Engineering inorganic lead halide perovskite deposition toward solar cells with efficiency approaching 20%

Abstract

Inorganic perovskite materials have gained tremendous attention due to their superior chemical and thermal stability than organic‐inorganic hybrid perovskites. In the past years, substantial research efforts have been devoted to developing uniform and pin‐hole free inorganic perovskite films with high electronic quality. As a result, power conversion efficiency of inorganic perovskite solar cells (PSCs) has boosted to over 19%, which presents a promising potential for technology commercialization. Herein, we give a comprehensive review of the recent progress on state‐of‐the‐art inorganic cesium lead halide based PSCs, particularly on the perovskite deposition approaches. We show a clear roadmap to fabricate high electronic quality inorganic perovskite films by tuning the precursor crystallization kinetics, performing post‐deposition treatments, and passivating surface/interface defects. Inorganic perovskite films prepared by these approaches exhibit not only high crystallinity, favorable morphology, and low trap densities but also improved phase stability. The advanced deposition approaches lay the foundation for further improving the performance and long‐term operational stability of inorganic perovskite photovoltaics.

INTRODUCTION

In recent years, perovskite solar cells (PSCs) have obtained great progress. The certified power conversion efficiency (PCE) has reached 25.5%, which is comparable to commercial photovoltaic technologies such as crystalline silicon, cadmium telluride (CdTe), and copper indium gallium selenide (CIGS) solar cells.[] Perovskite refers to a class of materials with the formula of ABX₃, in which A is monovalent organic cations such as methylammonium (CH₃NH₃⁺, MA⁺), formamidinium (NH₂ = CHNH₂⁺, FA⁺) and inorganic cations such as Cs⁺, B is divalent metal cations such as Pb²⁺ and Sn²⁺, and X is halide anion such as I^–, Br^– and Cl^–.[]

The most-investigated perovskite materials remain a major limitation to survival under moisture, thermal, and UV irradiation stress due to the intrinsic volatility of organic cations. [] To mitigate the moisture-induced degradation pathway, many strategies have been reported, e.g., optimization of crystallization process and film morphology, substitution of the small organic cation with bulky or long-chain organic cations at A site, deposition of a hydrophobic charge transfer layer or a surface modification layer, and employment of encapsulation, etc.[] Significant stability enhancement has been realized in different research labs. On the other hand, thermal instability is still a huge challenge for organic perovskites due to the high volatility nature of organic species at elevated temperature (e.g., 85 °C for MAPbI₃).[]

Complete substitution of the organic cations with inorganic cation, i.e., cesium (Cs⁺) to form CsPbI₃ prevents the degradation pathways due to the much stronger Cs-I ionic bond in inorganic perovskite than the H-I bond in organic perovskite []. As a result, the inorganic perovskites show substantially enhanced stability against heat and illumination. In addition, partial substitution of I^– with Br^– to form the halide alloy allows stabilization of the inorganic perovskite in the photoactive phase at the device operating temperature and broadening its optical bandgap.[] This opens up the application of inorganic perovskite as absorbers of the top cells in tandem and triple-junction solar cells. With the great research efforts made in the community, inorganic PSC has reached over 19% efficiency and enhanced stability in a very short period (Figure ).[]

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Quality of inorganic perovskite films, such as the crystallinity, surface roughness, film thickness, defect level and density, etc., determine the carrier transport and recombination kinetics and therefore the device performance.[] Besides, phase stability of inorganic perovskite films determines the device long-term operational stability.[] These properties can be adjusted by the development of appropriate perovskite film deposition approaches. In this review, we aim to give a comprehensive overview of the recent advancement of deposition methods for inorganic perovskite films with favorable optoelectronic properties. First, we briefly introduce the fundaments of CsPbX₃. Then, we systematically review the representative deposition methods for four inorganic perovskite systems, i.e., CsPbI₃, CsPbI₂Br, CsPbI₂Br₂ and CsPbBr₃ with the following two focuses: i) preparation of thick inorganic perovskite films with low defect density and ii) stabilization of the inorganic perovskite in the photoactive phase. Specifically, the effect of tuning the film crystallization and growth kinetics, defect passivation, interface optimization, post-deposition treatment, and two-dimensional passivation are discussed in detail. Finally, we conclude the review with perspectives for the future development of inorganic PSCs toward an efficiency beyond 20%.

FUNDAMENTS OF INORGANIC PEROVSKITES

For the inorganic perovskite of CsPbX₃ (X refers to iodine or bromine), the three-dimensional network of PbX₆ octahedra is formed by Pb and X. Cs ions are occupied at the octahedral voids between halogens.[] The Goldschmidt tolerance factor is used to estimate the phase stability according to Equation () 1 $t=\frac{r_{\mathrm{A}}+r_{\mathrm{X}}}{\sqrt{2}\left(r_{\mathrm{B}}+r_{\mathrm{X}}\right)}$where r_A, r_B and r_X are the radii of A, B, and X ions, respectively.[] Empirically, the stable cubic phase could be obtained when the t is between 0.9 and 1. If the t value is smaller than 0.9, the PbX₆ titling will induce perovskite structure distortion. Meanwhile, there will be non-perovskite phases when the t value is smaller than 0.8 or larger than 1. Take CsPbI₃ as an example, four complex phases exist, which are cubic (α-, Pm3m), tetragonal (β-, P4/mbm), orthorhombic (γ-, Pbnm) and the non-perovskite (δ-, Pnma) phases.[, ] Due to the small Cs ionic radii, CsPbI₃ exhibits a small Goldschmidt tolerance of 0.81. As a result, CsPbI₃ is thermodynamically stable in the yellow δ-phase at room temperature.[] The δ-CsPbI₃ has a wide bandgap of 2.82 eV and low carrier mobility, which are unsuitable for photovoltaic application. Upon high-temperature annealing at 350 °C δ-CsPbI₃ transfers to cubic α-CsPbI₃ with a favorable bandgap of 1.73 eV. The α-CsPbI₃ converts to the tetragonal β phase at 260 °C, and subsequently the orthorhombic γ phase at 175 °C during the cooling process. [] The β-CsPbI₃ and γ-CsPbI₃ are both black phases but tend to convert to the δ phase when exposed to humidity at room temperature.[, ] Due to the strong relationship between perovskite crystal structure and device performance, stabilization of CsPbI₃ in the black α phase is essential to obtain high efficiency and stable inorganic PSCs.

By partially replacing I^– with Br^–, the Goldschmidt tolerance factor of CsPbX₃ slightly increases, indicating improved phase stability.[] The representative inorganic perovskite, i.e., CsPbI₂Br (bandgap 1.9 eV), indeed exhibits improved phase stability at room temperature, but still suffers from phase transition under high humidity exposure.[] Theoretical calculations demonstrated that CsPbI₂Br exhibits a stable alloying phase due to strong coulomb interactions, which is beneficial for suppressing phase segregation.[] Further increase Br^– concentration in the X cite results in CsPbIBr₂ and CsPbBr₃ with enlarged bandgaps of ∼2.05 and ∼2.3 eV, respectively. The wide bandgaps dramatically constrain the absorption edges of absorbers, leading to a low photovoltaic performance for single-junction solar cells. [] On the other hand, these perovskites are promising top cell candidates for tandem and triple-junction perovskite-based solar cells.

To address these challenges of inorganic perovskite, it is urgent to develop new perovskite deposition approaches to simultaneously increase solar cell efficiency and long-term operational stability. In the following section, we have summarized the deposition methods for a series of perovskite systems, i.e., CsPbI₃ (Section 3.2), CsPbI₂Br (Section 3.3), CsPbIBr₂ (Section 3.4) and CsPbBr₃ (3.5), respectively.

RECENT ADVANCES IN PEROVSKITE DEPOSITION APPROACHES

Methodologies for inorganic perovskite deposition

The preparation of high electronic quality perovskite thin films lays the foundation for the realization of high-performance PSCs. Similar to the hybrid organic-inorganic perovskite materials, all-inorganic perovskite films can be prepared by the solution processing techniques, including one-step and two-step spin-coating and blade coating methods (Figure ). The solution processing technique is a relatively facile approach to fabricate perovskite films with a low manufacturing cost. In general, the precursors are dissolved in a proper solvent to form a solution, which is spin- or blade-coated on a substrate and followed by a thermal annealing process. Using the perovskite quantum dots (QDs) precursor is an alternative method. The dispersions of CsPbX₃ QDs are applied as “inks” to fabricate perovskite films by spin-casting at room temperature.[] Compared with the solution processing method, QDs deposition is almost unaffected by external conditions, and allows multiple deposition steps to adjust film thickness. In addition, vapor deposition techniques including co-evaporation and sequential deposition, are effective strategies to prepare high-quality all-inorganic perovskite films. The vapor deposition techniques have demonstrated special advantages in obtaining uniform and smooth perovskite films over large area with great repeatability, and are especially suitable for the perovskite materials that show low solubility.

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CsPbI₃

CsPbI₃ is thermodynamically stable in a non-perovskite (δ) phase at room temperature and the black cubic (α) phase at a temperature over 350 °C.[] In 2014, Choi et al. reported the first CsPbI₃ PSCs by substitution of MA⁺ from Cs_xMA_1−xPbI₃, resulting in a low PCE of 0.09% (see Table ).[] Snaith and co-workers introduced a small amount of hydroiodic acid (HI) into the CsPbI₃ precursor solution to prepare black phase films. These films exhibited a stable α phase with small grains at room temperature, yielding an improved PCE of 2.9%.[] Later on, Swarnkar et al. synthesized CsPbI₃ nanocrystals (NC) to stabilize the α phase, which demonstrated a high PCE of 1.77% with a large open-circuit voltage (V_oc) of 1.23 V.[] In 2018, Hu and co-workers presented a new method for preparation of orthorhombic γ-CsPbI₃, which is thermodynamically stable at room temperature and shows ideal optoelectronic properties (Figure ).[] The stabilization of γ-CsPbI₃ was realized by adding a small amount of H₂O to manipulate the size-dependent phase formation, resulting in lower free energy. As a result, the inorganic PSCs achieved a high PCE of 11.3% and exhibited high stability after storage in ambient condition for several months. Zhang et al. developed a low-temperature CsPbI₃ fabrication method by using PbI₂·xHI (x > 1.3) instead of PbI₂ as a precursor (Figure ).[] Besides, a small amount of two-dimensional EDAPbI₄ (EDA refers to ethylenediamine) was added to the perovskite precursor to mitigate the black phase transition to γ-CsPbI₃. By employing this bication EDA stabilization method, a high efficiency of 11.8% was reached.

TABLE 1 Deposition methods for CsPbI₃ and the photovoltaic parameters of PSCs

Method	Architecture	PCE)%(	Voc (V)	Jsc(mA cm–2)	FF(%)	Ref.
One-step (Cs substitution)	ITO/PEDOT:PSS/CsPbI3/PCBM/Al	0.09	0.79	0.26	45.0	[]
One-step (additive: HI)	FTO/TiO2/CsPbI3/Spiro-OMeTAD/Au	2.9	0.85	12.0	28.4	[]
One-step (nanocrystals)	FTO/TiO2/CsPbI3/Spiro-OMeTAD/MoOx/Al	10.77	1.23	13.47	65.0	[]
One-step (passivation: PVP)	FTO/c-TiO2/mp-TiO2/CsPbI3/Spiro-OMeTAD/Au	10.74	1.11	14.88	65.0	[]
One-step (additive: HI+H2O)	FTO/TiO2/CsPbI3/P3HT/Au	11.30	1.04	16.53	65.7	[]
One-step (additive: EDAPbI4)	FTO/TiO2/CsPbI3-0.025EDAPbI4/Spiro-OMeTAD/Ag	11.86	1.15	14.53	71.0	[]
One-step (solvent control)	ITO/SnO2/CsPbI3/Spiro-OMeTAD/Au	15.71	1.08	18.41	79.3	[]
One-step (additive: HI+PEAI)	FTO/TiO2/CsPbI3/PTAA/Au	15.07	1.06	18.95	74.9	[]
One-step (post treatment: PEAI)	FTO/TiO2/CsPbI3/PEAI/Spiro-OMeTAD/Ag	14.3	1.11	18.50	69.6	[]
One-step (post treatment: PTABr)	FTO/TiO2/CsPbI3/PTABr/Spiro-OMeTAD/Ag	17.06	1.10	19.15	80.6	[]
One-step (post treatment: CHI)	FTO/TiO2/CsPbI3/CHI/Spiro-OMeTAD/Ag	18.4	1.11	20.23	82.0	[]
One-step (post treatment: PTACl)	FTO/TiO2/CsPbI3/PTACl/Spiro-OMeTAD/Ag	19.03	1.14	20.23	82.7	[]
One-step (Heterojunction:0D/3D)	ITO/SnO2/3D CsPbI3 (0D Cs4PbI6)/Spiro-OMeTAD/Au	16.39	1.09	18.84	80.0	[]
One-step (Antisolvent: MAI+PCBM)	ITO/SnO2/CsPbI3/Spiro-OMeTAD/Au	16.04	1.06	20.1	75.3	[]
One-step (Print: Zn(C6F5)2)	FTO/TiO2/CsPbI3/Spiro-OMeTAD/Au	19.00	1.12	20.67	82.0	[]

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In 2018, Zhao and co-workers developed an organic salt post-treatment on CsPbI₃ films and improved the CsPbI₃-based PSCs efficiency (Figure ).[] The large phenylethylammonium (PEA) cations were physically absorbed on the CsPbI₃ films without forming a 2D perovskite capping layer by ion exchange (Figure ). The organic cations exhibited effective defect passivation, which was responsible for improved phase stability and moisture resistance. Accordingly, PSC with PEAI post-treatment obtained a champion PCE of 13.5%. Motivated by this work, a series of organic salts such as phenyltrimethylammonium bromide (PTABr), choline iodine (CHI) and phenyltrimethylammonium chloride (PTACl) were successfully employed to continuously improve the recording-PCE of CsPbI₃-based PSCs up to 19.03%.[]

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Based on the fact that the residual DMSO in the as-coated film increases the mass transport and diffusion, Wang et al. developed a solvent-controlled growth (SCG) method to prepare high crystallization and pinhole-free α-CsPbI₃ films (Figure ).[] The CsPbI₃-based PSCs showed a PCE of 15.7% and over 500 h stability under continuous light soaking. To improve the phase stability at room temperature, Bai et al. constructed a heterojunction between 0D Cs₄PbI₆ and 3D CsPbI₃ (Figure ).[] The 0D Cs₄PbI₆, located at the grain boundaries of CsPbI₃, stabilized the black phase and passivated the surface defects, leading to an impressive PCE of 16.39%.

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The long-term stable CsPbI₃ was synthesized by using a polymer-induced surface passivation engineering (Figure ).[] A thin poly-vinylpyrrolidone (PVP) layer enhanced electron cloud density and lower surface energy of CsPbI₃, therefore stabilized it in the α-phase. An ultra-long carrier diffusion length of over 1.5 μm was obtained which contributed to improved charge collection efficiency and PCE. Moreover, a mediator-antisolvent strategy (MAS) was demonstrated combining phenyl-C61-butyric acid methyl ester (PCBM) in chlorobenzene (CBZ) antisolvent and methylammonium iodide (MAI) mediator. This strategy allowed synthesizing high crystalline and pinhole-free CsPbI₃ films (Figure ).[] PCBM in CBZ antisolvent greatly reduced the CsPbI₃ grain sizes. Meanwhile, MAI in perovskite precursor served as a crystallization mediator and subsequently induced oriented crystal growth. Taking advantage of the film morphology and electronic quality, the devices exhibited a significant high PCE of 16.04% and maintained 95% of their initial efficiency for over 1000 h storage in N₂ condition. In view of processing upscalability, all-printable CsPbI₃-based PCSs were reported by Liu and co-workers. They utilized multifunctional molecular additive, i.e., Zn(C₆F₅)₂, to bridge SnO₂ and perovskite, which not only improved the perovskite crystallinity but also allowed a favorable energy level alignment (Figure ).[] As a result, a PCE of about 19% was obtained with increased open-circuit voltage.

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CsPbI₂Br

CsPbI₂Br, with a bandgap of approximately 1.93 eV, is a popular composition for PSCs considering the efficiency and stability. The CsPbI₂Br film is more stable in the α phase compared with the pure CsPbI₃ at room temperature, although the transition to the δ phase still occurs in the high humidity environment.[] Such a bandgap makes CsPbI₂Br a promising top cell candidate for tandem and triple-junction solar cells.

Although solubility of bromide salt is low in the common solvents, e.g., N,N-dimethylformamide (DMF), solution processing was used because of the relatively low bromide content in CsPbI₂Br. Snaith et al. first obtained high-quality CsPbI₂Br films via a one-step spin-coating method. The PSCs achieved a PCE of 9.8%.[] However, CsPbI₂Br shows limited solubility in DMF (0.4 M). Therefore thickness of this CsPbI₂Br film is only 150 nm. McGehee et al. employed DMSO, a solvent with stronger polarity, to fabricate the invert-structured CsPbI₂Br PSCs, and achieved a champion PCE of 6.69%.[] Soon after, Park et al. demonstrated that the formation of black polymorph is critical to both solar cell efficiency and phase stability.[] By carefully tuning the annealing condition, the PSCs presented an efficiency of 10.7% and an open-circuit voltage of 1.23 V. Later on, Chen and co-workers systematically studied the effects of solvent composition and precursor concentration on film electronic quality and thickness (Figure ).[] They showed that a suitable solvent composition with 40% DMSO is favorable for the preparation of a pinhole-free and surface-smooth perovskite film with improved crystallinity. Chen et al. modulated the film growth kinetics via optimizing solvent composition (DMF: DMSO volume ratio) to enhance mass transport during film growth and achieved a stabilized PCE of 14.31% (see Table ).[] Zheng and co-workers developed a hot-casting method to deposit CsPbI₂Br films (Figure ). They casted the perovskite precursor solution on a hot substrate (maintained at 55 °C) and annealed the sample at a low-temperature to obtain CsPbI₂Br films with large grains. These perovskite films outperformed the conventional room temperature (RT)-casted and high-temperature (340 °C) annealed perovskite films. The PSCs reached an outstanding PCE of 13.8%.[]

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TABLE 2 Deposition methods for CsPbI₂Br and the photovoltaic parameters of PSCs

Methods	Architecture	PCE(%)	Voc(V)	Jsc(mA cm–2)	FF(%)	Ref.
Vacuum deposited (vacuum-controlled growth)	ITO/NiOx/CsPbI2Br/PCBM/Ag	12.32	1.02	15.79	76.68	[]
One-step (methylamine gas healing)	FTO/c-TiO2/m-TiO2/CsPbI2Br/Spiro-OMeTAD/Ag	13.14	1.25	14.6	0.72	[]
One-step (nickel iodide incorporation)	ITO/P3CT/CsPb0.98Ni0.02I2Br/PCBM/C60BCP/Ag	13.88	1.141	16.02	75.96	[]
One-step (low-temperature solution process)	ITO/SnO2/CsPbI2Br/PTAA/Au	14.30	1.162	15.565	79.06	[]
One-step (2D/3D heterostructure)	FTO/TiO2/CsPbI2Br/Spiro-OMeTAD /Au	14.5	1.079	16.80	80.10	[]
One-step (hot-casting)	ITO/SnO2/CsPbI2Br/PTAA/MoO3/Al	14.54	1.19	15.66	74.1	[]
One-step (Lewis base adducts)	FTO/TiO2/CsPbBrI2/Spiro-OMeTAD /Au	14.78	1.22	15.33	78.70	[]
One-step (crystallization engineering)	FTO/TiO2/CsPbBrI2/CsPbBrI2 QDs/PTAA/Au	14.81	1.221	15.10	80.3	[]
One-step (oxygen passivation)	ITO/SnO2/CsPbI2Br/Spiro-OMeTAD /Au	15.17	1.18	16.31	78.8	[]
One-step (Pre-annealing)	ITO/SnO2/CsPbI2Br/P3HT/Au	15.50	1.261	15.62	78.7	[]
One-step (hot Air assisted)	TCO/MgZnO/CsPbI2Br/P3HT/Au	15.52	1.271	15.68	77.87	[]
One-step (TMTA incorporation)	ITO/SnO2/CsPbI2Br/Spiro-OMeTAD /Au	15.58	1.274	16.30	75.01	[]
One-step (methyl acetate antisolvent)	ITO/SnO2/TiO2/CsPbI2Br/Spiro-OMeTAD /MoO3/Ag	15.86	1.23	15.67	82.29	[]
One-step (IPA antisolvent)	ITO/TiO2/CsPbI2Br/Spiro-OMeTAD /Au	16.07	1.23	16.79	77.81	[]
One-step (HCOOCs, HPbI3, and HPbBr3)	FTO/TiO2/CsPbI2Br/PTAA/Au	16.14	1.32	15.34	79.54	[]
One-step (water assisted)	FTO/c-TiO2/CsPbI2Br/P3HT/Au	16.47	1.33	15.98	78	[]
One-step (hot air assisted)	ITO/Bl-TiO2/mp-TiO2/CsPbI2Br/P3HT/Au	17.16	1.32	16.25	80.03	[]
Two-step (antisolvent)	FTO/TiO2/CsPbBrI2/C	9.38	1.14	13.36	61.6	[]
Two-step (DMSO and methanol mixed solvent)	FTO/TiO2/CsPb(IxBr1−x)3/Spiro-OMeTAD /Au	13.27	1.32	13.98	71.91	[]
Blade coating (control the fluid dynamics)	FTO/c-TiO2/CsPbI2Br/Spiro-OMeTAD /Au	14.69	1.15	16.82	75.73	[]
Vacuum deposited (control the stoichiometric ratios of the precursors)	ITO/Ca/C60/CsPbI2Br/TAPC/TAPC:MoO3/Ag	11.8	1.15	15.2	67	[]

The solvent engineering method is commonly used to prepare high electronic quality organic-inorganic hybrid perovskite films. Upon dripping the antisolvent on the precursor film, solubility of the solute in solution rapidly decreases, accelerating the nucleation and later on the crystal growth. This method can be applied to prepare CsPbI₂Br films. Dong et al. used a low boiling point green EA solvent to fabricate inorganic perovskite film.[] The EA-treated film presented compact morphology and a large grain size. Soon after, the authors developed an antisolvent assisted multi-step deposition strategy for fabrication of CsPbI₂Br film with a pure phase, high crystallinity and full coverage. Different antisolvents were introduced in the PbI₂ precursor via processing to construct porous PbI₂(DMSO) films.[] The porous film was coated with multiple layers of CsBr to ensure sufficient reaction between the PbI₂ and the CsBr. PSCs using this film achieved a champion PCE of 10.21% (Figure ). Li et al. reported a multi-step annealing approach to precisely control the crystal growth kinetics, involving a gradient thermal annealing (GTA) process and IPA antisolvent post-treatment (Figure ).[] The CsPbI₂Br film showed an averaged grain size of one-micron and reduced defect density. The multi-step annealing approach led to a high PCE of 16.07%.

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Inspired by the solvent engineering method, You et al. reported a SCG method to retard the solvent evaporation rate. The residual DMSO in the wet CsPbI₂Br film allows decreasing the nucleation and growth rate. An impressive PCE of 14.21% was obtained from the corresponding PSCs.[] Liu et al. carefully controlled the temperature of the perovskite precursor solution. They fabricated thick and pinhole-free CsPbI₂Br films with large crystalline grain size and high homogeneity. Devices with the perovskite films achieved a maximum PCE up to 14.81%.[] Recently, Hu and co-workers developed a pre-annealing strategy for perovskite film formation, which allows to fine-tune the perovskite film nucleation and crystallization kinetics (Figure ).[]

Fabrication of CsPbI₂Br films using highly crystalline CsPbI₂Br NCs solution is another effective approach. The surfactants capping the nanocrystals (NCs) surfaces allow the NCs dispersing homogeneously in a nonpolar solvent. This allows fabricating uniform perovskite films independent of substrates. Yang et al. prepared the CsPbI₂Br layers with the NC solution (Figure ). [] A solvent soaking process in anhydrous isopropanol (IPA) was applied to the NC film to not only remove the ligands but also allow the NCs to reassemble and form a continuous film. PSCs with absorber prepared by this method achieved a PCE of 12.02%, a V_oc of up to 1.32 V, and a V_oc deficit of 0.5 eV. It worth noting that the whole device fabrication process was conducted under ambient air conditions.

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Moreover, vacuum deposition has also been demonstrated to be an effective method for fabrication of perovskite films. The method offers several advantages such as high film uniformity, good batch to batch reproducibility, and is suitable for integration into industrial facilities.[] Lin and co-workers first applied a co-deposition technique to fabricate CsPbI₂Br films (Figure ).[] The CsPbI₂Br films presented a substantially small crystalline domain size of 100 nm. After annealing, the grain size becomes as large as 3 μm. As a result, the vacuum-deposited CsPbI₂Br PSCs achieved a remarkable PCE of 11.8%, a high V_oc of 1.13 V and negligible J-V hysteresis.

Zhao et al. developed a blade coating technique to fabricate the large area CsPbI₂Br film (Figure ). [] The results indicate that both Benard-Marangoni instability and the moisture attack can be eliminated at a moderate processing temperature, leading to the formation of a high-quality film. The blade-coated device efficiencies reach 14.7% on an area of 0.03 cm² cell and 12.5% on an area of 1.0 cm², respectively.

CsPbIBr₂

CsPbIBr₂, with a bandgap of 2.05 eV, appears in red color in the cubic phase and nearly transparent in the orthorhombic phase. Due to high bromide content, the CsPbIBr₂ devices exhibit much higher stability than the CsPbI₃ devices but suffer from a lower PCE. The one-step and two-step spin-coating, and dual source thermal evaporation have been reported for CsPbIBr₂ film preparation. To obtain the desired perovskite phase, the reported annealing temperatures varied from 135 to 320 °C.

In 2016, Ma et al. fabricated the CsPbIBr₂ films by thermal evaporation of PbBr₂ and CsI precursors (Figure ).[] They kept the substrate at 75 °C during the film deposition and performed a post-annealing at 250 °C to achieve the films with a grain size reaching 500–1000 nm. PSCs based on the optimal film achieved a remarkable PCE of 4.7% without an HTL (see Table ). Soon after, they also developed a spray assisted solution strategy to fabricate high-quality CsPbIBr₂ film, which overcame the low solubility issue of the bromide ion in the precursor solution (Figure ).[] The best-performing device fabricated at the optimized conditions achieved a stabilized efficiency of 6.3% and negligible hysteresis. Song et al. employed the antisolvent and organic ion surface passivation strategies to precisely control the growth of CsPbIBr₂ crystals (Figure ).[] A pure phase CsPbIBr₂ film of full coverage and high crystallinity with preferable (100) orientation was successfully obtained by introducing diethyl ether as the antisolvent, followed by guanidinium surface passivation. The optimal CsPbIBr₂ film showed large grains with an average size of 950 nm, few grain boundaries, and high hydrophobicity. The device based on this film exhibited a PCE of 9.17%.

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TABLE 3 Deposition methods for CsPbIBr₂ and the photovoltaic parameters of PSCs

Methods	Architecture	PCE(%)	Voc(V)	Jsc(mA cm–2)	FF(%)	Ref.
One-step (gas-assisted)	FTO/c-TiO2/CsPbIBr2/Spiro-OMeTAD/Au	8.02	1.227	9.69	67	[]
One-step (light processing)	FTO/c-TiO2/CsPbIBr2/C	8.6	1.283	11.17	60	[]
One-step (intermolecular exchange)	FTO/c-TiO2/CsPbIBr2/C	9.16	1.245	10.66	69	[]
One-step (Pre-heating treatment)	ITO/SnO2/CsPbIBr2/Spiro-OMeTAD /Au	9.86	1.267	10.69	71	[]
One-step (intermolecular exchange)	FTO/TiO2/CsBr/CsPbIBr2/C	10.71	1.261	11.80	72	[]
One-step (Sn doping and antisolvent)	FTO/SnO2/C60/CsPb1−xSnxIBr2/Spiro-OMeTAD/Au	11.53	1.21	12.57	75.8	[]
Two-step (Precursor solution aging time)	FTO/c-TiO2/CsPbIBr2/C	6.55	1.142	9.11	63	[]
Two-step (antisolvent)	FTO/c-TiO2/CsPbIBr2/Spiro-OMeTAD/Au	9.17	1.2	10.24	74.6	[]
Two-step (Sn doping)	FTO/m-TiO2/CsPb0.9Sn0.1IBr2/C	11.33	1.26	14.3	63	[]
Vacuum deposited (dual source thermal evaporation)	FTO/c-TiO2/CsPbIBr2/Au	4.7	0.96	8.7	56	[]

In addition, one-step spin-coating method is also recognized as an effective and convenient method for CsPbIBr₂ thin film fabrication. Zhang et al. demonstrated a light processing technology that enabled pure-phase CsPbIBr₂ films with large grains, high crystallinity with [] grains orientation, and favorable electronic structure> (Figure ).[] The resulting carbon-based, all-inorganic planar cells showed a PCE of 8.60% with the V_oc of 1.283 V. Que et al. reported a pre-heating assisted one-step spin-coating method (Figure ).[] During spin-coating, the high-temperature substrate accelerates volatilization of the solvent molecule. As a result, CsPbIBr₂ films show complete coverage and higher crystallinity. PSCs exhibited a superior power conversion efficiency (PCE) of 9.86% with a stabilized output of 8.78% and a high V_oc of 1.267 V. Zhang et al. developed a facile intermolecular exchange strategy, wherein a CsI solution is spin-coated on CsPbIBr₂ precursor film (Figure ).[] Microscopy and spectroscopy characterization indicated that the CsPbIBr₂ films were composed of high-crystallinity, (100)-oriented, micrometer-sized crystalline grains. As a result, the cost-effective, carbon-based all-inorganic planar cells fabricated by this route delivered an optimized PCE of 9.16% and a stabilized PCE of 8.46% in ambient condition.

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CsPbBr₃

CsPbBr₃ shows a direct bandgap of 2.25-2.37 eV, which varies with different fabrication approaches.[] The absorption edge of the CsPbBr₃ film is shorter than 540 nm, and the characteristic absorption peak is located at 520 nm. Tuning the ratio between CsBr and PbBr₂ further increases the bandgap from 2.3 to 4.0 eV because the phase transition occurs from the cubic perovskite structure to the derivative phases (Cs₄PbBr₆ and CsPb₂Br₅). Notably, this compound is among the most stable lead halide perovskite materials under ambient condition. The solution-processed method was usually employed to prepare CsPbBr₃ films due to low cost. Meanwhile, vapor-based deposition approaches, including co-evaporation and sequential deposition, were also investigated. These approaches allow for the fabrication of thick CsPbBr₃ films that are technically challenging for solution-processed approaches because of limited precursor solubility.

Usually, DMF, DMSO, γ-butyrolactone (GBL), and a combination of mixed solvents are used to dissolve perovskite precursor materials. However, due to the poor solubility of CsBr, it is difficult to prepare CsPbBr₃ precursor with a molar ratio of 1:1. For example, You et al. demonstrated that the maximum concentration of CsPbBr₃ precursor solution was only 0.4 M in the mixed DMF and DMSO solvent. It was difficult to form a thick and full-coverage CsPbBr₃ film by the one-step spin-coating method.[] To overcome the insolubility issue of CsBr, Zhong et al. used ionic liquid methylammonium acetate (MAAc) and cesium acetate (CsAc) to increase the concentration of the CsPbBr₃ precursor (i.e., 1.0 M) and tailor the crystallization kinetics. CsPbBr₃ films with large sized crystalline grains, high uniformity and coverage were formed (Figure ) (see Table ). [] The champion CsPbBr₃ PSCs achieved an efficiency of 7.37%. Meanwhile, the two-step solution-coating method was developed to prepare CsPbBr₃ films. For example, Tang and coworkers spin-coated the PbBr₂ and CsBr solution sequentially (Figure ). [] The CsBr solution was spin-coated six times to realize an optimized material composition. The all-inorganic perovskite solar cell reached a PCE of 9.72 %.

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TABLE 4 Deposition methods for CsPbBr₃ and the photovoltaic parameters of PSCs

Methods	Configuration	PCE (%)	Voc(V)	Jsc(mA cm–2)	FF(%)	Ref.
Two-step (face-down liquid-space-restricted deposition)	FTO/c-TiO2/CsPbBr3/C	5.86	1.34	6.46	68.04	[]
Two-step (Sm Doping)	FTO/TiO2/CsPbBr3/Cu(Cr,M)O2/C	10.79	1.615	7.81	85.5	[]
One-step (cesium acetate and ionic liquid methylammonium acetate)	FTO/c-TiO2/m-TiO2/CsPbBr3/C	7.37	1.22	7.4	84.1	[]
Vacuum deposited (dual-source vacuum co-evaporation)	FTO/ZnO/CsPbBr3/Spiro-OMeTAD/Au	7.78	1.44	7.01	77.11	[]
Vacuum deposited (sequential-vapor deposition)	FTO/TiO2/CsPbBr3/Spiro-OMeTAD/Ag	8.34	1.296	8.48	75.9	[]
Vacuum deposited (sequential-vapor)	FTO/TiO2/CsPbBr3/MXene/C	9.01	1.444	8.54	73.08	[]
Vacuum deposited (formation of the perovskite derivative phases)	FTO/c-TiO2/CsPbBr3/Spiro-OMeTAD/Ag	10.91	1.498	9.78	74.47	[]

Vapor deposition method was also employed to fabricate CsPbBr₃ film. Qi et al. controlled the ratio of CsBr and PbBr₂ to fabricate the perovskite derivative phases (CsPb₂Br₅/Cs₄PbBr₆) via a vapor growth method (Figure ). [] Upon post-annealing, the perovskite derivative phases, acting as nucleation sites, transformed into the pure CsPbBr₃ phase via a crystal rearrangement process. The method effectively retarded the perovskite recrystallization rate and delivered CsPbBr₃ films of high electronic quality. Using this method, a PCE of 10.91% was achieved for n-i-p structured PSCs. Besides, Yang et al. reported a vapor-assisted solution technique to prepare uniform and pure CsPbBr₃ films (Figure ). [] By tuning the thickness of CsBr, a series of phase conversion from CsPb₂Br₅ to CsPbBr₃ and to Cs₄PbBr₆ could be precisely controlled. The optimized CsPbBr₃ PSC delivered a PCE of 10.45%.

CONCLUSIONS AND PERSPECTIVES

Within a decade of intensive research and development, perovskite solar cells have witnessed substantial progress. The organic-inorganic hybrid perovskite, being the most successful perovskite system, has reached a certified efficiency of 25.5%, which surpasses all the other thin-film solar cell technologies i.e., CIGS, CdTe, amorphous Si, and is on the way to catch up with crystalline Si and GaAs. The outstanding PCE has attracted considerable attention not only in the research labs but also in the photovoltaics companies for various applications. Despite the impressive performance, organic–inorganic hybrid PSCs still present significant challenges in terms of stability. The relatively weak bonding between organic cation and halide ion and the volatile nature of the organic cations makes organic-inorganic hybrid perovskite intrinsically unstable when exposed to oxygen, moisture, illumination and temperature. By substitution of organic cations with Cs⁺, the inorganic perovskite, i.e., CsPbX₃ offers the promise for thermodynamical stability, therefore mitigating many external impact-induced degradation pathways.

In the past years, much research effort has been devoted to the development of deposition approaches for achieving high electronic quality CsPbX₃ perovskite films. Efficiency of CsPbX₃ PSCs has boosted to over 19% (Figure ) with V_oc deficit (E_g-eV_oc) approaching 0.5 eV (Figure ). Nevertheless, many problems still exist, deteriorating the device efficiency, stability and upscalability. The following investigation directions have been proposed to further unleash the efficiency potential of CsPbX₃ beyond 20%, improve the long-term operational stability and large-area processability (Figure ).

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I) Development of advanced inorganic perovskite deposition methods. So far, precisely control the composition and structure of inorganic perovskite films at the microscopic scale is still a challenge. Investigation of chemically coordinated additives, such as low-dimensional perovskite, ammonium salts and organic passivation molecule, would be beneficial for adjusting crystal nucleus formation and grain growth kinetics, functioning grain boundaries and protecting layers against external impact. In addition, a systematical study of the solvent composition will further provide insights into solution-processed inorganic perovskite formation kinetics and allow fine-tuning the solvent drying and crystal growth.

II) Design rationale interfaces between perovskite and charge transfer layers. Energy mismatch between inorganic perovskite and charge transfer layer results in an energy barrier for carriers extracting and increases interface recombination. A recent report has shown that an optimized interface with favorable energy alignment between CsPbI₂Br and P3HT can effectively decrease interface recombination and increase the device open-circuit voltage. [] Different inorganic perovskite materials, i.e., CsPbI₃, CsPbI₂Br, CsPbIBr₂, and CsPbBr₃, show different band structures. Therefore, it is of significant importance to develop a series of new electron or hole transfer layers that are suitable for different inorganic perovskite materials from an energy level alignment point of view.

III) Strategies to improve device stability. So far, stabilization of inorganic perovskite in the photoactive phase over the device lifetime is still a challenge.[] To unleash the stability potential, it is urgent to gain fundamental understandings of the material chemistry and device physics. From this viewpoint, the partial incorporation or doping of suitable metal ions might improve the intrinsic stability, which in turn allows all-inorganic PSCs to withstand severe environmental conditions. Furthermore, the preparation of inorganic perovskite films with large grains and dense morphology, the construction of continuous organic or inorganic transport layer with high moisture resistance and optimization of encapsulation methods are also critical strategies to improve the stability of all-inorganic PSCs.

IV) Strategies to improve processing upscalability. At present, there is still a considerable efficiency gap between the inorganic PSCs and large-area solar modules. Along with the continuous improvement of device efficiency and stability, more and more researches should be done in these fields. The knowledge obtained on the vacuum- and solution-processed methods provides significant insight for the development of large-scale preparation of inorganic perovskite films. Moreover, to further achieve ultra-uniform fabrication of perovskite thin films on large scale, it is necessary to understand in depth the mechanism of the solution- to solid-phase nucleation and crystal growth in the future.

ACKNOWLEDGEMENTS

F.-Z.Q. and M.-H.L. contributed equally to this work. This work was supported by funding from the Chinese Academy of Sciences, Beijing Natural Science Foundation (2202030) and Energy Materials and Optoelectronics Unit of Songshan Lake Materials Laboratory.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interest.

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