Challenges and modification strategies of air-processed all-inorganic CsPbX₃ perovskite films for efficient photovoltaics

Intrinsic characteristics

The ideal three-dimensional (3D) CsPbX₃ crystal structure is a cubic phase with an octahedron structure^[5]. As shown in Figure 2A, the cation Pb²⁺ is located at the center of a regular octahedron of anion X^-, and the inorganic cation Cs⁺ is surrounded by eight lead halide [PbI₆]^4- octahedra. Under this circumstance, the crystal structure of CsPbX₃ presents a highly symmetrical feature. However, owing to the small radius of Cs cation, the Goldschmidt tolerance factor t of CsPbX₃ is lower than the organic-inorganic counterparts as t can be determined by

Figure 2. (A) Schemes of the 3D cubic CsPbX₃ perovskite structure. (B) Crystal structure of CsPbI₃ perovskite with different phases and their relative phase transitions. Copyright from Ref^[13]. (C) Morphologies of CsPbI₃ films prepared under different RH conditions before and after annealing. Copyright from Ref^[20].

where r_A, r_B, and r_X are the ionic radii for cation A, cation B, and anion X, respectively^[12]. As a result, the lead iodide octahedra is prone to distort and the corner-sharing [PbI₆]^4- octahedra tends to rotate, lowering its symmetry. For example, three types of photoactive black crystal polymorphs are reported for CsPbI₃ along decreasing the temperature: the cubic (α-, Pm3m), tetragonal (β-, P4/mbm), and orthogonal black phases (γ-, Pbnm), following a tendency of decreased symmetry [Figure 2B]^[13]. Moreover, the black phases prefer to further distort and degrade into the photo-inactive yellow phase (δ-, Pnma), owing to the lowest formation energy. This explains the phase instability of CsPbI₃. The phase transition greatly influences the optoelectronic properties, including the density of states, electronic band structure, exciton binding energy, etc. Substituting the I ion with smaller Br or Cl ions would increase the t value, thus enhancing the structure stability of all-inorganic perovskites. However, other problems, such as widened bandgap for reduced photo absorption, difficulty dissolving in commonly used solvents, etc., may appear. Therefore, the reported all-inorganic PSCs with high performance mostly rely on CsPbI₃ or CsPbI₃ with a small amount of Br substitution.

Apart from the phase instability, ion migration would destroy the perovskite lattice and degrade the optoelectronic properties. As ionic materials with low formation energies and migration activation energies, the triggered tremendous vacancies provide pathways for ion migration to the adjacent contact layers, and this process more easily occurs under heat and light. It was reported that vacancy-assisted ion migration is one of the most common processes owing to the low defect formation energy in CsPbI₃^[14]. Such induced iodine vacancy defects may not aggravate the non-radiative recombination as shallow-level defects are mainly formed in this case. However, it will speed up the decomposition rate of the perovskite structure. Apart from the vacancy-assisted ion migration, the grain boundaries formed during fabrication may also facilitate the ion migration of the perovskite films compared to the interior of the grains^[15,16]. Therefore, decreasing the number of grain boundaries is also considered an effective way to mitigate the ion migration. Although a “self-healing” process was reported to partially suppress the ion migration^[17], the device’s photovoltaic performances, especially the hysteresis and long-term stability, are still far from satisfactory.

External atmosphere

Although the stability of CsPbX₃ perovskite films depends on the intrinsic characteristics, the external atmosphere, such as humid air, can accelerate the degeneration process. To develop complete air processing strategies for all-inorganic PSCs, the role of the environmental atmosphere for manufacturing CsPbX₃ perovskite films should be highlighted. In this section, the instability caused by environmental factors, including water and oxygen, is discussed.

Composition/precursor engineering

Increasing the tolerance factor of CsPbI₃ by doping with other ions, e.g., a larger A-site cation, a smaller B-site cation, or a smaller X-site cation, has been reported as an effective method to enhance the stability. For example, Sn and Ge, the same group elements with Pb, have been incorporated into the perovskite lattice to improve the phase stability of all-inorganic perovskites fabricated in ambient air^[28,30]. Besides, lanthanide elements, such as Eu³⁺, La³⁺, Yb²⁺, Ho³⁺, etc., are also introduced to regulate the crystal structure and help improve the phase stability of CsPbX₃ perovskite, therefore enhancing the PCE of devices^[33-36]. Jena et al. prepared α-CsPbI₃ films at a lower annealing temperature (85 °C) by incorporating Eu³⁺ into perovskite^[29]. After optimization, the CsPbI₃ perovskite films could maintain α-phase stability for more than 30 days under ambient air. Mali et al. employed a dual bulk and surface passivation strategy with CsPb_1-xTb_xI₂Br QDs to stabilize the photoactive γ-phase CsPb_1-xTb_xI₂Br absorber in ambient air [Figure 3A], thereby promoting the PCE to 17.51% with negligible hysteresis and significantly enhanced operational stability, and further to 19.01% for γ-CsPb_0.95Tb_0.05I_2.5Br_0.5-based devices^[63]. Additionally, Ba²⁺, In³⁺, and Cd²⁺ cations are also used to partially substitute Pb²⁺ for stabilizing black-phase CsPbI₂Br in ambient conditions^[22,31]. For example, In cation is introduced into the CsPbI₂Br crystal for fabricating pinhole-free perovskite films with improved stability under ambient conditions, thus obtaining a record efficiency for the devices with dopant-free hole transport layer at that time and > 95% retaining of the PCE after 1,600 h of operation^[22]. Moreover, it was reported that smooth and compact Cd-doped CsPbI_2.5Br_0.5 perovskite thin films can be obtained on a spray-deposited electron transport layer under ambient air^[22]. The optimized devices show a PCE of up to 19.75% and a stable performance in the air for more than 300 h.

Figure 3. (A) Schematic illustration of nucleation and crystal growth procedures of CsPb_1-xTb_xI₂Br perovskite films and passivation by γ-CsPb_1-xTb_xI₂Br QDs. Copyright from Ref^[63]. (B) Schematics of the crystallization processes of the control and optimized perovskite with precursors of HCOOCs and HPbX₃. Copyright from Ref^[38]. (C) Schematically illustrated formation process of CsPbI₂Br with Cs-acid and DMAPbX₃ as precursor Copyright from Ref^[39].

For the A-site incorporation, it is difficult to seek an inorganic cation for substituting or partially substituting Cs⁺ due to its largest radius among the monovalent metals. However, Patil et al. found that Rb incorporation at the A-site helps obtain high-quality CsPbI₂Br perovskite thin films with high compactness, uniform and pinhole-free morphology together with improved thermal stability, revealing that partial incorporation with Rb⁺ cation can stabilize black phase all-inorganic perovskites under ambient conditions^[32].

Apart from the cation engineering, reasonable incorporation of relatively smaller anions in the X-site of the CsPbX₃ perovskite material can also improve the structure stability. Substitution of I with Br is a mostly reported approach to increase the tolerance factor and improve the phase stability when processing in ambient air. However, the subsequently increased bandgap of the material is inevitably induced, resulting in lower light harvesting. Hence, adjusting the amount of Br doping and looking for more appropriate anions to balance the stability and the bandgap is of significant importance.

Precursor engineering is also a facile strategy for fabricating stable all-inorganic PSCs under ambient conditions. In earlier studies, the commonly used precursor solutions contain CsI and PbI₂ for fabricating CsPbI₃. Xiang et al. proposed a strategy to prepare air-stable α-CsPbI₃ PSCs using HPbI₃ to replace PbI₂^[37]. In 2020, Duan et al. proposed a new precursor system (HCOOCs, HPbI₃, and HPbBr₃) to prepare ambient-compatible, high-efficient and stable CsPbI₂Br PSCs under high-humidity conditions (91% RH) [Figure 3B]^[38]. In this precursor system, a new complex (HCOOH•Cs⁺) is formed, which is crucial for controlling the crystallization process, leading to compact and uniform CsPbI₂Br films with larger grain sizes. Recently, Yue et al. used Cs acid and DMAPbX₃ (DMA: dimethylammonium) as a precursor to assemble the CsPbI₂Br crystals [Figure 3C]^[39]. In this case, the evaporation enthalpy of the DMA-acid side product is modified, making the CsPbI₂Br perovskites insensitive to environment humidity. Therefore, PSCs with a PCE exceeding 17% have been achieved, with enhanced shelf storage and 85/85 (temperature/humidity) tests.

Solvent engineering

For fabricating inorganic PSCs, the most generally employed solvents are N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) owing to the dissolvability of lead halides and cesium halides in other popular solvents^[40]. Apart from the single DMF or DMSO solvent^[31,35,64], mixed solvents of DMF and DMSO are also used, and the ratio between them has been adjusted to enhance the device performance^[65,66]. In more recent work, methylammonium acetate (MAAc) was reported to be an excellent and eco-friendly solvent for CsPbX₃ precursor solution in one-step humidity-insensitive fabrication^[40,41]. The coordination interaction plays an important role in perovskite crystallization. In the MAAc system, the interaction is stronger than in the DMF:DMSO system due to strong N-H···I hydrogen bonds and Pb-O bonds. Wang et al. reported ambient processed CsPbI_2.5Br_0.5 PSCs using MAAc as a solvent, achieving a PCE of 17.10 % and V_OC of 1.30 V [Figure 4A]^[40].

Figure 4. (A) Schematics of interaction in the precursor solutions using solvents of DMF: DMSO and MAAc, respectively. Copyright from Ref^[40]. (B) Schematic illustration of the crystallization process for unbathed and anisole-bathed CsPbI₃ perovskite films in ambient air. Copyright from Ref^[62]. (C) Schematics of PHPS-containing anti-solvent treated CsPbI₃ perovskite crystal growth when processing in ambient air. Copyright from Ref^[42].

Owing to high boiling point of the frequently used DMF and DMSO solvents, the slow evaporation of solvent would readily cause the slow nucleation rate. It has been indicated that anti-solvents, including the widely employed chlorobenzene (CB), ethyl acetate (EA), etc., rapidly reduce the solubility of the precursor and accelerate the nucleation^[67]. Adjusting the operation of anti-solvents during the spin-coating process of perovskite films can mediate the nucleation and grain growth of the perovskite films, thus modifying the morphology, film coverage and defect density^[68,69]. Liang et al. conducted an anti-solvent hot substrate spin-coating method to construct air-fabricated CsPbI₃ PSCs, realizing good optoelectronic properties similar to the N₂-fabricated devices and a PCE of 15.91%^[20]. Jeong et al. proposed an anisole antisolvent bathing strategy for preparing DMAI-assisted CsPbI₃ films [Figure 4B]^[62]. During this process, rapid crystallization kinetics can be attributed to the formation of the Cs₄PbI₆-antisolvent adduct which lowers the transformation energy barrier, therefore resulting in a more uniform film with better coverage, fewer defects and pinholes. The obtained devices show a PCE of 18.84% under moisture processing conditions (30%-40% RH). Wang et al. used methyl acetate as an anti-solvent and introduced perhydropolysilazane (PHPS) into this anti-solvent to develop a curing-anti-solvent strategy for fabricating black-phase CsPbI₃ perovskite films under ambient conditions [Figure 4C]^[42]. Under humid conditions, the hydrolyzed products of PHPS can adjust the grain growth by forming Lewis acid-base adducts. The pyrolysis-generated Si-O-Pb bonding to CsPbI₃ grains can create a shield layer to hinder phase transition. The resulting PSCs show a remarkably improved PCE of 19.17%, even at RH 60%.

Additive engineering

Introducing additives into the perovskite precursor solutions can influence perovskite crystallization and film formation, passivate the defects, and tune the structure and energetics, either in N₂ atmosphere or ambient air^[70,71]. This section overviews the recent progress in additive engineering to improve the air-processed all-inorganic perovskite film quality and device performance.

Luo et al. first fabricated CsPbI₃ PSCs under fully open-air conditions by adding HI, which improves the solubility of CsI and PbI₂ precursors in DMF solution and helps CsPbI₃ transform from the yellow non-perovskite to the black perovskite under low temperature^[43]. After that, various additives are used to improve the air-processed film quality and the resultant effects on the PCE of devices have also been deeply investigated. DMAI is one of the most widely used additives for fabricating stable CsPbX₃ and introducing HI and HPbI₃ routes share the same mechanism due to the generated DMA through the reaction of HI with DMF^{[44,51,60,62]}. Wang et al. investigated the role of DMAI in CsPbI₃ perovskite films fabricated in a dry box with an accurately controlled humidity (5%~10% RH), which indicated that DMAI is a volatile additive to control the crystalline perovskite, rather than alloying into the crystal lattice of CsPbI₃ perovskite [Figure 5A]^[44]. Besides, they stated that the DMAI residues in perovskite film can deteriorate the device performance. Together with phenyltrimethylammonium chloride (PTACl) passivation, the photovoltaic device exhibits a high efficiency of 19.03 %. Recently, Li et al. developed a hydrogen-bonding-facilitated DMA extraction strategy to prepare γ-CsPbI₃ films [Figure 5B]^[21]. With the formation of hydrogen bonds between polyacrylic acid (PAA) and DMA cation, the escape energy barrier of DMA is reduced, accelerating the decomposition of DMAPbI₃ and the crystallization kinetics of CsPbI₃. Therefore, pinhole-free CsPbI₃ films can be obtained without any trace of DMAPbI₃ residue and the fabrication humidity can be extended up to 80%. Consequently, dopant-free poly(3-hexylthiophene) (P3HT)-based photovoltaic devices with an outstanding PCE of 20.25% and superior moisture and operational stability can be achieved. Meanwhile, Jiang et al. used ultra-low dose transmission electron microscopy to identify the structure of DMA⁺-incorporated CsPbI₃ perovskites^[72]. Interestingly, they found that Cs⁺ can be substituted by the larger organic cation DMA⁺ to form tetragonal β-(DMA, Cs)PbI₃, which shows better optoelectronic properties than inorganic orthorhombic γ-phase CsPbI₃ [Figure 5C]. Although the devices were fabricated in a N₂ glove box, the air humidity stability was tested and the tetragonal β-(DMA, Cs)PbI₃ PSC shows an extended storage lifetime than that of orthorhombic γ-CsPbI₃ devices.

Figure 5. (A) Schematics for DMAI additive induced CsPbI₃ perovskite formation and the device stability based on Cs-1.5DMAI perovskite with and without DMAI residues. Copyright from Ref^[44]. (B) Schematics of hydrogen bonding-assisted CsPbI₃ perovskite crystallization and the measured J-V curves of dopant-free P3HT-based CsPbI₃ PSCs. Copyright from Ref^[21]. (C) Schematic illustration of DMAI-assisted CsPbI₃ perovskite formation and time-resolved photoluminescence decay spectra of tetragonal, coexistence phase and orthorhombic perovskite films. Copyright from Ref^[72].

Based on the DMA-assisted route, Chang et al. introduced a multifunctional molecular additive Zn(C₆F₅)₂ to obtain printable CsPbI₃ films in ambient conditions [Figure 6A]. During printing, the contradiction between air-flow-assisted fast drying and low-quality film is reconciled, reducing trap density and improving energy level alignment^[45]. Mali et al. fabricated β-CsPbI₃ with additive Zn(C₆F₅)₂ in ambient air with RH of 25%-30% and constructed a phase-heterojunction with γ-CsPbI₃, therefore producing a 21.59% PCE for a small area-device (0.09 cm²) and an 18.43% PCE for a large area-device (18.08 cm²)^[53]. Additionally, ionic liquid, with high ionic conductivity and good thermal stability^[61,73,74], was used by Du et al. to regulate the printed thin-film growth in ambient air. It is found that grain boundaries are decreased, iodide vacancy defects are passivated, the interfacial energy barrier is reduced, and the lattice strain is relaxed with 1-ethyl-3-methylimidazolium hydrogen sulfate (EMIMHSO₄) [Figure 6B]^[66]. Resultantly, the ambient printed CsPbI₃ PSCs achieve a PCE of 20.01% under 1 sun illumination (100 mW cm^-2) and 37.24% under indoor light illumination (1,000 lux, 365 µW cm^-2).

Figure 6. (A) Schematics of printed CsPbI₃ films modified by Zn(C₆F₅)₂. Copyright from Ref^[45]. (B) Schematic illustration of defect passivation for CsPbI₃ perovskites enabled by EMIMHSO₄. Copyright from Ref^[66]. (C) Schematic diagram of CsPbI₂Br perovskite crystallization process via low-dimensional intermediate-assisted growth strategy and top-view scanning electron microscope images of the control and optimized perovskite films. Copyright from Ref^[48]. (D) Schematics of the chlorination and solid protection of the perovskites with humidity-assisted strategy via Si-Cl. Copyright from Ref^[47]. (E) Schematic illustration of CsPbI₃ perovskite formation via humidity-assisted polymerization strategy by MMDS. Copyright from Ref^[51].

Regulating both the nucleation and crystal growth kinetics is also widely reported to realize high-quality and stable all-inorganic perovskite films under ambient conditions^[48,50]. Yang et al. introduced imidazole halide (IMX: IMI and IMBr) into CsPbI₂Br perovskite, which can form a low-dimensional intermediate phase to help improve the film quality in ambient air [Figure 6C]^[48].

Besides, by introducing effective additives, Pb-related defects could be passivated, further improving the PCE and stability of PSCs. Wang et al. prepared CsPbI₃ perovskite films by introducing an additive 1,2-di(thiophen-2-yl)ethane-1,2-dione (DED) to form a chelate for effectively passivating Pb-related defects and suppressing ion migration, thus boosting a PCE to 21.15%^[52].

Considering the negative effect of moisture on the performance of CsPbX₃ PSCs, constructing a hydrophobic layer to protect perovskite films from water invasion is an effective way to improve the stability^[46,47,49]. For instance, Fu et al. introduced 3,5-difluorobenzoic acid hydrazide (FBJ), which features a stronger absorbance on CsPbI₃ than water, to regulate traps and prevent external erosions, thus realizing a high V_oc of 1.225 V for inverted CsPbI₃ devices^[49]. Differently, humidity-assisted strategy based on hydrolysis reaction to consume water has been developed by the same group in recent years. Moisture-sensitive 1,2-bis(chlorodimethylsilyl)ethane (Si-Cl) molecules were reported to rapidly react with ambient moisture to reduce erosion during coating of perovskite films in air conditions [Figure 6D]^[47]. The hydrolysis products of hydrogen chloride (HCl) can chlorinate CsPbI₃, and the spontaneous polymerization of 1,2-bis(hydroxydimethylsilyl)ethane (Si-OH) could fill the grain boundaries for solid protection, which could regulate grain growth for better crystallization and enhanced the stability of CsPbI₃. In addition, maleic anhydride (MAAD) works similarly due to hydrolysis to stabilize CsPbI₃ films prepared with a wide humidity operating window. The hydrolysis product of maleic acid (MAAC) contains two carboxylic groups, which have strong Lewis acid-based interaction with CsPbI₃, thus improving the crystallization, trap passivation, and phase stability^[46]. Apart from hydrolysis reaction, a humidity-assisted polymerizable additive was also used to improve the film stability and passivate defects by coordinating with Pb²⁺ [Figure 6E]. It is reported that mercaptopropylme-methyldimethoxysilane (MMDS) can react with water and form polymers in humid air^[51]. Its existence at grain boundaries could not only prevent moisture invasion but also passivate under-coordinated Pb²⁺ via the interaction with the -SH group. Therefore, regardless of fabrication in relatively high humidity (40%~80%), a PCE of 19% can be obtained for CsPbI₃ PSCs, in association with enhanced operational stability.

Interface engineering

Interfaces between perovskites and charge transport layers play an important role in PSCs with high PCE and stability since charge carriers need to be extracted and the moisture invasion generally happens from the surface. Thereby, modifying the interface is regarded as an effective approach to reducing defects, improving the energy alignment, increasing the charge extraction rate, enhancing the stability, etc., which render the high-performance air-processed inorganic PSCs^[61,63].

Wang et al. fabricated thermodynamically stabilized β-CsPbI₃ and developed a crack-filling modification strategy to improve the PCE of air-processed PSCs via surface treatment with choline iodide (CHI) [Figure 7A]^[60]. It is indicated that CHI can go through the pinholes and cracks and then penetrate into the bulk CsPbI₃ films. As a result, the traps are greatly passivated and the energy-level alignments at both the upper and bottom interfaces are improved, thus boosting the PCE to 18%. Chen et al. utilized 18-crown-6 ether (Crown) to stabilize/passivate α-CsPbI₃ fabricated in dry air with RH of 10%-15%^[54]. The strong interaction between Crown and Cs⁺ on the surface not only reduces humidity sensitivity but also concurrently passivates surface defects and reduces related migrations. Therefore, an efficiency of 16.9% is achieved for small-area devices (0.1 cm²) and 11.8% for large-area modules (16 cm²).

Figure 7. (A) Schematics of crack-filling interface engineering and the energy diagram of the CsPbI₃ layers. Copyright from Ref^[60]. (B) Schematic diagram of perovskite crystallization process before and after methyl ammonium chloride treatment and the J-V curves. Copyright from Ref^[55]. (C) X-ray photoelectron spectroscopy results at the surface and 10 nm depth and schematic diagram of the electron-transport dynamics based on the reference and polished films. Copyright from Ref^[56].

Additionally, the morphology and crystal structure of CsPbX₃ films can be regulated by intermediate treatment before annealing. Yoon et al. dropped the solution of methyl ammonium chloride (MACl) successively to control the intermediate state of the crystallization process of CsPbI₃ perovskites deposited in ambient air (RH: 15%-30%) [Figure 7B]^[55]. It is concluded that the crystallization process is accelerated, leading to a uniform and dense film with few pinholes. Together with the post-surface passivation with octyl ammonium iodide, the efficiency of PSCs is improved to 20.37%. Recently, Li et al. comparatively investigated the influence of intermediate- and post-treatment on the PCE of air-prepared inverted CsPbI₃ PSCs^[57]. Using formamidinium (FA) salts for intermediate treatment, crystallization rate is accelerated, and the shortened exposure time is found beneficial for water resistance. Compared to the post-treatment method, the films finely tuned by intermediate treatment show pinhole-free morphology with reduced defects and better energy level alignment, significantly enhancing PCE and stability.

In addition to molecular passivation at the film surface, Fu et al. reported that the 1,4-butanediamine (DAB) modification could polish the intrinsically formed lead-poor surface and eliminate the nonstoichiometric components [Figure 7C]^[56]. Therefore, the deep-energy-level p-type traps are reduced, improving charge transfer at the CsPbI₃/PCBM interface. This polishing treatment promotes the efficiency to 19.84% and improves the operational stability to 1,000 h. Recently, Guo et al. introduced N,N′-diphenylthiourea (DPhTA) to construct a gradual CsPbI₃/PbS heterojunction at the top of CsPbI₃ surface, thus resulting in a V_oc of 1.20 V and a PCE over 20%^[58].

Besides, Qiu et al. proposed a strategy via dipolar chemical bridge (DCB) to facilitate the charge transport at the perovskite/TiO₂ interface^[59]. By employing 4-aminobutyric acid (CA) and 3-amino-1-propanesulfonic acid (SA) at the buried interface, the interfacial energy loss is dramatically reduced and the crystallinity of perovskites in air (RH = 30%) is improved. Therefore, the optimized CsPbI₃ PSCs achieve a PCE of 21.86 % with a high V_oc of 1.26 V.