All-inorganic CsPbBr3 perovskite: a promising choice for photovoltaics

In recent years, inorganic CsPbBr3-based perovskites have accomplished considerable progress owing to their superior stability under harsh humid environment. The power conversion efficiency (PCE) of CsPbBr3 perovskite solar cells (PSCs) has seen an unprecedented development from 5.74% to 10.91% with the improvement of the CsPbBr3 crystal quality. Despite extensive research efforts, the device efficiency of the CsPbBr3-based PSCs still lags behind that of other hybrid perovskite materials. Therefore, there is a significant interest in further boosting the performance of all-inorganic CsPbBr3 PSCs by the synergic optimization of films and device interfaces. In this review, we have discussed and summarized recent advances and methodologies related to CsPbBr3 films and PSCs. Furthermore, we discuss different fabrication strategies such as solution-based and vapor-based methods and their influence on the properties of CsPbBr3, particularly the morphology of films. Moreover, the timeline of improvement of the device efficiency from 2015 to 2020 is comprehensively addressed and developments are clearly sorted out by addressing critical factors influencing the photovoltaic performance. We further highlight state-of-the-art engineering strategies for CsPbBr3 PSCs that facilitate the crystallization control, charge extraction, suppression of charge recombination, and defect passivation in a systematic manner. At the end of the review, the summary and perspectives are presented along with beneficial guidelines for developing highly efficient and stable CsPbBr3 PSCs.


Introduction
Solar energy released by solar radiation accounts for almost 99% of the energy produced on Earth. However, the global warming and over-reliance of humans on fossil fuels pose a threat of climate change. Therefore, there is a desperate need to address this possible threat by harvesting clean energy using energy conversion devices. Photovoltaics are an expedient and sustainable method that can deliver inexhaustible clean solar energy to mankind with a low cost of electricity. Since the development of first solar panel in 1954 by Bell laboratories, the conversion of solar energy has always been a hot topic among researchers. 1  silicon solar cells have been at the commercial forefront of the photovoltaic industry. 2 However, the fabrication of silicon solar cells is considered to be expensive owing to the high melting point of silicon, which limits their long-term potential. In last few years, organic-inorganic hybrid perovskite solar cells have attracted considerable attention in the photovoltaic community owing to their high efficiency, cost-effective fabrication, low exciton binding energies, balanced carrier mobility, high absorption coefficients, long carrier diffusion lengths, and suitable energy bandgaps. [3][4][5][6][7] The extraordinary development has seen an increase in PCE from 3.8% to the state-of-art values of 25.2%. [8][9][10][11][12][13][14][15][16][17] To commercialize perovskite solar cells, their largescale fabrication, improvement in the stability and efficiency, and efficient reproducibility is crucial. However, organic-inorganic hybrid PSCs suffer from compositional degradation under heat and moisture mainly due to the high volatility of organic cations. [18][19][20] Organic cations such as MA + and FA + are expected to react with water molecules or under the ambient thermal environment to form hydrates, which cause instability and the degradation of the device. [21][22][23][24][25][26] The instability of organic cationbased PSCs is a critical flaw that impedes the commercialization of PSCs. Recent studies have suggested that the environmental tolerance of PSCs can be improved by the introduction of inorganic cations such as (Cs + ) instead of organic (MA + , FA + ) ions. 27,28 To enhance the efficiency of PSCs, many methods such as solvent engineering, interfacial engineering, and bandgap engineering have been developed. Generally, there are three main types of halides, namely, CsPbI 3 , CsPbBr 3 , and CsPbCl 3 . [29][30][31][32] The intrinsic tolerance to defects is one of the exceptional qualities of these halide perovskites as the defect states exist within the valence and conduction bands of these materials. In last few years, significant research attention has been given to improve the device efficiency of inorganic halide perovskites. Fig. 1 shows the comparison between the theoretically-predicted (Shockley-Queisser limit) and experimentally-attained PCEs of inorganic halide perovskites. Among all exploited halide perovskites, CsPbI 3 is the most studied perovskite due to its optimal bandgap of 1.73 eV and high efficiency 419%. However, the desired perovskite phase of CsPbI 3 is only stable at higher temperatures (4300) and the material experiences an undesirable phase transition to the non-perovskite phase under ambient conditions. 33,34 This undesirable phase transition, followed by the presence of moisture, hinders the potential of CsPbI 3 as an efficient solar cell device. Although, CsPbCl 3 is stable at the room temperature, due to its high bandgap of 3.0 eV, it is undesirable for solar cell applications. CsPbBr 3 also possesses a larger bandgap of 2.3 eV but demonstrates appropriate light harvesting characteristics and superior stability in ambient conditions. CsPbBr 3 halide single-crystals have demonstrated ultra-high electron mobility of 1000 cm 2 V s À1 and an electron lifetime of 2.5 ms, which is relatively higher than that of full cesium-based perovskites. [35][36][37][38] Based on these characteristics, one may further envisage the potential of CsPbr 3 as a promising material for photovoltaic applications.
In this review, we have summarized the challenges and strategies for the purposes of performance enhancement and commercialization of CsPbBr 3 PSCs. The viewpoints of stability issues related with CsPbBr 3 PSCs and the comparison with the other PSCs is systematically addressed, followed by the discussion on different fabrication approaches for designing highquality films and their role in improving the photovoltaic performance of CsPbBr 3 PSCs. The importance of the fundamental configuration of a perovskite photo-absorption layer, transport layers (ETL, HTL), and the ''golden triangle'' criteria for carbon-based CsPbBr 3 PSCs is reviewed. Furthermore, we have comprehensively analyzed the potential engineering strategies including compositional engineering, additive engineering, and interfacial engineering. At the end of the review, discussions on the possible solutions to the developmental bottlenecks are suggested to provide beneficial guidance for improving the device efficiency of CsPbBr 3 PSCs.

Crystal structure and properties of CsPbBr 3
The CsPbBr 3 perovskite has a similar structure to the mineral CaTiO 3 and is designated by the chemical formula ABX 3 (X = I, Br, and Cl). In general, CsPbBr 3 possesses an octahedron structure in which Pb 2+ and Br À forms a 3D network of corner-sharing [PbBr 6 ] 4À octahedra with Cs + ions occupying the octahedral voids (Fig. 2a). The structural tenability of the ABX 3 perovskite is limited due to the presence of rigid structural constraints. The phase stability and distortion of the CsPbBr 3 structure is predicted by the Goldschmidt tolerance factor (t), which is given as Here, R is the ionic radii of the CsPbBr 3 states and t represents the octahedral factor. By controlling these two factors, the perovskite crystal structure can be adjusted with regards to its composition. Generally, a stable cubic phase is formed when the value of t is in the range of 0.9-1 and the cubic crystal structure is distorted if the value of t is below 0.9, which is attributed to the tilting of PbX 6 . [39][40][41][42][43][44][45] The t value of CsPbBr 3 is 0.92. Due to the smaller radius of Br À (1.96 Å) compared to that of I À (2.2 Å), the geometric structure of CsPbBr 3 does not exhibit any prominent difference between the different phases. 46,47 The favorable value of the tolerance factor of CsPbBr 3 facilitates the stabilization of the perovskite phase in a broader temperature range and improves the thermal stability. CsPbBr 3 possesses two derivative phases, i.e., CsPb 2 Br 5 and Cs 4 PbBr 6 , as shown in Fig. 2c. The former exhibits a twodimensional layer structure in which Cs + ions are interposed between the two layers of Pb-Br coordinated polyhedrons. The latter shows a 0-D structure based on the [PbBr] 4À octahedra, which are disconnected from each other by CsBr bridges due to abundant CsBr. [48][49][50][51][52][53][54] The formation mechanism of both the phases is shown in the following equations.
The g-phase is stable at room temperature and tends to convert to the b-phase and a-phase upon heating at 88 1C and 130 1C, respectively. 46,57,58 Interestingly, the a-phase returns to the g-phase after cooling to room temperature. However, the three phases of CsPbBr 3 possess similar properties, indicating a wide temperature operation range for this material. CsPbBr 3 possesses various morphologies such as nanocrystals, single crystals, and bulk films. CsPbBr 3 nanocrystals exhibit a maximum photoluminescence quantum yield (PLQY) of 95%, while the other halide perovskites such as CsPbI 3 and CsPbCl 3 exhibit lower PLQY of 70% and 10%, respectively. 59 CsPbBr 3 perovskite is intended to detect wide-band absorption of 270-532 nm, which is important for increasing the power output of PSCs. The carrier diffusion length of 80 nm was first reported for the CsPbBr 3 films, while an electron mobility of 1000 cm 2 V À1 s À1 and electron lifetime of 2.5 ms were realized for the CsPbBr 3 single-crystals. 60,61 For single crystals of CsPbBr 3 , the electron and hole diffusion lengths were reported as 1 mm and 12 mm, respectively. 62 High diffusion lengths, wide-band absorption, and high carrier mobility make CsPbBr 3 a promising material for achieving high PCE and photocurrent density. Owing to the high bandgap of 2.3 eV, [63][64][65][66] CsPbBr 3 PSC exhibits high open-circuit voltage (V oc ) exceeding 1.6 V, 67,68 which can also be accredited to the diminishing of the film defects with the precise control of crystal growth.

Stability of CsPbBr 3
The stability of the halide perovskites is greatly affected by the external environmental conditions, such as heat, water, light illumination, and air (Fig. 3a). Compared with the other organicinorganic halide perovskites, CsPbBr 3 shows superior moisture, light, and thermal stability under ambient environments. In this section, we will discuss the highly stable nature of CsPbBr 3 in comparison with other organic-inorganic halide PSCs.

Stability comparison between the hybrid and all-inorganic perovskites
The stability issue of the organic-inorganic hybrid perovskites appears to be the bottleneck that hinders their industrialization. Although MAPbI 3 and CsPbI 3 are highly efficient materials, their low stability in ambient conditions has been noticed since the early stage of research on perovskite materials. Controlling the interplay between the bandgap and phase stability of inorganic PSCs is a challenging task for researchers. The larger ionic radii of I À as compared to that of Br À affects the phase stability and restricts the processing fabrication of devices under the ambient environment. By enhancing the Br content, the phase stability can be improved significantly. Abate's group 80 examined the phase stability of CsPbX 3 perovskites and found out that a clear boundary situated at the I/Br ratio of 3 : 2 separates the stable and distorted perovskite lattices (Fig. 3b). Although the materials (E g o 2.0 eV) offer an ideal bandgap for highly efficient devices but due to their unstable nature, their photovoltaic performance is restricted. The photovoltaic performance evolution of the perovskite nanocomposites with respect to time was further investigated by Jiang and coworkers. 81 They found that all the iodide-rich nanocomposites were unstable and suffer from unfavorable phase transition, while the CsPbBr 3 perovskite exhibited a superior device, as shown in Fig. 3c. Also, the other inorganic counterparts of CsPbBr 3 such as CsPbI 3 , CsPbI 2 Br, and CsPbIBr 2 degrade rapidly under ambient conditions, as suggested in Fig. 3d. However, CsPbBr 3 shows superior stability in ambient conditions without any changes in its color and shape, 82 and thus can be utilized as a promising light harvesting material. Halide perovskites suffer from photo-induced degradation, which affects their long-term stability. 245,246 However, limited research has been done on the exploration of the origins of this photoinduced degradation. It has been reported that Cs-based materials exhibit superior light stabilities compared to MA-based materials. Previously, Hodes group 247 studied the impact of electron beam irradiation and light on different halide perovskites. Their findings highlighted that the CsPbBr 3 -based cells showed no prominent signs of degradation when exposed to the electron beam, while the MAPbBr 3 -based cells showed a rapid decay and dramatic change in the morphology from crystalline to amorphous films. Zhou et al. 248 reported that the Cs-based perovskite solar cells can maintain 499% of their initial efficiency (10.3%) under AM1.5G illumination after 1500 h, while the MA-based devices were harshly degraded after 50 h of tracking operation. Furthermore, comprehensive research on the photochemical stability of halide perovskites was made by the Akbulatov's group. 249 From their reported work, it can be inferred that CsPbBr 3 exhibits a higher degree of photostability without any prominent degradation as compared with MAPbBr 3 (Fig. 3e). After continuous illumination, the CsPbBr 3 absorption bands surprisingly revealed an increase in the intensity, which was further supported by the morphological investigations, indicating no significant variation in the composition of the CsPbBr 3 perovskite films. A comparison of the photostability of different halide perovskites is depicted in Fig. 3f, which also clearly indicates that CsPbBr 3 exhibits superior stability compared to its counterparts. The origins of photo-stability in the CsPbBr 3 QDs were addressed in the study made by Chen et al. 250 They found out that the presence of non-radiative recombination was indicated in the PL QY spectra of the degraded samples, while PL decays indicate the presence of trap site emission in CsPbBr 3 , which was prominent in the PL decays. The aggregation of CsPbBr 3 QDs and elimination of the capping agent will produce several surface/interface dangling bonds, which can act as emissive and non-emissive trap states. It was further proposed that the photo-stability of CsPbBr 3 can be improved with surface passivation strategies using polymer layers or tightly bonded agents.
In the pioneer report on CsPbBr 3 PSCs, it was demonstrated that CsPbBr 3 can work equally well as MAPbBr 3 , showcasing high open circuit voltage. What was more interesting about CsPbBr 3 was its superior stability compared with MAPbBr 3 . 86,87 Under an illumination period of 5 h, CsPbBr 3 exhibited a photocurrent density decay of just 13% from the maximum value, in contrast to MAPbBr 3 , which showed a faster decay of 55%. When aging tests were performed in relative humidity in the range of 60-70%, CsPbBr 3 showed no significant decay in the efficiency for 2 weeks while MAPbBr 3 suffered heavy efficiency loss of about 83%. Furthermore, the thermal stability comparison of CsPbBr 3 and MAPbI 3 revealed that inorganic CsPbBr 3 can show excellent thermal stability up to 580 1C, while MAPbI 3 started to lose mass at 200 1C (Fig. 4a). The firstly prepared carbon-based CsPbBr 3 perovskite films 32 were one of the most stable PSC to date, showing no signs of degradation under high-humidity conditions for about 720 h (RH 90-95%, 100 1C) and 3 months (RH 90-95%, 25 1C) (Fig. 4b and c). The higher thermal stability of the carbon-based CsPbBr 3 is due to the absence of metal contacts (Ag and Au) inward diffusion. Nagabhushana et al. 88 also reported the thermodynamically unstable nature of MAPbI 3 , making it vulnerable to decomposition under ambient conditions. Furthermore, Zhou et al. 89 tested the stability of the CsPbBr 3 thin films stored in ambient conditions (T = 298 K, 40% humidity). The UV absorption spectra showed no prominent change in the absorption after nearly 2000 h of storage, which is due to the stable nature of CsPbBr 3 (Fig. 4d). Furthermore, the thermal stabilities of the CsPbBr 3 and MAPbI 3 PSCs using MnS as an HTL were also compared. 120 After 100 days of harsh humidity exposure (80% RH, 85 1C), CsPbBr 3 yet again proved to be more moisture tolerant than MAPbI 3 as it retained 80% of its initial PCE, as depicted in Fig. 4e and f. However, the referenced devices showed poor stability without MnS HTL, which shows that HTL has a strong influence in improving the stability of CsPbBr 3 . This also shows the importance of inorganic HTL over organic HTL in improving the device stability in ambient conditions.

Thermal and humidity stability
Generally, CsPbBr 3 is annealed at higher temperatures to achieve good morphology and crystallinity. It has been reported that CsPbBr 3 exhibits negligible degradation below 350 1C and the mass loss temperature of CsPbBr 3 starts at temperatures higher than 350 1C. 90 This also highlights the interesting fact that unlike its other counterparts, CsPbBr 3 is not prone to decomposition at temperatures attained during light illumination (o85 1C). The thermal stability of CsPbBr 3 was further compared with organic MAPbI 3 solar cells. 91 After 300 h, the inorganic CsPbBr 3 devices exhibited superior stabilities by maintaining 80% of their initial efficiency. However, rapid decomposition and efficiency loss were observed for the MAPbI 3 -based devices. In situ transmission electron microscopy of CsPbBr 3 nanocrystals revealed no phase transformations and perfect thermal stability below the sublimation point of 690 K. 92 Hu et al. 93 reported the excellent structure and photostability of solution-processed CsPbBr 3 microcubes. As shown in Fig. 5a and b, the CsPbBr 3 microcubes, after being stored for numerous months under ambient conditions (RH = 35-40%) showed negligible change, suggesting the excellent crystalline quality and photostability of CsPbBr 3 . Further, Yuan et al. 94 employed NiO x as HTL achieved a high PCE of 10.26% and high moisture stability under ambient environment (RH = 80%), as shown in Fig. 5c. Liu et al. 95 reported the stability test of the TiO 2 -based and TiO 2 /SnO 2 -based CsPbBr 3 PSCs with both the devices showing excellent stabilities. The fabricated devices were stored at ambient conditions at RH = 40% for over 1000 h and exhibited no decline in the initial PCE. Moreover, both the devices showed no prominent thermal degradation at 60 1C in air for 1 month. The enhanced thermal stability of CsPbBr 3 can be attributed to the choice of ETL/HTL and the high decomposition temperature of over 467 1C for CsPbBr 3 .
Recently, Cao and coworkers 112 performed air stability tests (relative humidity of B30% and temperature of B25 1C) on solution-processed CsPbBr 3 PSCs as shown in Fig. 5d. The results presented excellent air stability with no evident change in the color of the CsPbBr 3 layer, implying that there is no certain decomposition from the CsPbBr 3 perovskite phase to white PbBr 2 and CsBr. Moreover, the device maintained its initial efficiency even without any encapsulation in air atmosphere for 90 days. The thermal stability studies of Sm-doped CsPbBr 3 PSCs 230 suggested no prominent degradation behaviour for 60 days at 80 1C, as shown in Fig. 5e. Duan's group 113 also suggested that the carbon-CsPbBr 3 PSCs exhibit high moisture stability (90% RH) by maintaining 87% of the initial PCE. Interestingly, CsPbBr 3 PSCs with HTL showed inferior stability performance as compared with the carbon electrodebased PSCs without the HTL, which is due to the hygroscopic nature of the HTLs. Based on these findings, one may envisage the further potential of highly stable CsPbBr 3 perovskites. However, more importance should be given to address the thermal stability of carbon-based CsPbBr 3 because at higher temperatures, the binding characteristics of the polymer binders in the carbon paste are easily damaged, which in turn affect the PCE of the device.
Based on the above findings, it can be established that inorganic CsPbBr 3 , as a light-absorbing material, exhibits better moisture and thermal stability than its counterparts; however, the study on the operational stability of CsPbBr 3 is very limited, which should be an essential consideration to evaluate the future commercialization aspects of CsPbBr 3 PSCs.

Fabrication strategies
The perovskite layer is the heart of an efficient solar device; fabricating a perovskite layer with uniform coverage and large grains is essential for improving the stability and PCE of the PSCs. Generally, the perovskite layer, which is frequently used in PSCs, is usually fabricated by solution-processing methods, such as conventional spin-coating and dipping processes. Meanwhile, vapor and vacuum deposition, comprising sequential deposition and co-evaporation processes, are other efficient strategies for preparing polycrystalline CsPbBr 3 films. In addition, the vapor-assisted solution strategy, which is based on combining the spin-coating and vapor treatment methods, is also exploited to prepare uniform and compact CsPbBr 3 films. In this section, we will discuss the different strategies for the preparation of high-quality CsPbBr 3 films.

Solution-processed methods
Solution-processed methods offer low-cost preparation of PSC devices. Generally, solution-processed methods are further classified into one-step, 98-101 two-step, and multi-step strategies, [101][102][103][104][105] as depicted in Fig. 6a. 106 4.1.1 One-step solution method. In the conventional onestep solution method, precursors such as PbX 2 and covalent halide salts are dissolved in a conventional solvent such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or a mixture, followed by spin-coating the precursor solution and the annealing process at a desired temperature. Unfortunately, in the case of CsPbBr 3 , the low solubility of cesium precursors in conventional solvents makes it difficult to adapt one-step solution-based techniques. It is noteworthy that these methods are also sensitive to processing conditions as the crystal quality, morphology, and photovoltaic performance of the CsPbX 3 perovskites are controlled in the same optimization step.
Wang et al. 107 fabricated the CsPbBr 3 films via the one-step solution method by dissolving the precursors into the mixed DMF and DMSO solvents and reported the maximum concentration of only 0.4 M for the CsPbBr 3 precursor solution. Also, the morphology of the films is affected by the one-step treatment as it is difficult to form uniform CsPbBr 3 films via the one-step method. To tackle this issue, they suggested that if the evaporation rate of the solvent is slowed down, it can enhance the mass transport and diffusion, which could lead to a better film quality.
Generally, to prepare acceptable CsPbBr 3 thin films, a mesoporous oxide substrate is used. However, to understand the fundamental properties for photovoltaic applications, it is important to use a regular substrate. Yu's 118 and Gupta's groups 119 adapted a gas flow-assisted process to deposit CsPbBr 3 films on a regular substrate, as shown in Fig. 6c. This process involves the gas flow-assisted spin-coating process to deposit the perovskite precursor solution onto the flat substrate. The resulting films showed uniform behavior with increased density of the nucleation sites due to rapid cooling by solvent evaporation.
4.1.2 Two-step solution method. To further tackle the solubility concern of CsPbBr 3 , the two-step spin-coating process has been widely proposed for the fabrication of perovskite films with full coverage. 108 The conventional two-step solution approach involves the spin-coating of the PbBr 2 solution in a solvent onto a glass or a TiO 2 layer in the first-step. In the second-step, CsBr is reacted with PbBr 2 to form the CsPbBr 3 films via evaporation/spin-coating or dipping process. Presently, with the optimization of the preparation process, a PCE of over 10% has been achieved for CsPbBr 3 PSCs. 109 A remarkable difference in the solubilities of PbBr 2 and CsBr in water offers the leeway to fabricate CsPbBr 3 films via the two-step spin-coating method. 110 Wan et al. 111 developed an effective and facile two-step spin coating process using methanol/H 2 O mixed solvent to prepare CsPbBr 3 films. The crystallization process and film morphology were controlled and high-quality CsPbBr 3 films consisting of large crystalline grains and low defect density were obtained. Furtherly, CsPbBr 3 films were successfully prepared using water as a green solvent. 112 In this process, concentrated CsBr/H 2 O solution was spin-coated onto the PbBr 2 film, followed by the annealing process. The interesting part of using water as the solvent is that it offers adequate CsBr to react with PbBr 2 without destroying the film. On the one hand, the exceptionally low solubility of PbBr 2 in water evades the destruction triggered by water during the dropping of the CsBr/H 2 O solution. On the other hand, water offers the possibility to prepare a high-concentration CsBr/H 2 O solution owing to the high solubility of CsBr in water.
In recent years, several novel processing strategies such as vapor-assisted annealing, delayed annealing, and gas-blown anti-solvent washing have been introduced to significantly ameliorate the crystallization mechanism and to prepare uniform and pure CsPbBr 3 films. Among these methods, solvent engineering is regarded as the most productive and easily functioning approach. As discussed earlier, choosing an appropriate solvent to dissolve PbBr 2 and CsBr is a problematic step owing to their remarkable difference in solubility. While the two-step solution process does facilitate the preparation of CsPbBr 3 films, the use of toxic solvents is a worrisome practice as toxic solvents such as methanol and DMF are commonly used, which may affect the health of research workers by permeating through the skin or respiration. A comprehensive research based on the use of different solvents for improving the uniformity and PCE, and decreasing the use of toxic solvents was made by Cao's group. 112,122 As discussed in the previous section, water was used as a solvent to replace toxic methanol in the conventional two-step solution process. 112 Although a promising PCE of 6.12% was achieved, the first step still involved the use of DMF to dissolve PbBr 2 . To overcome this issue, they further developed a solution process based on the formation of the PbBr 2 solution from a mixed green solvent of polyethylene glycol (PEG) and g-butyrolactone (GBL). A promising PCE of 8.11% was realized with the device configuration of FTO/TiO 2 /CsPbBr 3 /carbon. Furthermore, a series of green solvents such as acetone, anisole, water, ethanol, acetic acid, isopropyl alcohol, and ethyl acetate were employed for the extraction of residual PEG200, followed by a reaction with CsBr to form the CsPbBr 3 films. It can be clearly seen that acetic acid as an anti-solvent resulted in the formation of a yellow CsPbBr 3 film with smooth morphology. The SEM images of the CsPbBr 3 films with different anti-solvents also suggested that using acetic acid as an anti-solvent resulted in pinholefree and uniform film morphology with full coverage (Fig. 7a).
The photographs of the prepared films with different green solvents are shown in Fig. 7b. In the commonly adopted twostep process, the fundamental issue of unavoidable CsPb 2 Br 5 and Cs 4 PbBr 6 impurity phases establishes a long-lasting bottleneck that hinders the prospective of CsPbBr 3 -based PSCs. To enable the formation of the pure CsPbBr 3 phase in the Cs-Pb-Br system, it is important to control the precursor stoichiometry at both the macroscopic and molecular scales. Recently, this aforementioned issue was resolved via the solvent engineering strategy, as reported by Feng et al. 123 In this strategy, they replaced conventionally used methanol with 2-methoxyethanol as the solvent to precisely control the CsBr : PbBr 2 ratio and prepared phase-pure CsPbBr 3 films. In contrast to the methanol-derived films, 2-methoxyethanol (EGME) solventassisted films demonstrated insignificant phase variation over the film matrix (Fig. 7c). Furthermore, with the regulation of crystal growth, stoichiometric and high quality CsPbBr 3 films with reduced electron trap density were realized. Using the bi-solvents of EGME and isopropanol (IPA) instead of EGME, pure phase, compact, and smooth CsPbBr 3 films were prepared, and a PCE of 7.29% was obtained for the PSCs with the structure of FTO/c-TiO 2 /CsPbBr 3 /carbon. 243 More importantly, the crystal growth of solution-deposited devices can be effectively tuned by solvent engineering.
To enhance the performance of the PSCs, it is essential to control the crystallization dynamics. Therefore, a more feasible approach is required to inhibit the decomposition of the precursor films in the solution and to improve the crystallization dynamics of the CsPbBr 3 perovskite. However, inhibiting the decomposition of the CsPbBr 3 thin-film is a great challenge in the conventional two-step solution process. At the initial stage, sequential dipping methods were employed to fabricate CsPbBr 3 films in mesoscopic PSCs. However, the fabricated films decomposed rapidly in the precursor solution and showed poor morphology. Teng et al. 117 proposed an elegant face-down liquid-space-restricted process to inhibit the decomposition of CsPbBr 3 . As a consequence, highly reproducible, smooth, and uniform films with large grain size of about 860 nm were obtained. The schematics of the face-up dipping process and face-down liquid-space-restricted process are depicted in Fig. 7d. The first step involves the deposition of the PbBr 2 / DMF solution via spin-coating onto the TiO 2 layer at 2000 rpm for 30 s. After drying the substrate on a hot plate at 80 1C for 30 min, the PbBr 2 films were face-down and face-up dipped in a methanol solution, followed by annealing the dried films at 250 1C for 5 min to obtain the CsPbBr 3 films. It is noteworthy that in conventional two-step solution methods, the CsPbBr 3 films decomposes quickly in the CsBr/methanol solution unlike that in the presented case. Furthermore, the optical investigation of the films via both the processes showed an emission peak at 527 nm. However, the films prepared by the face-down approach showed higher PL intensity than the films prepared by the face-up approach, as shown in Fig. 7e. Similarly, the facedown processed film showed much higher absorption intensity than the face-up processed film. It can be concluded that the films prepared with the face-down approach have fewer defects

Materials Advances Review
and higher crystallinity. Moreover, the planar CsPbBr 3 PSC with carbon electrode prepared via the face-down approach showed a promising efficiency of 5.86% with a V oc of 1.34 V.
To further tackle the solubility issues of bromide in conventional solvents, Luo's group 121 reported a novel Br 2 vaporassisted CVD process to realize fast anion-exchange from unstable CsPbI 3 to stable CsPbBr 3 . In this method, iodides were selected as the raw materials to prepare the CsPbI 3 films by spin-coating, followed by fast transformation into CsPbBr 3 via Br 2 vapor-assisted method (Fig. 8a-c). Here, the CsPbI 3 precursor was heated to 150 1C in a tube furnace and the Br 2 vapors were introduced into the hot quartz tube. The reaction time of the transformation process was optimized and the light green-yellow CsPbI 3 transformed into the bright yellow CsPbBr 3 film. The reported methods give interesting insights into the fabrication of multiple compositions such as CsPb(I 1Àx Br x ) 3 with the optimization of the reaction conditions and the injection dose of the Br 2 vapor. Li et al. 120 developed a vaporassisted solution method to prepare high quality and pure CsPbBr 3 films. This method involves the deposition of the PbBr 2 /DMF solution via the spin-coating process, followed by the vapor-assisted deposition of CsBr onto the PbBr 2 in a vacuum, as shown in Fig. 8d.

Multistep solution method.
One of the major drawbacks of the traditional two-step solution deposition route is that for CsPbBr 3 , the perovskite always suffers from low phasepurity and poor morphology. Generally, in the conventional one-step and two-step solution-based processes, enhancing the device efficiency of CsPbBr 3 PSCs is difficult due to the large difference in the concentration between the CsBr and PbBr 2 solutions and the formation of mixed phases. Also, the phase conversion of CsPbBr 3 to Cs 2 PbBr 5 and Cs 4 PbBr 6 substantially reduces the PCE of the devices. To overcome this issue, multistep solution process was developed to prepare high-quality CsPbBr 3 films for the fabrication of efficient PSCs. 113 In this process, the formation of mixed phases is optimized by tuning the number of deposition cycles. Furthermore, multistep treatment also facilitates the formation of vertical and monolayer-aligned grains by optimizing the morphological alignment between PbBr 2 and CsBr. Fig. 9a shows the fabrication of CsPbBr 3 PSCs based on this multistep solution process. A solid layer of PbBr 2 was formed by spin-coating PbBr 2 DMF onto the ETL substrate. Furthermore, the CsBr/ethanol solution was spin-coated for one to six cycles, followed by the deposition of carbon electrode. It was observed that the phase formation of the inorganic halides is highly dependent on the number of spin-coating cycles and below the n o 3 cycles. The formation of Cs 2 PbBr 5 was evident at n = 3, CsPbBr 3 at n = 4, and Cs 4 PbBr 6 at n = 5. Liao et al. 114 proposed a multi-step solution strategy to decrease the ideal number of CsBr deposition cycles by dipping the as-prepared PbBr 2 film into the CsBr solution (Fig. 9b). The perovskite film morphology is strongly dependent on the properties of the PbBr 2 sublayer. Zhao et al. 115 investigated the effect of the crystallization temperature of the PbBr 2 film on the morphology of the CsPbBr 3 perovskite film and the final device performance. It was found that the crystallization temperature has a strong impact on the morphology and optical behaviors of the as-prepared PbBr2 films. The transparency of the films changed from semitransparent to opaque on varying the crystallization temperatures from 25 to 120 1C (Fig. 9c). It was suggested that a high crystallization temperature enlarges the porosity volume, while a low crystallization temperature leads to a relatively uniform PbBr 2 film, thus generating compressive stress along the in-plane direction. Further, it was demonstrated that precise stress control of the CsPbBr 3 film can facilitate charge transfer and improve the grain size. Recently, Tang's group 116 also proposed a multistep solution process for the preparation of the CsPbBr 3 /bulkheterojunction solar cells, as shown in Fig. 9d. In this strategy, the perovskite layer was deposited on the FTO glass/TiO 2 substrate, followed by spin-coating organic J61 and ITIC blend onto the CsPbBr 3 film and heating at 100 1C for 10 min, which established a tightly-contacted CsPbBr 3 /bulk-heterojunction interface. Recently, a novel antisolvent washing treatment to prepare high-quality films was developed by Liao's group. 231 For this approach, chlorobenzene (CB) was used as the anti-solvent during the spin-coating process to effectively eliminate the excess DMF solvent in the wet PbBr 2 film (Fig. 9e). It was suggested that untimely anti-solvent treatment is undesirable for PbBr 2 and CsPbBr 3 crystallization. Therefore, optimization was done by dropping the CB antisolvent at different delay times of 2 s, 5 s, and 8 s. A highly covered PbBr 2 film was obtained due to the modified crystallization and fast nucleation of PbBr 2 induced by solvent engineering. Subsequently, the elegant antisolventwashing assisted in promoting the perovskite crystallization process, leading to higher crystallinity, structural purity, stronger light absorption, and uniform coverage compared with that of the untreated films ( Fig. 9e and f). The antisolvent-treated PSC showed a high efficiency of 8.55% in comparison to that of the untreated PSC (6.94% PCE).

Vacuum/vapor methods
The vapor processing of the perovskite films enables researchers to understand the properties of perovskites by precisely controlling the film thickness and fabricating highly reproducible PSCs with high efficiency. Vacuum deposition approaches involve the sublimation of CsX and PbX 2 precursors in a vacuum chamber and provide fine thickness control and film homogeneity. One of the benefits of vacuum deposition approach is that it does not require a solvent, which eliminates the risk of solvent damage to the underlying layer. Generally, vacuum thermal evaporation consists of single source and multi-source evaporation processes. As discussed earlier, the fabrication of CsPbBr 3 perovskite films via solution-based techniques is a challenge for large scale commercialization. Nevertheless, vacuum/vapor deposition approaches offer high reproducibility but the energy requirements related to vacuum processing are quite high and could impede the benefit of the rapid energy payback time of halide perovskites. Therefore, cost-effective vapor fabrication approaches should be developed for the fabrication of compact and large area CsPbBr 3 perovskite films. Compared with the solution-based approaches, there are relatively few investigations discussing non-solution approaches  for the preparation of CsPbBr 3 films. In this section, we will discuss the potential non-solution fabrication approaches for the fabrication of high-quality CsPbBr 3 films. 4.2.1 Dual-source co-evaporation. The film deposition ratio and the precursor ratio are the main factors that influence the morphology and quality of the perovskite films, whereas the subsequent thermal annealing process is a critical factor for promoting the crystal growth. Vacuum thermal evaporation (VTE) is a frequently used approach for the preparation of homogenous perovskite films. In the dual-source deposition method, PbX 2 and CsX are evaporated onto the substrate, followed by the annealing process, in order to form highly uniform polycrystalline films. The dual-source VTE deposition process as reported by Fei's group 124 is illustrated in Fig. 10a. In this process, compact CsPbBr 3 films were prepared via the thermal co-evaporation of CsBr and PbBr 2 precursors in vacuum, followed by thermal annealing. The optimization of the evaporation rate ratio of CsBr to PbBr 2 and the substrate and the annealing temperatures is crucial as they affect the stoichiometry and crystallinity of the films. In this case, high-quality CsPbBr 3 films were obtained via the tuning of the substrate temperature (25-300 1C), annealing temperature (400-550 1C), and evaporation rate of CsBr to PbBr 2 (0.5 : 1-1.2 : 1). The deposited films showed extreme uniformity and optimum conditions was achieved at a substrate temperature of 300 1C, post-annealing temperature of 500 1C for 15 s, and evaporation rate of CsBr to PbBr 2 of 0.7 : 1. Furthermore, these conditions were applied to fabricate CsPbBr 3 solar cells with device architecture of FTO/c-TiO 2 /CsPbBr 3 /spiroMeOTAD/Au; a high filling factor of 78% and a promising PCE of 6.95% were achieved. Chen et al. 125 reported a dual-source vacuum evaporation (DSVE) process for the fabrication of pinhole-free CsPbBr 3 films with negligible grain boundaries and low defect concentration. Both the CsBr 2 and PbBr 2 precursors were simultaneously evaporated to avoid the vapor gas-like behavior and films with semi-transparent characteristics were obtained at a low evaporation rate. They found that the crystallinity and homogeneity of the CsPbBr 3 films are highly dependent on the evaporation rate ( Fig. 10b). For example, when the evaporation rate was too high, the prepared film showed inhomogeneous behavior and low crystallinity, while a low evaporation rate facilitated homogenous film formation. The resultant opaque planar PSC exhibits an ultrahigh open-circuit voltage of 1.44 V and the highest reported PCE of 7.78%. Unfortunately, the PCE of the vacuum/evaporation-processed CsPbBr 3 devices is still lower than that of solution-processed devices owing to the fact that it is problematic to control the two precursors' homogenous reaction with each other so as to render a precise 1 : 1 M ratio. 4.2.2 Single-source deposition. Single-source vapor deposition is an alternative and advantageous approach to dual source evaporation that not only provides uniform pin-hole free films but also benefits the PCE and fill factor of devices. Li and coworkers 127 reported a facile single-source vacuum thermal evaporation (VTE) method for the deposition of uniform and compact CsPbBr 3 films ( Fig. 11a and b). As the molar ratio of the precursors is a crucial factor, they optimized the processing conditions by controlling the thickness of the films and the molar ratio of CsBr and PbBr 2 . However, it is difficult to detect the actual molar ratio of PbBr 2 to CsBr that reaches the substrate due discrepancy between their melting points. The VTE-fabricated CsPbBr 3 devices exhibited a very high PCE of 8.65% with good long-term stability. Nasi et al. 244 obtained films consisting of a mixture of CsPbBr 3 , CsPb 2 Br 5 , and Cs 4 PbBr 6 due to a vertical composition gradient during the evaporation of the CsPbBr 3 precursor by single-source thermal ablation and found that the mild post-deposition treatments lead to the conversion of CsPb 2 Br 5 and Cs 4 PbBr 6 into CsPbBr 3 due to its higher thermodynamic stability (Fig. 11c-e). Therefore, developing alternative post-treating processes is important to promote the growth of CsPbBr 3 , especially for the vacuum deposition of films.
Another promising alternative of solution-based methods is pulsed laser deposition (PLD), which offers low cost, suitable operation, and preparation of high-quality perovskite films by controlling the deposition conditions such as the vacuum pressure and substrate temperature. Using this process, Wang and coworkers 133 fabricated high-quality and densely-packed CsPbBr 3 films, which showed outstanding stability under high humidity conditions ( Fig. 12e and f). They also suggested that CsPbB 3 can completely penetrate in the m-TiO 2 layer owing to the collisions among the ions with TiO 2 particle surfaces. Pulsed laser deposited films were utilized as photoactive layers in CsPbBr 3 PSCs, attaining ultrahigh V oc of 1.37 V, fill factor (FF) of 72%, and a promising efficiency of 6.3%.

Sequential evaporation methods.
Recently, sequential dual-source vacuum deposition (S-DSVD) processes have been developed for the fabrication of uniform and efficient CsPbBr 3 films. 128 In traditional dual-source vacuum deposition process, the CsBr and PbBr 2 precursors are deposited on the substrate by controlling the thickness, followed by the annealing process to form the CsPbBr 3 film (Fig. 12a). As both the precursors are separately evaporated, the CsPbBr 3 films in equal stoichiometric molar ratio could be formed by controlling the thickness of the film instead of lowering down the evaporation rate. However, in contrast with traditional S-DSVD, the multistep sequential dual-source vacuum deposition (MS-DSVD) process offers more homogenous reaction between CsBr and PbBr 2 . This was further verified by Zhang's group 129 with the development of the moisture-assisted growth of CsPbBr 3 films via the MS-DSVD process ( Fig. 12b and c). Their findings suggested that the perovskite films with large grain size (above 2 mm) can be efficiently prepared by increasing the quantity of moisture during the annealing process (Fig. 12d). Based on these moisture-assisted grown films, the fabricated devices achieved a champion PCE of 8.86% with enhanced stability in ambient conditions. Jiang et al. 130 also developed a sequential evaporation process to fabricate CsPbBr 3 and achieved a PCE of 8.34%, which is one of the highest PCE of CsPbBr 3 PSCs based on the vacuum-evaporation process. Further, it was suggested that by regulating the thickness ratio of CsBr : PbBr 2 to 12 : 8, uniform CsPbBr 3 films with low defect densities can be achieved by the sequential evaporation method. By optimizing the thickness (500 nm), the fabricated CsPbBr 3 /carbon devices exhibited a champion PCE of 7.58%. 131 The preparation conditions such as sintering also play an important part in the preparation of compact CsPbBr 3 films with large grains. Xian et al. 132 introduced a sequential evaporation technology based on a two-step sintering (TSS) strategy to fabricate CsPbBr 3 perovskite films. The TSS strategy, which was carried out in a muffle furnace, comprises of two steps: (1) preparing the CsPbBr 3 perovskite at 320 1C; (2) crystal growth of dense CsPbBr 3 films and defect reformation via sintering. Firstly, CsBr and PbBr 2 are thermally evaporated by controlling the evaporation rates onto the substrates in the vacuum chamber. Then, a two-step sintering process is carried out on the obtained films in the muffle furnace under ambient conditions. With the optimization of the sintering conditions, compact CsPbBr 3 perovskite films with an average grain size up to 2 mm are obtained. Moreover, the TSS strategy prolonged the charge lifetimes due to the suppression of recombination mechanisms and defect reduction in the crystal. Furthermore, the CsPbBr 3 devices based on the TSS strategy exhibited tremendous thermal stability and achieved a champion efficiency of 9.35% under 1 sun illumination, which is comparable to that of solution-processed CsPbBr 3 devices.
Further, Hua et al. 126 reported the pressure-assisted multi-step sequential dual-source vacuum deposition (MS-DSVD) method to prepare compact CsPbBr 3 perovskite films (Fig. 12e). The effect of pressure on the crystallization of the CsPbBr 3 film was explored by adjusting the vessel pressure from 2 MPa to 10 MPa. To compare the performance of the films, another CsPbBr 3 film was prepared at the annealing temperature of 335 1C under a standard atmospheric pressure of 0.1 MPa. Not surprisingly, the morphology of the prepared film at standard atmospheric pressure showed prominent pinholes and small grains. However, with the increase in the vessel pressure to 10 MPa, the pinholes vanished and the grain size increased substantially owing to the complete reaction between the precursors. Further, planar CsPbBr 3 devices with the architecture of FTO/c-TiO 2 /perovskite/carbon were prepared using the vessel pressure of 10 MPa and a champion PCE of 7.22% was achieved.

Device architecture
The architecture of organic-inorganic PSCs dictates the choice of materials, preparation methods for the materials, and compatibility between the components in the device. These characteristics make the device performance highly reliant on the device architecture, where the perovskite (PVK) layer is sandwiched between the electron transport layer (ETL) and the hole transport layer (HTL). Two major device architecture design of PSCs have been established so far, i.e., planar and mesoscopic structures. Planar architecture further exists in either the conventional (n-i-p) or inverted (p-i-n) configuration. 69 In the mesoscopic configuration, the perovskite either forms an overlayer on top of the infiltrated oxide scaffold or it is presented as a thin layer that will sufficiently cover the oxide scaffold with the pores in the scaffold, as shown in Fig. 13. To reduce the interface charge recombination and to simplify the device deposition process, ETL, HTL-free devices have also been developed.
Moreover, the inverted PSCs shows less hysteresis as compared with the conventional ones.
Although both these types of PSCs have shown tremendous growth in the last decade, the stability concerns related with them is a stumbling block in their commercialization. Therefore, all-inorganic perovskite materials, such as CsPbX 3 , with high stability have become a hot topic in research.

CsPbBr 3 -based PSCs
One of the exceptional characteristics of CsPbBr 3 -based PSCs is their high V oc thanks to the wide bandgap of 2.3 eV with the optical absorption range of about 300-540 nm. Owing to its high stability, CsPbBr 3 was the first lead halide perovskite material to be investigated as a light harvester. Kulbak and coworkers 64 first fabricated the CsPbBr 3 PSCs using different HTMs including spiro-OMETAD and PTAA in a device structure of FTO/c-TiO 2 /m-TiO 2 /CsPbBr 3 /HTL/Au. A champion PCE of 5.95% was achieved by using PTAA HTL. Fascinated by their findings, they immediately fabricated CsPbBr 3 PSC via the twostep sequential deposition technique and boosted the PCE to B6%, which was comparable to that of the MAPbBr 3 PSC but with enhanced stability. 86 Crystal growth on the CsPbBr 3 films can be enhanced by the annealing process, as suggested by Hoffman et al. 134 By optimizing the thickness of the film and carefully tuning the concentration of the precursor, they achieved a PCE of 5.6%. However, CsPbBr 3 is usually annealed at a high temperature of 4250 1C, which not only reduces the selection of materials for other layers but also increases the processing cost of the solar cells, which limits their potential in flexible electronics. To alleviate this issue, Yan's group 135 proposed a pyridine (Py)-assisted process and reduced the prerequisite annealing temperature of the CsPbBr 3 film down to 160 1C and yielded an efficiency of 6.04%. However, in the year 2017, pyridine was identified as a carcinogen by the World Health Organization, which makes this strategy unsuitable for practical applications.
As previously discussed, several derivative phases such as CsBr-rich Cs 4 PbBr 6 and PbBr 2 -rich CsPb 2 Br 5 have been revealed in the peripheral of the CsPbBr 3 film. Some reports suggest that the presence of these non-perovskite derivative phases can passivate the grain boundaries and the interfaces; however, excess of any phase will introduce defects, which will degrade the photovoltaic efficiency of the devices. By utilizing phase conversion among CsPbBr 3 -CsPb 2 Br 5 -Cs 4 PbBr 6 and the multistep processing of the films, the device efficiency was significantly improved to 9.7% (Fig. 14a). 97 Jiang and coworkers 128 also pointed out the benefits of the non-perovskite CsPb 2 Br 5 phase in passivating the surface defects and increasing the grain size of CsPbBr 3 PSCs. Owing to the reduced charge recombination, a device efficiency of 8.34% was achieved for the CsPb 2 Br 5 -based device prepared with a PbBr 2 : CsBr ratio of 1.1. Further, the device efficiency of CsPbBr 3 PSCs was boosted to 9.81% by the one-step solvent growth strategy. 107 By tuning the CsBr deposition time, reducing the carrier recombination, and accelerating the charge transfer, the PCE of CsPbBr 3 PSCs based on HTL of manganese sulfide (MnS) was further improved up to 10.45% (Fig. 14b). 120 However, the drawback of such solution-based strategies is the low solubility of Br ions in the solvents as well as difficulties in controlling the over-rapid liquid-phase reaction and the imbalance between low densities of the heterogeneous nuclei, which ultimately produces non-uniform thin films. Liu's group 124 further highlighted the influence of the substrate temperature, post-annealing temperature, and the ratio of evaporation rates in depositing uniform CsPbBr 3 perovskite films. Through the tuning of these parameters, a stable PCE of 6.95% was achieved. Using the same method, Li's group 125 fabricated opaque planar perostructure CsPbBr 3 PSCs and made significant improvement in the device efficiency, achieving a champion PCE of 7.78% with an ultrahigh V oc of 1.44 V (Fig. 14c). Fig. 14d shows that the architectural context can be clearly seen through the fabricated semitransparent CsPbBr 3 device. Moreover, they further suggested that CsPbBr 3 , owing to its wide bandgap of 2.3 eV, can be an excellent choice for fabricating the top cell in tandem photovoltaic devices in order to improve their efficiency and stability. However, the device efficiency of vacuum-processed PSCs was much lower than that of the solution-processed CsPbBr 3 PSCs.
Solution-processing approaches were also used to fabricate CsPbBr 3 nanocrystals (NCs) and QDs films. The application of colloidal semiconductor nanocrystals (NCs) to fabricate optoelectronic devices is another promising strategy but it is a difficult task to transform the NCs solution to the NCs films by maintaining their properties. To overcome this issue, Akkerman's group 136 reported a large-scale approach by employing low-boiling point ligands (propionic acid (PrAc) and butylamine (BuAm)) and ecologically-friendly solvents (isopropanol (IPrOH) and hexane (HEX)) for the preparation of CsPbBr 3 NCs. They also suggested the importance of increasing the deposition cycles so as to promote the device efficiency and V oc of the CsPbBr 3 PSCs. Based on the increase in thickness of 550 nm after 9 deposition cycles, an ultra-high V oc of 1.5 and a decent efficiency of 5.4% was obtained. Zhang et al. 137   solution treatment, where Cs 2 PbBr 5 can help in surface passivation and reduce the Br vacancies. The device efficiency improved to 6.81% by the incorporation of ZnO QDs as ETL and spiro-OMeTAD as HTL. By introducing a facile templateassisted spin-coating process, Zhou and coworkers 150 developed a novel carbon quantum dot (CQD)-embedded CsPbBr 3 inverse opal (IO) structure. The preparation protocol of CQDembedded CsPbBr 3 IO film is presented in Fig. 14e. The devices based on CQD-embedded CsPbBr 3 IO film with the architecture of FTO/TiO 2 /CQD-perovskite IO/spiro-OMeTAD/Au generated a high efficiency of 8.29%, which was almost two times higher than that of planar structure devices.
Further, major progress was made by Tong's group 139 with the development of conventional and phase transition induced (PTI) methods for the preparation of uniform CsPbBr 3 films. In the PTI method, the CsPbBr 3 @CsPb 2 Br 5 core-shell structured layer was first prepared by depositing CsBr and excess PbBr 2 , followed by the sequential vapor deposition of excess CsBr and PbBr 2 to form the CsPbBr 3 @Cs 4 PbBr 6 thin film. The phase transformation of the corresponding perovskites is shown in Fig. 14h. Based on the growth process induced by the phase transition strategy, a highly crystalline CsPbBr 3 film with uniform grain-sizes and reduced trap density as well as lower surface potential barrier existing between the crystals and grain boundaries was obtained. A record efficiency of 10.91% was achieved for the n-i-p structured CsPbBr 3 -based PSCs, which is the highest reported PCE for CsPbBr 3 devices till date. Moreover, the prepared devices exhibited excellent stability over 2000 h in high humidity conditions and 1400 h in 100 1C heating conditions. Such novel strategies, as proposed by Tong's group, should be further developed with special importance towards improving the film quality.
Despite extensive efforts, the device efficiency for CsPbBr 3 PSCs is still comparatively lower than that of CsPbI 3 PSCs, mainly due to wide-range absorption and defect states (Table 1). Therefore, the processing conditions of CsPbBr 3 PSCs are of key importance to achieve a negligible defect state in order to trap photo-induced carriers. Within the perovskite, a majority of the intrinsic defects encourage a shallow transition level, while some defects with high formation energy will lead to deep transition levels, thus signifying the CsPbBr 3 perovskite as a defect tolerance semiconductor with efficient photovoltaic response. We believe that further development should be made in enhancing the device efficiency by taking the advantage of the superior stability of CsPbBr 3 perovskites.

Role of HTL
Hole-transporting materials (HTMs) are an important part of PSCs that enable efficient extraction of photo-induced holes from the perovskite layer to the back electrodes. Throughout the literature, numerous types of HTMs have been employed to improve the device efficiency. Stability, and charge mobility. [140][141][142] Discouragingly, these conventional HTMs offer unfavorable drawbacks such as high cost, low hole mobility or low conductivity, and instability, thus seriously hampering the practicable industrialization of the developing PSC technology. To boost the device efficiency of CsPbBr 3 PSCs and future upscaling, novel, costeffective, durable, and scalable alternative HTMs are suggested to be used. Recently, p-type Cu-phthalocyanine (CuPc) was used as an HTL for organic PSCs. [143][144][145] The favorable effect of selecting CuPc as the HTM layer is that it can construct a Schottky barrier at the perovskite/electrode interfaces and reduce the carrier recombination, which will lead to fewer defects and pinholefree film formation. Secondly, introducing the CuPc HTM layer will deliver a leveled energy-level transition, thus suppressing monomolecular recombination and trap states, which will be beneficial for improving the device efficiency of the PSCs. 146, 147 Liu's group 66 incorporated CuPc as HTL in the CsPbBr 3 PSCs and reported that CuPc can efficiently decrease charge recombination and facilitate charge transfer in CsPbBr 3 PSCs. As shown by the PSCs models in Fig. 15a, the positive effects of the CuPc HTM layer is evident on the device process. Based on these favorable results, the optimized device yielded a promising PCE of 6.21%, which was 60% higher than the PCE of HTM-free PSCs. Further, CuPc was employed as an HTM for bilayer-structured CsPbBr 3 devices, which not only resulted in improved stability but also boosted the PCE to 8.79% with a high V oc of 1.310 V and a FF of 0.814. 95 Although organic HTMs, as mentioned above, can facilitate hole extraction and decrease charge recombination for improving the device efficiency of CsPbBr 3 , the hygroscopic and unstable nature as well as the complex processing of organic HTMs is a major issue, which causes a negative impact on the long-term device stability. Recently, inorganic HTMs, together with high conductivity carbon materials, have shown promise in replacing expensive HTMs owing to their low-cost, easy processing, highhole mobility, and non-hygroscopicity. This was further highlighted by introducing intermediate energy levels of MnS as the HTL to facilitate charge extraction in the CsPbBr 3 device and to reduce electron-hole recombination. 120 By optimizing the deposition thickness of CsBr and incorporating MnS as HTL, a high PCE of 10.45% was achieved, which was much higher than 8.16% for the device without HTL.
Zhao et al. 148 introduced several HTLs such as PEDOT, PPy, and PANi in CsPbBr 3 PSC with the device architecture of the m-TiO 2 /perovskite/HTL/carbon. The corresponding SEM micrographs in Fig. 15b exhibit a multi-layered structure with a welldefined boundary between each layer and an average thickness of 200 nm, 400 nm, and 15 mm for ETL, CsPbBr 3 , and the carbon electrode, respectively. Through systematic characterization, it was revealed that the incorporation of organic hole-transporting materials resulted in the suppression of charge recombination and improved charge extraction at the cell interfaces, which resulted in boosted device efficiency. A champion PCE of 9.32% 8.36%, 8.33% 7.69%, and 6.10%, was achieved for the BT-BTH, PEDOT, PPy, PANi, and HTM-free CsPbBr 3 PSC, respectively (Fig. 15c). Duan et al. 113 incorporated Cu(Cr,M)O 2 (M = Ca 2+ , Ni 2+ , or Ba 2+ ) nanocrystals as HTL for all-inorganic CsPbBr 3 PSC. Owing to their enhanced hole-transporting characteristics, the device efficiency of the CsPbBr 3 PSCs increases to 8.41% with the addition of the CuCrO 2 layer, which further increased to 9.44%, 10.18%, and 10.03% with the incorporation of Cu(Cr,Ni)O 2 , Cu(Cr,Ba)O 2 , and Cu(Cr,Ca)O 2 layers, and to 10.79% with the doping of Sm 3+ ions (Fig. 15d). In addition, Materials Advances Review the graded energy-level alignment, as shown in Fig. 15e, is favorable for robust charge carrier collection. Zong et al. 151 introduced an HTL of the MoO 2 /N-doped carbon nanosphere (NC) composite into the CsPbBr 3 /carbon PSCs to enhance the energy level alignment, regulate the work function, and charge extraction as well as to passivate the surface defects in the CsPbBr 3 perovskite, to suppress the charge recombination and to reduce the energy loss at the interface. Fig. 15f and g exhibits the multilayer structure and the corresponding SEM micrograph of FTO/c-TiO 2 /m-TiO 2 /CsPbBr 3 /(MoO 2 /NC composites)/ carbon. Energy level diagrams of the CsPbBr 3 PSC indicate that the transition of the photo-generated electrons occurs from the valence band (VB) of CsPbBr 3 to the conduction band (CB) and then drift into the CB of the ETL, causing the holes to transfer to the HTMs and the C-electrode (Fig. 15h). Further, the incorporation of MoO 2 /NC composites was beneficial as they reduced the energy loss and interface energy offset. By optimizing the N concentration in the MoO 2 /NC composite, the optimized device delivered a PCE of 9.40% in comparison to the reference device efficiency of 6.68% and showed excellent stability over 800 h in ambient conditions with RH = 80% (Fig. 15i).

Role of ETL
Similar to HTL, ETL is equally important and plays a vital role in inhibiting electron transfer and functionalizing hole extraction in PSCs. An ideal ETL with appropriate band structure, low trap state, good conductivity, and high density electron mobility is a prerequisite for maximizing the device efficiency of CsPbBr 3 PSCs. However, limited research has been done to exploit the characteristics of commonly used ETL such as TiO 2 and ZnO in order to minimize charge recombination in the CsPbBr 3 film. One of the major issues is the large energy barrier at the interface between TiO 2 (ECB = À4.24 eV) and the CsPbBr 3 perovskite (ECB = À3.3 eV), which restricts electron transportation from the perovskite to the TiO 2 layer. Moreover, the conduction band offset (CBO) between the ETL and the perovskites have a significant impact on interface recombination in the perovskite devices. For example, the CBO between the commonly used ETL TiO 2 and CsPbBr 3 is B0.94 eV, which deviates from the optimal range and thus limits the V oc and the performance of the CsPbBr 3 PSCs. Recently, Cao et al. 152 proposed a novel strategy for the preparation of CsPbBr 3 PSC by employing Sr-modified TiO 2 as the ETL for reducing the conduction band offset. The decrease in the conduction band offset efficiently enables the extraction of electrons from the perovskite to the ETL and as a result, the charge recombination significantly decreases. Based on these results, the carbon-based CsPbBr 3 PSCs yielded a PCE of 7.22%, which is much greater than the efficiency of 5.92% for the controlled device (Fig. 16a). Further, Xu et al. 153 boosted the mobility and electronic conductivity of the CsPbBr 3 film by incorporating the antimony (Sb) dopant into the lattice of lowtemperature processed TiO 2 nanocrystals. The CsPbBr 3 devices based on the Sb-doped TiO 2 ETL yielded a V oc of 1.654 V, a champion PCE of 8.91%, and reduced hysteresis from 32% to 15% owing to the suppressed charge recombination and improved perovskite film quality (Fig. 16b-d).

HTM-free CsPbBr 3 PSCS
Although HTM-based CsPbBr 3 PSCs have shown a relatively enhanced device efficiency, the device stability is still limited using organic HTMs. The conventionally used electrodes such as Au and Ag 156 can be employed as hole extraction electrodes in HTM-free CsPbBr 3 devices. However, in comparison with these electrode materials, carbon is economical, water-resistant, and inert towards ion migration originating from the perovskite and the metal electrodes, which makes it a promising electrode material for CsPbBr 3 PSCs. 153 In comparison with conventionally used electrodes such as Ag or Au, carbon is preferred due to the following reasons: (i) using an ultra-thick carbon electrode can improve the stability of PSC as it remarkably reduces H 2 O/O 2 penetration. 154 (ii) The HTL is the most expensive part in a PSC, which can be overcome by employing carbon as an electrode material. (iii) The hole extraction and current conduction is effectively improved, which boosts the efficiency of PSC. (iv) Usual Ag or Au electrodes react easily with halide ions in the perovskite materials.
(v) Carbon materials exhibit better water resistance and stability than Ag or Au. 155 (vi) Carbon offers greater benefit for commercialization due to its reduced cost. (vii) Carbon possesses ideal work function with the VBM of the perovskite layer. (viii) Usual Ag or Au electrodes react easily with the halide ions in the perovskite materials. (ix) The sticky nature of the carbon paste helps in tolerating some amounts of pinholes in the perovskite films, which benefits by minimizing the defects, increasing the crystallinity, and producing films with larger grains. Presently, various carbonaceous allotropes, 159 such as amorphous carbon, 155 carbon nanotubes, graphite, 157 and graphene, 158 have been successfully employed in inorganic PSCs (Fig. 17a).
HTM-free CsPbBr 3 PSC based on the architecture of FTO/TiO 2 /CsPbBr 3 /Au was first fabricated in 2015, yielding a promising PCE of 5.47%. HTM-free CsPbBr 3 PSCs using a carbon electrode were first fabricated by Chang's group. 64 By optimizing the reaction time and temperature, a promising efficiency of 5.0% was achieved. Further, Liang and coworkers 87 boosted the PCE to 6.7% and achieved a high V oc of 1.24 V and FF of 0.73 using an FTO/TiO 2 /perovskite/C structure. The interesting part of these findings was the long-term stability of the prepared devices in the ambient conditions, which paved the way for further research on HTM-free CsPbBr 3 PSCs. The carbon paste was also applied for HTM-free CsPb 1Àx Mn x I 1+2x Br 2À2x PSCs, which yielded a PCE 7.36% of with excellent stability in ambient conditions. 160 Liu's group 161 proposed an efficient strategy for the fabrication of uniform CsPbBr 3 films by introducing a porous CsPb 2 Br 5 intermediate layer. They further highlighted the importance of using isopropanol (IPA) as the solvent for the low-concentration CsBr solution and in doing so, an ultra-high V oc of 1.38 V and a PCE of 6.1% were realized for the carbon-based PSC. Further, a facile two-step spin-coating process based on isopropanolassisted post-treatment was again reported for the fabrication of highly efficient HTM-free CsPbBr 3 perovskite films. 111 Fig. 17b shows the device structure, the corresponding SEM image, and the energy diagram of the corresponding CsPbBr 3 devices. Based on the IPA-treated CsPbBr 3 films, an ultra-high V oc 1.49 and one of the highest PCE of 8.11% for planar carbon-based CsPbBr 3 PSCs was achieved (Fig. 17c). Presently, the highest PCE for CsPbBr 3 is 10.91%, which was also realized by employing carbon paste as the electrode material. 139 To further optimize the device efficiency, Ding and coworkers 162 doped the PtNi alloy nanowires (NWs) (from 0 to 7 wt%) into the carbon ink in order to regulate the work function of the carbon electrodes. By optimizing the doping quantity of PtNi NWs (3 wt%), they achieved a champion PCE of 7.17%.
One major obstacle is the large recombination at the interface, which affects the photocurrent of the devices. To alleviate this issue, Tong's group 138 further fabricated an HTL-free and gradient bandgap structure by introducing the CsPb 2 Br 5 and Cs 4 PbBr 6 derivative phases, which resulted in the boosting of the hole extraction efficiency and suppressed charge recombination at the interface. In this strategy, the derivative phase films were sandwiched between the ETL and carbon electrode layer, as shown in Fig. 17d. It was further suggested that the introduction of the derivative phases can produce the perfect charge generation/transport path for the PSC, as depicted in Fig. 17e. Eventually, a striking PCE of 10.17%, a V oc of 1.461 V, and photocurrent density of 9.24 mA cm À2 was realized through the CsPbBr 3 /CsPbBr 3 -CsPb 2 Br 5 /CsPbBr 3 -Cs 4 PbBr 6 architecture approach with outstanding stability above 3000 h, retaining 85% of the original performance in ambient conditions and 700 h at 100 1C under thermal conditions, retaining 83% of the initial efficiency. Similarly, a wide plateau of over 80% in the range of 350 to 500 nm and a photo-current density near 7.4 mA cm À2 can be seen in the EQE spectrum of the champion device, which matches well with that of the absorption spectra (Fig. 17f). Furthermore, Bu's group 157 added polyaniline/graphite (PANi/G) into the carbon electrode to enhance the hole extraction and tailor the work function of the back electrode for improved energy level arrangement. This strategy led to a remarkable improvement in the interfacial hole extraction and suppressed the charge recombination and energy loss. In doing so, a striking PCE of 8.87% and V oc of 1.59 V was realized, which is 43.8% higher than 6.17% PCE for the control device (Fig. 17g).
Further, Liao et al. 163 proposed a combination of multiwalled carbon nanotubes (MWCNT) and carbon black (CB) for a CsPbBr 3 /C-based PSC. By tuning the MWCNT/CB ratio, the electrical conductivity and work function (WF) of the carbon electrode was tuned to lessen the energy difference at the perovskite/carbon interface and to promote charge extraction. Based on this strategy, the 25 wt% champion device delivered a PCE of 7.62% and an outstanding stability at 80% RH (Fig. 17h). Recently, Mi et al. 164 incorporated 1-D structured carbon nanotubes (CNTs) and 2-D Ti 3 C 2 -MXene nanosheets into the carbon paste to facilitate the multi-dimensional charge transfer path. Owing to the improved carrier extraction and transport, a respectable PCE of 7.09% was achieved for the carbon/CNT/MXene-mixed electrode in the CsPbBr 3 -PSCs (Fig. 17i).
Although promising results have been realized for HTL-free CsPbBr 3 PSCs, the lethargic carrier dynamics accelerate substantial and unfavorable interfacial recombination arising from the large energy change between the CsPbBr 3 perovskite and the carbon electrode, which is still a deficiency ( Table 2). By introducing an HTL between the perovskite and carbon, a Schottky barrier can be established, which prevents the direct contact of carbon with the ETL through the pinholes, which can suppress carrier recombination. To overcome these issues, different intermediate level layers such as carbon and black phosphorus quantum dots, polyaniline (PANi), and poly(3-hexylthiophene) HTMs have been incorporated at the CsPbBr 3 /carbon interface to facilitate hole extraction and ameliorate the energy level alignment. Also, several strategies have been developed to overcome the severe energy losses at the perovskite/carbon interface and will be briefly discussed in the next section.

CsPbBr 3 nanoparticles (NPs) in PSCs
Recently, there has been an increased interest in the preparation of CsPbX 3 nanoparticles (NPs) for photovoltaic devices. Owing to their high absorption coefficient, high carrier mobility, tunable bandgap, low conduction band minimum (CBM), and high PL quantum yields (PLQY), it is believed that the introduction of CsPbX 3 nanoparticles (NPs) can facilitate the electron transport from the photoactive layer to the electrode layer. [252][253][254][255] Moreover, literature reports suggest that the CsPbX 3 nanoparticles (NPs) are more stable than their perovskite films, which make them interesting candidates to be employed in photovoltaic applications. Previously, MAPbBr 3Àx I x nanocrystals were introduced into the perovskite layer and the HTL to enhance hole extraction and to regulate the band structure of the perovskite. 256 However, MAPbBr 3Àx I x (NPs) suffers from poor chemical stability, which hinders their application potential in PSCs. Compared with organic (NPs), inorganic (NPs) such as CsPbBr 3 show excellent chemical stability and hold great potential for improving the device efficiency and stability when incorporated into the photoactive layers. As CsPbBr 3 possesses much higher stability, thus it is less challenging to prepare CsPbBr 3 (NPs) as compared with CsPbI 3 . Recently, Gao et al. 257 introduced CsPbBr 3 (NPs) to the chlorobenzene anti-solvent in order to control the perovskite film growth towards low defects, large grain sizes, and improved crystallinity of the MAPbI 3 films. The incorporation of CsPbBr 3 (NPs) resulted in improved device efficiency with the optimized device exhibiting a high PCE of 20.46% with improved stability and low hysteresis. Chen et al. 258 introduced a CsPbBr 3 @SiO 2 (NPs) coating for efficient and stable PSCs. This strategy resulted in the suppression of charge recombination and boosted the UV utilization. In doing so, the device efficiency of the photoactive layer was enhanced from 19.7% to 20.8%, which is a 5.6% improvement from that of the uncoated PSC. In addition, the CsPbBr 3 @SiO 2 (NPs)-coated device exhibited negligible hysteresis and could effectively block light-induced degradation, thus improving the lifetime over 100 h under UV illumination for the perovskite devices. These studies show that the incorporation of CsPbBr 3 (NPs) is a feasible strategy for improving charge collection and extraction, diminishing the high charge recombination, and reducing the grain boundaries.

Engineering technologies
The spectral response range of the CsPbBr 3 perovskite can be widened by substituting partial Br À with I À to boost the device efficiency. However, in doing so, the device stability of CsPbBr 3 is greatly affected. Therefore, the suppression of charge recombination is regarded as an efficient strategy to improve the device performance and to maintain the inherent stability of the CsPbBr 3 perovskite. Within the perovskite films, a larger part of the charge recombination arises at the grain boundaries as they induce a shallow state presence near the valence band (VB) edge of the perovskite, which restricts hole diffusion and offers sites for the position of many uncoordinated ions defects. 165,166 Therefore, the fabrication of dense and uniform CsPbBr 3 films with low grain boundaries and defect states is an essential requirement for realizing high-efficiency PSCs with improved stability. Remarkable research effort has been put in developing different technologies such as additive engineering, anti-solvent engineering, as well as compositional and interface engineering to suppress charge recombination and reduce grain boundaries in perovskite films (Fig. 18).

Additive engineering
The classical crystallization theory may be one of the promising approaches, which incorporates functional additives into the precursor solution to achieve compact and uniform perovskite films by regulating the crystallization dynamics and achieving the passivation of uncoordinated ions defects. Additive engineering is considered as an effective method for simultaneously annihilating the ionic traps as well as increasing the grain size of the photoactive layers. 167,168 To enhance the quality of perovskite films, a variety of additives including ammonium salts, 169,170 nanoparticles, 171 polymers, 167 Lewis acid or base, 172 and ionic liquids 173,174 have been employed to the precursor solution so as to bond with the halogen ionic defects and uncoordinated Pb 2+ . Previously, Yang et al. 175 reported an improvement in the PCE from 18.77% to 20.84% for MAPbI 3 perovskite due to defect passivation and regulation of the crystal orientation with additive engineering. Huang and coworkers 176 developed a new passivation molecule of D-4-tertbutylphenylalanine (D4TBP) and incorporated it into the precursor solution for the preparation of uniform perovskite films with suitable grain size and low defect states, achieving an improved efficiency up to 21.4% of the p-i-n structured devices. Zhu and coworkers 177 introduced a commonly available additive melamine in the PbBr 2 precursor solution to passivate the defects and simultaneously develop the crystal growth of CsPbBr 3 perovskite films. They found that the crystallinity of PbBr 2 is effectively reduced with the introduction of melamine and a high quality, pinhole free, large grain size CsPbBr 3 film is prepared (Fig. 19a). Compared with the pristine film, the melamine-incorporated CsPbBr 3 film exhibited low defects and significantly reduced the grain boundaries with an increase in the grain size from 682 nm to 945 nm. The VB of melamine-CsPbBr 3 (À5.60, À5.58, À5.61, and À5.63 eV for 0.5%, 1%, 2%, and 4% melamine-CsPbBr 3 , respectively) is shifted up to approach the work function of carbon, indicating a decreased energy level difference between the CsPbBr 3 layer and the carbon electrode, which promotes the transportation and extraction of holes and reduces energy loss (Fig. 19b). Moreover, the optimized CsPbBr 3 device showed a striking V oc of 1.584V with a champion device efficiency of 9.65% compared to the PCE of 6.07% and V oc of 1.584 of the pristine device (Fig. 19c). Notably, the champion device exhibited long-lasting stability under high temperature and high humidity conditions. Pei et al. 178 modified a layer of BiBr 3 as an additive on the PbBr 2 film before the CsBr coating to fabricate uniform CsPbBr 3 films. This strategy led to an improvement in the crystal quality as well as attuned the energy level so as to promote carrier extraction from the perovskite (Fig. 19d). As the result, the PCE was effectively improved to 8.73%. Recently, Wang et al. 179 incorporated the NH 4 SCN additive into the precursor solution to prepare uniform CsPbBr 3 perovskite films. It was suggested that films with low trap state density, good crystallinity, and reduced interface defects can be obtained by additive incorporation with NH 4 SCN. It is evident from Fig. 19e that the incorporation of NH 4 SCN additive led to a much flatter and cleaner surface compared to that of the pure one. Moreover, the energy-level alignment of the CsPbBr 3 -1.5% NH 4 SCN film is lower than that of the pure film, which indicates that the changed energy-level arrangement will not considerably influence the device performance. By optimizing the molar ratio of NH 4 SCN (1.5%), the CsPbBr 3 device delivered a champion efficiency of 8.47%, which was higher than that of the controlled one (7.12%) (Fig. 19f). The device efficiency of the CsPbBr 3 perovskite can be severely deteriorated due to the strong charge recombination arising from the ionic defects at the grain boundaries of the perovskite. To overcome this issue, Zhang et al. 180 adopted the strategy of incorporating the amino acid of L-lysine with two amino and one carboxyl groups into the CsPbBr 3 film to concurrently passivate the uncoordinated Pb 2+ (Cs + ) and halogen ion defects. The grain size of the films showed a significant improvement from 688 nm to over 1000 nm with the addition of L-lysine, which is attributed to the reduced nucleation rate and adequate growth of the perovskite, which decreases the grain boundaries. They found that by optimizing the concentration of the additive in the PbBr 2 solution, the defect density is effectively reduced, the carrier life time is prolonged, and the interfacial energy level alignment is greatly enhanced, which adds to a greatly improved charge extraction and transfer (Fig. 19g). Moreover, the incorporation of L-lysine resulted in improved quality and suppression of charge recombination of the CsPbBr 3 film, which is due to the development of the crystallinity of the film with the modulation of the surface morphology and reduced grain boundaries. Benefiting from these qualities, the photovoltaic efficiency of the optimized CsPbBr 3 device was strikingly improved from 6.01% to 9.69%. Moreover, the encapsulated devices showed longer-lasting stability under high humidity and maintained nearly 95% of the efficiency at 80% RH and 25 1C for over 45 days.
All of these studies highlight the fact that additive incorporation is an effective approach for grain size improvement, suppression of charge recombination, and defect passivation so as to fully improve the device performance of the CsPbBr 3 perovskite.

Interface engineering
Although additive engineering is a feasible strategy to reduce the defects and suppress charge recombination in CsPbBr 3 PSCs, yet, it necessitates complicated optimization as the mixed system undergoes severe phase transition due to its thermodynamic driving force. 181 Currently, new strategies such as interfacial engineering along with passivation methods have garnered significant attention due to manifold functions such as interface energy level compensation, surface defects passivation, and stability improvement. In the literature, the study on surface passivation of CsPbBr 3 by polymers or other chemical compounds and the influence of interface modification on mismatched energy levels between the perovskite and the electrode is very scarce.
To date, few interfacial engineering strategies such as the incorporation of inorganic salts, 182,183 organic halide salts, 184 electronic donor materials, 185,186 or Lewis base polymers including polyvinylpyrrolidone 187 have been employed to passivate the surface defects of the perovskite films for enhancing the PCE, lowering the V oc loss, and enhancing the stability.
7.2.1 Lewis bases' interface engineering. Ding et al. 188 developed the interface modification strategy by employing polyvinyl acetate (PVAc), a Lewis-base polymer, with easy dissolution, cost-effectiveness, and outstanding stability for the modification of the CsPbBr 3 perovskite surface. Systematic characterizations in this study indicated that not only were the CsPbBr 3 surface defects substantially reduced with the modification of PVAc but also the work function of the carbon electrode and the energy level alignment between the valence band (VB) of CsPbBr 3 perovskite is enhanced (Fig. 20a). The modification of PVAc effectively suppressed the charge recombination and energy loss at the perovskite/carbon interface, which led to enhanced PCE and V oc . To further improve the hole extraction and diminish the energy-level difference, graphene oxide (GO) was incorporated between PVAc and the carbon electrode. By tuning the concentration of PVAc and GO, a champion efficiency of 9.53% with an ultra-high V oc of 1.553 V was realized for the CsPbBr 3 device, which is much higher than the PCE of 6.62% for the pristine device (Fig. 20b).
7.2.2 Ionic liquids (ILs) interface engineering. Recently, the incorporation of ionic liquids (ILs) has been regarded as an efficacious way to stabilize the defects of the perovskite films, which further improves the device efficiency and stability of the film. [189][190][191][192] However, most ionic liquids such as methylammonium acetate, 193 1-methyl-3-imidazolium iodide, 194 1-alkyl-4-amino-1,2,4-triazolium, 195 1-hexyl-3-methylimidazolium chloride, 196 and 1-ethylpyridinium chloride 197 gave been incorporated in the perovskite solution, which requires complex optimization processes as the blending system can undergo severe phase transition. The interface modification of CsPbBr 3 via the incorporation of the ionic liquid (IL) of 1-butyl-2,3-dimethylimidazolium chloride ([BMMIm]Cl) was studied by Zhang's group. 198 In their study, they found out that the incorporation of the ionic liquid modifier led to the passivation of the surface defects and the valence band of perovskite was shifted close to the work function of the carbon electrode, which substantially suppressed the radiative energy loss and improved the energylevel matching and non-radiative charge recombination. Based on these characteristics, the optimized device exhibited a high V oc and a high efficiency of 9.92% with excellent air and thermal stability under a relative humidity of 70% RH at 20 1C or 0% RH

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at 80 1C as well as under continuous illumination for a month (Fig. 20c). The champion device showed a 61.3% increase in the device efficiency as compared with that of 6.15% for the pristine device, which suggests that the interface modification of ILs offers new prospects for defect passivation and interface energy-level alignment development in order to boost the device efficiency of inorganic CsPbBr 3 PSCs. 7.2.3 Nanocrystals (NCs) intermediate layer. Su et al. 199 introduced an intermediate energy level at the CsPbBr 3 /carbon interface and passivated the CsPbBr 3 perovskite film by spincoating the hexane solution of CsPbBr x I 3Àx nanocrystals (NCs) on the CsPbBr 3 layer. Based on detailed investigation, they found out that CsPbBr x I 3Àx NCs with high tunable energy level and high hole extraction capability significantly diminished the energy loss and blocked the electron backflow, while the passivation treatment via spin-coated hexane increased the grain size of CsPbBr 3 as well as reduced the grain boundaries and trap state density. The CsPbBr x I 3Àx NCs tailored device exhibited a remarkable improvement in the device efficiency and a champion PCE of 9.45% was obtained in comparison with that of 5.26% for the NC-free PSCs (Fig. 20d). Moreover, the incorporation of CsPbBr x I 3Àx NCs also improved the stability of the carbon-based CsPbBr 3 devices, maintaining an outstanding stability for over 900 h in 80% relative humidity air atmosphere at 25 1C. Moreover, the CsPbBr x I 3Àx NCs incorporated carbon-based CsPbBr 3 device exhibited outstanding stability for over 900 h in RH 80% at 25 1C.
7.2.4 HTM interface engineering. To further passivate the surface defects, another efficient approach is to diminish the energy level differences between the perovskite/carbon-electrode by setting an intermediate level layer of hole-transporting materials (HTMs) and quantum dots at the interface. We know that the use of expensive HTMs such as spiro-OMeTAD is always a financial liability for the commercialization of PSCs. Some other inorganic HTMs have been employed to increase the stability and hole mobility of PSCs but the photovoltaic performance is much lower than that of tailored organic HTMs. Therefore, it is important to further investigate effective inorganic HTMs to boost the efficiency of CsPbBr 3 and other PSCs. Recently, a more cost-effective and stable HTM P3HT, a thiophene-containing polymer, has been extensively explored, which has a passivation effect on the surface defect states of the perovskites due to the coordination of sulfur with undercoordinated Cs + and Pb 2+ ions, which suppress charge recombination, decrease the trap density states, and improve charge separation at the perovskite/P3HT interface. 200 As the carbonbased PSCs offer favorable characteristics for large area production and commercialization of the PSCs, the undesirable part is that the direct contact of the carbon electrode with the CsPbBr 3 layer is vulnerable to the development of interfacial recombination sites and the formation of an unwanted hole extraction barrier. To address this issue, Wang et al. 201 incorporated a P3HT interlayer as the interfacial modifier for the perovskite/ carbon interface to suppress interfacial recombination and to boost the PCE of CsPbBr 3 PSCs. The incorporation of the P3HT interlayer facilitated the energy level alignment for hole extraction and prohibited the photo-generated electron transfer from the CsPbBr 3 layer to the carbon electrode, which effectively suppressed the interface recombination and enhanced the charge extraction in the CsPbBr 3 PSCs (Fig. 20e). Based on these results, a decent PCE of 6.49% was obtained for the CsPbBr 3 PSC with the P3HT interlayer, which is 27% higher than the device without the P3HT interlayer. However, the unfortunate part of using P3HT as the HTM is the inferior hole mobility compared to that of spiro-OMeTAD, which reduces the PCE of the PSCs. To boost the carrier mobility, several organic, inorganic, and polymeric materials have been incorporated to P3HT to form the composite HTMs. [202][203][204][205][206][207] To date, a promising photovoltaic efficiency of 17.8% with excellent stability has been realized for phthalocyanine HTM devices, making it a promising candidate for composite HTMs. 208 Based on these findings, Liu et al. 209 incorporated a cost-effective and organic-semiconducting material zinc phthalocyanine (ZnPc) with an appropriate lowest unoccupied molecular orbital (LUMO) energy level of about À3.17 eV as well as the highest occupied molecular orbital (HOMO) energy level of about À5.33 eV into P3HT by tuning the mass ratios via solution fabrication to enhance the hole mobility of P3HT and optimize the energy level to match well with the CsPbBr 3 perovskite (Fig. 20f). The ZnPc/P3HT composite was employed as the HTM for PSCs with the device architecture of FTO/c-TiO 2 /m-TiO 2 /perovskite/ZnPc/P3HT/C. The incorporation of ZnPc with P3HT not only down-shifted the HOMO energy level and effectively enhanced the hole mobility of P3HT but also the surface defects states of the CsPbBr 3 perovskite were passivated. Due to the well-matched energy level and higher hole mobility, the optimized cell prepared in ambient conditions showed a short-circuit current density ( J sc ) of 7.652 mA cm À2 , an ultra-high V oc of 1.578 V, FF of 83.06%, and a high PCE of 10.03% with outstanding stability in ambient conditions (Fig. 20g).
Based on these findings, it is suggested that introducing an intermediate layer of composite HTMs is a feasible strategy for passivating the surface defects and optimizing the energy level difference as well as enhancing the device efficiency and stability of CsPbBr 3 PSCs. 7.2.5 Quantum dots' (QDs) interface engineering. Quantum dots' interfacial engineering is a feasible approach to enhance the charge extraction and to control the defect trap state via the introduction of an intermediate energy level between the perovskite and charge-contact layers. Quantum dots, owing to their high absorption coefficients and tunable bandgaps, exhibit promising potential as interfacial materials in PSCs. [210][211][212][213][214][215][216] Guided by this perception, Yuan et al. 216 fabricated CsPbBr 3 PSCs modified by phosphorus quantum dots (PQDs) and electron-transporting carbon quantum dots (CQDs). By modifying the lowest unoccupied molecular orbital (LUMO) of CQDs and highest occupied molecular orbital (HOMO) of PQDs, they addressed the serious charge recombination and large energy differences at the TiO 2 /CsPbBr 3 and CsPbBr 3 /carbon interfaces to extract holes and electrons, respectively. The preliminary outcome indicates an increase in the PCE from 6.05% for pristine device to 7.93% for the CQDs/PQDs tailored PSC owing to the matching intermediate energy levels and facilitated charge extraction. Moreover, the optimized device exhibited lasting stability in high humidity conditions over 1400 h.
Previous studies suggest that defects on the surface of traditional semiconductors can be significantly passivated by incorporating a shell on the surface of the QD, which leads to improved photoluminescence (PL) and quantum efficiency. 217 Li et al. 218 successfully fabricated core-shell constructed QDs on the CsPbBr 3 device, which exhibited outstanding photovoltaic properties including tunable energy levels and high luminescence and realized a champion PCE of 8.65%, which was 14.8% higher than that of the pristine cell (Fig. 20h). The same group further developed quantum interfacial engineering by setting an intermediate energy-level at the CsPbBr 3 /perovskite with CsSnBr 3Àx I x quantum dots (QDs). Their study suggested that maximized charge extraction can be achieved by tuning the Br : I ratio of the CsSnBr 3Àx I x quantum dots (QDs). By doing so, a champion PCE of 9.13% and excellent stability in high humidity conditions (80% RH or 80 1C) over 720 h is realized for the CsSnBr 2 I QDs-tailored CsPbBr 3 device. The benefit of selecting CsSnBr 3Àx I x quantum dots (QDs) as interfacial modification materials is their similar processing prerequisites to those of the CsPbBr 3 perovskite layers. They also introduced graphene quantum dots and CuInS 2 /ZnS QDs for the interfacial modification of CsPbBr 3 /carbon interface, which led to the enhanced PCE of 9.72% and 8.42%, respectively. 218 Recently, tin-oxide (SnO 2 ) QDs have been reported as promising electron transporting material owing to their high electron mobility, high transparency, wide bandgap, and good photostability, as suggested by Zhao et al. 219 By incorporating SnO 2 QDs as the ETL and CsMBr 3 (M 1 4 Sn, Bi, Cu) QDs as the HTL between the CsPbBr 3 layer and carbon electrode, better energy level alignment was realized, which contributed to the fast electron-hole separation and suppressed charge recombination, leading to an improved PCE of 10.6% and ultra-high open-circuit voltage of up to 1.610 V for CsPbBr 3 PSCs (Fig. 20i). Although quantum dots' interfacial engineering presented promising results, the introduction of HTMs at the interface is regarded to be a more suitable strategy due to the limited hole extraction ability of the quantum dots.

Precursor engineering
The quality of the perovskite films is a critical factor for increasing the efficiency of the solar cells. Therefore, designing high-quality pinhole-free films that possess large grain size with monolayer alignment is the key procedure in the preparation of highly efficient CsPbBr 3 devices. As discussed earlier, most of the preparation of CsPbBr 3 films is based on solutionbased technologies. Therefore, the preparation of high-quality CsPbBr 3 films with a highly concentrated precursor (CsBr, PbBr 2 ) solution is a prerequisite for photovoltaic applications via solution-based methods. However, the low solubility of CsBr in commonly used solvents such as DMF and DMSO is a challenging step to prepare the CsPbBr 3 film via solution methods with optimum thickness. The dilemma of low solubility of CsBr was previously discussed and solved through precursor engineering. 81 A facile single-step solution method using highly soluble cesium acetate (CsAc) and hydrogen lead trihalide (HPbX 3 ) was developed to tackle the aforementioned Cs-precursor solubility limitation and to prepare high-quality a-CsPbX 3 perovskite films. Uniform and ultra-smooth CsPbX 3 perovskite films were obtained at relatively low temperature due to the strong interaction between the CsAc and HPbX 3 induced precursors. However, the fabricated CsPbBr 3 devices delivered a low PCE of 5.96%, indicating that further optimization of the films is required.
Further study based on novel-precursor engineering strategy was developed by Huang et al. 106 by using a precursor pair of cesium acetate (CsAc) and ionic liquid methylammonium acetate (MAAc) so as to enhance the concentration of the CsPbBr 3 precursor solution to 1.0 M, obtaining high quality and large grain sized CsPbBr 3 films (Fig. 21a). It is well-known that most of the acetates such as lead acetate (PbAc 2 ) are considered as promising precursors due to their good solubility in conventional solvents such as (DMF) and (DMSO). It was suggested that using this precursor strategy, the film quality is substantially improved: (i) the crystallization dynamics can be tailored with the incorporation of the MAAc ionic liquid to the precursor solution due to strong interaction between Ac À and Pb 2+ ; (ii) the limitation of low concentration of the precursor solution is resolved by the introduction of highly soluble CsAc and a thickness of up to 600 nm is obtained. Correspondingly, with the implementation of this strategy, the trap state-related charge recombination was suppressed and improved light harvesting capacity with a champion PCE of 7.37% with a high fill factor of 0.841 was achieved.

Compositional engineering
Compositional engineering of doping hetero-ions in the host lattice has been broadly developed as an efficient approach to improve the PCE of CsPbBr 3 PSCs. It has been demonstrated that the partial replacement of the Cs-site with alkali metal cations (Na + , Li + , K + , and Rb + ) and the Pb-site with transition metal cations (Mn 2+ , Zn 2+ , Cu 2+ , Co 2+ , etc.), or rare earth cations such as Ln 3+ in the perovskite lattice of CsPbBr 3 is a preeminent strategy to improve the grain size of the perovskite with low trap states and suppression of charge recombination in order to boost the device efficiency. 220,221 Previously, Liu et al. 222 systematically examined the effect of Cs-site cations on the device performance, suggesting that the quality of the films can be enhanced by the doping of alkali metal cations. The incorporation of alkali metal cations in the perovskite lattice passivated the grain boundaries, enhanced the grain size, suppressed the non-recombination losses, and increased the built-in potential, which substantially improved the device efficiency. Previous studies have shown that large grain sizes of CsPbBr 3 and CsPbBr 2 I can be realized with the incorporation of alkali metal cations (e.g., Li + , Na + , K + , and Rb + ) at the Cs + -site for low trap state density and charge recombination. In addition to Cs-site doping, the incorporation of homovalent or hetrovalent ions at the Pb 2+ -site also causes a passivation effect on the perovskite grains. The non-recombination rate can also be suppressed with the partial replacement of Pb 2+ by lanthanide ions (Sm 3+ , Tb 3+ , Ho 3+ , Er 3+ , and Yb 3+ ) as well as the partial incorporation of In 3+ , Al 3+ , Ca 2+ , Cd 2+ , and Sr 2+ , as suggested by literature reports. [225][226][227][228][229] Duan's group 230 also proposed a compositional engineering approach to boost the efficiency of CsPbBr 3 by the lattice incorporation of lanthanide ions (Ln 3+ = La 3+ , Ce 3+ , Nd 3+ , Sm 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Ho 3+ , Er 3+ , Yb 3+ , and Lu 3+ ) into perovskite films. The partial introduction of lanthanide ions not only enhanced the grain size but the carrier lifetime was also considerably prolonged and charge recombination within the perovskite was greatly reduced. By tuning the stoichiometric ratio of the dopants, a promising efficiency of 10.14% with a V oc of 1.594 V was achieved for the HTM-free PSC with the FTO/c-TiO 2 /m-TiO 2 /CsPb 0.97 Sm 0.03 Br 3 /C architecture, which is considerably higher than the 6.99% efficiency of the pristine CsPbBr 3 device (Fig. 21b). Moreover, the doped fabricated devices exhibited good long-term moisture stability under harsh humidity conditions (RH 80%) in air atmosphere for over 110 days and sustained thermal-tolerance at 80 1C for over 60 days. These findings offer new prospects for endorsing the commercialization of highly efficient CsPbBr 3 PSCs with long-term stability.
However, the unfortunate part of lanthanide ions' incorporation is that they are predominantly located at the grain boundaries of the perovskite film apart from the incorporation into the perovskite lattice by a small quantity during the formation of the CsPbBr 3 perovskite due to the higher metal ionic values, which restricts the better understanding of the film quality and substitution of the Pb 2+ -sites with extrinsic ions in the perovskite lattice. In their recent study, 232 the same group further addressed this issue by incorporating divalent hard Lewis acids cations (M = Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ ) in the CsPbBr 3 perovskite lattice. It is evident from the average grain size histogram of the doped CsPbBr 3 that the grain size substantially increased to 870 nm as compared with that of the pristine CsPbBr 3 film. It is comprehensible that the grain size is mostly enhanced due to the suppressed crystal nucleus formation (Fig. 21c). Moreover, in comparison with the pristine sample, the surface roughness of the hard Lewis acid cationsdoped CsPbBr 3 films is substantially lowered. To the best of our knowledge, lower grain boundaries can markedly reduce the defect state and therefore increase the device efficiency of the perovskite. Based on these outcomes, a high PCE of 9.63% was realized for the optimal cell with the FTO/c-TiO 2 /m-TiO 2 / CsPb 0.97 Sr 0.03 Br 3 /C architecture, which is greater than the PCE of 7.25% for the undoped cell. Tang et al. 223 further employed the compositional engineering strategy by partially incorporating the divalent transition metal ions (TM 2+ = Mn 2+ , Ni 2+ , Cu 2+ , and Zn 2+ ) with smaller ionic radius at the Pb 2+ site to regulate the crystalline structure for HTM-free carbon-based CsPbBr 3 PSCs. Noticeably, the CsPbBr 3 films showed large grains with low grain boundaries due to the incorporation of divalent transition metal ions (Fig. 21d). Moreover, the performance of the carbon-based CsPbBr 3 devices improved significantly due to low trap state density and there was a reduction in the energy loss in the charge carrier transfer (Fig. 21e). Also, the incorporation of the TM 2+ ions undoubtedly suppressed the charge recombination at the CsPbBr 3 /carbon interface. Benefitting from these findings, the optimized Zn-doped film exhibited a champion PCE of 9.18% with excellent stability of over 760 h in high humidity conditions (RH 80%) (Fig. 21f). Similar results were reported by Li's group 224 by means of doping the CsPbBr 3 perovskite with various alkali metal cations. They reported that the lattice dimensions and energy levels of the Cs 1Àx R x PbBr 3 (R 1 4 Li, Na, K, Rb, x 1 4 0-1) perovskite can be optimized by adjusting the Cs/R ratio. According to their systematic investigations, it was suggested that the introduction of alkali metal cations can bring promotional effects on the CsPbBr 3 perovskite such as the shrinking of the lattice, electrical energy distribution, and crystallized dynamics. Furthermore, owing to the decreased charge-trap state, a high efficiency of 9.86% was realized for the HTM-free Cs 0.91 Rb 0.09 PbBr 3 device with enhanced air stability (Fig. 21g).
It is well known that halide regulation (Cl and I) is a feasible strategy, by which the optical and photo-physical characteristics of the CsPbBr 3 perovskites can be successfully tuned. As the CsPbBr 3 PSCs possess superior stability and ultra-high V oc , the significant loss in the J sc is a critical factor that hinders the  251 reported the compositional engineering of CsPbBr 3 perovskites and highlighted the role of iodine incorporation into the CsPbBr 3Àx I x perovskite. The addition of iodine resulted in a slight improvement in the device efficiency from 2.97% to 3.98% with a high V oc of 1.13 V (Fig. 21h). Obviously, with the incorporation of iodine, the bandgap descended and the hysteresis performance of the CsPbBr 2.9 I 0.1 perovskite solar cell, as compared to the CsPbBr 3 device, was successfully mitigated. Besides the doping of iodine ions, the doping of chloride anion has also been studied as an effective dopant for regulating the perovskite films with enlarged grains for low defects and ameliorating the crystalline quality of the perovskite film. Moreover, chloride anion substitution promotes the transport of charge carriers, which reduces the trap state density by promoting the transport of charge carriers, which decreases the trap state density of the perovskite. [233][234][235] Inspired by these findings, Li et al. 236 prepared Cl-doped CsPbBr 3 films via the multistep solution method to improve the charge extraction and separation of the perovskite and to reveal the effect of chlorine doping on the mobility rate of the carriers and interfacial energy level matching in the CsPbBr 3 system. Their findings suggested that doping chlorine not only suppresses charge recombination in the device but also improves the grain size, carrier mobility, and energy level alignment at the interface, leading to enhanced charge extraction and transportation as well as increased carrier lifetime within the cells. Based on these findings, the optimized chlorine-doped cell with the structure of FTO/c-TiO 2 /m-TiO 2 /CsPbBr 2.98 Cl 0.02 /C exhibited an ultra-high V oc of 1.571 V and a PCE of 9.73% was realized, which is a striking enhancement in comparison with the V oc of 1.479 and PCE of 6.69% for the pristine device (Fig. 21i).

Conclusion and prospects
Since the pioneering report in 2015, CsPbBr 3 has been a research hotspot in the photovoltaics community. After several years of development, all-inorganic CsPbBr 3 PSCs have accomplished the highest efficiency of 10.91%, approaching approximately 70% of the Shockley-Queisser (SQ) efficiency limits. The significant advances in the device efficiency are accredited to the superior thermal and moisture stability of the CsPbBr 3 perovskite. However, there is a large research space that is needed to be filled out to match the PCE of other counterparts, whose PCEs are over 80% of the SQ limits. Owing to the high bandgap of 2.3 eV, the S-Q limit model of CsPbBr 3 indicates a maximum PCE of 16.5%. Similarly, the theoretical calculations show a V oc of 1.98 V, which is much higher than the highest stated V oc of 1.615 V for CsPbBr 3 PSCs. In this review, we have systematically discussed the different aspects of CsPbBr 3 PSCs, starting with the basic properties to the development of different methodologies and engineering strategies to increase the stability and enhance the photovoltaic performance of CsPbBr 3 PSCs.
Unlike other halide perovskites, CsPbBr 3 does not suffer from stability issues; however, there is plenty of room for further development for achieving higher PCE. We believe that the following research directions could be beneficial for further enhancing the photovoltaic performance of CsPbBr 3 devices.
(i) The future advances of CsPbBr 3 perovskites necessitate the thoughtful design and preparation of each essential functional layer, such as the perovskite layer and the interfaces, and careful selection of the electron transport layer (ETL) and the hole transport layer (HTL) for controlling the carrier dynamics, suppression of charge recombination, and reducing the energy losses. We believe that extensive research is required to overcome the difficult challenge of designing optimal interfaces for stable and efficient CsPbBr 3 solar cells, which should fulfil the following merits: (i) have optimal surface energy; (ii) possess proper energy level alignment to decrease the energy barrier for charge transfer; (iii) efficient charge extraction; (iv) can passivate the films' trap-state densities; (v) facilitates lowtemperature processing and compact and pinhole-free film formation; (vi) high stability.
(ii) ETL and HTL are an important part of the PSCs structure, which can effectively accelerate carrier transport and reduce carrier recombination, thus contributing to improved device performance and stability. The development of ETL and HTL with optimal energy levels can significantly facilitate charge transfer. Secondly, the quality of the perovskite crystal structures plays a significant role as crystal structures with fewer defects enhance the carrier lifetime and reduce the energy losses in the perovskite. However, organic HTL, in particularl, spiro-OMeTAD, contributes to high cost as well as instability of the CsPbBr 3 PSCs. Moreover, the use of traditional ETLs such as TiO 2 contributes to the short lifetimes caused by the UV-induced instability, thus affecting the photovoltaic performance of the CsPbBr 3 PSCs. We suggest that other alternative ETLs and HTLs should be developed and special attention should be given to the ETL/perovskite interface and the HTL/perovskite interface in order to boost carrier transport. For the optimization of the transport layers, there are few critical points such as such as energy level alignment, trap states, morphology, charge mobility, and interfacial properties, which are crucial for shaping the final performances of the devices. The energy level alignment between the transport layers and the perovskite can efficiently accelerate the carrier transport to enhance the J sc and FF. Moreover, the wellmatched Fermi levels will contribute to enhanced V oc of the CsPbBr 3 -based devices. Therefore, it is important to develop new and effective strategies such as compositional engineering and interfacial engineering to passivate the interfacial defects and suppress non-radiative recombination.
(iii) In order to achieve compact and uniform CsPbBr 3 films, the following merits should be fulfilled: (i) have adequate thickness; (ii) pinhole-free and compact nature; (iii) good phase purity. Solution-based methods offer cost-effective and easy processing strategies but the unfavorable part is the low solubility of solvents, followed by several physical conditions relating to different factors such as vapor pressure, composition stoichiometry, boiling point, and dielectric constant, which should be fulfilled so as to resolve the possible incompatibility between solution-based processes and large area fabrication. For example, conventionally used solvents such as pure DMSO and DMF cannot meritoriously improve the solubility of the precursors for gaining a thick film. Using such solvents for the preparation of the CsPbBr 3 perovskite not only slows down the crystallization process but also affects the film morphology. Therefore, we suggest that new environment-friendly solvents should be employed and their mechanism of crystal growth and role in intermediate formation should be comprehensively investigated.
(iv) We believe that the fabrication approach should have the following merits in order to be considered for industrial-scale fabrication: (i) low cost; (ii) excellent reproducibility; (iii) feasible for large areas. However, the development of large-scale processing technology has become a serious issue that must be taken care of before industrialization. The commonly used solution methods such as drop-casting and spin-coating are unfavorable for large scale fabrication. Vapor/vacuum-based and printing methods are much more favorable for scale-up and large area devices. Presently, there have been limited studies on large active areas and a majority of fabricated PSCs have been reported for small active areas (o1 cm 2 ). It still remains a challenging task to scale up the size of the CsPbBr 3 PSCs without substantial efficiency loss.
(v) Furthermore, a variety of state-of-the-art engineering strategies such as compositional engineering, additive engineering, interfacial engineering, and precursor engineering have been established to enhance the photovoltaic performance of the CsPbBr 3 PSCs. We believe that the different engineering strategies mentioned in this review will positively influence the stability, film morphology, and PCE efficiency of the CsPbBr 3 PSCs. The reason for this is evident from the massive improvement in the device efficiency from 5.74 to 10.91% in a short period, which has been realized with the optimization of the crystal growth process, suppression of carrier recombination, and improved carried transport. It is also put forward that strategies such as doping and additive engineering of inorganic CsPbBr 3 facilitates the improvement in the stability as well as the reduction in the trap-state densities and suppression of charge recombination at the interface, thus contributing to the improvement of the device efficiency of CsPbBr 3 perovskites. Unfortunately, limited research attention has been paid to such strategies; thus, it is required to further develop and modify such new and existing strategies so as to boost the efficiency of CsPbBr 3 PSCs. It is suggested that introducing new materials as interface materials or as additives can reduce the surface defects and passivate the grain boundaries, thus improving the PCE and stability of the perovskite.
(vi) Apart from HTL-based CsPbBr 3 PSCs, stable electrode materials such as carbon have been employed to replace the conventional electrodes such as Ag, Au, and expensive HTMs for HTL-free CsPbBr 3 PSCs. The advances in carbon-based CsPbBr 3 PSCs show great promise in fulfilling the criteria of the golden triangle, i.e., (i) high PCE, (ii) low cost, and (iii) high stability. For example, the carbon electrode not only facilitates charge extraction but also offers device encapsulation function. Moreover, the development of carbon-based or HTM-free CsPbBr 3 PSCs have good prospect in meeting the challenge of large scale and low-cost fabrication and thus, should be further explored. However, the shortcomings of HTM-free PSCs need to be further studied before they can be considered as a permanent replacement to HTM-based PSCs.
(vii) Furthermore, we believe that strengthening the basic theoretical concepts through theoretical simulations and modeling are important for explaining the experimental results. The simulated device structures can provide important information about the materials and thickness of each layer, which can be further employed as a convincing reference for the experiments. Recently, our group 242 optimized the working mechanism of the CsPbBr 3 perovskite using simulation analysis and introduced a gradient junction design of the absorber, which resulted in the effective reduction in high interface recombination and an ultrahigh open-circuit voltage (V oc ) of 1.68 V; a record high PCE of 11.58% was realized for modulated CsPbBr 3 PSCs. Our findings propose that significant research efforts are still needed for the further improvement in the device efficiency as well as the opencircuit voltages of CsPbBr 3 PSCs.
(viii) To make the CsPbBr 3 perovskite feasible for flexible application, we believe that it is important to fabricate PSCs at low processing temperatures so as to accelerate the charge transport and to passivate the defects. For this purpose, it is important to understand the mechanism of crystal growth at low temperatures and to develop ways to decrease the activation energy of crystal growth without relinquishing the crystal quality. Since CsPbBr 3 has a large bandgap, combining with smaller bandgap photovoltaic materials to fabricate tandem solar cell can be a promising strategy for the further development of highly efficient tandem devices. We believe that the hybrid tandem solar cells based on CsPbBr 3 PSCs have great prospect and will generate more research attention in the future.
(ix) Long-term stability has been a major concern for other organic-inorganic PSCs such as CsPbI 3 . Compared with its counterparts, CsPbBr 3 exhibits good resistance against harsh humidity and thermal conditions. However, an essential consideration for large scale commercialization is the long-term operational stability of the CsPbBr 3 PSCs, which needs further studies. For this purpose, it is important to understand the degradation mechanism of selective contacts and electrodes as interfacerelated chemical reactions at the perovskite/electrodes have been considered as the possible cause for the degradation of the photovoltaic performance of the CsPbBr 3 PSCs. We suggest that the hydrophobic nature of the carbon electrode could be a game changer for prolonging the stability of CsPbBr 3 PSCs.