Instability of solution-processed perovskite films: origin and mitigation strategies

Future perspectivesPerovskite solar cells (PSCs) are promising next-generation photovoltaics due to their unique optoelectronic properties and rapid rise in power conversion efficiency (PCE). However, the instability of perovskite materials and devices is a serious obstacle hindering technology commercialization. Future research into the quality of precursor solution and the post-deposition treatment performed after perovskite formation will play a key role in the development of stable PSCs. For the perovskite precursor solution, designing novel solvent and reductants in perovskite precursors to inhibit the oxidization of I^- is the key to enhancing uniformity and stability. For post-deposition treatment, composition regulation, strain engineering, defect passivation and phase stabilization could further enhance the stability after the formation of perovskite films. Further efforts can be made in developing regulation of the crystallization kinetics, novel functional layer with superior stability and advanced encapsulation methods to realize high performance and stable PSCs.

1.   Introduction

Metal halide perovskites have become one of the current research hotspots of energy conversion materials due to their superior optoelectronic properties, such as high carrier mobility [1], long carrier diffusion length [2], adjustable band gap [3] and long carrier lifetime [4, 5]. Besides, solution processibility enables perovskites to be coated by simple solution-based techniques with a high deposition rate of up to 180 m h^-1 [6] and at a relatively low manufacturing cost. Solution-processed perovskites have been widely used in a variety of devices, including light-emitting diodes (LEDs) [7-9], photodetectors [10-13], x-ray detectors for imaging [14] and solar cells [15, 16]. Their employment as absorbers in solar cells is extremely successful. The CH₃NH₃PbI₃ (MAPbI₃) perovskite was first used as a sensitizer in dye-sensitized solar cells in 2009 with an efficiency of 3.8% [17]. Motivated by this pioneering study, a variety of solution-based methods (spin-coating [18], spray coating [19] and slot die coating [20], etc) and advanced strategies (e.g. anti-solvent [21], solvent annealing [22] and hot casting [23], etc) were developed to improve the quality of perovskite films. So far, the record power conversion efficiency (PCE) of solution-processed perovskite photovoltaics has achieved 25.8% (certified 25.5%) for small cell (0.1 cm²) [24] and 22.72% for minimodule (24 cm²) [25], which is much higher than other emerging solar cells [26] and is approaching single crystalline silicon cells [27]. When the comparison is made between solution- and vapor-based methods (e.g. thermal evaporation [28, 29], chemical vapor deposition [30, 31] and close-space sublimation, etc), solution-processed perovskite solar cells (PSCs) outperform vapor-processed PSCs because of better electronic quality (lower density of deep level defects) of perovskite absorber [32]. On the other hand, the stability of solution-processed PSCs is still not comparable to photovoltaics existing in the market (e.g. 20-25 years) and has become one of the most imperative challenges to be addressed before technology commercialization can be considered [33, 34].

Perovskites are unstable under many external stimuli, such as moisture, oxygen, heat, light and reverse bias [35, 36], which is a major obstacle hindering the long-term operational stability of PSCs [37-39]. In view of solution-processed perovskites, stability is closely related to the characteristics of the precursor solution and the quality of perovskite films. Purity and solubility of the solute, the interaction between solute and solvent, and the chemical reactions between different solutes may seriously affect the stability of the precursor solution and later on the quality of the perovskite films [40-49]. On the other hand, the chemical composition, defects chemistry and crystal phase of the as-prepared perovskite films determine their long-term stability. For example, the MAPbI₃ perovskite decomposes quickly under moisture or heat conditions via loss of the MA cation [50-53]. The FAPbI₃ perovskite demonstrates higher thermal stability but suffers from phase instability due to a mismatch of the ionic radius [39, 54, 55]. When multi-cations are introduced in the crystal lattice to form FAMA or CsFAMA perovskites, the phase instability of the FAPbI₃ can be mitigated. However, the stability of the complicated precursor solution and the thermal stability of the PSCs using these perovskites remains to be solved because of the presence of the MA cation [56-58]. Herein, we review the origin of instability of solution-processed PSCs mainly from the precursor solutions to the perovskite films (figure 1). In addition, we summarize the recent strategies for improving the stability of perovskite films. Finally, we pinpoint possible approaches to further advance their long-term stability.

Figure  1.  Summary of the main origin of instability of solution-processed perovskite solar cells.

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2.   Origin of instability in solution-processed perovskite

2.1   Perovskite precursor solution

2.1.1   Solute.

PSCs can be fabricated by solution processing methods. Purity, solubility of the solute and the chemical reactions between different solutes in the perovskite precursor solution affect not only the reproducibility of fabrication processes but also the quality of perovskite films (e.g. the defect densities, composition, uniformity, etc), which determines the stability of PSCs. As a consequence, understanding the characteristics of the solute is the key prerequisite for pursuing long-term stable PSCs. Recently, the degradation mechanism of the solute in the perovskite precursor solution was systematically studied.

The FAPbI₃ powder serves as an excellent raw material for fabrication of high-performance FAPbI₃ devices [24, 41, 59-61]. Shin et al investigated the effect of solutes on stability of the FAPbI₃ precursor solutions and the FAPbI₃ films [42]. Different precursor solutions were prepared either by dissolving a mixture of FAI/PbI₂ or the synthesized single-crystalline -FAPbI₃ in the same solvent (figure 2(a)). After aging the solution at ambient condition, the pH value of the conventional FAI/PbI₂ precursor solution was significantly decreased from 11 to 6 due to the formation of HI. The acidic condition prevents the formation of -FAPbI₃. On the other hand, the pH variation was less significant when single-crystalline -FAPbI₃ was used as solute. As a result, the PCE of PSCs prepared with the FAI/PbI₂ mixture decreased obviously with aging of the solution. While devices using -FAPbI₃ based solute were quite stable at the same time scale. Chen et al found that the I^- in the precursor solution was easily oxidized to I₂. And the ${\text{I}}_3^{-}$ appeared with the reaction of I₂ and I^- [45]. After aging the FAI/MAI solution in air for 2 d, color of the solution changed to yellow. This indicated the formation of ${\text{I}}_3^{-}$, which deteriorated the performance and reproducibility of the PSCs. They improved the stability of the precursor solution by adding benzylhydrazine hydrochloride as a reductant, which could effectively reduce the ${\text{I}}_3^{-}$ and stabilize the PSCs under operation condition.

Figure  2.  Effect of solute on the stability of perovskite precursor solution. (a) Preparation of perovskite precursor solutions with FAI + PbI₂ and FAPbI₃ crystal, and the PCE of perovskite solar cells. Reprinted with permission from [42]. Copyright (2020) American Chemical Society. (b) The deprotonation of FAI. (c) The addition-elimination reaction among three FA molecules in the solution. [47] John Wiley & Sons. [© 2021 Wiley-VCH GmbH]. (d) 1H NMR spectra of MAI + PbI₂, FAI + PbI₂, and 0.5MAI + 0.5FAI + PbI₂ after aging for 1 and 24 h. (e) The deprotonation of MAI. (f) The addition-elimination reaction between MA and FAI in perovskite solution. Reprinted from [56], Copyright (2020), with permission from Elsevier.

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Other degradation mechanisms of the precursor solutions were also proposed [47]. Chen et al found that the decomposition of FAI in the precursor solution could be divided into two steps, i.e. the deprotonation reaction and the addition-elimination reaction (figures 2(b) and (c)). In brief, the FAI was firstly deprotonated to form FA and HI. Then three FA molecules were self-condensation to form s-triazine. The HI and s-triazine were the main decomposition product. Wang et al found that the MA and FA mixed cations were also not stable in the precursor (figure 2(d)) [56]. They probed the composition evolution of the precursor solution by using ¹H nuclear magnetic resonance (NMR). The possible reaction mechanisms in the perovskite solutions were presented in figures 2(e) and (f). MA could be generated from the deprotonation of MAI, and then reacted with FAI to form N-methyl FAI and DMFAI.

2.1.2   Solvent.

Besides the solute, another major factor determining the characteristics of precursor solutions and quality of perovskite films (e.g. composition, crystallinity, morphology, and trap-state density) is the solvent. The solvents with different properties, such as boiling point, vapor pressure, viscosity, dipole moment and donor number (figure 3(a) and table 1), would not only affect the evaporation rate of the perovskite precursor solution but also the nucleation and crystal growth of the perovskite films [62]. The solvents with different solvating and coordination abilities play an important role in the formation of perovskite films. The affinity between PbI₂ and I^- is higher than that of PbI₂ and solvent molecules with low solvation ability (e.g. acetonitrile (ACN), isopropanol (IPA) et al), which facilitates the formation of iodoplumbate complexes (${\text{PbI}}_3^{-}$ and ${\text{PbI}}_4^{2-}$) before the perovskite formation. On the other hand, high-affinity solvent molecules (e.g. DMPU, HMPA et al) coordinate with PbI₂ and suppress the crystallization of perovskite film via formation of the PbI₂·solvent complex. The introduction of solvents with different coordination abilities would change the equilibrium between PbI₂·MAI and PbI₂·solvent, leading to the transformation of coordination complex species and perovskite crystallization kinetics.

Figure  3.  Effect of solvent on the stability of precursor solution and morphology of perovskite film. (a) Schematic illustration of equilibria among perovskite, iodoplumbate complexes, and PbI₂·solvent based on the competition between I^- and solvent to coordinate with PbI₂. [62] John Wiley & Sons. [© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. Low- and high-magnification SEM images of solution-processed perovskite films using (b), (c) DMF. [64] John Wiley & Sons. [© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. (d), (e) DMF-DMSO mixture. Reprinted from [63], Copyright (2021), with permission from Elsevier.

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Table  1.  Physical and chemical properties of common solvents for perovskite precursor solutions.

Solvent	Chemical formula	Boiling point (C)	Vapor pressure (mmHg, 20 C)	Viscosity (mPa s-1, 20 C)	Dipole moment (D)	Donor number (kcal mol-1)	References
N, N-dimethylformamide (DMF)	C3H7NO	153	2.7	0.92	3.86	26.6	[62]
Dimethyl sulfoxide (DMSO)	C3H7NO	189	0.42	1.996	3.96	29.8	[62]
-Butyrolactone (GBL)	C4H6O2	204	1.5	1.75	4.27	17.8	[62]
N-Methyl-2-pyrrolidone (NMP)	C5H9NO	202	0.29	1.67	4.09	27.3	[62]
Acetonitrile (ACN)	C2H3N	82	72.8	0.369	3.92	14.1	[62]
2-methoxyethanol (2-Me)	C3H8O2	124	6.17	1.7	2.04	19.8	[63]
1-cyclohexyl-2-pyrrolidone (CHP)	C10H17NO	284	0.05	11.6	4.22	28.9	[63]

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DMF and DMSO are the most conventional solvents for preparation of the perovskite precursor solution. These solvents show strong coordination ability and high boiling points. Xiao et al found that when DMF was used as the solvent, slow crystallization of the perovskite causing the formation of many uncovered pin-hole areas over the film (figures 3(b) and (c)) [64]. In addition, Yoo et al found that many pin-holes appeared in the perovskite films obtained from the precursor solution with a mixed solvent of DMF and DMSO (figures 3(d) and (e)) [63]. The balance between fast nucleation and slowed crystal growth was the key prerequisite for the formation of uniform and dense perovskite films.

2.2   Perovskite thin-film

2.2.1   Crystal structure and composition.

Crystal structures of semiconductors determine their electronic structures and optoelectronic characteristics. Zakutayev et al calculated the band structures of the III-V and II-VI semiconductors (e.g. GaN, GaAs, CdSe). The valence band maximum (VBM) of these semiconductors comprising the bonding states is easier to form deep trap states. These deep trap states can act as Shockley-Read-Hall recombination centers, significantly impeding carrier transport [65, 66]. Different from these conventional semiconductors, organometal halide perovskite shows an ABX₃ crystal structure with a soft lattice (figure 4(a)) [66]. Yin et al calculated the band structure of the MAPbI₃ perovskite using density functional theory (DFT)-PBE (figure 4(b)). They showed that MAPbI₃ is a direct bandgap semiconductor. The mixing of a Pb s orbital and an iodine p orbital constitutes an anti-bonding coupling in the VBM [37]. Meanwhile, the Pb p state contributes to the CBM. The unique band structure provides perovskite with better defect tolerance and extraordinary optoelectronic properties.

Figure  4.  Fundamental characteristics of metal halide perovskite. (a) Cubic crystal structure of perovskite. (b) Band structure and the density of states (DOS) of MAPbI₃. Reproduced from [37] with permission from the Royal Society of Chemistry. (c) C-AFM images of perovskite films subjected to 85 C for 24 h in different atmospheres. [68] John Wiley & Sons. [© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. (d), (e) Thermal decomposition (heating rate of 20 C min^-1) of (d) MAPbI₃ and (e) FAPbI₃. The TG (upper panel, dark green), DTA (upper panel, blue color) and the m/z peaks profiles (lower panel). Gray dash lines indicate the initial temperature (time) when gases release during the thermal degradation. (d) Reproduced from [50]. CC BY 3.0. (e) Reproduced from [69] with permission from the Royal Society of Chemistry.

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The FAPbI₃, FAMAPbI₃ and CsFAMAPbI₃ perovskites with MA, FA and Cs as the A-site cations are commonly used to prepare high-efficiency PSCs. MA and FA-cation are unstable in the precursor solution which has been discussed in section 2.1. Besides, the stability of MA- and FA-based PSCs is also unsatisfactory, especially under humidity or heat condition. Understanding the cation-induced device failure mechanism is crucial for improving the long-term stability of PSCs.

The instability of MAPbI₃ caused by humidity and oxygen could be solved after careful encapsulation. On the other hand, the heat-induced instability of MA-contained perovskite may be more severe. MAPbI₃ decomposes slowly at moderate temperature (65 C-85 C) and rapidly at high temperature (135 C-150 C) [35]. Conings et al compared the thermal stability of MAPbI₃ films under different atmospheres (figure 4(c)) [68]. After storage in N₂ atmosphere at 85 C for 24 h, the electrical conductivity of the perovskite film is slightly decreased, indicating the beginning of the decomposition process. The decomposition process is accelerated in O₂ atmosphere. When heating in air, the decomposition rate of the film is greatly accelerated and the conductivity is greatly reduced.

Later on, Juarez-Perez et al investigated the thermal decomposition of FAPbI₃ by using the same technique [69]. Two mass loss steps are observed, which can be assigned to the decomposition of FAI (330 C) and PbI₂ (400 C) (figure 4(e)). The increased decomposition temperature of FAI in FAPbI₃ compared to MAI in MAPbI₃ indicates higher thermal stability. FAI mainly undergoes the following three thermal decomposition reactions. Equation (8) is the FA self-condensation reaction at a low temperature. Equation (9) is the FA decomposition reaction at a high temperature. And equation (10) is the deprotonation reaction of FAI

Although FAPbI₃ has higher thermal stability, it suffers from poor phase stability, especially under humidity condition, which will be discussed in section 2.2.4.

Shi et al used gas chromatography-mass spectrometry to study the thermal decomposition products of FAI, MAI and MABr powders [53]. They found that both FAI and MAI decomposed significantly in the temperature range of 85 C-350 C. After heating at 85 C for 100 h, the decomposition products of MAI and MABr were CH₃I and CH₃Br. And the decomposition products of FAI were H₃C₃N₃ and NH₃. After aging at 140 C for 10 h, the additional decomposition product of MAI and MABr was NH₃. After heating at 350 C for 15 min, the decomposition products of MAI and MABr were the same as heating at 140 C. While the decomposition products of FAI changed from H₃C₃N₃ to HCN. These results were consistent with the thermal decomposition reactions raised by Juarez-Perez et al [50, 69].

2.2.2   Strain.

Strain in perovskite films is the main origin of instability and cannot be solved by the conventional extrinsic stabilization methods [70]. There are two types of strain in the PSCs, i.e. the local lattice strain and the external condition-induced strain [38]. Saidaminov et al reported that local lattice strain in perovskite films arises from the ionic size mismatch between the FA-cation and Pb-I cage. The local strain contributes to the cage distortions and BX6 octahedra tilting, facilitates the formation of vacancies and results in the degradation of PSCs. (figure 5(a)) [71]. Zhu et al revealed gradient evolution of residual strain in the vertical direction of the mixed halide perovskite film [72]. They performed cross-section TEM and nano-beam electron diffraction (NBED) measurements on three typical regions at different depths (figure 5(b)). The NBED patterns indicate the lattice distortion in the microscopic crystal structure, the increase in crystal plane distance and the decrease in lattice constant from the surface to the bottom.

Figure  5.  Strain in perovskite films. (a) Schematic illustration showing the relaxation of local strain in hybrid perovskite either by the formation of point defects or by incorporation of small ions. Reproduced from [71], with permission from Springer Nature. (b) A cross-sectional TEM image of a device and the nano-beam electron diffraction patterns ([100] zone axis measured at e, f and g points). Reproduced from [72]. CC BY 4.0. (c) Thermal expansion coefficients of widely-used functional layers in PSCs including substrates, ETLs, perovskites, and HTLs. (d) Annealing temperatures of hybrid and inorganic perovskite films during formation. (e) Scheme showing the formation of tensile and compressive strains. (f) Annealing temperature-dependent stress in perovskites. Reproduced from [73]. CC BY 4.0. (g) High-resolution XRD -2 scans of the (001) peaks of the epitaxial samples on different substrates. (h) Reciprocal space mapping with (104) asymmetric reflection of the -FAPbI₃, for different lattice mismatches with the substrate. The results show a decrease in the in-plane lattice parameter as well as an increase in the out-of-plane lattice parameter with larger compressive strain. Qx and Qz are the in-plane and out-of-plane reciprocal space coordinates. Reproduced from [75], with permission from Springer Nature.

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External condition-induced strain may arise from the mismatch of thermal expansion coefficient between the perovskite films and the substrates. Xue et al summarized the thermal expansion coefficient () of each functional layer (figures 5(c)-(f)) [73]. They found that perovskites have much higher values (ranging from 3.3 to 8.4 10^-5 K^-1) than the ITO-coated glass and metal oxide charge-transport layers (in the range of 0.37-1 10^-5 K^-1). A high-temperature annealing process (>100 C) is necessary to allow crystallization of the perovskite films. This result in the formation of a tensile strain during the cooling process. They also calculated the stress in different perovskite films. Stress in the CsPbI₂Br film is much larger than in the MAPbI₃ film because of a higher annealing temperature and a larger thermal expansion coefficient mismatch relative to the substrate [74]. In addition, mismatch of lattice constant between the perovskite and the substrate is another source of external condition-induced strain. Chen et al reported the strained epitaxial growth of -FAPbI₃ thin films on lattice-mismatched halide perovskite substrates (figures 5(g) and (h)) [75]. The XRD peaks of MAPbCl_xBr_3-x substrates shift to higher diffraction angles as the increase of x and the peak of FAPbI₃ shifts to the lower diffraction angles.

2.2.3   Defects.

Perovskite films usually have a polycrystalline nature with different types of defects. Although perovskites demonstrate high tolerance of defects, the performance and stability of the PSCs can still be affected especially in high-efficiency devices. From the performance perspective, the existence of defects affects the interfacial contact and the extraction of carriers, increasing series resistance and non-radiative recombination [76, 77]. From the stability perspective, the defects (especially the bulk defects) accelerate ion migration during the operation of the device [78], causing the decomposition of the crystal structure [79] or phase separation [80, 81]. Therefore, passivation of bulk, surface and interfacial defects is one of the effective methods to improve the efficiency and stability of the perovskite device.

Gao et al summarized the types of defects in perovskite films [82]. The typical defects in MAPbI₃ film could be divided into 0D and 2D defects (figure 6(a)). The Pb²⁺ vacancies, I^- vacancies, interstitial Pb²⁺, interstitial ${\text{I}}_3^{-}$ and Pb-I anti-site substitution are considered as the 0D defects. While those at the grain boundary and on the surface are the 2D defects. Furthermore, Buin et al calculated the formation energies of the point defects in the MAPbI₃ films by using the DFT [83]. Several types of point defects were presented in figure 6(b), such as the vacancy defects (MA (V_MA), Pb (V_Pb) and I (V_I)), interstitial defects (MA_i, Pb_i, I_i) and substitution defects (MA_Pb, Pb_MA, MA_I, Pb_I, I_MA, I_Pb). The vacancies defects are shallow-level defects. On the other hand, the interstitial and anti-site defects are deep-level defects. The low formation energies and deep-level point defects in the MAPbI₃ films would be detrimental to the stability and performance of the PSCs. Liu and Yam calculated the formation energies of the point defects in FAPbI₃ including interstitials (FA_i, Pb_i, I_i), vacancies (V_FA, V_Pb, V_I) and anti-sites (FA_Pb, Pb_FA, FA_I, Pb_I, I_FA, I_Pb) (figure 6(c)) [84]. They found that the FA-based defects (e.g. V_FA, FA_I and I_FA) have much lower formation energies. And anti-sites defects (e.g. FA_I and I_FA) are located in the deep levels of bandgap, which can act as recombination centers and increase the V_oc loss in PSCs. Furthermore, the defect formation energies in CsPbI₃ were calculated (figures 6(d) and (e)) by Liang et al [85] They found that CsPbI₃ has 12 point defects (the vacancies V_Cs, V_Pb, and V_I, the interstitials Cs_i, Pb_i, and I_i and anti-site occupations Cs_Pb, Cs_I, Pb_Cs, Pb_I, I_Cs, and I_Pb). Under the Pb-poor condition, I_i and V_Pb possess the lowest formation energies among all donor and acceptor defects, respectively. While under the Pb-rich condition, I_i and V_I possess the lowest formation energies among all donor and acceptor defects, respectively.

Figure  6.  Defects in perovskite films. (a) Schematic illustration of typical defects in PSCs. [82] John Wiley & Sons. [© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. (b) Energy levels associated with the defect states corresponding to neutral and charged vacancies (V_Pb, V_I, V_MA), interstitials (Pb_i, I_i, MA_i), and antisites (Pb_I and I_Pb). Reprinted with permission from [83]. Copyright (2014) American Chemical Society. (c) The charge transition energy levels of intrinsic defects in FAPbI₃. Reproduced from [84] with permission from the Royal Society of Chemistry. (d), (e) The defect formation energies as a function of Fermi level between the valence band maximum (VBM) and conduction band minimum (CBM) of native point defects in CsPbI₃ calculated under (d) lead-rich and (e) lead-poor growth conditions. [85] John Wiley & Sons. [© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. (f) Nanoscale photo-excited carrier trapping dynamics: (a) overlaid PL and PEEM images of (Cs_0.05FA_0.78MA_0.17)PbI₃ thin film. (b) Schematic of the TR-PEEM setup for time-resolved imaging of photoelectrons. (c) TR-PEEM signal as a function of delay time. (d), (e) TR-PEEM images of the marked areas in a. Reproduced from [89], with permission from Springer Nature. (g) Scheme of a MAPbI₃ thin single crystal on a PTAA/ITO substrate and (h) trap density near the junction barrier of a MAPbI₃ thin single crystal before mechanical polish, after mechanical polish, and after oxysalt [(C₈-NH₃)₂SO₄] treatment. Trap density depth profile of (i) a MAPbI₃ single crystal and (j) a double-layer MAPbI₃ thin single crystal measured by drive-level capacitance profiling (DLCP) methods. From [90]. Reprinted with permission from AAAS.

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In addition to theoretical calculation, several experimental techniques have been used to study the defects in perovskites [86-88]. Doherty et al used photoemission electron microscopy (PEEM) to image the trap distribution in perovskite films (figure 6(f)) [89]. The discrete, nanoscale trap clusters at the interfaces were observed. The regions with high photoluminescence (PL) efficiency showed little photo-excited hole trapping. And in regions with low PL efficiency, they see complex spatio-temporal dynamics with photo-excited holes being trapped at several discrete sites. Ni et al used drive-level capacitance profiling (DLCP) method to investigate the distributions of trap states in perovskite single-crystalline and polycrystalline solar cells (figures 6(g)-(j)) [90]. Most of the traps are distributed on the top and bottom surfaces of the single crystal. The trap density of the single-crystal was much lower than that of the polycrystalline films. Besides, the trap densities at the interfaces of the polycrystalline films were one to two orders of magnitude greater than that of the film interior. The results indicate that passivation of surface/interfacial defects may be the crucial issue to realizing the high performance and stability of PSCs.

Ions such as halogen [91, 92], Li⁺ [93] and Au/Ag [94] in PSCs can migrate under an external electric field, light, or heat mediated by vacancies and interstitial defects [95]. This usually leads to the failure of the perovskite device and cannot be solved by the encapsulation method. In figure 7(a), Domanski et al studied the migration of Au ion after heating the perovskite devices at different temperatures by TOF-SIMS (figure 7(b)) [94]. They showed that Au ion gradually diffused into the perovskite film and finally accumulated at the interface between ETL and perovskite upon increasing temperature. Kato et al detected the migration of I^- and corrosion of the Ag electrode by using XRD and XPS (figure 7(c)) [91]. Under humid air condition, the migration of I^- would be accelerated and moved through spiro-OMeTAD to the Ag electrode. The corrosion of Ag would occur by reacting with I^-. Finally, the AgI would be formed.

Figure  7.  The ion migration in perovskite solar cells. (a) ToF-SIMS depth profiles and (b) 3D maps showing the distribution of selected species across the device. Two Au^- profiles (devices aged at 30 and 70 C) were shown. Reprinted with permission from [94]. Copyright (2016) American Chemical Society. (c) Schematic illustration showing formation mechanism of AgI in a PSC. Step (1) H₂O penetration in the spiro-MeOTAD layer through pinholes. Step (2) Decomposition of MAPbI₃ accompanied by the formation of the iodine-containing volatile compound (MAI and/or HI). Step (3) Migration of the iodine-containing volatile compound from the MAPbI₃ layer corroding the electrode. Step (4) Surface diffusion of the iodine-containing volatile compound. Step (5) AgI formation. [91] John Wiley & Sons. [© 2015 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim].

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2.2.4   Phase transition.

Phase instability of FA-based perovskites is the major obstacle hindering the solar cell long-term stability. The origin of phase instability is ascribed to the mismatched ionic radius [39], resulting in an inappropriate tolerance factor (1.0) and the distortion of crystal structure. The photo-active black phase is obtained at high temperatures (T = 150 C-185 C) [96, 97]. And the black phase FAPbI₃ is metastable at room temperature, which turns to the yellow phase with the acceleration of the external condition (e.g. moisture). As a comparison, the ionic radius of MA⁺ is suitable and the photo-active phase can maintain in MAPbI₃ perovskite. However, MAPbI₃ suffers from thermal instability which has been discussed in section 2.2.1.

Masi et al summarized the phase types of FAPbI₃ (figure 8) [96], including the photoactive black phases of - (cubic), - (orthorhombic), and - (tetragonal) and the photoinactive yellow phase of - (hexagonal). At room temperature, the -phase is the one with the lowest free energy of formation. When the temperature is increased, the photoactive -phase will convert into the black phase with the expansion of the unit cell volume and the increase of the Pb-I-Pb tilting angle (). Moreover, Chen et al used neutron diffraction and first-principles calculations to study the structure of FAPbI₃ perovskite [98]. They obtained the cubic phase at 390 K, a hexagonal phase at 220 K, and another hexagonal phase at 15 K to systematically study the function of FA⁺ in the FAPbI₃ crystal structure and the importance of organic cations. They claimed that the entropy contribution to the Gibbs free energy caused by isotropic rotations of the FA⁺ cation plays a crucial role in the cubic-to-hexagonal structural phase transition. Stabilizing the metastable perovskite (black phase) of FAPbI₃ is crucial for photovoltaic applications.

Figure  8.  The phase transition in FAPbI₃ perovskite. Crystalline structure and polymorphic phase transitions of FAPbI₃ perovskites. The non-perovskite -yellow phase of both iodide perovskites transforms to black photoactive perovskite -phase at high temperature. The distortions into the [PbI₆]^4- octahedra promote the formation of lowered-symmetry - and -black phases after the temperature decreases. Reprinted with permission from [96]. Copyright (2020) American Chemical Society.

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2.2.5   Phase separation.

Mixed halide perovskites are usually used to regulate the tolerance factor and improve the phase stability of FAPbI₃. However, a new problem of phase separation appears in mixed halide perovskites, which becomes significant when increasing the Br^- to I^- ratios. Phase separation was first observed and reported by Hoke et al [99] They found that an additional PL peak forms at 1.68 eV on MAPb(Br_xI_1-x)₃, the intensity of which grew under continuous illumination (figure 9(a)). The position of this new peak is independent of halide composition (figure 9(b)). After continuous visible-light soaking of less than a minute, PL intensity from the new low-energy peak becomes more than an order of magnitude higher than the original peak (figure 9(c)). Furthermore, Li et al also observed the phase separation in all-inorganic CsPbIBr₂ films by using cathodoluminescence (CL) photomultiplier tube (PMT) mapping and secondary electron (SE) image [100]. The different colors between the grain boundaries and interiors indicated the formation of the I-rich phase at the grain boundaries. The I-rich phase would also segregate as clusters inside the films (figures 9(d) and (e)). The mechanism of the phase separation in CsPbIBr₂ was also explored. The mobile ions generated by the phase separation moved along grain boundaries as the path of ion migration.

Figure  9.  The phase separation in perovskite. (a) Evolution of photoluminescence (PL) spectra of MAPb(Br_0.4I_0.6)₃ thin film. A 457 nm laser with intensity of 15 Mw cm^-2 was used for excitation. The measurement was conducted every 5 s for 45 s at 300 K. Inset: temperature dependence of initial PL growth rate. (b) Normalized PL spectra of MAPb(Br_xI_1-x)₃ thin films after illuminating for 5-10 min. (c) PL spectra of MAPb(Br_0.6I_0.4)₃ thin film after sequential cycles of illumination for 2 min (457 nm, 15 mW cm^-2) followed by 5 min in the dark. Reproduced from [99]. CC BY 3.0. (d) Phase segregation on a CsPbIBr₂ film surface and (e) in film bulk obtained by the secondary electron SEM image and cathodoluminescence (CL) PMT mapping. The scale bars are 3 m. [100] John Wiley & Sons. [© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. (f) Scheme showing a reversible phase separation of MAPb(I_xBr_1-x)₃ trigged by light soaking. The yellow and blue spheres represent I_- and Br_-, respectively. The red and white pill shapes represent MA. And the lead atoms (not shown) are located in the center of the octahedra. (g) Secondary electron (SE) image, CL image and an SE/CL overlaying image after light soaking for 5 min at 100 mW cm^-2. The scale bars are 2 m. Reprinted with permission from [106]. Copyright (2017) American Chemical Society.

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Although the essential reason for phase separation is complex, it is widely accepted that the halogen-vacancy [80, 81, 101] and the excess charge-carrier [102] induced ionic migration in the perovskite films may be the main mechanism. Phase separation of perovskites happens either during the fabrication process or the aging period upon external stimuli (light [103, 104], bias [105], etc). Bischak et al reported that the separation of halide in the perovskite films with light soaking is distinct from macroscopic phase separation. They observed the light-induced phase separation in MAPb(I_0.1Br_0.9)₃ by CL imaging and multiscale simulations (figures 9(f) and (g)). The localized strain induced by the interaction between a photoexcited charge and lattice is sufficient to accelerate halide phase separation. And the low-bandgap, I-rich clusters are aggregated at the grain boundaries. Braly et al observed that the Br-rich composition of MAPb(I_0.6Br_0.4)₃ and (MA_0.9Cs_0.1)Pb(I_0.6Br_0.4)₃ experience rapid phase segregation upon 1-Sun equivalent current injection [105].

3.   Strategies to improve perovskite stability

3.1   Precursor solution advancement

The state of perovskite precursor solution (e.g. pH, solute, additive and solvent) could affect the stability of PSCs. In recent years, attentions have been paid on advancement of the precursor solutions from perspectives of the pH value [107], solute [41, 42, 108-110], additive [45-49, 56, 111, 112] and solvent [31, 63, 113-116].

The pH value of the precursor solution affects the formation of I₂ impurity in perovskite films, which acts as the deep-level defect and accelerates the degradation of perovskites under operational condition. As a consequence, regulating pH value of precursor solution is a rational method to stabilize perovskites [117]. Chen et al suppressed the formation of I₂ impurity by creation of an alkaline environment (figure 10(a)) [107]. The alkaline slowed down the crystallization kinetics of perovskite and retarded the formation of I₂ impurity (figure 10(b)). By using a residual-free weak alkaline (FAAc) as an additive, the device showed a PCE of 20.87% and less degradation after storing in N₂ condition for 1500 h. Zhang et al fabricated FAPbI₃ films by using the pre-synthesized FAPbI₃ powder to reduce the defect density [41]. The defect density in the perovskite films was reduced, and stable, high-efficiency devices were obtained (figures 9(c) and (d)). Min et al reported that the incorporation of S₈ could stabilize the (FAPbI₃)_0.95(MAPbBr₃)_0.05 precursor solution by the coordination of amine-sulfur (figures 10(e) and (f)) [112]. The (FAPbI₃)_0.95(MAPbBr₃)_0.05 PSC with S₈ maintained 90% of its initial PCE after continuous illumination for 500 min.

Figure  10.  Effect of precursor solution state on perovskite film and device. (a) SEM images of perovskite films prepared by using precursor solutions with various alkaline additives. (b) Normalized absorbance of organic cation solutions with different additives as a function of volume. Reproduced from [107]. CC BY 4.0. (c) Current density-voltage (J-V) curves and (d) stability of PSCs prepared with solutions using the powder and the precursor mixture. Reprinted with permission from [41]. Copyright (2020) American Chemical Society. (e) J-V curves of PSCs as a function of the aging time of the precursor solution. (f) PCE as a function of the sulfur content. [112] John Wiley & Sons. [© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. (g) The chemical structure of ITIC-Th. (h), (i) J-V curves of PSCs prepared with precursor solutions aged for different times (h) without and (i) with ITIC-Th additive. (j) PCE as a function of the solution aging time. [48] John Wiley & Sons.[© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim] [48].

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Furthermore, organic additives could be used to stabilize the perovskite precursor solution. Qin et al found that the small molecule (ITIC-Th) could facilitate the incorporation of MA cation and suppress the formation of yellow phase FAPbI₃ (figures 10(g)-(i)) [48]. The device with ITIC-Th showed improved stability (figure 10(j)). Wang et al introduced the triethyl borate into the FAMA mixed precursor solution to restrain the deprotonation of MAI. The device showed better performance and reproducibility. Liu et al added the MA/EtOH into the precursor solution to suppress the formation of I₂ impurities and coordinate with Pb²⁺ [44]. MA could adjust the perovskite colloidal size and stabilize the perovskite films. Li et al introduced the acetonitrile as the additive into the precursor solution to regulate the colloidal size and control the crystallization process. They obtained the device with a PCE of 19.7% [118].

The solvent is another crucial factor influencing the state of precursor solution and the quality of perovskite films. Many novel solvents and solvent mixture systems have been exploited. Chao et al introduced the room-temperature molten salt, methylammonium acetate (MAAc), as the solvent to prepare the MAPbI₃ precursor solution (figure 11(a)) [114]. The hydrogen bonds between methylammonium, lead salts and MAAc caused complete solubility of the solute. High-quality perovskite films could be processed in ambient air. The PSCs showed over 20% PCE and remained above 93% of original efficiency after storage in ambient air for more than 1000 h (figure 11(b)). Noel et al used the low-viscosity, low boiling point solvent system (i.e. methylamine (MA) and ACN mixture) to prepare large-scale perovskite films (figure 11(c)) [115]. The uniform, pinhole-free perovskite films were fabricated (figure 11(d)). Deng et al exploited a mixed solvent system containing 2-methoxyethanol (2-ME), ACN and dimethyl sulfoxide (DMSO) that could be applied for various perovskite compositions [116]. By tailoring solvent coordination capability, they obtained uniform perovskite films by blade-coating at an unprecedented speed of 99 mm s^-1. The device showed a PCE of 16.4% (63.7 cm²) and over 1000 h operational stability. Yoo et al employed 2-methoxyethanol (2-ME) as the solvent of precursor solution (figure 11(e)) [63]. Highly uniform and pinhole-free perovskite films could be achieved by adding n-cyclohexyl-2-pyrrolidone (CHP) into the 2-ME solution. They obtained the champion laser-patterned perovskite mini-module (figure 11(f)) with a PCE of 20.4% (31 cm²). What’s more, the mini-module was stable under ambient conditions for 50 d.

Figure  11.  Novel solvents and solvent mixture systems for perovskite films. (a) A picture of perovskite precursor solutions (300 mg ml^-1) with traditional solvents such as DMF, DMSO, NMP and the room-temperature molten salt, i.e. MAAc. (b) The I-V curves of the champion device. Reprinted from [114], Copyright (2019), with permission from Elsevier. (c) The perovskite precursor solution in ACN/MA solvent mixture and (d) the corresponding perovskite film. Reproduced from [115] with permission from the Royal Society of Chemistry. (e) Schematic of perovskite precursor solution formation in 2Me-CHP solvent and (f) the picture of perovskite mini-module obtained with the 2Me-CHP solvent. Reprinted from [63], Copyright (2021), with permission from Elsevier.

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3.2   Perovskite film optimization

3.2.1   Composition regulation.

The metal cations were introduced into the perovskite films as dopants with the aim to improve either efficiency or stability of PSCs. The performance and stability of PSCs with cations regulation were summarized in table 2. Among metal cation dopants, alkali metal cations have shown to improve the crystal structure stability and the crystallinity of the perovskite films [30, 58, 119-124]. In 2016, Saliba et al reported that the small and oxidation-stable Rb⁺ can be embedded into a cation cascade’ to form multi-cation perovskite film with excellent optoelectronic properties (figure 12(a)) [125]. The device showed 21.8% PCE and retained 95% of its initial performance after soaking under full sunlight at 85 C for 500 h. A similar phenomenon was observed when using Cs⁺ as the dopant in perovskite films [126]. In 2018, they incorporated both the Rb⁺ and Cs⁺ into FAPbI₃ films to realize highly crystalline formamidinium-based perovskites without any Br^- or MA⁺ (figure 12(b)) [127]. They obtained PSCs with an efficiency of 20.35%. Besides, the polymer-modified device maintained over 98% of the initial PCE after 1000 h of continuous maximum power point (MPP) tracking in a nitrogen atmosphere. The K⁺ was incorporated to reduce the J-V hysteresis and improve stability of the perovskite device [119]. Abdi-Jalebi et al found that the I^- in KI could compensate for the halide vacancies and the K⁺ could combine with the halides at the grain boundaries and surfaces, thereby inhibiting halide migration and suppressing the additional non-radiative decay arising from interstitial halides (figure 12(c)) [128]. Bu et al reported a universal potassium interfacial passivation strategy to improve the interfacial stability (figure 12(d)) [123]. The potassium passivated device showed excellent light stability and long-term storage stability.

Figure  12.  Metal-cation doping in perovskite. (a) Tolerance factor of metal halide perovskites and images of perovskite films at different temperatures. From [125]. Reprinted with permission from AAAS (b) Disproportionate bandgap tuning and cation stability. From [127]. Reprinted with permission from AAAS. (c) Scheme of a cross-section perovskite film showing that the surplus halide is immobilized through complexing with potassium into benign compounds at the grain boundaries and surfaces. Reproduced from [128], with permission from Springer Nature Copyright 2018, Springer Nature. (d) A schematic of the perovskite growth process on different substrates. Reproduced from [123]. CC BY 4.0. (e) Surface and cross-section SEM images of Cd-containing PSCs. Scale bars, 500 nm. Reproduced from [71], with permission from Springer Nature. (f) Scheme showing cyclical elimination of Pb⁰ and I⁰ defects and regeneration of Eu³⁺-Eu²⁺ metal ion pair in perovskite film. From [129]. Reprinted with permission from AAAS.Reproduced from [71], with permission from Springer Nature.

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Table  2.  The performance and stability of perovskite solar cells by metal-cation doping.

Name	Perovskite components	PCE [%] (before/after)	Stability	References
KI	(Cs,FA,MA)Pb(I0.85Br0.15)3	17.3/21.5	Over 80%, MPP for 300 h	[128]
RbI	Rb0.05(CsFAMA)0.95Pb(I,Br)3	NA/21.6	95%, 85 C MPP for 500 h	[125]
RbI	Cs0.1Rb0.05FAPbI3	NA/20.35	98%, MPP for 1000 h	[127]
CsI	Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3	NA/21.1	Over 90%, MPP for 250 h	[126]
CsCl	(Cs0.17FA0.83)Pb(I0.97-xBrxCl0.03)3	18.41/20.5	80%, MPP for 1000 h	[58]
GeI2	FA0.83MA0.17Ge0.03Pb0.97(I0.9Br0.1)3	21.27/22.09	80%, ambient condition	[43]
Zr(Ac)4	MAPbI3	NA/20.9	Over 98%, MPP for 1700 min	[130]
CdCl2	CsFAMAPb(I,Br)3	NA/20.5	Over 95%, ambient condition for 30 d	[71]
Eu(acac)3	(FA,MA,Cs)Pb(I,Br)3(Cl)	NA/21.89	92%, 1-sun illumination for 1500 h;	[129]
			89%, 85 C for 1500 h;
			91%, MPP for 500 h;
CoI2	MAPbI3	17.46/21.43	70%, N2 for 120 h	[131]

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Besides the alkali metal cations, other metal ions such as Cd²⁺ [71], Ge²⁺[43], Eu²⁺ [129], Zr⁴⁺ [130], Co²⁺ [131], etc have also been doped into the perovskite films to improve their thermal and illumination stability. Saidaminov et al found that the CdCl₂ could suppress the atomic vacancies in perovskite films (figure 12(e)) [71]. Doping with CdCl₂ improved the stability of PSCs by an order of magnitude. Wang et al reported that the Eu ion pair Eu³⁺-Eu²⁺ could act as the redox shuttle’ that selectively oxidized Pb⁰ and reduced I⁰ defects simultaneously via a cyclical transition process (figure 12(f)) [129]. They obtained the device with a certified PCE of 20.52%. In addition, the Eu-containing devices maintained 90% of the original PCE even after 8000 h of storage and 91% of the original stable PCE after MPP tracking for 500 h.

Anions were incorporated into the perovskite films as the Lewis-acid to passivate the Pb interstitial defects, under-coordinated Pb²⁺ and halide-vacancy defects. Generally, halide ions with a small radius (e.g. F^- and Cl^-) can be used to improve the crystallinity, passivate the vacancy defects and enhance the stability of the perovskite films [24, 132]. On the other hand, halide ions with a suitable radius (e.g. Br^-) can replace I^- in the [PbI₆]^- octahedron to adjust the bandgap and improve the phase stability [133, 134]. The performance and stability of PSCs with anions doping were summarized in table 3. Li et al employed fluoride to simultaneously passivate the anion and cation vacancy defects (figure 13(a)) [132]. They found that the extremely high electronegative of fluoride could enhance the hydrogen bond and ionic bond in the perovskite structure. With the incorporation of fluoride, the device demonstrated a high PCE of 21.46% and improved stability under stresses such as illumination, heat and humidity (figure 13(b)). Min et al reported a new interlayer (i.e. FASnCl_x) between the Cl-doped SnO₂ and Cl-containing perovskite films [24]. The interfacial defects and charge extraction were improved because of the atomically coherent features of the interlayer. With the interfacial passivation of Cl^-, they fabricated a device with a certified PCE of 25.5%. Besides, the unencapsulated device maintained 90% of the initial PCE after light exposure for 500 h. Graphene has been widely used to impede ion migration and perovskite decomposition [135, 136]. Wang et al modified the surface of the perovskite with the Pb(SCN)₂ and chlorinated graphene oxide to construct the Pb-rich surface and the strong Pb-Cl and Pb-O bonds [137]. The loss of decomposed components could be extremely impeded with the coverage of chlorinated graphene oxide and the operation stability could be enhanced. The device maintained 90% of its initial PCE after operation at MPP under AM1.5G solar light at 60 C for 1000 h.

Figure  13.  Anion doping in perovskite. (a) A schematic illustration of enhancing the hydrogen bond between the halogen and MA/FA ions, and strengthening the ionic bond between the halogen and metal ions by increasing the electronegativity of halogen. (b) Stability of PSCs under various conditions. Reproduced from [132], with permission from Springer Nature. (c) SEM image of the 2% formate-doped-FAPbI₃ film. The insets show a cross-sectional SEM image. Scale bar, 2 m. (d) Stability of PSCs under various conditions. Reproduced from [59], with permission from Springer Nature. (e) Schematic illustration of protection of perovskites by in situ formation of a lead sulfate top layer on the perovskite surface. (f) Stability of encapsulated solar cells based on control (blue) and sulfate-treated (red) CsFAMA perovskite active layers. From [139]. Reprinted with permission from AAAS.

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Table  3.  The performance and stability of perovskite solar cells by anion doping.

Name	Anion	Perovskite components	PCE [%] (before/after)	Stability	References
NaF	F-	(Cs0.05FA0.54MA0.41)	19.68/21.92	90%, MPP for 1000 h	[132]
		Pb(I0.98Br0.02)3		95%, continuous one-sun illumination in N2 for 1000 h;
				90%, 85 C for 1000 h;
				90%, 25%-45% RH, 25 C-40 C for 6000 h
SnO2-Cl (perovskite-Cl)	Cl-	FAPbI3	NA/25.8	90%, MPP according to the ISOS-L-1I protocol for 500 h	[24]
MAPbBr3	Br-	FAPbI3	NA/25.4	Stable, store for 3600 h	[134]
				80%, MPP for 500 h (encapsulated)
MABr, MACl	Br-, Cl-	FAPbI3	NA/22.51	97%, ambient condition for 2600 h	[133]
FACOOH	COOH-	FAPbI3	23.92/25.21	90%, 20% RH, 25 C for 1000 h;	[59]
				85%, MPP for 450 h;
				80%, 60 C for 1000 h
PEA(I0.25SCN0.75)	SCN-	(FA0.65MA0.2Cs0.15)	17.46/19.66	80%, continuous illumination for 1000 h	[138]
		Pb(I0.8Br0.2)3
(C8H17NH3)2SO4	SO42-	Cs0.05FA0.81MA0.14	NA/21.1	96.8%, MPP 65 C for 1200 h	[139]
		PbI2.55Br0.45

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Moreover, the non-halogen anions have been used to suppress the anion-vacancy defects at the grain boundaries and on the surface [138]. Jeong et al introduced pseudo-halide anion formate (HCOO^-) into the perovskite precursor solution, which is small enough to fit into the perovskite structure and fill the I-vacancy [59]. They obtained the perovskite films with grain sizes of up to 2 m (figure 13(c)) and PSCs with a PCE of 25.6% (certified 25.2%). The shelf-life, heat, and long-term operational stability were extremely improved (figure 13(d)). The sulfate or phosphate ions have also been used to passivate the defects and stabilize the perovskite material. Yang et al showed that treatment of the perovskite films with sulfate or phosphate ions resulted in the formation of water-insoluble lead oxysalt on the surface of the perovskite film (figure 13(e)) [139]. The wide-bandgap lead oxysalt could reduce the trap density on the perovskite surface by passivating the undercoordinated surface lead. As a consequence, the device maintained 96.8% of its initial efficiency after operation at MPP under simulated air mass (AM) 1.5G irradiation for 1200 h at 65 C (figure 13(f)).

Quasi-2D structured perovskites have presented better stability than 3D perovskites due to the higher hydrophobicity of the large organic cations and the higher formation energy [140]. Ruddlesden-Popper (RP), Dion-Jacobson (DJ) and the alternating cations in the interlayer space (ACI) phase are the most common 2D perovskites which are oriented along the (100)-plane [141, 142]. Though the stability of the quasi-2D perovskite is much more attractive [143], the performance of the 2D PSCs is unsatisfactory compared with the 3D PSCs. For the RP structured 2D perovskites, the van der waals gap between the adjacent inorganic [PbI₆]^4- sheets caused the formation of a large interlayer space, influencing the carrier transport across the inorganic layers. Moreover, the disorder of the quantum wells (QW) distribution and film growth orientation also inhibited the carrier transport [144]. For the DJ structured 2D perovskites, van der waals gap is avoided. However, the QW distribution and the crystallization of the DJ perovskite would affect the performance of the PSCs [145, 146]. The ACI perovskites are a new type of 2D halide perovskites that featured two different alternating cations in the interlayer space. Recently studies showed that the quality of the ACI perovskite films and the crystallization kinetics may determine the performance of the PSCs [142, 147]. The performance and stability of PSCs based on these 2D organic cations has been summarized in table 4.

Table  4.  The performance and stability of perovskite solar cells based on the 2D organic-cation.

	Name	Perovskite components	PCE [%] (before/after)	Stability	References
RP	MTEA	(MTEA)2(MA)4	15.94/18.06	Over 85%, MPP for over 1000 h	[154]
		Pb5I16
RP	BA	BA2MAn-1PbnI3n+1	13.81/16.25	Over 90%, 65 10% RH for 4680 h;	[155]
				Over 90%, 85 C for 558 h;
				Over 90%, continuous light illumination for 1100 h
RP	ThFA	(ThFA)2(MA)n-1	7.23/16.72	Over 99%, store in N2 for 3000 h;	[156]
		PbnI3n+1
RP	ThMA	(ThMA)2(FA)n-1	16.18/19.06	96%, 80 C for 576 h	[148]
		PbnI3n+1
RP	p-FPhFA	(p-FPhFA)2MAn-1	NA/17.37	99%, store in N2 for 3000 h;	[149]
		PbnI3n+1-xClx		82%, continuous light illumination for 450 h
RP	F-PEAI	(F-PEA)2(MA)4Pb5I16	12.6/14.5	90%, 40%-50% RH for 40 d	[150]
RP	BA	BA2MA3Pb4I13	NA/17.26	Over 95%, store in N2 for 2000 h	[151]
RP	BA	BA2MA3Pb4I13	NA/12.52	Over 60%, continuous light illumination for 2250 h	[153]
DJ	3AMP	(3AMP)(MA0.75FA0.25)3Pb4I13	NA/12.04	22%, 50%-70% RH for 47.5 h;	[145]
				Stable, store in N2 for 2760 h
DJ	CHDA	(CHDA)MA3Pb4I13	NA/15.01	80.7%, MPP for 270 min;	[157]
				96.5%, 60 C for 68 h;
				74.4%, 70 C for 68 h
DJ	BDA	BDAMA3Pb4I13	5.19/12.81	60%, ambient condition for 23 d	[158]
DJ	BDA	BDAMA4Pb5I16	NA/14.53	85%, 50 5% for 900 h	[159]
DJ	PDMA	PDMAMAn-1PbnI3n+1	13.73/15.81	93%, ambient condition for 744 h	[141]
DJ	4F-PhDMA	(4F-PhDMA)(MA)n-1PbnI3n+1	10.11/16.62	Over 93%, store in N2 for 1839 h;	[144]
				94%, continuous light soaking for 186 h;
				Over 94%, 80 C for 350 h
DJ	ThDMA	(ThDMA)(MA)n-1PbnI3n+1	6.11/15.75	Over 95%, store in N2 for 1655 h	[160]
ACI	GA	(GA)(MA)nPbnI3n+1	12.91/14.69	88%, ambient condition for 240 d	[142]
		(n = 3)
ACI	GA	(GA)(MA)nPbnI3n+1	15.96/19.18	95%, ambient condition for 123 d;	[147]
		(n = 5)
				80%, 80 C for 60 h
ACI	GA	GAMA3Pb3I10	NA/7.26		[161]
		(n = 3)
ACI	GA	GAMA4Pb4I13	8.62/15.27	75%, 50% RH for 480 h	[162]
		(n = 4)

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A large number of research works have focused on the RP structured PSCs [148-153]. Ren et al demonstrated that a sulfur-sulfur interaction was presented for a new bulky alkylammonium, 2-(methylthio)ethylamine hydrochloride (MTEACl) (figure 14(a)) [154]. The interaction between sulfur atoms in two MTEA molecules enabled the (MTEA)₂(MA)₄Pb₅I₁₆ (n = 5) perovskite framework with enhanced charge transport and stability. The PSC obtained 18.06% PCE, better moisture tolerance (1512 h under 70% humidity conditions), higher thermal stability (375 h at 85 C) and operational stability under continuous illumination (85% of the initial efficiency retained over 1000 h). Liang et al fabricated phase-pure QWs by introducing the molten salt spacer n-butylamine acetate (figure 14(b)) [155]. The high-quality phase-pure QW perovskite films in the 2D RP structure could be obtained by the strong ionic coordination between n-butylamine acetate and the perovskite framework. They obtained the PSCs with a PCE of 16.25% (n = 4). The device was stable under 65 10% humidity for 4680 h, heating at 85 C for 558 h) and light illumination for 1100 h. Dong et al introduced the 2-thiopheneformamidinium (ThFA) as the organic spacer to prepare the 2D (ThFA)₂(MA)_n-1Pb_nI_3n+1 (figure 14(c)) [156]. The 2D perovskite showed preferential vertical growth orientations, high charge carrier mobilities, and reduced trap density. The 2D RP device exhibited a high PCE of 16.72% with a low n-value of 3. The device also presented improved stability with less than 1% degradation after storing in N₂ for 3000 h.

Figure  14.  Dimensional engineering in perovskite. (a) Schematic crystal structures of the two-dimensional (2D) Ruddlesden-Popper (RP) perovskites (MTEA)₂(MA)_n-1Pb_nI_3n+1 and (BA)₂(MA)_n-1Pb_nI_3n+1. Reproduced from [154], with permission from Springer Nature. (b) Schematic structure of the phase-pure quantum well (QW) film prepared by n-butylamine acetate. Reproduced from [155], with permission from Springer Nature. (c) Schematic structure of (ThFA)₂MA₂Pb₃I₁₀ and the fabrication process of ThFA-based 2D perovskite films. [156] John Wiley & Sons. [© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. Copyright 2020, Wiley. (d) Crystal structure of the 2D DJ perovskite (PDMA)(MA)_n-1Pb_nI_3n+1 (n = 4). [141] John Wiley & Sons. [© 2021 Wiley-VCH GmbH]. (e) Chemical structures of PhDMA, 4F-PhDMA, the crystal structure of the (4F-PhDMA)PbI₄ perovskite and optimized structures of the 4F-PhDMA-Pb 2D perovskite by DFT calculations. Reprinted with permission from [144]. Copyright (2021) American Chemical Society. (f) Schematic crystal structure of 2D DJ (ThDMA)(MA)₄Pb₅I₁₆. Reprinted with permission from [160]. Copyright (2020) American Chemical Society.

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DJ structured perovskites have been developed to avoid the van der waals gap and enhance the carrier transport between the inorganic sheets [145, 157-159]. Zhang et al developed a DJ structured perovskite with the composition of (PDMA)(MA)_n-1Pb_nI_3n+1 (n = 4, PDMA refers to 1,4-phenylenedimethanammonium) (figure 14(d)) [141]. They found that the uniform thickness distribution of QWs could be obtained by the hot-casting or antisolvent processes. Using the hot-casting method, they prepared a DJ device with PCE reaching 15.81%. The thermal and humidity stabilities of the 2D perovskites were extremely enhanced compared with the 3D MAPbI₃ perovskite. Lv et al introduced a multifluorinated aromatic spacer, namely 4F-PhDMA, into the 2D DJ perovskite (figure 14(e)) [144]. The DJ perovskite with 4F-PhDMA spacer exhibited higher dissociation energy compared with the PhDMA because of the multiple noncovalent interactions such as NH···I and CH···F hydrogen-bonding and F···I electrostatic interactions. By incorporating the 4F-PhDMA organic spacer, they obtained the device (n = 4) with a PCE of 16.62%, advanced storage stability (>93% after storing in N₂ for 1839 h), operational stability (94% under continuous-light illumination for over 700 h) and heating stability (>94% under 80 C after 350 h). Di et al synthesized a thiophene-based bulky dication spacer, namely 2,5-thiophenedimethylammonium (ThDMA), to fabricate the high-quality 2D DJ perovskite (ThDMA)MA_n-1Pb_nI_3n+1 (nominal n = 5) (figure 14(f)) [160]. With the strong coordination molecule DMSO, The crystal growth and orientation of the 2D DJ perovskites could be enhanced, enabled by strong coordination between ThDMA and DMSO. A high PCE of 15.75% was achieved and the DJ device exhibited much better storage stability, light soaking stability and thermal stability than the 3D counterparts.

ACI structured 2D perovskite was firstly reported by Soe et al in 2017 ((GA)MA_nPb_nI_3n+1 (GA = guanidinium, MA = methylam-monium)) [161], and has become one of the hotspots in 2D perovskites [142, 147, 162]. Zhang et al carried out in situ studies on the solidification processes of ACI 2D perovskite (n = 3) by using in situ grazing-incidence x-ray scattering (GIWAXS) [142]. They found that the intermediate phases, e.g. 2D GA₂PbI₄ perovskite, provided a scaffold for the growth of the ACI perovskites during thermal annealing. Yang et al tailored the crystallization process of ACI perovskite (i.e. (GA)MA_nPb_nI_3n+1) via solvent engineering to achieve preferential QW distribution and improve the quality of perovskite films. The PSC obtained a high PCE of 19.18% and high environmental stability [147].

3.2.2   Strain engineering.

Tensile strain in perovskite films is an important source of instability. Many works have been reported for regulating the strain in perovskite devices, such as interfacial modification [163], doping [60, 71] and employment of suitable transport materials [73]. Dou et al introduced an ultrathin Eu-MOF layer between the electron transport layer and perovskite layer. The tensile strain in perovskite film was successfully converted into the compressive strain (figures 15(a) and (b)) [164]. The devices retained 96% of their original PCE after 2000 h under a relative humidity (RH) of 30% and 91% of the original PCE after 1200 h continuous heating at 85 C in N₂. Kim et al substituted Cs and methylenediammonium (MDA) cations in FAPbI₃ films (figure 15(c)) [60]. They found that adding a 0.03 mol fraction of both MDA and Cs cations could lower the lattice strain. The PSC reached a high PCE of 24.4% and maintained over 80% of the initial PCE after heating at 85 C for 1300 h. Zhang et al regulated the strain by a crosslinking-enabled strain-regulating crystallization (CSRC) method (figure 15(d)) [165]. A suitable concentration of the trimethylolpropane triacrylate (TMTA) was used to convert a tensile strain into strain-free perovskite film. The device with TMTA retained 80% of the initial PCE after light soaking for 1248 h (figure 15(e)). Wang et al used OAI as the A-site cation to release the interfacial stress (figure 15(f)) [166]. Soft structural subunits were realized and a bone-joint’ configuration was constructed at the interface between the absorber and the carrier transport layer. The treatment of OAI led to improved humidity and thermal stability (figure 15(g)).

Figure  15.  Strain engineering in perovskite. (a) Schematic illustration of the Eu-MOF modified perovskite film. (b) Residual strain distribution in the depth of 500 nm for the tensile-strained film. [164] John Wiley & Sons. [© 2021 Wiley-VCH GmbH]. (c) Residual strain in perovskites deposited on FTO/mp-TiO₂ substrates. From [60]. Reprinted with permission from AAAS (d) The schematic representation of the tensile strain state of the control perovskite film regulated by the CSRC method with TMTA in different concentrations. (e) Stability of the device under irradiation of a white-light-emitting diode array with 0.8 sun equivalent illumination, AM 1.5G. [165] John Wiley & Sons. [© 2021 Wiley-VCH GmbH]. (f) Schematic describing the residual stress relaxation. (g) The long-term stability of perovskite solar cells stored in air with humidity of 16%-50% for over 1000 h without encapsulation. [166] John Wiley & Sons. [© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim].

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3.2.3   Defects passivation.

Polycrystalline perovskite films exhibit a variety of defects, i.e. positively charged cationic defects (under-coordinated Pb²⁺, and halide vacancies), negatively charged anionic defects (cation vacancies, Pb-I antisites and halide-excess) and metallic lead (Pb⁰) defects, etc. These defects are ion migration channels and cause instability of the PSCs. In this section, we summarized the recent defect passivation methods for improving the stability of the PSCs.

Lewis acid is an electron-pair acceptor which can accept a foreign electron pair [167]. Besides the anions, the organic molecule with the Lewis-acid functional group, such as TPFPB [168], PCBB-3N-3I [169], TPFP [170], iodine-terminated self-assembled monolayer (I-SAM) [171], I-PEA [172], TMOS [173], PFTS [174], etc, could also be introduced to passivate the halide-vacancy and Pb-I antisite defects in perovskite and enhance the stability of the PSCs. The performance of perovskite device based on the Lewis acid-base passivation is summarized in table 5. Fu et al introduced a halogen-halogen bond at the grain boundaries of perovskite to suppress the ion migration and the phase separation (figure 16(a)) [172]. They found that the halogen atom with a positively charged hole acts as an electron acceptor (Lewis acid) and forms strong halogen-halogen bonds with the electron-rich halide anions (Lewis base). The binding energy of the halogen-halogen bond is higher than that of the hydrogen bond (figure 16(b)). The halogen-halogen bond-containing CsMAFAPb(I_xBr_1-x)₃ perovskite films enabled the encapsulated device to retain 90% of initial PCE after MPP tracking for 500 h. Dai et al found that the I-SAM could be used as the Lewis acid to form the electrostatic bonds with the perovskite films (figure 16(e)) [171]. Treatment of the buried interface with the I-SAM significantly suppressed the point defects in perovskite, leading to improved solar cell performance. The T₈₀ of the device reached 4000 h under 1-sun illumination with MPP tracking, benefiting from reduced ion migration.

Figure  16.  Defect passivation with Lewis acid and Lewis base additives. (a) Schematic of halogen-halogen bonds between halogenated molecules and halide anions and the electrostatic potential of different halogenated compounds (X-FEA, X=F, Cl, Br, I). (b) Charge transfer density difference plots from DFT calculations. Bonding between PbI₂-terminated -FAPbI₃ (001) surface and H(CH₂)₃H (H-SAM’) or H(CH₂)₃I (I-SAM’), dashed lines across the interface indicate no bonding. Reprinted from [172], Copyright (2021), with permission from Elsevier. (c) Surface-defect identification and constructive configuration of the C=O group in three different chemical environments. From [176]. Reprinted with permission from AAAS (d) Illustration of polymerization-assisted grain growth (PAGG) process. [177] John Wiley & Sons. [© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. (e) Binding energy between I-FEA and external halide anions (Br^- and I^-), and binding energy of different hydrogen bonds. From [171]. Reprinted with permission from AAAS (f) Stability test of the reference and 2-MP passivated devices stored in ambient air with a relative humidity of 60%-70% at room temperature. Inset is the molecular structure of passivating materials and schematic illustration of their interaction with metal ions. [183]. John Wiley & Sons. [© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. (g) Schematic illustration of the formation of zwitterion on SnO₂ layer. Reproduced from [193] with permission from the Royal Society of Chemistry.

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Table  5.  The performance and stability of perovskite solar cells by Lewis acid/base passivation.

	Name	Perovskite components	PCE [%] (before/after)	Stability	References
Lewis acid	PCBB-3N-3I	MAPbI3	17.7/21.1	83%, ambient condition (40%-50% RH) for 940 h	[169]
				62%, 75%-85% RH for 500 h
Lewis acid	TPFP	Cs0.05FA0.8MA0.15	18.05/22.02	63%, 75% RH for 14 d (Unencapsulated)	[170]
		Pb(I0.83Br0.17)3		80%, 85% RH for 14 d (encapsulated)
Lewis acid	Si(OCH3)3(CH2)3I	Cs0.05(FA0.85MA0.15)0.95	20.15/21.44	T80 4000 h, 1-sun MPP	[171]
		Pb(I0.85Br0.15)3
Lewis acid	I-PEA	Cs0.12MA0.2FA0.68	NA/19.19	90%, MPP for 500 h (encapsulated)	[172]
		Pb(I0.78Br0.22)3
Lewis acid	TMOS	FAMAPbI3(Cl)	20.96/22.49	91.4%, ambient condition for 30 d	[173]
Lewis acid	PFTS	Cs0.05(FA0.85MA0.15)0.95	20.167/21.34	90%, 70% RH for 115 d	[174]
		PbI2.55Br0.45		88%, 85 C for 500 h
				90%, 1-sun MPP for 150 h
Lewis base	Theophylline	(FAPbI3)x	21.02/23.48	Over 95%, ambient condition for 60 d	[176]
		(MAPbBr3)1-x		Over 80%, continuous light and open-circuit condition for 500 h
Lewis base	Dimethyl itaconate	FA1-xMAxPbI3	20.9/23	85.7%, continuous illumination for 504 h	[177]
				91.8%, ambient condition for 2208 h
Lewis base	PMMA	MAPbI3	12.16/16.32	95%, ambient condition for 33 d	[175]
Lewis base	FO-19	MAPbI3	19.14/21.23	73%, 80 C for 600 h	[178]
				78%, dry condition for 3000 h
Lewis base	TMTA	MAPbI3	19.08/20.22	92.3%, air condition for 1000 h	[179]
				Over 98%, 85 C in N2 for 930 h
				78.5%, MPP for 400 h
Lewis base	Caffeine	MAPbI3	17.59/20.25	Over 85%, 85 C for 1300 h	[180]
Lewis base	Capsaicin	MAPbI3	19.16/21.88	90%, ambient condition (45% RH) for 800 h	[181]
Lewis base	TPT-P6	Cs0.05MA0.12FA0.83	NA/21.43	90%, ambient condition for 3 months	[182]
		Pb(I0.85Br0.15)3
Lewis base	2-MP	MAPbI3	18.35/20.28	93%, 60%-70% RH for 60 d	[183]
Lewis base	PHMT	MAPbI3	18.11/21.11	Over 89%, 85 C for 500 h	[185]
				86%, continuous illumination for 600 h
Lewis base	Poly(TA)	MAPbI3	17.4/20.4	98%, UV illumination for 450 min	[186]
				97%, 10 5% RH for 2160 h
				92%, MPP for 600 h
Lewis base	MMI	MAPbI3	NA/20.1	Over 94%, store in N2 for 2184 h	[187]
				80%, continuous illumination for 672 h
Lewis base	PFA	MAPbI3	19.53/21.31	97%, 70 5% RH for 2500 h	[188]
Lewis base	D4TBP	Cs0.05FA0.81MA0.14PbI2.55Br0.45	19.7/21.4		[189]
Lewis base	POSS-NH2	MAPbI3	NA/20.5	Over 85%, continuous illumination for 600 h	[190]
Zwitterion	PPP	CsMAFA triple-cation	18.62/22.11	93%, 40% RH for 6000 h;	[192]
				Stable, 1-sun MPP at 45 C for 1000 h;
				91%, continuous illumination for 1000 h;
Zwitterion	3-(1-pyridinio)-1-propanesulfonate	FAMAPb(I, Br)3	19.63/21.43	80%, 150 C for 60 min	[193]
				93%, 85% RH and 85 C for 140 h
Zwitterion	CsCF3SO3	FACsPbI3	NA/22.06	92.9%, continuous illumination for 1000 h	[191]
				93%, 85 C in N2 for 1000 h
Zwitterion	DPSI	FA0.85MA0.15	19.1/21.1	88%, continuous illumination for 480 h	[194]
		Pb(I0.85Br0.15)3		96%, continuous illumination in 30%-70% RH for 60 h

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Lewis base is an electron-pair donor. It may provide the self-electron pair and coordinate with the Lewis acid type of defects (i.e. interstitial Pb²⁺ and under-coordinated Pb²⁺ defects) [167]. Organic molecules with the Lewis base functional groups, such as O-donor [175-182], S-donor [183-187], N-donor [188-190], have been widely used as Lewis acid defect passivators. Wang et al reported that the C=O and N-H groups in the theophylline, caffeine and theobromine could act as the Lewis base to passivate the antisite Pb defect to maximize surface-defect binding (figure 16(c)) [176]. They obtained the stabilized PCE of 22.6% with theophylline modified perovskite (FAPbI₃)_x(MAPbBr₃)_1-x. The encapsulated device maintained over 90% of its initial efficiency under continuous light at the open-circuit condition for 500 h. Zhao et al found that dimethyl itaconate (DI) with C=C and C=O functional groups could interact with PbI₂ while the polymerization triggered by the annealing process (figure 16(d)) [177]. The polymerization would be adhered to the grain boundaries and passivate the under-coordinated Pb²⁺ defects. The FA_1-xMA_xPbI₃ PSCs obtained 23% PCE and prolonged lifetimes when 85.7% and 91.8% of the initial PCEs remained after 504 h continuous illumination and 2208 h shelf storage. Zhang et al introduced the organic molecule with the S, N functional group to combine with Pb²⁺ in the perovskite film (figure 16(f)) [183]. The PCE of the 2-MP-passivated device achieved 20.28% efficiency. The unencapsulated device retained 93% of the initial efficiency under a RH of 60%-70% for 60 d.

Furthermore, the zwitterion with the Lewis acid and base functional groups showed an excellent bilateral effect by simultaneously passivating negatively and positively charged defects [167, 191-194]. Cao et al reported the employment of a star-shaped polymer at the perovskite interface to improve charge transport and inhibit ion migration [192]. The polymer with the Lewis acid functional group -CF₃ and the Lewis base functional group C=O formed the strong hydrogen bond of FH-N between the polymer and FA⁺ or MA^+, and the coordination between C=O and Pb²⁺. The crystallization process of perovskite film was well controlled and the films were prepared with lower trap density and higher carrier mobility. They obtained the device with a PCE of 22.1% and a FF of 0.862. The device exhibited excellent environmental stability, long-term operational stability and thermal stability. Choi et al employed the zwitterionic, 3-(1-pyridinio)-1-propanesulfonate, to modify the interface between SnO₂ and perovskite (figure 16(g)) [193]. The zwitterion at the interface could passivate the Pb-I antisite defects and enhance the electron transport ability of SnO₂. Finally, the device showed a PCE of 21.43% and excellent stability under 85 C, 85% RH.

Besides the Lewis-acid and base salts, organic ammonium salts have been widely used to simultaneously passivate the cation and anion defects by doping or interfacial modification. The role of the organic ammonium salts on the perovskite films mainly consists of the following aspects: (I) short-chain organic ammonium salts could passivate the defects, inhibit the ion migration and reduce leakage current loss through forming the wide-bandgap low-dimensional perovskite [195-204]; (II) long-chain organic ammonium salts could improve energy level structure, optimize the growth orientation of the perovskite films and enhance the humidity and thermal stability [205-209]. The performance and stability of PSCs by organic-ammonium salts passivation has been summarized in table 6.

Table  6.  The performance and stability of perovskite solar cells by organic-ammonium salts passivation.

Name		Perovskite components	PCE [%] (before/after)	Stability	References
CYCl		FAPbI3	-/24.98	91%, store in dark for 1300 h	[195]
VBABr		(MAPbBr3)0.15	18.2/20.2	90%, store in dark for 2300 h	[201]
		(FAPbI3)0.85
PI		Cs0.05(FA0.95MA0.05)0.95	20.76/23.37	93%, 55 5% RH for 1000 h (encapsulated)	[199]
		Pb(I0.95Br0.05)3		Over 98%, continuous illumination for 200 h (encapsulated)
BA2PbI4	BA:	(FAPbI3)0.95	22.52/24.63	94%, 85% RH and 85 C for 1056 h (encapsulated)	[202]
		(MAPbBr3)0.05		98%, full-sun illumination for 1620 h (encapsulated)
HAI		CsFAMA	18.83/20.62	Stable, ambient condition (55%-75% RH) for 45 d	[196]
		Pb(I, Br)3		80%, 85 C for 600 h
PhFACl		FAMAPbI3(Cl)	19.02/22.39	95%, ambient condition (30%-40% RH)	[198]
				87%, 80 C for 1480 h
CH3O-PEAI		(FAPbI3)1-x	19.98/22.98	Stable, ambient condition for 1000 h	[200]
		(MAPbBr3-yCly)x		89%, continuous illumination in N2 for 300 h
PREA		(FAPbI3)x	20.1/23	85%, continuous light under open-circuit	[203]
		(MAPbBr3)1-x		Conditions for 2000 h
EATZI		MAPbI3	16.13/20.03	80%, 40 5% RH for 3500 h;	[204]
PEAI		FA1-xMAxPbI3	-/23.32	Stable, MPP for 40 h	[210]
				80%, 85 C for 500 h
HTAB		(FAPbI3)0.95	15.8/22.8	80%, 85% RH for 1008 h	[205]
		(MAPbBr3)0.05		95%, MPP for 1370 h
OAm		Cs0.05(FA0.92MA0.08)0.95	-/23	Stable, MPP for 1000 h	[207]
		Pb(I0.92Br0.08)3
OAI		MAPbI3	18.4/20.6	80%, 85 C for 760 h	[206]
				90%, 65% RH for 430 h
CTAB		FA1-xMAxPbI3(Cl)	20.58/22.03	72%, MPP for 360 h	[208]
				78%, 66% RH for 110 h
OA		MAPbI3	17.4/20.7	94%, ambient condition (50 5% RH) for 1000 h	[209]
				96%, continuous illumination for 500 h
Choline chloride		FA0.83MA0.17Pb(I0.83Br0.17)3	12.6/17.2	Stable, ambient condition for 1 month	[211]
				86%, MPP for 26 h

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Short-chain organic ammonium salts were employed to passivate the surface defects and form the 2D wide-bandgap perovskite layer on the surface of 3D perovskite layer. Jiang et al found that the organic molecule phenethylammonium iodide (PEAI) could serve as a much more effective passivator than the traditional PEA₂PbI₄ for the FA-MA mixed perovskite films through the non-annealing process (figure 17(a)) [210]. The device obtained a certificated efficiency of 23.32% and a high V_oc of 1.18 V. Besides, the device was stable under 85 C for 500 h and continuous light soaking at the MPP (25 C, 100 mW cm^-2) for over 40 h. Proppe et al introduced the ligand 4-vinylbenzylammonium to form 2D perovskite quantum wells (PQWs) on the 3D perovskite layer (figure 17(b)) [201]. Especially, the vinyl group of 4-vinylbenzylammonium could be activated using 254 nm UV light to form new covalent bonds in the 2D PQWs. Based on the UV-cross-linked 2D/3D structure, they obtained the champion PCE of 20.4%. The device retained 90% of its initial efficiency after aging in dark for 2300 h and 75% after operating for 16 h. Jang et al employed a new method to prepare the 2D/3D heterojunction (figure 17(c)) [202]. They fabricated the perovskite films by growing a highly crystalline 2D (C₄H₉NH₃)₂PbI₄ (n = 1) film on top of the 3D film without the quasi-2D phase using a solvent-free solid-phase in-plane growth (SIG) method. The perovskite films demonstrated a prolonged carrier lifetime, well-passivated surface defects and enhanced build-in potential. They obtained the device with a high PCE of 24.35%. The encapsulated device retained 94% of its initial efficiency after 1056 h under the 85 C/85% RH condition and 98% after 1620 h under full-sun illumination.

Figure  17.  Defect passivation with organic ammonium salts. (a) Device structure, possible passivation mechanism and states of PEAI on the perovskite surface. Reproduced from [210], with permission from Springer Nature. (b) Schematic illustration of cross-linked 2D perovskites atop a 3D majority layer by using the ligand 4-vinylbenzylammonium bromide (VBABr). Reprinted with permission from [201]. Copyright (2019) American Chemical Society. (c) Top-view and cross-sectional sketches of the manufacturing of a (BA)₂PbI₄ film on a 3D perovskite substrate via the solid-state in-plane growth (SIG) method. The formation of crystal seeds and the in-plane grain growth are indicated with black dashed lines and black arrows, respectively, in the cross-sectional representation of the perovskite layer. Reproduced from [202], with permission from Springer Nature. (d) Left, the structure of an n-i-p perovskite solar cell based on a double-layered halide architecture (DHA) using P3HT as the hole-transport material. Right, schematic structure of the interface between the wide-bandgap halide (WBH) and P3HT. Reproduced from [205], with permission from Springer Nature. (e) Illustration of the influence of the short-chain surface-anchoring alkylamine ligands (AALs) and long-chain AALs on the crystallization of the perovskite films. (f) Illustration of long-chain AALs assembled on the perovskite film surface and blocking the holes at the perovskite and C₆₀ interfaces. Reproduced from [207], with permission from Springer Nature. (g) Device structure and passivation mechanism by quaternary ammonium halides. Reproduced from [211], with permission from Springer Nature.

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Long-chain organic ammonium salts were also used to passivate the defects and enhance the device stability, especially the humidity stability. Generally, the ability of electron blocking and humidity resistance characteristics is different for the organic ammonium salts with different chain lengths. Jung et al proposed a new device architecture for the high-performance P3HT-based PSCs (figure 17(d)) [205]. The wide-bandgap perovskite layer could be formed with the reaction of n-hexyl trimethyl ammonium bromide (HTAB). The long alkyl chain (C₆H₁₃^-) in HTAB could form favorable van der Waals interactions between the P3HT and the perovskite layer and the functionalized moiety (N⁺(CH₃)₃^-) in HTAB molecule could passivate the charge traps on the perovskite surface. As a result, they obtained a certified PCE of 22.7%. The device exhibited good stability at 85% RH without encapsulation, and excellent long-term operational stability by maintaining 95% of its initial efficiency under 1-sun illumination at room temperature for 1370 h with encapsulation. Zheng et al incorporated a trace amount of surface-anchoring alkylamine ligands (AALs) with different chain lengths into the perovskite precursor solution to passivate the defects and optimize the crystal orientation (figure 17(e)) [207]. They found that the perovskite films with the low concentration of surface-ALLs doping exhibited a prominent (100) orientation and the alkylamine ligands could act as the AALs to optimize the carrier transfer (figure 17(f)). They obtained the device with 23% PCE and with operational stability of over 1000 h. Zheng et al showed that the quaternary ammonium halides could passivate the ionic defects more effectively compared with PCBM in various perovskite components (figure 17(g)) [211]. This not only reduced the V_oc loss to 0.39 V but also boosted the PCE to 20.59 0.45%. The device modified with choline chloride was stable for over 35 d in air and maintain 86% of the initial PCE under 1 sun continuous illumination for 26 h.

Besides the aforementioned passivators, other passivation methods have also been employed to improve the stability of the PSCs [212, 213]. As shown in figure 18(a), Lin et al incorporated the ion liquid, i.e. 1-butyl-1-methylpiperidinium tetrafluoroborate (BMPBF₄), into the perovskite absorber to suppress compositional segregation and reduce the density of deep trap sites [214]. The champion device with 0.25% BMPBF₄ modified Cs_0.17FA_0.83Pb(I_0.9Br_0.1)₃ exhibited a PCE of 20.1%. The unencapsulated devices retained 80% of their peak steady-state power output (SPO) for 1010 h at 60 C and 95% of the post-burn-in efficiency (T_95,ave) for 1200 h at 85 C, respectively (figure 18(b)). Furthermore, the ion liquid, i.e. 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄) was also incorporated by their group to suppress the degradation of the PSCs (figures 18(c)-(e)) [215].

Figure  18.  Defect passivation with novel strategies. (a) Schematic of the p-i-n structured perovskite solar cell and the chemical structure of [BMP]⁺[BF₄]^-. (b) Evolution of SPOs of encapsulated Cs_0.17FA_0.83Pb(I_0.77Br_0.23)₃ cells aged under full-spectrum sunlight at 85 C in ambient air (six cells for each condition). From [214]. Reprinted with permission from AAAS. (c) Architecture of the planar heterojunction p-i-n structured perovskite solar cell. (d) Chemical structure of the ionic liquid BMIMBF₄. (e) Long-term stability of the most stable device with BMIMBF₄ treatment under full-spectrum sunlight and heat stressing at 70 C-75 C. Reproduced from [215], with permission from Springer Nature. (f) Diagram and mechanism of the liquid medium annealing (LMA) process. From [216]. Reprinted with permission from AAAS. (g) Schematic of peeling an adhesive tape off a MAPbI₃ film. (h) Photographs of MAPbI₃ films after light soaking for different time intervals. The right half of the film was treated with adhesive tape. Reprinted from [217], Copyright (2020), with permission from Elsevier.

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Li et al developed a liquid medium annealing (LMA) method to create a robust chemical environment-anisole for the crystal growth during the annealing process (figure 18(f)) [216]. They successfully prepared the perovskite films with high crystallinity, fewer defects and desired stoichiometry. The PSCs showed a PCE of 24.04% and 23.15% over areas of 0.08 cm² and 1 cm², respectively. The operational stability of the device was also extremely enhanced. Huang’s group developed the adhesive tape treatment method (figure 18(g)) [217] and the polish method [218] to remove the defective surface layers. As shown in figure 18(h), the right tape-treated part remained black after 8 h of light soaking, while the left part without tape-treatment already decomposed into yellow phases in less than 4 h. The tape-treated device retained 97.1% of its initial PCE after operation near MPP under 1-sun illumination for 1440 h at 65 C.

3.2.4   Phase stabilization.

FAPbI₃ is the most popular component for high-efficiency PSCs because of its suitable bandgap. However, the phase instability issue due to the ionic radius mismatch would inhibit the development of stable FAPbI₃ PSCs. Many methods, such as additive engineering, interfacial engineering, solvent engineering, etc, have been reported to stabilize the black-phase FAPbI₃ perovskite films [60, 61, 165, 219-224]. Kim et al doped the methylammonium chloride (MACl) into the FAPbI₃ precursor solution to stabilize the -phase FAPbI₃ (figure 19(a)) [225]. They found that the MA⁺ and Cl^- in MACl could be incorporated into the lattice of FAPbI₃ to reduce the formation energy -phase FAPbI₃ and enhance the crystallinity of FAPbI₃ (figure 19(b)). The optimized PSCs achieved a certified efficiency of 23.48% and the ideal stability. Xie et al used the MACl to modify the surface of FAPbI₃ films and assist the vertical recrystallization [226]. The FA-based device with the low MA content showed 20.65% PCE and 500 h thermal stability. Lu et al reported a methylammonium thiocyanate (MASCN) vapor treatment method to stabilize the -phase FAPbI₃ [227]. The phase transition temperature was decreased with the treatment of MASCN (figure 19(c)). Yoo et al incorporated the MAPbBr₃ and MACl into the precursor solution to stabilize the FAPbI₃ (figure 19(d)) [134]. The MAPbBr₃ additive not only enhanced the crystallinity but also increased the effective mobility of FAPbI₃. Based on the CBD processed SnO₂ and the passivation strategy, they obtained the 25.2% certified PCE (figure 19(e)) and excellent storage and illumination stability. Park et al used the isopropylammonium cations (iPAmH⁺) to stabilize the FAPbI₃ (figure 19(f)) [228]. They found that iPAmH⁺ appeared by the chemical reaction between isopropyl alcohol (IPA) and MACl. The device with iPAmH⁺ exhibited a PCE of 23.9% and better long-term operation stability.

Figure  19.  Phase stabilization in perovskite. (a) Schematic diagram of MACl-assisted formation of perovskite films. (b) Formation energies of bare -phase FAPbI₃ perovskite structure and that with Cl, MA, or MACl. Reprinted from [225], Copyright (2019), with permission from Elsevier. (c) Schematic diagram of vapor-treated FAPbI₃ films. From [227]. Reprinted with permission from AAAS. (d) X-ray diffraction of perovskite thin films with four different amounts of MAPbBr₃. (e) J-V curves of the champion device measured at Newport, showing both the conventional J-V sweep and the certified quasi-steady-state measurements. Reproduced from [134], with permission from Springer Nature. (f) Simulated formation energies of -FAPbI₃ and -FAPbI₃ crystals in the bulk and thin-film phases. Reproduced from [228], with permission from Springer Nature. (g) Images of PbI₂@MAFa and PbI₂@DMF: DMSO solutions and schematic diagram of interactions in the solutions. From [229]. Reprinted with permission from AAAS. (h) Schematic illustrations of the device structure, molecular structure of the 1-hexyl-3-methylimidazolium (HMI⁺) and the effects of HMI⁺ in FAPbI₃ active layer. [230] John Wiley & Sons. [© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. Reprinted from [225], Copyright (2019), with permission from Elsevier. Reproduced from [134], with permission from Springer Nature.

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Ionic liquid was widely used to enhance the crystallinity and stabilize the FAPbI₃. Hui et al used a novel solvent (i.e. methylamine formate (MAFa) ionic liquid) to fabricate the stable -FAPbI₃ (figure 19(g)) [229]. The MAFa exhibited strong interactions with PbI₂ through C=OPb and N-HI, which promoted the vertical growth of the FAPbI₃ crystals. The device showed the champion PCE of 24.1% and retained 80% of its initial PCE at 85 C for 500 h. Akin et al employed the 1-hexyl-3-methylimidazolium iodide (HMII) ionic liquid as the additive to solve the phase instability of FAPbI₃ (figure 19(h)) [230]. They found that the HMII could increase the grain size and reduce the activation energy of the grain-boundary migration. The FAPbI₃ device with the HMII additive achieved a PCE of 20.6% and maintained 80% and 95% of its initial PCE at 60 10% RH and 65 C, respectively.

4.   Conclusion and outlook

Efficiency, cost and lifetime are the solar cell performance golden triangle [231]. Until now, the certified record PCE of PSCs has reached 25.5%. The energy payback time of PSCs is estimated to be 2-3 months [232]. The lifetime becomes the key obstacle hindering technology commercialization. In this review, we reviewed the origin of instability of the solution-processed PSCs from the perspectives of the precursor solutions (i.e. solute and solvent) and the perovskite films (i.e. composition, strain, defect and phase). In addition, we summarized the recent strategies for improving stability of the perovskite films and solar cells, including perovskite precursor solution advancement, perovskite composition regulation, strain engineering, defect passivation and phase stabilization. The perspective for the stable PSCs was depicted in figure 20. Furthermore, we proposed the following strategies that may hold the key to further enhancing the stability of perovskite films and devices. Exploration of novel reductants in perovskite precursors to inhibit the oxidization of I^-. One of the key degradation pathways of the perovskite precursor solution is the oxidization of I^-. The formation of I₂ not only affects the reproductivity of the high-quality perovskite films but also deteriorates the long-term stability of the perovskite devices. The development of novel reductants in the precursors which effectively inhibit the oxidization of I^- will prolong the durability and guarantee period of the perovskite precursor solution. This is a prerequisite when solution-processed perovskites are to be considered at an industrial-relevant manufacturing scale.Regulation of the crystallization kinetics for the preparation of high-crystallinity (or single-crystalline-like) perovskite films with low defect densities and long carrier lifetime. Crystallinity and defect in perovskite semiconductors play a vital role in solar cell performance and stability [32]. The high density of defects and short carrier lifetimes usually exist in the perovskite films with low crystallinity, resulting in large electric loss and severe ion migration. The formation of defects (i.e. type, density and location) is determined by the crystallization kinetics. From this viewpoint, fundamental understanding of crystallization kinetics and defect chemistry allows the development of advanced perovskite precursor solutions and the fabrication of high-crystallinity perovskite films.Employment of suitable A-site cations for enabling quasi-2D perovskites with optimized crystal growth orientation and enhanced carrier transport. Quasi-2D structured perovskites have presented higher stability than 3D perovskites but unsatisfactory carrier transport characteristics. Previous studies showed that A-site cations affect the growth orientations of perovskites, which determine carrier transport and stability of the device. Understanding the effect of structures of A-site cations (e.g. length of the hydrophobicity chains and the functional groups) on the perovskite growth orientation, carrier and ion transport kinetics lay the foundation for high-efficiency and stable quasi-2D PSCs.Advancement of other functional layers (i.e. transport layers and electrodes) which demonstrate high intrinsic robustness and are chemically inert with the perovskite layer. Transport layers and electrodes are important components in PSCs and influence the long-term stability of PSCs. The mostly investigated hole transport materials, e.g. spiro-OMeTAD, are unstable under heating or moisture [233]. The electron transport materials, e.g. TiO₂, promote the decomposition of perovskite films under light illumination [234]. The Ag and Au electrodes react with the mobile halide ions, causing irreversible device degradation [91]. Therefore, designing dopant-free charge transport layers (e.g. inorganic semiconductors and conductive polymers) with high-temperature and humidity resistance and tailoring the interfaces between perovskite and charge transport layers may promote the stability of PSCs [235, 236]. Furthermore, the employment of stable electrodes (i.e. transparent conductive oxides and carbon) or the construction of double-layer metal electrodes (i.e. Bi/Au and ITO/Au) avoid the detrimental metal halide reactions.Development of advanced encapsulation methods to avoid the ingress of oxygen and moisture, and outgassing of perovskite degradation gaseous species. External encapsulations are key components of commercial photovoltaics such as silicon, copper indium gallium selenide and CdTe solar cells. The degradation pathways of PSCs induced by external stimuli (moisture, heat and oxygen) have been well-documented, and can be mitigated by reliable external encapsulation methods [237-239]. Several encapsulants (e.g. epoxy [240] and polyisobutylene [237]) and encapsulation strategies (e.g. edge sealing and blanket encapsulation [53]) have shown promising results under independent accelerated stress conditions including continuous light-soaking, damp heat and thermal cycling. In future, exploration of advanced encapsulation methods of PSCs is to be investigated, aiming to pass harsher combination stresses such as simultaneous light, temperature and RH [33].

Figure  20.  Perspectives for obtaining stable PSCs.

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