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AgBiI4 as a Lead-Free Solar Absorber with Potential Application in Photovoltaics

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† Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom
‡ Department of Physics, University of Liverpool, Oxford Street, Liverpool L69 7ZE, United Kingdom
§ The Stephenson Institute for Renewable Energy, University of Liverpool, Liverpool L69 3BX, United Kingdom
Cite this: Chem. Mater. 2017, 29, 4, 1538–1549
Publication Date (Web):January 23, 2017
https://doi.org/10.1021/acs.chemmater.6b04135

Copyright © 2017 American Chemical Society. This publication is licensed under CC-BY.

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Abstract

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AgBiI4 powder, crystals, and polycrystalline films were synthesized by sealed tube solid state reactions, chemical vapor transport (CVT), and solution processing, respectively, and their structural, optical and electronic properties are reported. The structure of AgBiI4 is based unambiguously upon a cubic close packed iodide sublattice, but it presents an unusual crystallographic problem: we show that the reported structure, a cubic defect-spinel, cannot be distinguished from a metrically cubic layered structure analogous to CdCl2 using either powder or single crystal X-ray crystallography. In addition, we demonstrate the existence a noncubic CdCl2-type polymorph by isolation of nontwinned single crystals. The indirect optical band gap of AgBiI4 is measured to be 1.63(1) eV, comparable to the indirect band gap of 1.69(1) eV measured for BiI3 and smaller than that reported for other bismuth halides, suggesting that structures with a close-packed iodide sublattice may give narrower band gaps than those with perovskite structures. Band edge states closely resemble those of BiI3; however, the p-type nature of AgBiI4 with low carrier concentration is more similar to MAPbI3 than the n-type BiI3. AgBiI4 shows good stability toward the AM1.5 solar spectrum when kept in a sealed environment and is thermally stable below 90 °C.

1 Introduction

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Lead halide based perovskites and perovskite-related semiconductors, particularly methylammonium lead iodide (MAPbI3), are of great interest due to their remarkable performance in photovoltaic devices. This stems from their possession of a suitable band gap as set by the Shockley-Queisser limit, (1) high absorption coefficients suitable for thin film technology, (2) good carrier mobilities, (3-5) and low recombination rates. (4, 6, 7) Furthermore, films can be cast from solution (8) or vapor deposited (9) and contain inexpensive, earth-abundant elements. The main issues associated with these materials are low stability against thermal and photoinduced degradation (e.g., MAPbI3 is sensitive to light and heat, (10, 11) moisture, (12) and oxygen (12)) and the potential environmental impact associated with scale up of new lead-based technologies, a concern which is shared with other families of functional materials such as ferroelectrics. (13) Studies exploring the chemical diversity of this class of materials have investigated substituting the methylammonium cation with other organic cations, (14-17) the Pb(II) cation with Sn(II) (14, 18, 19) or Ge(II), (20, 21) and replacing or mixing the halides between Cl, Br, and I. (22-25) Attempts to substitute Pb(II) with Sn(II) or Ge(II) have produced compounds that oxidize rapidly to Sn(IV) (14) and Ge(IV) (20) in air and more recently the similar chemistries of Pb(II) and Bi(III), which are isoelectronic 6s2 cations, has led to interest in developing Bi(III) halide based semiconductors for this purpose. Research has concentrated on producing direct analogues of the hybrid lead perovskites, with 3D networks of bismuth halide octahedra expected to produce narrower band gaps. (26) Several ternary and quaternary Bi-based compounds based on a perovskite-type halide sublattice have been investigated as potential solar absorbers, including (CH3NH3)BiI4, (27) (NH4)3Bi2I9, (28) A3Bi2I9 (A = K, Rb, Cs), (29) Cs2AgBiX6 (X = Cl, Br) (30) and (CH3NH3)2KBiCl6, (26) but all have been found to exhibit band gaps that are too wide to be used in single junction solar cells (1.90–3.04 eV). In contrast, BiI3, (31, 32) whose structure is based on a hexagonal close-packed iodide sublattice, has a smaller band gap of 1.69(1) eV, (33) similar to that reported for AgBi2I7, (34) which is thought to be close packed. (35) This indicates that selection of a suitable iodide packing array may be an important step in the development of bismuth based halide solar absorbers.
Several Bi-rich compounds based on a close-packed iodide sublattice have been reported in the AgI–BiI3 (Ag1–3xBi1+xI4) phase field. (36) The earliest study of Ag1–3xBi1+xI4 isolated the bismuth-rich composition Ag0.58Bi1.14I4 (x = 0.14), which was reported to crystallize in a cubic unit cell. (37) Subsequent reports suggested that this cubic phase is stable across a range of stoichiometries, as it is observed independently for AgBiI4 (36, 38) (x = 0) and for the compositions Ag0.73Bi1.09I4–Ag0.58Bi1.14I4 (39) (x = 0.09–0.14) using different synthetic routes. The structure of the cubic phase was solved using crystals of AgBiI4 grown by a solvothermal method (36) and is reported as a cubic close-packed iodide lattice where edge-sharing octahedra are occupied by either an Ag+ or Bi3+ cation, giving rise to an extended 3D network. It can be considered as a spinel type structure with all of the tetrahedral sites vacant (Figure 2b). Here we use three different routes to synthesize AgBiI4 as powder, single crystals, and solution-deposited films and evaluate its use as a lead-free solar absorber by detailed characterization of its structure, composition, and electronic and optical properties.

2 Experimental Section

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2.1 Synthesis Procedures

Commercial BiI3 powder (Alfa Aesar, 99.999%) was used for the preliminary syntheses (compositional screening) of Ag1–3xBi1+xI4 but was found to contain BiOCl as a crystalline impurity and was not used thereafter. The BiI3 powder used for the synthesis of high quality AgBiI4 samples (powders, crystals, and films) was synthesized directly from the elements: 0.43 mmol of Bi powder (Alfa Aesar, 99.999%, kept in a He-filled drybox) and 0.75 mmol of I2 powder (Sigma-Aldrich, 99.8%), corresponding to a 16 mol % excess, were combined in a borosilicate tube of length 150 mm, inner diameter of 7 mm, and wall thickness of 2 mm and sealed at a pressure of 10–4 mbar by using a liquid nitrogen bath to prevent the sublimation of I2. The tube was placed vertically inside a furnace with enclosed heating elements and heated to 427 °C to a melt overnight with slow heating and cooling rates of 1 °C min–1 due to the high vapor pressure of elemental I2. The resulting BiI3 powder was found to be phase pure by PXRD, and the composition was measured as Bi0.98(3)I3.00(3) by scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM EDX). The AgI starting material (Alfa Aesar, 99.999%) was found to be pure by PXRD with the composition Ag1.00(2)I1.00(2) as measured by SEM EDX and used throughout the study.
All powder samples were synthesized in sealed 200 mm long quartz tubes, with 1 mm thick walls and a 6 mm internal diameter. The tubes were vacuum sealed at 10–4 mbar and placed vertically inside a furnace with enclosed heating elements, and the temperature was ramped at 5 °C min–1 to 610 °C, held for 1 day, and then cooled at a rate of 5 °C min–1 to 350 °C and held for a further 5 days. The tubes were then quickly taken out of the furnace, and the bottom half (containing the powder) was quenched to room temperature in a water bath to suppress the formation of BiI3 and Ag2BiI5, which form when the reaction is cooled slowly (see Figure S1).
AgBiI4 single crystals were grown by chemical vapor transport (CVT). CVT growth was carried out by placing BiI3 (0.33 mmol) and AgI (0.25 mmol) in a 150 mm long quartz tube, with 1 mm thick walls and a 6 mm internal diameter, which was then vacuum sealed at 10–4 mbar. The tube was placed horizontally in the middle of a horizontal two-zone tube furnace between temperatures 363 and 350 °C for 2 weeks, with a temperature gradient of 0.8 °C cm–1, the powder being at the hot end. Black octahedral-faceted crystals of dimensions approximately 0.5 mm × 0.5 mm × 0.5 mm and black elongated plate crystals of faces approximately 0.1 mm × 0.5 mm suitable for single crystal X-ray diffraction studies were isolated.
Optimized films were deposited on 15 mm × 15 mm glass substrates, which had been prepared by the following protocol: they were cleaned with soap, sonicated in acetone for 10 min and methanol for 5 min, rinsed with DI water, dried under a nitrogen gas flow, sonicated in isopropanol for 10 min, and finally dried again under a nitrogen gas flow. BiI3 (0.117 mmol) and AgI (0.058 mmol) were dissolved in 4 cm3 DMSO by heating and sonicating at 50 °C for 30 min. The solution was then dropped onto a glass substrate until full coverage was achieved. The deposited solution was dried by placing the glass slide in an oven and heating to 200 °C at a rate of 3.4 °C min–1. Once at 200 °C, the oven was switched off, allowing the films to cool back to room temperature.

2.2 Characterization Methods

2.2.1 Structural

Initial compositional screening, light sensitivity, and solution casting experiments made use of powder X-ray diffraction (PXRD) data measured on a Panalytical X’Pert Pro diffractometer using Co Kα1 radiation (λ = 1.7890 Å) in Bragg–Brentano geometry and an X’Celerator detector. Phase identification was carried out using the X’Pert HighScore Plus (Version 2.2a) (40) with the PDF-2-ICDD database. PXRD data of AgBiI4 in capillaries used for photostability assessment were measured on a Bruker D8 Advance diffractometer using monochromated Mo Kα1 radiation (λ = 0.7093 Å).
PXRD patterns used for detailed structural analysis were collected using the MAC detectors on the I11 beamline at the synchrotron at Diamond Light Source (RAL, Oxfordshire, U.K.) with a wavelength of λ = 0.825898 Å at room temperature. Samples were mixed with 80 vol % amorphous boron to reduce absorption effects and contained within 0.3 mm diameter borosilicate capillaries. For stability measurements the sample was measured every 10 °C on heating from 60 to 350 °C, using a position sensitive detector for rapid data collection. Topas Academic (Version 5) (41) was used to perform Le Bail fittings and Rietveld refinements of the data. VESTA (42) was used for graphical representation of the structures.
Single crystal X-ray diffraction (SCXRD) data were collected at 100 K on a Rigaku MicroMax-007 HF diffractometer with a molybdenum rotating anode microfocus source and a Saturn 724+ detector using Rigaku Crystal Clear v2.0. Unit-cell indexation, data integration, and reduction were performed using Rigaku CrysAlisPro v171.38.43. (43) The structure was solved and refined using SHELX-2013, (44) implemented through Olex2. (45)

2.2.2 Compositional Analysis

SEM EDX was used to measure the composition as a direct elemental analysis technique. Measurements were carried out using a Hitachi S-4800 SEM and an Oxford Instruments model 7200 EDS X-ray detector with the quantification carried out using the microanalysis suite of the Inca Suite software (Version 4.15). Measurements on powders consisted of taking the spectra of nine different areas. Measurements on crystals consisted of taking the spectra at three different areas on each crystal; however, crystals used for structural analysis were subject to six measurements each. Powders of AgI and BiI3 were used as EDX standards (see Figure S2). All powders and crystals were sputtered with 15 nm Au to limit charging effects. Errors on reported average compositions correspond to the 1σ standard deviation of the measured distribution.

2.2.3 Optical and Electronic Properties

UV–visible spectra were taken on a Shimadzu UV-2600 instrument with an integrating sphere in both reflection and transmission mode to measure the absorbance, diffuse reflectance, and transmittance. The measurements on the powder sample consisted of a small amount of powder pressed between two glass slides that was exposed to light between wavelengths 600–1400 nm. The Kubelka–Munk function F(R) was obtained from the diffuse reflectance measurements R using the equation F(R) = (1 – R)2/2R. Tauc plots were then plotted using (hνF(R))1/n vs hν where n = 1/2 and 2 for direct and indirect band gaps, respectively. The absorption coefficients of the films deposited on glass slides were calculated using α = −ln(T/(1 – R))/d where α is the absorption coefficient (cm–1), T is the transmission, R is the reflection, and d is the average film thickness (cm). The thickness of each film was measured using a WYKO NT1100 Optical Profiling System. A strip of the film was wiped off the glass substrate to produce a step profile, which was measured in 16 areas for each film. The mean of these step heights is reported as the film thickness with errors corresponding to the standard deviation.

2.2.4 X-ray Photoelectron Spectroscopy Measurements

XPS measurements were conducted on powders of BiI3 and AgBiI4 using a SPECS monochromatic Al Kα (1486.6 eV) X-ray source and a PSP MCD5 analyzer. Further details, including spectrometer calibration, can be found elsewhere. (46) Use of 54.7° atomic scaling factors (ASFs) (47) applied to the Ag 3d5/2, Bi 4f7/2, and I 3d5/2 peak areas gave approximate surface compositions Bi1.0I2.9 and Ag0.2Bi1.0I3.1. Charge neutralization at the surface was achieved by means of a low energy electron flood gun and subsequent correction of the binding energy scale to the adventitious C 1s peak (284.8 eV).

2.2.5 Resistivity and Thermopower

Seebeck coefficient and resistivity were measured in a Quantum Design PPMS Dynacool system using the thermal transport and electrical transport options, respectively. The Seebeck coefficient was measured on a cold pressed pellet in a two probe configuration using disc shaped copper leads attached with conductive epoxy. The resistivity was measured on a bar shaped sample in a two probe configuration with gold wires attached using silver paint, achieving ohmic contacts.

2.2.6 Photostability

Two experiments were performed to measure the stability of the material toward the solar spectrum: the first under ambient conditions in an open system and the second under controlled atmospheres in sealed vessels. For both experiments, a Solar Light Model 16S-300-002 Solar Simulator was used which has a spectral output that complies with air mass 1.5 (AM1.5) per the ASTM standard definition. The combination of neutral density filters and lamp-to-sample distance allowed for the tuning of the intensity of the incident light to 1000 W m–2 as measured by a Solar Light Pyranometer PMA2144 and datalogging radiometer PMA2100. For measurements under an ambient atmosphere, AgBiI4 powder was sprinkled onto a glass slide. Sample temperatures were monitored using a T-type thermocouple and were found to stay below 35 °C. Second, for measurements in sealed atmospheres, AgBiI4 was loaded into thin-walled (0.01 mm wall thickness) borosilicate capillaries under ambient air, dry synthetic air, and helium, and these were sealed with a gas-oxygen torch. The capillaries were then placed in the solar simulator and subjected to the full solar spectrum at an intensity of 1000 W m–2.

2.3 Computational Details

Density functional theory calculations were performed with a plane-wave basis under periodic boundary conditions using VASP. (48) A plane-wave cutoff energy of 400 eV was used throughout, and core-electrons were treated using the projector augmented wave method. (49) Structural optimization was performed using the optB96b-vdW functional (50) without the inclusion of spin–orbit coupling until atomic forces were less than 0.001 eV Å–1. This was found to reproduce the experimental cell parameters of BiI3 to within 1% (51) (Figure S3). Electronic structure calculations (band structure and density of states, DOS) were then performed using the PBE (52) functional with spin–orbit coupling included. The inclusion of spin–orbit coupling has been found to be necessary for the correct description of BiI3. (33)
Structural models of AgBiI4 were constructed in rhombohedral cells with initial lattice parameters of a = b = c = 14.93 Å and α = β = γ = 33.56°, containing 8 metal cations and 16 iodide anions. This cell was chosen such that both the proposed spinel and the CdCl2 structures could be modeled in the same cell. With each structure, the symmetrically unique configurations of four Ag+ and four Bi3+ ions were constructed, resulting in nine configurations in the spinel structure and six in the CdCl2 structure. The cell and atomic positions of every configuration were then relaxed and the electronic structure calculated using a 7 × 7 × 7 k-point grid. Of all 15 calculations, one of the two lowest energy configurations had the defect-spinel structure and one the CdCl2 structure, the defect-spinel structure being more stable by only 18 meV per AgBiI4 formula unit (using optB86b-vdW energies, Table S1). The crystal and electronic band structures of these two lowest energy configurations are shown in Figures S4 and S5.

3 Results

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3.1 Synthesis and Composition

An initial compositional screening of the Ag1–3xBi1+xI4 system was performed by reacting AgI and BiI3 following the powder synthesis procedure described in Section 2.1, with a range of nominal compositions corresponding to 0 ≤ x ≤ 0.19, which covers the range of reported compositions resulting in compounds with the defect-spinel structure (x = 0 (36) and 0.09 (37)–0.14 (39)). It was necessary to quench the tubes from 350 °C to room temperature to prevent the formation of BiI3 and Ag2BiI5 impurities. For all starting compositions, this route resulted in a bulk powder phase at the bottom of the tube and a small quantity of fragile flat dendritic crystals, with widths and lengths varying between 50 and 300 μm, at the top of the tube. These crystals were found to be silver-poor AgBi2I7 (measured by SEM EDX), and their yield increased with decreasing AgI:BiI3 ratio. The powder retrieved from reactions with nominal compositions of x = 0.05 and 0.07 appear phase pure by PXRD, with the exception of the BiOCl impurity present in the purchased BiI3 (Figure S6).
Pure BiI3 synthesized directly from Bi and I2 was used with the same synthetic procedure and a nominal composition of x = 0.07 to synthesize the phase pure powders used for the remainder of this study. SEM EDX measurements show the resulting powder has an average composition of Ag0.97(8)Bi1.06(5)I4.00(10), within the error of AgBiI4 (Figure 1), which we will use in the rest of the paper. The composition is slightly Bi and I deficient compared to the starting mixture which is consistent with the small silver-poor crystals forming at the top of the tube. The spread in the composition of the AgBiI4 powder observed by SEM EDX is larger than that of the standards AgI and BiI3 (Figure S2), showing a degree of inhomogeneity that is comparable to a previous report on this system. (39) Efforts to homogenize the sample by a second firing at 350 °C for 5 days and quenching to room temperature further increased the spread of the composition along the charge balance line (Figure S7). This powder synthesis was found to be reproducible with subsequent reactions producing the same average compositions and spreads (Figure S8).

Figure 1

Figure 1. (a) Quarter of the Bi–Ag–I phase diagram showing SEM EDX compositional measurements of Ag1–3xBi1+xI4 powder (black), polycrystalline film (green), plate crystal with CdCl2 structure (red), and octahedral-faceted crystal (blue) on the Ag1–3xBi1+xI4 charge balance line with previously reported compositions (orange). (b) Dashed zone enlarged around the AgBiI4x = 0 point showing additional octahedral-faceted (blue) and plate (red) crystal measurements. Also shown is the nominal x = 0.07 composition used for sealed tube synthesis (orange). Circles represent average compositions, and hashed areas represent the 1σ statistical spread. Areas labeled A and B are the plate and octahedral-faceted crystals used for the structural study (Section 3.2). Areas labeled 1 and 2 correspond to the polycrystalline films used for absorption coefficient measurements (Section 3.3).

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Single crystals were grown by CVT, resulting in two sets of black crystals with different habits; octahedral-faceted crystals and elongated plate crystals (Figure S9). SEM EDX measured compositions of four octahedral-faceted crystals and four plate crystals are shown in Figure 1b and Table S2. An octahedral-faceted crystal of composition Ag1.16(6)Bi0.93(6)I4.00(5) and a plate-like crystal of composition Ag1.06(5)Bi0.97(5)I4.00(5), both within 3σ of AgBiI4, were used for determination of the crystal structure by SCXRD.
Solution processing of Ag1–3xBi1+xI4 was carried out by the method described in Section 2.1. This procedure resulted in polycrystalline films with PXRD patterns consistent with that of AgBiI4 powder (Figure S10), with a set of low-intensity peaks corresponding to a BiI3 impurity. The extremely intense (00l), l = 4n, peaks indicate that the polycrystalline films are textured. Two optimized films were used for measurements. A Le Bail fit to laboratory PXRD data gives a cubic lattice parameter for films 1 and 2 of a = 12.2104(3) Å and 12.2135(6) Å, respectively, which are consistent with that obtained from bulk powder samples (a = 12.21446(4) Å, see Table S6). The compositions of the films 1 and 2 were measured by SEM EDX as Ag0.79(10)Bi0.90(9)I4.00(10) and Ag0.72(13)Bi1.06(6)I4.00(10), respectively (Figure 1b). SEM micrographs of the polycrystalline films are shown in Figure S11. Measurements gave a mean thickness of 221(97) nm for film 1 and 343(116) nm for film 2 with a representative film profile shown in Figure S12.

3.2 Structure

The plate-like crystal (Section 3.1) was found by SCXRD at 100 K to be a single untwinned crystal of AgBiI4. The data were indexed to a rhombohedral unit cell with lattice parameters a = 4.3187(1) Å and c = 20.6004(8) Å, with the space group R3̅m (Table S3). Examination of the lattice parameter ratio c/2a shows that the rhombohedral cell cannot be indexed to an equivalent cubic cell (c/2a = 2.3850(1), instead of c/2a = √6 = 2.4495 for a metrically cubic cell); it is therefore distinct from the previously reported cubic structure (36) and would be distinguishable by PXRD, as shown by the simulated powder pattern in Figure S13. The structure was solved by direct methods and was found to belong to the layered CdCl2 structure type: (53) statistically disordered Ag+ and Bi3+ ions occupy every other ⟨111⟩ layer of octahedral interstices in the close packed iodide sublattice, forming the ordered rock-salt structure with layers of edge-sharing Ag+/Bi3+ octahedra (Figure 2a). Rhombohedral distortion by contraction along the cubic [111] iodide sublattice causes a decrease in I–I distances in layers unoccupied by cations and increases closest Bi–Bi distances compared to the previously reported (36) cubic structure.

Figure 2

Figure 2. PXRD and Rietveld refinements of AgBiI4 synchrotron data for (a) CdCl2-type and (b) defect-spinel structures, with the structures inset. Tick marks in (b) show the absence of expected Bragg intensity due to the increased cell size of the cubic cell.

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The previously reported cubic structure also consists of a face-centered cubic iodide sublattice but with differing occupation of the octahedral interstices, such that edge-sharing octahedra are occupied by statistically disordered Ag+ and Bi3+ cations giving rise to an extended 3D network with a defect-spinel structure where tetrahedral sites are entirely vacant (Figure 2b). The cell of the cubic defect-spinel structure in its trigonal setting has the same length along the c axis as the CdCl2 structure, but is doubled in the a and b directions (Figure 3) and has thus four times the volume of the CdCl2 cell.

Figure 3

Figure 3. (a) Reported crystal structure of BiI3, (51) showing its layered structure in which layers of edge-sharing octahedra are occupied by 2/3 Bi3+ cations and 1/3 vacancies and alternate with entirely vacant layers. The iodide packing is hexagonally close-packed (ABA). (b) Cubic defect-spinel structure of AgBiI4 is shown in its trigonal setting, for comparison to BiI3 and the CdCl2 structure. The interstitial octahedral sites are occupied by mixed silver and bismuth in alternating layers of 3/4 and 1/4 occupancy. (c) CdCl2-type structure of AgBiI4 with doubled a and b directions showing layered structure with alternating fully occupied and entirely vacant layers. In both (b) and (c) the iodide packing is cubic close-packed (ABC). The dimensions of a CdCl2 single cell are shown in blue in all panels to show the relationship between the different unit cells.

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The SCXRD data of the octahedral-faceted crystal (Section 3.1) could be indexed according to two different solutions. Initially, the data were reduced in a cubic cell and the structure was solved in the Fd3̅m space group. This produced the defect-spinel structure previously published (36) with a = 12.1075(1) Å (Table S4). On closer inspection of the data for the octahedral-faceted crystal, the indexed reflections in an Ewald sphere projection show systematic absences throughout the crystal lattice (Figure 4). If the cubic unit cell is correct, these absences must be coincidental based on the structure factor. Alternatively, the SCXRD data can be indexed to four twinned metrically cubic trigonal cells with cell dimensions corresponding to the CdCl2 structure, where the c axis (001) of each trigonal cell points along the body diagonal of the cubic defect-spinel cell. Fitting the reflections using the twinned trigonal cells removes the zero-intensity cubic reflections highlighted in Figure 4 by reducing the size of the unit cell. The structure was reduced and solved in the R3̅m CdCl2 structure and refined against the outputted combined twin reflection listing, with all four twins giving an equal fractional volume contribution (Table S5). The resulting cells have metrically cubic trigonal cells with a = 4.2816(4) Å and c = 20.975(3) Å (c/2a = 2.4494(4)). SCXRD is thus unable to determine whether the octahedral-faceted crystal has the defect-spinel structure or a twinned CdCl2 structure. SCXRD weighted R parameters for the defect-spinel (wR2 0.0377) and twinned CdCl2 (wR2 0.0898) models show that both are valid solutions, and neither model may be disregarded on this basis (the difference in values is due to the deconvolution of the overlapped reflections for each rhombohedral cell in the twinned refinement and that the twinned refinement is fitted against 7 times the number of data points). We note that these models could in principle be distinguished by performing electron diffraction on a single domain; however, these samples were found to be too unstable in an electron beam to achieve this.

Figure 4

Figure 4. Reconstructed SCXRD pattern from the ⟨011̅⟩ plane of the Fd3̅m cubic cell. Indexing this pattern to the cubic Fd3̅m cell reveals several allowed Bragg peaks with zero intensity (e.g., the expected [220] reflection is indicated by a green circle and arrow). Alternatively, the pattern can be indexed to two rhombohedral R3̅m twins (red and blue circles), with the [001] axis of the rhombohedral cell aligned along the [111] axis of the cubic cell. Uncircled peaks arise from the rock salt sublattice and are common to all twins. This solution has no zero-intensity allowed Bragg peaks. Note that while two rhombohedral R3̅m twins are required to fit this particular region of reciprocal space, by extension four rhombohedral R3̅m twins are required to fit the full data set.

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Analysis of PXRD data of AgBiI4 powder samples also cannot distinguish the two models. As for SCXRD data, Figure 2 shows a series of symmetry allowed reflections for the cubic defect-spinel model which do not correspond to observed Bragg intensity; these reflections are not allowed in the rhombohedral CdCl2 model. Rietveld refinements of the two candidate models, shown in Figure 2 and Table S6, produce equivalent fits to the data (χ2 = 1.229 for the cubic defect-spinel model, versus 1.227 for the CdCl2 model) and equivalent refined cell parameters: the refined rhombohedral model is metrically cubic with a = 4.31844(1) Å, c = 21.15553(8) Å, and c/2a = 2.44944(1), and no peak broadening due to a rhombohedral distortion is resolved.
To test whether the distinct defect-spinel and CdCl2 models could be distinguished from a statistical combination of the two, a new cell was constructed which accommodates both cation ordering motifs by using equivalent cell settings (the trigonal setting of the cubic defect-spinel model and the trigonal setting of the CdCl2 cell with doubled a, b parameters) (Figure S14). A series of Rietveld refinements was then carried out at fixed spinel:CdCl2 ratios, as defined by the occupancies of their characteristic octahedral sites in the close packed iodide sublattice. These refinements show a monotonic increase in χ2 up to a maximum at the 50:50 ratio (Figure S15), showing clearly that the structure must adopt either the defect-spinel structure or the CdCl2 structure but is not a statistical mixture of the two.
Density functional theory (DFT) calculations were performed to investigate the physical plausibility of the two candidate models. For both defect-spinel and CdCl2 models, the disorder of the Ag+ and Bi3+ cations was accounted for by computing energies for all possible Ag/Bi distributions within a supercell. No significant separation was found between the calculated energies of the defect-spinel and CdCl2 models, suggesting that the two models are expected to have a similar level of stability (Table S1): neither one of the structures can be disregarded on computational grounds.
In summary, it is shown unambiguously by SCXRD that AgBiI4 can crystallize in a layered CdCl2-type structure which is metrically rhombohedral. The existence of a second polymorph is also demonstrated, but its structure is more enigmatic: it must be either a metrically cubic equivalent of the layered CdCl2-type rhombohedral structure or the cubic defect-spinel structure which has been reported previously. (36)

3.3 Optical and Electronic Properties

UV–visible spectroscopy measurements of powder and thin-film samples show that AgBiI4 absorbs visible and near-infrared light (Figure 5a), with a band gap suitable for single junction solar cells. Tauc plots using diffuse reflectance data give indirect band gaps of 1.63(1) eV for powder samples of AgBiI4 and 1.75(1) eV and 1.73(1) eV for the polycrystalline films 1 and 2 (Figure 5b). Tauc plots for direct band gaps, which lie in the range 1.73(1)–1.80(1) eV, are shown in Figure S16. Similar measurements for BiI3 powder gave an indirect band gap of 1.69(1) eV, consistent with previous reports. (31-33) A small red shift in the optical absorbance is observed for AgBiI4 when compared with BiI3. Indirect band gaps are calculated in similar materials such as CdI2 (2.9 eV) (54) and AgBi2I7. (35) Absorption coefficients for films 1 and 2 were found to lie in the range 105–106 cm–1 at energies greater than the band gap (see Figure 5c), which is comparable to typical Pb-based perovskite systems, (55) and implying that similar film thicknesses would be required in a AgBiI4-based device.

Figure 5

Figure 5. (a) UV–visible spectroscopy optical absorbance data for sealed tube synthesized powder (red), polycrystalline films 1 (blue) and 2 (green), and BiI3 powder (black) plotted against the AM1.5 solar spectrum (gray). (69) (b) Tauc plot for indirect band gaps calculated using the Kubelka–Munk function F(R) obtained via diffuse reflectance measurements for the sealed tube synthesized powder (red), polycrystalline films 1 (blue) and film 2 (green), and BiI3 powder (black). (c) Absorption coefficient of films 1 (blue) and 2 (green). The shaded areas in (c) indicate the error limits, derived from the standard deviations of the measured film thicknesses.

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XPS results (Figure 6a) and DOS calculations carried out for the lowest energy CdCl2 and defect-spinel structures (Figures 6b,c) suggest that the states at the top of the valence band and bottom of the conduction band closely resemble those of BiI3 (Figure S3). The top of the valence band is dominated by I 5p states and the bottom of the conduction band by a mixture of Bi 6p and I 5p states. As the main contribution of the Ag 4d states is to the bottom of the valence band and the Ag states are absent from the conduction band, we have used a combination of the PBE functional (48) and spin–orbit coupling to study the states around the band gap. This method reproduces the electronic structure of BiI3 (49) well but performs less well when describing more localized Ag d states (the computed band gap of AgI of 1.13 eV with this approach is much narrower than the experimental band gap of 2.85 eV (56)). As a result, both structural models have calculated band gaps which are underestimates of the experimental values for (CdCl2-type AgBiI4: 0.51 eV, defect-spinel AgBiI4: 0.81 eV), though we expect this to have little effect on the nature and dispersion of the states close to the valence and conduction band edges. (57) Using a similar method, Xiao and co-workers computed a band gap of 1.04 eV for AgBiI4 in the defect-spinel structure, which increased to 1.70 eV when using a hybrid functional. (35)

Figure 6

Figure 6. (a) XPS spectra of the valence band of pellets of AgBiI4 (red) and BiI3 (black) showing the main Ag 4d contribution is to the bottom of the valence band. DOS calculations for the lowest energy defect-spinel and CdCl2 structures are shown in (b) and (c), respectively.

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Hole and electron effective masses for both the defect-spinel and CdCl2-type models were calculated by fitting the curvature of the bands along high symmetry directions (Figures S4 and S5). The defect-spinel model showed little difference in either direction, with hole effective masses of 1.0 me and 0.9 me and electron effective masses of 0.6 me and 0.8 me along the Γ–Z and Γ–L directions, respectively. In contrast, for the CdCl2-type model the effective masses of holes (1.3 me) and electrons (1.8 me) are much higher along the Γ–Z direction than along the Γ–L direction where the hole and electron effective mass is 0.4 me. This is expected since the Γ–Z direction represents transport normal to the Ag+ and Bi3+ filled planes in the structure. The same effect is seen for hole transport within BiI3, where we compute an effective mass of 1.5 me along Γ–Z and 0.9 me along Γ–L, though surprisingly not for electrons where the effective mass is lower (1.0 me) along Γ–Z than along Γ–L (1.2 me).
At 300 K, bulk AgBiI4 has a large positive Seebeck coefficient and resistivity of 500 μV/K and 1 MΩ·cm, respectively (Figure 7), comparable to that of MAPbI3 (820 μV/K and 38 MΩ·cm), (58, 59) which suggests a small hole majority carrier concentration and shows that AgBiI4 behaves as a p-type band semiconductor with low dopant concentration. This is in comparison to BiI3 which has been characterized as an n-type semiconductor. (60) Resistivity presents Arrhenius behavior between 190 and 300 K, giving an impurity level with an activation energy of 0.4 eV similar to those found in BiI3 (0.43 eV) (61) and MAPbI3 (0.48 eV). (58) The measured thermal conductivity of 0.6 W/K·m is also comparable to that of MAPbI3 (0.3–0.5 W/K·m). (58)

Figure 7

Figure 7. (a) Seebeck coefficient measured from 210 to 300 K on bulk AgBiI4. (b) Resistivity of bulk AgBiI4 measured from 190 to 300 K.

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3.4 Stability

Stability toward light and heat in ambient conditions is a major factor in the technical viability of a solar absorbing material under working conditions in a solar cell. A series of PXRD patterns collected in situ on heating from 60 to 350 °C show that the first indication of structural decomposition of AgBiI4 occurs at 90 °C where Ag2BiI5 appears as a minority phase as a shoulder to the main phase [111]c/[003]r peak, and this remains the sole minority phase until a major irreversible decomposition process occurs above 290 °C (Figure S17). No structural phase transitions (i.e., changes to the structure of AgBiI4 itself) are observed in this temperature range, as shown by the linear expansion of the unit cell up to 100 °C (Figure S18). This compares favorably to the thermal stability of Pb hybrid perovskites; e.g., the organic component of MAPbI3, CH3NH3+, decomposes into HI and CH3NH2 at 85 °C even in inert atmospheres, (62) while at lower temperatures (45–55 °C) this compound undergoes a tetragonal to cubic phase transition which may affect long-term device performance.
The photostability of AgBiI4 was investigated under ambient conditions (with a sample sprinkled on a glass slide in an open atmosphere) and under controlled gastight conditions (in thin-walled borosilicate capillaries sealed under air or inert gas), using a solar simulator as described in Section 2.2.6. AgBiI4 was found to decompose partially to form crystalline AgI under ambient atmospheric conditions after 144 min of exposure at 1000 W m–2 and showed a similar decomposition when a 500 nm band-pass filter was used; however, it did not decompose under these conditions when a 600 nm band-pass filter was used (Figure S19), implying stability to longer wavelengths. The sealed samples were found to be considerably more stable, with no apparent decomposition after 180 min exposure to the full solar spectrum at 1000 W m–2, while exposure for an extended period of 150 h produced less extensive decomposition than that observed in the open system (Figure S20). A control sample, kept in the dark under ambient conditions, was found to be stable for at least 6 weeks (Figure S21). These results imply that while wavelengths of 600–761 nm (approximately 21% of the AM1.5 solar spectrum) would be accessible to the photovoltaic process without causing structural decomposition of AgBiI4 regardless of atmospheric conditions, the sealed environment of a photovoltaic cell is likely to afford stability over the full solar spectrum for extended periods.

4 Discussion

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The results presented in Section 3.2 show that bulk powders, polycrystalline films, and octahedral-faceted crystals of AgBiI4 cannot be assigned unambiguously to either the cubic defect-spinel structure or the rhombohedral (metrically cubic) CdCl2-type structure but must be one of them and not a statistical combination of the two. However, the SCXRD of an untwinned plate-like crystal associates a metrically rhombohedral cell with the CdCl2 structure type, with no ambiguity. The two structures both have face-centered cubic iodide sublattices and can be described as different vacancy ordered derivatives of the rock-salt structure with half of the cation sites vacant. Both orderings can be treated as frozen modes associated with the L (1/2,1/2,1/2) point in the Brillouin zone of Fm3̅m and indeed both are products of the L2– irreducible representation. These distortions can be more explicitly labeled, in the format “Irreducible Representation (order parameter direction in representation space) Sub group (basis) (origin shift)” as L2– (a, 0, 0, 0) R3̅m(1/2, −1/2, 0), (0, 1/2, −1/2), (2, 2, 2) (0, 0, 1/2) and L2– (a, a, −a, a) Fd3̅m (2, 0, 0), (0, 2, 0), (0, 0, 2) (0, 1/2, 0) for CdCl2-type and defect-spinel, respectively. (63, 64) The difficulty in distinguishing these two models derives from the homometry (65) of the cubic structure and the twinned rhombohedral structure: in the absence of a detectable rhombohedral strain they possess identical pair distribution functions and thus powder diffraction patterns. Essentially, they are related as alternative cation decorations of an expanded face-centered cubic iodide sublattice. It can be shown that the bond lengths and angles are the same for the two structures (Tables S7, S8) with the first coordination spheres of Bi/Ag (octahedral coordination) and I (trigonal coordination) shown in Figure S22. It should be noted that this sort of ambiguity is common in modulated structures where only first order satellites are observed; (66) in this case the choice of the L point as the modulation vector precludes observation of higher order satellites. The defect-spinel and CdCl2 structures of AgBiI4 are computed to have similar energies, suggesting that it may be possible to access the different structures by chemical substitution. We note that the silver rich compound Ag2BiI5 has unambiguously been shown to have the CdCl2 structure (39) and that CuBiI4 is reported to have a defect-spinel structure. (67)
The structure of BiI3 consists of a hexagonal close-packed iodide sublattice, in which layers of edge-sharing octahedra occupied by 2/3 Bi3+ cations and 1/3 vacancies alternate with entirely vacant layers (Figure 3a). (51) Taking into account the unoccupied layers, 1/3 of the octahedral sites in the structure are filled. The inclusion of silver in AgBiI4 increases the coverage of octahedral sites to 1/2 and changes the close-packing of the iodide sublattice from hexagonal to cubic (Figures 3b,c).
Apart from BiI3, other recently reported bismuth halide semiconductors have wider band gaps not suitable for single junction devices which have led to them being suggested as useful for tandem devices (Table 1). The structures of all of the compounds with band gaps above 1.9 eV are perovskite-like, with large cations replacing 1/4 of the iodide anions within a close packed lattice (Figure 8, Table 1), whereas AgBiI4 and BiI3 with indirect band gaps of 1.63(1) and 1.69(1) eV, respectively, both have fully occupied close-packed iodide sublattices. The indirect band gap of 1.63(1) eV for AgBiI4 is to date the most suitable band gap for a bismuth halide solar absorber in single junction solar cells but is still wider than the ideal 1.1–1.5 eV range suggested by the Shockley–Queisser limit, (1) suggesting that further reduction of the optical gap in bismuth halides is desirable. This study suggests that materials with uninterrupted close-packed halide sublattices are more promising candidates for bismuth halides with band gaps in the 1.1–1.5 eV range than those based on the perovskite structure of MAPbI3 and related lead halides.

Figure 8

Figure 8. Halide (green) connectivity (red) is shown for AgBiI4 and the bismuth halide semiconductors with previously reported optical gaps. Bismuth is shown in purple and cations in yellow. BiI3 shows uninterrupted hexagonal close packed iodide sublattice, and both the CdCl2 and defect-spinel structures of AgBiI4 show uninterrupted cubic close packed iodide sublattices. In the double perovskites Cs2AgBiX6 (X = Br, Cl), one-fourth of the anions in the close packed iodide sublattice are replaced by large Cs+ cations, in the perovskite-type structure. The same level of cation substitution is observed in hexagonal iodide layers within (CH3NH3)BiI4, (NH4)3Bi2I9, A3Bi2I9 (A = K, Rb, Cs), and (CH3NH3)2KBiCl6. All compounds with structures where cations replace I– anions within the hexagonal layers have optical gaps above 1.9 eV, (26-30, 32) where those with pure iodide layers have optical gaps of 1.6–1.8 eV.

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Table 1. Reported Band Gap Values Obtained from Indirect and Direct Tauc Plots, DFT-Calculated Band Gap Transition Types, and the Iodide Sub-Lattice Type of Reported Bismuth Halidesa
compoundindirect Tauc (eV)Direct Tauc (eV)calculated typeiodide sublattice
BiI31.67(9) (33)1.76 (33)indirect (33),bCP
 1.69(1)b1.77(1)b  
AgBiI4b1.63(1)1.73(1) CP
AgBi2I7 (34)1.661.87indirectCP
(CH3NH3)BiI4 (27)2.042.63directP
(NH4)3Bi2I9 (28)2.04 directP
(CH3NH3)2KBiCl6 (26)3.043.37indirectP
K3Bi2I9 (29) 2.10directP
Rb3Bi2I9 (29) 2.10directP
Cs3Bi2I9 (29)1.90 indirectP
Cs2AgBiCl6 (30)2.77 indirectP
Cs2AgBiBr6 (30)2.190 indirectP
a

The most suitable, i.e., smallest, band gaps for single junction photovoltaics are for AgBiI4 and then BiI3, which have uninterrupted CCP and HCP iodide sublattices respectively. Perovskite-type iodide sublattices, which have 1/4 of anion sites replaced by cations, exhibit larger band gaps above 1.90 eV. Abbreviations CP and P stand for close packed and perovskite-type packing, respectively.

b

Results obtained from this work.

The electronic properties of AgBiI4 are more similar to those of MAPbI3 than to those of BiI3, in spite of the closer structural similarity with the latter. Previous reports of the resistivity of BiI3 at room temperature give values of 100–1000 MΩ·cm (60, 68) with n-type charge carriers, whereas AgBiI4 has an increased conductivity of 2–3 orders of magnitude with a small number of p-type charge carriers, as does MAPbI3. (59)

5 Conclusion

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The lead-free semiconductor AgBiI4 has been synthesized and characterized. SCXRD of a plate crystal shows that the CdCl2 structure is accessible, and for the octahedral-faceted crystals and powder samples another polymorph exists that adopts either a metrically cubic CdCl2 structure or a cubic defect-spinel structure but not a statistical mixture of both. The iodide sublattice is cubically close packed in AgBiI4 in comparison to the hexagonally close packed sublattice of BiI3. We suggest that retaining a close packed iodide sublattice is the best way to keep the band gap narrow because other bismuth halide compounds reported have significantly wider band gaps and all have cations replacing 1/4 of the anions within the anion sublattice. Although the band edge states closely resemble those of BiI3, the p-type nature of AgBiI4 with low carrier concentrations is more similar to the hybrid perovskites than the n-type BiI3. These properties suggest that for bismuth halide based semiconductors other structure types, rather than perovskite, may better mimic the properties of the hybrid lead-based systems. AgBiI4 is accessible via different synthesis routes: solid state sealed tube reactions, chemical vapor transport growth, and solution processing. The thermal stability of AgBiI4 up to 90 °C is above photovoltaic operating temperatures, but light sensitivity intrinsic to the lead and bismuth halides remains a key factor that must be addressed in order to secure the technical viability of these materials in the next generation of solar cells. AgBiI4 shows structural stability to red light corresponding to energies above the band gap that should be accessible to the photovoltaic process, and its enhanced photostability in sealed, controlled atmospheres improves its potential for device application.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b04135.

  • Additional data and figures (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
    • John B. Claridge - Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom
    • Matthew J. Rosseinsky - Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom;  Orcidhttp://orcid.org/0000-0002-1910-2483
  • Authors
    • Harry C. Sansom - Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom
    • George F. S. Whitehead - Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom
    • Matthew S. Dyer - Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom
    • Marco Zanella - Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom
    • Troy D. Manning - Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom;  Orcidhttp://orcid.org/0000-0002-7624-4306
    • Michael J. Pitcher - Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom
    • Thomas J. Whittles - Department of Physics, University of Liverpool, Oxford Street, Liverpool L69 7ZE, United Kingdom;  The Stephenson Institute for Renewable Energy, University of Liverpool, Liverpool L69 3BX, United Kingdom
    • Vinod R. Dhanak - Department of Physics, University of Liverpool, Oxford Street, Liverpool L69 7ZE, United Kingdom;  The Stephenson Institute for Renewable Energy, University of Liverpool, Liverpool L69 3BX, United Kingdom
    • Jonathan Alaria - Department of Physics, University of Liverpool, Oxford Street, Liverpool L69 7ZE, United Kingdom
  • Notes
    The authors declare no competing financial interest.

    Experimental data is available at the University of Liverpool’s DataCat repository at DOI: 10.17638/datacat.liverpool.ac.uk/240. The supporting crystallographic information files may also be obtained from FIZ Karlsruhe, 76344 Eggenstein- Leopoldshafen, Germany (e-mail: [email protected]), on quoting the deposition numbers CSD-432551, CSD-432552 and CSD-432553.

Acknowledgment

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We thank EPSRC for support under EP/N004884 and for a Ph.D. studentship for Harry Sansom. We thank the STFC for access to beam time at Diamond Light Source and ISIS Spallation Source, Dr. C. Murray, Dr. A. Baker, and Prof. C. Tang for assistance at I11, and Dr. D. Fortes and Dr. K. Knight for assistance at HRPD. We also thank Max Birkett, Dr. Laurie Phillips, Dr. Tim Veal, and Dr. Alex Cowan at the Stephenson Institute for Renewable Energy (SIRE), University of Liverpool, U.K., for helpful discussion and use of equipment. Matthew Rosseinsky is a Royal Society Research Professor.

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    Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber Science 2013, 342, 341– 344 DOI: 10.1126/science.1243982
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    7
    Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber
    Stranks, Samuel D.; Eperon, Giles E.; Grancini, Giulia; Menelaou, Christopher; Alcocer, Marcelo J. P.; Leijtens, Tomas; Herz, Laura M.; Petrozza, Annamaria; Snaith, Henry J.
    Science (Washington, DC, United States) (2013), 342 (6156), 341-344CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Org.-inorg. perovskites showed promise as high-performance absorbers in solar cells, 1st as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, the authors report transient absorption and photoluminescence-quenching measurements to det. the electron-hole diffusion lengths, diffusion consts., and lifetimes in mixed halide (MeNH3PbI3-xClx) and triiodide (MeNH3PbI3) perovskite absorbers. The diffusion lengths are >1 μm in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. But the triiodide absorber has electron-hole diffusion lengths of about 100 nm. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a crit. parameter to optimize for future perovskite absorber development.
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  8. 8
    Sharenko, A.; Toney, M. F. Relationships between Lead Halide Perovskite Thin-Film Fabrication, Morphology, and Performance in Solar Cells J. Am. Chem. Soc. 2016, 138, 463– 470 DOI: 10.1021/jacs.5b10723
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    8
    Relationships between Lead Halide Perovskite Thin-Film Fabrication, Morphology, and Performance in Solar Cells
    Sharenko, Alexander; Toney, Michael F.
    Journal of the American Chemical Society (2016), 138 (2), 463-470CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A review. Soln.-processed lead halide perovskite thin-film solar cells have achieved power conversion efficiencies comparable to those obtained with several com. photovoltaic technologies in a remarkably short period of time. This rapid rise in device efficiency is largely the result of the development of fabrication protocols capable of producing continuous, smooth perovskite films with micrometer-sized grains. Further developments in film fabrication and morphol. control are necessary, however, in order for perovskite solar cells to reliably and reproducibly approach their thermodn. efficiency limit. This Perspective discusses the fabrication of lead halide perovskite thin films, while highlighting the processing-property-performance relationships that have emerged from the literature, and from this knowledge, suggests future research directions.
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  9. 9
    Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition Nature 2013, 501, 395– 398 DOI: 10.1038/nature12509
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    9
    Efficient planar heterojunction perovskite solar cells by vapour deposition
    Liu, Mingzhen; Johnston, Michael B.; Snaith, Henry J.
    Nature (London, United Kingdom) (2013), 501 (7467), 395-398CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Many different photovoltaic technologies are being developed for large-scale solar energy conversion. The wafer-based 1st-generation photovoltaic devices have been followed by thin-film solid semiconductor absorber layers sandwiched between 2 charge-selective contacts and nanostructured (or mesostructured) solar cells that rely on a distributed heterojunction to generate charge and to transport pos. and neg. charges in spatially sepd. phases. Although many materials have been used in nanostructured devices, the goal of attaining high-efficiency thin-film solar cells in such a way has yet to be achieved. Organometal halide perovskites have recently emerged as a promising material for high-efficiency nanostructured devices. Nanostructuring is not necessary to achieve high efficiencies with this material: a simple planar heterojunction solar cell incorporating vapor-deposited perovskite as the absorbing layer can have solar-to-elec. power conversion efficiencies of over 15 per cent (as measured under simulated full sunlight). Perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures.
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  10. 10
    Habisreutinger, S. N.; Leijtens, T.; Eperon, G. E.; Stranks, S. D.; Nicholas, R. J.; Snaith, H. J. Carbon Nanotube/Polymer Composites as a Highly Stable Hole Collection Layer in Perovskite Solar Cells Nano Lett. 2014, 14, 5561– 5568 DOI: 10.1021/nl501982b
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    10
    Carbon Nanotube/Polymer Composites as a Highly Stable Hole Collection Layer in Perovskite Solar Cells
    Habisreutinger, Severin N.; Leijtens, Tomas; Eperon, Giles E.; Stranks, Samuel D.; Nicholas, Robin J.; Snaith, Henry J.
    Nano Letters (2014), 14 (10), 5561-5568CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Org.-inorg. perovskite solar cells have recently emerged at the forefront of photovoltaics research. Power conversion efficiencies have experienced an unprecedented increase to reported values exceeding 19% within just four years. With the focus mainly on efficiency, the aspect of stability has so far not been thoroughly addressed. In this paper, the thermal stability is identified as a fundamental weak point of perovskite solar cells, and an elegant approach is demonstrated to mitigating thermal degrdn. by replacing the org. hole transport material with polymer-functionalized single-walled carbon nanotubes (SWNTs) embedded in an insulating polymer matrix. With this composite structure, JV scanned power-conversion efficiencies are obtained of up to 15.3% with an av. efficiency of 10 ± 2%. Moreover, strong retardation is obsd. in thermal degrdn. as compared to cells employing state-of-the-art org. hole-transporting materials. In addn., the resistance to water ingress is remarkably enhanced. These are crit. developments for achieving long-term stability of high-efficiency perovskite solar cells.
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  11. 11
    Bag, M.; Renna, L. A.; Adhikari, R. Y.; Karak, S.; Liu, F.; Lahti, P. M.; Russell, T. P.; Tuominen, M. T.; Venkataraman, D. Kinetics of Ion Transport in Perovskite Active Layers and Its Implications for Active Layer Stability J. Am. Chem. Soc. 2015, 137, 13130– 13137 DOI: 10.1021/jacs.5b08535
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    11
    Kinetics of Ion Transport in Perovskite Active Layers and Its Implications for Active Layer Stability
    Bag, Monojit; Renna, Lawrence A.; Adhikari, Ramesh Y.; Karak, Supravat; Liu, Feng; Lahti, Paul M.; Russell, Thomas P.; Tuominen, Mark T.; Venkataraman, D.
    Journal of the American Chemical Society (2015), 137 (40), 13130-13137CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Solar cells fabricated using alkyl ammonium metal halides as light absorbers have the right combination of high power conversion efficiency and ease of fabrication to realize inexpensive but efficient thin film solar cells. However, they degrade under prolonged exposure to sunlight. Herein, we show that this degrdn. is quasi-reversible, and that it can be greatly lessened by simple modifications of the solar cell operating conditions. We studied perovskite devices using electrochem. impedance spectroscopy (EIS) with methylammonium (MA)-, formamidinium (FA)-, and MAxFA1-x lead triiodide as active layers. From variable temp. EIS studies, we found that the diffusion coeff. using MA ions was greater than when using FA ions. Structural studies using powder X-ray diffraction (PXRD) show that for MAPbI3 a structural change and lattice expansion occurs at device operating temps. On the basis of EIS and PXRD studies, we postulate that in MAPbI3 the predominant mechanism of accelerated device degrdn. under sunlight involves thermally activated fast ion transport coupled with a lattice-expanding phase transition, both of which are facilitated by absorption of the IR component of the solar spectrum. Using these findings, we show that the devices show greatly improved operation lifetimes and stability under white-light emitting diodes, or under a solar simulator with an IR cutoff filter or with cooling.
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  12. 12
    Aristidou, N.; Sanchez-Molina, I.; Chotchuangchutchaval, T.; Brown, M.; Martinez, L.; Rath, T.; Haque, S. A. The Role of Oxygen in the Degradation of Methylammonium Lead Trihalide Perovskite Photoactive Layers Angew. Chem., Int. Ed. 2015, 54, 8208– 8212 DOI: 10.1002/anie.201503153
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    12
    The Role of Oxygen in the Degradation of Methylammonium Lead Trihalide Perovskite Photoactive Layers
    Aristidou, Nicholas; Sanchez-Molina, Irene; Chotchuangchutchaval, Thana; Brown, Michael; Martinez, Luis; Rath, Thomas; Haque, Saif A.
    Angewandte Chemie, International Edition (2015), 54 (28), 8208-8212CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In this paper we report on the influence of light and oxygen on the stability of CH3NH3PbI3 perovskite-based photoactive layers. When exposed to both light and dry air the mp-Al2O3/CH3NH3PbI3 photoactive layers rapidly decomp. yielding methylamine, PbI2, and I2 as products. We show that this degrdn. is initiated by the reaction of superoxide (O2-) with the methylammonium moiety of the perovskite absorber. Fluorescent mol. probe studies indicate that the O2- species is generated by the reaction of photoexcited electrons in the perovskite and mol. oxygen. We show that the yield of O2- generation is significantly reduced when the mp-Al2O3 film is replaced with an mp-TiO2 electron extn. and transport layer. The present findings suggest that replacing the methylammonium component in CH3NH3PbI3 to a species without acid protons could improve tolerance to oxygen and enhance stability.
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  13. 13
    Panda, P. K. Review: Environmental Friendly Lead-Free Piezoelectric Materials J. Mater. Sci. 2009, 44, 5049– 5062 DOI: 10.1007/s10853-009-3643-0
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    13
    Review: environmental friendly lead-free piezoelectric materials
    Panda, P. K.
    Journal of Materials Science (2009), 44 (19), 5049-5062CODEN: JMTSAS; ISSN:0022-2461. (Springer)
    A review. Lead zirconate titanate (PZT)-based piezoelec. materials are well known for their excellent piezoelec. properties. However, considering the toxicity of lead and its compds., there is a general awareness for the development of environmental friendly lead-free materials as evidenced from the legislation passed by the European Union to this effect. Several classes of materials are now being considered as potentially attractive alternatives to PZTs for specific applications. In this paper, attempts have been made to review the recent developments on lead-free piezo materials emphasizing on their prepn., structure-property correlation, etc. In this context, perovskite systems such as bismuth sodium titanate, alkali niobates (ANbO3), etc. and non-perovskites such as bismuth layer-structured ferroelecs. are reviewed in detail. From the above study, it is concluded that some lead-free compns. show stable piezoelec. responses even though they do not match the overall performance of PZT. This has been the stimulant for growing research on this subject. This topic is of current interest to the researchers worldwide as evidenced from the large no. of research publications. This has motivated us to come out with a review article with a view that it would give further impetus to the researchers already working in this area and also draw the attention of others.
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  14. 14
    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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    14
    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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  15. 15
    Koh, T. M.; Fu, K.; Fang, Y.; Chen, S.; Sum, T. C.; Mathews, N.; Mhaisalkar, S. G.; Boix, P. P.; Baikie, T. Formamidinium-Containing Metal-Halide: An Alternative Material for Near-IR Absorption Perovskite Solar Cells J. Phys. Chem. C 2014, 118, 16458– 16462 DOI: 10.1021/jp411112k
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    15
    Formamidinium-Containing Metal-Halide: An Alternative Material for Near-IR Absorption Perovskite Solar Cells
    Koh, Teck Ming; Fu, Kunwu; Fang, Yanan; Chen, Shi; Sum, T. C.; Mathews, Nripan; Mhaisalkar, Subodh G.; Boix, Pablo P.; Baikie, Tom
    Journal of Physical Chemistry C (2014), 118 (30), 16458-16462CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Solid-state, soln. processed solar-cells based on org.-inorg. Me ammonium lead halide absorbers have achieved efficiencies >15%, which has superseded liq. dye sensitized cells, as well as various thin film-based photovoltaics. This report introduces a new metal-halide perovskite, based on the formamidinium cation (HC-(NH2)2+), that displays a favorable band gap (1.47 eV) and represents a broader absorption compared to previously reported absorbers that contained the methylammonium cation (MeNH3+). The high open-circuit voltage (Voc = 0.97 V) and promising fill-factor (FF = 68.7%) yield an efficiency of 4.3%, which make this material an excellent candidate for this new class of perovskite solar cell. This report also studies the formation of a black trigonal (P3m1) perovskite polymorph and a yellow hexagonal non-perovskite (P63mc) polymorph. Further solar cell development would entail the stabilization of the black trigonal (P3m1) perovskite polymorph over the yellow hexagonal nonperovskite (P63mc) polymorph.
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  16. 16
    Eperon, G. E.; Stranks, S. D.; Menelaou, C.; Johnston, M. B.; Herz, L. M.; Snaith, H. J. Formamidinium Lead Trihalide: a Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar Cells Energy Environ. Sci. 2014, 7, 982– 988 DOI: 10.1039/c3ee43822h
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    16
    Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells
    Eperon, Giles E.; Stranks, Samuel D.; Menelaou, Christopher; Johnston, Michael B.; Herz, Laura M.; Snaith, Henry J.
    Energy & Environmental Science (2014), 7 (3), 982-988CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    Perovskite-based solar cells have attracted significant recent interest, with power conversion efficiencies in excess of 15% already superceding a no. of established thin-film solar cell technologies. Most work has focused on a methylammonium lead trihalide perovskites, with a bandgaps of ∼1.55 eV and greater. Here, we explore the effect of replacing the methylammonium cation in this perovskite, and show that with the slightly larger formamidinium cation, we can synthesize formamidinium lead trihalide perovskites with a bandgap tunable between 1.48 and 2.23 eV. We take the 1.48 eV-bandgap perovskite as most suited for single junction solar cells, and demonstrate long-range electron and hole diffusion lengths in this material, making it suitable for planar heterojunction solar cells. We fabricate such devices, and due to the reduced bandgap we achieve high short-circuit currents of >23 mA cm-2, resulting in power conversion efficiencies of up to 14.2%, the highest efficiency yet for soln. processed planar heterojunction perovskite solar cells. Formamidinium lead triiodide is hence promising as a new candidate for this class of solar cell.
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  17. 17
    Kulbak, M.; Cahen, D.; Hodes, G. How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr3 Cells J. Phys. Chem. Lett. 2015, 6, 2452– 2456 DOI: 10.1021/acs.jpclett.5b00968
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    17
    How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr3 Cells
    Kulbak, Michael; Cahen, David; Hodes, Gary
    Journal of Physical Chemistry Letters (2015), 6 (13), 2452-2456CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Hybrid org.-inorg. lead halide perovskite photovoltaic cells have already surpassed 20% conversion efficiency in the few years that they have been seriously studied. However, many fundamental questions still remain unanswered as to why they are so good. One of these is "Is the org. cation really necessary to obtain high quality cells" In this study, we show that an all-inorg. version of the lead bromide perovskite material works equally well as the org. one, in particular generating the high open circuit voltages that are an important feature of these cells.
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  18. 18
    Noel, N. K.; Stranks, S. D.; Abate, A.; Wehrenfennig, C.; Guarnera, S.; Haghighirad, A.-A.; Sadhanala, A.; Eperon, G. E.; Pathak, S. K.; Johnston, M. B.; Petrozza, A.; Herz, L. M.; Snaith, H. J. Lead-Free Organic-Inorganic Tin Halide Perovskites for Photovoltaic Applications Energy Environ. Sci. 2014, 7, 3061– 3068 DOI: 10.1039/C4EE01076K
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    18
    Lead-free organic-inorganic tin halide perovskites for photovoltaic applications
    Noel, Nakita K.; Stranks, Samuel D.; Abate, Antonio; Wehrenfennig, Christian; Guarnera, Simone; Haghighirad, Amir-Abbas; Sadhanala, Aditya; Eperon, Giles E.; Pathak, Sandeep K.; Johnston, Michael B.; Petrozza, Annamaria; Herz, Laura M.; Snaith, Henry J.
    Energy & Environmental Science (2014), 7 (9), 3061-3068CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    Already exhibiting solar to elec. power conversion efficiencies of over 17%, org.-inorg. lead halide perovskite solar cells are one of the most promising emerging contenders in the drive to provide a cheap and clean source of energy. One concern however, is the potential toxicol. issue of lead, a key component in the archetypical material. The most likely substitute is tin, which like lead, is also a group 14 metal. While org.-inorg. tin halide perovskites have shown good semiconducting behavior, the instability of tin in its 2+ oxidn. state has thus far proved to be an overwhelming challenge. Here, we report the first completely lead-free, CH3NH3SnI3 perovskite solar cell processed on a mesoporous TiO2 scaffold, reaching efficiencies of over 6% under 1 sun illumination. Remarkably, we achieve open circuit voltages over 0.88 V from a material which has a 1.23 eV band gap.
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  19. 19
    Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.; Mhaisalkar, S. G.; Graetzel, M.; White, T. J. Synthesis and Crystal Chemistry of the Hybrid Perovskite (CH3NH3)PbI3 for Solid-State Sensitised Solar Cell Applications J. Mater. Chem. A 2013, 1, 5628– 5641 DOI: 10.1039/c3ta10518k
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    19
    Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3)PbI3 for solid-state sensitised solar cell applications
    Baikie, Tom; Fang, Yanan; Kadro, Jeannette M.; Schreyer, Martin; Wei, Fengxia; Mhaisalkar, Subodh G.; Graetzel, Michael; White, Tim J.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2013), 1 (18), 5628-5641CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    The hybrid org.-inorg. perovskite (CH3NH3)PbI3 may find application in next generation solid-state sensitized solar cells. Although this material and related perovskites were discovered many decades ago, questions remain concerning their diverse structural chem. and unusual properties. The article presents a review of previous work and provides a detailed description of the prepn., structural characterization and phys. characteristics of (CH3NH3)PbI3. The phase changes exhibited by (CH3NH3)PbI3 have been probed using variable temp. powder and single crystal x-ray diffraction, combined with differential scanning calorimetry, thermogravimetric anal. and phase contrast transmission electron microscopy. The optical band gap for (CH3NH3)PbI3 detd. by UV-visible spectroscopy was compared to values obtained from d.-of-state simulation of the electronic band structure.
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  20. 20
    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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    20
    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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    Clark, S. J.; Donaldson, J. D.; Harvey, J. A. Evidence for the Direct Population of Solid-State Bands by Non-Bonding Electron Pairs in Compounds of the Type CsMX3(M= Ge, Sn, Pb; X = Cl, Br, I) J. Mater. Chem. 1995, 5, 1813– 1818 DOI: 10.1039/jm9950501813
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    Aharon, S.; Cohen, B. E.; Etgar, L. Hybrid Lead Halide Iodide and Lead Halide Bromide in Efficient Hole Conductor Free Perovskite Solar Cell J. Phys. Chem. C 2014, 118, 17160– 17165 DOI: 10.1021/jp5023407
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    22
    Hybrid Lead Halide Iodide and Lead Halide Bromide in Efficient Hole Conductor Free Perovskite Solar Cell
    Aharon, Sigalit; Cohen, Bat El; Etgar, Lioz
    Journal of Physical Chemistry C (2014), 118 (30), 17160-17165CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    In this work we used CH3NH3PbInBr3-n (where 0 ≤ n ≤ 3) as hole conductor and light harvester is used in the solar cell. Various concns. of methylammonium iodide and methylammonium bromide were studied which reveal that any compn. of the hybrid CH3NH3PbInBr3-n can conduct holes. The hybrid perovskite was deposited in two steps, sepg. it to two precursors to allow better control of the perovskite compn. and efficient tuning of its band gap. The X-ray diffraction reveals the change in the lattice parameter due to the introduction of the Br- ions. The hybrid iodide/bromide perovskite hole conductor free solar cells show very good stability, their power conversion efficiency achieved 8.54% under 1 sun illumination with c.d. of 16.2 mA/cm2. The results of this work open the possibility for graded structure of perovskite solar cells without the need for hole conductor.
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  23. 23
    Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colorful, Efficient, and Stable Inorganic–Organic Hybrid Nanostructured Solar Cells Nano Lett. 2013, 13, 1764– 1769 DOI: 10.1021/nl400349b
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    23
    Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells
    Noh, Jun Hong; Im, Sang Hyuk; Heo, Jin Hyuck; Mandal, Tarak N.; Seok, Sang Il
    Nano Letters (2013), 13 (4), 1764-1769CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Chem. tuned inorg.-org. hybrid materials, based on MeNH3(=MA)Pb(I1-xBrx)3 perovskites, have been studied using UV-visible absorption and x-ray diffraction patterns and applied to nanostructured solar cells. The band gap engineering brought about by the chem. management of MAPb(I1-xBrx)3 perovskites can be controllably tuned to cover almost the entire visible spectrum, enabling the realization of colorful solar cells. The authors demonstrate highly efficient solar cells exhibiting 12.3% in a power conversion efficiency of under std. AM 1.5, for the most efficient device, as a result of tunable compn. for the light harvester in conjunction with a mesoporous TiO2 film and a hole conducting polymer. Probably the works highlighted in this paper represent one step toward the realization of low-cost, high-efficiency, and long-term stability with colorful solar cells.
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  24. 24
    Kitazawa, N.; Watanabe, Y.; Nakamura, Y. Optical Properties of CH3NH3PbX3 (X = Halogen) and their Mixed-Halide Crystals J. Mater. Sci. 2002, 37, 3585– 3587 DOI: 10.1023/A:1016584519829
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    24
    Optical properties of CH3NH3PbX3 (X = halogen) and their mixed-halide crystals
    Kitazawa, N.; Watanabe, Y.; Nakamura, Y.
    Journal of Materials Science (2002), 37 (17), 3585-3587CODEN: JMTSAS; ISSN:0022-2461. (Kluwer Academic Publishers)
    Thin films of microcryst. CH3NH3PbX3 (X = halogen) as well as their mixed-halide crystals were fabricated by the spin-coating technique, and their optical properties were investigated. X-ray diffraction investigation revealed that CH3NH3PbBr3-xClx (x = 0-3) were successfully formed on glass substrate self-assembly and oriented with the a-axis. Owing to due to their large exciton binding energy, these materials showed clear exciton absorption and free-exciton emission in the visible region at room temp. Replacing Br with Cl made it possible to control the band structure of these materials. As a result, the peak position of the exciton band shifted continuously towards blue region with increasing the Cl content in the films.
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  25. 25
    Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut Nano Lett. 2015, 15, 3692– 3696 DOI: 10.1021/nl5048779
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    25
    Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut
    Protesescu, Loredana; Yakunin, Sergii; Bodnarchuk, Maryna I.; Krieg, Franziska; Caputo, Riccarda; Hendon, Christopher H.; Yang, Ruo Xi; Walsh, Aron; Kovalenko, Maksym V.
    Nano Letters (2015), 15 (6), 3692-3696CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Metal halides perovskites, such as hybrid org.-inorg. MeNH3PbI3, are newcomer optoelectronic materials that have attracted enormous attention as soln.-deposited absorbing layers in solar cells with power conversion efficiencies reaching 20%. A new avenue for halide perovskites was demonstrated by designing highly luminescent perovskite-based colloidal quantum dot materials. Monodisperse colloidal nanocubes (4-15 nm edge lengths) of fully inorg. perovskites (CsPbX3, X = Cl, Br, and I or mixed halide systems Cl/Br and Br/I) were synthesized using inexpensive com. precursors. Through compositional modulations and quantum size-effects, the bandgap energies and emission spectra are readily tunable over the entire visible spectral region of 410-700 nm. The luminescence of CsPbX3 nanocrystals is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color std., high quantum yields of ≤90%, and radiative lifetimes at 1-29 ns. The compelling combination of enhanced optical properties and chem. robustness makes CsPbX3 nanocrystals appealing for optoelectronic applications, particularly for blue and green spectral regions (410-530 nm), where typical metal chalcogenide-based quantum dots suffer from photodegrdn.
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    Wei, F.; Deng, Z.; Sun, S.; Xie, F.; Kieslich, G.; Evans, D. M.; Carpenter, M. A.; Bristowe, P. D.; Cheetham, A. K. The Synthesis, Structure and Electronic Properties of a Lead-Free Hybrid Inorganic-Organic Double Perovskite (MA)2KBiCl6 (MA = Methylammonium) Mater. Horiz. 2016, 3, 328– 332 DOI: 10.1039/C6MH00053C
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    26
    The synthesis, structure and electronic properties of a lead-free hybrid inorganic-organic double perovskite (MA)2KBiCl6 (MA = methylammonium)
    Wei, Fengxia; Deng, Zeyu; Sun, Shijing; Xie, Fei; Kieslich, Gregor; Evans, Donald M.; Carpenter, Michael A.; Bristowe, Paul D.; Cheetham, Anthony K.
    Materials Horizons (2016), 3 (4), 328-332CODEN: MHAOBM; ISSN:2051-6355. (Royal Society of Chemistry)
    In a search for lead-free materials that could be used as alternatives to the hybrid perovskites, (MA)PbX3, in photovoltaic applications, we have discovered a hybrid double perovskite, (MA)2KBiCl6, which shows strong similarities to the lead analogs. Spectroscopic measurements and nanoindentation studies are combined with d. functional calcns. to reveal the properties of this interesting system.
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  27. 27
    Hoye, R. L. Z.; Brandt, R. E.; Osherov, A.; Stevanović, V.; Stranks, S. D.; Wilson, M. W. B.; Kim, H.; Akey, A. J.; Perkins, J. D.; Kurchin, R. C.; Poindexter, J. R.; Wang, E. N.; Bawendi, M. G.; Bulović, V.; Buonassisi, T. Methylammonium Bismuth Iodide as a Lead-Free, Stable Hybrid Organic–Inorganic Solar Absorber Chem. - Eur. J. 2016, 22, 2605– 2610 DOI: 10.1002/chem.201505055
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    27
    Methylammonium Bismuth Iodide as a Lead-Free, Stable Hybrid Organic-Inorganic Solar Absorber
    Hoye, Robert L. Z.; Brandt, Riley E.; Osherov, Anna; Stevanovic, Vladan; Stranks, Samuel D.; Wilson, Mark W. B.; Kim, Hyunho; Akey, Austin J.; Perkins, John D.; Kurchin, Rachel C.; Poindexter, Jeremy R.; Wang, Evelyn N.; Bawendi, Moungi G.; Bulovic, Vladimir; Buonassisi, Tonio
    Chemistry - A European Journal (2016), 22 (8), 2605-2610CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Methylammonium lead halide (MAPbX3) perovskites exhibit exceptional carrier transport properties. But their com. deployment as solar absorbers is currently limited by their intrinsic instability in the presence of humidity and their lead content. Guided by our theor. predictions, we explored the potential of methylammonium bismuth iodide (MBI) as a solar absorber through detailed materials characterization. We synthesized phase-pure MBI by soln. and vapor processing. In contrast to MAPbX3, MBI is air stable, forming a surface layer that does not increase the recombination rate. We found that MBI luminesces at room temp., with the vapor-processed films exhibiting superior photoluminescence (PL) decay times that are promising for photovoltaic applications. The thermodn., electronic, and structural features of MBI that are amenable to these properties are also present in other hybrid ternary bismuth halide compds. Through MBI, we demonstrate a lead-free and stable alternative to MAPbX3 that has a similar electronic structure and nanosecond lifetimes.
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  28. 28
    Sun, S.; Tominaka, S.; Lee, J.-H.; Xie, F.; Bristowe, P. D.; Cheetham, A. K. Synthesis, Crystal Structure, and Properties of a Perovskite-Related Bismuth Phase, (NH4)3Bi2I9 APL Mater. 2016, 4, 031101 DOI: 10.1063/1.4943680
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    28
    Synthesis, crystal structure, and properties of a perovskite-related bismuth phase, (NH4)3Bi2I9
    Sun, Shijing; Tominaka, Satoshi; Lee, Jung-Hoon; Xie, Fei; Bristowe, Paul D.; Cheetham, Anthony K.
    APL Materials (2016), 4 (3), 031101/1-031101/7CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    Org.-inorg. halide perovskites, esp. methylammonium lead halide, have recently led to remarkable advances in photovoltaic devices. However, due to environmental and stability concerns around the use of lead, research into lead-free perovskite structures has been attracting increasing attention. In this study, a layered perovskite-like architecture, (NH4)3Bi2I9, is prepd. from soln. and the structure solved by single crystal X-ray diffraction. The band gap, which is estd. to be 2.04 eV using UV-visible spectroscopy, is lower than that of CH3NH3PbBr3. The energy-minimized structure obtained from first principles calcns. is in excellent agreement with the X-ray results and establishes the locations of the hydrogen atoms. The calcns. also point to a significant lone pair effect on the bismuth ion. Single crystal and powder cond. measurements are performed to examine the potential application of (NH4)3Bi2I9 as an alternative to the lead contg. perovskites. (c) 2016 American Institute of Physics.
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  29. 29
    Lehner, A. J.; Fabini, D. H.; Evans, H. A.; Hébert, C.-A.; Smock, S. R.; Hu, J.; Wang, H.; Zwanziger, J. W.; Chabinyc, M. L.; Seshadri, R. Crystal and Electronic Structures of Complex Bismuth Iodides A3Bi2I9 (A = K, Rb, Cs) Related to Perovskite: Aiding the Rational Design of Photovoltaics Chem. Mater. 2015, 27, 7137– 7148 DOI: 10.1021/acs.chemmater.5b03147
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    29
    Crystal and Electronic Structures of Complex Bismuth Iodides A3Bi2I9 (A = K, Rb, Cs) Related to Perovskite: Aiding the Rational Design of Photovoltaics
    Lehner, Anna J.; Fabini, Douglas H.; Evans, Hayden A.; Hebert, Claire-Alice; Smock, Sara R.; Hu, Jerry; Wang, Hengbin; Zwanziger, Josef W.; Chabinyc, Michael L.; Seshadri, Ram
    Chemistry of Materials (2015), 27 (20), 7137-7148CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Ternary bismuth halides form an interesting functional materials class in the context of the closely related Pb halide perovskite photovoltaics, esp. given the significantly reduced toxicity of Bi when compared with Pb. The compds. A3Bi2I9 (A = K, Rb, Cs) examd. here crystallize in two different structure types: the layered defect-perovskite K3Bi2I9 type, and the Cs3Cr2Cl9 type. The latter structure type features isolated Bi2I93- anions. Here, the crystal structures of the ternary iodides are redetd. and a cor. structural model for Rb3Bi2I9, as established by single crystal x-ray diffraction and solid state 87Rb NMR spectroscopy and supported by d. functional theory (DFT) calcns. is presented. A variety of facile prepn. techniques for single crystals, bulk materials, as well as soln.-processed thin films are described. The optical properties and electronic structures were studied exptl. by optical absorption and UPS and computationally by DFT calcns. Abs. band positions of the valence and conduction bands of these semiconductors, with excellent agreement of exptl. and calcd. values, are reported, constituting a useful input for the rational interface design of efficient electronic and optoelectronic devices. The different structural connectivity in the two different structure types, somewhat surprisingly, appears to not impact band positions or band gaps in a significant manner. Computed dielec. properties, including the finding of anomalously large Born effective charge tensors on Bi3+, suggest proximal structural instabilities arising from the Bi3+ 6s2 lone pair. These anomalous Born effective charges are promising for defect screening and effective charge carrier transport. The structural, electronic, and optical properties of the complex bismuth iodides are to some extent similar to the related lead iodide perovskites. The deeper valence band positions in the complex bismuth iodides point to the need for different choices of hole transport materials for Bi-iodide based solar cell architectures.
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  30. 30
    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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    30
    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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  31. 31
    Lehner, A. J.; Wang, H.; Fabini, D. H.; Liman, C. D.; Hébert, C.-A.; Perry, E. E.; Wang, M.; Bazan, G. C.; Chabinyc, M. L.; Seshadri, R. Electronic Structure and Photovoltaic Application of BiI3 Appl. Phys. Lett. 2015, 107, 131109 DOI: 10.1063/1.4932129
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    31
    Electronic structure and photovoltaic application of BiI3
    Lehner, Anna J.; Wang, Hengbin; Fabini, Douglas H.; Liman, Christopher D.; Hebert, Claire-Alice; Perry, Erin E.; Wang, Ming; Bazan, Guillermo C.; Chabinyc, Michael L.; Seshadri, Ram
    Applied Physics Letters (2015), 107 (13), 131109/1-131109/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Rapid recent improvement in photovoltaic efficiency in hybrid lead halide perovskite materials provided the impetus for understanding other, related main-group halide systems. Here, the closely related but less toxic BiI3 can show promising optoelectronic properties. Layered binary BiI3 is used here as the active layer in planar solar cell architectures (efficiency ∼0.3%). Exptl. and computational studies of abs. band positions of BiI3 are also presented, to help in the rational design of device architectures that would allow efficient charge transfer at the interfaces. (c) 2015 American Institute of Physics.
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  32. 32
    Brandt, R. E.; Kurchin, R. C.; Hoye, R. L. Z.; Poindexter, J. R.; Wilson, M. W. B.; Sulekar, S.; Lenahan, F.; Yen, P. X. T.; Stevanović, V.; Nino, J. C.; Bawendi, M. G.; Buonassisi, T. Investigation of Bismuth Triiodide (BiI3) for Photovoltaic Applications J. Phys. Chem. Lett. 2015, 6, 4297– 4302 DOI: 10.1021/acs.jpclett.5b02022
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    Investigation of Bismuth Triiodide (BiI3) for Photovoltaic Applications
    Brandt, Riley E.; Kurchin, Rachel C.; Hoye, Robert L. Z.; Poindexter, Jeremy R.; Wilson, Mark W. B.; Sulekar, Soumitra; Lenahan, Frances; Yen, Patricia X. T.; Stevanovic, Vladan; Nino, Juan C.; Bawendi, Moungi G.; Buonassisi, Tonio
    Journal of Physical Chemistry Letters (2015), 6 (21), 4297-4302CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Guided by predictive discovery framework, the authors study bismuth triiodide (BiI3) as a candidate thin-film photovoltaic (PV) absorber. BiI3 was chosen for its optical properties and the potential for defect-tolerant charge transport properties, which the authors test exptl. by measuring optical absorption and recombination lifetimes. The authors synthesize phase-pure BiI3 thin films by phys. vapor transport and soln. processing and single-crystals by an electrodynamic gradient vertical Bridgman method. The bandgap of these materials is ∼1.8 eV, and they demonstrate room-temp. band-edge photoluminescence. The authors measure monoexponential recombination lifetimes at 180-240 ps for thin films, and longer, multiexponential dynamics for single crystals, with time consts. up to 1.3 to 1.5 ns. The outstanding challenges to developing BiI3 PVs, including mech. and elec. properties, which can also inform future selection of candidate PV absorbers are discussed.
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  33. 33
    Podraza, N. J.; Qiu, W.; Hinojosa, B. B.; Xu, H.; Motyka, M. A.; Phillpot, S. R.; Baciak, J. E.; Trolier-McKinstry, S.; Nino, J. C. Band Gap and Structure of Single Crystal BiI3: Resolving Discrepancies in Literature J. Appl. Phys. 2013, 114, 033110 DOI: 10.1063/1.4813486
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    Band gap and structure of single crystal BiI3: Resolving discrepancies in literature
    Podraza, Nikolas J.; Qiu, Wei; Hinojosa, Beverly B.; Xu, Haixuan; Motyka, Michael A.; Phillpot, Simon R.; Baciak, James E.; Trolier-McKinstry, Susan; Nino, Juan C.
    Journal of Applied Physics (Melville, NY, United States) (2013), 114 (3), 033110/1-033110/8CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Bismuth tri-iodide (BiI3) is an intermediate band gap semiconductor with potential for room temp. gamma-ray detection applications. Remarkably, very different band gap characteristics and values of BiI3 have been reported in literature, which may be attributed to its complicated layered structure with strongly bound BiI6 octahedra held together by weak van der Waals interactions. Here, to resolve this discrepancy, the band gap of BiI3 was characterized through optical and computational methods and differences among previously reported values are discussed. Unpolarized transmittance and reflectance spectra in the visible to near UV (UV-Vis) range at room temp. yielded an indirect band gap of 1.67 ± 0.09 eV, while spectroscopic ellipsometry detected a direct band gap at 1.96 ± 0.05 eV and higher energy crit. point features. The discrepancy between the UV-Vis and ellipsometry results originates from the low optical absorption coeffs. (α ∼ 102 cm-1) of BiI3 that renders reflection-based ellipsometry insensitive to the indirect gap for this material. Further, electronic-structure calcns. of the band structure by d. functional theory methods are also consistent with the presence of an indirect band gap of 1.55 eV in BiI3. Based on this, an indirect band gap with a value of 1.67 ± 0.09 eV is considered to best represent the band gap structure and value for single crystal BiI3. (c) 2013 American Institute of Physics.
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  34. 34
    Kim, Y.; Yang, Z.; Jain, A.; Voznyy, O.; Kim, G.-H.; Liu, M.; Quan, L. N.; García de Arquer, F. P.; Comin, R.; Fan, J. Z.; Sargent, E. H. Pure Cubic-Phase Hybrid Iodobismuthates AgBi2I7 for Thin-Film Photovoltaics Angew. Chem., Int. Ed. 2016, 55, 9586– 9590 DOI: 10.1002/anie.201603608
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    Pure Cubic-Phase Hybrid Iodobismuthates AgBi2I7 for Thin-Film Photovoltaics
    Kim, Younghoon; Yang, Zhenyu; Jain, Ankit; Voznyy, Oleksandr; Kim, Gi-Hwan; Liu, Min; Quan, Li Na; Garcia de Arquer, F. Pelayo; Comin, Riccardo; Fan, James Z.; Sargent, Edward H.
    Angewandte Chemie, International Edition (2016), 55 (33), 9586-9590CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Bismuth-based hybrid perovskites are candidates for lead-free and air-stable photovoltaics, but poor surface morphologies and a high band-gap energy have previously limited these hybrid perovskites. A new materials processing strategy to produce enhanced bismuth-based thin-film photovoltaic absorbers by incorporation of monovalent silver cations into iodobismuthates is presented. Soln.-processed AgBi2I7 thin films are prepd. by spin-coating silver and bismuth precursors dissolved in n-butylamine and annealing under an N2 atmosphere. X-ray diffraction anal. reveals the pure cubic structure (Fd3m) with lattice parameters of a=b=c=12.223 Å. The resultant AgBi2I7 thin films exhibit dense and pinhole-free surface morphologies with grains ranging in size from 200-800 nm and a low band gap of 1.87 eV suitable for photovoltaic applications. Initial studies produce solar power conversion efficiencies of 1.22 % and excellent stability over at least 10 days under ambient conditions.
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  35. 35
    Xiao, Z.; Meng, W.; Mitzi, D. B.; Yan, Y. Crystal Structure of AgBi2I7 Thin Films J. Phys. Chem. Lett. 2016, 7, 3903– 3907 DOI: 10.1021/acs.jpclett.6b01834
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    35
    Crystal Structure of AgBi2I7 Thin Films
    Xiao, Zewen; Meng, Weiwei; Mitzi, David B.; Yan, Yanfa
    Journal of Physical Chemistry Letters (2016), 7 (19), 3903-3907CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Synthesis of cubic-phase AgBi2I7 iodobismuthate films and fabrication of air-stable Pb-free solar cells using the AgBi2I7 absorber have recently been reported. From XRD anal. and nominal compn., probably the synthesized films have a cubic ThZr2H7 crystal structure with AgBi2I7 stoichiometry. Through careful examn. of the proposed structure and computational evaluation of the phase stability and bandgap, the reported AgBi2I7 films cannot be forming with the ThZr2H7-type structure, but rather more likely adopt an Ag-deficient AgBiI4 type. Both the exptl. x-ray diffraction pattern and bandgap can be better explained by the AgBiI4 structure. The proposed AgBiI4 structure, with octahedral Bi coordination, removes unphys. short Bi-I bonding within the [BiI8] hexahedra of the ThZr2I7 model. The results provide crit. insights for assessing the photovoltaic properties of AgBi2I7 iodobismuthate materials.
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  36. 36
    Oldag, T.; Aussieker, T.; Keller, H.-L.; Preitschaft, C.; Pfitzner, A. Solvothermale Synthese und Bestimmung der Kristallstrukturen von AgBiI4 und Ag3BiI6 Z. Anorg. Allg. Chem. 2005, 631, 677– 682 DOI: 10.1002/zaac.200400508
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    36
    Solvothermal synthesis and crystal structure determination of AgBiI4 and Ag3BiI6
    Oldag, Thorsten; Aussieker, Thomas; Keller, Hans-Lothar; Preitschaft, Christian; Pfitzner, Arno
    Zeitschrift fuer Anorganische und Allgemeine Chemie (2005), 631 (4), 677-682CODEN: ZAACAB; ISSN:0044-2313. (Wiley-VCH Verlag GmbH & Co. KGaA)
    AgBiI4 and Ag3BiI6 were synthesized by solvothermal reaction from AgI and BiI3 in dild. HI-soln. (20%) at a temp. of 160°. The grayish-black crystals grow as octahedra (Ag-BiI4) or hexagonal/trigonal platelets (Ag3BiI6). AgBiI4 crystallizes in space group Fd‾3m with a = 1222.3(1) pm (300 K), Z = 8, R1 = 0.0151, wR2 = 0.0226, whereas Ag3BiI6 shows the space group R‾3m with a = 435.37(6) pm, c = 2081.0(4) pm (300 K), Z = 1, 154 independent reflections, 8 refined parameters, R1 = 0.0220, wR2 = 0.0573. Both crystal structures show stacking sequence abcabc... of hexagonal layers contg. I. Bi and Ag are sharing octahedral sites with different mass ratio in both structures. The part of Ag which could be localized varies with temp. This behavior indicates mobility of Ag within the crystal structure. The ionic cond. of AgBiI4 is explored. AgBiI4 and Ag3BiI6 show close structural relationship, with AgBiI4 as a variant with a higher degree of order.
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  37. 37
    Fourcroy, P. H.; Palazzi, M.; Rivet, J.; Flahaut, J.; Céolin, R. Etude du Systeme AgIBiI3 Mater. Res. Bull. 1979, 14, 325– 328 DOI: 10.1016/0025-5408(79)90096-5
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    Study of the silver iodide-bismuth(III) iodide system
    Fourcroy, P. H.; Palazzi, M.; Rivet, J.; Flahaut, J.; Ceolin, R.
    Materials Research Bulletin (1979), 14 (3), 325-8CODEN: MRBUAC; ISSN:0025-5408.
    The pseudo-binary system AgI-BiI3 was studied by DTA and by microcalorimetry. An eutectic point was found at 389° at the compn. 85 mol. % BiI3. Two new phases Ag2BiI5 and AgBi2I7 were identified, and their existence confirmed by x-ray diffraction. Ag2BiI5 is rhombohedral, with hexagonal a 4.34 and c 20.77 Å. AgBi2I7 is cubic with a 3.05 Å.
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    Dzeranova, K. B.; Kaloev, N. I.; Bukhalova, G. A. The BiI3 - AgI System Russ. J. Inorg. Chem. 1985, 30, 1700– 1701
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    Mashadieva, L. F.; Aliev, Z. S.; Shevelkov, A. V.; Babanly, M. B. Experimental Investigation of the Ag–Bi–I Ternary System and Thermodynamic Properties of the Ternary Phases J. Alloys Compd. 2013, 551, 512– 520 DOI: 10.1016/j.jallcom.2012.11.033
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    Experimental investigation of the Ag-Bi-I ternary system and thermodynamic properties of the ternary phases
    Mashadieva, Leyla F.; Aliev, Ziya S.; Shevelkov, Andrei V.; Babanly, Mahammad B.
    Journal of Alloys and Compounds (2013), 551 (), 512-520CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
    The phase equil. in the Ag-Bi-I ternary system and thermodn. properties of the ternary phases were exptl. detd. by using DTA and x-ray diffraction techniques and EMF measurements with the Ag4RbI5 solid electrolyte. According to the obtained exptl. results, the polythermal sections of the ternary phase diagram, its isothermal section at 300 K as well as the projection of the liqs. surface were revised. The fields of the primary crystn. and types and coordinates of non-variant and mono-variant equil. were detd. The partial molar functions of silver iodide and silver in the alloys as well as the std. thermodn. functions of formation and the std. entropies of Ag2BiI5 and AgBi2I7 were calcd. based on EMF measurements.
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    Degen, T.; Sadki, M.; Bron, E.; König, U.; Nénert, G. The HighScore Suite Powder Diffr. 2014, 29, S13– S18 DOI: 10.1017/S0885715614000840
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    The HighScore suite
    Degen, Thomas; Sadki, Mustapha; Bron, Egbert; Koenig, Uwe; Nenert, Gwilherm
    Powder Diffraction (2014), 29 (S2), S13-S18CODEN: PODIE2; ISSN:0885-7156. (Cambridge University Press)
    HighScore with the Plus option (HighScore Plus) is the com. powder diffraction anal. software from PAnal. It has been in const. development over the last 13 years and has evolved into a very complete and mature product. In this paper, we present a brief overview of the suite focusing on the latest addns. and its user-friendliness. The introduction briefly touches some basic ideas behind HighScore and the Plus option.
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    TOPAS, version 5; Bruker AXS: Karlsruhe, Germany, 2011.
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    Momma, K.; Izumi, F. VESTA: a Three-Dimensional Visualization System for Electronic and Structural Analysis J. Appl. Crystallogr. 2008, 41, 653– 658 DOI: 10.1107/S0021889808012016
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    VESTA: a three-dimensional visualization system for electronic and structural analysis
    Momma, Koichi; Izumi, Fujio
    Journal of Applied Crystallography (2008), 41 (3), 653-658CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
    A cross-platform program, VESTA, has been developed to visualize both structural and volumetric data in multiple windows with tabs. VESTA represents crystal structures by ball-and-stick, space-filling, polyhedral, wire frame, stick, dot-surface and thermal-ellipsoid models. A variety of crystal-chem. information is extractable from fractional coordinates, occupancies and oxidn. states of sites. Volumetric data such as electron and nuclear densities, Patterson functions, and wavefunctions are displayed as isosurfaces, bird's-eye views and two-dimensional maps. Isosurfaces can be colored according to other phys. quantities. Translucent isosurfaces and/or slices can be overlapped with a structural model. Collaboration with external programs enables the user to locate bonds and bond angles in the 'graphics area', simulate powder diffraction patterns, and calc. site potentials and Madelung energies. Electron densities detd. exptl. are convertible into their Laplacians and electronic energy densities.
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    CrysAlisPro, version 171.38.48; Agilent Technologies: Yarton, Oxfordshire, U.K., 2013.
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    Sheldrick, G. A Short History of SHELX Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112– 122 DOI: 10.1107/S0108767307043930
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    A short history of SHELX
    Sheldrick, George M.
    Acta Crystallographica, Section A: Foundations of Crystallography (2008), 64 (1), 112-122CODEN: ACACEQ; ISSN:0108-7673. (International Union of Crystallography)
    An account is given of the development of the SHELX system of computer programs from SHELX-76 to the present day. In addn. to identifying useful innovations that have come into general use through their implementation in SHELX, a crit. anal. is presented of the less-successful features, missed opportunities and desirable improvements for future releases of the software. An attempt is made to understand how a program originally designed for photog. intensity data, punched cards and computers over 10000 times slower than an av. modern personal computer has managed to survive for so long. SHELXL is the most widely used program for small-mol. refinement and SHELXS and SHELXD are often employed for structure soln. despite the availability of objectively superior programs. SHELXL also finds a niche for the refinement of macromols. against high-resoln. or twinned data; SHELXPRO acts as an interface for macromol. applications. SHELXC, SHELXD and SHELXE are proving useful for the exptl. phasing of macromols., esp. because they are fast and robust and so are often employed in pipelines for high-throughput phasing. This paper could serve as a general literature citation when one or more of the open-source SHELX programs (and the Bruker AXS version SHELXTL) are employed in the course of a crystal-structure detn.
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    Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: a Complete Structure Solution, Refinement and Analysis Program J. Appl. Crystallogr. 2009, 42, 339– 341 DOI: 10.1107/S0021889808042726
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    OLEX2: a complete structure solution, refinement and analysis program
    Dolomanov, Oleg V.; Bourhis, Luc J.; Gildea, Richard J.; Howard, Judith A. K.; Puschmann, Horst
    Journal of Applied Crystallography (2009), 42 (2), 339-341CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
    New software, OLEX2, was developed for the detn., visualization and anal. of mol. crystal structures. The software has a portable mouse-driven workflow-oriented and fully comprehensive graphical user interface for structure soln., refinement and report generation, as well as novel tools for structure anal. OLEX2 seamlessly links all aspects of the structure soln., refinement and publication process and presents them in a single workflow-driven package, with the ultimate goal of producing an application which will be useful to both chemists and crystallographers.
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  46. 46
    Whittles, T. J.; Burton, L. A.; Skelton, J. M.; Walsh, A.; Veal, T. D.; Dhanak, V. R. Band Alignments, Valence Bands, and Core Levels in the Tin Sulfides SnS, SnS2, and Sn2S3: Experiment and Theory Chem. Mater. 2016, 28, 3718– 3726 DOI: 10.1021/acs.chemmater.6b00397
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    46
    Band Alignments, Valence Bands, and Core Levels in the Tin Sulfides SnS, SnS2, and Sn2S3: Experiment and Theory
    Whittles, Thomas J.; Burton, Lee A.; Skelton, Jonathan M.; Walsh, Aron; Veal, Tim D.; Dhanak, Vin R.
    Chemistry of Materials (2016), 28 (11), 3718-3726CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Tin sulfide solar cells show relatively poor efficiencies despite attractive photovoltaic properties, and there is difficulty in identifying sep. phases, which are also known to form during Cu2ZnSnS4 depositions. The authors present x-ray photoemission spectroscopy (XPS) and inverse photoemission spectroscopy measurements of single crystal SnS, SnS2, and Sn2S3, with electronic-structure calcns. from d. functional theory (DFT). Differences in the XPS spectra of the three phases, including a large 0.9 eV shift between the 3d5/2 peak for SnS and SnS2, make this technique useful when identifying phase-pure or mixed-phase systems. Comparison of the valence band spectra from XPS and DFT reveals extra states at the top of the valence bands of SnS and Sn2S3, arising from the hybridization of lone pair electrons in Sn(II), which are not present for Sn(IV), as found in SnS2. This results in relatively low ionization potentials for SnS (4.71 eV) and Sn2S3 (4.66 eV), giving a more comprehensive explanation as to the origin of the poor efficiencies. The authors also demonstrate, by a band alignment, the large band offsets of SnS and Sn2S3 from other photovoltaic materials and highlight the detrimental effect on cell performance of secondary tin sulfide phase formation in SnS and CZTS films.
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    Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D.; Chastain, J.; King, R. C. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics, Inc.: Eden Prairie, MN, 1995.
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    Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations using a Plane-Wave Basis Set Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169– 11186 DOI: 10.1103/PhysRevB.54.11169
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    Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
    Kresse, G.; Furthmueller, J.
    Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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    Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758– 1775 DOI: 10.1103/PhysRevB.59.1758
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    From ultrasoft pseudopotentials to the projector augmented-wave method
    Kresse, G.; Joubert, D.
    Physical Review B: Condensed Matter and Materials Physics (1999), 59 (3), 1758-1775CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived. The total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addn., crit. tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed-core all-electron methods. These tests include small mols. (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
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    Klimeš, J.; Bowler, D. R.; Michaelides, A. Van der Waals Density Functionals Applied to Solids Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 195131 DOI: 10.1103/PhysRevB.83.195131
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    Van der Waals density functionals applied to solids
    Klimes, Jiri; Bowler, David R.; Michaelides, Angelos
    Physical Review B: Condensed Matter and Materials Physics (2011), 83 (19), 195131/1-195131/13CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The van der Waals d. functional (vdW-DF) of M. Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)] is a promising approach for including dispersion in approx. d. functional theory exchange-correlation functionals. Indeed, an improved description of systems held by dispersion forces has been demonstrated in the literature. However, despite many applications, std. general tests on a broad range of materials including traditional "hard" matter such as metals, ionic compds., and insulators are lacking. Such tests are important not least because many of the applications of the vdW-DF method focus on the adsorption of atoms and mols. on the surfaces of solids. Here we calc. the lattice consts., bulk moduli, and atomization energies for a range of solids using the original vdW-DF and several of its offspring. We find that the original vdW-DF overestimates lattice consts. in a similar manner to how it overestimates binding distances for gas-phase dimers. However, some of the modified vdW functionals lead to av. errors which are similar to those of PBE or better. Likewise, atomization energies that are slightly better than from PBE are obtained from the modified vdW-DFs. Although the tests reported here are for hard solids, not normally materials for which dispersion forces are thought to be important, we find a systematic improvement in cohesive properties for the alkali metals and alkali halides when nonlocal correlations are accounted for.
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    Trotter, J.; Zobel, T. The Crystal Structure of Sbl3 and Bil3 Z. Kristallogr. 1966, 123, 67
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    The crystal structure of SbI3 and BiI3
    Trotter, J.; Zobel, T.
    Zeitschrift fuer Kristallographie, Kristallgeometrie, Kristallphysik, Kristallchemie (1966), 123 (1), 67-72CODEN: ZKKKAJ; ISSN:0044-2968.
    Crystals of antimony triiodide, SbI3, are rhombohedral, with aH 7.48, cH 20.90 A., Z = 6, and space group R‾3. The structure was detd. from Patterson, Fourier and difference projections, by using F(h0.l) data, the final discrepancy factor being 0.131. The Sb atoms are significantly displaced from the centers of I octahedra, and have 3 near-neighbor I atoms at 2.686 ± 0.0010 A., with I-Sb-I 95.8 ± 0.3°, and 3 farther at 3.316 ± 0.010 A. The structure is thus intermediate between that of a mol. crystal, as in AsI3, and an ionic arrangement. Crystals of BiI3 have aH 7.52, and cH 20.72 A. A comparison of measured and calcd. powder intensities suggests that Bi is situated at the center of an I octahedron, so that the nonbonded electron pair is not sterically active and the structure is probably largely ionic.
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    Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple Phys. Rev. Lett. 1996, 77, 3865– 3868 DOI: 10.1103/PhysRevLett.77.3865
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    Generalized gradient approximation made simple
    Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
    Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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    Pauling, L.; Hoard, J. XXXVII. The Crystal Structure of Cadmium Chloride Z. Kristallogr. - Cryst. Mater. 1930, 74, 546– 551 DOI: 10.1524/zkri.1930.74.1.546
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    The crystal structure of cadmium chloride
    Pauling, Linus; Hoard, J. L.
    Zeitschrift fuer Kristallographie, Kristallgeometrie, Kristallphysik, Kristallchemie (1930), 74 (), 546-51CODEN: ZKKKAJ; ISSN:0044-2968.
    The unit of structure for CdCl2 is a rhombohedron with α = 36°02' and a = 6.23 A. U. contg. 1 mol. There is a layer structure along [0001], closely related to that of Cdl2. The Cl atoms are in approximate cubic closepacking. The relation of the CdCl2 and the Cdl2 structures is discussed, and a list of similar compds. which crystallize in each type is given.
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    Bordas, J.; Robertson, J.; Jakobsson, A. Ultraviolet Properties and Band Structure of SnS2, SnSe2, CdI2, PbI2, BiI3 and BiOI Crystals J. Phys. C: Solid State Phys. 1978, 11, 2607 DOI: 10.1088/0022-3719/11/12/021
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    Ultraviolet properties and band structure of tin disulfide, tin diselenide, cadmium diiodide, lead diiodide, bismuth triiodide, and bismuth iodide oxide crystals
    Bordas, J.; Robertson, J.; Jakobsson, A.
    Journal of Physics C: Solid State Physics (1978), 11 (12), 2607-21CODEN: JPSOAW; ISSN:0022-3719.
    The UV reflectivity spectra of the layer materials SnS2, SnSe2, CdI2, PbI2, BiI3, and BiOI were detd. at 8-40 eV. The band structures for SnS2, CdI2, and PbI2 were calcd. using an LCAO approach and the partial and total d. of states for the valence and conduction bands were compared with the optical consts. detd. from Kramers-Kronig anal. of the exptl. data. The obsd. optical transitions from the metal d-core states were discussed in terms of an at.-like picture.
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    Nanoscale (2016), 8 (12), 6317-6327CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    The optical consts. of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calcns. using the relativistic quasiparticle self-consistent GW approxn. (QSGW). Good agreement between the optical consts. derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their resp. inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calcns. indicate that the orientation of [CH3NH3]+ cations has a significant influence on the position of the bandgap suggesting that collective orientation of the org. moieties could result in significant local variations of the optical properties. The optical consts. and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS).
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    The authors report on the temp. dependence of thermal cond. of single cryst. and polycryst. organometallic perovskite CH3NH3PbI3. The comparable abs. values and temp. dependence of the two samples' morphologies indicate the minor role of the grain boundaries on the heat transport. Theor. modeling demonstrates the importance of the resonant scattering in both specimens. The interaction between phonon waves and rotational degrees of freedom of CH3NH3+ sublattice emerges as the dominant mechanism for attenuation of heat transport and for ultralow thermal cond. of 0.5 W/(Km) at room temp.
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    The hybrid halide perovskites, the very performant compds. in photovoltaic applications, possess large Seebeck coeff. and low thermal cond., making them potentially interesting high figure of merit (ZT) materials. For this purpose, one needs to tune the elec. cond. of these semiconductors to higher values. The authors have studied the CH3NH3MI3 (M = Pb,Sn) samples in pristine form showing very low ZT values for both materials; however, photoinduced doping (in M = Pb) and chem. doping (in M = Sn) indicate that, by further doping optimization, ZT can be enhanced toward unity and reach the performance level of the presently most efficient thermoelec. materials.
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    Journal of Physical Chemistry C (2014), 118 (6), 3244-3250CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Undoped and Sb-doped BiI3 (SBI) single crystals are grown via the vertical Bridgman growth technique. Elec. properties, such as resistivity and leakage current, in addn. to radiation response measurements were performed on both BiI3 and SBI single crystal detectors. The resistivity of SBI (2.63109 Ω cm) increases by an order of magnitude compared to that of BiI3 (1.45 × 108 Ω cm). Also, leakage currents of SBI (10-2 μA/cm2) decrease by four orders magnitude relative to BiI3. The radiation response of the SBI indicates that less polarization exists under bias for prolonged periods of time, making SBI a promising material for use in gamma-ray detector applications. D. functional theory (DFT) calcns. predict that Sb forms strong covalent bonds with neighboring iodine ions and that the Sb-I dimer can be formed when Sb is doped into the BiI3 lattice. Defect modeling verifies that substitution of Bi ions with Sb and incorporation of Sb in iodine vacancy sites can effectively decrease the formation and migration of iodine vacancies, which significantly improves radiation detection performance of the material.
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    Noting that BiI3 and the known topol. insulator (TI) Bi2Se3 have the same high symmetry parent structures, and that it is desirable to find a wide-band gap TI, the authors det. here the effects of pressure on the structure, phonons and electronic properties of rhombohedral BiI3. The authors report a pressure-induced insulator-metal transition near 1.5 GPa, using high pressure elec. resistivity and Raman measurements. X-ray diffraction studies, as a function of pressure, reveal a structural peculiarity of the BiI3 crystal, with a drastic drop in c/a ratio at 1.5 GPa, and a structural phase transition from rhombohedral to monoclinic structure at 8.8 GPa. The metallic phase, at relatively low pressures, exhibits minimal resistivity at low temps., similar to that in Bi2Se3. The authors corroborate these findings with 1st-principles calcns. and suggest that the drop in the resistivity of BiI3 in the 1-3 GPa range of pressure arises possibly from the appearance of an intermediate crystal phase with a lower band-gap and hexagonal crystal structure. Calcd. Born effective charges reveal metallic states in the structural vicinity of rhombohedral BiI3. Changes in the topol. of the electronic bands of BiI3 with pressure, and a sharp decrease in the c/a ratio <2 GPa, give rise to changes in the slope of phonon frequencies near that pressure.
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    Perovskite and dye-sensitized solar cells are PV technologies which hold promise for PV application. Arguably, the biggest issue facing these technologies is stability. The vast majority of studies have been limited to small area lab. cells. Moisture, oxygen, UV light, thermal and elec. stresses are leading the degrdn. causes. There remains a shortage of stability investigations on large area devices, in particular modules. At the module level there exist particular challenges which can be different from those at the small cell level such as encapsulation (not only of the unit cells but of interconnections and contacts), non-uniformity of the layer stacks and unit cells, reverse bias stresses, which are important to investigate for technologies that aim for industrial acceptance. Herein we present a review of stability investigations published in the literature pertaining large area perovskite and dye-sensitized solar devices fabricated both on rigid (glass) and flexible substrates.
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    ISODISPLACE is a new internet-server tool for exploring structural phase transitions. Given parent-phase structural information, it generates at. displacement patterns induced by irreducible representations of the parent space-group symmetry and allows a user to visualize and manipulate the amplitude of each distortion mode interactively. ISODISPLACE is freely accessible at http://stokes.byu.edu/isodisplace.html via common internet browsers.
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    (TaSe4)2I is a quasi-one-dimensional (1D) elec. conductor. It exhibits a phase transition at TCDW = 263 K towards a charge-d.-wave (CDW) state at low temps. We report a full structure refinement of the incommensurately modulated structure in the CDW state at T = 110 K against synchrotron radiation, single-crystal x-ray diffraction data. At room temp. the crystal structure has tetragonal symmetry with space group I422. In the CDW state each main reflection in the x-ray scattering is surrounded by eight incommensurate satellites at (±0.064, ±0.064, ±0.151). The CDW state is found to comprise four domains, and it is characterized by one modulation wavevector. With respect to a 2 × 2 × 1 supercell it has the symmetry of the superspace group F2(0, β, γ) with β = 0.128 and γ = 0.151. The first part of the modulation is found to be a transverse acoustic wave, involving amplitudes of similar magnitudes of about 0.13 Å on all atoms. The second part of the modulation involves displacements of the Ta atoms of about 0.03 Å, that are parallel to the 1D chains. These are interpreted as reflecting the CDW. A Landau free-energy model is developed, that shows that symmetry arguments allow the phase transition to be second order.
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    Bismuth tri-iodide is a wide band-gap semiconductor material that may be able to operate as a radiation detector without any cooling mechanism. This material has a higher effective at. no. than germanium and CdZnTe, and thus should have a higher gamma-ray detection efficiency, particularly for moderate and high energy gamma-rays. Unfortunately, not much is known about bismuth tri-iodide, and the general properties of the material need to be investigated. Bismuth tri-iodide does not suffer from some of the material issues, such as a solid state phase transition and dissocn. in air, that mercuric iodide (another high-Z, wide band-gap semiconductor) does. Thus, bismuth tri-iodide is both easier to grow and handle than mercuric iodide. A modified vertical Bridgman growth technique is being used to grow large, single bismuth tri-iodide crystals. Zone refining is being performed to purify the starting material and increase the resistivity of the crystals. The single crystals being grown are typically several hundred mm3. The larger crystals grown are approx. 2 cm3. Initial detectors are being fabricated using both gold and palladium electrodes and palladium wire. The electron mobility measured using an alpha source was detd. to be 260±50 cm2/Vs. An alpha spectrum was recorded with one of the devices; however the detector appears to suffer from polarization.
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  • Abstract

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    Figure 1

    Figure 1. (a) Quarter of the Bi–Ag–I phase diagram showing SEM EDX compositional measurements of Ag1–3xBi1+xI4 powder (black), polycrystalline film (green), plate crystal with CdCl2 structure (red), and octahedral-faceted crystal (blue) on the Ag1–3xBi1+xI4 charge balance line with previously reported compositions (orange). (b) Dashed zone enlarged around the AgBiI4x = 0 point showing additional octahedral-faceted (blue) and plate (red) crystal measurements. Also shown is the nominal x = 0.07 composition used for sealed tube synthesis (orange). Circles represent average compositions, and hashed areas represent the 1σ statistical spread. Areas labeled A and B are the plate and octahedral-faceted crystals used for the structural study (Section 3.2). Areas labeled 1 and 2 correspond to the polycrystalline films used for absorption coefficient measurements (Section 3.3).

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    Figure 2

    Figure 2. PXRD and Rietveld refinements of AgBiI4 synchrotron data for (a) CdCl2-type and (b) defect-spinel structures, with the structures inset. Tick marks in (b) show the absence of expected Bragg intensity due to the increased cell size of the cubic cell.

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    Figure 3

    Figure 3. (a) Reported crystal structure of BiI3, (51) showing its layered structure in which layers of edge-sharing octahedra are occupied by 2/3 Bi3+ cations and 1/3 vacancies and alternate with entirely vacant layers. The iodide packing is hexagonally close-packed (ABA). (b) Cubic defect-spinel structure of AgBiI4 is shown in its trigonal setting, for comparison to BiI3 and the CdCl2 structure. The interstitial octahedral sites are occupied by mixed silver and bismuth in alternating layers of 3/4 and 1/4 occupancy. (c) CdCl2-type structure of AgBiI4 with doubled a and b directions showing layered structure with alternating fully occupied and entirely vacant layers. In both (b) and (c) the iodide packing is cubic close-packed (ABC). The dimensions of a CdCl2 single cell are shown in blue in all panels to show the relationship between the different unit cells.

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    Figure 4

    Figure 4. Reconstructed SCXRD pattern from the ⟨011̅⟩ plane of the Fd3̅m cubic cell. Indexing this pattern to the cubic Fd3̅m cell reveals several allowed Bragg peaks with zero intensity (e.g., the expected [220] reflection is indicated by a green circle and arrow). Alternatively, the pattern can be indexed to two rhombohedral R3̅m twins (red and blue circles), with the [001] axis of the rhombohedral cell aligned along the [111] axis of the cubic cell. Uncircled peaks arise from the rock salt sublattice and are common to all twins. This solution has no zero-intensity allowed Bragg peaks. Note that while two rhombohedral R3̅m twins are required to fit this particular region of reciprocal space, by extension four rhombohedral R3̅m twins are required to fit the full data set.

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    Figure 5

    Figure 5. (a) UV–visible spectroscopy optical absorbance data for sealed tube synthesized powder (red), polycrystalline films 1 (blue) and 2 (green), and BiI3 powder (black) plotted against the AM1.5 solar spectrum (gray). (69) (b) Tauc plot for indirect band gaps calculated using the Kubelka–Munk function F(R) obtained via diffuse reflectance measurements for the sealed tube synthesized powder (red), polycrystalline films 1 (blue) and film 2 (green), and BiI3 powder (black). (c) Absorption coefficient of films 1 (blue) and 2 (green). The shaded areas in (c) indicate the error limits, derived from the standard deviations of the measured film thicknesses.

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    Figure 6

    Figure 6. (a) XPS spectra of the valence band of pellets of AgBiI4 (red) and BiI3 (black) showing the main Ag 4d contribution is to the bottom of the valence band. DOS calculations for the lowest energy defect-spinel and CdCl2 structures are shown in (b) and (c), respectively.

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    Figure 7

    Figure 7. (a) Seebeck coefficient measured from 210 to 300 K on bulk AgBiI4. (b) Resistivity of bulk AgBiI4 measured from 190 to 300 K.

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    Figure 8

    Figure 8. Halide (green) connectivity (red) is shown for AgBiI4 and the bismuth halide semiconductors with previously reported optical gaps. Bismuth is shown in purple and cations in yellow. BiI3 shows uninterrupted hexagonal close packed iodide sublattice, and both the CdCl2 and defect-spinel structures of AgBiI4 show uninterrupted cubic close packed iodide sublattices. In the double perovskites Cs2AgBiX6 (X = Br, Cl), one-fourth of the anions in the close packed iodide sublattice are replaced by large Cs+ cations, in the perovskite-type structure. The same level of cation substitution is observed in hexagonal iodide layers within (CH3NH3)BiI4, (NH4)3Bi2I9, A3Bi2I9 (A = K, Rb, Cs), and (CH3NH3)2KBiCl6. All compounds with structures where cations replace I– anions within the hexagonal layers have optical gaps above 1.9 eV, (26-30, 32) where those with pure iodide layers have optical gaps of 1.6–1.8 eV.

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  • References

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    This article references 69 other publications.

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      Aristidou, Nicholas; Sanchez-Molina, Irene; Chotchuangchutchaval, Thana; Brown, Michael; Martinez, Luis; Rath, Thomas; Haque, Saif A.
      Angewandte Chemie, International Edition (2015), 54 (28), 8208-8212CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In this paper we report on the influence of light and oxygen on the stability of CH3NH3PbI3 perovskite-based photoactive layers. When exposed to both light and dry air the mp-Al2O3/CH3NH3PbI3 photoactive layers rapidly decomp. yielding methylamine, PbI2, and I2 as products. We show that this degrdn. is initiated by the reaction of superoxide (O2-) with the methylammonium moiety of the perovskite absorber. Fluorescent mol. probe studies indicate that the O2- species is generated by the reaction of photoexcited electrons in the perovskite and mol. oxygen. We show that the yield of O2- generation is significantly reduced when the mp-Al2O3 film is replaced with an mp-TiO2 electron extn. and transport layer. The present findings suggest that replacing the methylammonium component in CH3NH3PbI3 to a species without acid protons could improve tolerance to oxygen and enhance stability.
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    13. 13
      Panda, P. K. Review: Environmental Friendly Lead-Free Piezoelectric Materials J. Mater. Sci. 2009, 44, 5049– 5062 DOI: 10.1007/s10853-009-3643-0
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      13
      Review: environmental friendly lead-free piezoelectric materials
      Panda, P. K.
      Journal of Materials Science (2009), 44 (19), 5049-5062CODEN: JMTSAS; ISSN:0022-2461. (Springer)
      A review. Lead zirconate titanate (PZT)-based piezoelec. materials are well known for their excellent piezoelec. properties. However, considering the toxicity of lead and its compds., there is a general awareness for the development of environmental friendly lead-free materials as evidenced from the legislation passed by the European Union to this effect. Several classes of materials are now being considered as potentially attractive alternatives to PZTs for specific applications. In this paper, attempts have been made to review the recent developments on lead-free piezo materials emphasizing on their prepn., structure-property correlation, etc. In this context, perovskite systems such as bismuth sodium titanate, alkali niobates (ANbO3), etc. and non-perovskites such as bismuth layer-structured ferroelecs. are reviewed in detail. From the above study, it is concluded that some lead-free compns. show stable piezoelec. responses even though they do not match the overall performance of PZT. This has been the stimulant for growing research on this subject. This topic is of current interest to the researchers worldwide as evidenced from the large no. of research publications. This has motivated us to come out with a review article with a view that it would give further impetus to the researchers already working in this area and also draw the attention of others.
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    14. 14
      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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      14
      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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    15. 15
      Koh, T. M.; Fu, K.; Fang, Y.; Chen, S.; Sum, T. C.; Mathews, N.; Mhaisalkar, S. G.; Boix, P. P.; Baikie, T. Formamidinium-Containing Metal-Halide: An Alternative Material for Near-IR Absorption Perovskite Solar Cells J. Phys. Chem. C 2014, 118, 16458– 16462 DOI: 10.1021/jp411112k
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      15
      Formamidinium-Containing Metal-Halide: An Alternative Material for Near-IR Absorption Perovskite Solar Cells
      Koh, Teck Ming; Fu, Kunwu; Fang, Yanan; Chen, Shi; Sum, T. C.; Mathews, Nripan; Mhaisalkar, Subodh G.; Boix, Pablo P.; Baikie, Tom
      Journal of Physical Chemistry C (2014), 118 (30), 16458-16462CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Solid-state, soln. processed solar-cells based on org.-inorg. Me ammonium lead halide absorbers have achieved efficiencies >15%, which has superseded liq. dye sensitized cells, as well as various thin film-based photovoltaics. This report introduces a new metal-halide perovskite, based on the formamidinium cation (HC-(NH2)2+), that displays a favorable band gap (1.47 eV) and represents a broader absorption compared to previously reported absorbers that contained the methylammonium cation (MeNH3+). The high open-circuit voltage (Voc = 0.97 V) and promising fill-factor (FF = 68.7%) yield an efficiency of 4.3%, which make this material an excellent candidate for this new class of perovskite solar cell. This report also studies the formation of a black trigonal (P3m1) perovskite polymorph and a yellow hexagonal non-perovskite (P63mc) polymorph. Further solar cell development would entail the stabilization of the black trigonal (P3m1) perovskite polymorph over the yellow hexagonal nonperovskite (P63mc) polymorph.
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    16. 16
      Eperon, G. E.; Stranks, S. D.; Menelaou, C.; Johnston, M. B.; Herz, L. M.; Snaith, H. J. Formamidinium Lead Trihalide: a Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar Cells Energy Environ. Sci. 2014, 7, 982– 988 DOI: 10.1039/c3ee43822h
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      16
      Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells
      Eperon, Giles E.; Stranks, Samuel D.; Menelaou, Christopher; Johnston, Michael B.; Herz, Laura M.; Snaith, Henry J.
      Energy & Environmental Science (2014), 7 (3), 982-988CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      Perovskite-based solar cells have attracted significant recent interest, with power conversion efficiencies in excess of 15% already superceding a no. of established thin-film solar cell technologies. Most work has focused on a methylammonium lead trihalide perovskites, with a bandgaps of ∼1.55 eV and greater. Here, we explore the effect of replacing the methylammonium cation in this perovskite, and show that with the slightly larger formamidinium cation, we can synthesize formamidinium lead trihalide perovskites with a bandgap tunable between 1.48 and 2.23 eV. We take the 1.48 eV-bandgap perovskite as most suited for single junction solar cells, and demonstrate long-range electron and hole diffusion lengths in this material, making it suitable for planar heterojunction solar cells. We fabricate such devices, and due to the reduced bandgap we achieve high short-circuit currents of >23 mA cm-2, resulting in power conversion efficiencies of up to 14.2%, the highest efficiency yet for soln. processed planar heterojunction perovskite solar cells. Formamidinium lead triiodide is hence promising as a new candidate for this class of solar cell.
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    17. 17
      Kulbak, M.; Cahen, D.; Hodes, G. How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr3 Cells J. Phys. Chem. Lett. 2015, 6, 2452– 2456 DOI: 10.1021/acs.jpclett.5b00968
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      17
      How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr3 Cells
      Kulbak, Michael; Cahen, David; Hodes, Gary
      Journal of Physical Chemistry Letters (2015), 6 (13), 2452-2456CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Hybrid org.-inorg. lead halide perovskite photovoltaic cells have already surpassed 20% conversion efficiency in the few years that they have been seriously studied. However, many fundamental questions still remain unanswered as to why they are so good. One of these is "Is the org. cation really necessary to obtain high quality cells" In this study, we show that an all-inorg. version of the lead bromide perovskite material works equally well as the org. one, in particular generating the high open circuit voltages that are an important feature of these cells.
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    18. 18
      Noel, N. K.; Stranks, S. D.; Abate, A.; Wehrenfennig, C.; Guarnera, S.; Haghighirad, A.-A.; Sadhanala, A.; Eperon, G. E.; Pathak, S. K.; Johnston, M. B.; Petrozza, A.; Herz, L. M.; Snaith, H. J. Lead-Free Organic-Inorganic Tin Halide Perovskites for Photovoltaic Applications Energy Environ. Sci. 2014, 7, 3061– 3068 DOI: 10.1039/C4EE01076K
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      18
      Lead-free organic-inorganic tin halide perovskites for photovoltaic applications
      Noel, Nakita K.; Stranks, Samuel D.; Abate, Antonio; Wehrenfennig, Christian; Guarnera, Simone; Haghighirad, Amir-Abbas; Sadhanala, Aditya; Eperon, Giles E.; Pathak, Sandeep K.; Johnston, Michael B.; Petrozza, Annamaria; Herz, Laura M.; Snaith, Henry J.
      Energy & Environmental Science (2014), 7 (9), 3061-3068CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      Already exhibiting solar to elec. power conversion efficiencies of over 17%, org.-inorg. lead halide perovskite solar cells are one of the most promising emerging contenders in the drive to provide a cheap and clean source of energy. One concern however, is the potential toxicol. issue of lead, a key component in the archetypical material. The most likely substitute is tin, which like lead, is also a group 14 metal. While org.-inorg. tin halide perovskites have shown good semiconducting behavior, the instability of tin in its 2+ oxidn. state has thus far proved to be an overwhelming challenge. Here, we report the first completely lead-free, CH3NH3SnI3 perovskite solar cell processed on a mesoporous TiO2 scaffold, reaching efficiencies of over 6% under 1 sun illumination. Remarkably, we achieve open circuit voltages over 0.88 V from a material which has a 1.23 eV band gap.
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    19. 19
      Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.; Mhaisalkar, S. G.; Graetzel, M.; White, T. J. Synthesis and Crystal Chemistry of the Hybrid Perovskite (CH3NH3)PbI3 for Solid-State Sensitised Solar Cell Applications J. Mater. Chem. A 2013, 1, 5628– 5641 DOI: 10.1039/c3ta10518k
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      19
      Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3)PbI3 for solid-state sensitised solar cell applications
      Baikie, Tom; Fang, Yanan; Kadro, Jeannette M.; Schreyer, Martin; Wei, Fengxia; Mhaisalkar, Subodh G.; Graetzel, Michael; White, Tim J.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2013), 1 (18), 5628-5641CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      The hybrid org.-inorg. perovskite (CH3NH3)PbI3 may find application in next generation solid-state sensitized solar cells. Although this material and related perovskites were discovered many decades ago, questions remain concerning their diverse structural chem. and unusual properties. The article presents a review of previous work and provides a detailed description of the prepn., structural characterization and phys. characteristics of (CH3NH3)PbI3. The phase changes exhibited by (CH3NH3)PbI3 have been probed using variable temp. powder and single crystal x-ray diffraction, combined with differential scanning calorimetry, thermogravimetric anal. and phase contrast transmission electron microscopy. The optical band gap for (CH3NH3)PbI3 detd. by UV-visible spectroscopy was compared to values obtained from d.-of-state simulation of the electronic band structure.
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    20. 20
      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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      20
      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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    21. 21
      Clark, S. J.; Donaldson, J. D.; Harvey, J. A. Evidence for the Direct Population of Solid-State Bands by Non-Bonding Electron Pairs in Compounds of the Type CsMX3(M= Ge, Sn, Pb; X = Cl, Br, I) J. Mater. Chem. 1995, 5, 1813– 1818 DOI: 10.1039/jm9950501813
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    22. 22
      Aharon, S.; Cohen, B. E.; Etgar, L. Hybrid Lead Halide Iodide and Lead Halide Bromide in Efficient Hole Conductor Free Perovskite Solar Cell J. Phys. Chem. C 2014, 118, 17160– 17165 DOI: 10.1021/jp5023407
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      22
      Hybrid Lead Halide Iodide and Lead Halide Bromide in Efficient Hole Conductor Free Perovskite Solar Cell
      Aharon, Sigalit; Cohen, Bat El; Etgar, Lioz
      Journal of Physical Chemistry C (2014), 118 (30), 17160-17165CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      In this work we used CH3NH3PbInBr3-n (where 0 ≤ n ≤ 3) as hole conductor and light harvester is used in the solar cell. Various concns. of methylammonium iodide and methylammonium bromide were studied which reveal that any compn. of the hybrid CH3NH3PbInBr3-n can conduct holes. The hybrid perovskite was deposited in two steps, sepg. it to two precursors to allow better control of the perovskite compn. and efficient tuning of its band gap. The X-ray diffraction reveals the change in the lattice parameter due to the introduction of the Br- ions. The hybrid iodide/bromide perovskite hole conductor free solar cells show very good stability, their power conversion efficiency achieved 8.54% under 1 sun illumination with c.d. of 16.2 mA/cm2. The results of this work open the possibility for graded structure of perovskite solar cells without the need for hole conductor.
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    23. 23
      Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colorful, Efficient, and Stable Inorganic–Organic Hybrid Nanostructured Solar Cells Nano Lett. 2013, 13, 1764– 1769 DOI: 10.1021/nl400349b
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      23
      Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells
      Noh, Jun Hong; Im, Sang Hyuk; Heo, Jin Hyuck; Mandal, Tarak N.; Seok, Sang Il
      Nano Letters (2013), 13 (4), 1764-1769CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Chem. tuned inorg.-org. hybrid materials, based on MeNH3(=MA)Pb(I1-xBrx)3 perovskites, have been studied using UV-visible absorption and x-ray diffraction patterns and applied to nanostructured solar cells. The band gap engineering brought about by the chem. management of MAPb(I1-xBrx)3 perovskites can be controllably tuned to cover almost the entire visible spectrum, enabling the realization of colorful solar cells. The authors demonstrate highly efficient solar cells exhibiting 12.3% in a power conversion efficiency of under std. AM 1.5, for the most efficient device, as a result of tunable compn. for the light harvester in conjunction with a mesoporous TiO2 film and a hole conducting polymer. Probably the works highlighted in this paper represent one step toward the realization of low-cost, high-efficiency, and long-term stability with colorful solar cells.
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    24. 24
      Kitazawa, N.; Watanabe, Y.; Nakamura, Y. Optical Properties of CH3NH3PbX3 (X = Halogen) and their Mixed-Halide Crystals J. Mater. Sci. 2002, 37, 3585– 3587 DOI: 10.1023/A:1016584519829
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      24
      Optical properties of CH3NH3PbX3 (X = halogen) and their mixed-halide crystals
      Kitazawa, N.; Watanabe, Y.; Nakamura, Y.
      Journal of Materials Science (2002), 37 (17), 3585-3587CODEN: JMTSAS; ISSN:0022-2461. (Kluwer Academic Publishers)
      Thin films of microcryst. CH3NH3PbX3 (X = halogen) as well as their mixed-halide crystals were fabricated by the spin-coating technique, and their optical properties were investigated. X-ray diffraction investigation revealed that CH3NH3PbBr3-xClx (x = 0-3) were successfully formed on glass substrate self-assembly and oriented with the a-axis. Owing to due to their large exciton binding energy, these materials showed clear exciton absorption and free-exciton emission in the visible region at room temp. Replacing Br with Cl made it possible to control the band structure of these materials. As a result, the peak position of the exciton band shifted continuously towards blue region with increasing the Cl content in the films.
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    25. 25
      Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut Nano Lett. 2015, 15, 3692– 3696 DOI: 10.1021/nl5048779
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      25
      Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut
      Protesescu, Loredana; Yakunin, Sergii; Bodnarchuk, Maryna I.; Krieg, Franziska; Caputo, Riccarda; Hendon, Christopher H.; Yang, Ruo Xi; Walsh, Aron; Kovalenko, Maksym V.
      Nano Letters (2015), 15 (6), 3692-3696CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Metal halides perovskites, such as hybrid org.-inorg. MeNH3PbI3, are newcomer optoelectronic materials that have attracted enormous attention as soln.-deposited absorbing layers in solar cells with power conversion efficiencies reaching 20%. A new avenue for halide perovskites was demonstrated by designing highly luminescent perovskite-based colloidal quantum dot materials. Monodisperse colloidal nanocubes (4-15 nm edge lengths) of fully inorg. perovskites (CsPbX3, X = Cl, Br, and I or mixed halide systems Cl/Br and Br/I) were synthesized using inexpensive com. precursors. Through compositional modulations and quantum size-effects, the bandgap energies and emission spectra are readily tunable over the entire visible spectral region of 410-700 nm. The luminescence of CsPbX3 nanocrystals is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color std., high quantum yields of ≤90%, and radiative lifetimes at 1-29 ns. The compelling combination of enhanced optical properties and chem. robustness makes CsPbX3 nanocrystals appealing for optoelectronic applications, particularly for blue and green spectral regions (410-530 nm), where typical metal chalcogenide-based quantum dots suffer from photodegrdn.
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    26. 26
      Wei, F.; Deng, Z.; Sun, S.; Xie, F.; Kieslich, G.; Evans, D. M.; Carpenter, M. A.; Bristowe, P. D.; Cheetham, A. K. The Synthesis, Structure and Electronic Properties of a Lead-Free Hybrid Inorganic-Organic Double Perovskite (MA)2KBiCl6 (MA = Methylammonium) Mater. Horiz. 2016, 3, 328– 332 DOI: 10.1039/C6MH00053C
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      26
      The synthesis, structure and electronic properties of a lead-free hybrid inorganic-organic double perovskite (MA)2KBiCl6 (MA = methylammonium)
      Wei, Fengxia; Deng, Zeyu; Sun, Shijing; Xie, Fei; Kieslich, Gregor; Evans, Donald M.; Carpenter, Michael A.; Bristowe, Paul D.; Cheetham, Anthony K.
      Materials Horizons (2016), 3 (4), 328-332CODEN: MHAOBM; ISSN:2051-6355. (Royal Society of Chemistry)
      In a search for lead-free materials that could be used as alternatives to the hybrid perovskites, (MA)PbX3, in photovoltaic applications, we have discovered a hybrid double perovskite, (MA)2KBiCl6, which shows strong similarities to the lead analogs. Spectroscopic measurements and nanoindentation studies are combined with d. functional calcns. to reveal the properties of this interesting system.
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    27. 27
      Hoye, R. L. Z.; Brandt, R. E.; Osherov, A.; Stevanović, V.; Stranks, S. D.; Wilson, M. W. B.; Kim, H.; Akey, A. J.; Perkins, J. D.; Kurchin, R. C.; Poindexter, J. R.; Wang, E. N.; Bawendi, M. G.; Bulović, V.; Buonassisi, T. Methylammonium Bismuth Iodide as a Lead-Free, Stable Hybrid Organic–Inorganic Solar Absorber Chem. - Eur. J. 2016, 22, 2605– 2610 DOI: 10.1002/chem.201505055
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      27
      Methylammonium Bismuth Iodide as a Lead-Free, Stable Hybrid Organic-Inorganic Solar Absorber
      Hoye, Robert L. Z.; Brandt, Riley E.; Osherov, Anna; Stevanovic, Vladan; Stranks, Samuel D.; Wilson, Mark W. B.; Kim, Hyunho; Akey, Austin J.; Perkins, John D.; Kurchin, Rachel C.; Poindexter, Jeremy R.; Wang, Evelyn N.; Bawendi, Moungi G.; Bulovic, Vladimir; Buonassisi, Tonio
      Chemistry - A European Journal (2016), 22 (8), 2605-2610CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Methylammonium lead halide (MAPbX3) perovskites exhibit exceptional carrier transport properties. But their com. deployment as solar absorbers is currently limited by their intrinsic instability in the presence of humidity and their lead content. Guided by our theor. predictions, we explored the potential of methylammonium bismuth iodide (MBI) as a solar absorber through detailed materials characterization. We synthesized phase-pure MBI by soln. and vapor processing. In contrast to MAPbX3, MBI is air stable, forming a surface layer that does not increase the recombination rate. We found that MBI luminesces at room temp., with the vapor-processed films exhibiting superior photoluminescence (PL) decay times that are promising for photovoltaic applications. The thermodn., electronic, and structural features of MBI that are amenable to these properties are also present in other hybrid ternary bismuth halide compds. Through MBI, we demonstrate a lead-free and stable alternative to MAPbX3 that has a similar electronic structure and nanosecond lifetimes.
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    28. 28
      Sun, S.; Tominaka, S.; Lee, J.-H.; Xie, F.; Bristowe, P. D.; Cheetham, A. K. Synthesis, Crystal Structure, and Properties of a Perovskite-Related Bismuth Phase, (NH4)3Bi2I9 APL Mater. 2016, 4, 031101 DOI: 10.1063/1.4943680
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      28
      Synthesis, crystal structure, and properties of a perovskite-related bismuth phase, (NH4)3Bi2I9
      Sun, Shijing; Tominaka, Satoshi; Lee, Jung-Hoon; Xie, Fei; Bristowe, Paul D.; Cheetham, Anthony K.
      APL Materials (2016), 4 (3), 031101/1-031101/7CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
      Org.-inorg. halide perovskites, esp. methylammonium lead halide, have recently led to remarkable advances in photovoltaic devices. However, due to environmental and stability concerns around the use of lead, research into lead-free perovskite structures has been attracting increasing attention. In this study, a layered perovskite-like architecture, (NH4)3Bi2I9, is prepd. from soln. and the structure solved by single crystal X-ray diffraction. The band gap, which is estd. to be 2.04 eV using UV-visible spectroscopy, is lower than that of CH3NH3PbBr3. The energy-minimized structure obtained from first principles calcns. is in excellent agreement with the X-ray results and establishes the locations of the hydrogen atoms. The calcns. also point to a significant lone pair effect on the bismuth ion. Single crystal and powder cond. measurements are performed to examine the potential application of (NH4)3Bi2I9 as an alternative to the lead contg. perovskites. (c) 2016 American Institute of Physics.
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    29. 29
      Lehner, A. J.; Fabini, D. H.; Evans, H. A.; Hébert, C.-A.; Smock, S. R.; Hu, J.; Wang, H.; Zwanziger, J. W.; Chabinyc, M. L.; Seshadri, R. Crystal and Electronic Structures of Complex Bismuth Iodides A3Bi2I9 (A = K, Rb, Cs) Related to Perovskite: Aiding the Rational Design of Photovoltaics Chem. Mater. 2015, 27, 7137– 7148 DOI: 10.1021/acs.chemmater.5b03147
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      29
      Crystal and Electronic Structures of Complex Bismuth Iodides A3Bi2I9 (A = K, Rb, Cs) Related to Perovskite: Aiding the Rational Design of Photovoltaics
      Lehner, Anna J.; Fabini, Douglas H.; Evans, Hayden A.; Hebert, Claire-Alice; Smock, Sara R.; Hu, Jerry; Wang, Hengbin; Zwanziger, Josef W.; Chabinyc, Michael L.; Seshadri, Ram
      Chemistry of Materials (2015), 27 (20), 7137-7148CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Ternary bismuth halides form an interesting functional materials class in the context of the closely related Pb halide perovskite photovoltaics, esp. given the significantly reduced toxicity of Bi when compared with Pb. The compds. A3Bi2I9 (A = K, Rb, Cs) examd. here crystallize in two different structure types: the layered defect-perovskite K3Bi2I9 type, and the Cs3Cr2Cl9 type. The latter structure type features isolated Bi2I93- anions. Here, the crystal structures of the ternary iodides are redetd. and a cor. structural model for Rb3Bi2I9, as established by single crystal x-ray diffraction and solid state 87Rb NMR spectroscopy and supported by d. functional theory (DFT) calcns. is presented. A variety of facile prepn. techniques for single crystals, bulk materials, as well as soln.-processed thin films are described. The optical properties and electronic structures were studied exptl. by optical absorption and UPS and computationally by DFT calcns. Abs. band positions of the valence and conduction bands of these semiconductors, with excellent agreement of exptl. and calcd. values, are reported, constituting a useful input for the rational interface design of efficient electronic and optoelectronic devices. The different structural connectivity in the two different structure types, somewhat surprisingly, appears to not impact band positions or band gaps in a significant manner. Computed dielec. properties, including the finding of anomalously large Born effective charge tensors on Bi3+, suggest proximal structural instabilities arising from the Bi3+ 6s2 lone pair. These anomalous Born effective charges are promising for defect screening and effective charge carrier transport. The structural, electronic, and optical properties of the complex bismuth iodides are to some extent similar to the related lead iodide perovskites. The deeper valence band positions in the complex bismuth iodides point to the need for different choices of hole transport materials for Bi-iodide based solar cell architectures.
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    30. 30
      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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      30
      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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    31. 31
      Lehner, A. J.; Wang, H.; Fabini, D. H.; Liman, C. D.; Hébert, C.-A.; Perry, E. E.; Wang, M.; Bazan, G. C.; Chabinyc, M. L.; Seshadri, R. Electronic Structure and Photovoltaic Application of BiI3 Appl. Phys. Lett. 2015, 107, 131109 DOI: 10.1063/1.4932129
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      31
      Electronic structure and photovoltaic application of BiI3
      Lehner, Anna J.; Wang, Hengbin; Fabini, Douglas H.; Liman, Christopher D.; Hebert, Claire-Alice; Perry, Erin E.; Wang, Ming; Bazan, Guillermo C.; Chabinyc, Michael L.; Seshadri, Ram
      Applied Physics Letters (2015), 107 (13), 131109/1-131109/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Rapid recent improvement in photovoltaic efficiency in hybrid lead halide perovskite materials provided the impetus for understanding other, related main-group halide systems. Here, the closely related but less toxic BiI3 can show promising optoelectronic properties. Layered binary BiI3 is used here as the active layer in planar solar cell architectures (efficiency ∼0.3%). Exptl. and computational studies of abs. band positions of BiI3 are also presented, to help in the rational design of device architectures that would allow efficient charge transfer at the interfaces. (c) 2015 American Institute of Physics.
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    32. 32
      Brandt, R. E.; Kurchin, R. C.; Hoye, R. L. Z.; Poindexter, J. R.; Wilson, M. W. B.; Sulekar, S.; Lenahan, F.; Yen, P. X. T.; Stevanović, V.; Nino, J. C.; Bawendi, M. G.; Buonassisi, T. Investigation of Bismuth Triiodide (BiI3) for Photovoltaic Applications J. Phys. Chem. Lett. 2015, 6, 4297– 4302 DOI: 10.1021/acs.jpclett.5b02022
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      32
      Investigation of Bismuth Triiodide (BiI3) for Photovoltaic Applications
      Brandt, Riley E.; Kurchin, Rachel C.; Hoye, Robert L. Z.; Poindexter, Jeremy R.; Wilson, Mark W. B.; Sulekar, Soumitra; Lenahan, Frances; Yen, Patricia X. T.; Stevanovic, Vladan; Nino, Juan C.; Bawendi, Moungi G.; Buonassisi, Tonio
      Journal of Physical Chemistry Letters (2015), 6 (21), 4297-4302CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Guided by predictive discovery framework, the authors study bismuth triiodide (BiI3) as a candidate thin-film photovoltaic (PV) absorber. BiI3 was chosen for its optical properties and the potential for defect-tolerant charge transport properties, which the authors test exptl. by measuring optical absorption and recombination lifetimes. The authors synthesize phase-pure BiI3 thin films by phys. vapor transport and soln. processing and single-crystals by an electrodynamic gradient vertical Bridgman method. The bandgap of these materials is ∼1.8 eV, and they demonstrate room-temp. band-edge photoluminescence. The authors measure monoexponential recombination lifetimes at 180-240 ps for thin films, and longer, multiexponential dynamics for single crystals, with time consts. up to 1.3 to 1.5 ns. The outstanding challenges to developing BiI3 PVs, including mech. and elec. properties, which can also inform future selection of candidate PV absorbers are discussed.
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    33. 33
      Podraza, N. J.; Qiu, W.; Hinojosa, B. B.; Xu, H.; Motyka, M. A.; Phillpot, S. R.; Baciak, J. E.; Trolier-McKinstry, S.; Nino, J. C. Band Gap and Structure of Single Crystal BiI3: Resolving Discrepancies in Literature J. Appl. Phys. 2013, 114, 033110 DOI: 10.1063/1.4813486
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      33
      Band gap and structure of single crystal BiI3: Resolving discrepancies in literature
      Podraza, Nikolas J.; Qiu, Wei; Hinojosa, Beverly B.; Xu, Haixuan; Motyka, Michael A.; Phillpot, Simon R.; Baciak, James E.; Trolier-McKinstry, Susan; Nino, Juan C.
      Journal of Applied Physics (Melville, NY, United States) (2013), 114 (3), 033110/1-033110/8CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Bismuth tri-iodide (BiI3) is an intermediate band gap semiconductor with potential for room temp. gamma-ray detection applications. Remarkably, very different band gap characteristics and values of BiI3 have been reported in literature, which may be attributed to its complicated layered structure with strongly bound BiI6 octahedra held together by weak van der Waals interactions. Here, to resolve this discrepancy, the band gap of BiI3 was characterized through optical and computational methods and differences among previously reported values are discussed. Unpolarized transmittance and reflectance spectra in the visible to near UV (UV-Vis) range at room temp. yielded an indirect band gap of 1.67 ± 0.09 eV, while spectroscopic ellipsometry detected a direct band gap at 1.96 ± 0.05 eV and higher energy crit. point features. The discrepancy between the UV-Vis and ellipsometry results originates from the low optical absorption coeffs. (α ∼ 102 cm-1) of BiI3 that renders reflection-based ellipsometry insensitive to the indirect gap for this material. Further, electronic-structure calcns. of the band structure by d. functional theory methods are also consistent with the presence of an indirect band gap of 1.55 eV in BiI3. Based on this, an indirect band gap with a value of 1.67 ± 0.09 eV is considered to best represent the band gap structure and value for single crystal BiI3. (c) 2013 American Institute of Physics.
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    34. 34
      Kim, Y.; Yang, Z.; Jain, A.; Voznyy, O.; Kim, G.-H.; Liu, M.; Quan, L. N.; García de Arquer, F. P.; Comin, R.; Fan, J. Z.; Sargent, E. H. Pure Cubic-Phase Hybrid Iodobismuthates AgBi2I7 for Thin-Film Photovoltaics Angew. Chem., Int. Ed. 2016, 55, 9586– 9590 DOI: 10.1002/anie.201603608
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      Pure Cubic-Phase Hybrid Iodobismuthates AgBi2I7 for Thin-Film Photovoltaics
      Kim, Younghoon; Yang, Zhenyu; Jain, Ankit; Voznyy, Oleksandr; Kim, Gi-Hwan; Liu, Min; Quan, Li Na; Garcia de Arquer, F. Pelayo; Comin, Riccardo; Fan, James Z.; Sargent, Edward H.
      Angewandte Chemie, International Edition (2016), 55 (33), 9586-9590CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Bismuth-based hybrid perovskites are candidates for lead-free and air-stable photovoltaics, but poor surface morphologies and a high band-gap energy have previously limited these hybrid perovskites. A new materials processing strategy to produce enhanced bismuth-based thin-film photovoltaic absorbers by incorporation of monovalent silver cations into iodobismuthates is presented. Soln.-processed AgBi2I7 thin films are prepd. by spin-coating silver and bismuth precursors dissolved in n-butylamine and annealing under an N2 atmosphere. X-ray diffraction anal. reveals the pure cubic structure (Fd3m) with lattice parameters of a=b=c=12.223 Å. The resultant AgBi2I7 thin films exhibit dense and pinhole-free surface morphologies with grains ranging in size from 200-800 nm and a low band gap of 1.87 eV suitable for photovoltaic applications. Initial studies produce solar power conversion efficiencies of 1.22 % and excellent stability over at least 10 days under ambient conditions.
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    35. 35
      Xiao, Z.; Meng, W.; Mitzi, D. B.; Yan, Y. Crystal Structure of AgBi2I7 Thin Films J. Phys. Chem. Lett. 2016, 7, 3903– 3907 DOI: 10.1021/acs.jpclett.6b01834
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      35
      Crystal Structure of AgBi2I7 Thin Films
      Xiao, Zewen; Meng, Weiwei; Mitzi, David B.; Yan, Yanfa
      Journal of Physical Chemistry Letters (2016), 7 (19), 3903-3907CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Synthesis of cubic-phase AgBi2I7 iodobismuthate films and fabrication of air-stable Pb-free solar cells using the AgBi2I7 absorber have recently been reported. From XRD anal. and nominal compn., probably the synthesized films have a cubic ThZr2H7 crystal structure with AgBi2I7 stoichiometry. Through careful examn. of the proposed structure and computational evaluation of the phase stability and bandgap, the reported AgBi2I7 films cannot be forming with the ThZr2H7-type structure, but rather more likely adopt an Ag-deficient AgBiI4 type. Both the exptl. x-ray diffraction pattern and bandgap can be better explained by the AgBiI4 structure. The proposed AgBiI4 structure, with octahedral Bi coordination, removes unphys. short Bi-I bonding within the [BiI8] hexahedra of the ThZr2I7 model. The results provide crit. insights for assessing the photovoltaic properties of AgBi2I7 iodobismuthate materials.
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    36. 36
      Oldag, T.; Aussieker, T.; Keller, H.-L.; Preitschaft, C.; Pfitzner, A. Solvothermale Synthese und Bestimmung der Kristallstrukturen von AgBiI4 und Ag3BiI6 Z. Anorg. Allg. Chem. 2005, 631, 677– 682 DOI: 10.1002/zaac.200400508
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      36
      Solvothermal synthesis and crystal structure determination of AgBiI4 and Ag3BiI6
      Oldag, Thorsten; Aussieker, Thomas; Keller, Hans-Lothar; Preitschaft, Christian; Pfitzner, Arno
      Zeitschrift fuer Anorganische und Allgemeine Chemie (2005), 631 (4), 677-682CODEN: ZAACAB; ISSN:0044-2313. (Wiley-VCH Verlag GmbH & Co. KGaA)
      AgBiI4 and Ag3BiI6 were synthesized by solvothermal reaction from AgI and BiI3 in dild. HI-soln. (20%) at a temp. of 160°. The grayish-black crystals grow as octahedra (Ag-BiI4) or hexagonal/trigonal platelets (Ag3BiI6). AgBiI4 crystallizes in space group Fd‾3m with a = 1222.3(1) pm (300 K), Z = 8, R1 = 0.0151, wR2 = 0.0226, whereas Ag3BiI6 shows the space group R‾3m with a = 435.37(6) pm, c = 2081.0(4) pm (300 K), Z = 1, 154 independent reflections, 8 refined parameters, R1 = 0.0220, wR2 = 0.0573. Both crystal structures show stacking sequence abcabc... of hexagonal layers contg. I. Bi and Ag are sharing octahedral sites with different mass ratio in both structures. The part of Ag which could be localized varies with temp. This behavior indicates mobility of Ag within the crystal structure. The ionic cond. of AgBiI4 is explored. AgBiI4 and Ag3BiI6 show close structural relationship, with AgBiI4 as a variant with a higher degree of order.
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      Fourcroy, P. H.; Palazzi, M.; Rivet, J.; Flahaut, J.; Céolin, R. Etude du Systeme AgIBiI3 Mater. Res. Bull. 1979, 14, 325– 328 DOI: 10.1016/0025-5408(79)90096-5
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      Study of the silver iodide-bismuth(III) iodide system
      Fourcroy, P. H.; Palazzi, M.; Rivet, J.; Flahaut, J.; Ceolin, R.
      Materials Research Bulletin (1979), 14 (3), 325-8CODEN: MRBUAC; ISSN:0025-5408.
      The pseudo-binary system AgI-BiI3 was studied by DTA and by microcalorimetry. An eutectic point was found at 389° at the compn. 85 mol. % BiI3. Two new phases Ag2BiI5 and AgBi2I7 were identified, and their existence confirmed by x-ray diffraction. Ag2BiI5 is rhombohedral, with hexagonal a 4.34 and c 20.77 Å. AgBi2I7 is cubic with a 3.05 Å.
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      Dzeranova, K. B.; Kaloev, N. I.; Bukhalova, G. A. The BiI3 - AgI System Russ. J. Inorg. Chem. 1985, 30, 1700– 1701
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      Mashadieva, L. F.; Aliev, Z. S.; Shevelkov, A. V.; Babanly, M. B. Experimental Investigation of the Ag–Bi–I Ternary System and Thermodynamic Properties of the Ternary Phases J. Alloys Compd. 2013, 551, 512– 520 DOI: 10.1016/j.jallcom.2012.11.033
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      Experimental investigation of the Ag-Bi-I ternary system and thermodynamic properties of the ternary phases
      Mashadieva, Leyla F.; Aliev, Ziya S.; Shevelkov, Andrei V.; Babanly, Mahammad B.
      Journal of Alloys and Compounds (2013), 551 (), 512-520CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      The phase equil. in the Ag-Bi-I ternary system and thermodn. properties of the ternary phases were exptl. detd. by using DTA and x-ray diffraction techniques and EMF measurements with the Ag4RbI5 solid electrolyte. According to the obtained exptl. results, the polythermal sections of the ternary phase diagram, its isothermal section at 300 K as well as the projection of the liqs. surface were revised. The fields of the primary crystn. and types and coordinates of non-variant and mono-variant equil. were detd. The partial molar functions of silver iodide and silver in the alloys as well as the std. thermodn. functions of formation and the std. entropies of Ag2BiI5 and AgBi2I7 were calcd. based on EMF measurements.
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      Degen, T.; Sadki, M.; Bron, E.; König, U.; Nénert, G. The HighScore Suite Powder Diffr. 2014, 29, S13– S18 DOI: 10.1017/S0885715614000840
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      The HighScore suite
      Degen, Thomas; Sadki, Mustapha; Bron, Egbert; Koenig, Uwe; Nenert, Gwilherm
      Powder Diffraction (2014), 29 (S2), S13-S18CODEN: PODIE2; ISSN:0885-7156. (Cambridge University Press)
      HighScore with the Plus option (HighScore Plus) is the com. powder diffraction anal. software from PAnal. It has been in const. development over the last 13 years and has evolved into a very complete and mature product. In this paper, we present a brief overview of the suite focusing on the latest addns. and its user-friendliness. The introduction briefly touches some basic ideas behind HighScore and the Plus option.
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      TOPAS, version 5; Bruker AXS: Karlsruhe, Germany, 2011.
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      Momma, K.; Izumi, F. VESTA: a Three-Dimensional Visualization System for Electronic and Structural Analysis J. Appl. Crystallogr. 2008, 41, 653– 658 DOI: 10.1107/S0021889808012016
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      VESTA: a three-dimensional visualization system for electronic and structural analysis
      Momma, Koichi; Izumi, Fujio
      Journal of Applied Crystallography (2008), 41 (3), 653-658CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
      A cross-platform program, VESTA, has been developed to visualize both structural and volumetric data in multiple windows with tabs. VESTA represents crystal structures by ball-and-stick, space-filling, polyhedral, wire frame, stick, dot-surface and thermal-ellipsoid models. A variety of crystal-chem. information is extractable from fractional coordinates, occupancies and oxidn. states of sites. Volumetric data such as electron and nuclear densities, Patterson functions, and wavefunctions are displayed as isosurfaces, bird's-eye views and two-dimensional maps. Isosurfaces can be colored according to other phys. quantities. Translucent isosurfaces and/or slices can be overlapped with a structural model. Collaboration with external programs enables the user to locate bonds and bond angles in the 'graphics area', simulate powder diffraction patterns, and calc. site potentials and Madelung energies. Electron densities detd. exptl. are convertible into their Laplacians and electronic energy densities.
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      CrysAlisPro, version 171.38.48; Agilent Technologies: Yarton, Oxfordshire, U.K., 2013.
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      Sheldrick, G. A Short History of SHELX Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112– 122 DOI: 10.1107/S0108767307043930
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      A short history of SHELX
      Sheldrick, George M.
      Acta Crystallographica, Section A: Foundations of Crystallography (2008), 64 (1), 112-122CODEN: ACACEQ; ISSN:0108-7673. (International Union of Crystallography)
      An account is given of the development of the SHELX system of computer programs from SHELX-76 to the present day. In addn. to identifying useful innovations that have come into general use through their implementation in SHELX, a crit. anal. is presented of the less-successful features, missed opportunities and desirable improvements for future releases of the software. An attempt is made to understand how a program originally designed for photog. intensity data, punched cards and computers over 10000 times slower than an av. modern personal computer has managed to survive for so long. SHELXL is the most widely used program for small-mol. refinement and SHELXS and SHELXD are often employed for structure soln. despite the availability of objectively superior programs. SHELXL also finds a niche for the refinement of macromols. against high-resoln. or twinned data; SHELXPRO acts as an interface for macromol. applications. SHELXC, SHELXD and SHELXE are proving useful for the exptl. phasing of macromols., esp. because they are fast and robust and so are often employed in pipelines for high-throughput phasing. This paper could serve as a general literature citation when one or more of the open-source SHELX programs (and the Bruker AXS version SHELXTL) are employed in the course of a crystal-structure detn.
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    45. 45
      Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: a Complete Structure Solution, Refinement and Analysis Program J. Appl. Crystallogr. 2009, 42, 339– 341 DOI: 10.1107/S0021889808042726
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      OLEX2: a complete structure solution, refinement and analysis program
      Dolomanov, Oleg V.; Bourhis, Luc J.; Gildea, Richard J.; Howard, Judith A. K.; Puschmann, Horst
      Journal of Applied Crystallography (2009), 42 (2), 339-341CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
      New software, OLEX2, was developed for the detn., visualization and anal. of mol. crystal structures. The software has a portable mouse-driven workflow-oriented and fully comprehensive graphical user interface for structure soln., refinement and report generation, as well as novel tools for structure anal. OLEX2 seamlessly links all aspects of the structure soln., refinement and publication process and presents them in a single workflow-driven package, with the ultimate goal of producing an application which will be useful to both chemists and crystallographers.
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    46. 46
      Whittles, T. J.; Burton, L. A.; Skelton, J. M.; Walsh, A.; Veal, T. D.; Dhanak, V. R. Band Alignments, Valence Bands, and Core Levels in the Tin Sulfides SnS, SnS2, and Sn2S3: Experiment and Theory Chem. Mater. 2016, 28, 3718– 3726 DOI: 10.1021/acs.chemmater.6b00397
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      46
      Band Alignments, Valence Bands, and Core Levels in the Tin Sulfides SnS, SnS2, and Sn2S3: Experiment and Theory
      Whittles, Thomas J.; Burton, Lee A.; Skelton, Jonathan M.; Walsh, Aron; Veal, Tim D.; Dhanak, Vin R.
      Chemistry of Materials (2016), 28 (11), 3718-3726CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Tin sulfide solar cells show relatively poor efficiencies despite attractive photovoltaic properties, and there is difficulty in identifying sep. phases, which are also known to form during Cu2ZnSnS4 depositions. The authors present x-ray photoemission spectroscopy (XPS) and inverse photoemission spectroscopy measurements of single crystal SnS, SnS2, and Sn2S3, with electronic-structure calcns. from d. functional theory (DFT). Differences in the XPS spectra of the three phases, including a large 0.9 eV shift between the 3d5/2 peak for SnS and SnS2, make this technique useful when identifying phase-pure or mixed-phase systems. Comparison of the valence band spectra from XPS and DFT reveals extra states at the top of the valence bands of SnS and Sn2S3, arising from the hybridization of lone pair electrons in Sn(II), which are not present for Sn(IV), as found in SnS2. This results in relatively low ionization potentials for SnS (4.71 eV) and Sn2S3 (4.66 eV), giving a more comprehensive explanation as to the origin of the poor efficiencies. The authors also demonstrate, by a band alignment, the large band offsets of SnS and Sn2S3 from other photovoltaic materials and highlight the detrimental effect on cell performance of secondary tin sulfide phase formation in SnS and CZTS films.
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    47. 47
      Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D.; Chastain, J.; King, R. C. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics, Inc.: Eden Prairie, MN, 1995.
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      Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations using a Plane-Wave Basis Set Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169– 11186 DOI: 10.1103/PhysRevB.54.11169
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      Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
      Kresse, G.; Furthmueller, J.
      Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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      Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758– 1775 DOI: 10.1103/PhysRevB.59.1758
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      From ultrasoft pseudopotentials to the projector augmented-wave method
      Kresse, G.; Joubert, D.
      Physical Review B: Condensed Matter and Materials Physics (1999), 59 (3), 1758-1775CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived. The total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addn., crit. tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed-core all-electron methods. These tests include small mols. (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
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      Klimeš, J.; Bowler, D. R.; Michaelides, A. Van der Waals Density Functionals Applied to Solids Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 195131 DOI: 10.1103/PhysRevB.83.195131
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      Van der Waals density functionals applied to solids
      Klimes, Jiri; Bowler, David R.; Michaelides, Angelos
      Physical Review B: Condensed Matter and Materials Physics (2011), 83 (19), 195131/1-195131/13CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The van der Waals d. functional (vdW-DF) of M. Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)] is a promising approach for including dispersion in approx. d. functional theory exchange-correlation functionals. Indeed, an improved description of systems held by dispersion forces has been demonstrated in the literature. However, despite many applications, std. general tests on a broad range of materials including traditional "hard" matter such as metals, ionic compds., and insulators are lacking. Such tests are important not least because many of the applications of the vdW-DF method focus on the adsorption of atoms and mols. on the surfaces of solids. Here we calc. the lattice consts., bulk moduli, and atomization energies for a range of solids using the original vdW-DF and several of its offspring. We find that the original vdW-DF overestimates lattice consts. in a similar manner to how it overestimates binding distances for gas-phase dimers. However, some of the modified vdW functionals lead to av. errors which are similar to those of PBE or better. Likewise, atomization energies that are slightly better than from PBE are obtained from the modified vdW-DFs. Although the tests reported here are for hard solids, not normally materials for which dispersion forces are thought to be important, we find a systematic improvement in cohesive properties for the alkali metals and alkali halides when nonlocal correlations are accounted for.
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      Trotter, J.; Zobel, T. The Crystal Structure of Sbl3 and Bil3 Z. Kristallogr. 1966, 123, 67
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      The crystal structure of SbI3 and BiI3
      Trotter, J.; Zobel, T.
      Zeitschrift fuer Kristallographie, Kristallgeometrie, Kristallphysik, Kristallchemie (1966), 123 (1), 67-72CODEN: ZKKKAJ; ISSN:0044-2968.
      Crystals of antimony triiodide, SbI3, are rhombohedral, with aH 7.48, cH 20.90 A., Z = 6, and space group R‾3. The structure was detd. from Patterson, Fourier and difference projections, by using F(h0.l) data, the final discrepancy factor being 0.131. The Sb atoms are significantly displaced from the centers of I octahedra, and have 3 near-neighbor I atoms at 2.686 ± 0.0010 A., with I-Sb-I 95.8 ± 0.3°, and 3 farther at 3.316 ± 0.010 A. The structure is thus intermediate between that of a mol. crystal, as in AsI3, and an ionic arrangement. Crystals of BiI3 have aH 7.52, and cH 20.72 A. A comparison of measured and calcd. powder intensities suggests that Bi is situated at the center of an I octahedron, so that the nonbonded electron pair is not sterically active and the structure is probably largely ionic.
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      Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple Phys. Rev. Lett. 1996, 77, 3865– 3868 DOI: 10.1103/PhysRevLett.77.3865
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      Generalized gradient approximation made simple
      Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
      Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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    53. 53
      Pauling, L.; Hoard, J. XXXVII. The Crystal Structure of Cadmium Chloride Z. Kristallogr. - Cryst. Mater. 1930, 74, 546– 551 DOI: 10.1524/zkri.1930.74.1.546
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      The crystal structure of cadmium chloride
      Pauling, Linus; Hoard, J. L.
      Zeitschrift fuer Kristallographie, Kristallgeometrie, Kristallphysik, Kristallchemie (1930), 74 (), 546-51CODEN: ZKKKAJ; ISSN:0044-2968.
      The unit of structure for CdCl2 is a rhombohedron with α = 36°02' and a = 6.23 A. U. contg. 1 mol. There is a layer structure along [0001], closely related to that of Cdl2. The Cl atoms are in approximate cubic closepacking. The relation of the CdCl2 and the Cdl2 structures is discussed, and a list of similar compds. which crystallize in each type is given.
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    54. 54
      Bordas, J.; Robertson, J.; Jakobsson, A. Ultraviolet Properties and Band Structure of SnS2, SnSe2, CdI2, PbI2, BiI3 and BiOI Crystals J. Phys. C: Solid State Phys. 1978, 11, 2607 DOI: 10.1088/0022-3719/11/12/021
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      Ultraviolet properties and band structure of tin disulfide, tin diselenide, cadmium diiodide, lead diiodide, bismuth triiodide, and bismuth iodide oxide crystals
      Bordas, J.; Robertson, J.; Jakobsson, A.
      Journal of Physics C: Solid State Physics (1978), 11 (12), 2607-21CODEN: JPSOAW; ISSN:0022-3719.
      The UV reflectivity spectra of the layer materials SnS2, SnSe2, CdI2, PbI2, BiI3, and BiOI were detd. at 8-40 eV. The band structures for SnS2, CdI2, and PbI2 were calcd. using an LCAO approach and the partial and total d. of states for the valence and conduction bands were compared with the optical consts. detd. from Kramers-Kronig anal. of the exptl. data. The obsd. optical transitions from the metal d-core states were discussed in terms of an at.-like picture.
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    55. 55
      Leguy, A. M. A.; Azarhoosh, P.; Alonso, M. I.; Campoy-Quiles, M.; Weber, O. J.; Yao, J.; Bryant, D.; Weller, M. T.; Nelson, J.; Walsh, A.; van Schilfgaarde, M.; Barnes, P. R. F. Experimental and Theoretical Optical Properties of Methylammonium Lead Halide Perovskites Nanoscale 2016, 8, 6317– 6327 DOI: 10.1039/C5NR05435D
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      Experimental and theoretical optical properties of methylammonium lead halide perovskites
      Leguy, Aurelien M. A.; Azarhoosh, Pooya; Alonso, M. Isabel; Campoy-Quiles, Mariano; Weber, Oliver J.; Yao, Jizhong; Bryant, Daniel; Weller, Mark T.; Nelson, Jenny; Walsh, Aron; van Schilfgaarde, Mark; Barnes, Piers R. F.
      Nanoscale (2016), 8 (12), 6317-6327CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      The optical consts. of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calcns. using the relativistic quasiparticle self-consistent GW approxn. (QSGW). Good agreement between the optical consts. derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their resp. inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calcns. indicate that the orientation of [CH3NH3]+ cations has a significant influence on the position of the bandgap suggesting that collective orientation of the org. moieties could result in significant local variations of the optical properties. The optical consts. and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS).
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      Sankapal, B.; Baviskar, P.; Salunkhe, D. Synthesis and Characterization of AgI Thin Films at Low Temperature J. Alloys Compd. 2010, 506, 268– 270 DOI: 10.1016/j.jallcom.2010.06.190
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      Synthesis and characterization of AgI thin films at low temperature
      Sankapal, Babasaheb; Baviskar, Prashant; Salunkhe, Dipak
      Journal of Alloys and Compounds (2010), 506 (1), 268-270CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      The growth of AgI (silver iodide) thin films has been performed in aq. medium using simple chem. method at room temp. (25 °C). Silver nitrate and potassium iodide have been used as source materials. Thin films were characterized by X-ray diffraction (XRD), SEM, at. force microscopy (AFM), surface photovoltage (SPV) and optical absorption spectroscopy. Glass and indium doped tin oxide (ITO) coated glass was used as substrates. The thin films were surface homogeneous with mixed β and γ-phases with surface roughness value of 21 nm. Optical transmission on glass exceeds 80% for 150 nm thick film with the direct band gap value of 2.85 eV. The change in crystal phases after transition temp. is studied by SPV measurements.
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      Godby, R. W.; Schlüter, M.; Sham, L. J. Self-Energy Operators and Exchange-Correlation Potentials in Semiconductors Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 10159– 10175 DOI: 10.1103/PhysRevB.37.10159
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      Self-energy operators and exchange-correlation potentials in semiconductors
      Godby; Schluter; Sham
      Physical review. B, Condensed matter (1988), 37 (17), 10159-10175 ISSN:0163-1829.
      There is no expanded citation for this reference.
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    58. 58
      Pisoni, A.; Jaćimović, J.; Barišić, O. S.; Spina, M.; Gaál, R.; Forró, L.; Horváth, E. Ultra-Low Thermal Conductivity in Organic–Inorganic Hybrid Perovskite CH3NH3PbI3 J. Phys. Chem. Lett. 2014, 5, 2488– 2492 DOI: 10.1021/jz5012109
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      Ultra-Low Thermal Conductivity in Organic-Inorganic Hybrid Perovskite CH3NH3PbI3
      Pisoni, Andrea; Jacimovic, Jacim; Barisic, Osor S.; Spina, Massimo; Gaal, Richard; Forro, Laszlo; Horvath, Endre
      Journal of Physical Chemistry Letters (2014), 5 (14), 2488-2492CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The authors report on the temp. dependence of thermal cond. of single cryst. and polycryst. organometallic perovskite CH3NH3PbI3. The comparable abs. values and temp. dependence of the two samples' morphologies indicate the minor role of the grain boundaries on the heat transport. Theor. modeling demonstrates the importance of the resonant scattering in both specimens. The interaction between phonon waves and rotational degrees of freedom of CH3NH3+ sublattice emerges as the dominant mechanism for attenuation of heat transport and for ultralow thermal cond. of 0.5 W/(Km) at room temp.
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      Mettan, X.; Pisoni, R.; Matus, P.; Pisoni, A.; Jaćimović, J.; Náfrádi, B.; Spina, M.; Pavuna, D.; Forró, L.; Horváth, E. Tuning of the Thermoelectric Figure of Merit of CH3NH3MI3 (M = Pb,Sn) Photovoltaic Perovskites J. Phys. Chem. C 2015, 119, 11506– 11510 DOI: 10.1021/acs.jpcc.5b03939
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      Tuning of the Thermoelectric Figure of Merit of CH3NH3MI3 (M= Pb, Sn) Photovoltaic Perovskites
      Mettan, Xavier; Pisoni, Riccardo; Matus, Peter; Pisoni, Andrea; Jacimovic, Jacim; Nafradi, Balint; Spina, Massimo; Pavuna, Davor; Forro, Laszlo; Horvath, Endre
      Journal of Physical Chemistry C (2015), 119 (21), 11506-11510CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The hybrid halide perovskites, the very performant compds. in photovoltaic applications, possess large Seebeck coeff. and low thermal cond., making them potentially interesting high figure of merit (ZT) materials. For this purpose, one needs to tune the elec. cond. of these semiconductors to higher values. The authors have studied the CH3NH3MI3 (M = Pb,Sn) samples in pristine form showing very low ZT values for both materials; however, photoinduced doping (in M = Pb) and chem. doping (in M = Sn) indicate that, by further doping optimization, ZT can be enhanced toward unity and reach the performance level of the presently most efficient thermoelec. materials.
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    60. 60
      Han, H.; Hong, M.; Gokhale, S. S.; Sinnott, S. B.; Jordan, K.; Baciak, J. E.; Nino, J. C. Defect Engineering of BiI3 Single Crystals: Enhanced Electrical and Radiation Performance for Room Temperature Gamma-Ray Detection J. Phys. Chem. C 2014, 118, 3244– 3250 DOI: 10.1021/jp411201k
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      Defect Engineering of BiI3 Single Crystals: Enhanced Electrical and Radiation Performance for Room Temperature Gamma-Ray Detection
      Han, HyukSu; Hong, Minki; Gokhale, Sasmit S.; Sinnott, Susan B.; Jordan, Kelly; Baciak, James E.; Nino, Juan C.
      Journal of Physical Chemistry C (2014), 118 (6), 3244-3250CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Undoped and Sb-doped BiI3 (SBI) single crystals are grown via the vertical Bridgman growth technique. Elec. properties, such as resistivity and leakage current, in addn. to radiation response measurements were performed on both BiI3 and SBI single crystal detectors. The resistivity of SBI (2.63109 Ω cm) increases by an order of magnitude compared to that of BiI3 (1.45 × 108 Ω cm). Also, leakage currents of SBI (10-2 μA/cm2) decrease by four orders magnitude relative to BiI3. The radiation response of the SBI indicates that less polarization exists under bias for prolonged periods of time, making SBI a promising material for use in gamma-ray detector applications. D. functional theory (DFT) calcns. predict that Sb forms strong covalent bonds with neighboring iodine ions and that the Sb-I dimer can be formed when Sb is doped into the BiI3 lattice. Defect modeling verifies that substitution of Bi ions with Sb and incorporation of Sb in iodine vacancy sites can effectively decrease the formation and migration of iodine vacancies, which significantly improves radiation detection performance of the material.
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    61. 61
      Devidas, T. R.; Shekar, N. V. C.; Sundar, C. S.; Chithaiah, P.; Sorb, Y. A.; Bhadram, V. S.; Chandrabhas, N.; Pal, K.; Waghmare, U. V.; Rao, C. N. R. Pressure-Induced Structural Changes and Insulator-Metal Transition in Layered Bismuth Triiodide, BiI3: a Combined Experimental and Theoretical Study J. Phys.: Condens. Matter 2014, 26, 275502 DOI: 10.1088/0953-8984/26/27/275502
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      Pressure-induced structural changes and insulator-metal transition in layered bismuth triiodide, BiI3: a combined experimental and theoretical study
      Devidas, T. R.; Chandra Shekar, N. V.; Sundar, C. S.; Chithaiah, P.; Sorb, Y. A.; Bhadram, V. S.; Chandrabhas, N.; Pal, K.; Waghmare, U. V.; Rao, C. N. R.
      Journal of Physics: Condensed Matter (2014), 26 (27), 275502/1-275502/9, 9 pp.CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
      Noting that BiI3 and the known topol. insulator (TI) Bi2Se3 have the same high symmetry parent structures, and that it is desirable to find a wide-band gap TI, the authors det. here the effects of pressure on the structure, phonons and electronic properties of rhombohedral BiI3. The authors report a pressure-induced insulator-metal transition near 1.5 GPa, using high pressure elec. resistivity and Raman measurements. X-ray diffraction studies, as a function of pressure, reveal a structural peculiarity of the BiI3 crystal, with a drastic drop in c/a ratio at 1.5 GPa, and a structural phase transition from rhombohedral to monoclinic structure at 8.8 GPa. The metallic phase, at relatively low pressures, exhibits minimal resistivity at low temps., similar to that in Bi2Se3. The authors corroborate these findings with 1st-principles calcns. and suggest that the drop in the resistivity of BiI3 in the 1-3 GPa range of pressure arises possibly from the appearance of an intermediate crystal phase with a lower band-gap and hexagonal crystal structure. Calcd. Born effective charges reveal metallic states in the structural vicinity of rhombohedral BiI3. Changes in the topol. of the electronic bands of BiI3 with pressure, and a sharp decrease in the c/a ratio <2 GPa, give rise to changes in the slope of phonon frequencies near that pressure.
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      Perovskite and dye-sensitized solar cells are PV technologies which hold promise for PV application. Arguably, the biggest issue facing these technologies is stability. The vast majority of studies have been limited to small area lab. cells. Moisture, oxygen, UV light, thermal and elec. stresses are leading the degrdn. causes. There remains a shortage of stability investigations on large area devices, in particular modules. At the module level there exist particular challenges which can be different from those at the small cell level such as encapsulation (not only of the unit cells but of interconnections and contacts), non-uniformity of the layer stacks and unit cells, reverse bias stresses, which are important to investigate for technologies that aim for industrial acceptance. Herein we present a review of stability investigations published in the literature pertaining large area perovskite and dye-sensitized solar devices fabricated both on rigid (glass) and flexible substrates.
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      ISODISPLACE is a new internet-server tool for exploring structural phase transitions. Given parent-phase structural information, it generates at. displacement patterns induced by irreducible representations of the parent space-group symmetry and allows a user to visualize and manipulate the amplitude of each distortion mode interactively. ISODISPLACE is freely accessible at http://stokes.byu.edu/isodisplace.html via common internet browsers.
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      A review. The description of displacive distorted structures in terms of symmetry-adapted modes is reviewed. A specific parameterization of the symmetry-mode decompn. of these pseudosym. structures defined on the setting of the exptl. space group is proposed. This approach closely follows crystallog. conventions and permits a straightforward transformation between symmetry-mode and conventional descriptions of the structures. Multiple examples are presented showing the insight provided by the symmetry-mode approach. The methodol. is shown at work, illustrating its various possibilities for improving the characterization of distorted structures, for example: detection of hidden structural correlations, identification of fundamental and marginal degrees of freedom, redn. of the effective no. of at. positional parameters, quant. comparison of structures with the same or different space group, detection of false refinement min., systematic characterization of thermal behavior, rationalization of phase diagrams and various symmetries in families of compds. etc. The close relation of the symmetry-mode description with the superspace formalism applied to commensurate superstructures is also discussed. Finally, the application of this methodol. in the field of ab initio or first-principles calcns. is outlined. At present, there are several freely available user-friendly computer tools for performing automatic symmetry-mode analyses. The use of these programs does not require a deep knowledge of group theory and can be applied either a posteriori to analyze a given distorted structure or a priori to parameterize the structure to be detd. It is hoped that this article will encourage the use of these tools. All the examples presented here have been worked out using the program AMPLIMODES.
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      (TaSe4)2I is a quasi-one-dimensional (1D) elec. conductor. It exhibits a phase transition at TCDW = 263 K towards a charge-d.-wave (CDW) state at low temps. We report a full structure refinement of the incommensurately modulated structure in the CDW state at T = 110 K against synchrotron radiation, single-crystal x-ray diffraction data. At room temp. the crystal structure has tetragonal symmetry with space group I422. In the CDW state each main reflection in the x-ray scattering is surrounded by eight incommensurate satellites at (±0.064, ±0.064, ±0.151). The CDW state is found to comprise four domains, and it is characterized by one modulation wavevector. With respect to a 2 × 2 × 1 supercell it has the symmetry of the superspace group F2(0, β, γ) with β = 0.128 and γ = 0.151. The first part of the modulation is found to be a transverse acoustic wave, involving amplitudes of similar magnitudes of about 0.13 Å on all atoms. The second part of the modulation involves displacements of the Ta atoms of about 0.03 Å, that are parallel to the 1D chains. These are interpreted as reflecting the CDW. A Landau free-energy model is developed, that shows that symmetry arguments allow the phase transition to be second order.
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      Fourcroy, P. H.; Carre, D.; Thevet, F.; Rivet, J. Structure du Tetraiodure de Cuivre(I) et de Bismuth(III), CuBiI4 Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1991, 47, 2023– 2025 DOI: 10.1107/S0108270191005309
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      Bismuth tri-iodide is a wide band-gap semiconductor material that may be able to operate as a radiation detector without any cooling mechanism. This material has a higher effective at. no. than germanium and CdZnTe, and thus should have a higher gamma-ray detection efficiency, particularly for moderate and high energy gamma-rays. Unfortunately, not much is known about bismuth tri-iodide, and the general properties of the material need to be investigated. Bismuth tri-iodide does not suffer from some of the material issues, such as a solid state phase transition and dissocn. in air, that mercuric iodide (another high-Z, wide band-gap semiconductor) does. Thus, bismuth tri-iodide is both easier to grow and handle than mercuric iodide. A modified vertical Bridgman growth technique is being used to grow large, single bismuth tri-iodide crystals. Zone refining is being performed to purify the starting material and increase the resistivity of the crystals. The single crystals being grown are typically several hundred mm3. The larger crystals grown are approx. 2 cm3. Initial detectors are being fabricated using both gold and palladium electrodes and palladium wire. The electron mobility measured using an alpha source was detd. to be 260±50 cm2/Vs. An alpha spectrum was recorded with one of the devices; however the detector appears to suffer from polarization.
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