Lead Halide Perovskite Quantum Dots for Photovoltaics and Photocatalysis: A Review

: Lead halide-based perovskite quantum dots (PQDs) have recently emerged as an important class of nanocrystal (NC) materials for optoelectronic and photoelec-trochemical applications. Thanks to their intriguing features including tunable band gap, narrow emission, high charge carrier mobility, remarkable light-absorbing factors, and long charge diffusion length, there has been a surge in research on lead halide-based PQDs and their applications. In this review, we showcase the fundamentals of PQDs and two principal applications including PQD solar cells (PQDSCs) and photocatalytic conversion. First, a thorough discussion on PQDSCs, their structure, surface treat-ment, and interface engineering along with their recent progress are presented. It is highlighted that the improvement of the efficiency of PQDSCs from below 10% to beyond 16% in a matter of a few years has turned them into promising candidates for future SC applications. Subsequently, the application of PQDs in photocatalytic reactions such as hydrogen production, CO 2 reduction, and organic compounds’ degradation is summarized. Not to mention that, despite the remarkable properties of PQDs in SCs and photocatalysis, the inferior stability of PV devices based thereon under operation as well as their poor tolerance under air, water, light, and heat impede their widespread application. For this, the practical efforts and possible solutions are extensively addressed. Finally, an outlook is provided, addressing further merits, and demerits of each application as well as prospective opportunities.


INTRODUCTION
−10 Lead halide PQDs (LHPQDs) own special light-emitting traits and have compatibility with industrialized printing technology, which exhibit a promising prospect in next-generation flexible and high-definition displays. 11In addition, colloidal HPQDs on account of their long lifetime, outstanding conductivity, and diffusion distance have been identified as superb photodetectors. 12What is more, inorganic halide perovskite quantum dots (IHPQDs) materials with relatively high stability, 13 ultrahigh photoluminescence quantum yield (PLQY), and widely absorption spectra covering the entire visible range 14 have recently been used in photoelectrochemistry.PQDs' unique properties combined with solution-processable device fabrication enable novel forms of advanced optoelectronic devices for energy harvesting, light generation, photon detection, and photoelectrochemical reactions.
Application-wise and for the sake of energy harvesting, herein, the two intriguing cases are addressed.First, PQDs offer a promising solution for SCs.Their spectral absorption window covering the UV−visible (UV−vis) range up to the near-infrared range combined with short-wavelength infrared (SWIR) range allowed for a jump in power conversion efficiency (PCE) levels from 3.8% 5,15,16 to beyond 16.6%. 17econd, in the interest of photoelectrochemical (PEC) systems, the following features could be achieved: (i) wide excitation with which IHPQDs can be excited by a single light source in the range of 350−400 nm, and (ii) narrow full width at half-maximum (fwhm) of the emission peak (only 12−42 nm) which is much narrower than that of traditional quantum dots and organic dyes, and its color gamut range can reach 140% of the National Television System Committee (NTCS) standard. 18Hence, in the last years, PQDs appeared as gamechanging nanomaterials for a wide range of optoelectronic devices.
In this review, we thoroughly explore the fundamentals of PQDs in two prominent applications (Figure 1).To begin with, the relentless search for sustainable forms of energy is a pressing solution to the severe energy crunch and a strategy to confront the brutal environmental concerns. 19Over the last few years, the photovoltaic (PV) community has fiddled with the idea of the fabrication of PVs with halide perovskite solar cells (PSCs) thanks to their high-power conversion efficiency (PCE). 20−25 Thanks to various advantages, QDs are able to exceed the Shockley−Queisser limit 26 and further enhance the efficiency of tandem devices.For this, we provide a detailed description of the architecture of PQDSCs composed of multiple layers accompanying the function of each layer.
The story of PQDs within the scope of this review includes their application in photocatalysis as well to contribute to the solution of energy and environment-related issues.Despite the tremendous success of organometallic halide perovskites (OHPs) in optoelectronic applications, 27,28 they fail to fulfill the criteria in other applications like photocatalysis mainly due to their instability. 29That is why IHPQDs have received significant attention in the field of photoelectrochemistry.For this reason, here we have compiled various applications of PQD-based materials in photoelectrochemical reactions differing from photocatalytic hydrogen generation, degradation of organic compounds and antibiotic residues in organic systems, CO 2 reduction, stereoselective C−C oxidative coupling, polymerization reactions, and NO removal to organic transformations.
In essence, herein, we discuss compendious aspects of PQDs including their fundamentals and two pioneer applications.

FUNDAMENTALS OF PEROVSKITES: CRYSTAL STRUCTURES, DIFFERENT PHASES, AND BANDGAP ENGINEERING
Perovskites are a large family of ceramic crystalline materials having three-dimensional (3D) structures similar to natural mineral calcium titanium oxide. 30They are the main members of the perovskites NCs (PNCs), which can be described by the general chemical formula of ABX 3 (see Figure 2a), in which B cations are 6-fold coordinated to X anions creating cornersharing BX 6 octahedra and A cations fill the cubo−octahedral holes for maintaining charge neutrality of the system. 31,32For the case of metal halide perovskite, B can be a divalent metal such as Pb 2+ , Sn 2+ , Cu 2+ , Ni 2+ , Co 2+ , or Mn 2+ and X a halogen (I − , Br − , or Cl − ).Generally, "A" cations such as Cs + , methylammonium (MA + ), or formamidinium (FA + ) form a 3D framework with the octahedral network.−38 The low stability and highly ionic nature of 3D perovskites urge researchers to work on the low dimensional perovskites.
For that, 2D LHPs having a tetragonal phase of Cs + and [Pb 2 X 5 ] − polyhedron layers were explored. 37,39Their structure can be considered as a 3D ABX 3 perovskite cut into one repeat unit thick slice along the ⟨100⟩ direction, where A and X ions within the slice planes are cut into halves. 32,40A 2 PbX 4 perovskites are another type of 2D perovskites consisting of corner-sharing [PbX 6 ] 4− octahedra and bulky cations layers. 37,41On the other hand, 0D perovskites (Cs 4 PbX 6 ) were studied recently, 42,43 where the PbX 6 4− octahedra are individually disconnected and are completely decoupled in all dimensions, and the halide ions are no longer shared between them. 37,44,45ven though thin film or bulk perovskite materials possess higher power efficiency, they have inferior phase stability at room temperature compared to PQDs.Their sufficient stability can be ascribed to nanocrystal surfaces and electronic coupling between the QDs.As can be seen from Figure 2b, the transmission electron microscopy (TEM) micrograph of CsPbI 3 QDs reveals an interplanar distance of 0.62 nm, which is consistent with the (100) plane of cubic phase stable at room temperature. 46Besides distinctive structural properties, the quantum confinement effect of metal halide perovskites has received significant attention for the facile tuning of bandgap. 37,47,48ince metal halide perovskites possess a soft ionic framework, they exhibit significant polymorphism, manifested by the presence of several different phases within one ABX 3 composition.Therefore, the phase transition between hightemperature low-bandgap (1.5 eV) and low-temperature highbandgap (>2.8 eV) hexagonal perovskites has been well documented for widely used FAPbI 3 and CsPbI 3 compositions.Density functional theory (DFT) calculations revealed that bandgap was affected by two main factors, including the ratio of cubic (c) and hexagonal (h) close-packed layers and the thickness of blocks of cubic layers.It was reported that there is a nonlinear relationship between the band gap and the ratio of cubic and hexagonal layers in these structures.Since crystallization of perovskites is often complicated, and the formation of various phases with the same stoichiometry is inevitable, the presence of such polytypes in the perovskite matrix is responsible for a decrease in the mobility of charge carriers. 49he change of the divalent metal and anionic components change the X−B−X bond impact and tune the bandgap of a metal halide perovskite. 32,50By substitution of larger cations such as FA and MA, the lattice of metal halide perovskite expands, and the bandgap decreases. 32,51Moreover, by changing the halide from iodide to bromide and chloride, the bandgap increases, and thus emission wavelength produces a blueshift.The composition of divalent metal can tune not only the bandgap but also the absolute energy levels of the valence and conduction bands. 52,53Meanwhile, the band gap of QDs can be tunable via size control due to quantum confinement (Figure 2c,d). 46At the moment, a large number of perovskites have been identified with a broad spectrum, covering from conductors to insulators. 54,55So far, the most intensively investigated colloidal QDs (CODs) have been manufactured from chalcogenides within the zinc blend or wurtzite crystal structure, whereas their perovskite counterparts outperform them in the context of superconductivity, ferroelectricity, colossal magnetoresistance, and so forth. 56

APPLICATIONS OF PEROVSKITE QUANTUM DOTS
3.1.Perovskite Quantum Dot Solar Cells.Despite considerable attention swayed to dye-sensitized solar cells (DSSCs) because of their facile synthesis and low price, 57−59 the thick absorbing layer (>10 μm) and the discoloration of organic dyes induced by light are still their major demerits.The controversial bottlenecks have convinced researchers to seek another candidate composed of all-solid dyes.This is where PQDs have come under the spotlight.Primarily, perovskite materials were exploited solely as a dye alternative material.Later, the substitution of liquid electrolyte with a solid-state hole conductor acted as a game changer. 60,61he intriguing optoelectronic characteristics of perovskites including broadband light absorption and high absorption coefficient in the visible range (∼10 5 cm −1 ), to name but two, have made them simultaneously employed as an electron/hole transport layer and light-absorbing layer.It should be noted that a significant reduction of the desired thickness as well as the elimination of barriers to harvesting photogenerated carriers can be achieved as a result of these materials' strong optical absorption.In 2009 Kojima and co-workers 62 realized that metal halide perovskites play the role of dyes in the harvesting of sunlight.Following this, employing the organic metal halide perovskite (MHP) as a sensitizer under a liquid electrolyte in DSSC delivered a PCE of 3.8%.
Compared to well-known N719 dye sensitizers, perovskites exhibited improved absorption; however, their fragile tolerance in polar solutions resulted in their instant deterioration. 63In 2012, solid-state-based PSCs exhibited an over 2.5-fold improvement in terms of PCE in the absence of liquid electrolytes and reached 9.7%. 60Over the last few years, we have witnessed the rapid development of PSCs by virtue of their inexpensive manufacturing process along with high efficiency.In this regard, the highest PCE of 25.5% was obtained for organic−inorganic lead halide PSC in 2020, showing substantial durability 64 and approaching the 27.6% efficiency for commercially single crystalline silicon SCs. 65onetheless, large-scale implementation of PSCs appears to be far-fetched due to their insufficient durability when they undergo treatment in different conditions. 66To enhance the performance of PSCs and strengthen their durability, different perovskite compounds such as QDs have been manipulated.The objective of this section is to introduce multiple layers of PSC, followed by a detailed description of the function of each layer.Subsequently, a new generation of active layers as QDs for a typical PSC is presented.Eventually, the latest developments of PQDSCs are delineated.A representative PSC is made up of multiple layers comprising a conductive glass electrode, an electron transport layer (ETL), an active layer (perovskite layer), a hole transport layer (HTL), and a back-contact electrode layer.Once the PSC receives the sunlight, the perovskite layer absorbs photons, generating electrons and holes so-called excitons.Smaller binding energies of excitons in perovskites arising from the large dielectric constant can lead to free electrons and holes, ending up either as current or recombination of carriers to form excitons. 67 The long lifespan and diffusion length of perovskites come as no surprise, since the mobility of charge carriers is quite high, while recombination rates remain low.Both extended lifespan and long diffusion length are prime reasons behind the remarkable performance of PSCs. 68Once the electrons and holes are free, they are assembled by respective ETLs and HTLs.Thereafter electrons are delivered to TiO 2 as the electron-transporting material (ETM) layer and assembled by fluorine-doped tin oxide (FTO) electrode.Holes, on the other hand, are delivered to the holetransporting material (HTM) layer and assembled by the counter electrode.The photocurrent is produced throughout the external circuit via the connection between the counter electrode and FTO.
Figure 3a depicts the arrangement of energy levels for the transport process of carriers within an HTM/perovskite/TiO 2 cell.The beneficial processes shown in Figure 3a encompass excitation induced by light in perovskite (process (i)) followed by electron injection to titania (process (ii)).It should be pointed out that electron and hole splitting takes place at the interfacial segment of TiO 2 /perovskite and Spiro-OMeTAD/ perovskite accompanied by electron delivery to TiO 2 (process (i)) and hole delivery to HTM (process (ii)) with which charge transport process is accomplished. 69Besides the mentioned events, unwanted processes namely recombination of photoexcited carriers, back charge transfer at the interfacial segments of TiO 2 and the HTM with the perovskite (photoluminescence), a backward motion of carriers (process (iv) and (v)), and electrons and holes recombination at the interfacial junction of TiO 2 and the HTM (process (vi)) occur concurrently within the cell, undermining its performance.
3.1.1.Device Architecture.Manufacturing PSCs can be achieved either through a normal arrangement or inverted configuration based on the carrier assemblage electrode, which can be further modulated by employing multiple layers.The structure of PSCs is defined depending on the position of the conductive substrate�FTO or indium tin oxide (ITO)�and deposited layer.For this, if the conductive substrate is deposited on the ETL; it is a normal arrangement or n-i-p configuration.On the other hand, if an HTL is placed on a conductive substrate, it is called an inverse arrangement or p-in configuration.Figure 3b illustrates both configurations. 70,71SCs with the configuration of n-i-p can be classified into two planar or mesoporous structures with reference to their ETL construction.The n-i-p mesoporous structures are the original framework of the perovskite PV devices that are currently in common use.Keeping this in mind, the mesoporous layer can intensify charge assemblage through lowering the distance of charge transit, preclusion of current leaking at interfacial segments, and enhancing of absorption of photons because of the light scattering phenomenon.In the early stages, people postulated that mesoporous layers of ETM with large areas affect positively the performance of perovskite devices, considering that electron removal at the interfaces of ETM is more cumbersome compared to hole removal at the HTM. 72hat being said, careful control of the fabrication of perovskites and their interfaces, formation of carrier layers, as well as electrodes result in high-efficiency devices in the absence of a mesoporous layer. 73t the same time, for the fabrication of the p-i-n type perovskite device, the following procedure can be adopted.First, a p-type conductive polymer is grown on an ITO-coated substrate.Following the deposition of a thin layer of perovskite, a hole-blocking layer and a cathode either Au or Al are placed on one another, respectively.For the initial constructions, the materials employed as donor and acceptor were perovskites and fullerene, separately. 74During the recent rapid development of PV devices, the organic acceptor has been replaced by a thin layer of ETM, resulting in planar perovskites in between two organic charges transiting layers. 75or the additional advances in this field, one can mention the switching in selective contact from organic to inorganic compounds.Continuing on this line, the recent usage of NiO and ZnO/TiO 2 layers as respective hole and electron selective contacts has turned perovskites into superior materials as opposed to their analogues. 76,77Moreover, manufacturing the mesoscopic p-i-n configuration is subject to the exploitation of HTMs such as NiO/mp-Al 2 O 3 or c-NiO/mp-NiO. 70,71.1.1.1.Conductive Glass Electrode.A conducting glass electrode acts as both a current collector and a substrate to grow multiple layers on it.The transparency of the glass electrode plays a pivotal role in letting sunlight pass through and reach the active region of the cell.Sheet resistance, on the contrary, is in inverse relation to the carrier assemblage of the cell: the lower the resistance, the higher the charge collection.To this end, FTO, with regard to its low sheet resistance and thermal stability is the first choice to be used as a conducting electrode.Sima et al. 78 proved that contrary to FTO, ITO is not thermally stable, showing a resistance increase from 18 to 52 Ω/cm 2 at a high-temperature heat treatment.Hence, ITO is applicable for the low-temperature fabrication of cells.Moreover, Xu et al. and Hu et al. 3,6 reported the application of SnO 2 at low temperatures of 120−150 °C as ETL on ITO to construct the SCs.On the flip side, in connection with the application of TiO 2 as ETL at higher temperatures, FTO is preferred. 46,79In addition, polymers are alternated by dint of their versatility and affordability.Despite these advantages, they are restrained by the applicable temperatures.For flexible SCs, the conductive electrode should possess several major exclusivities including good endurance, high mechanical flexibility along with high conductivity, and transparency.Up to this point, ITO, silver nanowires, Al-doped ZnO, carbon nanotubes, and organic materials have been utilized among which ITO still preserves its prevalence as a conductive electrode. 80Hu and colleagues fabricated efficient flexible QDSCs by using conventional polyethylene terephthalate (PET)/ITO substrates. 3In this sense, the significance of further research on high performance and flexible SCs via using higher mechanical endurance of QD films has been emphasized.Up to now, certified efficiencies of 25.5 and 19.5% have been recorded on rigid and flexible conductive substrates, respectively. 81.1.1.2.Electron Transport Layer (ETL).The presence of an ETL in a cell device is a prerequisite to obtaining a PSC with higher efficiency.An ETL expedites the assemblage of electrons and their journey to the corresponding electrodes. 82ompetent ETLs should hold a few features such as (i) decent electron mobility to ease electron transit within the layer, (ii) a broad bandgap to allow more light to travel through and thereby increase the light absorption amount by perovskites, and (iii) suitable energy levels with consistency between the conduction band minimum (CBM) and the valence band maximum (VBM) in connection with perovskite materials as light collectors, alleviating electron transit and obstructing hole motion through the perovskite layer.A compact ETL encourages the excited electrons from the active layer to the external circuit and at the same acts as a hole blocking layer (HBL) to obstruct exciting holes to FTO glass. 83Likewise, compact ETL restricts backward electron transit from FTO to the active layer. 84As a result, a compact ETL needs to consist of a structure that is regular, stable, as well as ultrathin.An ETL with a mesoporous structure in a PSC device accommodates hollow spaces within its framework, which is a path for perovskites to permeate.A mesoporous structured ETL expands the interfacial surface area between the active layer and ETM, providing facile charge transfer kinetics. 85To prevent direct contact between the active layer and the conducting substrate, a compact ETL or so-called HBL is placed under the mesoporous ETL.Lacking the HBL gives rise to back recombination of electrons and holes, and in turn, reduces values of open-circuit voltage (V OC ) and fill factor (FF) of the device. 86So far, many inorganic compounds have been nominated as promising ETLs for PSC devices, some of which are TiO 2 , 87 SnO 2 , 88 and ZnO. 89Various manufacturing techniques have been identified for the deposition of ETLs on the conducting electrode.Based on the utilized conducting substrate, ETLs can be obtained via high-or low-temperature processes.
3.1.1.3.ETL/QDs Chemical Interface Engineering.Engineering the active layer (perovskite) and correlated interfaces (active layer-ETM and active layer-HTM) are critical parameters to develop PSC devices with high performances.−92 To date, the most successful trial in terms of application of ETM in PSCs has been attributed to mesoscopic TiO 2 (m-TiO 2 ) thanks to its perfectly large interfacial contact with the active layer 93−95 Considering the fact that film growth takes place in a brief period, the homogeneous introduction of QDs into the mesoporous structure is not a straightforward task to fulfill.Therefore, researchers have mostly used compact TiO 2 (c-TiO 2 ) in preference to m-TiO 2 to develop all inorganic lead halide QDbased SCs. 90,96In addition to this, m-TiO 2 films contain much larger irregularities on the surface as opposed to c-TiO 2 films that bring forth the formation of a QD capping layer, leaving negative effects on the interfacial properties of TiO 2 /QD.In this respect, proper modulation of both the ETM layer and ETM/QDs interface are the fundamental ingredients to attain α-CsPbI 3 QD-based PSCs with high performances. 97hen et al. 97 have studied the chemical interface of ETL/ QDs by using the mesoporous TiO 2 /QDs layer.The results of current−potential (J−V) characteristics for both c-TiO 2 (∼40 nm) and m-TiO 2 (∼200 nm) based CsPbI 3 PQDSCs were examined.c-TiO 2 -based PSCs (c-PSCs) delivered a shortcircuit current density (J SC ) value of 14.44 mA•cm −2 and a PCE of 8.77%, which are appreciably higher compared to those of m-TiO 2 -based PSCs (m-PSCs) (10.07 mA cm −2 , 5.01%), suggesting better charge split-up and improved carrier transit for planar structure.The lower performance of m-PSCs can be ascribed to the inhomogeneous distribution of QDs in the framework of m-TiO 2 ETM along with improper engineering of the m-TiO 2 /CsPbI 3 QDs interface.
Figure 4a,b demonstrates the schematic view of the CsPbI 3 QD-based PSC with mesoporous structure and the introduction process of QDs into the structure of the mesoporous layer that has been treated by Cs.Lack of Cs-treatment results in easy adsorption of CsPbI 3 QDs on the surface of the mesoporous layer, blocking m-TiO 2 porous structure and in turn discouraging migration of QDs into the mesoporous layer.On the contrary, treatment with a cesium acetate (CsOAc)/ methyl acetate (MeOAc) solution lowers the adsorption energy of the QDs on the surface of TiO 2 , leading to the effective introduction of QDs into the mesoporous layer.In brief, the Cs-m-TiO 2 /QDs film exhibited a 2-and 3-fold higher rates of electron delivery compared to respective films of c-TiO 2 /QDs and m-TiO 2 /QDs.It is noteworthy that the observed irregularities on the surface of m-TiO 2 film were correlated with the spin-coating process used for its fabrication.Chen and colleagues 97 conducted research to flatten the m-TiO 2 film through a postdeposition process called the smoothing (s-m-TiO 2 ) approach by employing ethanol.All the PQDSCs developed by Cs-s-m-TiO 2 /QD films delivered PCE values beyond 13%, confirming the efficacy of the smoothing approach by ethanol.
Hu et al. 3 succeeded to build a thin hybrid interfacial architecture (HIA) via incorporating phenyl-C61-butyric acid methyl ester (PCBM) into the structure of CsPbI 3 QD taking most of QDs' massive surface areas.Figure 4c depicts the related QD film deposition.The PCBM forms bonds with Pb 2+ ions on the surface of QD by means of carboxyl groups leading to the creation of an exciton cascade between SnO 2 as ETL and the CsPbI 3 QD layer leading to enhanced generation of excitons at the QD/ETL interfaces.From Figure 4d, it can be seen that PCBM with sufficient electron affinity acts as an adjustment layer between the SnO 2 and the CsPbI 3 QDs, restricting charge carrier recombination between the two layers.Consequently, PCBM is interpreted as a road for swift electron delivery in QD devices.It is pronounced from Figure 4e that integration of PCBM with CsPbI 3 QDs at the ideal ratio leads to efficacious electron transfer at the interfaces.Recently, Lim and colleagues 98 have reported that TiO 2 nanoparticles can expedite the degradation of cubic-phase CsPbI 3 -PQDs due to their intense photocatalytic activity.
Hence, they developed chloride-passivated SnO 2 (Cl@SnO 2 ) QDs as the ETL with poor photocatalytic activity.Compared to nonpassivated SnO 2 , the passivated Cl@SnO 2 ETL exhibited more hydrophobic characteristics and an enhanced Fermi level, which contributes to the improvement of device stability and V OC , respectively.Given these advantages, the PQDSC based on Cl@SnO 2 ETLs demonstrated an enhanced PCE up to 14.5% compared to that of TiO 2 -based control devices (PCE of 13.8%) and tremendously improved stability, maintaining 80% of the initial PEC, without encapsulation under 50% RH conditions for 8 h compared to the TiO 2 -based control device (maintaining 28% of the initial PEC, at the same condition).
An overview of recent works on the interface between ETL and the perovskite layer may serve as a prototype for future research.Zhang and colleagues 99 have also used carbon dots (CDs) to modify interfaces between metal oxide ETLs and perovskite active layers.On the report of this, CD modifiers can reduce the ITO work function allowing to significantly improve the performance of PSC devices.Zhong and coworkers 100 lately documented that tetrabutylammonium acetate (TBAAc) as a buffer layer between the SnO 2 ETL and CsPbI 3 interface not only improved the conductivity of ETL but also formed a 1D TBAPbI 3 layer between the ETL and the perovskite film resulting in enhanced stability and perovskite cell performance.Likewise, black phosphorus QDs have been employed as an additive component to a precursor solution of SnO 2 nanoparticles and it could effectively improve the performance of ETL in the PSC device. 101.1.1.4.Active Layer.A perovskite layer�active layer�is the core of a PSC device, containing a photoresponsive material and responsible for encouraging exciton generation.PQDs exceed their bulk counterparts in many aspects.The merits that they offer incude but are not limited to tunable bandgap through a quantum-confinement effect, 21 multiple exciton generation, 22 a PLQY of nearly one, 21,23 and photon up-and down-conversion. 24Thereupon, QDs are expected to surpass the Shockley−Queisser limit 26 and excel in the tandem PSCs.Furthermore, PQDs can be more fascinating owing to their solution processability at ambient temperature, providing a facile technique and dissociation of film deposition and grain growth. 46PQDs with satisfactory endurance against defects are rising as potential materials to supersede conventional chalcogenide QDs in PV applications. 21Yet, both CsPbI 3 and FAPbI 3 QDs for instance are extremely vulnerable to exposure to wetness and polar solvents after synthesis treatment. 6,46,102The preceding setback creates complications in ligand modulation and thereby escalates the number of defects throughout the framework, restricting carrier transit and minimizing PCE values.Figure 5 shows the plot of the year versus the record PCE of QD-based SCs.Table 1 summarizes the chronological improvement of PQDSCs with respect to their device architectures and photovoltaic performance.
3.1.1.4.1.Surface Ligand Management of PQDs.It is widely believed that the first surface layer of a 10 nm PQDs contains approximately 20% of the atoms.Various factors affect the surface chemistry of semiconductor nanocrystals, including crystal facets, stoichiometry, organic ligands, and the ionic state.Therefore, to rationally improve and tune the photophysical properties of halide perovskite NCs, it is essential to understand their surface chemistry. 119As reported by earlier research, X-ray photoelectron spectroscopy (XPS) analysis revealed that on the surface of the CsPbBr 3 NCs, the Br:Cs elemental ratio is ∼5 and it decreases to ∼3 in the bulk, indicating a surface excess of Br.However, for a CsBr termination, some of the surface Cs-sites could be occupied by alkylammonium cations, resulting in Br:Pb ratios exceeding 3 and Cs:Pb ratios of ∼1. 120Moreover, the measured internuclear distances of H−Cs and H−Pb between dodecylammonium −NH 3 + ligand protons and surface and subsurface Cs and Pb spins confirmed termination of CsBr with alkylammonium ligands which substituted into some surface A-sites. 121t is believed that the native ligands influence particle morphology, stability, and solution dispersibility, and they could passivate electronic trap states.It is also well-known that there are two kinds of native ligands on CsPbBr 3 NCs: longchain primary alkylammonium ligands (oleylammonium) and long-chain carboxylate ligands (oleate).The maximum density of organic ligands has been demonstrated to be approximately 2−3 ligands per nm 2 for ligands with 0.3−0.5 nm −2 ligand footprints. 121It should be highlighted that even in nonpolar solvents, the native surface ligands are unstable, which results in poor stability NCs. 122It was determined by Brutchey et al. 123 that the native oleylammonium and oleate ligands can be exchanged with free alkylamines and carboxylic acids inside a solution.Inferred from this dynamic equilibrium of ligand, it is favorable for oleate to exchange with 10-undecenoic acid at room temperature or higher.On the contrary, with increasing temperatures, the exchange of oleylammonium with 10undeceneamine becomes less favorable.A collection of amine ligands was examined for the exchange of native amine ligands of CsPbBr 3 NCs in order to effectively passivate the surface. 124In this sense, the stoichiometric addition of strongly basic primary alkylamines (short C4−C8) could effectively bind to the surface and improve colloidal stability.
In 2020, Wang et al. 17 introduced Cs 1−x FA x PbI 3 without Br and volatile MA as a decent perovskite, demonstrating better moisture, thermal, and photostability.With reference to stability and charge transport traits, mixed-cation Cs 1−x FA x PbI 3 QDs are more worthwhile than their pure ingredients�CsPbI 3 and FAPbI 3 QDs.The reason is that the resulting Cs 1−x FA x PbI 3 QDs possess higher stability under ambient conditions or in contact with polar solvents in comparison with pure CsPbI 3 or FAPbI 3 QDs.Besides, they display extremely smaller trap densities and longer carrier lifespans.The simultaneous introduction of FA and Cs generates entropic stability in the structure of perovskite under surrounding conditions. 125In addition, the swift rotation of FA brings about intensified orbital overlap and simpler establishment of polaron, giving rise to lower nonradiative recombination and longer carrier lifespans. 126Furthermore, the enhanced light collection followed by greater J SC is because of the wider spectral absorption as a result of the introduction of FA.The development and diffusion of A-site cation vacancies, which are of significant importance for inducing the crossexchange of cations, are subject to the surface ligands.Wang et al. 17 suggested that the oleic acid (OA) ligands can be utilized to remove obstacles by solvation of A-site cations of QDs including Cs-oleate and FA-oleate in the colloidal solution.They stated that once the original colloidal solutions (CsPbI 3 and FAPbI 3 ) were treated twice with MeOAc prior to mixing, a great portion of OA ligands was eliminated, leading to the generation of an OA-deficient environment (Cs 0.5 FA 0.5 PbI 3 -QD-OD).To integrate the two distinctive photoluminescence (PL) peaks of CsPbI 3 and FAPbI 3 QDs peaks into a single and wide peak, tens of hours were spent.The presence of a high kinetic barricade for cations to leave the initial QDs and incorporate them into other QDs is signified through the sluggish reaction.To this end, to produce an OA-enriched environment (Cs 0.5 FA 0.5 PbI 3 -QD-OE), Wang and colleagues 17 preserved more OA ligands in the original colloidal solutions.As a result, the two above-written PL peaks were swiftly integrated into one on account of mixing and the produced emission spectrum did not show alteration within 30 min, manifesting high-speed cation exchange completion (see Figure 6a,b).As a result, the cation-exchange process under OA-rich conditions is promoted compared with that in the OAless environment.Reproduced with permission. 17Copyright 2020, Nature.(e) Schematic illustrations of MeOAc Treatment on the Surface of CsPbI 3 PQDs.Reproduced with permission. 127Copyright 2021, The American Chemical Society.(f) Schematic illustration of the size selection via GPC and (g) variation in solution-phase absolute PLQYs according to the elution sequence of the Pe-CQD eluate after GPC.Reproduced with permission. 112Copyright 2021, The American Chemical Society.
The fundamental characteristic in ligand tuning is the significant difference that maintains high radiative performance by restraining the growth of surface defects in addition to the promotion of the cation-exchange reaction (shown in Figure 6c,d).In light of the results, the CsPbI 3 -QD device revealed a J SC of 15.4 mA cm −2 , a V OC of 1.16 V, and an FF value of 53.9%, producing a PCE of 9.6%.Surprisingly, SCs based on Cs 0.5 FA 0.5 PbI 3 -QD-OD yielded a low V OC of 1.08 V and a J SC of 14.6 mA cm −2 , delivering a PCE value similar to the reference device�10.1%.On the contrary, the PCE values obtained from SCs based on Cs 0.5 FA 0.5 PbI 3 -QD-OE were beyond 16% owing to significantly improved J SC .Copyright 2020, The American Chemical Society.
Since MeOAc is a well-known solvent in the ligand exchange of PQDs, the reproducibility of PQDSCs exploiting MeOAc treatment has been considered one of the bottlenecks for these cells.The importance of keeping long-chain organic ligands for preserving the stability of the crystal phase and getting rid of the ligands for high charge transport drive researchers to investigate the degree of MeOAc treatment on CsPbI 3 PQD solid films.In this regard, recently Han and colleagues 127 studied the effect of MeOAc treatment on PQDSC performance.Based on that, using several cycles of MeOAc treatment within a certain range they could eliminate the ligands and passivate the defects of the PQD film surface, thereby increasing the charge transport inside the film.But excessive MeOAc treatment can induce undesired phase transition and degradation of the device.In general, I-and A-site vacancies are the main factors in creating defects.Thus, Lewis base groups including carboxylate and thiol groups might be absorbed to the uncoordinated Pb atom formed due to the I vacancy, while Lewis acids such as the ammonium group could be adsorbed to the uncoordinated I formed due to the Cs vacancy (see Figure 6e).Keeping these in mind, it was proven that optimizing the MeOAc treatment could enhance carrier lifetime and reduce trap density throughout the QD film. 127im et al. 128 improved the electronic coupling and photovoltaic performance of CsPbI 3 PQDs by using sodium acetate (NaOAc) in MeOAc solution to remove the long-chain oleate ligands.In the conventional method, acetic acid and methanol are produced as intermediate substances in the hydrolysis of MeOAc.Consequently, the metal hydroxide (i.e., Pb-OH) is formed from methanol produced during the process.In the light of experimental results, by efficient removal of long-chain oleate ligands using a solution of NaOAc in MeOAc (which results in the direct generation of OAc ions without forming protons and methanol), surface trap states originating from metal hydroxide formation could be minimized, leading to an improved device performance of 13.3%, compared to a lead nitrate-treated control device (with a PCE of 12.4%).
It should be overemphasized that polar-antisolvent-based purification induces agglomeration of PQDs after ligand management steps bringing about the irregular size distribution of QDs.Therefore, the polydispersity of PQDs has been also considered one of the bottlenecks for the PQDSC fabrication due to bandtail broadening originating from irregular-sized PQDs.It is well established that these irregular-sized PQDs could enhance the energetic disorder in the PQD films.Recently, Lim and co-workers applied monodisperse of PQDs in identical sizes and band gaps in the PQDSCs.They demonstrated that well-purified monodisperse CsPbI 3 QDs could be obtained via size selection by using gel permeation chromatography (GPC).These monodisperse QDs exhibited higher PLQY in comparison to irregular-sized PQDs (see Figure 6f,g).In this respect, the SCs fabricated by well-purified monodisperse CsPbI 3 QDs have encompassed the highest PCE of 15.3% amidst all inorganic PQDSCs reported so far. 112ue et al. 6 categorized the solvents into three grades based on the magnitude of polarity in order to lay down a practical process for surface ligand treatment.For this, "grade I" indicates those solvents with a strong polarity that are able to annihilate ionic bonds in FAPbI 3 .On the other hand, "grade III" is denoted to those solvents containing the least polarity, unable to split the bond between the ligand and FAPbI 3 . 129In this regard, successful treatment of ligands without the annihilation of CQDs needs solvents with medium polarity ("grade II").Accordingly, water, methanol, ethanol, and isopropyl alcohol labeled as grade I are strong solvents; tbutanol, 2-pentanol, acetonitrile (ACN)/toluene, and EtOAc as grade II are solvents with medium polarity; chlorobenzene, toluene, octane, and hexane as grade III bear weak polarity.Xue and colleagues 6 were able to design a method that gradually decrease the density of ligand in solution and the solid phase, while the unity of FAPbI 3 CQDs remained largely intact.The gradual reducing the polarity of a grade II solvent such as 2-pentanol in each run of surface ligand treatment played an essential role in this procedure.
3.1.1.4.2.Surface Passivation and Post Treatment of PQDs.3.1.1.4.2.1.Cesium Cation (Cs + ) Passivation.Previous reports on PbS QDs have proved that surface ligands can modify some extraordinary properties of QDs such as solution processability, electron coupling between QDs in film, trap densities, and long-run durability. 24,68Thus, the extensive realization of the ligand exchange process during manufacturing CsPbI 3 QDSC along with the acceleration of charge transit is still of substantial importance.Thus, the development of a practical approach for surface treatment of PQDs is a critical key.To achieve this objective, a straightforward strategy for surface passivation of CsPbI 3 QD films employing several Cssalts (CsOAc, cesium iodide (CsI), cesium carbonate (Cs 2 CO 3 ), and cesium nitrate (CsNO 3 ) was performed by Ling et al. 105 They substantiated that cesium salts can boost both atomic defects by filling the vacancies and electron coupling between CsPbI 3 QDs.
To conduct surface passivation, pure MeOAc is applied to play two roles: (i) to eradicate intrinsic ligands on the CsPbI 3 QD during its fabrication and (ii) to provide proper grounds for succeeding layer growth.Once the required CsPbI 3 QD film with a suitable thickness (300−400 nm) is achieved, it is then washed with a saturated Cs-salt solution in ethyl acetate (EtOAc).As it was confirmed by Ling et al. 105 different Cssalts can enhance the device performance of CsPbX 3 PQDSCs.Amidst the above-written salts, one stands out, CsOAc.The QD film treated by CsOAc, displayed boosted carrier mobility besides reduced recombination thanks to both improved electron coupling and surface passivation.To this end, the PEC values secured by CsOAc-, Cs 2 CO 3 -, CsI-, and CsNO 3treated CsPbI 3 QDSCs are as follows 14.10%, 13.14%, 13.74%, and 13.67%, respectively, outrunning the PCE value of the control device, 12.59%.From Figure 7a, the solution processable CsPbI 3 QDs contain long intrinsic ligands remaining from a spin coating that can act as insulators.MeOAc is applied to wash the QD layer and remove insulating ligands, turning it into a conductive layer.Cs atoms can be lost in the course of this process together with bound Cs-oleate, creating Cs vacancies in perovskite structure. 130,131For the case of the QD layer treatment with the CsOAc solution in EtOAc, further washing with the EtOAc reinforced electron coupling between QDs, which in turn led to higher carrier delivery in CsPbI 3 QD layers. 132.1.1.4.2.2.Post-Treatment.QDs enjoy the wide bandgap tunability and its associated energy levels.Moreover, about CsPbI 3 , there is additional merit�that is, phase durability.Yet, QDSCs display insufficient V OC values compared to their wide bandgap and unsatisfactory efficiency when it comes to carrier assemblage.These drawbacks can be solved to a great extent by coupling CsPbI 3 QD layers.The AX technique is a general strategy to adjust the electronic attributes of the CsPbI 3 QD films.In this respect, Sanehira and co-workers 79 exploited an AX method (A-site cation halide salt) and confirmed the reliability of this method to modulate the electronic properties of QDs and subsequently improve carrier mobility.
Based on the results, in the case of addition of FAI to a CsPbI 3 QD film, although FA + was clearly present in the treated films, the chemical composition of the QD core was unlikely to undergo drastic changes.It has been demonstrated that the AX salt-containing materials neither alloy with QDs nor encourage grain growth.Instead, they coat the QD layer.Following deposition of the QD with an adequate thickness, it is afterward soaked in a saturated X salt solution in EtOAc.The comparison of J−V characterizations for devices with different AX salt treatments is illustrated in Figure 7b.Postsynthesis treatment that was carried out by each AX salt revealed a notable improvement in terms of the performance of the CsPbI 3 QDSCs compared with the control device, manifesting the general applicability and efficacy of this technique.Figure 7c depicts a flow diagram, illustrating this process for film assembly.Amidst these AX salts, the device treated by FAI showed the highest PCE, 13.4%, and a J SC of 14.37 mA/cm 2 which were considerably greater than those of the pure EtOAc control with a PCE and J SC of 8.5% and 9.22 mA/cm 2 , individually.
Khan and colleagues 133 reported the utilization of a pmercaptopyridine (4-MP) ligand for post-treatment of PQDs and acquired not only boosted electronic coupling but also cubic phase robustness in post-treated PQDs in comparison with the ones treated with o-mercaptopyridine (2-MP) and pyridine ligands.They demonstrated that the residual oleic acid (OA) and oleylamine (OLA) native ligands can proficiently exchange with 4-MP ligands (schematic illustration shown in Figure 7d), which leads to the best efficiency of 14.25% due to the enhanced electronic coupling and fewer trap states.What is more, the intense anchoring strength of the bifunctional 4-MP linker leads to the improvement of the SC stability.The investigation manifested that the ligand surface coverage and their strength can be promoted by regulating the anchoring position, following an enhancement in the stability and performance of the PQDSC devices. 133Analogous to this, Xue et al. 107 used conjugated small molecules ITIC in fabricating PQDSC to provide an additional driving force to separate charges more effectively.In this sense, boosted device performance approaching 13% in FAPbI 3 PQDSCs was recorded by removing the original insulating surface ligands of QDs and replacing ITIC in QD film by taking advantage of the layer-by-layer (LBL) deposition technique.Besides, an efficient solid-state-ligand exchange process was performed on a CsPbI 3 QD film via exploiting organic ligand triphenyl phosphite (TPPI) by Wang and co-workers. 117They proved that TPPI could balance the carrier delivery and thereby passivate the QD surface.In this sense, the CsPbI 3 PQDSC with a champion efficiency of 15.21% was achieved with an improved V OC and a high FF.
3.1.1.4.2.3.Dual Passivation of PQDs.Jia et al. 109 studied the application of amino acids as dual-functional ligands that are able not only to enhance the defect passivation of PQDs but also to retain the cubic phase of PQDs.For the construction of the PQD film, first, a short ligand solution is prepared in the MeOAc solvent.Thereafter, for ligand exchange, the PQD solid film with OA and OLA ligands on the PQD surface is completely rinsed with a short ligand solution.Applying amino acids as dual-passivation ligands can passivate both the A-site (cesium) and iodine vacancies on the PQD surface, resulting in enhanced defect passivation.The results proved improvement of the charge transport property between individual PQDs through a short alkane chain of amino acids.In this respect, Jia and colleagues 109 constructed PQDSC devices containing planar structure to explore the PV characteristics of amino-acid-based PQD solid films.For the sake of comparison, conventional PQDSC was also developed by exploiting the lead(II) nitrate (Pb(NO 3 ) 2 )-based PQD solid film as a light absorber in the device.From the results, the Pb(NO 3 ) 2 -based PQDSC yields a J SC of 16.30 mA cm −2 , a V OC of 1.19 V, and an FF value of 0.60 with a high PCE of 11.69%, confirming the fact that the obtained results are analogous to or even better than the reported values for PQDSCs in the literature. 79On the contrary, the PQDSCs based on glycine displayed 17.66 mA cm −2 , 1.22 V, and 0.63 for J SC , V OC , and FF, respectively, along with an improved PCE of about 13.66%.
3.1.1.4.2.4.Layer-by-Layer FAI Treatment.Shivarudraiah et al. 110 studied LBL FAI treatment on inverted systems containing all-inorganic charge transport layers with the following arrangement: FTO/NiO x /CsPbI 3 QDs/C 60 /ZnO/ Ag.Compared to a typical single-step postdeposition FAI treatment, in this method, each layer is treated separately with FAI, which brings about notable PV performance.A thorough investigation conducted on the device configuration shows that the LBL FAI treatment approach discards intrinsic ligands including oleate and oleylammonium and offers better surface passivation and electron coupling.In this technique, a layer of QD solution is spin-cast onto the substrate several times.The chemical substitution of oleate ligands can be carried out via two sequential treatments including saturated salts of Pb-(NO 3 ) 2 in MeOAc solution and AX salts.The Pb(NO 3 ) 2 in MeOAc solution is applied to treat control QD films in each layer individually, and subsequently, a single post-treatment is conducted through FAI in an EtOAc solution.Figure 8a depicts the relevant ligand exchange protocol.Shivarudraiah and colleagues 110 proved the complete elimination of nearly all oleylammonium, oleate, or octadecene, confirming the substantial degradation of all intrinsic ligands for the FAI-QD films.As a result, it can be stated that the LBL FAI treatments appear to be quite useful not only to discard a great deal of carbonaceous materials but also to create a thick layer and produce better films.It should be noted that the FAI treatment does not alleviate the formation procedure of QDs alloy (Cs x FA 1−x PbI 3 ) which is substantiated by the lack of noticeable change in the absorption and PL peaks.Consequently, the following statement can be drawn to attention that the effect of the LBL FAI process is restricted to improved surface passivation and QD coupling.Based on the results, control QDSCs delivered a V OC of 1.17 V, a J SC of 13.12 mA/cm 2 , and an FF value of 0.752 with a PCE of 11.5%.Encouragingly, the FAI QD-based SCs displayed a V OC of 1.19 V, a J SC of 14.25 mA/cm 2 , and an FF value of 0.776 with an enhanced PCE of 13.10%.
3.1.1.4.3.PQDs Heterostructure.Heterostructure-containing optoelectronics can remarkably regulate the energy positions of electrons and holes within the device. 134,135Not to mention that in PV devices, fine-tuning the energy band promotes the collection of photoinduced charge carriers. 136,137he role of solvents in PQDSCs is not to dissolve starting materials, but precisely, to form a suspension of QDs through their surface ligands under a colloidal solution.In a typical LBL technique, initially, the deposition of a layer of QDs is executed, followed by ligand removal, making the resulting layer nonsoluble in the nonpolar solvent, preparing fine conditions for succeeding film deposition without impairing the layer underneath. 46,79,102The active layer can be made up of either one type of QDs or a composition of various QDs, enabling the modification of the perovskite layer according to the required target across the film.For instance, Zhao et al. 106 conducted a spin-coating technique containing four courses, with which a 300 nm-thick layer was produced with respect to the concentration of QD and solution properties.
It was discovered that underlying film engenders a certain shift in the absorption onset of the composite.Besides, the effect of the thickness ratio of the Cs 0.25 FA 0.75 PbI 3 layer� composed of different compositions�to the CsPbI 3 layer was examined via the J−V characterizations.The results revealed that the device with a Cs 0.25 FA 0.75 PbI 3 : CsPbI 3 of ratio 1:3 was named the best-performing device with a PCE of 15.52%.This outstanding performance stems from intensified J SC followed by higher photoinduced carrier collection with this construction.As shown in Figure 8b, the external quantum efficiency (EQE) of cells is subject to the internal heterostructure interface position of the layers in which the collection efficiency was diminished for those devices including higher amounts of FA.Meanwhile, a systematic shift in the absorption onset is also caused by the composition of the bottom layer.What is more, it was found that device performance is quite similar for all following compositions Cs 0.75 FA 0.25 PbI 3 , Cs 0.5 FA 0.5 PbI 3 , Cs 0.25 FA 0.75 PbI 3 , or FAPbI 3 , when they are used as underneath layer in the absorber.The reason may lie in identical energy levels and charge split up in every one of the mentioned compositions. 106he accomplishment of production of competent and durable single-junction PQDSCs is subject to meeting two requirements including (i) long-distance electron delivery and (ii) prolonged photon absorption.Li and co-workers 104 made an effort on the α-CsPbI 3 /FAPbI 3 bilayer structure to adjust simultaneously both light collection and the electronic traits.Based on the results, the best-performing title was devoted to the single α-CsPbI 3 QD device with a PCE of 12.3% and a J SC of 13.80 mA/cm 2 comprising of four-layer α-CsPbI 3 QDs.At the same time, the single FAPbI 3 QD device comprising threelayer FAPbI 3 QDs displayed a PCE and J SC of 12.0% and 15.97 mA/cm 2 , separately.The higher J SC of FAPbI 3 -based devices, as opposed to α-CsPbI 3 -based SCs, is associated with the former's wider absorption.Conversely, the devices composed of both α-CsPbI 3 and FAPbI 3 QDs demonstrated meaningfully enhanced performance.To this end, a device consisting of three layers of α-CsPbI 3 and two layers of FAPbI 3 QDs delivered a PCE, V OC , and J SC of 15.6%, 1.22 V, and17.26mA/ cm 2 , respectively, along with a relatively higher FF value of 0.74.Analogous to this work, Park et al. 138 reported a multinary PQD layer by combining CsPbI 3 and FAPbI 3 PQD layers to develop a heterostructured PQDSC.In this sense, a PCE of 16.07% with negligible hysteresis and significantly enhanced stability was obtained.
Yuan and colleagues 114 benefitted from the gradient-band alignment (GBA) homojunction of PQDs in SCs.Consequently, CsPbI 3 PQDs with three different E g values (by controlling the size of the PQDs) were used to fabricate SCs with a gradient-band homojunction structure (see Figure 8c,d).In this structure, due to the additional driving force for the electrons, the GBA could provide promoted charge extraction and carrier diffusion length in the PQD film.As a result, by reducing the potential loss of the SC, high efficiency of 13.2% could be achieved along with improved stability.In this heterojunction, the migration of doped ions could be also suppressed due to homojunction formed from samecomposition PQDs.
Lately, Zhang et al. 118 reported an internal P/N homojunction by using CsPbI 3 PQDs over 1 μm-thick.In this context, to produce different carrier-type QD arrays, an organic dopant named F6TCNNQ was used in the arrays to transform the carrier-type from n-type to p-type.These different carrier- ).(d) Atomic models of cubes and platelets having 10 and 20% Sb (w.r.t.Pb) obtained from reaction with different Sb intakes.These percentages were calculated from EDS in the nanocrystals, but originally, the reaction intakes were 25 and 50% Sb with respect to Pb for cube and platelet formations.Reproduced with permission. 152Copyright 2019, The American Chemical Society.
type QDs were used in assembling P/N homojunction PQDSCs generating a boosted efficiency of 15.29%.
Hybrid PQD/nonfullerene molecule SCs have recently attracted great attention; however, their construction is still challenging.Yuan and co-workers 115 reported hybrid SCs with superior PV performance consisting of CsPbI 3 PQDs and Y6 series nonfullerene molecules.In this work, CsPbI 3 QDs and nonfullerene molecules were used as a hybrid active layer.It was proven that the nonfullerene molecules not only could increase the charge transfer and extraction in the hybrid active layer by introducing a type-II energy alignment but also could passivate the QDs by diminishing surface defects and energetic disorder.Based upon the results, the high-performance hybrid SC with an efficiency of 15.05% was acquired, proving the rational engineering of the solution-processable hybrid active layer to adjust efficient optoelectronic devices.
3.1.1.4.4.PQDs Compositional Engineering.Although the phase stability of perovskites has been improved in the form of QDs compared to their bulk counterparts, 139 under operation, the stability of PQDSCs has been considered one of the bottlenecks for the PV devices.To circumvent this issue, we witnessed a lot of efforts by using fundamental strategies including compositional engineering, QD surface treatment, and optimization of the SC layered structure.Recently, compositional engineering has aroused great research interest in Cs-based PQDs due to its enormous capacity to tune the structures to avoid the formation of undesirable phase transitions.It is widely postulated that the stability of the SCs can be enhanced by modifying the perovskite composition to receive satisfactory tolerance factors.For the ABX 3 perovskite structure, appropriate doping of the A-site, B-site, and X-site can stabilize the crystal structure and improve the optoelectronic properties of the device.In the case of A-site doping, filling the Cs vacancies on the surface of QDs could promote the stability of perovskite structure in the air environment. 105On the contrary, B-site doping could strengthen the structure along with adjusting the electronic properties of QDs.Besides, as the B-site cation is Pb 2+ , partially replacing Pb with a proper atom could also diminish nonradiative recombination, improving the device performance. 105To obtain desired tolerance factor, the ionic radii of Asite, B-site, and X-site ions must be correlated to the Goldschmidt tolerance factor in the following relation: where r A , r X , and r B stand for ionic radii of A-site, X-site, and Bsite ions, respectively.It has been demonstrated that in the view of SC with superior performance, the ABX 3 should be in a cubic phase structure consisting of corner-sharing [BX 6 ] octahedra with the A cation staying in the 12-fold coordination sites surrounded by the cube of eight octahedra. 140For the stable cubic perovskite, the Goldschmidt tolerance factor, t, should be between 0.9 and 1. 141 It should be noted that CsPbI 3 QDs hold a t value as low as 0.8.For this, the rational engineering of ionic radii�that is, diminishing the r B and enlarging the r A is accepted as an effective scheme to obtain a reliable tolerance factor.Likewise, the octahedral factor, μ, is another critical parameter to evaluate octahedral stability that is defined as the following: To achieve the stable octahedral [BX 6 ] 4− , the required μ should be between 0.442 and 0.895.Figure 9a depicts some of the reported t and μ values for A-site and B-site doping in the PQD structure.Therefore, for B-site doping, satisfying the t along with μ is essential to meet the requirements of getting a stable cubic structure.
It should be stressed that although FA + and MA + as A-site candidates have larger ionic radii than Cs + , producing a highly anisotropic strain of cubic phase lattice (therefore creating deviation from the equilibrium interplanar distances) 11 along with the volatile nature of these cations are the major drawbacks of replacing the A-site with FA + and MA + cations. 143The unsatisfactory efficiency and poor stability of organic−inorganic PQDSCs owing to the preceding bottlenecks have made researchers move to and adopt using allinorganic CsPbI 3 QDs.Thereby, B-site doping serves as a prototype for future research in the development of allinorganic CsPbI 3 QDs.
In addition, in the case of B-site doping, replacing Pb with a tiny amount of B-site dopant with a smaller ionic radius than that of Pb 2+ can alter the phase structure and enhance optoelectronic properties. 11Zou et al. 144 investigated Mn doping in CsPbBr 3 structure by DFT and proved that the much shorter Mn−Br bonds compared with Pb−Br bonds cause a lattice distortion that leads to better thermal stabilities of target PQD.Analogous to the Zou et al. work, Akkerman et al. 21showed that shorter Mn−I bonds in Mn-doped CsPbI 3 are responsible for the lattice contraction along with the negligible variation of the electronic structures at the band edges.Consistent with the tunable bandgaps, B-site doping could shift the PL position.In this way, proper doping can also boost the PLQY.In fact, B-site doping could regulate the lightemitting aspects of PQDs on account of the critical status of Pb in the PQDs' structure.It is also noteworthy to underline that according to eq 1, to increase the t value, replacing X-site with smaller elements e.g., Cl or Br would be even more beneficial.Nevertheless, these alloying elements induce an unintended blue shift in the absorption edge due to the bandgap broadening. 145As a result, as a paradigm for future research of stable PQDSCs, developing new strategies toward improving the perovskite phase stability without increasing the bandgap via doping the B-sites with smaller metal cations is essential.
To date, several metal ions have been introduced to replace Pb 2+ in PQDs including Mn 2+ , 146 Sn 2+ , 147 and Sr 2+ . 148Zhang et al. 149 reported Zn-doped CsPbI 3 QDs exploiting ZnI 2 as the dopant with enhanced thermodynamic stability.Based on the results, the additional iodine ions could reduce the halogen vacancies on the QD surface (schematic illustration shown in Figure 9b), leading to diminished nonradiative recombination and improved PLQY.Furthermore, the Zn-doped CsPbI 3 PQDSC exhibited an enhanced efficiency from 13.98 to 16.07% compared to the undoped device.Analogous to this work, Bi and colleagues 150 reported CsPb 1−x Zn x I 3 QDSCs with improved efficiency and stability.Behera et al. 151 reported Ni(II) ion as a dopant in red-emitting CsPbI 3 QDs structure with a maximum PLQY of 81% and the stability recorded for 2 months.Liu et al. 147 also reported that CsSn 1−x Pb x I 3 QDs alloy with superior stability to the CsPbI 3 and CsSnI 3 QDs parent structures.
Heterovalent B-site doping has been also used to enhance the optoelectronic properties of PQDs along with stabilizing the cubic phase at room temperature.Bera and colleagues 152 doped CsPbI 3 QDs via a tiny amount of Sb 3+ ions to obtain the phase and optical stability.The phase stability of Sb-doped α-CsPbI 3 QDs after 5 months without turning to yellow δ-CsPbI 3 is shown in Figure 9c.Likewise, for heterovalent Sb doping, the atomic model is shown in Figure 9d.Based on the model, cubes were formed when Sb(III) was assumed to be 25% of Pb; while adding a higher amount of Sb(III) resulted in platelets.To further study, the Yb 3+ doping strategy has been also proposed by Shi and co-workers 153 to improve device efficiency and stability with reduced defects and trap states.Ghosh and colleagues 154 reported CsPbBr 1.5 I 1.5 QDs doped with monovalent Ag + ions.The authors claimed that partial replacement of Pb 2+ by Ag + not only maintained the crystal structure but also reduced intrinsic defects, resulting in a long carrier lifetime.In this sense, the Ag-doped PQDSC with an increase in efficiency up to 20% and the device performance for at least 24 days with only a 5% drop in efficiency was achieved.
3.1.1.5.Hole Transport Layer.There are a few features that describe an excellent HTL, including but not limited to fast hole mobility, high conductivity, suitable alignment with the valence band of perovskite, as well as decent durability.HTLs in PSCs can be constructed from a wide spectrum of materials such as organics, inorganics, and polymers. 155.1.1.5.1.Organic and Polymer Hole Transport Materials.Small organic materials that are readily soluble in organic solvents are considered promising HTMs since they provide better contact with the active layer regardless of the morphology of the active layer.Knowing that organics are mostly composed of earth-abundant elements such as carbon, hydrogen, and nitrogen, they are potentially cheap materials.Spiro-OMeTAD is a small organic material that is commonly used as the HTL in PSCs.The application of Spiro-OMeTAD as an alternative to the liquid electrolyte in PSCs does not merely extend the device lifespan, but at the same time enhances its efficiency. 61,156Even though the triarylamine and spiro structure hamper the crystallization process in this layer, it suffers from both low mobility (<10 −4 cm 2 V −1 s −1 ) as well as conductivity (10 −5 S cm −1 ), requiring additional species to encourage the mentioned characteristics.Thus, far, different small organics have been identified as HTMs including pyrene derivatives, thiophene derivatives, triptycene derivatives, triazine derivatives, and triphenylamines derivatives, some of which can exhibit a performance equal to Spiro-OMeTAD. 157he mobility and conductivity properties of polymeric HTMs surpass those of organics.Nonetheless, identifying a legitimate organic solvent that is able to dissolve the polymeric layer without affecting the perovskite layer is challenging.Due to the larger size of polymeric materials, the contact between active and hole transport layers particularly in the case of PSCs�consisting of a mesoporous electrode�is insufficient. 158It should be noted that polymeric HTMs are intrinsically more stable compared to small organics.Poly[bis-(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and poly(3hexylthiophene-2,5-diyl) (P3HT) have been the most commonly employed polymeric HTMs.Yet, their stability demands further improvement. 157Yuan et al. 159 have employed a series of dopant-free polymeric HTMs in CsPbI 3 PQDSCs.Using QD/polymer interfaces, efficient charge extraction is achieved, and device instability (due to oxidation processes required by conventional Spiro-OMeTAD) is avoided.In this sense, the PQDSC with an efficiency of 13% and a very low energy loss of 0.45 eV was generated.
3.1.1.5.2.Inorganic Hole Transport Materials.Considering that the crystallization of the HTL lowers the contact at the interface of perovskite/HTL, the amorphous nature of inorganics seems such a necessity.Following depositing these materials on the substrate, some of them require a sintering process.It is worth noting that high-temperature heat treatment can destroy perovskite materials; on the flip side, the volatile organic compounds in the HTL pose a challenge regarding their stability issue.Thus, some of the inorganic HTMs are inevitably utilized in the inverted configuration PSCs.The higher durability of inorganic HTMs is their unique merit which is not the case for both organics and polymers.
Two quintessential examples of inorganic HTMs can be named as NiO x and CuSCN bearing high stability as well as performance.NiO x which has been used in the inverted configuration PSCs demands a temperature as high as 600 °C during its heat-treatment process.Once it is assembled with the ZnO as the ETL in PSCs, the efficiency of the device is sustained for 60 days under light ambient conditions. 76Liu et al. 111   charge transport. 160For this reason, fine-tuning the interfaces of QD/HTL is a feasible way to promote the charge dynamic process and thereby intensify the PV performance.Ji et al. 108 synthesized a solution-based conjugated polymer-PQD hybrid BHJ layer to achieve the preceding purpose (see Figure 10a).A proper selection of polymer brings about energy level hierarchy as QD/polymer/PTAA which in turn promotes charge carrier transport and minimizes recombination at PQD/HTL interfacial spots. 161The deposition of a conjugated polymer-QD hybrid layer on the QD film was carried out prior to spincoating the organic HTL.The activity of CsPbI 3 and FAPbI 3 PQDSCs was explored utilizing different conjugated polymers (PBDB-T, PTP8, PTB7-Th, and PTB7). 159,162Energy level alignment on the interface of QD/polymer is an essential parameter, affecting carrier recombination loss and thereby improving carrier transport.Figure 10b displays a thorough comparison of the energy levels of PQDs and conjugated polymers as well as the HTL material PTAA.Ji et al. 108 selected conjugated polymers with distinctive HOMO energy levels positioned between the valence band of PQD and the HOMO of PTAA.CsPbI 3 -and FAPbI 3 -based PQDSCs deliver respective PCEs up to 13.1% and 11.6%.Interestingly, the addition of a layer of PBDB-T/QD BHJ promoted the PCEs of CsPbI 3 and FAPbI 3 PQD-based to 13.8% and 13.2%, separately.This betterment can be attributed to the improved J SC value, stemming from the reduced carrier recombination at the interfaces.The charge-harvesting performance of the polymer-QD hybrid layer adjusted PQDSCs and demonstrated a noticeable improvement as against the control original PQD devices.It is postulated that a conjugated polymer such as PBDB-T is vital for the charge carrier mobility in the PQD in general and in particular at interfaces of PQD/HTL, suggesting effective charge separation and diminished charge recombination.
3.1.1.7.Back-Contact Electrode.Lastly, the finishing step involved in the fabrication of PQDSC is the deposition of metallic electrodes via thermal evaporation.Au 6,111,163,164 and Ag 16,107,109,110 electrodes are suitable candidates employed for the collection of holes.The metal oxides in combination with low work function metals such as MoO x /aluminum 46,79,103,106 and MoO 3 /Ag 3,5,104,105,108 are the other alternatives used for the back-contact electrode.
3.1.2.Stability Improvement.As of today, the highest efficiencies obtained for hybrid organic−inorganic halide perovskites are for those with the poorest stability, raising the most formidable challenge: device stability. 165Although relative efficiencies to commercial silicon PV cells have been acquired for PQD cells, still comparing the lifespan of the former roughly 25 years with the latter from a few days to months simply indicates it is fairly difficult to fully supersede silicon PV cells with PQD cells.The components of PQDSCs seem to deteriorate immediately, which eventually leads the device to fall apart mainly because of the light-induced volatile organic components of the various layers.Several factors such as oxygen, moisture, temperature, illumination, biasing, and UV are affecting the stability of PQDSCs.Despite the improved device stability until now, it still has space to grow and be comparable to previously reported PbS-based QDSCs and some PSCs, 166,167 implying that more investigations are required to excel the long-term performance of SCs for largescale applications.
Up to this point, there have been some successful constructions of PQDs with high PLQYs such as MAPbX 3 , FAPbX 3 , and CsPbX 3 . 103,168Nonetheless, MA-based LHP QDs have low chemical tolerance because of their low formation energy, bringing forth their readily degradation in the course of separation and purification treatments. 169,170onsidering the metastable form of the 3D phases of Cs-and FA-based LHPs under ambient conditions, downsizing the crystallite size from bulk to QD regime can indeed strengthen their tolerance. 6,171Cs-based LHPQDs, however, fail to be stable enough after a few weeks owing to the small-sized Cs + . 171In principle, phase transformation of CsPbI 3 QD films takes place in a matter of days to weeks, suggesting its close correlation with surface treating as well as storing conditions. 79,172Albeit the CsPbI 3 QD-based devices deliver insignificantly higher PCE value than that of FAPbI 3 QD-based SCs, the latter outweighs the former in terms of long-run stability. 103,170,171an et al. 173 investigated the photo-and air-tolerance of an inorganic−organic hybrid ion pair as the capping ligand which showed the superior durability potential of CsPbBr 3 QDs with high PLQY.Dastidar and co-workers 174 proved that incorporation of chloride into CsPbI 3 by its maximum capacity of solubility can stabilize the α-phase.Moreover, Swarnkar et al. 46 witnessed the QD-triggered phase stabilization of α-CsPbI 3 with the help of two well-known ligands, OA and OLA.Wang et al. 175 used bis(2,2,4-trimethylpentyl)phosphinic acid (TMPPA), confirming its potential to replace OA to fabricate α-CsPbI 3 .Considering numerous reports, the instability of α-CsPbI 3 still lingers.Although the chemical removal of surface ligands stabilizes the α-CsPbI 3 from the device's point of view, it ends up with phase degradation.In contrast, α-CsPbBr 3 , an alternative absorber is a substantially stable phase.Pb-based perovskites are notoriously famous for their toxicity.The decomposition of these materials into Pb(OH)I complex and HI can be the reason behind their toxic nature.In addition, decomposition of the organo-halides can be accelerated once I − ions migrate in perovskite.Theoretical research corroborates that replacement of I − with Br − ions by 50% not only alleviates the diffusion barrier but also stabilizes the perovskite. 176CsPbBr 1.5 I 1.5 constructed by Ghosh et al. 5 was proven to be the best of all among halide compositions in terms of long-run PV performance with an infinitesimal drop in the PCE value.Accordingly, the CsPbBr 1.5 I 1.5 QDs, showed a longer lifespan for charge carriers and thereby higher assemblage efficiencies prior to recombination.
The presence of defects in the crystal lattice of perovskite gives rise to device instability. 132Ling et al. 105 employed CsAc to treat CsPbI 3 after synthesis.They stated that post-treatment results in stabilization of CsPbI 3 crystal lattice as well as mitigation of vulnerability of QDs against humidity as a consequence of filling of Cs vacancies.The device that underwent post-treatment with CsAc preserved 87% of its original PCE after its storage for a week in N 2 , while the control device displayed 64% of its initial PCE under the same conditions.This finding suggests that Cs vacancies are an influential factor in chemical and/or structural degradation besides acting as charge trap and recombination sites.Wang and colleagues 17 explored the long-run durability of the CsPbI 3 QD-based SCs under exposure to atmospheric conditions accompanying a relative humidity of 50−70% following the cation-exchange method.They concluded that Cs 0.5 FA 0.5 PbI 3 QD-based devices maintained 97% of their initial performance after storing for 20 days.The formation and migration of ionic defects inside the halide perovskite lattice are the major contributors to ultimate device degradation. 143,177Therefore, this conclusion can be drawn to attention that superb device tolerance of Cs 1−x FA x PbI 3 QD films under exposure to light is mainly due to appreciable suppression of defects in the alloy.It was stated by Jia et al. 109 that higher long-run performance is on account of the presence of denser hydrophobic glycine on the surface of PQD, offering better surface passivation and diminishing surface defects, as a result, the interaction between atmospheric vapor and PQDs is avoided. 46Finally, Liu and colleagues 113 also reported that Br doping can enhance the tolerance factor of the CsPbI 3 structure and consequently improve phase stability.From the results, the CsPbI 2.4 Br 0.6 PQDSC with the PCE of 12.31%, is close to that of CsPbI 3based PQDSC (14.13%) and may retain 87% of its original efficiency after 15 days of storage in ambient air.
Even though colloidal PQDs exhibit significantly improved phase stability at room temperature in comparison to bulk counterparts, 178 the stability of PV devices based thereon under operation remains one of the most pressing research challenges facing the perovskites field.A significant amount of effort has been invested by the scientific community in order to address this issue.To this end, three commonly used fundamental strategies have been employed to improve the stability and durability of PQDSCs.In the first one, the focus is on the fabrication of PQD materials through compositional engineering and/or doping.The second strategy involves the surface treatment of QD films, and the third one focuses on optimizing SCs to protect the active absorber perovskite layer, including optimizing the transporting layer and deposition procedures. 179s discussed in the PQDs compositional engineering section, Cs-based PQDs are prone to phase transitions due to their low structural tolerance factor.As a result, by adjusting the composition of perovskites to obtain appropriate tolerance factors, the stability of SCs can be improved.In this sense, a compositional engineering approach relaxes the lattice strain and suppresses phase transitions by controlling the A-site, Bsite, and X-site atoms (see Section 3.1.1.4.4).While A-site doping could fill the Cs vacancies of QDs on the surface and improve the stability of perovskite structure in the air environment, 105 B-site doping could promote the stability of the structure along with adjusting the electronic properties of QDs. 105ot to mention that, under operating conditions, light illumination and changes in the electric field cause ions to migrate toward the interfaces, resulting in a decrease in device stability.As a means of suppressing migration, it is necessary to reduce the defects and increase the vacancy formation energy.Since halide anions have a lower activation energy than A-site and B-site cations, they migrate more frequently.It is a wellknown fact that substituting some portion of I anions with the smaller-size Br or Cl is another way to increase the tolerance factor (eq 1).Nevertheless, the Br and Cl alloying gives rise to an undesired blue shift of the absorption edge and an enlargement of the band gap. 145he surface treatment of QD films is another important strategy to enhance the stability of PQDSCs.Although QDs are capped by ligands, they are largely removed during the purification and fabrication stages of the device, resulting in surface vacancies and QD agglomeration.These surface ligands, in addition to improving stability, also play an important role in the transport of photogenerated carriers.As compared to thin-film base devices, the relatively low J SC of CsPbI 3 PQD is mainly due to inefficient carrier transport.To achieve desired charge transport, following hot-injection synthesis, QDs capped by ligands with long alkyl chains should be partially removed.The antisolvent MeOAc is widely used in the purification of CsPbI 3 ligand PQDs and the reduction of the surface-bound oleate.Surface ligand density is critical to determining the balance between carrier transport and PQD phase stability.In specific, if the ligands are too dense, interdots charge hopping will be hampered, whereas too few ligands will prevent cubic CsPbI 3 PQDs phase stabilization.As documented by the literature, FAI salt can remove OA oleylammonium ligands by cation exchange and MeOAc can hydrolyze to acetic acid and methanol to remove oleic acid ligand from the surface. 102It should be noted that the hydrolysis of MeOAc without the aid of acids and bases is slow.Therefore, a mild process for controlling the density of surface ligands can be achieved by using a secondary amine, such as di-n-propylamine (DPA).It is reported that by treating CsPbI 3 PQDs with DPA, it is possible to promote efficient ligand removal and maintain the integrity of the devices at the same time. 180ikewise, it is possible to enhance device stability through the rational design of devices.The spiro-OMeTAD HTL is one of the major factors contributing to the degradation of perovskite PVs, while polymeric HTMs are intrinsically more stable compared to small organics.It is also possible to prevent the degradation of cells using PTB7 as a promising substitute to spiro-OMeTAD. 159In this regard, Yang et al. 181 reported the unencapsulated polymer-modified devices by depositing polythiophene on the top of CsPbI 2 Br QDs with great stability maintained around 90% of their initial PCE after 960 h of storage in a dry glovebox.
Ultimately, the fabrication of SC devices can also have a direct impact on their operational stability.The layer-by-layer deposition could deteriorate the stability due to the washing of each layer and the resulting defects in the QD film.As a matter of fact, it is highly recommended that QD layers be deposited in sequence.
3.1.3.Summary.Altogether, in this review, the PQD function in SCs and the recent advances of emerging PQDSCs were summarized from the viewpoints of device structure and improving performance.In this regard, we outlined the structure and chemical interface engineering of PQDSC, surface ligand management, surface passivation and posttreatment of PQDs, and their charge separation management and stability improvement.Through the research avenue, it was recognized that the addition of volatile organic cations into PQDs would raise concerns about thermal stability during long-term operational conditions.Consequently, improving the efficiency of all-inorganic PQDSCs remains a challenge.However, thanks to extensive research efforts, the PQDSCs' efficiency has been boosted from 10.8% in 2016 to over 16% in 2020.Based on this success, the authorized efficiency for PQDSCs is now comparable with organic SCs and other thinfilm technologies, transforming PQDSCs into an emerging candidate for the next-generation SCs.
3.1.4.Future Perspectives.While colloidal PQDs exhibit significantly enhanced phase stability at room temperature in comparison with their bulk counterparts, the stability of PV devices based thereon under operation has still emerged as the most urgent problem confronting the perovskite research community.Scientists have invested considerable efforts in enhancing the stability of PQDSCs that have been focused primarily on three common fundamental strategies.The first strategy focuses on compositional engineering and/or doping of PQDs, the second one is based on the surface treatment of QD film and the third strategy relies on optimization of SC device structure (ETL, active layer, and HTL).However, an extensive investigation is still necessary to address many open questions to fully understand their impact on next-generation applications.What is more, although CsPbI 3 PQDSCs have demonstrated a high V OC in comparison to thin-film PSCs in the same bandgap energy, they still suffer from a poor J SC due to surface defects and insulating organic ligands between PQD interfaces.Hence, the issue of low carrier mobility in PQD films should be addressed to achieve high-efficiency PQDSCs.Recently, Han and colleagues 116 reported a novel strategy in which utilizing ionic liquids with high polarity and electrondonating character could boost the mobility of PQD films in PQDSCs resulting in a significant increase in efficiency.Chen et al. 182 also reported a "surface surgery treatment" of PQDs for resurfacing PQDs by exploiting multidentate ligands, in which the surface defect passivation of PQDs by occupying I − vacancies and cross-link PQDs with multidentate ligands as a charger bridge could substantially boost the efficiency.The incidence of these concerns will eventually lead to a greater emphasis on finding other novel pathways to improve PQD device efficiency and stability through enhanced optoelectronic properties.

PQDs in Photocatalysis
Application.Photocatalysis introduces one of the most beneficial approaches to tackling the controversial concerns related to the energy crisis and environmental issues. 183Over the past few years, halide perovskites by showing peculiar characteristics in the field of photovoltaics, have been included in the photocatalysis library and thus, displayed huge potential for energy conversion through harnessing solar power.Some of the exceptional features which make these materials favorable for photo-catalysis are as follows, superior light-absorbing factors, small binding energies of the exciton, long lifespan of charge transporter, and diffusion lengths (≈1 μm). 183dditionally, LHP-based CQDs fabricated by the hot injection method 184 containing neat size and shape may hold further merit as opposed to those of LHP NC with larger size and irregular shapes in photocatalysis, three of which are further elaborated.First of all, they intrinsically enjoy a larger surface area, and the energy levels can be adjusted in terms of size 184 to regulate the electronic transmission systems with the substrates.Second of all, CQDs can create a homogeneous colloidal solution in many organic solvents regarding their high dispersibility property, while maintaining their heterogeneouslike catalytic traits including recyclability and separability.Last but not least, surface-controlled catalysis with particular specifications for different substrates can be achieved owing to their monodisperse, fine-tuned size, and morphological configuration of the surface. 184Recently, several reports have been released in which the diverse attributes of these perovskites have been exploited for: photocatalytic hydrogen generation, degradation of organic compounds and antibiotic residues in organic systems, CO 2 reduction, stereoselective C− C oxidative coupling, polymerization reactions, NO removal, and organic transformations, each of which is briefly discussed hereunder.

Photocatalytic Hydrogen Generation.
−189 The photocatalysis through lead halide perovskite materials came into the picture only after Park et al. 185 demonstrated the photocatalytic hydrogen generation from hydriodic acid using methylammonium lead iodide for the first time in 2016.By this time, there had been enormous research on the thermochemical water splitting of HI to produce H. 190−192 Yet, hydrogen production on a large scale is still a Reproduced with permission. 183Copyright 2019, Wiley.
formidable challenge.One promising technology for mass-scale hydrogen production is the sulfur−iodine cycle, which has been used by researchers over the past few decades. 193This cycle has high efficiency of around 50% and this can be further improved by optimizing the parameters involved in the sulfur− iodine process. 194,195It consists of the following reactions: 196,197 Bunsen reaction: Sulfuric-acid decomposition: Hydrogen-iodide decomposition: Net reaction: In eq 3 (Bunsen reaction), the reaction of sulfur dioxide, iodine, and water produces a mixture of hydrogen iodide and sulfuric acid.After the separation of hydrogen iodide and sulfuric acid, the two products are decomposed further in eq 4 (sulfuric-acid decomposition) and ( 5) (hydrogen-iodide decomposition).Equation 4generates water, sulfur dioxide, and oxygen, whereas eq 5 produces hydrogen and iodine.The products sulfur dioxide and water from eq 4 and iodine from eq 3 are recycled back to the Bunsen reaction and this process goes on.The net sum of all the reactions involved in this cycle is the splitting of water into hydrogen and oxygen in a carbon dioxide-free environment.
In addition to this process, the solar-induced splitting of hydrohalic acids (HX) is another favorable alternative for conventional water splitting.For the splitting of HX which involves two electrons, a smaller overpotential is demanded compared to water splitting with four electrons. 198,199It can be concluded that the overpotential for iodide ion oxidation is close to zero. 200,201The key challenge in materials design for the photoinduced HI splitting originates from the strong acidity of aqueous HI, implying that most materials are not able to tolerate acidic conditions. 202Considering the abovewritten points, Park et al. 185 prepared methylammonium lead iodide (MAPbI 3 ) to be utilized for hydrogen generation via HI splitting.
The photocatalytic reaction proceeded by irradiation of visible light (λ ≥ 475 nm) (Figure 11a) over precipitated MAPbI 3 powder as the photocatalyst to dissociate HI into H 2 and I 3 − .To evaluate the role of visible light and a photocatalyst, the reaction was conducted in the absence of light and a catalyst.It was observed that no H 2 gas was identified in the absence of each of these parameters, signifying that the generated H 2 gas can be solely correlated to MAPbI 3 powder.The presence of I 3 − ions was proved by the UV−vis spectra (Figure 11b).The quality check of the products confirmed that overall HI splitting took place with no side reactions (Figure 11c).Over time, the evolved hydrogen rate dwindled as a consequence of interference caused by the light absorption of I 3 − .To overcome this problem, the authors selected H 3 PO 2 �known as a good reducing agent for I 3 − �to be added to the saturated solution to reduce I 3 − to I − .Based upon the results, Park et al. 185 claimed that MAPbI 3 was maintained photocatalytically active after 160 h of continuous illumination, as depicted in Figure 11d.the remarkable durability was ascribed to the quasi-stable property of the dynamic equilibrium between MAPbI 3 powder and the saturated solution.
Following this study, an explosion of research on PQDbased photocatalysis occurred.Undoubtedly the photocatalyst is the crucial ingredient for photocatalysis.Yet, insufficient tolerance of ionic compounds in the vicinity of moisture or air is a formidable challenge that restricts the rapid advance in photocatalysis. 203,204Due to the far-reaching impact of the photocatalysts in various areas, the development of tolerant photocatalysts able to function under ambient conditions cannot be ignored.As mentioned, in the case of PQDs, one of the options is through modifying the surface with surface ligands that so far worked well for optoelectronic devices. 6,205or the light-driven photocatalytic hydrogen evolution, however, Xiao et al. 183 took advantage of this strategy for the first time to stabilize CsPbBr 3 QDs against destruction in moisture while sustaining efficient charge transfer for redox reactions.
Xiao et al. 183 synthesized CsPbBr 3 QDs with the aid of a hot-injection method, which on the surface were protected by OLA and OA as surface ligands (Figure 11e).Samples were labeled CsPbBr 3 -1, CsPbBr 3 -2, and CsPbBr 3 -3 indicating the CsPbBr 3 QDs rinsing one to three times, individually.To improve the photocatalytic performance, a heterojunction was developed between CsPbBr 3 QDs and TiO 2 (anatase) nanoparticles (NPs) with which charge separation could be promoted. 206,207Prior to this combination, TiO 2 was decorated with Pt nanoparticles as the cocatalyst by the previously reported method. 208In addition, the ratio of CsPbBr 3 QDs/Pt-TiO 2 were selected to be 9.86, 9.10, and 8.63 wt % for CsPbBr 3 -1, CsPbBr 3 -2, and CsPbBr 3 -3, respectively.Finally, CsPbBr 3 QDs/Pt-TiO 2 photocatalysts were exploited in a gas/solid photocatalytic system to generate hydrogen (Figure 11e).
The authors performed control experiments to realize the mechanism of the reaction.With only methanol, the rate of generated H 2 was merely 20% of the solution of water/ methanol, indicating that water was an essential factor in completely oxidizing the methanol under anaerobic conditions. 209Nonetheless, for the reaction with only water as the solution, no H 2 generation was reported, emphasizing the high barrier for water splitting under light illumination.Furthermore, the photocatalytic reactions conducted on the composites substantiated that these materials were active under visible-light illumination (λ > 420 nm) (Figure 11f).In addition, control experiments showed that in the presence of neither light nor catalyst, no H 2 was detected.Moreover, no H 2 was produced using Pt-TiO 2 photocatalyst or CsPbBr 3 QDs individually within 5 h.This behavior was justified by the following facts; first, TiO 2 is active under the UV region, and second, as an individual photocatalyst CsPbBr 3 QDs are not competent enough to drive H 2 from water.
From photocatalytic results illustrated in Figure 11f

Photocatalytic Degradation of Organic Compounds.
Man-made environmental contamination has turned into a consistent challenge with the galloping rate of population expansion and economic growth.More to the point, the released sewage from industrial plants usually contains dyes, metal ion-containing compounds, antibiotics remaining, harmful microorganisms, and other strong contaminants, which severely threaten the safety and security of the ecosystems. 210Needless to mention that the transformation of organic dyes into benign materials is critical for human life and eco-friendly growth. 211In addition to this, antibiotic residues, one of the medicine contaminants, leave pernicious effects on human health and the ecosystem, which have sparked a great deal of interest.Tetracycline hydrochloride (TC-HCl) for example is one of the most widespread antibiotics, and its residues inflict substantial harm on the ecological environment.Considering the above-written con- cerns, it is of significant importance to transform the detrimental residues into nontoxic and innocuous contents for the sake of protection of the environment and long-term development. 212,213everal methods including catalytic oxidation, electrolyzing, and adsorption via activated carbon have been employed to remove antibiotics remaining from water.Due to some flaws correlated with each mentioned method, photocatalysis has arisen as one of the research hotspots thanks to its high performance; without any peripheral contaminations as a result of the degradation of antibiotics remaining.Therefore, enormous efforts have been put forward to craft and harness visible-light-driven photocatalysts based on perovskite quantum dots with suitable traits to take the most advantage of solar energy. 210To this end, Gao et al. 211 released a report on the photocatalytic degradation of methylene orange (MO) for the first time with the aid of CsPbX 3 (X = Cl, Br, and I) QDs.The CsPbX 3 QDs were prepared by an emulsion and demulsion method at room temperature.Thereafter, the photocatalytic performance of as-obtained QDs was examined through decomposition of an ordinary organic pollutant.CsPbX 3 QDs were added in dissimilar quantities: 0.1 mg, 0.5 mg, 1 mg, 1.5 mg, 2 mg, and 3 mg.These samples were denoted as  223 Copyright 2017, The American Chemical Society.The yields for CO 2 reduction to CH 4 and CO with PCN-221(Fe x ) and MAPbI 3 @PCN-221(Fe x ) as photocatalysts in the CO 2 -saturated ethyl acetate/water solution after (c) 25 h, (d) 80 h of irradiation under 300 W Xe lamp, with the light intensity of 100 mW cm −2 .Reproduced with permission. 220Copyright 2019, Wiley.Solar CO 2 reduction into fuels under 300 W Xe lamp irradiation for CsPbBr 3 QDs, (e) 8.5 nm CsPbBr 3 QDs, and (f) tunable CsPbBr 3 QDs with different particle sizes.Reproduced with permission. 224Copyright 2017, Wiley.
Under visible-light illumination, merely 17% of MO degradation was achieved after 80 min without the presence of the catalyst (see Figure 12a−c).In sharp contrast, utilizing the CsPbCl 3 as the photocatalyst, the experiments showed a direct relationship between the efficiency of photocatalytic degradation of MO and CsPbCl 3 content.The concentration (C t /C 0 ) changes of MO with various photocatalysts can be seen in Figure 12c.In the absence of the catalyst, the photodegradation efficiency is insignificant.The same fashion can be followed in the UV−vis absorption spectra.The conspicuous photocatalytic performance can be discerned following the addition of CsPbX 3 QDs (Figure 12d,f,g,i). 211inally, the MO degradation efficiency (1 − C t /C 0 ) was conducted over two catalysts, namely, CsPbCl 3 -0.1 mg and CsPbCl 3 -2 mg, and the outcomes were summarized in Figure 12a,h.
Currently, the majority of photocatalysts are designed to eliminate pollutants in aqueous solutions in preference to nonaqueous systems. 215,216Nonetheless, some detrimental agents such as tetrachloroethylene and trichloroethylene in chemical and pharmaceutical applications can be dispersed via utilizing organic systems. 217,218To preclude the widespread diffusion of these hazardous chemicals into the soil or water, practical degradation approaches are needed for the treatment of nonaqueous solutions. 217For the sole sake of this purpose, Qian et al. 214 prepared CsPbBr 3 QDs by antisolvent precipitation and subsequently applied for the photocatalytic degradation of both tetracycline hydrochloride (TC-HCl) and MO in ethanol solution.
From Figure 12j for the case of the control experiment, no apparent decrease in TC-HCl concentration was detected in the absence of the catalyst; in fact, within the running period, TC-HCl concentration negligibly increased, which was attributed to the evaporation of ethanol during the test process.Moreover, only 18% of TC-HCl was decomposed after the addition of prepared CsPbBr 3 into the water solution of TC-HCl.In contrast, once ethanol was used as the solvent, photocatalytic activity was tremendously promoted, and consequently, the TC-HCl removal reached 76% within 30 min under visible-light exposure.In addition, the photodegradation of TC-HCl with varied concentrations including 5, 10, 15, and 20 mg/L was realized maintaining identical reaction conditions, individually, as portrayed in Figure 12k.On the basis of the findings, the best degradation performance on TC-HCl belonged to the catalyst in alcohol at the TC-HCl concentration of 10 mg/L.These results demonstrated that CsPbBr 3 QDs own remarkable photocatalytic activity once they are exposed to visible light.
Even though PQD-based photocatalysts have exhibited tremendous potential for the deterioration of organic dyes and antibacterial species, they sometimes are constrained by the immediate recombination of inner paired-up electron− hole.To circumvent this bottleneck, the combination of individual PQDs with another promising semiconducting material to construct a composite could be a possible solution reported. 210The combination of AgBr/Ag with PbBiO 2 Br showed an improvement in terms of performance and stability.

Visible-Light-Driven
Photocatalytic CO 2 Reduction.The rapid depletion of fossil fuel resources (coal, oil, natural gas), and inordinate emission of carbon dioxide (CO 2 ) have urged the scientific community to seek viable clean energy and in turn, dwindle the greenhouse effect in current society. 219−222 For this purpose, LHP materials have received considerable value in photovoltaics and optoelectronic applications by virtue of their distinctive optical attributes. 221Yet, the inorganic halide PQDs such as CsPbBr 3 , being remarkable materials for PSCs and light-emitting diode devices, have poorly been studied for photochemical CO 2 conversion to chemicals following their fragile stability in the vicinity of wetness or polar solvent. 219eeping in mind the above-written merits of CsPbBr 3 , its catalytic performance is still under question by the serious charge recombination.Thus, far, a wide variety of cocatalysts have been applied to integrate with CsPbBr 3 to form composites, such as rGO, g-C 3 N 4 , and Pd nanosheets, metal−organic frameworks (MOFs), and MXene nanosheets. 222The typical example that we can name is Xu et al. 223 who manufactured CsPbBr 3 QDs and a CsPbBr 3 QD/ graphene oxide (CsPbBr 3 QD/GO) composite for the photocatalytic reduction of CO 2 to EtOAc.In this work, the photocatalytic CO 2 reduction measurements were carried out in a sealed Pyrex bottle with EtOAc as the solvent.The photocatalytic CO 2 reduction to produce CH 4 and H 2 was evaluated after 12 h, and the results are summarized in Figure 13a,b and Table 2. Xu et al. 223 further claimed that there was a linear relationship between the concentrations of all the products and time.In this sense, the averaged electron consumption (R electron ) for the CsPbBr 3 QDs was 23.7 μmol/g h in the course of the reaction, and a R electron of 29.8 μmol/g h was achieved for the CsPbBr 3 QD/GO composite.In light of the findings, the photocatalysis of synthesized QDs in this study�at the time�surpassed those of CdS and other newly manufactured photocatalysts under visible-light irradiation.
As mentioned, the combination of LHPQDs and graphene oxide 223 or g-C 3 N 4 221 leads to efficient charge separation, bringing about enhanced catalytic activity toward CO 2 reduction.Nonetheless, the poor stability of LHPQDs and the scarcity of active catalytic sites hinder their performance in photocatalytic CO 2 reduction.To tackle these challenges, Wu et al. 220 came up with a brand new solution, which was to wrap LHPQDs of MAPbI 3 in the pores of Fe-porphyrin-based MOFs of PCN-221(Fe x ) to manufacture a series of MAPbI 3 @ PCN-221(Fe x ) composites as photocatalysts for visible-lightinduced CO 2 reduction (x = 0−1).−229 Besides their interesting catalytic properties, the framework of MOFs can elevate the longevity of LHP QDs. 230,231The close attachment of LHPQDs to the catalytic active spots in MOFs can minimize the charge-transfer distance, thus promoting the photoinduced charge separation efficiency of LHPQDs and catalytic performance of MOFs.To fulfill this objective, Wu et al. 220  Under the same exposure period of 25 h, Fe-free MAPbI 3 @ PCN-221 likewise displayed negligible photocatalytic performance toward CO 2 reduction (refer to Figure 13c).On the contrary, the composite photocatalysts containing both MAPbI 3 QDs and Fe demonstrated substantial enhancement of photocatalytic activity for CO 2 reduction, bringing about notably improved yields for both CO and CH 4 and thereby confirming the indispensable role of Fe as a catalytic site for photocatalytic CO 2 reduction.Finally, this work proved that the as-obtained composites of MAPbI 3 @PCN-221(Fe x ) possess greater durability compared with other LHPQDsbased photocatalysts for CO 2 reduction. 221,223,224,232n another study, Hou et al. 224 did survey the in-depth sizedependency of the photocatalytic CO 2 reduction system at the atomic level through a governable colloidal approach.Not to mention that CsPbBr 3 QDs, besides possessing a suitable bandgap, are favorable semiconducting materials with a high capacity to absorb sunlight. 13,14,233,234In addition, the optical band gaps and PL spectra are adjustable over the whole visible region owing to quantum size effects.Moreover, the prolonged carrier lifespan which can be uncovered by time-resolved photoluminescence (TRPL) elucidates the electron−hole separation efficiency.Consequently, the CsPbBr 3 QDs bearing an optimum size yield the highest photochemical conversion efficiency for the solar CO 2 reduction reaction.
Keeping in mind the above-written advantages, Hou et al. 224 developed CsPbBr 3 QDs adopting the solution phase synthesis technique.To put the as-prepared CsPbBr 3 QDs into practice, Hou et al. 224 utilized them as photocatalysts for the photocatalytic CO 2 reduction reaction.Following the photo- Experiments were also done in the absence of water vapor and in the presence of CO 2 in visible light.Another control study was done in the absence of a catalyst but in presence of CO 2 and water vapor.ND: Not detected.Reproduced with permission. 244Copyright 2021, The American Chemical Society.catalysis, the major products (CO, CH 4, and H 2 ).There was a linear growth between all the product generation and the prolonged time.The averaged electron yield (R electron ) rate for the CsPbBr 3 QDs�20.9μmol g −1 h −1 during the entire reaction�was estimated based on the following equation: R electron = 2R CO + 8R CH4 + 2R H2 .−237 Needless to mention that colloidal PQDs are attractive candidates for the photoreduction of CO 2 on account of their abilities to modulate their catalytic features through size modifications. 13,14,233,234,238To build a logical connection between the quantum size effect and solar-induced CO 2 , sizecontrollable CsPbBr 3 QDs with different sizes were employed as photocatalysts for the CO 2 reduction experiments.From Figure 13f, the highest yields of CO, CH 4, and H 2 in the span of 8 h under solar light exposure were obtained for the 8.5 nm CsPbBr 3 QDs.The CsPbBr 3 QDs with a size of 8.5 nm enjoy a strong optical absorption capacity, high surface area, and large active sites, all of which encourage the faster charge transfer to the surface, shorten the charge transference path, and at the same time impede the electron−hole recombination.As a whole, while downsizing brings about extreme aggregation, the large size diminishes the surface area, 13,14,233,234,238 regulating the optical absorption and charge transfer, and separation. 239n a conclusion, the engineering of the particle size (refer to Figure 13e,f) is of paramount significance to promote solar CO 2 reduction.
Considering the huge attention given to this research, water has been selected as the prime candidate for the conversion of CO 2 . 240Nevertheless, the overall efficiency of this process is highly dependent on the solubility of CO 2 in water, which is low and thus not satisfactory. 241To overcome this problem, people have been using other solvents such as ACN, EtOAc, aqueous carbonate solutions, and aqueous NaOH solutions along with several hole scavengers like triethanol amine (TEOA), triethyl amine (TEA), etc. 223,242,243 Being said this, some solvents can be photoactive under light illumination and converted to one of the products of CO 2 reduction, resulting in faulty conclusions on the efficiency of the catalyst.This fact is often ignored when selecting the appropriate solvent.
Das et al. 244 performed a thorough investigation to determine the effect of the solvent on the photocatalytic CO 2 reduction performance over a halide perovskite catalyst BCN (boron carbon nitride)/CsPbBr 3 in the presence of hole scavengers under artificial UV−vis light and visible light.Das and colleagues illustrated the rate of products under various reaction conditions in Figure 14.From this figure, pristine CsPbBr 3 (CPB) and the BCN/CsPbBr 3 composite (BCN/ CPB) displayed CO and H 2 production under full arc (200− 2400 nm) in vapor phase conditions.In contrast, in liquid phase conditions, the composite catalyst generated a very high amount of CO, CH 4 , C 2 H 4 , and H 2 (Figure 14a), which could be ascribed to ACN photolysis.As depicted in Figure 14b both CPB and BCN/CPB produced CO and H 2 under visible light in vapor phase conditions.Interestingly, in the liquid phase condition (10 mL of ACN and 40 μL of water) the photocatalytic activity was negligible.The authors related this phenomenon to poor water adsorption on the catalyst surface due to an insufficient supply of water in the medium.
The authors further determined the quantity of oxidation product (O 2 ) to validate whether the products developed from the photocatalytic redox process or solvent photolysis.In gas medium BCN/CPB or CPB both catalysts under visible light showed a mass balance between reduced products (CO and H 2 ) and oxidized product (O 2 ) (Figure 14c).However, some amount of CO was generated from photodegradation of BCN under full arc for BCN/CPB and only BCN in the absence of CO 2 (Figure 14d), which might be the reason for a slightly lower O 2 evolution rate compared to the expected value for BCN/CPB(g) (Figure 14c).
On the other side, for the liquid phase reaction, O 2 was not produced in the presence of CPB and BCN/CPB catalysts under the full arc or visible region, demonstrating the fact that the obtained carbonaceous products in the ACN/water mixture under full arc did not originate from CO 2 reduction.The produced CO originating from the reduction of CO 2 by CPB/BCN composite under visible light in vapor phase conditions was further confirmed using 13 CO 2 .Furthermore, no gas formation was detected under visible light in the absence of water vapor or the absence of catalyst, which confirmed that CO 2 to CO production was just not a stoichiometric reaction where the O of CO 2 was left behind in the solid material.The observed CO came from the photocatalytic process where water oxidation to O 2 happened by photogenerated holes.
The control experiments in this study verified the photolysis of CH 3 CN, EtOAc, TEA, and TEOA under UV−vis light even in the absence of any catalyst is plausible to produce CO, CH 4 , C 2 H 4 , and H 2 .The selection of the right solvent for photocatalysis is critical as if it is overlooked, it can lead to overestimation of catalytic efficiency and product selectivity.

Stereoselective C−C Oxidative Coupling Reactions.
Colloidal QDs as photocatalytic materials demonstrate a few distinctive benefits in contrast to metal-based coordination complexes and organic dyes.First, colloidal QDs can be simultaneously utilized as both photosensitizers and catalysts, outperforming common photocatalytic approaches in which photosensitizers and catalysts are usually separate components. 245Second, frequently used precious metal-containing complex-based catalysts impose a pricy procedure which is not the case in typical QDs (CdS, CdSe, InP QDs, or perovskite QDs).Third, the modulation of surface ligands of the QDs can be obtained without affecting the redox properties or the absorption characteristics of the QDs, compared to complexbased materials in which electronic structure can be significantly tuned by ligand species.Lastly, the redox potentials of the QDs, which are dependent on their bandgaps, can be modified for diverse catalysis by means of engineering the size, morphology, and/or composition of the QDs. 245t was only recently that we have begun to comprehend the photocatalytic reactions with stereoselectivity over PQDs.Despite their superb qualities in phytochemistry and optoelectronics, additional investigations are required to make the most of their photocatalytic potential.In this respect, Yuan et al. 245 demonstrated that the stereoselective C−C oxidative coupling of α-aryl ketonitriles can be accomplished over CsPbBr 3 perovskite QDs as a photocatalyst under exposure to visible light.In this study, to examine and compare various QDs in photocatalysis of the C−C oxidative coupling of α-aryl ketonitriles, eight types of QDs with varied compositions and capping ligands were developed.In particular, the zwitterionic ligand (that is, 3-(N,N-dimethyloctadecylammonio)propanesulfonate, DMOA-PS) capped colloidal CsPbBr 3 PQDs (ZW-CsPbBr 3 ) were achieved manipulating an already documented report. 246hey discovered that the surface tuning of CsPbBr 3 PQDs with zwitterionic ligands can lead to the simultaneous improvement of the NC stability and rate of coupling reaction as well as the stereoselectivity.Moreover, a mechanistic insight study confirmed that for effective dimerization electron donors or large conjugated π systems were essential parameters.The study claimed that the reaction experiences a radical intermediate reaction route on the surface of the PQDs, implying that a less steric hindrance brings forth the desired dlisomer.The constructed dimers, which are seldom accessible using other oxidants or catalysts 247,248 can be utilized as intermediates to form indenes with significant pharmaceutical potentials. 249This study revealed that colloidal PQDs with suitable alterations can be favorable photocatalysts in organic manufacturing processes. 245.2.5.Photocatalytic Polymerization of 3,4-Ethylenedioxythiophene.The synthesis procedure of halide PQDs necessitates the presence of surfactants to not only boost their durability but also preserve their morphology. 250,251espite that, the surfactants may interfere with or even obstruct the interfacial charge carrier flow.Encapsulation is an effective tool to preclude the aggregation of the PQDs.The two frequently employed materials for the stabilization of PQDs are insulating organic 252,253 and silica protecting layers, 254−256 both of which can hamper the flow of charge carriers between PQDs and the other functional materials, which means restricting their application.To resolve this issue, wrapping the QDs within a conductive protecting layer could be an alternative strategy.It can be expected that the protecting layer simultaneously encourages electron transfer, maintains the morphology, and enhances the durability of the PQDs.
Poly(3,4-ethylenedioxythiophene) (PEDOT), a commonly utilized conducting polymer in optoelectronic devices, is generally fabricated via electro/chemical oxidation of 3,4ethylenedioxythiophene. Nonetheless, these techniques are not suitable for incorporating the PQD into the PEDOT structure; in that, the oxidizing environment may damage or even devastate the PQD.Hence, by virtue of the long-run durability of carriers and decent absorption coefficient of halide perovskites, the photodeposition route seems a rational solution to introduce the PQD into the structure of PEDOT.Further, it is postulated that photodeposition takes the advantage of photoinduced electron transmission from the PQD to the polymer, boosting the charge carrier transfer and thereby giving forth enhanced optoelectronic properties. 257s stated by Chen and colleagues 257 all-inorganic halide perovskites with higher chemical stability as compared to organic−inorganic hybrid halide perovskites but identical preparation procedures are considered more favorable photocatalysts.In this regard, they examined both the photocatalytic activity and phase stability of all-inorganic perovskite for the polymerization of 2,2′,5′,2″-ter-3,4-ethylenedioxythiophene (TerEDOT) to PEDOT, which is usually manipulated as a hole transfer layer for manufacturing of the PSC.The photocatalytic process is presented in Figure 15a.For the photocatalytic reaction, TerEDOT and 1,4-benzoquinone (Qu) or molecular oxygen were used as the hole and electron acceptors, respectively.
Initially, the photocatalytic reaction was conducted in the open air in the presence of CsPbI 3 QDs.The findings showed that the increase in absorbance can be even more noticeable once both molecular oxygen and the CsPbI 3 QDs are present.This outcome substantiated that the synergy between molecular oxygen and CsPbI 3 QDs can indeed improve the polymerization process.The authors correlated this phenomenon with the formation of O 2

•−
. For this, it was found that dissolved oxygen/Qu can function as an electron acceptor and accepts one electron to generate O 2 •−259 with which the oxidization of TerEDOT can take place and the polymerization can be expedited.
The photoinduced electron transfer from the TerEDOT precursor to the CsPbI 3 QDs plays an important role in the reaction process.It is well-known that the charge carrier transfer across the interface between different functional layers of the optoelectronic device is critical for the device's performance. 260The photoinduced electron transfer may optimize exciton dissociation in hybrid interfaces in devices 261,262 and thus improve the interfacial carrier transfer.Thus, the presented photocatalytic reaction displays potential applications in optoelectronic material preparation and device fabrication.
The principal drawback of the common long-chain OA/ OLA ligands is their poor conductivity, which could slow down the charge flow between the QDs and hole/electron acceptor. 263,264Hence, the logical treatment of the surface ligands can encourage the charge flow and exciton separation, for which a simple strategy has been documented by treating the QDs with MeOAc. 185As a result, the replacement of the OA ligand with a shorter acetic acid may accelerate the charge flow between QDs and hole/electron acceptor.To accelerate the polymerization process of TerEDOT, inorganic LHPQDs (pristine-QDs, p-QDs) with various halide compositions were constructed and employed as photocatalysts.The inorganic LHPQDs treated with MeOAc (MeOAc-QDs) proved an enhanced photocatalytic activity thanks to the exchange of the OA ligand with shorter acetic acid and/or the defects created in the post-treating step that expedite the charge flow and restrict the notorious recombination of electrons and holes.
Li et al. 258 prepared a series of CsPbX 3 p-QDs (X = I, I 0.67 Br 0.33 , I 0.5 Br 0.5 , I 0.33 Br 0.67 , Br) based on previously documented work with some modifications. 234To explore the role of halide composition on the qualities of QDs, the morphology was regulated in uniform by adjusting the reaction time.The photocatalytic investigations were performed to ascertain the role of halide composition in the photocatalytic performance of the CsPbI 3 , CsPbI 1.5 Br 1.5 , and CsPbBr 3 p-QDs for the polymerization of TerEDOT to PEDOT.It was found that CsPbI 3 had higher photocatalytic activity relative to CsPbI 1.5 Br 1.5 PQDs, whereas CsPbBr 3 p-QDs demonstrated extremely negligible performance (Figure 15b−e).This finding is in accord with the fact that the narrower the bandgap the higher the photocatalytic activity as a result of the broad absorbance window.
3.2.6.Photocatalytic NO Removal.Tremendous attempts have been made to design an effective and reliable method for NO removal.Catalytic elimination of NO contaminations is accepted as an encouraging scheme, in which some developments have already been obtained in a few years.The NO removal induced by photocatalytic oxidation using semiconductors under visible-light exposure is a promising approach; however, it still suffers from low conversion efficiency.The bandgap and the locations of the conduction band and valence band are the two critical attributes of the photocatalysts to make sure whether the coming photon energy can be put to work or not.The proper bandgap guarantees the efficacious visible-light absorption and the locations of the CB and VB redox competence of the photocatalysts.
Despite several photocatalytic-oriented advantages of hybrid PQDs, their utilization as photocatalysts for the direct degradation of NO has rarely been explored.In this regard, hybrid PQD-based photocatalysts have received enormous research interest.Not to mention that their insufficient stability toward air, temperature, and light exposure resulting from high surface energy and myriad unpaired bonds at the surface impedes their mass-scale implementation. 265Up until now, plenty of schemes such as surface modification engineering, 183,256 core−shell structure, 255 and the embedment in microcrystals or glass, 266−268 have been designed to enhance the tolerance of HPQDs.The formamidinium lead halide (FAPbX 3 , X = Cl, Br, I) a member of the large family of PQDs possesses the characteristic credits of methylammonium lead halide MAPbX 3 (MA = CH 3 NH 3 , X = Cl, Br, I), 269 and at the same time ensures the tolerance to some extent.This can be explained by the fact that FA has a larger size compared to MA, meaning that ion exchange favors the composition containing FA cation, forming a more symmetric crystal structure and a broader absorption spectrum. 270eng et al. 265 recently conducted a one-pot synthesis route to attach QDs onto the amino-containing silica spheres (A-SiO 2 ), resulting in an improvement in the durability of random lasers built by the CsPbBr 3 QDs.Inspired by this, Huo et al. 265 have also used the A-SiO 2 for a completely different story, yet a relatively similar application�that is�promoting the photocatalytic durability of the FAPbBr 3 in the NO degradation reaction.Therefore, manufacturing of the composite was carried out via attachment of FAPbBr 3 on A-SiO 2 via N−H bonding.
The photocatalytic performance of virgin FAPbBr 3 and FAPbBr 3 /A-SiO 2 were assessed via varying the concentration of NO gas every 30 min under both visible-and UV-light irradiation.From Figure 16a, one can notice that under UV light, the material could deteriorate easily.In addition, the photogenerated electrons and holes recombine instead of assisting the charge carrier conduction.Figure 16b compares the photocatalytic capacity of neat FAPbBr 3 with that of FAPbBr 3 /ASiO 2 , the presence of plentiful NH x groups on the surface of A-SiO 2 alleviated the development of hybrid PQDs nucleation, leading to less agglomeration of the bare FAPbBr 3 and in turn, availability of more active sites.In this way, the photocatalytic conversion performance is significantly boosted because of the N−H chemical bonding between NH x and the FA cation.For this reason, the photocatalytic activity of FAPbBr 3 /A-SiO 2 composites was much higher than their bare counterpart, FAPbBr 3 for the NO removal. 265Finally, Huo et al. 265 attempted to examine the durability and recyclability of photocatalytic performance by multicycle experiments for FAPbBr 3 /ASiO 2 photocatalyst (Figure 16c).The NO removal ratios were calculated to be 70%, 60%, and 57% from the first to the third cycles, respectively.
3.2.7.Visible-Light Organic Transformations.In addition to all the above-mentioned applications, the LHPs can activate and catalyze the organic substrates in photoredox reactions in which light absorption, charge separation, and transfer are of high interest.Until now, studies conducted on organic transformation via photocatalysis have all exploited polydisperse LHP NCs (ca.2−100 nm), which are not included in quantum confinement regimes, owing to their large sizes of  265 Copyright 2020, Elsevier.(d) Initial result of oxidative aromatization of 1a in the presence of QD1 acting as the photocatalyst.Reproduced with permission. 184Copyright 2021, The Royal Society of Chemistry.(e) Formation of pyridazine 1b from surface-bound diamine substrates (e.g., 1a).(f) Optimized Cu loading for yield of 1b; 0.5 mg CuI per 2 mg NCs used for ion-exchange under different time.(g) Catalytic performance using excess CuI for ion-exchange with 2 mg NCs under 2 h ion-exchange time.(h) FTIR comparison indicating the surface binding of substrate through Cu.Reproduced with permission. 271Copyright 2021, The American Chemical Society.crystals. 184However, by contrast with these large-sized PNCs, colloidal LHPQDs fabricated by common techniques have not shown decent catalytic efficiency, indicating their low performance against moisture, air, substrates, and solvent.There have been numerous efforts to stabilize CsPbI 3 and CsPbBr 3 -based perovskites with the aid of some additives such as halide salt, 272 ammonium halide, 273 2,2′-iminodibenzoic acid, 23 phosphinic acid, 175 sulfides and metal ion, 274 and polymers 275 or adopting the application of specific postsynthetic purification steps. 46,276Despite the success of these approaches in SCs and LEDs, they are not practical candidates for photocatalysis.
−282 Those photocatalytic reactions accompanying nitrogen heterocycles such as 1,2-dihydropyridines (1,2-DHP), 1,4-dihydropyridines (1,4-DHP), dihydrobenzothiazoles and pyrazolines are critical; nevertheless, they are extremely sluggish; meaning that either an effective photocatalyst or a strong oxidant should be implemented.Earlier studies' focus on the aromatization of 1,2-dihydropyridine (1,2-DHP), 1,4-DHP, and 1,3,5-triaryl-pyrazolines as substrates were to some extent successful 283−286 using Ru-, Pd-, and Pt-based molecular catalysts but at the expense of high costs imposed by the noble metal catalyst that are not tolerant against oxygen, which rules out the possibility of recyclability.To this end, Pradhan et al. 184 reported the manufacturing of the air-tolerant CsPbBr 3 perovskite CQDs (QD1) via the "three precursors method" and their passivation with the help of halide to oxidize and subsequently aromatize a wide spectrum of azaheterocycles.
To appraise the photocatalytic value of QD1 in aromatization reactions under air exposure, the authors selected Hantzsch ester, i.e., diethyl 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate (Figure 16d, 1a) as a model substrate on account of its synthetic accessibility.The production of 2a with 85% yield was verified by nuclear magnetic resonance (NMR) (Figure 16d).To further shed light on the importance of the presence of a catalyst for the aromatization of 1a (Figure 16d), the reaction was conducted in the absence of the catalyst as well.The results demonstrated only a 20% yield of 2a production which pointed out the crucial role of QD1 to catalyze the reaction.In addition, the reaction was carried out in the absence of light and a negligible amount of 2a was observed, implying the significance of light for the completion of the reaction.
For another important organic reaction, Martin et al. 271 developed a novel heterogeneous photocatalytic system that exploited a lead-halide perovskite NC to absorb photons and encouraged the transfer of the photogenerated holes to a surface-bound transition metal Cu-site, bringing about an N− N heterocyclization reaction.In this work, the authors employed a controlled cation exchange reaction to fabricate a unique "merged" photocatalysis that utilized the NCs as the light absorber decorated with Cu catalyst incorporated onto the NC surfaces.The photocatalytic NCs were fabricated by exposing as-synthesized CsPbBr 3 NCs to a solution of Cu(I)X (X = Cl, Br, I) at ambient temperature, resulting in lightly doped Cu:CsPbBr 3 NCs.
An intramolecular N−N heterocyclization was examined to inspect the catalytic activity of the Cu:CsPbBr 3 NC system.Such reactions can be directly rendered from surface-bound diamine substrates (e.g., 1a, Figure 16e), in the presence of ambient air and visible light as the terminal oxidant and the energy source, respectively.The photocatalysis showed that the Cu:CsPbBr 3 photocatalyst suspended in dichloromethane under air with 450 nm LED irradiation, without any other additives, produced pyridazine 1b from 1a (Figure 16e) with 90% yield throughout 18 h.the control experiments excluding any component of the catalytic system, light, or air did not give rise to any product.From the combination of various reaction conditions, overall, a 2 h CuI-exchange brought about the highest yield of 1b (Figure 16f).The authors found that a higher Cu-loading did not necessarily correspond to a higher reactivity.The optimal catalytic activity was attributed to a balanced and synergistic effect of the Cu cation exchange and Cu catalytic activity.Imperfect Cu-loading indicated a lower number of Cu-catalytic sites and accordingly a lower yield.On the contrary, higher Cu incorporation gave rise to phase segregation, forming CsCuX 3 rather than further substitution of Cu in the CsPbBr 3 .
With the designed ion-exchange system, the amount of Cuincorporation was determined via the exchange reaction time, that is, when an excess of Cu(I) was applied but under the same exchange, the same photocatalytic activity (yield of 1b, Figure 16g) was found.The observations showed that only cation-exchanged Cu sites were responsible for the catalytic reaction and one that stood out among other samples, appeared to be Cu 0.01 :CsPbBr 3 .The origin of oxidative intramolecular N−N coupling is the diradical formation that must take place on a single diamine molecule.To achieve this, it was suggested that the diamine substrate coordinates to Cusites on the surface of the perovskite NCs in such a manner that consecutive excitation of the NC leads to the diradical formation.The diamine coordination to Cu surface sites was directly proved by an IR analysis, the comparison between pristine CsPbBr 3 and Cu-exchange NCs (Figure 16h).

Stability Issues of PQDs under Different Environments.
Considering that the stability of PQDs under different environments in photocatalysis plays a pivotal role, hereunder the protecting methods to modify and engineer the APbX 3 PQDs are addressed.Passivation of ligands or modifying ligands to protect APbX 3 PQDs from degradation is normally carried out by ligand exchange, which is a well-known approach for this purpose.Ligands used for the preparation of PQDs have a strong effect on the stability and dispersibility of PQDs in various solvent media. 287Moreover, long chain ligands like OA and OLA, which are the prominent capping ligands for the synthesis of CsPbBr 3 PQDs, are highly vulnerable to exposed air, water, light, and temperature. 288,289o this end, ligand exchange is an established method among chalcogenide quantum dots. 290,291Recently, people have adapted and extended this strategy to PQDs with the utilization of different types of ligands. 292,293ypes of ligands based on the different properties and applications required could be as follows, short chainmonofunctional, bidentate, bifunctional, multidentate, and so on.Ligands with high molecular length can form larger interparticle spacing inducing lower charge/energy transfer. 294ence, employing short chain alkyl ligands 295 is desired for photo applications.Ligand exchange with short-chain ligands like decanoic acid (DA), 296 cinnamic acid, 297 2-hexyldecanoic acid, and phosphoric acid 292 provide facilitated charge transfer, lower recombination, and longer carrier lifetimes.Nonetheless, they can only enhance the stability by a couple of hours, which cannot be applied to various solvent systems.Ligands that contain bicarboxylic acid groups namely succinic acid (SA) 298 have been found to exhibit enhanced stability and optical properties. 23rom the literature, bidentate ligands have shown both core stability and ligand stability.For instance, zwitterionic ligands like alpha amino butyric acid, iminodibenzoic acid, 23,246 and lysine allow both functional groups in them to bind to Pb 2+ ions, enhancing fluorescence and stability. 298In 2020, the Zhang group 299 documented air-stable CsPbBr 3 PQDs by introducing an L-lysine ligand via a spin coating method.The resulting material showed that L-lysine had stronger binding energy to CsPbBr 3 PQDs than OA binding.This resulted in reduced recombination and enhanced the carrier lifetime.The surface passivated L-lysine CsPbBr 3 PQDs material demonstrated improved environmental stability for more than 45 days (in exposed air with high humidity) and 500 h under the illumination of daylight.
In another work, perfluoro decanoic acid (PFDA) and DA ligands were exploited to promote thermal stability and strengthen the tolerance of CsPbBr 3 PQDs against polar environments. 296The fluorocarbon component easily coordinates with metal ions, causing the reduction of surface defects.Their thermal stability study also showed good enhancement.PFDA-CsPbBr 3 PQDs were subjected to 100 °C for 4 h.The PL intensity was constant at 92.1% for PFDA-CsPbBr 3 PQDs, while it showed a drastic reduction for pristine CsPbBr 3 PQDs and DA-CsPbBr 3 PQDs.Resistivity in polar solvents (ethanol) followed the same trend with PL maintained at 36.5% for PFDA-CsPbBr 3 PQDs while it reduced to 9.53% and 13.6% for the other two.Thus, surface passivation using a PFDA ligand conferred good photo and thermal stability together with solvent resistivity, helping in the construction of stable electroluminescent devices.
In addition to the above-written research, Pan et al. 300 introduced a quite durable CsPbX 3 QD film with the aid of a halide ion pair (such as didodecyl dimethylammonium bromide (DDAB)), which was a relatively short ligand.These types of ligands are effective for charge transport in the QD film which can be good candidates for the development of highly efficient perovskite LEDs.This group found out that surface-treated CsPbI 3 with 2,2′-iminodibenzoic acid (IDA) contains a double carboxyl group compared to surface-capped CsPbI 3 with a single carboxyl group which means it was bound to two Pb atoms on the surface.The theoretical calculations validated that the double carboxylterminated surface has higher binding energy (1.4 eV) compared to the single carboxyl-terminated surface using an OA ligand (1.14 eV). 23he structural stability of perovskite materials can be related to the Goldschmidt tolerance factor, which was mentioned earlier.From the reports, perovskites can preserve their 3D structure while the tolerance factor would be between 0.813 and 1.107. 301The change from one crystal structure to another can be pertained to this factor and with slight structure distortion.However, slight distortion may not be sufficient to modify the photoelectric properties of the material's energy band.In this sense, a dramatic change in the A-site of ABX 3 could result in the collapse of the perovskite structure. 302This phenomenon is even more evident when it comes to lead iodine perovskite materials (CsPbI 3 , MAPbI 3 , and FAPbI 3 ).The 3D lead iodine perovskite materials are thermodynami-cally metastable at room temperature, easily transformed into the 1D structure of a broad band, and this incident can be escalated as the crystal size shrinks. 270,303Similar to studies reported on CsPbBr 3 PQDs, ligand modification has a strong impact on the stability of other APbX 3 PQDs.For instance, Tan et al. 304 developed the CsPbX 3 PQDs using octylphosphonic acid (OPA) rather than conventional OA and OLA ligands.Their results showed that OPA-CsPbX 3 quantum dots can not only achieve high-quality dispersion in solvents after several purification treatments but also exhibit ultrahigh PLQY (>90%) because of the powerfulness of OPA and lead ions effect.
In addition, Jasieniak et al. 175 used bis(2,2,4-trimethylpentyl) phosphinic acid (TMPPA) for the preparation of CsPbI 3 PQDs instead of an OA ligand.They found that the PL intensity of CsPbI 3 -TMPPA sustained its initial value after 20 days compared to the decomposition of CsPbI 3 -OA after 3 days.It was shown that the following molecules with large steric hindrance can induce high proton solvent resistance in PQDs; highly branched (3-aminopropyl) triethoxysilane (APTES), 305 polyhedral silsesquioxane-[3-(2-aminoethyl) amino] propylheptylbutyl substitution (NH 2 -POSS), 306 1tetradecylphosphonic acid (TDPA), 307 2-adamantyl ammonium bromide (ADBr), 308 trioctylphosphine oxide (TOPO), 309 mercapto-β-cyclodextrin (SH-β-CD), 310 poly-(lactic acid) (PLA), 311 and cage polyhedral oligosilsesquioxane (POSS). 256The experiments verified that highly branched ligands can obstruct the penetration of proton solvent into the surface of PQDs.As a whole, ligand exchange is an effective and simple scheme for the surface passivation of PQDs to finetune surface compatibility, creating PQDs with improved properties to stand in air, water, light, and heat.It must be noted that the binding energy of the new ligand with the core ought to be much stronger than the existing system.
Apart from this, an aqueous solution is the main medium for photocatalysis.However, LHPQDs are poor materials in contact with water.To overcome this challenge, Zhang et al. 312 came up with a brilliant idea, which was to embed CsPbX 3 (X = Cl, Br, I) NCs in polystyrene-based microhemispheres (MHSs) to produce "waterproof" NCs@MHSs hybrids.They initially used polyvinylpyrrolidone (PVP) to adsorb and wrap around the surface of PQDs to act as a protective layer.The PVP could enhance the quantum yield of CsPbX 3 NCs and at the same time acted as the interface layer to combine PQDs with polystyrene polymers.The prepared CsPbX 3 NCs@MHSs was demonstrated to be stable and nontoxic as multicolored luminescent probes in living cells.Finally, Yang et al. 313 wrapped CsPbX 3 (X = Cl, Br, I) NCs inside the phospholipids to acquire CsPbX 3 -phospholipid micelles (CsPbX 3 @phospholipids).The phospholipid layer endows CsPbX 3 NC with remarkable water resistance and the capability to further biofunctionalized and eventually be more biocompatible.Prepared CsPbX 3 @phospholipid microclusters displayed strong fluorescence and narrow half-peak widths in water for over 4 months.
3.2.9.Summary.Halide PQDs have recently emerged as competent photocatalysts in the energy and environmentoriented fields including hydrogen production, CO 2 reduction, and organic compounds degradation to name but three following their exceptional properties such as superior lightabsorbing factors and small binding energies of the exciton.Knowing that environmental concerns namely rapid depletion of fossil fuels, excessive emission of CO 2 , and contamination-containing sewages released from industrial plants demand urgent solutions, e.g., photocatalysis by state-of-the-art PQDs under visible light to take the most advantage of renewable solar energy.Furthermore, PQDs can catalyze the organic substrates in photoredox reactions in which light absorption plays a pivotal role.Halide PQDs, regardless of their prominent traits, do not perform well under open air and in the vicinity of a wet environment.One approach that has been suggested is wrapping the QDs within a conductive protecting layer which not only can encourage electron transfer, but also maintains the morphology and enhances the stability of the PQDs.Moreover, the swift recombination of electrons and holes is another barrier to their far-reaching implementation.The solution to this impediment, i.e., the presence of longchain OA/OLA that could slow down the charge transfer, has been responded to by the exchange of long-chain ligands with short-chain ligands to accelerate the charge flow between QDs and the hole/electron acceptor.
3.2.10.Future Perspectives.Despite striking properties coming along with the utilization of HPQDs, their practical photocatalytic applications require further investigations.The utmost challenging parameter in PQDs' widespread utilization is their poor stability in the air or under aqueous solutions.To overcome this impediment, efforts have been made.For this, surface modification engineering, a core−shell structure, and the embedment in microcrystals or glass are proposed schemes that have been designed to enhance the tolerance of HPQDs.Reports revealed that coupling LHPQDs with graphene oxide or g-C 3 N 4 brings forth efficient charge separation, leading to an enhanced catalytic activity toward CO 2 reduction.Moreover, the surface tuning of CsPbBr 3 PQDs with zwitterionic ligands can lead to the simultaneous improvement of the NC stability and rate of coupling reaction as well as stereoselectivity.Thus, the addition of a second component to HPQDs or surface and compositional engineering can alter the structural and chemical features, resulting in a more stable PQD-based photocatalyst.
Even though Pb-based HPQDs have been the archetype in PQD-derived photocatalysts, the wide applications of halide PQD-based materials are restrained due partially to the presence of the Pb component.Substitution of Pb with other environmentally benign elements can overcome this bottleneck and pave the way for nontoxic PQD-based photoelectrochemical devices.For this, the combination of various photophysical characterizations and DFT calculations to deepen our insights into the basic luminescence properties and dynamic processes of halide PQDs can be helpful for the fabrication of highquality materials.The proposed Pb-free PQD photocatalyst not only should hold a competent photocatalytic capacity, but also should bear sufficient stability under different reaction environments.
The real application of photocatalysts based on HPQDs is subject to favorable photocatalytic attributes such as efficient light absorption, charge generation, separation, and transfer as well as durability.Encouraging one aspect may generate other challenges.For instance, the synthesis procedure of HPQDs necessitates the presence of surfactants to not only boost their durability but also preserve their morphology.Despite that, the surfactants may interfere with or even obstruct the interfacial charge carrier flow.Encapsulation is an effective tool to preclude the aggregation of the PQDs.To resolve this issue, wrapping the QD within a conductive protecting layer could be an alternative strategy.It can be expected that the protecting layer simultaneously encourages electron transfer, maintains the morphology, and enhances the durability of the PQDs.Based on the discussion mentioned above, material/compositional engineering should be at the forefront.

CONCLUSION
In summary, even though there has been an explosion of research on the development of next-generation SCs, and photocatalysts based on halide PQDs in the span of a few years, their real application is still a distant dream due to many key challenges that require further consideration.With the progress of the research, the photo-, solvent-, and thermaltolerance of halide PQDs still lag behind traditional QDs, which demand to be addressed immediately and properly.Thanks to various studies in the PQDSCs up to now, the enhanced electron coupling, charge transport, and reduced recombination loss in QD film led to improved device performance.With the tremendous efforts in the perovskite optoelectronic device research, the PQD-based SCs, LEDs, lasers, and photodetectors have mainly been the center of attention, while they can be realized in top-named photocatalytic reactions such as hydrogen and oxygen evolution.Taking into account the mentioned pros and cons, halide PQDs have opened a new era, especially in the fields of SCs and photocatalysis.Thereupon, despite persistent flaws, brilliant solutions are already underway.

Figure 1 .
Figure 1.Overall framework of the review.

Figure 3 .
Figure 3. (a) Schematic diagram of energy levels and transport processes of electrons and holes in an HTM/perovskite/TiO 2 cell.(b) Schematic structure and energetics of different configurations of PQDSCs.

Figure 4 .
Figure 4. (a) Schematic view of the CsPbI 3 QD-based PSC with mesoporous structure.(b) Schematic of QD/TiO 2 mesoporous structure before and after Cs-treatment.Reproduced with permission. 97Copyright 2020, The American Chemical Society.(c) Schematic of the fabrication of pristine, control, and hybrid CsPbI 3 QD films.(d) Band energy level diagram of control and target devices.(e) Schematic diagram of the hydrophobic PCBM passivating surface of CsPbI 3 QDs.Reproduced with permission. 3Copyright 2021, Nature.

Figure 5 .
Figure 5. Plot of the record efficiency of PQDSCs each year.

Figure 6 .
Figure 6.(a,b) Evolution of PL emission peaks with time to form Cs 0.5 FA 0.5 PbI 3 QDs by combining parent QD solutions that were purified twice (OA-less, a) and once (OA-rich, b), respectively.The bottom-most spectra in these two figures show the individual PL emissions from CsPbI 3 (blue) and FAPbI 3 (orange).The remaining emission spectra (black) are shown for the temporal evolution with time.Before mixing, they exhibit a PL peak at 683 and 774 nm for CsPbI 3 (blue dashed line) and FAPbI 3 (orange dashed line), respectively.(c) Proposed A-site cation-exchange reaction mechanism.Desorption of solvated cations from QDs leaves behind cation vacancies at the surface for the new cations to occupy.(d) Schematic illustration of cation exchange in different environments.Shuttling of FA + and Cs + cations between QDs is facilitated by the diffusion of Cs-oleate and FA-oleate in solution.As a result, the cation-exchange process under OA-rich conditions is promoted compared with that in the OAless environment.Reproduced with permission.17Copyright 2020, Nature.(e) Schematic illustrations of MeOAc Treatment on the Surface of CsPbI 3 PQDs.Reproduced with permission.127Copyright 2021, The American Chemical Society.(f) Schematic illustration of the size selection via GPC and (g) variation in solution-phase absolute PLQYs according to the elution sequence of the Pe-CQD eluate after GPC.Reproduced with permission.112Copyright 2021, The American Chemical Society.

Figure 8 .
Figure 8.(a) Schematic diagram of solid-state ligand exchange of control QDs and FAI QDs.Reproduced with permission.110Copyright 2020, The American Chemical Society.(b) EQE spectra of SCs with a varying thickness ratio of the Cs 0.25 FA 0.75 PbI 3 layer to CsPbI 3 layer in the perovskite QD absorber along with EQE spectra of SCs with different compositions of the bottom layer in the perovskite QD absorber where the thickness ratio of the mixed-cation Cs x FA 1−x PbI 3 QD layers to CsPbI 3 QD layers is 1:3 in all cases.Reproduced with permission.106Copyright 2019, Nature.(c) UV−vis spectra and normalized PL spectra of the CsPbI 3 PQDs obtained at different synthesis temperatures along with (d) the GBA homojunction structure in SCs.Reproduced with permission.114Copyright 2021, The American Chemical Society.

Figure 9 .
Figure 9. (a) Tolerance factor of 3D lead halide perovskite with general A-site cations, X-site anions, and octahedral factor of [BX 6 ] 4− octahedra with possible B-site cations.Reproduced with permission. 11Copyright 2019, Elsevier.(b) Schematic illustration of VI defect state control by ZnI 2 during synthesis and ligand removal processes of CsPbI 3 QDs.Reproduced with permission. 149Copyright 2020, Wiley.(c) Atomic model of the crystal structure of α-CsPbI 3 and digital image of the nanocrystals of stored α-(cubic) CsPbI 3 , which turned to yellow-colored δ-(orthorhombic) CsPbI 3 within 3 days along with atomic model showing that Sb(III) occupied the Pb(II) position in the α-CsPbI 3 crystal structure and Digital image of as-synthesized α-CsPbI 3 nanocrystals having ∼10% Sb(III) incorporated and stored for 5 months (These nanocrystals were stored at a relative humidity of 44 ± 6%.).(d) Atomic models of cubes and platelets having 10 and 20% Sb (w.r.t.Pb) obtained from reaction with different Sb intakes.These percentages were calculated from EDS in the nanocrystals, but originally, the reaction intakes were 25 and 50% Sb with respect to Pb for cube and platelet formations.Reproduced with permission.152Copyright 2019, The American Chemical Society.
developed PQDSC having the structure of FTO/c-TiO 2 / m-TiO 2 /CsPbI 3 QDs/Cu 12 Sb 4 S 13 QDs/Au, to use Cu 12 Sb 4 S 13 QDs as HTL to increase device stability.In light of the results, Cu 12 Sb 4 S 13 QD-based SCs generated a PCE of 10.02%, approaching the PCE of spiro-MeOTAD-based PQDSCs (12.14%).What is more, by improving the light absorption and the hole extraction ability of Cu 12 Sb 4 S 13 QDs, a J SC of 18.28 mA/cm 2 was reached, and after 360 h of storage in ambient air, the PQDSCs retained 94% of their initial PCE.3.1.1.6.QD/HTL Chemical Interface Engineering.Because of the huge difference in surface energy of QDs and polymers, the interfaces of PQDs and the organic HTL resulting from consecutive deposition processes are not perfect for efficacious

Figure 10 .
Figure 10.(a) Device structure of PQDSCs with polymer-QD BHJ connecting layers, and (b) energy levels of PQDs, conjugated polymers, and PTAA in this work.Reproduced with permission.Copyright 2020, The Royal Society of Chemistry.

Figure 11 .
Figure 11.(a) Photocatalytic H 2 evolution from the MAPbI 3 powder in a saturated solution.The aqueous HI solutions were prepared by electrochemical reduction (black squares) and H 3 PO 2 addition (red circles), respectively.(b) Photocatalytic reaction time-dependent UV−vis spectra of diluted saturated solutions for I 3 − titration.The saturated solution was made by adding MAPbI 3 powder to the electrochemically reduced HI solution.(c) Quantitative comparison between the evolved H 2 and I 3 − .(d) Stable photocatalytic H 2 evolution produced by the MAPbI 3 powder in the saturated solution for 160 h.H 3 PO 2 was added to the HI solution.Reproduced with permission. 185Copyright 2016, Nature.(e) Illustration of the photocatalytic activity on the CsPbBr 3 QDs/Pt-TiO 2 composite for H 2 production.(f) Photocatalytic H 2 evolution activity of various samples during the first several hours (visible light λ > 420 nm).(g) Long-term H 2 generation of CsPbBr 3 -1/Pt-TiO 2 photocatalyst under visible light irradiation.(h) Duration of photocatalytic H 2 evolution reaction for CsPbBr 3 -1/Pt-TiO 2 , CsPbBr 3 -2 /Pt-TiO 2 , and CsPbBr 3 -3/Pt-TiO 2 .Reproduced with permission.183Copyright 2019, Wiley.

Figure 13 .
Figure 13.(a) Photocatalytic performance: yield of the CO 2 reduction products after 12 h of photochemical reaction.(b) UV−vis absorption spectra and the EQE plots.Reproduced with permission.223Copyright 2017, The American Chemical Society.The yields for CO 2 reduction to CH 4 and CO with PCN-221(Fe x ) and MAPbI 3 @PCN-221(Fe x ) as photocatalysts in the CO 2 -saturated ethyl acetate/water solution after (c) 25 h, (d) 80 h of irradiation under 300 W Xe lamp, with the light intensity of 100 mW cm −2 .Reproduced with permission.220Copyright 2019, Wiley.Solar CO 2 reduction into fuels under 300 W Xe lamp irradiation for CsPbBr 3 QDs, (e) 8.5 nm CsPbBr 3 QDs, and (f) tunable CsPbBr 3 QDs with different particle sizes.Reproduced with permission.224Copyright 2017, Wiley.
conducted a consecutive deposition technique to wrap the MAPbI 3 QDs within the pores of PCN-221(Fe x ), as illustrated in Figure 13c,d.The photoreduction experiments of CO 2 were carried out in a CO 2saturated EtOAc solution containing a small amount of water.As depicted in Figure 13c, concerning the bare samples of PCN-221(Fe x ), CO 2 reduction increased with an increase in Fe amount owing to the increase of Fe catalytic sites.With reference to the results, among PCN-221(Fe x ) catalysts, the most efficient one generated only 13 and 38 mmolg −1 of CO and CH 4 , respectively, after 25 h of illumination.After that time, the PCN-221(Fe x ) photocatalysts were demolished and lost their catalytic activity.

Figure 14 .
Figure 14.Rate of product formation with BCN/CsPbBr 3 composite catalyst in different reaction conditions.A photocatalytic study was done under (a) UV−vis light (λ > 100 nm) and (b) visible light (λ > 400 nm).CPB, CsPbBr 3 ; BCN, boron carbon nitride.The "g" and "l" refer to the reaction condition.Catalysis was done at catalyst−gas interface conditions (g) and catalyst−liquid interface conditions (l).The rate is shown on a logarithmic scale.(c) Rate of O 2 evolution with respect to theoretically calculated O 2 evolution rate for CPB and BCN/CPB composite catalyst in gas phase condition.(d) Control experiments with BCN and BCN/CPB catalyst in absence of CO 2 and water with 5 mg of catalyst.Experiments were also done in the absence of water vapor and in the presence of CO 2 in visible light.Another control study was done in the absence of a catalyst but in presence of CO 2 and water vapor.ND: Not detected.Reproduced with permission.244Copyright 2021, The American Chemical Society.

Figure 15 .
Figure 15.(a) Illustration of the proposed mechanism for photocatalytic polymerization of TerEDOT over CsPbI 3 QD under visible-light Illumination.Reproduced with permission. 257Copyright 2017, The American Chemical Society.Time-dependent UV−vis spectra of the photocatalytic reaction of (b) CsPbI 3 p-QDs, (c) CsPbI 1.5 Br 1.5 p-QDs, (d) CsPbBr 3 p-QDs, and (e) the absorbance profile over reaction time at 510 nm for tracking PEDOT formation with different p-QDs.Reproduced with permission. 258Copyright 2019, The American Chemical Society.a A indicates the electron acceptor.E e indicates the lowest energy electron state, and E h indicates the highest energy hole state.Inset: photographs of (i) CsPbI 3 QD under light illumination for 90 min; (ii) CsPbI 3 QD and TerEDOT in the absence of light; (iii) the solution of TerEDOT after light illumination for 90 min; and (iv) CsPbI 3 QD and TerEDOT after light illumination for 90 min.The solvent is a mixture of dichloromethane and toluene (v/v, 1:1).

Figure 16 .
Figure 16.(a) Photocatalytic activity of FAPbBr 3 /A-SiO 2 composites under visible light and UV light.(b) Photocatalytic performance of pristine FAPbBr 3 and FAPbBr 3 /A-SiO 2 composites under visible light.(c) Stability test of FAPbBr 3 /A-SiO 2 composites in three consecutive runs under visible light.Reproduced with permission.265Copyright 2020, Elsevier.(d) Initial result of oxidative aromatization of 1a in the presence of QD1 acting as the photocatalyst.Reproduced with permission.184Copyright 2021, The Royal Society of Chemistry.(e) Formation of pyridazine 1b from surface-bound diamine substrates (e.g., 1a).(f) Optimized Cu loading for yield of 1b; 0.5 mg CuI per 2 mg NCs used for ion-exchange under different time.(g) Catalytic performance using excess CuI for ion-exchange with 2 mg NCs under 2 h ion-exchange time.(h) FTIR comparison indicating the surface binding of substrate through Cu.Reproduced with permission.271Copyright 2021, The American Chemical Society.

Table 1 .
Summary of the Device Architectures and Photovoltaic Performances of PQDSCs