Recent Progress of All‐Bromide Inorganic Perovskite Solar Cells

Energy TechnologyVolume 8, Issue 4 1900961 ReviewOpen Access Recent Progress of All-Bromide Inorganic Perovskite Solar Cells Guoqing Tong, Guoqing Tong Energy Materials and Surface Sciences Unit (EMSSU), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Okinawa, 904-0495 JapanSearch for more papers by this authorLuis K. Ono, Luis K. Ono Energy Materials and Surface Sciences Unit (EMSSU), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Okinawa, 904-0495 JapanSearch for more papers by this authorYabing Qi, Corresponding Author Yabing Qi Yabing.Qi@OIST.jp orcid.org/0000-0002-4876-8049 Energy Materials and Surface Sciences Unit (EMSSU), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Okinawa, 904-0495 JapanSearch for more papers by this author Guoqing Tong, Guoqing Tong Energy Materials and Surface Sciences Unit (EMSSU), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Okinawa, 904-0495 JapanSearch for more papers by this authorLuis K. Ono, Luis K. Ono Energy Materials and Surface Sciences Unit (EMSSU), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Okinawa, 904-0495 JapanSearch for more papers by this authorYabing Qi, Corresponding Author Yabing Qi Yabing.Qi@OIST.jp orcid.org/0000-0002-4876-8049 Energy Materials and Surface Sciences Unit (EMSSU), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Okinawa, 904-0495 JapanSearch for more papers by this author First published: 04 October 2019 https://doi.org/10.1002/ente.201900961Citations: 47AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Inorganic perovskite solar cells (PSCs) have attracted enormous attention during the past 5 years. Many advanced strategies and techniques have been developed for fabricating inorganic PSCs with improved efficiency and stability to realize commercial applications. CsPbBr3 is one of the representative materials of inorganic perovskites and has demonstrated excellent stability against thermal and high humidity environmental conditions. The power conversion efficiency of CsPbBr3-based PSCs has increased significantly from 5.95% in 2015 to 10.91%, and the storage stability under moisture (≈80% relative humidity) and heat (≈80 °C) is more than 2000 h. The outstanding performance of CsPbBr3 PSCs shows great potential in light-to-electricity conversion applications. In this review, recent developments of CsPbBr3-based PSCs including the physico-chemical as well as optoelectronic properties, processing techniques for fabricating CsPbBr3 films, derivative phase structures, efficiency, and stability of devices are reviewed and discussed. Finally, the challenges and outlook of CsPbBr3 PSCs for future research directions are outlined. 1 Introduction Currently, the unprecedented development of organic–inorganic hybrid perovskite solar cells (OIH-PSCs) has drawn much attention because of high efficiency, low-cost processing techniques, and abundant availability of raw materials.[ 1-5 ] The power conversion efficiency (PCE) of OIH-PSCs, in the past decade, increased rapidly from 3.8% as reported in 2009 to a certified record of 25.2% in 2019.[ 6, 7 ] At the same time, researchers demonstrated large-scale hybrid PSCs with certified PCEs of 17.25% and 11.7% for minimodules (17.277 cm2) and submodules (703 cm2), respectively, under stabilized output conditions.[ 8, 9 ] However, phase instability of OIH-PSCs under external stimuli such as moisture, heat, light, and oxygen impedes their further application because of the volatility of the organic constituents in these materials (such as MA+, FA+, or mixed cations) and the weak chemical bonding energies between metal cations (Pb2+) and halide anions (I−, Br−, or Cl−).[ 10-15 ] Despite of several strategies developed to improve the stability of OIH-PSCs (e.g., employment of inorganic cation doping,[ 16-19 ] surface modification with stable materials,[ 20-22 ] advanced encapsulation techniques[ 23-25 ]), the intrinsically unstable nature of hybrid perovskite materials is still a pending issue for commercial applications with long-term stability. Different from OIH-PSCs, inorganic PSCs demonstrate excellent moisture and thermal stability by substituting the organic cations (MA+/FA+) with inorganic ones (Cs+).[ 26, 27 ] There are three basic types, based on different halides, i.e., CsPbI3, CsPbBr3, and CsPbCl3.[ 28-31 ] Detailed discussions about perovskite formation can be found in several review papers focusing on this topic.[ 26, 32 ] CsPbI3, as a representative of inorganic PSCs, has a suitable bandgap of ≈1.73 eV.[ 33 ] Despite the fact that CsPbI3 PSCs can already achieve PCE as high as 19%, the undesirable phase transition from the black cubic phase to the yellow non-perovskite phase at room temperature makes the material unstable.[ 29, 34 ] For CsPbCl3, the optical bandgap is above ≈3.0 eV, which makes it unsuitable for solar cell applications. Alternatively, CsPbBr3 perovskite has also a larger bandgap of 2.3 eV compared with CsPbI3, but shows a much better phase stability in ambient, which ensures appropriate light harvesting characteristics with long-term stability when CsPbBr3 is incorporated into a solar cell device structure. In this article, we review the recent developments of CsPbBr3 PSCs. We first introduce the basic properties of CsPbBr3 including the crystal structure, physico–chemical, and optoelectronic properties. Then, we discuss the reported fabrication techniques to prepare CsPbBr3 films highlighting the morphology differences of as-prepared films based on the solution- and vapor-based deposition techniques. Furthermore, we summarize the device structures and corresponding efficiencies (in Table 1 ) and the recent developments of CsPbBr3-based PSCs. Finally, we discuss the storage stability under moisture (≈20–80% relative humidity, RH) and elevated temperatures (50–100 °C) as well as operational stability to provide further insights into the advantages of CsPbBr3PSCs. We end this review outlining a few promising future research directions. Table 1. Summary of CsPbBr3-based PSCs in the literature with CsPbBr3 thin-films synthesized by the different methods. Solar cell parameters of power conversion efficiency (PCE), open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF) as well as lifetimes are provided Device Method PCE [%] V oc [V] J sc [mA cm−2] FF [%] Lifetime (moisture/heat) Ref ITO/PEDOT:PSS/CsPbBr3/PCBM/Ag Co-vapor 3.9 0.94 5.9 70 / / 35 FTO/TiO2/CsPbBr3 QD/spiro-OMeTAD/Au 2-Solutionb) 4.21 0.859 8.55 57 / / 36 FTO/TiO2/CsPbBr3/CZTS/spiro-OMeTAD/Ag 2-Solution 5.36 1.12 7.04 68.11 2500 h / 37 Air FTO/TiO2/CsPbBr3/PTAA/Au 2-Solution 5.95 1.28 6.24 74 / / 38 FTO/TiO2/CsPbBr3/spiro-OMeTAD/Au 2-Solution 6.05 1.34 6.52 69 / / 39 FTO/TiO2/CsPbBr3/PTAA/Au 2-Solution 6.2 1.25 6.7 73 14 days / 40 RH ≈ 20% FTO/TiO2/CsPbBr3(Cl)/spiro-OMeTAD/Ag 2-Solution 6.21 1.02 8.47 71.6 300 h 300 h 41 RH ≈ 40% T ≈ 50 °C FTO/ZnO/CsPbBr3-QDs/spiro-OMeTAD/Au 2-Solution 6.81 1.43 6.17 77.2 100 days / 42 RH ≈ 45% FTO/TiO2/CsPbBr3/spiro-OMeTAD/Au Co-vapor 6.95 1.27 6.97 78.5 60 days / 43 RH ≈ 20% FTO/ZnO/CsPbBr3/spiro-OMeTAD/Au Co-vapor 7.78 1.44 7.01 77.11 / / 44 FTO/TiO2/CsPbBr3/Si QDs/spiro-OMeTAD/Ag 1-Solutiona) 8.31 1.42 7.8 75 7 days / 45 RH ≈ 70% FTO/TiO2/CsPbBr3/spiro-OMeTAD/Ag Sequential-vapor 8.34 1.296 8.48 75.9 1000 h / 46 RH ≈ 45% ITO/SnO2/CsPbBr3/spiro-OMeTAD/Au 1-Solution 9.81 1.26 10.33 75.34 / / 47 FTO/TiO2/PTI-CsPbBr3/spiro-OMeTAD/Ag Sequential-vapor 10.91 1.498 9.78 74.47 1000 h 20 h 48 RH ≈ 45% T ≈ 100 °C FTO/CsPbBr3/carbon 2-Solution 2.35 1.05 4.64 48.2 / / 49 FTO/TiO2/CsPbBr3/carbon Solution and vapor 5.38 1.13 6.79 70.0 Air / 43 Air FTO/TiO2/CsPbBr3/carbon 2-Solution 5.86 1.34 6.46 68.04 240 h / 50 RH ≈ 60% FTO/TiO2/CsPbBr3/carbon 2-Solution 6.1 1.38 7.13 62 200 days 1080 h 51 RH ≈ 25%–85% T ≈ 80 °C FTO/TiO2/CsPbBr3/CuPc/carbon 2-Solution 6.21 1.26 6.62 74.4 2000 h 944 h 52 RH ≈ 40% RH ≈ 80%, T ≈ 100 °C FTO/TiO2/CsPbBr3/P3HT/carbon 2-Solution 6.49 1.36 7.02 68 40 days / 53 Air FTO/TiO2/CsPbBr3/carbon 2-Solution 6.7 1.24 7.4 73 2640 h / 31 RH ≈ 90% FTO/TiO2/CsPbBr3/carbon:PtNiNW 2-Solution 7.86 1.432 6.78 81.0 20 days / 54 RH ≈ 80% FTO/TiO2/m-ZrO2/CsPbBr3/carbon 2-Solution 8.19 1.44 7.75 73.52 / / 55 FTO/TiO2/CsPbBr3/carbon 2-Solution 8.63 1.37 7.66 82.22 90 days / 56 RH ≈ 10% FTO/TiO2/SnO2/CsPbBr3/CuPc/carbon 2-Solution 8.79 1.31 8.24 81.4 1000 h 30 days 57 RH ≈ 40% T ≈ 60 °C FTO/TiO2/CsPbBr3/carbon Co-vapor 8.86 1.522 7.24 80.4 / 30 days 58 RH ≈ 35%, T ≈ 85 °C FTO/TiO2/CsPbBr3/MXene/carbon Sequential-vapor 9.01 1.444 8.54 73.08 1900 h 600 h 59 RH ≈ 45% T ≈ 80 °C FTO/TiO2/CsPbBr3/carbon 2-Solution 9.72 1.458 8.12 82.1 130 days 40 days 60 RH ≈ 90% T ≈ 80 °C FTO/TiO2/CsPbBr3/P3HT/ZnPC/carbon 2-Solution 10.03 1.578 7.652 83.06 30 days / 61 RH ≈ 70% FTO/TiO2/CsPb0.97Sm0.03Br3/carbon 2-Solution 10.14 1.594 7.48 85.1 110 days 60 days 62 RH ≈ 80% T ≈ 80 °C FTO/TiO2/CsPbBr3/carbon Sequential-vapor 10.17 1.461 9.24 75.39 3000 h 700 h 63 RH ≈ 45% T ≈ 100 °C FTO/TiO2/CsPb0.97Tb0.03Br3/SnS:ZnS/NiO x /carbon 2-Solution 10.26 1.57 8.21 79.6 40 days 14 days 64 RH ≈ 80% T ≈ 80 °C FTO/TiO2/CsPbBr3/MnS/carbon Solution and vapor 10.45 1.52 8.28 83 150 days 100 days 65 RH ≈ 80% T ≈ 80 °C FTO/SnO2 QDs/CsPbBr3/CsSnBr3 QD/carbon 2-Solution 10.6 1.61 7.8 84.4 10 days 10 days 66 RH ≈ 80% T ≈ 80 °C FTO/TiO2/CsPbBr3/Cu(Cr,M)O2/carbon 2-Solution 10.79 1.615 7.81 85.5 60 days 40 days 67 RH ≈ 80% T ≈ 80 °C a) 1-Solution stands for the one-step solution method; b) 2-Solution stands for the two-step solution method. 2 CsPbBr3 Properties 2.1 Crystal Structure of CsPbBr3 In the general crystal structure of CsPbBr3, Pb2+ and Br− ions form a 3D framework of corner-sharing [PbBr6]4− octahedra with Cs+ ions incorporated between the octahedral spaces (Figure 1 ).[ 26, 68 ] On the basis of single-crystal X-ray diffraction (XRD) data, CsPbBr3 crystallizes in the orthorhombic (Pnma) space group at room temperature and transforms to the tetragonal (P4/mbm) and cubic (Pm-3m) phases at 88 and 130 °C, respectively.[ 69 ] The geometric stability of CsPbBr3 structure can be determined by Goldschmidt tolerance factor (t) as follows t = R Cs + R Pb 2 ( R Cs + R Br ) (1)where R is the ionic radius. In general, to maintain the high symmetry of the perovskite structure, it is desirable to have a tolerance factor value between 0.81 and 1.02. Although the Cs+ ion has a smaller ionic radius (1.81 Å) than that of MA+ (2.70 Å) or FA+ (2.79 Å),[ 17 ] Cs+ still satisfies the requirement of the tolerance factor in the CsPbBr3 inorganic perovskite (t = 0.82).[ 70 ] In contrast, the radius of other cations, i.e., ethylammonium (EA) [(C2H5)NH3]+, guanidinium (GA) [C(NH2)3]+, and imidazolium (IA) [C3N2H5]+ are 2.74, 2.78, and 2.58 Å, respectively, which are larger than that of Cs+.[ 71 ] We note that the tolerance factor of EA+(1.06), GA+(1.06), and IA+ (1.02) exceed 1, which indicates phase instability. Figure 1Open in figure viewerPowerPoint a) Schematic crystal structures of CsBr, PbBr2, and formation of inorganic perovskite CsPbBr3. Reprinted with permission.[ 50 ] Copyright 2018, American Chemical Society. b) Atomic Cs/Pb ratios determined by EDS inferring the different phases by the multistep spin-coating by increasing deposition cycles of CsBr solution. Reprinted with permission.[ 60 ] Copyright 2018, Wiley-VCH. c) Schematic crystal structures and formation of inorganic perovskite derivative phases (CsPb2Br5 and Cs4PbBr6). Reprinted with permission.[ 63 ] Copyright 2019, Wiley-VCH. In addition to the CsPbBr3 phase, there are two other derivative phases, i.e., CsPb2Br5 and Cs4PbBr6 structures (Figure 1c). The tetragonal CsPb2Br5 shows a 2D layer structure. In this derivative phase, Cs+ ions are sandwiched between the two layers of Pb–Br-coordinated polyhedrons. The formation of the CsPb2Br5 phase is induced by the excessive PbBr2 in the structure and can be understood as follows[ 72 ] CsBr + 2 PbBr 2 → CsPb 2 Br 5 (2) CsPbBr 3 + PbBr 2 → CsPb 2 Br 5 (3) The reverse direction in Reactions (2) and (3) takes place if the tetragonal CsPb2Br5 phase is annealed at a high temperature (e.g., 300 °C). CsPb2Br5, eventually, fully disappears above 400 °C by the following mechanism[ 73 ]: CsPb2Br5 → CsPbBr3 + PbBr2. In addition, Cs4PbBr6 exhibits a 0D structure based on the [PbBr6]4− octahedra. The octahedra are separated from each other by CsBr bridges. Similar to the CsPb2Br5, the influence of excessive CsBr leading to the formation of Cs4PbBr6 can be explained by the following reactions 4 CsBr + PbBr 2 → Cs 4 PbBr 6 (4) 3 CsBr + CsPbBr 3 → Cs 4 PbBr 6 (5) 2.2 Properties of CsPbBr3 CsPbBr3 films are yellow in color after annealing, which indicates that only the portion of light (spectrum) in the short wavelength range can be absorbed. The absorption spectrum edge of the CsPbBr3 film is shorter than 540 nm, and the exciton-induced absorption peak is located at ≈520 nm. Tauc plot analyses suggest that the CsPbBr3 thin films possess a large optical bandgap of ≈2.3 eV. However, by controlling the ratio of CsBr and PbBr2, the bandgap will further increase significantly from 2.3 to 4.0 eV because of the phase transition from the cubic perovskite structure to the derivative phases (Cs4PbBr6 and CsPb2Br5). As a countermeasure, substituting the bromide with iodine or replacing lead dication with tin dication can realize a narrower optical bandgap. However, these strategies usually result in inorganic perovskite films with inferior stability. Not only the optical bandgap, but the high carrier mobility and long diffusion length are also important parameters for solar cells. Stoumpos et al. found that the high electron mobility of CsPbBr3 (in the form of single crystals) was up to 1000 cm2 V−1 s−1, and the electron lifetime was 2.5 μs.[ 69 ] Recently, Zhu et al. demonstrated the carrier mobility of 38 ± 11 cm2 V−1 s−1 for CsPbBr3 single-crystal microplates.[ 74 ] The mobility-lifetime product of CsPbBr3 single crystals was investigated by Dirin et al.[ 75 ] CsPbBr3 showed a smaller mobility-lifetime product (≈2 × 10−4 cm2 V−1) than that of hybrid perovskites, which was attributed to shorter carrier lifetime. Charge carrier lifetimes of 2–7 μs were previously reported for CsPbBr3 single crystals.[ 26 ] The carrier diffusion length of 80 nm was reported for CsPbBr3 films.[ 76 ] For CsPbBr3 single crystals, the diffusion lengths for electrons and holes were reported to be ≈10 and ≈12 μm, respectively.[ 77 ] Lead halide perovskite systems (APbBr3, A = MA/FA/Cs) favor hole diffusion over electron diffusion, which was demonstrated by Elbaz et al.[ 78 ] They found that the diffusion length of holes (≈10–50 μm) was on average an order of magnitude larger than that of electrons (1–5 μm). These high carrier mobilities and diffusion lengths ensure fast charge injection and transport in devices, which is essential for achieving a high photocurrent density. 3 Fabrication Methods for Preparation of CsPbBr3 Films The full coverage and high crystallinity of perovskite films are required to achieve a high performance in solar cells. Generally, similar to the hybrid organic–inorganic perovskite materials, CsPbBr3 films are usually prepared by the solution-processing technique, for example, spin-coating and dipping. Meanwhile, vapor deposition, including co-evaporation and sequential deposition, is another effective strategy to prepare high-quality CsPbBr3 films because the solubility limitation of precursor materials in solvents is not a concern in vapor deposition. In addition, the vapor-assisted solution method, which is based on combining the spin-coating processing and vapor treatment techniques, is also utilized to produce high-quality CsPbBr3 films. 3.1 Solution Methods for Preparation of CsPbBr3 Films 3.1.1 One-Step Solution Method Solution-processing techniques are a simple strategy based on dissolving the precursor materials into a solvent with a fixed ratio. Usually, for OIH-PSCs, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), γ-butyrolactone (GBL), and a combination of mixed solvents are used to dissolve the PbX2 and MAI/FAI. However, CsBr is less soluble than PbBr2 in the aforementioned aprotic solvents. The maximized concentrations of CsBr in methanol and DMSO are only 0.07 and 0.25 m, respectively.[ 31, 60, 79 ] The poor solubility of CsBr makes it difficult to prepare the CsPbBr3 precursor solution with a molar ratio of 1:1. The side products, for example, CsBr-rich or PbBr2-rich phase, have a significant influence on the morphology and quality of as-prepared CsPbBr3 films. Therefore, it is still challenging to fabricate CsPbBr3 films by the one-step solution method.[ 80 ] For example, You and coworkers reported a one-step spin-coating method to fabricate the CsPbBr3 films by dissolving the CsBr and PbBr2 into the mixed DMF and DMSO solvents (Figure 2 a).[ 47 ] The maximum concentration of CsPbBr3 precursor solution was only 0.4 m, and it was shown that it was difficult to form a thick and full-coverage CsPbBr3 films by the one-step spin-coating method. Figure 2Open in figure viewerPowerPoint a) Schematic diagram showing the one-step solution method and top-view SEM images of an inorganic CsPbBr3 film. Reproduced with permission.[ 47 ] Copyright 2018, Nature Publishing Group. b) Schematic diagram showing the two-step solution method via dipping and top-view SEM images of an inorganic CsPbBr3 film. Reproduced with permission.[ 50 ] Copyright 2018, American Chemical Society. c) Schematic diagram showing the two-step solution method via multistep spin-coating and top-view SEM images of an inorganic CsPbBr3 film. Reproduced with permission.[ 60 ] Copyright 2018, Wiley-VCH. 3.1.2 Two-Step Solution Method To overcome the insolubility issue of CsBr encountered in the one-step solution-coating method, the two-step solution-coating method has been developed for CsPbBr3-based PSCs. One molar concentration of PbBr2 was first dissolved in DMF and spin-coated on substrates. After the subsequent annealing at an optimized temperature (80 °C) for 30 min, the samples were dipped in a CsBr/methanol solution for several minutes (Figure 2b).[ 38, 50 ] The key parameters in the two-step solution method are the precursor concentration, temperature of methanol solution, and dipping time.[ 26 ] The as-prepared CsPbBr3 films were rinsed with isopropanol and annealed at 250 °C for 5–10 min. This strategy has been widely used in CsPbBr3-based PSCs to form thick, full-coverage, and uniform CsPbBr3 films. In addition, Tang and coworkers proposed a multistep solution method by spin-coating the PbBr2 and CsBr solution sequentially.[ 60 ] As shown in Figure 2c, the CsBr solution was spin-coated six times to optimize material composition. Each of the CsBr spin-coating step leads to stepwise phase transition between the derivative phases (CsPb2Br5/Cs4PbBr6) and CsPbBr3 phase. The ideal coverage and grain size can be obtained after four cycles. Liu et al. also developed the same strategy to deposit the CsPbBr3 films.[ 57 ] They found that the as-prepared CsPbBr3 films had homogeneous, uniform, and highly crystalline grains. The average grain size was up to 1 μm after annealing. Furthermore, the root-mean-square (RMS) roughness of CsPbBr3 films was below 50 nm. 3.2 Vapor Deposition Methods for Preparation of CsPbBr3 Films Solubility limitation of precursor materials in solvents is still a bottleneck to fabricate high-quality CsPbBr3 perovskite films. Pal et al. reported an all-solid-state mechanochemical grinding method to synthesize CsPbBr3 powder, which can effectively avoid the solubility limitation of precursor materials and CsPbBr3.[ 81 ] This indicates that, there is an increasing interest in vapor deposition strategy. The melting/boiling points of CsBr (≈630/1300 °C) and PbBr2 (≈370/892 °C) powders are still feasible temperatures in standard physical vapor deposition (PVD) techniques. In addition, the thickness of inorganic perovskite films or precursor layers can be precisely controlled by the quartz crystal monitor (QCM) to satisfy the ratio of each component.[ 82, 83 ] Furthermore, the vapor deposition strategy can realize the application in large-scale devices with good uniformity and high reproducibility. Therefore, vapor deposition is a promising and effective strategy to fabricate high-quality perovskite films with full coverage and good uniformity.[ 84 ] 3.2.1 Sequential Vapor Deposition Tong et al. reported a sequential vapor deposition method to produce uniform inorganic perovskite films with controllable chemical composition (Figure 3 a).[ 63 ] They first deposited CsBr on a substrate with an optimized evaporation rate and then coated it with a PbBr2 film on top. By controlling the thickness ratio of the two precursor materials, they obtained pure CsPbBr3 and mixed phases (CsPbBr3–CsPb2Br5 and CsPbBr3–Cs4PbBr6) as follows[ 63, 86, 87 ] CsBr + PbBr 2 → CsPbBr 3 (6) CsBr + 2 PbBr 2 → CsPb 2 Br 5 (7) 4 CsBr + PbBr 2 → Cs 4 PbBr 6 (8) Figure 3Open in figure viewerPowerPoint a) Schematic diagram showing the sequential vapor deposition method for inorganic CsPbBr3 solar cells and corresponding top-view SEM images of CsPbBr3 films. Reproduced with permission.[ 63 ] Copyright 2019, Wiley-VCH. b) Schematic diagram showing the co-evaporation method to deposit inorganic CsPbBr3 thin films in vacuum and corresponding top-view SEM images of CsPbBr3 films. Reproduced with permission.[ 44 ] Copyright 2018, Wiley-VCH. c) Schematic diagram showing the vapor-assisted CVD process to produce CsPbBr3 films and corresponding top-view and cross-sectional SEM images of CsPbBr3 films. Reproduced with permission.[ 85 ] Copyright 2018, Elsevier Ltd. The as-prepared CsPbBr3 films displayed large crystalline grains after annealing in air. The largest size of perovskite grains was up to 1 μm, which ensured efficient carrier transport properties in addition to enhanced light absorption. In addition, a similar strategy was also developed by Li et al., who prepared CsPbBr3 films by controlling the thickness ratio of CsBr and PbBr2 films to achieve high-quality CsPbBr3 films.[ 46 ] The films deposited by this strategy showed an ultrasmooth surface with a RMS of 17.3 nm, which was much lower than the films made by solution methods. The small RMS led to a better interface contact with hole transport layer (HTL). This strategy is usually performed in high vacuum systems with a typical pressure of 10−3 Pa that has the added advantage of minimizing extrinsic impurities. 3.2.2 Co-Evaporation Deposition The co-evaporation method was also developed to fabricate CsPbB3 thin films by heating CsBr and PbBr2 simultaneously.[ 43, 44 ] Chen et al. investigated the effect of the evaporation rates on the formation of CsPbBr3 thin films at the rates of 5, 15, and 25 Å s−1 (Figure 3b). They found that the evaporation rate showed a significant influence on the crystallinity and crystal orientation.[ 44 ] For example, the lower rate was usually favorable for high crystallinity and (100) and (200) crystal orientation. In contrast, an inhomogeneous thin film with smaller grain sizes was found when the evaporation rate was too high. Recently, Lei et al. also developed the co-evaporation technique to investigate the formation of CsPbB3 films by investigating the effects of the substrate temperature and evaporation rate ratio of precursor materials.[ 43 ] They found that the CsPbBr3 films showed a high crystallinity when the substrate temperature was 300 °C, and the evaporation rate ratio of CsBr and PbBr2 was 0.7:1. In addition, a higher annealing temperature could significantly increase the grain size and reduce the roughness of the as-prepared films from 60 to 43.7 nm. However, many voids appeared as the treatment temperature was above 550 °C. 3.2.3 Vapor-Assisted Solution Process To lower the cost associated with the high vacuum system, Luo et al. developed a vapor-assisted CVD process (Figure 3c), to realize the fast anion-exchange from CsPbI3 to CsPbBr3 by placing the sample into Br2 environment in a vacuum chamber.[ 85 ] In this case, the CsPbI3 precursor films were first deposited by spin-coating onto desired substrates, and then transferred into a quartz tube furnace and heated to 150 °C. Br2 vapor was generated from a commercially available bromine water solution and injected into the hot quartz tube. After several minutes of reaction, CsPbI3 transformed to CsPbBr3 (from light green-yellow to bright yellow color) as follows: CsPbI3 + Br2 (g) → CsPbBr3 + I2 (g). Different from the CsPbI3 films that showed a thick but porous layer characterized by the existence of several random cracks within the film, the CsPbBr3 films presented a dense and compact thin layer with a smooth surface because of crystal lattice compression, and the flaws in the CsPbI3 precursor films were healed by the smaller Br− ions. 4 CsPbBr3 Solar Cells The efficiency of OIH-PSCs is now up to 25.2%, and the highest reported PCE of CsPbI3 PSCs is over 19%.[ 7, 34 ] However, the record PCE of CsPbBr3 is still substantially lower (10.91%) because of its large bandgap of 2.3 eV and energy mismatch in the HTL-free structure.[ 48 ] To alleviate these issues, some strategies are developed such as A-site[ 88, 89 ] or B-site doping,[ 62, 90 ] electron transport layer (ETL)/HTL modification,[ 37, 66 ] phase transition control[ 48, 60 ] to improve the perovskite quality, and reduce the energy alignment mismatch in order to achieve high performance. Therefore, there is still room to further improve the performance of CsPbBr3 solar cells. It is worth noting that CsPbBr3 films are much more stable than hybrid organic–inorganic perovskite films and pure CsPbI3 films in terms of thermal/moisture stability and phase stability. Recently, research efforts are focused on the HTL-free structure (fluorine-doped tin oxide (FTO)/ETL/CsPbBr3/carbon) in CsPbBr3 PSCs instead of the conventional configuration (FTO/ETL/CsPbBr3/HTL/Au) to realize high stability against moisture and thermally induced degradation. Furthermore, CsPbBr3 can also act as an interlayer to provide a better energy alignment and to passivate detrimental defects in the hybrid PSCs.[ 91-93 ] 4.1 Electron Transport Layer ETLs should show a high light transmittance and an appropriate conduction band energy level that is slightly lower than that of the inorganic CsPbBr3 perovskite film, which favors electron injection from the absorber layer to ETL. In parallel, an ETL also serves as the hole blocking layer to effectively impede hole transfer to the FTO electrode because of a large gap between the valence band maximum (VBM) of CsPbBr3 and the VBM of ETL. Usually, the choice of ETL materials (Figure 4 ) includes inorganic metal oxide materials (TiO2, ZnO, SnO2, WO3, and so on)[ 94-97 ] and organic carbon-base materials (phenyl-C61-butyric acid methyl ester (PCBM), C60).[ 98 ] Figure 4Open in figure viewerPowerPoint Schematic illustration of CsPbBr3 solar cells with different ETLs: a) TiO2 ETL. Reproduced with permission.[ 31 ] Copyright 2016, American Chemical Society. b) SnO2 ETL. Reproduced with permission.[ 66 ] Copyright 2018, Wiley-VCH. c) ZnO. Reproduced with permission.[ 44 ] Copyright 2018, Wiley-VCH. d) Without ETL. Reproduced with permission.[ 49 ] Copyright 2018, Wiley-VCH. The TiO2 film shows a high crystallinity after annealing at high temperatures (over 450 °C) and can be fabricated as mesoporous structure, which can serve as a scaffold in solution processing and favorable for the nucleation and crystallization of CsPbBr3 films (Figure 4a).[ 31 ] Li et al. used a compact (c-) TiO2 layer with a thickness of 20 nm as ETL by spin-coating precursor solution and treated the c-TiO2 using a 40 mm aqueous solution of TiCl4 at 70 °C.[ 46 ] CsPbBr3 films were deposited by the vapor deposition strategy. Even though TiO2 is widely applied in both inorganic and hybrid PSCs, the lower electron mobility (0.1–10 cm2 V−1 s−1) of TiO2 layer leads to inefficient charge carrier transport to the transparent electrode, which results in recombination in the PSCs.[ 95 ] In parallel, the UV photocatalytic effect of TiO2 films usually causing potential degradation of perovskite films is another issue for the stability of devices.[ 99 ] In addition to TiO2 films, SnO2 can also serve as an alternative ETL in CsPbBr3 solar cells because of high electron mobility (240 cm2 V−1 s−1), high light transmittance (E g = 3.8 eV), and low temperature fabrication (less than 150 °C).[ 66, 95 ] You et al. and Tang et al. used SnO2 nanoparticles (NPs) and SnO2 quantum dots (QDs) (Figure 4b), respectively, as the ETL in CsPbBr3 solar cells and achieved high eff