly performing semi-transparent APbI3-xBrx perovskites of a higher band gap (close to 1.6-1.8 eV). We discuss these fascinating results, focusing on some key points such as, among others, the role of the tin compensator/reducing agent (usually SnF2) during perovskite crystn. In addn., we present the crit. challenges that currently limit the efficiency/stability of these systems and propose prospects for future directions.
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    Li, W.; Wang, Z.; Deschler, F.; Gao, S.; Friend, R. H.; Cheetham, A. K. Chemically Diverse and Multifunctional Hybrid Organic-Inorganic Perovskites. Nat. Rev. Mater. 2017, 2, 16099,  DOI: 10.1038/natrevmats.2016.99
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    Li, M.; Li, F.; Gong, J.; Zhang, T.; Gao, F.; Zhang, W.-H.; Liu, M. Advances in Tin(II)-Based Perovskite Solar Cells: From Material Physics to Device Performance. Small Struct. 2022, 3, 2100102,  DOI: 10.1002/sstr.202100102
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    Advances in Tin(II)-Based Perovskite Solar Cells: From Material Physics to Device Performance
    Li, Man; Li, Faming; Gong, Jue; Zhang, Tiankai; Gao, Feng; Zhang, Wen-Hua; Liu, Mingzhen
    Small Structures (2022), 3 (1), 2100102CODEN: SSMTB2; ISSN:2688-4062. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. During the past decade, metal halide perovskites are widely studied in the field of optoelectronic materials due to their unique optical and elec. properties. Lead-based halide perovskite solar cells (PSCs), in particular, currently achieve a record efficiency of 25.5%, thus showing strong potential in industrial application. However, toxicity of lead-based perovskite materials possesses great concerns to natural environment and human body. Therefore, the quest for nontoxic and eco-friendly elements to replace lead in perovskites is of great interest. Among all the element choices, tin(II) (Sn2+) is the most promising candidate. As a rising star of lead-free PSCs, Sn-based PSCs have drawn much attention and made promising progress during the past few years. While the rapid oxidn. and decompn. of Sn-based perovskites result in poor stability and low efficiency of PSCs. In this review, structural, optoelectronic properties and the crit. issues of Sn-based perovskite materials are analyzed. Then, a detailed discussion on the recent methods in solving crit. issues of Sn-based perovskite devices, from optimization on materials physics to device performance, is also presented. Finally, remaining challenges and future perspective are given to advance the progression of Sn-based PSCs.
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    Sun, N.; Gao, W.; Dong, H.; Liu, Y.; Liu, X.; Wu, Z.; Song, L.; Ran, C.; Chen, Y. Architecture of p-i-n Sn-Based Perovskite Solar Cells: Characteristics, Advances, and Perspectives. ACS Energy Lett. 2021, 6, 2863– 2875,  DOI: 10.1021/acsenergylett.1c01170
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    Architecture of p-i-n Sn-Based Perovskite Solar Cells: Characteristics, Advances, and Perspectives
    Sun, Nan; Gao, Weiyin; Dong, He; Liu, Yanghua; Liu, Xin; Wu, Zhongbin; Song, Lin; Ran, Chenxin; Chen, Yonghua
    ACS Energy Letters (2021), 6 (8), 2863-2875CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    A review. During the past few years, Sn-based perovskites have been extensively investigated in the field of photovoltaics and are considered as one of the most promising alternatives for their Pb counterparts. For perovskite solar cells (PSCs), the rational design of the device architecture plays a crit. role in obtaining high-performing devices, and architecture engineering has made a significant contribution to the development of Sn-based PSCs. In this Review, we summarize the advanced development of Sn-based PSCs from the viewpoint of architecture engineering. We begin with a demonstration of the distinctive characteristics of Sn-based perovskites, aiming at providing important guidance for architecture design toward high-performing PSCs. Next, up-to-date studies on the architecture engineering of Sn-based PSCs are comprehensively reviewed. Finally, the current challenges and future perspectives regarding architecture engineering of Sn-based PSCs are discussed, which we hope will guide further development toward efficient and stable Sn-based PSCs.
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    Yang, W. F.; Igbari, F.; Lou, Y. H.; Wang, Z. K.; Liao, L. S. Tin Halide Perovskites: Progress and Challenges. Adv. Energy Mater. 2020, 10, 1902584,  DOI: 10.1002/aenm.201902584
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    Tin Halide Perovskites: Progress and Challenges
    Yang, Wen-Fan; Igbari, Femi; Lou, Yan-Hui; Wang, Zhao-Kui; Liao, Liang-Sheng
    Advanced Energy Materials (2020), 10 (13), 1902584CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
    A review. The chem. compn. engineering of lead halide perovskites via a partial or complete replacement of toxic Pb with tin has been widely reported as a feasible process due to the suitable ionic radius of Sn and its possibility of existing in the +2 state. Interestingly, a complete replacement narrows the bandgap while a partial replacement gives an anomalous phenomenon involving a further narrowing of bandgap relative to the pure Pb and Sn halide perovskite compds. Unfortunately, the merits of this anomalous behavior have not been properly harnessed. Although promising progress has been made to advance the properties and performance of Sn-based perovskite systems, their photovoltaic (PV) parameters are still significantly inferior to those of the Pb-based analogs. This review summarizes the current progress and challenges in the prepn., morphol. and photophys. properties of Sn-based halide perovskites, and how these affect their PV performance. Although it can be argued that the Pb halide perovskite systems may remain the most sought after technol. in the field of thin film perovskite PV, prospective research directions are suggested to advance the properties of Sn halide perovskite materials for improved device performance.
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    Ke, W.; Stoumpos, C. C.; Kanatzidis, M. G. ″Unleaded″ Perovskites: Status Quo and Future Prospects of Tin-Based Perovskite Solar Cells. Adv. Mater. 2019, 31, e1803230  DOI: 10.1002/adma.201803230
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    "Unleaded" Perovskites: Status Quo and Future Prospects of Tin-Based Perovskite Solar Cells
    Ke, Weijun; Stoumpos, Constantinos C.; Kanatzidis, Mercouri G.
    Advanced Materials (Weinheim, Germany) (2019), 31 (47), 1803230CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. The tremendous interest focused on org.-inorg. halide perovskites since 2012 derives from their unique optical and elec. properties, which make them excellent photovoltaic materials. Pb-based halide perovskite solar cells, in particular, currently stand at a record efficiency of ≈ 23%, fulfilling their potential toward commercialization. However, because of the toxicity concerns of Pb-based perovskite solar cells, their market prospects are hindered. In principle, Pb can be replaced with other less-toxic, environmentally benign metals. Sn-based perovskites are thus the far most promising alternative due to their very similar and perhaps even superior semiconductor characteristics. After years of effort invested in Sn-based halide perovskites, sufficient breakthroughs have finally been achieved that make them the next runners up to the Pb halide perovskites. To help the reader better understand the nature of Sn-based halide perovskites, their optical and elec. properties are systematically discussed. Recent progress in Sn-based perovskite solar cells, focusing mainly on film fabrication methods and different device architectures, and highlighting roadblocks to progress and opportunities for future work are reviewed. Finally, a brief overview of mixed Sn/Pb-based systems with their anomalous yet beneficial optical trends are discussed. The current challenges and a future outlook for Sn-based perovskites are discussed.
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    Stoumpos, C. C.; Frazer, L.; Clark, D. J.; Kim, Y. S.; Rhim, S. H.; Freeman, A. J.; Ketterson, J. B.; Jang, J. I.; Kanatzidis, M. G. Hybrid Germanium Iodide Perovskite Semiconductors: Active Lone Pairs, Structural Distortions, Direct And Indirect Energy Gaps, and Strong Nonlinear Optical Properties. J. Am. Chem. Soc. 2015, 137, 6804– 6819,  DOI: 10.1021/jacs.5b01025
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    50
    Hybrid Germanium Iodide Perovskite Semiconductors: Active Lone Pairs, Structural Distortions, Direct and Indirect Energy Gaps, and Strong Nonlinear Optical Properties
    Stoumpos, Constantinos C.; Frazer, Laszlo; Clark, Daniel J.; Kim, Yong Soo; Rhim, Sonny H.; Freeman, Arthur J.; Ketterson, John B.; Jang, Joon I.; Kanatzidis, Mercouri G.
    Journal of the American Chemical Society (2015), 137 (21), 6804-6819CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The synthesis and properties of the hybrid org./inorg. Ge perovskite compds., AGeI3, are reported (A = Cs, org. cation). The systematic study of this reaction system gave 6 new hybrid semiconductors. Using CsGeI3 (1) as the prototype compd., methylammonium, MeNH3GeI3 (2), formamidinium, HC(NH2)2GeI3 (3), acetamidinium, CH3C(NH2)2GeI3 (4), guanidinium, C(NH2)3GeI3 (5), trimethylammonium, Me3NHGeI3 (6), and isopropylammonium, Me2C(H)NH3GeI3 (7) analogs were prepd. The crystal structures of the compds. are classified based on their dimensionality with 1-4 forming 3D perovskite frameworks and 5-7 1D infinite chains. Compds. 1-7, with the exception of compds. 5 (centrosym.) and 7 (nonpolar acentric), crystallize in polar space groups. The 3D compds. have direct band gaps of 1.6 eV (1), 1.9 eV (2), 2.2 eV (3), and 2.5 eV (4), while the 1D compds. have indirect band gaps of 2.7 eV (5), 2.5 eV (6), and 2.8 eV (7). The 2nd harmonic generation (SHG) properties are reported of the compds., which display remarkably strong, type I phase-matchable SHG response with high laser-induced damage thresholds (up to ∼3 GW/cm2). The 2nd-order nonlinear susceptibility, χ(2)S, is 125.3 ± 10.5 pm/V (1), (161.0 ± 14.5) pm/V (2), 143.0 ± 13.5 pm/V (3), and 57.2 ± 5.5 pm/V (4). First-principles d. functional theory electronic structure calcns. indicate that the large SHG response is attributed to the high d. of states in the valence band due to sp-hybridization of the Ge and I orbitals, a consequence of the lone pair activation. Crystallog. data are given.
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    Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin And Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019– 9038,  DOI: 10.1021/ic401215x
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    51
    Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties
    Stoumpos, Constantinos C.; Malliakas, Christos D.; Kanatzidis, Mercouri G.
    Inorganic Chemistry (2013), 52 (15), 9019-9038CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    A broad org.-inorg. series of hybrid metal iodide perovskites AMI3, where A is the methylammonium (MeNH3+) or formamidinium (HC(NH2)2+) cation and M is Sn (1 and 2) or Pb (3 and 4) are reported. The compds. were prepd. through a variety of synthetic approaches, and the nature of the resulting materials is discussed in terms of their thermal stability and optical and electronic properties. The chem. and phys. properties of these materials strongly depend on the prepn. method. Single crystal x-ray diffraction anal. of 1-4 classifies the compds. in the perovskite structural family. Structural phase transitions were obsd. and studied by temp.-dependent single crystal x-ray diffraction in the 100-400 K range. The charge transport properties of the materials are discussed in conjunction with diffuse reflectance studies in the mid-IR region that display characteristic absorption features. Temp.-dependent studies show a strong dependence of the resistivity as a function of the crystal structure. Optical absorption measurements indicate that 1-4 behave as direct-gap semiconductors with energy band gaps distributed at 1.25-1.75 eV. The compds. exhibit an intense near-IR luminescence (PL) emission in the 700-1000 nm range (1.1-1.7 eV) at room temp. Solid solns. between the Sn and Pb compds. are readily accessible throughout the compn. range. The optical properties such as energy band gap, emission intensity, and wavelength can be readily controlled for the isostructural series of solid solns. MeNH3Sn1-xPbxI3 (5). The charge transport type in these materials was characterized by Seebeck coeff. and Hall-effect measurements. The compds. behave as p- or n-type semiconductors depending on the prepn. method. The samples with the lowest carrier concn. are prepd. from soln. and are n-type; p-type samples can be obtained through solid state reactions exposed in air in a controllable manner. In the case of Sn compds., there is a facile tendency toward oxidn. which causes the materials to be doped with Sn4+ and thus behave as p-type semiconductors displaying metal-like cond. The compds. appear to possess very high estd. electron and hole mobilities that exceed 2000 cm2/(V s) and 300 cm2/(V s), resp., as shown in the case of MeNH3SnI3 (1). The authors also compare the properties of the title hybrid materials with those of the all-inorg. CsSnI3 and CsPbI3 prepd. using identical synthetic methods.
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    Ke, W.; Kanatzidis, M. G. Prospects For Low-Toxicity Lead-Free Perovskite Solar Cells. Nat. Commun. 2019, 10, 965,  DOI: 10.1038/s41467-019-08918-3
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    Prospects for low-toxicity lead-free perovskite solar cells
    Ke Weijun; Kanatzidis Mercouri G
    Nature communications (2019), 10 (1), 965 ISSN:.
    Since the 2012 breakthroughs(1-3), it is now very much accepted that halide perovskite solar cells may have a strong practical impact in next-generation solar cells. The most efficient solar cells are using Pb-based halide perovskites. The presence of Pb in these devices, however, has caused some concerns due to the high perceived toxicity of Pb, which may slow down or even hinder the pace of commercialization. Therefore, the science community has been searching for lower-toxicity perovskite-type materials as a back-up strategy. The community is paying significant attention to Pb-free materials and has achieved promising results albeit not yet approaching the spectacular performance of APbI3 materials. In this comment, we summarize the present status and future prospects for Pb-free perovskite materials and their devices.
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    Hu, H.; Dong, B.; Zhang, W. Low-Toxic Metal Halide Perovskites: Opportunities and Future Challenges. J. Mater. Chem. A 2017, 5, 11436– 11459,  DOI: 10.1039/C7TA00269F
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    Low-toxic metal halide perovskites: opportunities and future challenges
    Hu, Hang; Dong, Binghai; Zhang, Wei
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2017), 5 (23), 11436-11449CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    A review. Over the past few years, lead halide perovskites have emerged as a class of dominant semiconductor materials in the photovoltaic (PV) field with an unprecedented sharp enhancement of power conversion efficiencies (PCEs) up to 22.1%, as well as in other promising optoelectronic applications due to their extraordinary and unique properties. However, the lead toxicity and long-term stability of these lead-based perovskites have raised considerable concerns for their real applications. Exploration of potentially low-toxic metal halide perovskite materials becomes one of the significant pivotal challenges in this century for PV, optoelectronic, and other unexplored applications. In this review, the recent progress is summerized on the development of low-toxic metal halide perovskites with a particular focus on their structures and properties, and discuss their potential applications in PV and optoelectronic devices. Moreover, current challenges and future research directions are suggested with the goal of stimulating further research interest and potential applications.
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    Liang, L.; Gao, P. Lead-Free Hybrid Perovskite Absorbers for Viable Application: Can We Eat the Cake and Have It too?. Adv Sci. 2018, 5, 1700331,  DOI: 10.1002/advs.201700331
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    Lead-Free Hybrid Perovskite Absorbers for Viable Application: Can We Eat the Cake and Have It too?
    Liang Lusheng; Gao Peng; Liang Lusheng; Gao Peng
    Advanced science (Weinheim, Baden-Wurttemberg, Germany) (2018), 5 (2), 1700331 ISSN:2198-3844.
    Many years since the booming of research on perovskite solar cells (PSCs), the hybrid perovskite materials developed for photovoltaic application form three main categories since 2009: (i) high-performance unstable lead-containing perovskites, (ii) low-performance lead-free perovskites, and (iii) moderate performance and stable lead-containing perovskites. The search for alternative materials to replace lead leads to the second group of perovskite materials. To date, a number of these compounds have been synthesized and applied in photovoltaic devices. Here, lead-free hybrid light absorbers used in PV devices are focused and their recent developments in related solar cell applications are reviewed comprehensively. In the first part, group 14 metals (Sn and Ge)-based perovskites are introduced with more emphasis on the optimization of Sn-based PSCs. Then concerns on halide hybrids of group 15 metals (Bi and Sb) are raised, which are mainly perovskite derivatives. At the same time, transition metal Cu-based perovskites are also referred. In the end, an outlook is given on the design strategy of lead-free halide hybrid absorbers for photovoltaic applications. It is believed that this timely review can represent our unique view of the field and shed some light on the direction of development of such promising materials.
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  55. 55
    Park, B. W.; Philippe, B.; Zhang, X.; Rensmo, H.; Boschloo, G.; Johansson, E. M. Bismuth Based Hybrid Perovskites A3Bi2I9 (A: Methylammonium or Cesium) for Solar Cell Application. Adv. Mater. 2015, 27, 6806– 6813,  DOI: 10.1002/adma.201501978
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    55
    Bismuth Based Hybrid Perovskites A3Bi2I9 (A: Methylammonium or Cesium) for Solar Cell Application
    Park, Byung-Wook; Philippe, Bertrand; Zhang, Xiaoliang; Rensmo, Hakan; Boschloo, Gerrit; Johansson, Erik M. J.
    Advanced Materials (Weinheim, Germany) (2015), 27 (43), 6806-6813CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Bismuth based hybrid perovskites A3Bi2I9 (A: Methylammonium or Cesium) are synthesized for solar cell applications.
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  56. 56
    Saparov, B.; Hong, F.; Sun, J.-P.; Duan, H.-S.; Meng, W.; Cameron, S.; Hill, I. G.; Yan, Y.; Mitzi, D. B. Thin-Film Preparation and Characterization of Cs3Sb2I9: A Lead-Free Layered Perovskite Semiconductor. Chem. Mater. 2015, 27, 5622– 5632,  DOI: 10.1021/acs.chemmater.5b01989
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    56
    Thin-Film Preparation and Characterization of Cs3Sb2I9: A Lead-Free Layered Perovskite Semiconductor
    Saparov, Bayrammurad; Hong, Feng; Sun, Jon-Paul; Duan, Hsin-Sheng; Meng, Weiwei; Cameron, Samuel; Hill, Ian G.; Yan, Yanfa; Mitzi, David B.
    Chemistry of Materials (2015), 27 (16), 5622-5632CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Computational, thin-film deposition, and characterization approaches have been used to examine the ternary halide semiconductor Cs3Sb2I9. Cs3Sb2I9 has two known structural modifications, the 0-D dimer form (space group P63/mmc, no. 194) and the 2-D layered form (P‾3m1, no. 164), which can be prepd. via soln. and solid-state or gas-phase reactions, resp. Our computational investigations suggest that the layered form, which is a one-third Sb-deficient deriv. of the ubiquitous perovskite structure, is a potential candidate for high-band gap photovoltaic (PV) applications. In this work, we describe details of a two-step deposition approach that enables the prepn. of large grain (>1 μm) and continuous thin films of the lead-free layered perovskite deriv. Cs3Sb2I9. Depending on the deposition conditions, films that are c-axis oriented or randomly oriented can be obtained. The fabricated thin films show enhanced stability under ambient air, compared to methylammonium lead(II) iodide perovskite films stored under similar conditions, and an optical band gap value of 2.05 eV. Photoelectron spectroscopy study yields an ionization energy of 5.6 eV, with the valence band max. approx. 0.85 eV below the Fermi level, indicating near-intrinsic, weakly p-type character. D. functional theory (DFT) anal. points to a nearly direct band gap for this material (less than 0.02 eV difference between the direct and indirect band gaps) and a similar high-level of absorption compared to CH3NH3PbI3. The photoluminescence peak intensity of Cs3Sb2I9 is substantially suppressed compared to that of CH3NH3PbI3, likely reflecting the presence of deep level defects that result in nonradiative recombination in the film, with computational results pointing to Ii, ISb, and VI as being likely candidates. A key further finding from this study is that, despite a distinctly layered structure, the electronic transport anisotropy is less pronounced due to the high ionicity of the I atoms and the strong antibonding interactions between the Sb s lone pair states and I p states, which leads to a moderately dispersive valence band.
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  57. 57
    McClure, E. T.; Ball, M. R.; Windl, W.; Woodward, P. M. Cs2AgBiX6 (X = Br, Cl): New Visible Light Absorbing, Lead-Free Halide Perovskite Semiconductors. Chem. Mater. 2016, 28, 1348– 1354,  DOI: 10.1021/acs.chemmater.5b04231
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    57
    Cs2AgBiX6 (X = Br, Cl): New Visible Light Absorbing, Lead-Free Halide Perovskite Semiconductors
    McClure, Eric T.; Ball, Molly R.; Windl, Wolfgang; Woodward, Patrick M.
    Chemistry of Materials (2016), 28 (5), 1348-1354CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The double perovskites Cs2AgBiBr6 and Cs2AgBiCl6 have been synthesized from both solid state and soln. routes. X-ray diffraction measurements show that both compds. adopt the cubic double perovskite structure, space group Fm‾3m, with lattice parameters of 11.2711(1) Å (X = Br) and 10.7774(2) Å (X = Cl). Diffuse reflectance measurements reveal band gaps of 2.19 eV (X = Br) and 2.77 eV (X = Cl) that are slightly smaller than the band gaps of the analogous lead halide perovskites, 2.26 eV for CH3NH3PbBr3 and 3.00 eV for CH3NH3PbCl3. Band structure calcns. indicate that the interaction between the Ag 4d-orbitals and the 3p/4p-orbitals of the halide ion modifies the valence band leading to an indirect band gap. Both compds. are stable when exposed to air, but Cs2AgBiBr6 degrades over a period of weeks when exposed to both ambient air and light. These results show that halide double perovskite semiconductors are potentially an environmentally friendly alternative to the lead halide perovskite semiconductors.
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  58. 58
    Slavney, A. H.; Hu, T.; Lindenberg, A. M.; Karunadasa, H. I. A Bismuth-Halide Double Perovskite with Long Carrier Recombination Lifetime for Photovoltaic Applications. J. Am. Chem. Soc. 2016, 138, 2138– 2141,  DOI: 10.1021/jacs.5b13294
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    58
    A Bismuth-Halide Double Perovskite with Long Carrier Recombination Lifetime for Photovoltaic Applications
    Slavney, Adam H.; Hu, Te; Lindenberg, Aaron M.; Karunadasa, Hemamala I.
    Journal of the American Chemical Society (2016), 138 (7), 2138-2141CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Despite the remarkable rise in efficiencies of solar cells contg. the Pb-halide perovskite absorbers RPbX3 (R =org. cation; X =Br- or I-), the toxicity of Pb remains a concern for the large-scale implementation of this technol. This has spurred the search for Pb-free materials with similar optoelectronic properties. We use the double-perovskite structure to incorporate nontoxic Bi3+ into the perovskite lattice in Cs2AgBiBr6. The solid shows a long room-temp. fundamental photoluminescence (PL) lifetime of ∼660 ns, which is very encouraging for photovoltaic applications. Comparison between single-crystal and powder PL decay curves of Cs2AgBiBr6 suggests inherently high defect tolerance. The material has an indirect bandgap of 1.95 eV, suited for a tandem solar cell. Cs2AgBiBr6 is significantly more heat and moisture stable compared to (MA)PbI3. The extremely promising optical and phys. properties of Cs2AgBiBr6 shown here motivate further exploration of both inorg. and hybrid halide double perovskites for photovoltaics and other optoelectronics.
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  59. 59
    Boopathi, K. M.; Karuppuswamy, P.; Singh, A.; Hanmandlu, C.; Lin, L.; Abbas, S. A.; Chang, C. C.; Wang, P. C.; Li, G.; Chu, C. W. Solution-Processable Antimony-Based Light-Absorbing Materials Beyond Lead Halide Perovskites. J. Mater. Chem. A 2017, 5, 20843– 20850,  DOI: 10.1039/C7TA06679A
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    Solution-processable antimony-based light-absorbing materials beyond lead halide perovskites
    Boopathi, Karunakara Moorthy; Karuppuswamy, Priyadharsini; Singh, Anupriya; Hanmandlu, Chintam; Lin, Lin; Abbas, Syed Ali; Chang, Chien Cheng; Wang, Pen Cheng; Li, Gang; Chu, Chih Wei
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2017), 5 (39), 20843-20850CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Org.-inorg. lead halide perovskites have recently emerged as highly competitive light absorbing materials for low cost soln.-processable photovoltaic devices. With the high efficiency already achieved, removing the toxicity, i.e., lead-free and stability are the key obstacles for perovskite solar cells. Here, we report the synthesis of an antimony (Sb)-based hybrid material having the compn. of A3Sb2I9 [A = CH3NH3 (MA), Cs] and an investigation of its potential photovoltaic applications. Sb-based perovskite-like materials exhibited attractive absorbance properties, with the band gaps of MA3Sb2I9 and Cs3Sb2I9 measured to be 1.95 and 2.0 eV, resp. XPS confirmed the formation of stoichiometric perovskites from appropriate precursor molar ratios incorporated with hydroiodic acid (HI). Planar hybrid Sb-based solar cells exhibited negligible hysteresis and reproducible power output under working conditions. A power conversion efficiency of 2.04% was achieved by the MA3Sb2I9 perovskite-based device-the highest reported to date for a Sb-based perovskite solar cell.
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  60. 60
    Hu, W.; He, X.; Fang, Z.; Lian, W.; Shang, Y.; Li, X.; Zhou, W.; Zhang, M.; Chen, T.; Lu, Y. Bulk Heterojunction Gifts Bismuth-Based Lead-Free Perovskite Solar Cells with Record Efficiency. Nano Energy 2020, 68, 104362,  DOI: 10.1016/j.nanoen.2019.104362
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    Bulk heterojunction gifts bismuth-based lead-free perovskite solar cells with record efficiency
    Hu, Wanpei; He, Xin; Fang, Zhimin; Lian, Weitao; Shang, Yanbo; Li, Xingcheng; Zhou, Weiran; Zhang, Mengmeng; Chen, Tao; Lu, Yalin; Zhang, Lijun; Ding, Liming; Yang, Shangfeng
    Nano Energy (2020), 68 (), 104362CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
    Bismuth-based lead-free perovskite solar cells are promising alternatives to the lead-based org.-inorg. hybrid cells which suffer from the environmental toxicity of lead and poor ambient stability, but the devices based on single-component ternary bismuth halides exhibit inferior power conversion efficiency. Herein, for the first time we construct bulk heterojunction (BHJ) bismuth-based perovskite solar cells with the photoactive layer consisting of in-situ phase-sepd. Cs3Bi2I9 and Ag3Bi2I9 components, achieving a record efficiency of approx. 3.6% and an unprecedented open-circuit voltage reaching 0.89 V. Formation of BHJ structure leads to increased crystal grain size of Cs3Bi2I9 and optimized grain orientation of Ag3Bi2I9, and a type-II energy band alignment is achieved, benefiting exciton sepn. and charge carrier transport. Cs3Bi2I9-Ag3Bi2I9 BHJ devices exhibit superb thermal stability, retaining ~ 90% of the initial efficiency after 450 h heating under 85°C in glove box. Moreover, the universality of BHJ concept in boosting device performance of perovskite solar cells based on other reported AgxBiyIx+3y light-absorbers is verified. Our proof-of-concept breakthrough paves the way toward high-efficiency lead-free perovskite solar cells.
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  61. 61
    Shi, Z.; Guo, J.; Chen, Y.; Li, Q.; Pan, Y.; Zhang, H.; Xia, Y.; Huang, W. Lead-Free Organic-Inorganic Hybrid Perovskites for Photovoltaic Applications: Recent Advances and Perspectives. Adv. Mater. 2017, 29, 1605005,  DOI: 10.1002/adma.201605005
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    Jin, Z.; Zhang, Z.; Xiu, J.; Song, H.; Gatti, T.; He, Z. A Critical Review on Bismuth and Antimony Halide Based Perovskites and Their Derivatives for Photovoltaic Applications: Recent Advances and Challenges. J. Mater. Chem. A 2020, 8, 16166– 16188,  DOI: 10.1039/D0TA05433J
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    A critical review on bismuth and antimony halide based perovskites and their derivatives for photovoltaic applications: recent advances and challenges
    Jin, Zhixin; Zhang, Zheng; Xiu, Jingwei; Song, Haisheng; Gatti, Teresa; He, Zhubing
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2020), 8 (32), 16166-16188CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    A review. In the past decade, lead halide perovskites experienced impressive progress in photovoltaics with the certified device conversion efficiency over 25%, owing to their outstanding optoelectronic properties. However, the toxicity and environmental instability of the core lead halide materials would strongly limit their commercialization. Within this scenario, research investigations directed at assessing the properties and opportunities offered by emerging lead-free halide perovskites are becoming everyday more relevant to pinpoint new low-cost/low-toxicity solns. for solar-to-electricity conversion. In this review, group VA metal halide based perovskites, namely those of bismuth (Bi) and antimony (Sb), and their derivs. with different valence states are classified based on the formulas A3B2X9 and A2AgBX6, also known as double perovskites, and AgaBibXa+3b, called rudorffites (A = MA, FA, Cs, Rb, etc.; B = Bi, Sb; X = I, Br, Cl). Here, we summarize the recent progress in the exploitation of these materials, with special attention devoted to the description of the crystal structures, thin film prepn. methods and performances in real devices, including both theor.
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  63. 63
    Filip, M. R.; Liu, X.; Miglio, A.; Hautier, G.; Giustino, F. Phase Diagrams and Stability of Lead-Free Halide Double Perovskites Cs2BB′X6: B = Sb and Bi, B′ = Cu, Ag, and Au, and X = Cl, Br, and I. J. Phy. Chem. C 2018, 122, 158– 170,  DOI: 10.1021/acs.jpcc.7b10370
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    Phase Diagrams and Stability of Lead-Free Halide Double Perovskites Cs2BB'X6: B = Sb and Bi, B' = Cu, Ag, and Au, and X = Cl, Br, and I
    Filip, Marina R.; Liu, Xinlei; Miglio, Anna; Hautier, Geoffroy; Giustino, Feliciano
    Journal of Physical Chemistry C (2018), 122 (1), 158-170CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Lead-free pnictogen/noble metal halide double perovskites Cs2BiAgCl6, Cs2BiAgBr6, and Cs2SbAgCl6 are some of the most promising environmentally friendly alternatives to lead-halide perovskites. However, due to their relatively large band gaps (1.9-2.2 eV), they are not yet competitive candidates for use in photovoltaic devices. In this work, we perform a systematic study of the thermodn. stability of the entire family of Cs2BB'X6 compds. (B = Bi and Sb, B' = Cu, Ag, and Au, and X = Cl, Br, and I), and we explore the possibility of chem. mixing as a route to stabilize pnictogen/noble metal halide perovskites with low band gaps. Our calcns. indicate that Cs2BiAg1-xCuxCl6 mixes should be amenable to synthesis and could reduce the band gap down to 1.6-1.9 eV.
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    Fridkin, F. M.; Gerzanich, E. I.; Groshik, I. I.; Lyakhovitskaya, V. A. Absorption Edge in the Semiconducting Ferroelectrics SbSBr, BiSBr, SbSI. JETP Lett. 1966, 4, 201– 205
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    Harbeke, G. Absorption Edge on Ferroelectric SbSi under Electric Fields. J. Phys. Chem. Solids 1963, 24, 957– 963,  DOI: 10.1016/0022-3697(63)90074-X
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    Starczewska, A.; Nowak, M.; Szperlich, P.; Bednarczyk, I.; Mistewicz, K.; Kepinska, M.; Duka, P. Antimony Sulfoiodide as Novel Material for Photonic Crystals. Front. Opt. 2014, JW3A, JW3A.28,  DOI: 10.1364/FIO.2014.JW3A.28
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    Sanghera, J. S.; Heo, J.; Mackenzie, J. D. Chalcogenide Glasses. J. Non-Cryst. Solids 1988, 103, 155– 178,  DOI: 10.1016/0022-3093(88)90196-2
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    Chalcohalide glasses
    Sanghera, J. S.; Heo, J.; Mackenzie, J. D.
    Journal of Non-Crystalline Solids (1988), 103 (2-3), 155-78CODEN: JNCSBJ; ISSN:0022-3093.
    A review is given with 67 refs. on the chem., phys. elec., and optical properties of known chalcohalide glass forming systems.
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    Alward, J. F.; Fong, C. Y. Electronic and Optical Properties of SbSBr, SbSI and SbSeI. Solid State Commun. 1978, 25, 307– 310,  DOI: 10.1016/0038-1098(78)90964-X
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    Electronic and optical properties of antimony monosulfide monobromide, antimony monosulfide monoiodide, and antimony monoselenide monoiodide
    Alward, J. F.; Fong, C. Y.; El-Batanouny, M.; Wooten, F.
    Solid State Communications (1978), 25 (5), 307-10CODEN: SSCOA4; ISSN:0038-1098.
    Band structures of SbSBr and SbSeI were obtained by using the empirical pseudopotential method (EPM) to fit measured optical reflectivity data and earlier gap measurements. An SbI band structure was detd. by fitting to earlier reflectivity and Raman spectroscopic data, and the results agree better with the data than do the results of an earlier preliminary EPM calcn. Secondary conduction band min. may in part be responsible for the obsd. microwave oscillation (Gunn effect) in SbSI. Similar min. in SbSBr and SbSeI are reported, suggesting these crystals might also show microwave properties. The total ds. of states are presented.
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    Fridkin, V. M.; Gorelov, I. M.; Grekov, A. A.; Lyakhovitskaya, V. A.; Rodin, A. I. Phase Boundry in Ferroelectric SbSI as the Analog of an Electric Domain in a Semiconductor. ZhETF Pis’ma 4 1966, 11, 461– 468
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    Nitsche, R.; Merz, W. J. Photoconduction in Ternary V-VI-VII Compounds. J. Phys. Chem. Solids 1960, 13, 154– 155,  DOI: 10.1016/0022-3697(60)90136-0
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    Photoconduction in ternary V-VI-VII compounds
    Nitsche, R.; Merz, W. J.
    Physics and Chemistry of Solids (1960), 13 (), 154-5CODEN: PCSOA7; ISSN:0369-8726.
    Some photoelec. properties of single crystals of ternary compds. of the type V-VI-VII (V = Sb, Bi; VI = S, Se, Te; VII = Cl, Br, I) were studied. The wave lengths of max. photocurrent, λmax. shift in a regular way towards longer wave lengths with increasing at. weight of the components. For the compds. SbSBr and SbSI the dependence of photocurrent, dark current, and λmax. on temp. was measured between -140 and 120°.
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    Fatuzzo, E.; Harbeke, G.; Merz, W. J.; Nitsche, R.; Roetschi, H.; Ruppel, W. Ferroelectricity in SbSI. Phy. Rev. 1962, 127, 2036– 2037,  DOI: 10.1103/PhysRev.127.2036
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    Ferroelectricity in SbSI
    Fatuzzo, E.; Harbeke, G.; Merz, W. J.; Nitsche, R.; Roetschi, H.; Ruppel, W.
    Physical Review (1962), 127 (), 2036-7CODEN: PHRVAO; ISSN:0031-899X.
    The photoconducting compd. SbSI was found to be ferroelectric with a Curie point at 22°. It has a high spontaneous polarization of P8 = 25 μcoulombs/cm.2 and a small coercive field strength of Ec = 100 v./cm. at 0°. The dielec. const. at the Curie point reaches values as high as 50,000. The switching is relatively fast and depends on the applied field according to a power law. It appears that the unusually large field-induced shift in absorption edge in SbSI and its electromech. behavior can be explained by the fact that the material becomes ferroelectric below room temp.
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    Li, T.; Wang, X.; Yan, Y.; Mitzi, D. B. Phase Stability and Electronic Structure of Prospective Sb-Based Mixed Sulfide and Iodide 3D Perovskite (CH3NH3)SbSI2. J. Phys. Chem. Lett. 2018, 9, 3829– 3833,  DOI: 10.1021/acs.jpclett.8b01641
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    Phase Stability and Electronic Structure of Prospective Sb-Based Mixed Sulfide and Iodide 3D Perovskite (CH3NH3)SbSI2
    Li, Tianyang; Wang, Xiaoming; Yan, Yanfa; Mitzi, David B.
    Journal of Physical Chemistry Letters (2018), 9 (14), 3829-3833CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Pb-free Sb-based mixed sulfide and iodide perovskite phases have recently been reported to be synthesized exptl. and to exhibit reasonable photovoltaic performance. Through a combination of exptl. validation and computational anal., we show no evidence of the formation of the mixed sulfide and iodide perovskite phase, MASbSI2 (MA =CH3NH3+), and instead that the main products are a mixt. of the binary and ternary compds. (Sb2S3 and MA3Sb2I9). D. functional theory calcns. also indicate that such a mixed sulfide and iodide perovskite phase should be thermodynamically less stable compared with binary/ternary anion-segregated secondary phases and less likely to be synthesized under equil. conditions. Band structure calcns. show that this mixed sulfide and iodide phase, if possible to synthesize (e.g., under nonequil. conditions), should have a suitable direct band gap for photovoltaic application.
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    Sun, Y. Y.; Shi, J.; Lian, J.; Gao, W.; Agiorgousis, M. L.; Zhang, P.; Zhang, S. Discovering Lead-Free Perovskite Solar Materials with A Split-Anion Approach. Nanoscale 2016, 8, 6284– 6289,  DOI: 10.1039/C5NR04310G
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    Discovering lead-free perovskite solar materials with a split-anion approach
    Sun, Yi-Yang; Shi, Jian; Lian, Jie; Gao, Weiwei; Agiorgousis, Michael L.; Zhang, Peihong; Zhang, Shengbai
    Nanoscale (2016), 8 (12), 6284-6289CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Org.-inorg. hybrid perovskite solar materials, being low-cost and high-performance, are promising for large-scale deployment of the photovoltaic technol. A key challenge that remains to be addressed is the toxicity of these materials since the high-efficiency solar cells are made of lead-contg. materials, in particular, CH3NH3PbI3. Here, based on first-principles calcn., we search for lead-free perovskite materials based on the split-anion approach, where we replace Pb with non-toxic elements while introducing dual anions (i.e., splitting the anion sites) that preserve the charge neutrality. We show that CH3NH3BiSeI2 and CH3NH3BiSI2 exhibit improved band gaps and optical absorption over CH3NH3PbI3. The split-anion approach could also be applied to pure inorg. perovskites, significantly enlarging the pool of candidate materials in the design of low-cost, high-performance and environmentally-friendly perovskite solar materials.
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    Butler, K. T.; Frost, J. M.; Walsh, A. Ferroelectric Materials for Solar Energy Conversion: Photoferroics Revisited. Energy Environ. Sci. 2015, 8, 838– 848,  DOI: 10.1039/C4EE03523B
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    Ferroelectric materials for solar energy conversion: photoferroics revisited
    Butler, Keith T.; Frost, Jarvist M.; Walsh, Aron
    Energy & Environmental Science (2015), 8 (3), 838-848CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    A review. The application of ferroelec. materials (i.e. solids that exhibit spontaneous elec. polarisation) in solar cells has a long and controversial history. This includes the first observations of the anomalous photovoltaic effect (APE) and the bulk photovoltaic effect (BPE). The recent successful application of inorg. and hybrid perovskite structured materials (e.g. BiFeO3, CsSnI3, CH3NH3PbI3) in solar cells emphasizes that polar semiconductors can be used in conventional photovoltaic architectures. We review developments in this field, with a particular emphasis on the materials known to display the APE/BPE (e.g. ZnS, CdTe, SbSI), and the theor. explanation. Crit. anal. is complemented with first-principles calcn. of the underlying electronic structure. In addn. to discussing the implications of a ferroelec. absorber layer, and the solid state theory of polarisation (Berry phase anal.), design principles and opportunities for high-efficiency ferroelec. photovoltaics are presented.
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    Ran, Z.; Wang, X.; Li, Y.; Yang, D.; Zhao, X.-G.; Biswas, K.; Singh, D. J.; Zhang, L. Bismuth and Antimony-Based Oxyhalides and Chalcohalides As Potential Optoelectronic Materials. npj Comput. Mater. 2018, 4, 1– 7,  DOI: 10.1038/s41524-018-0071-1
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    Bismuth and antimony-based oxyhalides and chalcohalides as potential optoelectronic materials
    Ran, Zhao; Wang, Xinjiang; Li, Yuwei; Yang, Dongwen; Zhao, Xin-Gang; Biswas, Koushik; Singh, David J.; Zhang, Lijun
    npj Computational Materials (2018), 4 (1), 1-7CODEN: NCMPCS; ISSN:2057-3960. (Nature Research)
    In the last decade the ns2 cations (e.g., Pb2+ and Sn2+)-based halides have emerged as one of the most exciting new classes of optoelectronic materials, as exemplified by for instance hybrid perovskite solar absorbers. These materials not only exhibit unprecedented performance in some cases, but they also appear to break new ground with their unexpected properties, such as extreme tolerance to defects. However, because of the relatively recent emergence of this class of materials, there remain many yet to be fully explored compds. Here, we assess a series of bismuth/antimony oxyhalides and chalcohalides using consistent first principles methods to ascertain their properties and obtain trends. Based on these calcns., we identify a subset consisting of three types of compds. that may be promising as solar absorbers, transparent conductors, and radiation detectors. Their electronic structure, connection to the crystal geometry, and impact on band-edge dispersion and carrier effective mass are discussed.
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    Kunioku, H.; Higashi, M.; Abe, R. Low-Temperature Synthesis of Bismuth Chalcohalides: Candidate Photovoltaic Materials with Easily, Continuously Controllable Band gap. Sci. Rep. 2016, 6, 32664,  DOI: 10.1038/srep32664
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    Low-Temperature Synthesis of Bismuth Chalcohalides: Candidate Photovoltaic Materials with Easily, Continuously Controllable Band gap
    Kunioku, Hironobu; Higashi, Masanobu; Abe, Ryu
    Scientific Reports (2016), 6 (), 32664CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    Although bismuth chalcohalides, such as BiSI and BiSeI, have been recently attracting considerable attention as photovoltaic materials, the methods available to synthesize them are quite limited thus far. In this study, a novel, facile method to synthesize these chalcohalides, including BiSBr1-xIx solid solns., at low temps. was developed via the substitution of anions from O2- to S2- (or Se2-) using bismuth oxyhalide precursors. Complete phase transition was readily obsd. upon treatment of BiOI particles with H2S or H2Se at surprisingly low temps. of less than 150 °C and short reaction times of less than 1 h, producing BiSI and BiSeI particles, resp. This method was also applied for synthesizing BiSBr1-xIx, where continuous changes in their band gaps were obsd. depending on the ratio between iodine and bromine. The compn. of all elements (except oxygen) in the chalcohalides thus produced was almost identical to that of the oxyhalide precursors, attributed to the suppressed volatilization of halogens at such low temps. All chalcohalides loaded on FTO clearly exhibited an anodic photocurrent in an acetonitrile soln. contg. I-, attributed to their n-type nature, e.g., the BiSI electrode exhibited high IPCE (64% at 700 nm, +0.2 V vs. Ag/AgCl).
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    Peng, B.; Xu, K.; Zhang, H.; Ning, Z.; Shao, H.; Ni, G.; Li, J.; Zhu, Y.; Zhu, H.; Soukoulis, C. M. 1D SbSeI, SbSI, and SbSBr With High Stability and Novel Properties for Microelectronic, Optoelectronic, and Thermoelectric Applications. Adv. Theory Simul. 2018, 1, 1700005,  DOI: 10.1002/adts.201700005
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    Fenner, J.; Rabenau, A.; Trageser, G. Solid-State Chemistry of Thio-, Seleno-, and Tellurohalides of Representative and Transition Elements. Adv. Ing. Chem. Radiochem 1980, 23, 329– 425,  DOI: 10.1016/S0065-2792(08)60096-5
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    Nowak, M.; Jesionek, M.; Mistewicz, K. Industrial Applications of Nanomaterials;. Elsevier 2019, 225– 282,  DOI: 10.1016/B978-0-12-815749-7.00009-8
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    Nowak, M.; Jesionek, M.; Mistewicz, K. Nanomaterials Synthesis; Elsevier, 2019; pp 337– 384.
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    Wlazlak, E.; Blachecki, A.; Bisztyga-Szklarz, M.; Klejna, S.; Mazur, T.; Mech, K.; Pilarczyk, K.; Przyczyna, D.; Suchecki, M.; Zawal, P. Heavy Pnictogen Chalcohalides: The Synthesis, Structure And Properties of These Rediscovered Semiconductors. Chem Commun. 2018, 54, 12133– 12161,  DOI: 10.1039/C8CC05149F
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    Heavy pnictogen chalcohalides: the synthesis, structure and properties of these rediscovered semiconductors
    Wlazlak, Ewelina; Blachecki, Andrzej; Bisztyga-Szklarz, Magdalena; Klejna, Sylwia; Mazur, Tomasz; Mech, Krzysztof; Pilarczyk, Kacper; Przyczyna, Dawid; Suchecki, Maciej; Zawal, Piotr; Szacilowski, Konrad
    Chemical Communications (Cambridge, United Kingdom) (2018), 54 (86), 12133-12162CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    This review focuses on the synthesis, properties and selected applications of heavy pnictogen chalcohalides, i.e. compds. of the MQX stoichiometry, where M = As, Sb, and Bi; Q = O, S, Se, and Te; and X = F, Cl, Br and I. The first section focuses on their synthesis and crystal structures, and the second section discusses the electronic structure on the basis of quantum chem. modeling and selected exptl. data. Finally, the third section discusses their elec., photoelectrochem. and photocatalytic properties and applications. In contrast to perovskites, chalcopyrites and kesterites, chalcohalides have attracted relatively less attention, but their structure and properties are well suited for numerous applications.
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    Wu, L. M.; Wu, X. T.; Chen, L. Structural Overview and Structure-Property Relationships of Iodoplumbate and Iodobismuthate. Coord. Chem. Rev. 2009, 253, 2787– 2804,  DOI: 10.1016/j.ccr.2009.08.003
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    Structural overview and structure-property relationships of iodoplumbate and iodobismuthate
    Wu, Li-Ming; Wu, Xin-Tao; Chen, Ling
    Coordination Chemistry Reviews (2009), 253 (23-24), 2787-2804CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    This review discusses the main structural features of the inorg. of the iodoplumbate and iodobismuthate moieties according to the aggregation and connections of the primary MI6 octahedron building unit. The cation effect, ligand effect and hetero metal-iodine bonding effect are summarized. The aggregation d. of the inorg. moiety (ADIM) is an important structural parameter that is related to the solid-state properties. An empirical r value is thus defined as r = Σ(Nμn-I)/n, where Nμn-I means the no. of n-fold-coordinated I atoms per building unit and n = 1-6; M/r value as M/r = M:r (M = the no. of metal centers per building unit). The M/r value-anionic structure relationship and M/r value-band gap correlation are described. Several interesting optical, thermal and ferroelec. properties that are related to the dimensionality, compn., configuration, distortion, and bonding are discussed in detail.
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    Xiao, J. R.; Yang, S. H.; Feng, F.; Xue, H. G.; Guo, S. P. A Review of The Structural Chemistry and Physical Properties of Metal Chalcogenide Halides. Coord. Chem. Rev. 2017, 347, 23– 74,  DOI: 10.1016/j.ccr.2017.06.010
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    A review of the structural chemistry and physical properties of metal chalcogenide halides
    Xiao, Jin-Rong; Yang, Si-Han; Feng, Fang; Xue, Huai-Guo; Guo, Sheng-Ping
    Coordination Chemistry Reviews (2017), 347 (), 23-47CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A review. Given the collaborative effect between different anions, compds. contg. two types of anions can have improved phys. performance compared with some aspects of those contg. only one type of anion. To date, many such compds. have been investigated. This review focuses on metal chalcogenide halides, namely, M-Q-X (M = metal element; Q = S, Se, Te; X = F, Cl, Br, I) compds. Although they have similar at. radii, chalcogen atoms have much smaller electronegativities than those of its neighboring halogen atoms, which results in their different bonding styles. The inclusion of X- into chalcogenides can move their optical band gaps to higher values and their coordination polyhedra can become more distorted, albeit with little d. change. These structural effects make metal chalcogenide halides a good choice to fine-tune the structures and phys. properties of chalcogenides or halides. Several hundreds of metal chalcogenide halides already have been discovered. They demonstrate rich structures and versatile phys. properties, enabling their diverse applications in the fields of solid-state electrolytes, second-order non-linear optics and thermoelectricity. Considering their flourishing development, this paper provides an overview of recent achievements in the structural chem. and phys. properties of metal chalcogenide halides.
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    Nie, R.; Sumukam, R. R.; Reddy, S. H.; Banavoth, M.; Seok, S. I. Lead-free perovskite solar cells enabled by hetero-valent substitutes. Energy Environ. Sci. 2020, 13, 2363– 2385,  DOI: 10.1039/D0EE01153C
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    Lead-free perovskite solar cells enabled by hetero-valent substitutes
    Nie, Riming; Sumukam, Ranadeep Raj; Reddy, Sathy Harshavardhan; Banavoth, Murali; Seok, Sang Il
    Energy & Environmental Science (2020), 13 (8), 2363-2385CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    A review. Striving for clean and economical methods of energy prodn., lead-based perovskites have become the key materials in photovoltaics owing to their facile soln. processability and phenomenal performance. Unfortunately, lead toxicity poses a major hurdle to their scalability and widespread commercialization. In this perspective, we provide a comprehensive assessment of lead-free perovskite solar cells enabled by hetero-valent substitutes. These comprise A3B2X9-structured perovskites, halide double perovskites, and mixed metal halide-chalcogenide perovskites. Importantly, we emphasize the effects of cationic-anionic sites, metal substitutions, and solvents on the chem. and structural properties of A3B2X9-structured perovskites. Moreover, we also focus on antimony(V) perovskite-like materials and antimony-based perovskite nanocrystals (NCs) in the fabrication of devices. Two types of halide double perovskites are described, including A2B(I)B(III)X6 and A2B(IV)X6 double perovskites. Subsequently, chalcogenide perovskite solar cells are also discussed. The primary purpose of this perspective is to explicitly describe lead-free perovskite solar cells enabled by hetero-valent substitutions more broadly as a category of next-generation optoelectronic materials. Finally, we propose that mixed halide-chalcogenide perovskites offer a promising pathway towards achieving highly efficient and stable perovskite solar cells.
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    Palazon, F. Metal Chalcohalides: Next Generation Photovoltaic Materials?. Sol. RRL 2022, 6, 2100829,  DOI: 10.1002/solr.202100829
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    Metal Chalcohalides: Next Generation Photovoltaic Materials?
    Palazon, Francisco
    Solar RRL (2022), 6 (2), 2100829CODEN: SRORAW; ISSN:2367-198X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Metal chalcohalides have recently been highlighted as so-far overlooked semiconductors that could play an important role in the future of photovoltaics (PV). Indeed, the blooming field of emergent PV technologies is still in search for stable, efficient, and environmentally-friendly light-harvesting materials to be used either in single-junction solar cells or multijunction devices in combination with silicon or another absorbers. Under the broad terms of metal chalcohalides, there exists a plethora of semiconductor materials with different chem., structural, and optoelectronic characteristics. While some have already been implemented in solar cells with power conversion efficiencies up to 4-5%, others are only theor. described. This perspective article offers a general overview of these materials as potential next-generation absorbers in PV and also discusses possible limitations, not only related to intrinsic materials' properties but also to processing conditions.
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    Choi, Y. C.; Jung, K. W. Recent Progress in Fabrication of Antimony/Bismuth Chalcohalides for Lead-Free Solar Cell Applications. Nanomater. 2020, 10, 2284,  DOI: 10.3390/nano10112284
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    Recent progress in fabrication of antimony/bismuth chalcohalides for lead-free solar cell applications
    Choi, Yong Chan; Jung, Kang-Won
    Nanomaterials (2020), 10 (11), 2284CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
    A review. Despite their comparable performance to com. solar systems, lead-based perovskite (Pb-perovskite) solar cells exhibit limitations including Pb toxicity and instability for industrial applications. To address these issues, two types of Pb-free materials have been proposed as alternatives to Pb-perovskite: perovskite-based and non-perovskite-based materials. In this review, we summarize the recent progress on solar cells based on antimony/bismuth (Sb/Bi) chalcohalides, representing Sb/Bi non-perovskite semiconductors contg. chalcogenides and halides. Two types of ternary and quaternary chalcohalides are described, with their classification predicated on the fabrication method. We also highlight their utility as interfacial layers for improving other solar cells. This review provides clues for improving the performances of devices and design of multifunctional solar systems.
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    Behler, J. Atom-Centered Symmetry Functions for Constructing High-Dimensional Neural Network Potentials. J. Chem. Phys. 2011, 134, 074106– 13,  DOI: 10.1063/1.3553717
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    Atom-centered symmetry functions for constructing high-dimensional neural network potentials
    Behler, Joerg
    Journal of Chemical Physics (2011), 134 (7), 074106/1-074106/13CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    Neural networks offer an unbiased and numerically very accurate approach to represent high-dimensional ab initio potential-energy surfaces. Once constructed, neural network potentials can provide the energies and forces many orders of magnitude faster than electronic structure calcns., and thus enable mol. dynamics simulations of large systems. However, Cartesian coordinates are not a good choice to represent the at. positions, and a transformation to symmetry functions is required. Using simple benchmark systems, the properties of several types of symmetry functions suitable for the construction of high-dimensional neural network potential-energy surfaces are discussed in detail. The symmetry functions are general and can be applied to all types of systems such as mols., cryst. and amorphous solids, and liqs. (c) 2011 American Institute of Physics.
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    Curtarolo, S.; Setyawan, W.; Wang, S.; Xue, J.; Yang, K.; Taylor, R. H.; Nelson, L. J.; Hart, G. L. W.; Sanvito, S.; Buongiorno-Nardelli, M. AFLOWLIB.ORG: A Distributed Materials Properties Repository from High-Throughput Ab Initio Calculations. Comput. Mater. Sci. 2012, 58, 227– 235,  DOI: 10.1016/j.commatsci.2012.02.002
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    AFLOWLIB.ORG: A distributed materials properties repository from high-throughput ab initio calculations
    Curtarolo, Stefano; Setyawan, Wahyu; Wang, Shidong; Xue, Junkai; Yang, Kesong; Taylor, Richard H.; Nelson, Lance J.; Hart, Gus L. W.; Sanvito, Stefano; Buongiorno-Nardelli, Marco; Mingo, Natalio; Levy, Ohad
    Computational Materials Science (2012), 58 (), 227-235CODEN: CMMSEM; ISSN:0927-0256. (Elsevier B.V.)
    Empirical databases of crystal structures and thermodn. properties are fundamental tools for materials research. Recent rapid proliferation of computational data on materials properties presents the possibility to complement and extend the databases where the exptl. data is lacking or difficult to obtain. Enhanced repositories that integrate both computational and empirical approaches open novel opportunities for structure discovery and optimization, including uncovering of unsuspected compds., metastable structures and correlations between various characteristics. The practical realization of these opportunities depends on a systematic compilation and classification of the generated data in addn. to an accessible interface for the materials science community. In this paper we present an extensive repository, aflowlib.org, comprising phase-diagrams, electronic structure and magnetic properties, generated by the high-throughput framework AFLOW. This continuously updated compilation currently contains over 150,000 thermodn. entries for alloys, covering the entire compn. range of more than 650 binary systems, 13,000 electronic structure analyses of inorg. compds., and 50,000 entries for novel potential magnetic and spintronics systems. The repository is available for the scientific community on the website of the materials research consortium, aflowlib.org.
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    Stevanović, V.; Lany, S.; Zhang, X.; Zunger, A. Correcting Density Functional Theory for Accurate Predictions of Compound Enthalpies of Formation: Fitted Elemental-Phase Reference Energies. Phys. Rev. B 2012, 85, 115104– 12,  DOI: 10.1103/PhysRevB.85.115104
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    Correcting density functional theory for accurate predictions of compound enthalpies of formation: fitted elemental-phase reference energies
    Stevanovic, Vladan; Lany, Stephan; Zhang, Xiuwen; Zunger, Alex
    Physical Review B: Condensed Matter and Materials Physics (2012), 85 (11), 115104/1-115104/12CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Despite the great success that theor. approaches based on d. functional theory have in describing properties of solid compds., accurate predictions of the enthalpies of formation (ΔHf) of insulating and semiconducting solids still remain a challenge. This is mainly due to incomplete error cancellation when computing the total energy differences between the compd. total energy and the total energies of its elemental constituents. In this paper we present an approach based on GGA + U calcns., including the spin-orbit coupling, which involves fitted elemental-phase ref. energies (FERE) and which significantly improves the error cancellation resulting in accurate values for the compd. enthalpies of formation. We use an extensive set of 252 binary compds. with measured ΔHf values (pnictides, chalcogenides, and halides) to obtain FERE energies and show that after the fitting, the 252 enthalpies of formation are reproduced with the mean abs. error MAE = 0.054 eV/atom instead of MAE ≈ 0.250 eV/atom resulting from pure GGA calcns. When applied to a set of 55 ternary compds. that were not part of the fitting set the FERE method reproduces their enthalpies of formation with MAE = 0.048 eV/atom. Furthermore, we find that contributions to the total energy differences coming from the spin-orbit coupling can be, to a good approxn., sepd. into purely at. contributions which do not affect ΔHf. The FERE method, hence, represents a simple and general approach, as it is computationally equiv. to the cost of pure GGA calcns. and applies to virtually all insulating and semiconducting compds., for predicting compd. ΔHf values with chem. accuracy. We also show that by providing accurate ΔHf the FERE approach can be applied for accurate predictions of the compd. thermodn. stability or for predictions of Li-ion battery voltages.
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    Lany, S. Band-Structure Calculations For The 3d transition Metal Oxides In-GW. Phys. Rev. B 2013, 87, 085112– 9,  DOI: 10.1103/PhysRevB.87.085112
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    Band-structure calculations for the 3d transition metal oxides in GW
    Lany, Stephan
    Physical Review B: Condensed Matter and Materials Physics (2013), 87 (8), 085112/1-085112/9CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Many-body GW calcns. have emerged as a std. for the prediction of band gaps, band structures, and optical properties for main-group semiconductors and insulators, but it is not well established how predictive the GW method is in general for transition metal (TM) compds. Surveying the series of 3d oxides within a typical GW approach using the RPA reveals mixed results, including cases where the calcd. band gap is either too small or too large, depending on the oxidn. states of the TM (e.g., FeO/Fe2O3, Cu2O/CuO). The problem appears to originate mostly from a too high av. d-orbital energy, whereas the splitting between occupied and unoccupied d symmetries seems to be reasonably accurate. It is shown that augmenting the GW self-energy by an attractive (neg.) and occupation-independent on-site potential for the TM d orbitals with a single parameter per TM cation can reconcile the band gaps for different oxide stoichiometries and TM oxidn. states. In Cu2O, which is considered here in more detail, std. GW based on wave functions from initial d. or hybrid functional calcns. yields an unphys. prediction with an incorrect ordering of the conduction bands, even when the magnitude of the band gap is in apparent agreement with expt. The correct band ordering is restored either by applying the d-state potential or by iterating the wave functions to self-consistency, which both have the effect of lowering the Cu-d orbital energy. While it remains to be detd. which improvements over std. GW implementations are needed to achieve an accurate ab initio description for a wide range of transition metal compds., the application of the empirical on-site potential serves to mitigate the problems specifically related to d states in GW calcns.
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    Kirklin, S.; Saal, J. E.; Meredig, B.; Thompson, A.; Doak, J. W.; Aykol, M.; Rühl, S.; Wolverton, C. The Open Quantum Materials Database (OQMD): assessing the accuracy of DFT formation energies. npj Comput. Mater. 2015, 1, 15010,  DOI: 10.1038/npjcompumats.2015.10
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    The Open Quantum Materials Database (OQMD): assessing the accuracy of DFT formation energies
    Kirklin, Scott; Saal, James E.; Meredig, Bryce; Thompson, Alex; Doak, Jeff W.; Aykol, Muratahan; Ruhl, Stephan; Wolverton, Chris
    npj Computational Materials (2015), 1 (), 15010CODEN: NCMPCS; ISSN:2057-3960. (Nature Publishing Group)
    The Open Quantum Materials Database (OQMD) is a high-throughput database currently consisting of nearly 300,000 d. functional theory (DFT) total energy calcns. of compds. from the Inorg. Crystal Structure Database (ICSD) and decorations of commonly occurring crystal structures. To maximise the impact of these data, the entire database is being made available, without restrictions, at www.oqmd.org/download. In this paper, we outline the structure and contents of the database, and then use it to evaluate the accuracy of the calcns. therein by comparing DFT predictions with exptl. measurements for the stability of all elemental ground-state structures and 1,670 exptl. formation energies of compds. This represents the largest comparison between DFT and exptl. formation energies to date. The apparent mean abs. error between exptl. measurements and our calcns. is 0.096 eV/atom. In order to est. how much error to attribute to the DFT calcns., we also examine deviation between different exptl. measurements themselves where multiple sources are available, and find a surprisingly large mean abs. error of 0.082 eV/atom. Hence, we suggest that a significant fraction of the error between DFT and exptl. formation energies may be attributed to exptl. uncertainties. Finally, we evaluate the stability of compds. in the OQMD (including compds. obtained from the ICSD as well as hypothetical structures), which allows us to predict the existence of ∼3,200 new compds. that have not been exptl. characterised and uncover trends in material discovery, based on historical data available within the ICSD.
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    Luo, S.; Li, T.; Wang, X.; Faizan, M.; Zhang, L. High-Throughput Computational Materials Screening and Discovery Of Optoelectronic Semiconductors. WIREs Computat. Mol. Sci. 2021, 11, e1489  DOI: 10.1002/wcms.1489
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    Ma, X. Y.; Lewis, J. P.; Yan, Q. B.; Su, G. Accelerated Discovery of Two-Dimensional Optoelectronic Octahedral Oxyhalides via High-Throughput Ab Initio Calculations and Machine Learning. J. Phys. Chem. Lett. 2019, 10, 6734– 6740,  DOI: 10.1021/acs.jpclett.9b02420
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    93
    Accelerated Discovery of Two-Dimensional Optoelectronic Octahedral Oxyhalides via High-Throughput Ab Initio Calculations and Machine Learning
    Ma, Xing-Yu; Lewis, James P.; Yan, Qing-Bo; Su, Gang
    Journal of Physical Chemistry Letters (2019), 10 (21), 6734-6740CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Traditional trial-and-error methods are obstacles for large-scale searching of new optoelectronic materials. Here, we introduce a method combining high-throughput ab initio calcns. and machine-learning approaches to predict two-dimensional octahedral oxyhalides with improved optoelectronic properties. We develop an effective machine-learning model based on an expansive data set generated from d. functional calcns. including the geometric and electronic properties of 300 two-dimensional octahedral oxyhalides. Our model accelerates the screening of potential optoelectronic materials of 5,000 two-dimensional octahedral oxyhalides. The distorted stacked octahedral factors proposed in our model play essential roles in the machine-learning prediction. Several potential two-dimensional optoelectronic octahedral oxyhalides with moderate band gaps, high electron mobilities, and ultrahigh absorbance coeffs. are successfully hypothesized.
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  94. 94
    Davies, D. W.; Butler, K. T.; Jackson, A. J.; Morris, A.; Frost, J. M.; Skelton, J. M.; Walsh, A. Computational Screening of All Stoichiometric Inorganic Materials. Chem 2016, 1, 617– 627,  DOI: 10.1016/j.chempr.2016.09.010
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    Computational Screening of All Stoichiometric Inorganic Materials
    Davies, Daniel W.; Butler, Keith T.; Jackson, Adam J.; Morris, Andrew; Frost, Jarvist M.; Skelton, Jonathan M.; Walsh, Aron
    Chem (2016), 1 (4), 617-627CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
    A review. Forming a four-component compd. from the first 103 elements of the periodic table results in more than 1012 combinations. Such a materials space is intractable to high-throughput expt. or first-principle computation. We introduce a framework to address this problem and quantify how many materials can exist. We apply principles of valency and electronegativity to filter chem. implausible compns., which reduces the inorg. quaternary space to 1010 combinations. We demonstrate that ests. of band gaps and abs. electron energies can be made simply on the basis of the chem. compn. and apply this to the search for new semiconducting materials to support the photoelectrochem. splitting of water. We show the applicability to predicting crystal structure by analogy with known compds., including exploration of the phase space for ternary combinations that form a perovskite lattice. Computer screening reproduces known perovskite materials and predicts the feasibility of thousands more. Given the simplicity of the approach, large-scale searches can be performed on a single workstation.
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    Cai, W. B.; Abudurusuli, A.; Xie, C. W.; Tikhonov, E.; Li, J. J.; Pan, S. L.; Yang, Z. H. Toward the Rational Design of Mid-Infrared Nonlinear Optical Materials with Targeted Properties via a Multi-Level Data-Driven Approach. Adv. Funct. Mater. 2022, 32, 2200231,  DOI: 10.1002/adfm.202200231
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    Toward the Rational Design of Mid-Infrared Nonlinear Optical Materials with Targeted Properties via a Multi-Level Data-Driven Approach
    Cai, Wenbing; Abudurusuli, Ailijiang; Xie, Congwei; Tikhonov, Evgenii; Li, Junjie; Pan, Shilie; Yang, Zhihua
    Advanced Functional Materials (2022), 32 (23), 2200231CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Design and exploratory synthesis of new mid-IR (mid-IR) nonlinear optical (NLO) materials are urgently needed for modern laser science and technol. because the widely used IR NLO crystals still suffer from their inextricable drawbacks. Herein, a multi-level data-driven approach to realize fast and efficient structure prediction for the exploration of promising mid-IR NLO materials is proposed. Techniques based on machine learning, crystal structure prediction, high-throughput calcn. and screening, database building, and exptl. verification are tightly combined for creating pathways from chem. compns., crystal structures to rational synthesis. Through this data-driven approach, not only are all known structures successfully predicted but also five thermodynamically stable and 50 metastable new selenides in AIBIIISe2 systems (AI = Li, Na, K, Rb, and Cs; BIII = Al and Ga) are found, among which eight outstanding compds. with wide bandgaps (> 2.70 eV) and large SHG responses (>10 pm V-1) are suggested. Moreover, the predicted compds. I42d-LiGaSe2 and I4/mcm-KAlSe2 are successfully obtained exptl. In particular, LiGaSe2 exhibits a robust SHG response (≈2 x AGS) and long IR absorption edge that can cover two atm. windows (3-5, 8-12 μm). Simultaneously, this new research paradigm is also applicative for discovering new materials in other fields.
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  96. 96
    Wang, P.; Chu, Y.; Tudi, A.; Xie, C. W.; Yang, Z. H.; Pan, S. L.; Li, J. J. The Combination of Structure Prediction and Experiment for the Exploration of Alkali-Earth Metal-Contained Chalcopyrite-Like IR Nonlinear Optical Material. Adv. Sci. 2022, 9, 2106120,  DOI: 10.1002/advs.202106120
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    96
    The Combination of Structure Prediction and Experiment for the Exploration of Alkali-Earth Metal-Contained Chalcopyrite-Like IR Nonlinear Optical Material
    Wang, Peng; Chu, Yu; Tudi, Abudukadi; Xie, Congwei; Yang, Zhihua; Pan, Shilie; Li, Junjie
    Advanced Science (Weinheim, Germany) (2022), 9 (15), 2106120CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Design and fabrication of new IR (IR) nonlinear optical (NLO) materials with balanced properties are urgently needed since com. chalcopyrite-like (CL) NLO crystals are suffering from their intrinsic drawbacks. Herein, the first defect-CL (DCL) alkali-earth metal (AEM) selenide IR NLO material, DCL-MgGa2Se4, has been rationally designed and fabricated by a structure prediction and expt. combined strategy. The introduction of AEM tetrahedral unit MgSe4 effectively widens the band gap of DCL compds. The title compd. exhibits a wide band gap of 2.96 eV, resulting in a high laser induced damage threshold (LIDT) of ≈3.0 x AgGaS2 (AGS). Furthermore, the compd. shows a suitable second harmonic generation (SHG) response (≈0.9 x AGS) with a type-I phase-matching (PM) behavior and a wide IR transparent range. The results indicate that DCL-MgGa2Se4 is a promising mid-to-far IR NLO material and give some insights into the design of new CL compd. with outstanding IR NLO properties based on the AEM tetrahedra and the structure predication and expt. combined strategy.
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  97. 97
    Brandt, R. E.; Stevanović, V.; Ginley, D. S.; Buonassisi, T. Identifying Defect-Tolerant Semiconductors With High Minority-Carrier Lifetimes: Beyond Hybrid Lead Halide Perovskites. MRS Commun. 2015, 5, 265– 275,  DOI: 10.1557/mrc.2015.26
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    97
    Identifying defect-tolerant semiconductors with high minority-carrier lifetimes: beyond hybrid lead halide perovskites
    Brandt, Riley E.; Stevanovic, Vladan; Ginley, David S.; Buonassisi, Tonio
    MRS Communications (2015), 5 (2), 265-275CODEN: MCROF8; ISSN:2159-6867. (Cambridge University Press)
    The emergence of methyl-ammonium lead halide (MAPbX 3) perovskites motivates the identification of unique properties giving rise to exceptional bulk transport properties, and identifying future materials with similar properties. Here, we propose that this "defect tolerance" emerges from fundamental electronic-structure properties, including the orbital character of the conduction and valence band extrema, the charge-carrier effective masses, and the static dielec. const. We use MaterialsProject.org searches and detailed electronic-structure calcns. to demonstrate these properties in other materials than MAPbX 3. This framework of materials discovery may be applied more broadly, to accelerate discovery of new semiconductors based on emerging understanding of recent successes.
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    Chatterjee, S.; Pal, A. J. Influence of Metal Substitution On Hybrid Halide Perovskites: Towards Lead-Free Perovskite Solar Cells. J. Mater. Chem. A 2018, 6, 3793– 3823,  DOI: 10.1039/C7TA09943F
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    98
    Influence of metal substitution on hybrid halide perovskites: towards lead-free perovskite solar cells
    Chatterjee, Soumyo; Pal, Amlan J.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (9), 3793-3823CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    The revolutionary impact of hybrid halide perovskites in the field of soln.-based photovoltaics has made them one of the most-promising technologies for next-generation solar cells. However, such a breakthrough has natural drawbacks, since all these perovskite materials yielding high efficiency contain lead as a primary element in their chem. compn. There is hence a substantial hurdle for their acceptance in industries and society alike. Substitution of lead in the perovskite structure by a suitable nontoxic metal has therefore become one of the significant pivotal challenges assocd. with these wonder materials of the present decade. We hereby review the progress in this emerging field of research to summarize the influence of alternative elements in replacing lead vis-´a-vis the materials' properties and characteristics of solar cells based on the metal-substituted hybrid halide perovskites. Moreover, we have discussed the prospects of next-generation of lead-free perovskites, namely 2D layered perovskites, defect ordered and double perovskites with a focus on their properties, stability, and photovoltaic applications.
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  99. 99
    Huang, Y. T.; Kavanagh, S. R.; Scanlon, D. O.; Walsh, A.; Hoye, R. L. Z. Perovskite-Inspired Materials for Photovoltaics And Beyond-From Design to Devices. Nanotechnol. 2021, 32, 132004,  DOI: 10.1088/1361-6528/abcf6d
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    Perovskite-inspired materials for photovoltaics and beyond-from design to devices
    Huang, Yi-Teng; Kavanagh, Sean R.; Scanlon, David O.; Walsh, Aron; Hoye, Robert L. Z.
    Nanotechnology (2021), 32 (13), 132004CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    Lead-halide perovskites have demonstrated astonishing increases in power conversion efficiency in photovoltaics over the last decade. The most efficient perovskite devices now outperform industry-std. multi-cryst. silicon solar cells, despite the fact that perovskites are typically grown at low temp. using simple soln.-based methods. However, the toxicity of lead and its ready soly. in water are concerns for widespread implementation. These challenges, alongside the many successes of the perovskites, have motivated significant efforts across multiple disciplines to find lead-free and stable alternatives which could mimic the ability of the perovskites to achieve high performance with low temp., facile fabrication methods. This Review discusses the computational and exptl. approaches that have been taken to discover lead-free perovskite-inspired materials, and the recent successes and challenges in synthesizing these compds. The atomistic origins of the extraordinary performance exhibited by lead-halide perovskites in photovoltaic devices is discussed, alongside the key challenges in engineering such high-performance in alternative, next-generation materials. Beyond photovoltaics, this Review discusses the impact perovskite-inspired materials have had in spurring efforts to apply new materials in other optoelectronic applications, namely light-emitting diodes, photocatalysts, radiation detectors, thin film transistors and memristors. Finally, the prospects and key challenges faced by the field in advancing the development of perovskite-inspired materials towards realization in com. devices is discussed.
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  100. 100
    Li, T.; Luo, S.; Wang, X.; Zhang, L. Alternative Lone-Pair ns2 -Cation-Based Semiconductors beyond Lead Halide Perovskites for Optoelectronic Applications. Adv. Mater. 2021, 33, e2008574  DOI: 10.1002/adma.202008574
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    100
    Alternative Lone-Pair ns2-Cation-Based Semiconductors beyond Lead Halide Perovskites for Optoelectronic Applications
    Li, Tianshu; Luo, Shulin; Wang, Xinjiang; Zhang, Lijun
    Advanced Materials (Weinheim, Germany) (2021), 33 (32), 2008574CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review on lead halide perovskites have emerged in the last decade as advantageous high-performance optoelectronic semiconductors, and have undergone rapid development for diverse applications such as solar cells, light-emitting diodes , and photodetectors. While material instability and lead toxicity are still major concerns hindering their commercialization, they offer promising prospects and design principles for developing promising optoelectronic materials. The distinguished optoelectronic properties of lead halide perovskites stem from the Pb2+ cation with a lone-pair 6s2 electronic configuration embedded in a mixed covalent-ionic bonding lattice. Herein, we summarize alternative Pb-free semiconductors contg. lone-pair ns2 cations, intending to offer insights for developing potential optoelectronic materials other than lead halide perovskites. We then review the emerging Pb-free semiconductors contg. ns2 cations in terms of structural dimensionality, which is crucial for optoelectronic performance. For each category of materials, the research progresses on crystal structures, electronic/optical properties, device applications, and recent efforts for performance enhancements are overviewed. Finally, the issues hindering the further developments of studied materials are surveyed along with possible strategies to overcome them, which also provides an outlook on the future research in this field.
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    Zhao, X.-G.; Yang, D.; Ren, J.-C.; Sun, Y.; Xiao, Z.; Zhang, L. Rational Design of Halide Double Perovskites for Optoelectronic Applications. Joule 2018, 2, 1662– 1673,  DOI: 10.1016/j.joule.2018.06.017
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    Rational Design of Halide Double Perovskites for Optoelectronic Applications
    Zhao, Xin-Gang; Yang, Dongwen; Ren, Ji-Chang; Sun, Yuanhui; Xiao, Zewen; Zhang, Lijun
    Joule (2018), 2 (9), 1662-1673CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
    Practical application of hybrid Pb-based halide perovskites needs not only to fabricate high-quality film samples on a large scale but also to properly overcome the issues of Pb toxicity and materials instability. Finding new, stable, Pb-free perovskites currently attracts significant research interest. Among various strategies, hetero-substitution of Pb to form quaternary halide double perovskites represents a promising direction to keep the high structural (and likely electronic) dimensionality nature of perovskite lattice and meanwhile offers rich chem. compns. a degree of freedom for discovering new perovskite materials. We herein present a perspective that concisely reviews the progress of rational design of Pb-free halide double perovskites by both theor. and exptl. efforts as well as their current and potential applications in various optoelectronic categories. We also envision the future research directions to realize new materials by exploring broader chem. compn. space and to better utilize existing materials by considering optoelectronic properties modulation.
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    Horak, J.; Turjanica, I. D.; Nejezchleb, K. Synthesis, Optical and Photoelectric Properties Of Semiconducting BiSeI. Crystals. Krist. Tech 1968, 3, 231– 240,  DOI: 10.1002/crat.19680030209
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    Turyanitsa, I. D.; Zayachkovskij, M. P.; Zayachkovskaya, N. F.; Kozmanko, I. I. Bisei Crystal Growing By The Stockbarger Method. Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy 1974, 10 (1884), 1884– 1885
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    Growing bismuth iodide selenide (BiSeI) crystals by the Stockbarger technique
    Turyanitsa, I. D.; Zayachkovskii, N. F.; Kozmanko, I. I.
    Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy (1974), 10 (10), 1884-5CODEN: IVNMAW; ISSN:0002-337X.
    Crystals of BiSeI were grown by the Stockbarger method, and the properties of these crystals compared to those grown by chem. transport. The BiI3-Bi2Se3 phase diagram was studied to select the optimum conditions for growing BiSeI crystals. The phys. phys. properties of the crystals grown by the two methods agreed quite well.
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    Yoo, B.; Ding, D.; Marin-Beloqui, J. M.; Lanzetta, L.; Bu, X.; Rath, T.; Haque, S. A. Improved Charge Separation and Photovoltaic Performance of BiI3 Absorber Layers by Use of an In Situ Formed BiSI Interlayer. ACS Appl. Energy Mater. 2019, 2, 7056– 7061,  DOI: 10.1021/acsaem.9b00838
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    Improved Charge Separation and Photovoltaic Performance of BiI3 Absorber Layers by Use of an In Situ Formed BiSI Interlayer
    Yoo, Bowon; Ding, Dong; Marin-Beloqui, Jose M.; Lanzetta, Luis; Bu, Xiangnan; Rath, Thomas; Haque, Saif A.
    ACS Applied Energy Materials (2019), 2 (10), 7056-7061CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
    Stable and nontoxic bismuth iodide (BiI3) is emerging as a promising absorber material for solar cell applications as it possesses favorable optical properties such as a narrow band gap (1.7 eV) and a high absorption coeff. (105 cm-1) in the visible region. Despite these promising features, solar cells employing this material have only achieved power con-version efficiencies in the region of 1% yet, which is distant from the theor. efficiency limit of 28%. It is reasonable to suppose, that the relatively low performance of BiI3 based solar cells may originate from very short carrier lifetimes (180-240 ps) in BiI3, which makes efficient sepn. of mobile charges a crucial factor for the improvement of the photovoltaic performance of this material. Herein, transient optical spectroscopy is employed to show that the use of a bismuth sulfide iodide interlayer between the electron transport layer (ETL) and the bismuth iodide absorber promotes efficient charge sepn. Based on this knowledge, we report BiI3 solar cells with a with a power conversion efficiency of 1.21% using a solar cell architecture comprising: ITO/SnO2/BiSI/BiI3/org. HTM.
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    Xiao, B.; Zhu, M.; Ji, L.; Zhang, B.-B.; Dong, J.; Yu, J.; Sun, Q.; Jie, W.; Xu, Y. Centimeter Size BiSeI Crystal Grown by Physical Vapor Transport Method. J. Cryst. Growth 2019, 517, 7– 11,  DOI: 10.1016/j.jcrysgro.2019.04.003
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    Centimeter size BiSeI crystal grown by physical vapor transport method
    Xiao, Bao; Zhu, Mengqin; Ji, Leilei; Zhang, Bin-Bin; Dong, Jiangpeng; Yu, Jingyi; Sun, Qihao; Jie, Wanqi; Xu, Yadong
    Journal of Crystal Growth (2019), 517 (), 7-11CODEN: JCRGAE; ISSN:0022-0248. (Elsevier B.V.)
    On the purpose to get large sized BiSeI crystals for diverse applications based on its phys. properties, such as thermoelec., photoelec. and ferroelec., etc. We developed a synthesis process to obtain pure BiSeI polycrystal, which was used as source material for further phys. vapor transport (PVT) growth of BiSeI single crystal. Strip-shaped BiSeI single crystal with the size of ∼80 × 4 × 0.5 mm3 was obtained. Needle-like and layered microstructures with homogeneous compn. were obsd. in BiSeI crystal by the SEM. The band-gap of as-grown BiSeI crystal was estd. to be ∼1.29 eV according to the UV-Vis diffuse reflectance spectra. Hall measurement shows that BiSeI single crystal is n-type cond. with the anisotropic resistivity of 39.3, 3.4, and 1650.4 Ω·cm along a-, b- and c-axis, resp.
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    Hahn, N. T.; Rettie, A. J. E.; Beal, S. K.; Fullon, R. R.; Mullins, C. B. n-BiSI Thin Films: Selenium Doping and Solar Cell Behavior. J. Phys. Chem. C 2012, 116, 24878– 24886,  DOI: 10.1021/jp3088397
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    n-BiSI Thin Films: Selenium Doping and Solar Cell Behavior
    Hahn, Nathan T.; Rettie, Alexander J. E.; Beal, Susanna K.; Fullon, Raymond R.; Mullins, C. Buddie
    Journal of Physical Chemistry C (2012), 116 (47), 24878-24886CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    BiSI (indirect band gap = 1.57 eV) is a recently discovered photoelectrode material possessing promising optical properties for use in alternative thin film solar cells. In this work, the effects are studied of selenium doping on BiSI film properties and also demonstrate the incorporation of BiS1-xSexI films into both electrochem. and solid state solar cells. Tuning the band gap of BiS1-xSexI by substituting selenium for sulfur was accomplished by substituting various amts. of SeO2 for thiourea in the BiSI spray pyrolysis precursor solns. This strategy was employed to reduce the direct band gap of BiS1-xSexI films from 1.63 eV to as low as 1.48 eV, as measured by UV-vis-NIR diffuse reflectance spectroscopy for x = 0.4. Both electrochem. and solid state solar cell devices utilizing n-BiSI as the light absorbing material demonstrated open circuit voltages of nearly 0.4 V. The electrochem. devices showed much higher short circuit currents and power conversion efficiencies than the solid state devices. Power conversion efficiencies of up to 0.25 and 0.012% were measured for electrochem. and solid state devices, resp., under AM1.5G illumination.
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    Groom, R. A.; Jacobs, A.; Cepeda, M.; Drummey, R.; Latturner, S. E. Structural and Optical Properties of Sb-Substituted BiSI Grown from Sulfur/Iodine Flux. Inorg Chem. 2017, 56, 12362– 12368,  DOI: 10.1021/acs.inorgchem.7b01839
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    107
    Structural and Optical Properties of Sb-Substituted BiSI Grown from Sulfur/Iodine Flux
    Groom, Ryan A.; Jacobs, Allison; Cepeda, Marisa; Drummey, Rachel; Latturner, Susan E.
    Inorganic Chemistry (2017), 56 (20), 12362-12368CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Bi and Sb were reacted in S/I flux mixts. at various temps. and I concns. to explore the effects of these variables on the synthesis and properties of Bi1-xSbxSI products. The products grow as crystals; microprobe elemental anal. and UV/visible/NIR spectroscopy of the Bi1-xSbxSI solid solns. indicate that substitution is homogeneous within individual crystals but varies up to 15% between crystals within each synthesis batch. Raman spectra show a 2-mode behavior upon substitution, indicating covalent bonding within the structure, and TEM/SEM data confirm no presence of nanoclustering or segregation within the crystals.
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    Zhou, C.; Wang, R.; Jiang, C.; Chen, J.; Wang, G. Dynamically Optimized Multi-interface Novel BiSI-Promoted Redox Sites Spatially Separated n-p-n Double Heterojunctions BiSI/MoS2/CdS for Hydrogen Evolution. Ind. Eng. Chem. Res. 2019, 58, 7844– 7856,  DOI: 10.1021/acs.iecr.9b00234
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    108
    Dynamically Optimized Multi-interface Novel BiSI-Promoted Redox Sites Spatially Separated n-p-n Double Heterojunctions BiSI/MoS2/CdS for Hydrogen Evolution
    Zhou, Chengxin; Wang, Ruilin; Jiang, Chunping; Chen, Jinwei; Wang, Gang
    Industrial & Engineering Chemistry Research (2019), 58 (19), 7844-7856CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
    Novel BiSI promoted n-p-n double heterojunction multi-interface photocatalyst BiSI/MoS2/CdS was constructed. BiSI is applied to the photocatalytic hydrogen evolution. It possesses a small band gap and a strong optical absorption coeff.; therefore, the optical absorption scope and coeff. of MoS2/CdS have been effectively enhanced by compounding with BiSI. The continuous heterojunctions strengthened the function of the single junction and guided the carriers' transfer direction; thus, the redox reactions occur at spatially sepd. sites. The built-in elec. field along the radial direction of the BiSI nanorod and MoS2 interlayer helps to transport carriers within the lifetime. Carrier dynamics is optimized by the multi-interface structure. In general, a new material BiSI is introduced to construct a multi-interface structure to optimize carrier dynamics, which resulted in a 46-fold increase in hydrogen prodn. efficiency.
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    Islam, S. M.; Malliakas, C. D.; Sarma, D.; Maloney, D. C.; Stoumpos, C. C.; Kontsevoi, O. Y.; Freeman, A. J.; Kanatzidis, M. G. Direct Gap Semiconductors Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5. Chem. Mater. 2016, 28, 7332– 7343,  DOI: 10.1021/acs.chemmater.6b02691
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    109
    Direct Gap Semiconductors Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5
    Islam, Saiful M.; Malliakas, Christos D.; Sarma, Debajit; Maloney, David C.; Stoumpos, Constantinos C.; Kontsevoi, Oleg Y.; Freeman, Arthur J.; Kanatzidis, Mercouri G.
    Chemistry of Materials (2016), 28 (20), 7332-7343CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    New quaternary thioiodides Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5 have been synthesized by isothermal heating as well as chem. vapor transport. Pb2BiS2I3 and Sn2BiS2I3 crystallize in the space group, Cmcm, with unit cell parameters a = 4.3214 (9), b = 14.258 (3), and c = 16.488 (3) Å; a = 4.2890 (6), b = 14.121(2), and c = 16.414 (3) Å, resp. Sn2BiSI5 adopts a unique crystal structure that crystallizes in C2/m with cell parameters a = 14.175 (3), b = 4.3985 (9), c = 21.625 (4) Å, and β = 98.90(3)o. The crystal structures of Pb2BiS2I3 and Sn2BiS2I3 are strongly anisotropic and can be described as three dimensional networks that composed of parallel infinite ribbons of [M4S2I4] (M = Pb, Sn, Bi) running along the crystallog. c-axis. The crystal structure of Sn2BiSI5 is a homolog of the M2BiS2I3 (M=Pb, Sn) which has two successive ribbons of [M4S2I4] sepd. by two interstitial (Sn1-xBixI6) octahedral unit. These compds. were characterized by SEM, DTA, and XPS. Pb2SbS2I3, Pb2BiS2I3, "Pb2Sb1-xBixS2I3" (x∼0.4), Sn2BiS2I3 and Sn2BiSI5 are highly resistive that exhibit elec. resistivity of 3.0 GΩ.cm, 100 MΩ.cm, 65 MΩ.cm, 1.2 MΩ.cm and 34 MΩ.cm, resp. at room temp. Pb2BiS2I3, Sn2BiS2I3, Pb2SbS2I3, "Pb2Sb1-xBixS2I3" (x∼0.4), and Sn2BiSI5 are semiconductors with bandgaps of 1.60, 1.22, 1.92, 1.66 and 1.32 eV, resp. The electronic band structures of Pb2BiS2I3, Sn2BiS2I3 and Sn2BiSI5, calcd. using d. functional theory, show that all compds. are direct bandgap semiconductors.
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  110. 110
    Pai, N.; Lu, J.; Gengenbach, T. R.; Seeber, A.; Chesman, A. S. R.; Jiang, L.; Senevirathna, D. C.; Andrews, P. C.; Bach, U.; Cheng, Y. B. Silver Bismuth Sulfoiodide Solar Cells: Tuning Optoelectronic Properties by Sulfide Modification for Enhanced Photovoltaic Performance. Adv. Energy Mater. 2019, 9, 1803396,  DOI: 10.1002/aenm.201803396
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  111. 111
    Lin, W.; Stoumpos, C. C.; Kontsevoi, O. Y.; Liu, Z.; He, Y.; Das, S.; Xu, Y.; McCall, K. M.; Wessels, B. W.; Kanatzidis, M. G. Cu2I2Se6: A Metal-Inorganic Framework Wide-Bandgap Semiconductor for Photon Detection at Room Temperature. J. Am. Chem. Soc. 2018, 140, 1894– 1899,  DOI: 10.1021/jacs.7b12549
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    111
    Cu2I2Se6: A Metal-Inorganic Framework Wide-Bandgap Semiconductor for Photon Detection at Room Temperature
    Lin, Wenwen; Stoumpos, Constantinos C.; Kontsevoi, Oleg Y.; Liu, Zhifu; He, Yihui; Das, Sanjib; Xu, Yadong; McCall, Kyle M.; Wessels, Bruce W.; Kanatzidis, Mercouri G.
    Journal of the American Chemical Society (2018), 140 (5), 1894-1899CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Cu2I2Se6 is a new wide-band gap semiconductor with high stability and great potential toward hard radiation and photon detection. Cu2I2Se6 crystallizes in the rhombohedral R‾3m space group with a d. of d = 5.287 g cm-3 and a wide band gap Eg of 1.95 eV. First-principles electronic band structure calcns. at the d. functional theory level indicate an indirect band gap and a low electron effective mass me* of 0.32. The congruently melting compd. was grown in cm-size Cu2I2Se6 single crystals using a vertical Bridgman method. A high elec. resistivity of ∼1012 Ω cm is readily achieved, and detectors made of Cu2I2Se6 single crystals demonstrate high photosensitivity to Ag Kα x-rays (22.4 keV) and show spectroscopic performance with energy resolns. under 241Am α-particles (5.5 MeV) radiation. The electron mobility is measured by a time-of-flight technique to be ∼46 cm2 V-1 s-1. This value is comparable to that of 1 of the leading γ-ray detector materials, TlBr, and is a factor of 30 higher than mobility values obtained for amorphous Se for x-ray detection.
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  112. 112
    Toso, S.; Akkerman, Q. A.; Martin-Garcia, B.; Prato, M.; Zito, J.; Infante, I.; Dang, Z.; Moliterni, A.; Giannini, C.; Bladt, E. Nanocrystals of Lead Chalcohalides: A Series of Kinetically Trapped Metastable Nanostructures. J. Am. Chem. Soc. 2020, 142, 10198– 10211,  DOI: 10.1021/jacs.0c03577
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    112
    Nanocrystals of Lead Chalcohalides: A Series of Kinetically Trapped Metastable Nanostructures
    Toso, Stefano; Akkerman, Quinten A.; Martin-Garcia, Beatriz; Prato, Mirko; Zito, Juliette; Infante, Ivan; Dang, Zhiya; Moliterni, Anna; Giannini, Cinzia; Bladt, Eva; Lobato, Ivan; Ramade, Julien; Bals, Sara; Buha, Joka; Spirito, Davide; Mugnaioli, Enrico; Gemmi, Mauro; Manna, Liberato
    Journal of the American Chemical Society (2020), 142 (22), 10198-10211CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We report the colloidal synthesis of a series of surfactant-stabilized lead chalcohalide nanocrystals. Our work is mainly focused on Pb4S3Br2, a chalcohalide phase unknown to date that does not belong to the ambient-pressure PbS-PbBr2 phase diagram. The Pb4S3Br2 nanocrystals herein feature a remarkably narrow size distribution (with a size dispersion as low as 5%), a good size tunability (from 7 to ~ 30 nm), an indirect bandgap, photocond. (responsivity = 4 ± 1 mA/W), and stability for months in air. A crystal structure is proposed for this new material by combining the information from 3D electron diffraction and electron tomog. of a single nanocrystal, X-ray powder diffraction, and d. functional theory calcns. Such a structure is closely related to that of the recently discovered high-pressure chalcohalide Pb4S3I2 phase, and indeed we were able to extend our synthesis scheme to Pb4S3I2 colloidal nanocrystals, whose structure matches the one that has been published for the bulk. Finally, we could also prep. nanocrystals of Pb3S2Cl2, which proved to be a structural analog of the recently reported bulk Pb3Se2Br2 phase. It is remarkable that one high-pressure structure (for Pb4S3I2) and two metastable structures that had not yet been reported (for Pb4S3Br2 and Pb3S2Cl2) can be prepd. on the nanoscale by wet-chem. approaches. This highlights the important role of colloidal chem. in the discovery of new materials and motivates further exploration into metal chalcohalide nanocrystals.
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  113. 113
    Sun, Y. Y.; Agiorgousis, M. L.; Zhang, P.; Zhang, S. Chalcogenide Perovskites For Photovoltaics. Nano. Lett. 2015, 15, 581– 585,  DOI: 10.1021/nl504046x
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    113
    Chalcogenide Perovskites for Photovoltaics
    Sun, Yi-Yang; Agiorgousis, Michael L.; Zhang, Peihong; Zhang, Shengbai
    Nano Letters (2015), 15 (1), 581-585CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Chalcogenide perovskites are proposed for photovoltaic applications. The predicted band gaps of CaTiS3, BaZrS3, CaZrSe3, and CaHfSe3 with the distorted perovskite structure are within the optimal range for making single-junction solar cells. The predicted optical absorption properties of these materials are superior compared with other high-efficiency solar-cell materials. Possible replacement of the alk.-earth cations by mol. cations, e.g., (NH3NH3)2+, as in the org.-inorg. halide perovskites (e.g., MeNH3PbI3), are also proposed and are stable. The chalcogenide perovskites provide promising candidates for addressing the challenging issues regarding halide perovskites such as instability in the presence of moisture and contg. the toxic element Pb.
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  114. 114
    Li, J.; Huang, J.; Li, K.; Zeng, Y.; Zhang, Y.; Sun, K.; Yan, C.; Xue, C.; Chen, C.; Chen, T. Defect-Resolved Effective Majority Carrier Mobility in Highly Anisotropic Antimony Chalcogenide Thin-Film Solar Cells. Solar RRL 2021, 5, 2000693,  DOI: 10.1002/solr.202000693
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    114
    Defect-Resolved Effective Majority Carrier Mobility in Highly Anisotropic Antimony Chalcogenide Thin-Film Solar Cells
    Li, Jianjun; Huang, Jialiang; Li, Kanghua; Zeng, Yiyu; Zhang, Yuanfang; Sun, Kaiwen; Yan, Chang; Xue, Chaowei; Chen, Chao; Chen, Tao; Green, Martin A.; Tang, Jiang; Hao, Xiaojing
    Solar RRL (2021), 5 (3), 2000693CODEN: SRORAW; ISSN:2367-198X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Majority carrier mobility is one of the most fundamental and yet important carrier transport parameters detg. the optimal device architecture and performance of the emerging antimony chalcogenide solar cells. However, carrier mobility measurements based on the Hall effect have limitations for these highly anisotropy materials due to the discrepancy of transport directions under Hall measurement and device operation. Herein, a defect-resolved mobility measurement (DRMM) method enabling the evaluation of effective majority carrier mobility from a working device without such limitations is presented. Using this method, comprehensive information about the carrier transport in representative Sb2S3 and Sb2Se3 solar cells is extd. Though with preferred [hk1]-cryst. orientation, Sb2S3 and Sb2Se3 still suffer from extremely low carrier mobility and low carrier d., resp., resulting in large bulk resistance and poor carrier collection efficiency. Further cryst. structure anal. discloses that cryst. defects such as dislocations may significantly constrain carrier transport in these low-dimensional materials. These results suggest that a p-i-n device architecture with fully depleted absorber is a promising optimization approach for further efficiency advances of antimony chalcogenide solar cells.
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  115. 115
    Yu, L.; Lany, S.; Kykyneshi, R.; Jieratum, V.; Ravichandran, R.; Pelatt, B.; Altschul, E.; Platt, H. A. S.; Wager, J. F.; Keszler, D. A. Iron Chalcogenide Photovoltaic Absorbers. Adv. Energy Mater. 2011, 1, 748– 753,  DOI: 10.1002/aenm.201100351
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    115
    Iron chalcogenide photovoltaic absorbers
    Yu, Liping; Lany, Stephan; Kykyneshi, Robert; Jieratum, Vorranutch; Ravichandran, Ram; Pelatt, Brian; Altschul, Emmeline; Platt, Heather A. S.; Wager, John F.; Keszler, Douglas A.; Zunger, Alex
    Advanced Energy Materials (2011), 1 (5), 748-753CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
    The properties were studied of Fe2SiS4, Fe2GeS4, FeS2, FeS2-x, and related compds. The suitability of these materials as solar cells is discussed.
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  116. 116
    Agiorgousis, M. L.; Sun, Y. Y.; Choe, D. H.; West, D.; Zhang, S. Machine Learning Augmented Discovery of Chalcogenide Double Perovskites for Photovoltaics. Adv. Theor. Simul. 2019, 2, 1800173,  DOI: 10.1002/adts.201800173
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    Frost, J. M. Calculating Polaron Mobility in Halide Perovskites. Phys. Rev. B 2017, 96, 195202– 10,  DOI: 10.1103/PhysRevB.96.195202
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    117
    Calculating polaron mobility in halide perovskites
    Frost, Jarvist Moore
    Physical Review B (2017), 96 (19), 195202/1-195202/10CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Lead halide perovskite semiconductors are soft, polar materials. The strong driving force for polaron formation (the dielec. electron-phonon coupling) is balanced by the light band effective masses, leading to a strongly-interacting large polaron. A first-principles prediction of mobility would help understand the fundamental mobility limits. Theories of mobility need to consider the polaron (rather than free-carrier) state due to the strong interactions. In this material we expect that at room temp. polar-optical phonon mode scattering will dominate and so limit mobility. We calc. the temp.-dependent polaron mobility of hybrid halide perovskites by variationally solving the Feynman polaron model with the finite-temp. free energies of ‾Osaka. This model considers a simplified effective-mass band structure interacting with a continuum dielec. of characteristic response frequency. We parametrize the model fully from electronic-structure calcns. In methylammonium lead iodide at 300K we predict electron and hole mobilities of 133 and 94 cm2V-1s-1, resp. These are in acceptable agreement with single-crystal measurements, suggesting that the intrinsic limit of the polaron charge carrier state has been reached. Repercussions for hot-electron photoexcited states are discussed. As well as mobility, the model also exposes the dynamic structure of the polaron. This can be used to interpret impedance measurements of the charge-carrier state. We provide the phonon-drag mass renormalization and scattering time consts. These could be used as parameters for larger-scale device models and band-structure dependent mobility simulations.
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  118. 118
    Wilson, J. N.; Frost, J. M.; Wallace, S. K.; Walsh, A. Dielectric and Ferroic Properties of Metal Halide Perovskites. APL Mater. 2019, 7, 010901– 14,  DOI: 10.1063/1.5079633
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    118
    Dielectric and ferroic properties of metal halide perovskites
    Wilson, Jacob N.; Frost, Jarvist M.; Wallace, Suzanne K.; Walsh, Aron
    APL Materials (2019), 7 (1), 010901/1-010901/14CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    Halide perovskite semiconductors and solar cells respond to elec. fields in a way that varies across time and length scales. We discuss the microscopic processes that give rise to the macroscopic polarization of these materials, ranging from the optical and vibrational response to the transport of ions and electrons. The strong frequency dependence of the dielec. permittivity can be understood by sepg. the static dielec. const. into its constituents, including the orientational polarization due to rotating dipoles, which connects theory with exptl. observations. The controversial issue of ferroelectricity is addressed, where we highlight recent progress in materials and domain characterization but emphasize the challenge assocd. with isolating spontaneous lattice polarization from other processes such as charged defect formation and transport. We conclude that CH3NH3PbI3 exhibits many features characteristic of a ferroelastic electret, where a spontaneous lattice strain is coupled to long-lived metastable polarization states. (c) 2019 American Institute of Physics.
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  119. 119
    Su, R.; Xu, Z.; Wu, J.; Luo, D.; Hu, Q.; Yang, W.; Yang, X.; Zhang, R.; Yu, H.; Russell, T. P. Dielectric Screening in Perovskite Photovoltaics. Nat. Commun. 2021, 12, 2479,  DOI: 10.1038/s41467-021-22783-z
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    119
    Dielectric screening in perovskite photovoltaics
    Su, Rui; Xu, Zhaojian; Wu, Jiang; Luo, Deying; Hu, Qin; Yang, Wenqiang; Yang, Xiaoyu; Zhang, Ruopeng; Yu, Hongyu; Russell, Thomas P.; Gong, Qihuang; Zhang, Wei; Zhu, Rui
    Nature Communications (2021), 12 (1), 2479CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    The performance of perovskite photovoltaics is fundamentally impeded by the presence of undesirable defects that contribute to non-radiative losses within the devices. Although mitigating these losses has been extensively reported by numerous passivation strategies, a detailed understanding of loss origins within the devices remains elusive. Here, we demonstrate that the defect capturing probability estd. by the capture cross-section is decreased by varying the dielec. response, producing the dielec. screening effect in the perovskite. The resulting perovskites also show reduced surface recombination and a weaker electron-phonon coupling. All of these boost the power conversion efficiency to 22.3% for an inverted perovskite photovoltaic device with a high open-circuit voltage of 1.25 V and a low voltage deficit of 0.37 V (a bandgap ∼1.62 eV). Our results provide not only an in-depth understanding of the carrier capture processes in perovskites, but also a promising pathway for realizing highly efficient devices via dielec. regulation.
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  120. 120
    Baranowski, M.; Plochocka, P. Excitons in Metal-Halide Perovskites. Adv. Energy Mater. 2020, 10, 1903659,  DOI: 10.1002/aenm.201903659
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    120
    Excitons in Metal-Halide Perovskites
    Baranowski, Michal; Plochocka, Paulina
    Advanced Energy Materials (2020), 10 (26), 1903659CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
    A review. The unprecedented increase of the power conversion efficiency of metal-halide perovskite solar cells has significantly outpaced the understanding of their fundamental properties. One of the biggest puzzles of perovskites has been the exciton binding energy, which has proved to be difficult to det. exptl. Many contradictory reports can be found in the literature with values of the exciton binding energy from a few meV to a few tens of meV. In this review the results of the last few years of intense investigation of the exciton physic in perovskite materials are summarized. In particular a crit. overview of the different exptl. approaches used to det. exciton binding energy is provided. The problem of exciton binding energy in the context of the polar nature of perovskite crystals and related polaron effects which have been neglected to date in most of work is discussed. It is shown that polaron effects can reconcile at least some of the exptl. observations and controversy present in the literature. Finally, the current status of the exciton fine structure in perovskite materials is summarized. The peculiar carrier-phonon coupling can help to understand the intriguing efficiency of light emission from metal-halide perovskites.
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    Blancon, J. C.; Even, J.; Stoumpos, C. C.; Kanatzidis, M. G.; Mohite, A. D. Semiconductor Physics of Organic-Inorganic 2D Halide Perovskites. Nat Nanotechnol. 2020, 15, 969– 985,  DOI: 10.1038/s41565-020-00811-1
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    Semiconductor physics of organic-inorganic 2D halide perovskites
    Blancon, Jean-Christophe; Even, Jacky; Stoumpos, Costas. C.; Kanatzidis, Mercouri. G.; Mohite, Aditya D.
    Nature Nanotechnology (2020), 15 (12), 969-985CODEN: NNAABX; ISSN:1748-3387. (Nature Research)
    Abstr.: Achieving technol. relevant performance and stability for optoelectronics, energy conversion, photonics, spintronics and quantum devices requires creating atomically precise materials with tailored homo- and hetero-interfaces, which can form functional hierarchical assemblies. Nature employs tunable sequence chem. to create complex architectures, which efficiently transform matter and energy, however, in contrast, the design of synthetic materials and their integration remains a long-standing challenge. Org.-inorg. two-dimensional halide perovskites (2DPKs) are org. and inorg. two-dimensional layers, which self-assemble in soln. to form highly ordered periodic stacks. They exhibit a large compositional and structural phase space, which has led to novel and exciting phys. properties. In this Review, we discuss the current understanding in the structure and phys. properties of 2DPKs from the monolayers to assemblies, and present a comprehensive comparison with conventional semiconductors, thereby providing a broad understanding of low-dimensional semiconductors that feature complex org.-inorg. hetero-interfaces.
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    Yang, Y.; Gu, J.; Young, J. L.; Miller, E. M.; Turner, J. A.; Neale, N. R.; Beard, M. C. Semiconductor Interfacial Carrier Dynamics via Photoinduced Electric Fields,. Science 2015, 350, 1062– 1065,  DOI: 10.1126/science.aad3459
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    Eskandari, R.; Zhang, X.; Malkinski, L. M. Polarization-dependent photovoltaic effect in ferroelectric-semiconductor system. Appl. Phys. Lett. 2017, 110, 121105– 5,  DOI: 10.1063/1.4978749
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    Polarization-dependent photovoltaic effect in ferroelectric-semiconductor system
    Eskandari, Rahmatollah; Zhang, Xiaodong; Malkinski, Leszek M.
    Applied Physics Letters (2017), 110 (12), 121105/1-121105/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Radio-frequency (RF) magnetron sputtering method was used to fabricate ferroelec. films of hafnium oxide doped with 6 mol. % silicon. The effect of polarization of the Si doped HfO2 layer on photovoltaic properties of this ferroelec.-semiconductor system was investigated. Piezoresponse force microscopy method provided clear evidence for ferroelec. properties of HfO2 films with 10 nm thickness. Kelvin probe force microscopy showed that change in the surface potential of the neg. poled sample due to illumination is opposite to the response from unpoled and pos. poled samples. Transport measurements also revealed a significant difference between photo-responses of the ferroelec. films that were polarized in opposite directions. (c) 2017 American Institute of Physics.
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    Shahrokhi, S.; Gao, W.; Wang, Y.; Anandan, P. R.; Rahaman, M. Z.; Singh, S.; Wang, D.; Cazorla, C.; Yuan, G.; Liu, J. M. Emergence of Ferroelectricity in Halide Perovskites. Small Methods 2020, 4, 200149,  DOI: 10.1002/smtd.202000149
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    Li, X.; Chen, S.; Liu, P. F.; Zhang, Y.; Chen, Y.; Wang, H. L.; Yuan, H.; Feng, S. Evidence for Ferroelectricity of All-Inorganic Perovskite CsPbBr3 Quantum Dots. J. Am. Chem. Soc. 2020, 142, 3316– 3320,  DOI: 10.1021/jacs.9b12254
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    Evidence for ferroelectricity of all-inorganic perovskite CsPbBr3 quantum dots
    Li, Xia; Chen, Shaoqing; Liu, Peng-Fei; Zhang, Yuelan; Chen, Yan; Wang, Hsing-Lin; Yuan, Hongming; Feng, Shouhua
    Journal of the American Chemical Society (2020), 142 (7), 3316-3320CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The combination of ferroelec.-optical properties in halide perovskites has attracted tremendous interest because of its potential for optoelectronic and energy applications. However, very few reports focus on the ferroelectricity of all-inorg. halide perovskites quantum dots. Herein, the authors report a excellent ferroelectricity in CsPbBr3 quantum dots (QDs) with a satn. polarization of 0.25μC/cm2. Differential scanning calorimetry, x-ray diffraction, and transmission electronic microscopy revealed that the mechanism of ferroelec.-paraelec. switching of CsPbBr3 QDs can be attributed to the phase transition from cubic phase (Pm‾3m) to the orthorhombic phase (Pna21). In the orthorhombic CsPbBr3, the distortion of octahedral [PbBr6]4- structural units and the off-center Cs+ generated the slightly sepd. centers of pos. charge and neg. charge, resulting in the ferroelec. properties. The variable-temp. emission spectrum from 328 to 78 K exhibits green luminescence and a gradual red shift due to the phase transition. This finding opens up an avenue to explore the ferroelec.-optical properties of inorg. lead halide perovskites for high-performance multifunctional materials.
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  126. 126
    Liao, W. Q.; Zhang, Y.; Hu, C. L.; Mao, J. G.; Ye, H. Y.; Li, P. F.; Huang, S. D.; Xiong, R. G. A Lead-Halide Perovskite Molecular Ferroelectric Semiconductor. Nat. Commun. 2015, 6, 7338,  DOI: 10.1038/ncomms8338
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    A lead-halide perovskite molecular ferroelectric semiconductor
    Liao Wei-Qiang; Zhang Yi; Ye Heng-Yun; Li Peng-Fei; Hu Chun-Li; Mao Jiang-Gao; Huang Songping D; Xiong Ren-Gen
    Nature communications (2015), 6 (), 7338 ISSN:.
    Inorganic semiconductor ferroelectrics such as BiFeO3 have shown great potential in photovoltaic and other applications. Currently, semiconducting properties and the corresponding application in optoelectronic devices of hybrid organo-plumbate or stannate are a hot topic of academic research; more and more of such hybrids have been synthesized. Structurally, these hybrids are suitable for exploration of ferroelectricity. Therefore, the design of molecular ferroelectric semiconductors based on these hybrids provides a possibility to obtain new or high-performance semiconductor ferroelectrics. Here we investigated Pb-layered perovskites, and found the layer perovskite (benzylammonium)2PbCl4 is ferroelectric with semiconducting behaviours. It has a larger ferroelectric spontaneous polarization Ps=13 μC cm(-2) and a higher Curie temperature Tc=438 K with a band gap of 3.65 eV. This finding throws light on the new properties of the hybrid organo-plumbate or stannate compounds and provides a new way to develop new semiconductor ferroelectrics.
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    Shu, L.; Ke, S.; Fei, L.; Huang, W.; Wang, Z.; Gong, J.; Jiang, X.; Wang, L.; Li, F.; Lei, S. Photoflexoelectric Effect in Halide Perovskites. Nat. Mater. 2020, 19, 605– 609,  DOI: 10.1038/s41563-020-0659-y
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    127
    Photoflexoelectric effect in halide perovskites
    Shu, Longlong; Ke, Shanming; Fei, Linfeng; Huang, Wenbin; Wang, Zhiguo; Gong, Jinhui; Jiang, Xiaoning; Wang, Li; Li, Fei; Lei, Shuijin; Rao, Zhenggang; Zhou, Yangbo; Zheng, Ren-Kui; Yao, Xi; Wang, Yu; Stengel, Massimiliano; Catalan, Gustau
    Nature Materials (2020), 19 (6), 605-609CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
    Harvesting environmental energy to generate electricity is a key scientific and technol. endeavor of our time. Photovoltaic conversion and electromech. transduction are two common energy-harvesting mechanisms based on, resp., semiconducting junctions and piezoelec. insulators. However, the different material families on which these transduction phenomena are based complicate their integration into single devices. Here, the authors demonstrate that halide perovskites, a family of highly efficient photovoltaic materials, display a photoflexoelec. effect whereby, under a combination of illumination and oscillation driven by a piezoelec. actuator, they generate orders of magnitude higher flexoelectricity than in the dark. The authors also show that photoflexoelectricity is not exclusive to halides but a general property of semiconductors that potentially enables simultaneous electromech. and photovoltaic transduction and harvesting in unison from multiple energy inputs.
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    Kooi, B. J.; Noheda, B. Ferroelectric chalcogenides-materials at the edge. Science 2016, 353, 221– 222,  DOI: 10.1126/science.aaf9081
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    Ferroelectric chalcogenides-materials at the edge
    Kooi, Bart J.; Noheda, Beatriz
    Science (Washington, DC, United States) (2016), 353 (6296), 221-222CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    There is no expanded citation for this reference.
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    Zhang, Y.; Shimada, T.; Kitamura, T.; Wang, J. Ferroelectricity in Ruddlesden-Popper Chalcogenide Perovskites for Photovoltaic Application: The Role of Tolerance Factor. J. Phys. Chem. Lett. 2017, 8, 5834– 5839,  DOI: 10.1021/acs.jpclett.7b02591
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    129
    Ferroelectricity in Ruddlesden-Popper chalcogenide perovskites for photovoltaic application: the role of tolerance factor
    Zhang, Yajun; Shimada, Takahiro; Kitamura, Takayuki; Wang, Jie
    Journal of Physical Chemistry Letters (2017), 8 (23), 5834-5839CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Chalcogenide perovskites with optimal band gap and desirable light absorption are promising for photovoltaic devices, whereas the absence of ferroelectricity limits their potential in applications. On the basis of first-principles calcns., the authors reveal the underlying mechanism of the paraelec. nature of Ba3Zr2S7 obsd. in expts. and demonstrate a general rule for the appearance of ferroelectricity in chalcogenide perovskites with Ruddlesden-Popper (RP) A3B2X7 structures. Group theor. anal. shows that the tolerance factor is the primary factor that dominates the ferroelectricity. Both Ba3Zr2S7 and Ba3Hf2S7 with large tolerance factor are paraelec. because of the suppression of in-phase rotation that is indispensable to hybrid improper ferroelectricity. In contrast, Ca3Zr2S7, Ca3Hf2S7, Ca3Zr2Se7, and Ca3Hf2Se7 with small tolerance factor exhibit in-phase rotation and can be stable in the ferroelec. Cmc21 ground state with nontrivial polarization. These findings not only provide useful guidance to engineering ferroelectricity in RP chalcogenide perovskites but also suggest potential ferroelec. semiconductors for photovoltaic applications.
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    Moriwake, H.; Konishi, A.; Ogawa, T.; Fujimura, K.; Fisher, C. A. J.; Kuwabara, A.; Shimizu, T.; Yasui, S.; Itoh, M. Ferroelectricity in wurtzite structure simple chalcogenide. Appl. Phys. Lett. 2014, 104, 242909– 3,  DOI: 10.1063/1.4884596
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    Ferroelectricity in wurtzite structure simple chalcogenide
    Moriwake, Hiroki; Konishi, Ayako; Ogawa, Takafumi; Fujimura, Koji; Fisher, Craig A. J.; Kuwabara, Akihide; Shimizu, Takao; Yasui, Shintaro; Itoh, Mitsuru
    Applied Physics Letters (2014), 104 (24), 242909/1-242909/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The possibility of the new class ferroelec. materials of wurtzite structure simple chalcogenide is discussed using modern 1st-principles calcn. technique. Ferroelectricity in the wurtzite structure (P63mc) can be understood by structure distortion from centrosym. P63/mmc by relative displacement of cation against anion along c-axis. Calcd. potential surface of these compds. shows typical double well between two polar variants. The potential barriers for the ferroelec. polarization switching are 0.25 eV/f.u. for ZnO. It is slightly higher energy to the common perovskite ferroelec. compd. PbTiO3. Epitaxial tensile strain on the ab-plane (0001) is effective to lower the potential barrier. The potential barrier decreased from 0.25 to 0.15 eV/f.u. by 5% ab-plane expansion in wurtzite structure ZnO. Epitaxial ZnO thin film with donor type defect redn. should be a possible candidate to confirm this ferroelectricity in wurtzite structure simple chalcogenide. (c) 2014 American Institute of Physics.
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  131. 131
    Wang, C.; Zhang, M.; Wang, R.; Zhang, C.; Meng, X.; Xi, Y.; Li, S.; Yan, H. First-Principles Investigation into Hybrid Improper Ferroelectricity in Ruddlesden-Popper Perovskite Chalcogenides Sr3B2X7 (B = Ti, Zr, Hf; X = S, Se). J. Phys. Chem. C 2021, 125 (25), 13971– 1383,  DOI: 10.1021/acs.jpcc.1c00720
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    131
    First-principles investigation into hybrid improper ferroelectricity in ruddlesden-popper perovskite chalcogenides Sr3B2X7 (B = Ti, Zr, Hf; X = S, Se)
    Wang, Chao; Zhang, Ming; Wang, Ruzhi; Zhang, Chi; Meng, Xianrui; Xi, Yupeng; Li, Sainan; Yan, Hui
    Journal of Physical Chemistry C (2021), 125 (25), 13971-13983CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Recently, hybrid improper ferroelectricity in layered perovskites is becoming an effective way to expand the compositional palette to find new ferroelecs. In the present work, the ground state, electronic structure, optical properties, effective carrier masses, ferroelec. polarization, and polarization switching pathways of the Ruddlesden-Popper perovskite chalcogenides Sr3B2X7 (B = Ti, Zr, Hf; X = S, Se) were thoroughly investigated by first-principles calcns. Some new hybrid improper ferroelecs. with the A21am polar ground state, i.e., Sr3Zr2S7, Sr3Zr2Se7, Sr3Hf2S7, and Sr3Hf2Se7, were predicted by confirming the elec. polarization induced from a trilinear coupling of octahedral rotation and tilting modes. The results showed that the ferroelec. photovoltaic-related properties such as band gaps and elec. polarizations depend crucially on the amplitude of octahedral rotations and tilts of perovskite blocks. It was predicted that Sr3Zr2Se7 could be a good candidate for ferroelec. photovoltaic device applications by a systematical evaluation of ferroelec. photovoltaic properties of all hypothetical compds.
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    Allan, G.; Delerue, C.; Lannoo, M.; Martin, E. Hydrogenic Impurity Levels, Dielectric Constant, and Coulomb Charging Effects in Silicon Crystallites. Phys. Rev. B Condens. Matter 1995, 52, 11982– 11988,  DOI: 10.1103/PhysRevB.52.11982
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    Hydrogenic impurity levels, dielectric constant, and Coulomb charging effects in silicon crystallites
    Allan, G.; Delerue, C.; Lannoo, M>; Martin, E.
    Physical Review B: Condensed Matter (1995), 52 (16), 11982-8CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The notion and usefulness of the effective dielec. const. in silicon nanocrystallites are analyzed using a self-consistent linear screening calcn. of hydrogenic impurities. The self-consistent screened potential induced by the defects can be reasonably approximated by the classical electrostatics expression of the Coulomb potential because the impurity energy levels are dominated by the surface polarization effects. The impurity binding energy, the exciton binding energy, and the exchange splitting are estd. taking into account the modified dielec. properties of the crystallites. The consequences of charging effects on carrier injection are discussed and shown to important.
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    Ganose, A. M.; Butler, K. T.; Walsh, A.; Scanlon, D. O. Relativistic Electronic Structure and Band Alignment of BiSI and BiSeI: Candidate Photovoltaic Materials. J. Mater. Chem. A 2016, 4, 2060– 2068,  DOI: 10.1039/C5TA09612J
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    133
    Relativistic electronic structure and band alignment of BiSI and BiSeI: candidate photovoltaic materials
    Ganose, Alex M.; Butler, Keith T.; Walsh, Aron; Scanlon, David O.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (6), 2060-2068CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Bi-based solar absorbers are of interest due to similarities in the chem. properties of Bi halides and the exceptionally efficient Pb halide hybrid perovskites. While they both experience the same beneficial relativistic effects acting to increase the width of the conduction band, Bi is non-toxic and non-bioaccumulating, meaning the impact of environmental contamination is greatly reduced. We use hybrid d. functional theory, with the addn. of spin orbit coupling, to examine 2 candidate bismuth contg. photovoltaic absorbers, BiSI and BiSeI, and show that they possess electronic structures suitable for photovoltaic applications. We calc. band alignments against commonly used hole transporting and buffer layers, which indicate band misalignments are likely to be the source of the poor efficiencies reported for devices contg. these materials. We suggest alternative device architectures expected to result in improved power conversion efficiencies.
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    Xiao, Z.; Yan, Y. Progress in Theoretical Study of Metal Halide Perovskite Solar Cell Materials. Adv. Energy Mater 2017, 7, 1701136,  DOI: 10.1002/aenm.201701136
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    Wang, K.; Yang, D.; Wu, C.; Sanghadasa, M.; Priya, S. Recent Progress In Fundamental Understanding of Halide Perovskite Semiconductors. Prog. Mater. Sci. 2019, 106, 100580,  DOI: 10.1016/j.pmatsci.2019.100580
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    135
    Recent progress in fundamental understanding of halide perovskite semiconductors
    Wang, Kai; Yang, Dong; Wu, Congcong; Sanghadasa, Mohan; Priya, Shashank
    Progress in Materials Science (2019), 106 (), 100580CODEN: PRMSAQ; ISSN:0079-6425. (Elsevier Ltd.)
    A review. The rapid progress in the field of org.-inorg. halide perovskite (OIHP) has led to not only >24% power conversion efficiency for photovoltaics, but also provided breakthroughs in processing of materials with tailored functional behavior. This ability to design and synthesize engineered OIHP materials has opened the possibility to develop various other optoelectronic applications. In addn. to that of photovoltaics, this includes photodetector, laser, light emitting diode, X-ray and gamma detector, photocatalyst, memory, transducer, transistor, and more. At this stage, the emphasis is on fundamental understanding of the underlying physics and chem. of OIHP materials, which will assist the evaluation of device performance and provide explanations for some of the contradictory results reported in literature. This review discusses the theor. and exptl. anal. of the OIHP materials reported from various sources and considers the chem. and structural origin of their unique optoelectronic properties, correlated microstructures, and newly discovered extraordinary properties. In the first few sections, we summarize and discuss the crystallog., chem. bonding, and substitutional effects, followed by the discussion of correlated photophysics including the optical, electronic, excitonic, charge transport, and ion migration characteristics. Next, we revisit and discuss the in-depth behavior of films with unique defect structure, structural disorder, morphol., and crystn. thermodn. Novel thermal-elec.-optical properties including ferroelectricity, hot-carrier contribution, spin-orbit coupling effect, terahertz time response, edge-state discovery, etc., are rationalized considering the results debated in the community. We elaborate on the opportunities and challenges regarding stability, toxicity, and hysteresis. The viewpoint on commercialization of OIHP based solar module is presented with the goal of identifying near-term opportunities. Throughout this review, the overarching goal is to provide a simplified explanation for the complex phys. effects and mechanisms, underlying interconnections between different mechanisms, uncertainties reported in literature, and recent important theor. and exptl. discoveries.
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    Davies, D. W.; Butler, K. T.; Skelton, J. M.; Xie, C.; Oganov, A. R.; Walsh, A. Computer-Aided Design of Metal Chalcohalide Semiconductors: From Chemical Composition to Crystal Structure. Chem. Sci. 2018, 9, 1022– 1030,  DOI: 10.1039/C7SC03961A
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    136
    Computer-aided design of metal chalcohalide semiconductors: from chemical composition to crystal structure
    Davies, Daniel W.; Butler, Keith T.; Skelton, Jonathan M.; Xie, Congwei; Oganov, Artem R.; Walsh, Aron
    Chemical Science (2018), 9 (4), 1022-1030CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    The std. paradigm in computational materials science is INPUT: STRUCTURE; OUTPUT: PROPERTIES, which has yielded many successes but is ill-suited for exploring large areas of chem. and configurational hyperspace. We report a high-throughput screening procedure that uses compositional descriptors to search for new photoactive semiconducting compds. We show how feeding high-ranking element combinations to structure prediction algorithms can constitute a pragmatic computer-aided materials design approach. Techniques based on structural analogy (data mining of known lattice types) and global searches (direct optimization using evolutionary algorithms) are combined for translating between chem. compn. and crystal structure. The properties of four novel chalcohalides (Sn5S4Cl2, Sn4SF6, Cd5S4Cl2 and Cd4SF6) are predicted, of which two are calcd. to have bandgaps in the visible range of the electromagnetic spectrum.
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    Mao, X.; Han, K.-L.; Deng, W.-Q.; Sun, L. First-Principles Screening of Lead-Free Mixed-Anion Perovskites for Photovoltaics. J. Phys. Chem. C 2020, 124, 1303– 1308,  DOI: 10.1021/acs.jpcc.9b10217
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    137
    First-Principles Screening of Lead-Free Mixed-Anion Perovskites for Photovoltaics
    Mao, Xin; Han, Ke-Li; Deng, Wei-Qiao; Sun, Lei
    Journal of Physical Chemistry C (2020), 124 (2), 1303-1308CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Org.-inorg. hybrid lead perovskites have made rapid progress in photovoltaic fields. However, the toxicity and poor long-term stability of these materials still limit their further com. application. Herein, we proposed a system of lead-free mixed-anion perovskites in which a chalcogen element is incorporated into the perovskite octahedrons to improve the system stability. We performed first-principles calcns. of the band gaps of 192 lead-free mixed-anion perovskites belonging to the class of ABX'X''2 where A = Cs+, CH3NH3+, and HC(NH2)2+; B = Ga3+, In3+, Sb3+, and Bi3+; X' = O2-, S2-, Se2-, and Te2-, and X'' = F-, Cl-, Br-, and I-. The band gap shows a linear relationship with the av. anion electronegativity. The contribution of anions to the band-edge states is related to electron affinity and structure parameters. Considering multiple factors forming perovskites, we screened out a promising candidate, CsInOBr2, with a suitable band gap (1.3 eV) for application in photovoltaics.
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  138. 138
    Farooq, U.; Ishaq, M.; Shah, U. A.; Chen, S.; Zheng, Z.-H.; Azam, M.; Su, Z.-H.; Tang, R.; Fan, P.; Bai, Y. Bandgap Engineering of Lead-Free Ternary Halide Perovskites for Photovoltaics And Beyond: Recent Progress And Future Prospects. Nano Energy 2022, 92, 106710,  DOI: 10.1016/j.nanoen.2021.106710
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    138
    Bandgap engineering of lead-free ternary halide perovskites for photovoltaics and beyond: Recent progress and future prospects
    Farooq, Umar; Ishaq, Muhammad; Shah, Usman Ali; Chen, Shuo; Zheng, Zhuang-Hao; Azam, Muhammad; Su, Zheng-Hua; Tang, Rong; Fan, Ping; Bai, Yang; Liang, Guang-Xing
    Nano Energy (2022), 92 (), 106710CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
    A review. Hybrid lead-halide perovskite materials have attracted enormous attention due to their remarkable optoelectronics properties. Within just a few years of research efforts, lead-based perovskite solar cells have attained power conversion efficiencies (PCEs) comparable to that of current-state-of-the-art silicon-based counterparts. However, their further development is hindered by threatening human health owing to toxic lead and severe instability issues. To address these challenges, numerous low toxic substitutes have been reported. Among them, antimony (Sb) and bismuth (Bi) ternary halide perovskites (THPs) with a compn. of A3M2X9 are mostly focused for photovoltaic applications due to their long-term stability and high absorption coeff. This emerging family of THPs is considered highly feasible for next-generation photovoltaics technol. However, these perovskites encounter two main issues: large bandgap and dimer phase, which are unfavorable for single-junction solar cells. We take this as an incentive to the approaches for reducing the bandgap of THPs and making them more promising for photovoltaic applications. Moreover, choosing appropriate charge transport layers could further boost the device performance. We highlighted other potential applications of THPs in various fields such as photodetectors, x-rays detection, and light emitting diode. With the perspective of their properties and recent challenges, we provide an outlook for the future development of A3M2X9 THPs to achieve high-quality layered-phase devices for a broader range of fundamental research and their potential in single-junction or tandem solar cells.
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  139. 139
    Baranwal, A. K.; Masutani, H.; Sugita, H.; Kanda, H.; Kanaya, S.; Shibayama, N.; Sanehira, Y.; Ikegami, M.; Numata, Y.; Yamada, K. Lead-Free Perovskite Solar Cells using Sb And Bi-Based A3B2X9 And A3BX6 Crystals with Normal and Inverse Cell Structures. Nano Converg. 2017, 4, 26,  DOI: 10.1186/s40580-017-0120-3
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    139
    Lead-free perovskite solar cells using Sb and Bi-based A3B2X9 and A3BX6 crystals with normal and inverse cell structures
    Baranwal Ajay Kumar; Masutani Hideaki; Sugita Hidetaka; Kanda Hiroyuki; Kanaya Shusaku; Shibayama Naoyuki; Ito Seigo; Sanehira Yoshitaka; Ikegami Masashi; Numata Youhei; Miyasaka Tsutomu; Yamada Kouji; Umeyama Tomokazu; Imahori Hiroshi
    Nano convergence (2017), 4 (1), 26 ISSN:2196-5404.
    Research of CH3NH3PbI3 perovskite solar cells had significant attention as the candidate of new future energy. Due to the toxicity, however, lead (Pb) free photon harvesting layer should be discovered to replace the present CH3NH3PbI3 perovskite. In place of lead, we have tried antimony (Sb) and bismuth (Bi) with organic and metal monovalent cations (CH3NH3(+), Ag(+) and Cu(+)). Therefore, in this work, lead-free photo-absorber layers of (CH3NH3)3Bi2I9, (CH3NH3)3Sb2I9, (CH3NH3)3SbBiI9, Ag3BiI6, Ag3BiI3(SCN)3 and Cu3BiI6 were processed by solution deposition way to be solar cells. About the structure of solar cells, we have compared the normal (n-i-p: TiO2-perovskite-spiro OMeTAD) and inverted (p-i-n: NiO-perovskite-PCBM) structures. The normal (n-i-p)-structured solar cells performed better conversion efficiencies, basically. But, these environmental friendly photon absorber layers showed the uneven surface morphology with a particular grow pattern depend on the substrate (TiO2 or NiO). We have considered that the unevenness of surface morphology can deteriorate the photovoltaic performance and can hinder future prospect of these lead-free photon harvesting layers. However, we found new interesting finding about the progress of devices by the interface of NiO/Sb(3+) and TiO2/Cu3BiI6, which should be addressed in the future study.
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    Kumar, D.; Kaur, J.; Mohanty, P. P.; Ahuja, R.; Chakraborty, S. Recent Advancements in Nontoxic Halide Perovskites: Beyond Divalent Composition Space. ACS Omega 2021, 6, 33240– 33252,  DOI: 10.1021/acsomega.1c05333