Role of Ionic Liquids in Perovskite Solar Cells

Although the power conversion efficiency of perovskite solar cells (PSCs) has reached 25.7%, there is still great potential for improvement in their performance and stability. In the past few years, ionic liquids (ILs) have been extensively investigated and demonstrated to enhance the efficiency and stability of devices substantially. Herein, the role of ILs in PSCs as additives, solvents, interface engineering, and charge transport layer is reviewed. Also, this review will guide the researchers in understanding bulk doping and interface engineering for efficient and stable PSCs.


Introduction
Perovskite solar cells (PSCs) have drawn significant attention due to their low-cost fabrication routes and excellent optoelectronic properties, such as high light absorption coefficient, tunable bandgap, and long carrier diffusion length. [1] After significant efforts over the past decade, the power conversion efficiency (PCE) of PSCs has dramatically increased from only 3.8% to 25.7%, reaching close to the Shockley-Queisser limit of 33%. [2] However, thermal-and humidity-induced degradation as well as long-term photoinstability has hindered the commercialization of PSCs. [3] To resolve these issues, several groups are developing strategies to fabricate more efficient and stable PSCs by using ionic liquids (ILs) as additives. [4] ILs have a low melting point (<100°C) and consist of various organic cations (e.g., imidazolium, pyridinium, pyrrolidinium) and inorganic or organic anions (e.g., halides, tetrafluoroborate, hexafluorophosphate). ILs feature many unique properties, such as low volatility, low flammability, high ionic conductivity, and excellent thermal stability. [4d,g] By molecular engineering, chemical structure engineering, and selecting appropriate functional groups, the physicochemical properties of ILs can be readily adjusted for application in PSCs.
In this review, a systematic summary of the research progress of ILs as applied in PSCs is presented. ILs in PSCs are classified into four categories according to their role, including additives, solvents, interface modifiers, and charge transport layers ( Figure 1). From nucleation to crystal growth, the mechanisms by which ILs can influence the perovskite crystallization kinetics are discussed. An overview of the relationship between device performance and the positional distribution of ILs in PSCs is provided. In particular, a series of novel poly(ionic liquid)s (PILs) are highlighted, which are promising candidates to replace ILs. The ILs applied in the perovskite solar modules are also discussed. The review closes with an outlook and discussion of possible future directions for the application of ILs in PSCs.

Ionic Liquids as Additives for Perovskite Films
ILs as additives in PSCs have been widely investigated, and it has been evidenced that ILs can significantly improve device performance and stability. This is because ILs can regulate the perovskite crystal growth yielding higher-quality perovskite films. In this section, a summary of how ILs regulate perovskite crystallization and its chemical structure, and how ILs are spatially distributed in the films and alternative additives to ILs is provided.

Ionic Liquids Regulate the Crystallization Dynamics of Perovskites
The crystallization process can be depicted by the LaMer model ( Figure 2a). [5] The model is comprised of three stages. In the first (I) stage, as the solvent evaporates, the precursor concentration exceeds the solute solubility limit (C s ). Since the precursor solution has not reached supersaturation, no nucleation occurs. In the second (II) stage, the precursor solution reaches the minimum (C * min ) supersaturation limits, and nucleation occurs immediately. A larger area under the curve between C * min and the maximum supersaturation limit (C * max ) implies a higher nucleation density. In the third (III) stage, the solvent continues to evaporate, which will lead to an increased concentration and promote nucleation. At the same time, the crystal growth will decrease the concentration. The competition between nucleation and crystal growth largely determines the grain size. Therefore, the nucleation rate and nucleation density need to be inhibited to achieve large-size perovskite grains.
Characterized by a high boiling point and high hydrophobicity, ILs can increase the nucleation barrier of perovskites by adjusting the wetting of the precursor solution on the substrate, and forming hydrogen bonds or intermediates with perovskites to achieve homogeneous nucleation. Uzzaman et al. employed IL 1-hexyl-3-methylimidazolium chloride (HMImCl) as an additive in a MAPbI 3 perovskite precursor to control the morphology. [6] The author hypothesized that small clusters are formed prior to the nucleation of perovskites. Since HMImCl has a higher boiling point than N,N-dimethylformamide (DMF), it remains in the film when DMF evaporates. The remaining HMImCl is then absorbed on the surface of the clusters. It is more conducive to the homogenous nucleation of clusters due to the protection of HMImCl. As a result, the HMImCl-modified perovskite films showed a more uniform morphology than that of perovskite films without HMImCl (Figure 2b). Uzzaman et al. then used three ILs of different viscosities, namely, tetrabutylammonium chloride ([TBAM]Cl), 1-benzyl-3-methylimidazolium chloride ([BMIM]Cl), and 1-allyl-3-methylimidazolium chloride ([AMIM] Cl), to study the effect of IL viscosity on MAPbI 3 films. [7] The results revealed that the lower viscosity ILs induce the formation of nanoparticulate MAPbI 3 films. Liu et al. employed the polar-hydrophobic IL methylammonium trifluoroacetate (MA þ CF 3 COO À , MA þ TFA À ) as an additive to adjust the hydrophobicity of the precursor solution. [8] The MA þ TFA À regulated the wetting of the precursor solution on the substrate, increasing the water contact angle from 5°to 30°, thus reducing the nucleation density and enlarging the grain size ( Figure 2c). Consequently, the MA þ TFA À -modified device achieved an improved PCE from 17.9% to 20.1%. Zhou et al. found that IL 1-ethylamine hydrobromide-3-methylimidazolium hexafluorophosphate (ILPF 6 ) can suppress nucleation, attributing to the formation of N-H···I hydrogen bonds between ammonium cations of Due to the Ostwald ripening effect, the average grain size remarkably increased from 319.1 to 798.0 nm. As a result, the PCE of the MAFa-modified PSCs was elevated from 21.26% to 23.11%. The increased grain size can also be explained by the reduced grain boundary migration activation energy. Ali et al. spin-coated IL 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF 4 ) on the perovskite layer, after dropping antisolvent and before annealing (Figure 2g). [20] The average grain size of the BMIMBF 4 -modified film (786 nm) is four times larger in the reference film, resulting in an enhancement of PCE from 19.1% to 20.4%. The calculation results show that the activation energy for grain boundary migration of BMIMBF 4 -modified films (0.2 eV) is only half of that of the reference film (0.44 eV), indicating that the ILs can make grain boundary migration easier. Akin et al. added IL 1-hexyl-3-methylimidazolium iodide (HMII) into FAPbI 3 perovskite precursor. [21] HMII provided a liquid domain for reducing the activation energy of grain boundary migration, thus forming micron-sized grains. Consequently, the HMII-modified PSCs achieved an improved PCE from 17.1% to 20.6%.
The examples provided above demonstrate that ILs can increase grain size, reduce defects, and improve film quality by suppressing nucleation, delaying crystal growth, and recrystallization, thanks to their unique physicochemical properties and chemical interactions with perovskites. In particular, ILs with different chemical structures and functional groups can form diverse chemical interactions with perovskites, such as hydrogen bonds and chelate bonds. These interactions significantly impact the ability to regulate the crystallization of perovskites, and will be discussed in Section 2.2.

Influence of Ionic Liquids Chemical Structure and Functional Groups
The chemical structures of ILs are highly designable. By introducing different functional groups into the anions and cations of ILs, the properties required for PSC applications can be achieved. Therefore, to design effective IL additives, it is necessary to investigate the influence of the ILs chemical structures and functional groups on PSCs.
The chemical structure of cations in ILs has mainly been studied in terms of two parts: the backbone and side chains. Various ILs with different backbones (such as imidazole, pyridine, and pyrrolidine) have been applied to improve the performance and stability of PSCs. Imidazolium is the cation widely used in ILs because the nitrogen atom of imidazole can provide a lone pair electron that can passivate defects in the perovskite film. Luo et al. incorporated IL 1-methyl-3-propylimidazolium bromide (MPIB) into the perovskite precursor solution. [22] As shown in high-resolution X-ray photoelectron spectroscopy (XPS) result (Figure 3a), the binding energy of Pb 4f was reduced by 0.2 eV in the MPIB-modified perovskite, which can be attributed to the chemical interaction between MPIB and uncoordinated Pb 2þ . The Fourier transform infrared (FTIR) results ( Figure 3b) indicated that the C=N band of imidazolium splits into two C-N bonds and the electron cloud density of the imidazole ring was changed. Therefore, the passivation of uncoordinated Pb 2þ occurs through the formation of Pb-N bond, which Figure 3. a) High-resolution XPS of Pb 4f core level measured on the perovskite and MPIB perovskite films. Reproduced with permission. [22] Copyright 2020, Royal Society of Chemistry. b) FTIR spectroscopy for bare MPIB, PbI 2 , and MPIB-PbI 2 powder. Reproduced with permission. [22] Copyright 2020, Royal Society of Chemistry. c) The variable-temperature 1 H NMR spectra of MAI þ BMIBr from 25 to 100°C and the MAI spectrum at 25°C; samples were dissolved in deuterated DMSO. The dash represents the signal of MA at 25°C. Reproduced with permission. [23] Copyright 2018, Wiley-VCH. d) Water contact angle measurements of the b1) MAPbI 3 , b2) MA/EATZ, b3) MA/BATZ, and b4) MA/OATZ films. Reproduced with permission. [27] Copyright 2019, Wiley-VCH. e) Under the same scale bar, the c-AFM topographic images of the a) MAPbI 3 , b) MA/EATZ, c) MA/BATZ, and d) MA/OATZ active layer films recorded under dark and À0.3 V bias. The scale bar in c-AFM images is 500 nm. Reproduced with permission. [27] Copyright 2019, Wiley-VCH. f ) Top-view SEM images of the perovskite films with different additives. Reproduced with permission. [29] Copyright 2021, American Chemical Society. g) Mechanism analyses of improved solar cell performance and stability by the ZIL. Reproduced with permission. [31] Copyright 2021, Wiley-VCH. h) View showing the [PbI 3 ] À anion chain surrounded by six groups of [C3CNmim] þ cations, forming an infinite 1D polymer. Reproduced with permission. [16] Copyright 2022, Elsevier. i) Theoretically determined adsorption structure of the 6CNBP-N molecule on MAPbI 3 . Reproduced with permission. [34] Copyright 2021, American Chemical Society.  [23] The variable-temperature 1 H NMR spectra (Figure 3c) showed that the hydrogen bond between BMI þ and MA þ was strong and existed at relatively high temperatures (100°C). The results showed that the BMIBr-modified device can retain 85% of the original device efficiency at 85°C for 20 min, which is higher than that of the control device (70%). The pyridine ring is an electron-rich chemical structure that can provide electrons to neutralize trap states. Wan et al. used IL 1-ethylpyridinium chloride (1-EC) as the additive of MAPbI 3 to passivate uncoordinated Pb 2þ and suppress the charge recombination. [24] As a result, the average current density (from 11.56 to 17.34 mA cm À2 ) and fill factor (from 51.10 to 75.24%) were remarkably enhanced. Li et al. introduced two ILs, namely, 1-butyl-1-methylpyrrolidinium chloride (BMPyrCl) and 1-butyl-3-methylpyridinium chloride (BMPyCl), into Cs 2 AgBiBr 6 perovskite precursor solution. [25] The fivemembered ring of pyrrolidinium has a higher charge density than the six-membered ring of pyridinium, resulting in stronger BMPyr þ -Br À interaction and a better pinning effect of Br À . [26] Consequently, the BMPyrCl-modified device showed superior performance because of its higher V oc and FF than that of the device treated with BMPyCl. Thus, the backbone of cations in ILs should possess a strong ability to donate electrons to effectively passivate defects. Regarding the side chain part, the impact of side chain length was first introduced. Wang et al. employed a series of ILs named [RATZ]I (R = ethyl, butyl, octyl, and ATZ represents 4-amino-1,2,4-triazolium) to study the effect of the alkyl chain length of ILs on the performance and stability of PSCs. [27] According to the water contact angle measurement results (Figure 3d), the hydrophobicity of RATZ-modified perovskite films was directly correlated to the alkyl chain length. The conductive atomic force microscopy (c-AFM) measurements showed that the conductivity of perovskite films decreased with increasing alkyl chain length, and the conductivity of [EATZ]I-, [BATZ]I-, and [OATZ]I-modified films was 2.07, 1.59, and 0.93 mS cm À1 , respectively (Figure 3e). From the spatial hindrance point of view, the cation with a shorter alkyl chain is beneficial to the ion migration in ILs and improves the possibility of interaction with perovskites. [22] The [EATZ]I-modified device (shortest side chains) showed the greatest improvement from 16.13% to 20.03%, and the [OATZ]I-modified device (longest side chains) exhibited the best stability and retained around 80% of the initial efficiency after 3500 h under 40 AE 5% relative humidity (RH). Haris et al. used two ILs, namely, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate) (EMIFAP) and 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (HMIFAP), as additives to again demonstrate that the longer alkyl side chain of the IL increased the hydrophobicity. [28] The water contact angle of the control, EMIFAP-, and HMIFAP-modified films was 92°, 98°, and 101°, respectively. Li et al. introduced a series of ILs with varied fluorocarbon chain lengths, namely, 1-methyl-3-(3 0 ,3 0 ,4 0 ,4 0 , 4 0 -pentafluorobutyl)imidazolium tetrafluoroborate (MFIM-2), 1-methyl-3-(3 0 ,3 0 ,4 0 ,4 0 ,5 0 , 5 0 , 6 0 , 6 0 , 6 0 -pentafluorobutyl)imidazolium tetrafluoroborate (MFIM-4), and 1-methyl-3-(3 0 ,3 0 ,4 0 ,4 0 ,5 0 , 5 0 , 6 0 , 6 0 , 7 0 , 7 0 , 8 0 , 8 0 , 8 0 -pentafluorobutyl)imidazolium tetrafluoroborate (MFIM-6), into FAPbI 3 . [29] Scanning electron microscopy (SEM) images showed that MFIM-2 suppressed the formation of PbI 2 and improved the film morphology but MFIM-4 and MFIM-6 with longer fluorocarbon chains promoted the formation of the PbI 2 ( Figure 3f ). The water contact angle of control, MFIM-2-, MFIM-4-, and MFIM-6-modified films was 60.0°, 79.5°, 102.9°, and 118.2°, respectively. This confirms that increasing the fluorocarbon chains length of ILs will enhance the hydrophobicity of perovskite films. The MFIM-2-treated PSCs showed an enhanced PCE of 19.4%, and no phase transition occurred during a 10-month aging test. In conclusion, a long side chain benefits the stability of PSCs but sacrifices the performance.
In addition to adjusting the length of side chains, various functional groups can be introduced into the side chains to tune the properties of ILs. The amino group (-NH 2 ) is often used for functionalization of ILs. Wang et al. employed an amino groupfunctionalized IL, 3-(3-aminopropyl)-1-methylimidazolium hexafluorophosphate (APMimPF 6 ), as the functional monomer of perovskites. [30] The APMimPF 6 reacted with PbI 2 and formed a 2D structure, which improved the stability of PSCs with respect to humidity. As a result, the device retained 98% of its initial PCE after 40 days under 57% RH. Zhou et al. showed that the amino group of ILPF 6 can slow the rapid growth of perovskite crystals by forming a N-H···I hydrogen bond. [9] Yang et al. introduced 4-fluoro-phenylammonium tetrafluoroborate-based zwitterionic ionic liquid (4FB-BF 4 ) into (FAPbI 3 ) 1-x (MAPbBr 3 ) x perovskites. [31] The amino group of 4FB-BF 4 can bind to the uncoordinated Pb 2þ and fill the A-site vacancies (Figure 3g [TFSI]), as an additive in (Cs 0.08 FA 0.8 MA 0.12 )Pb(I 0.88 Br 0.12 ) 3 precursor solutions. [32] The nitrile groups can coordinate with the uncoordinated Pb 2þ to enhance the thermal stability of perovskites, which were verified by XPS measurements. The XPS results showed that after thermal aging at 150°C for 30 [16] During the nucleation process, the PbI 2 , methylammonium iodide (MAI)/formamidinium iodide (FAI), and [C3CNmim]Cl were transformed into a 1D-PbI 3 salt ([C3CNmim]PbI 3 ) and MACl/FACl, which further www.advancedsciencenews.com www.solar-rrl.com slowed the crystallization process and yielded a higher quality perovskite film (Figure 3h). [33] [34] Density functional theory (DFT) calculations showed that the uncoordinated Pb 2þ in MAPbI 3 can be passivated by the nitrile group of 6CNBP-N through the formation of Pb-N bonds ( Figure 3i). As a result, the PCE of 6CNBP-N-modified device significantly increased from 18.07% to 20.45%. Wang et al. investigated the effect of ILs with different anions, namely, thiocyanate (SCN À ), dicyandiamide (DCA À ), TFSI À , and BF 4 À , on the interaction between ILs and perovskites. [35] The 1 H NMR spectra showed that the anion of IL can modulate the interaction between the cation of IL and PbI 2 , and the interaction strength followed the trend of SCN À > DCA À > TFSI À > BF 4 À . Interestingly, the trend in the device performance is the same. As a result, the BMIMSCN-modified PSCs achieved a PCE of 24.4% and the encapsulated devices retained over 90% of their original efficiency under 65 AE 5°C for 1900 h.
In addition to those mentioned above, many other functional groups have been introduced into ILs to improve the efficiency and stability of PSCs (Table 1), such as benzal groups (-CH 2 C 6 H 5 ), [7,36] vinyl groups (-CH=CH 2 ), [7,33,36] amide groups (-CONH 2 ), [37] and carboxyl groups (-COOH). [8,[11][12][13]15,19,38] In conclusion, to design an effective IL, first, an electron-rich chemical structure is necessary, such as imidazole rings, pyridine rings, and pyrrolidine rings, which can provide electrons to neutralize trap states. Second, the spatial hindrance effect should be considered. The IL with longer alkyl chain is beneficial to enhance the hydrophobicity and humidity stability of perovskite, but at the same time sacrifices the performance. Finally, the addition of functional groups, such as -NH 2 (hydrogen bonds, N-H···I) and -COOH (chelating bonds, C=O-Pb), can further improve the passivation ability of ILs.

Positional Distribution of Ionic Liquids in Perovskite Solar Cells
The various anions and cations of ILs often have different spatial distributions in PSCs, such as the buried interface, the top interface, and in the bulk ( Table 2). To some extent, the different positional distributions of ILs determine their role in passivating defects, tuning the work function, and inhibiting ion migration. Exploring the origins and evolution of different spatial distributions of ILs appears to be a key to advancing the application of ILs in PSCs.
It is challenging to distinguish between ILs and the organic components (MA þ and FA þ ) of perovskites using conventional techniques such as XPS, due to the similar atom components (C, N, and H). Before discussing the spatial distribution of ILs in PSCs, it is important to introduce a key technique called time-of-flight secondary ion mass spectrometry (ToF-SIMS). In this technique, secondary ions are first ejected from the sample by beams (metal or ion cluster) and then analyzed in the mass spectrometer. The secondary ion species are then labeled according to their mass-to-charge ratio (m/z). Unlike other techniques, ToF-SIMS does not break down organic molecules into their constituent atoms, and is, thus, one of the few techniques that can directly obtain information on the organic molecular species. In addition, ToF-SIMS can capture a wide range of unique information through 1D profiling, 2D imaging, and 3D tomography. With the help of ToF-SIMS, it is possible to determine the positional distribution of various ILs in PSCs. [39] Bai et al. applied the ILs BMIMBF 4 and 1-butyl-3-methylimidazolium chlorine (BMIMCl) as additives to (FA 0.83 MA 0.17 ) 0.95 Cs 0.05 Pb(I 0.9 Br 0.1 ) 3 perovskites. [39a] Both BMIMBF 4 and BMIMCl can enhance the stability of perovskites, but only BMIMBF 4 can improve the device performance. The ToF-SIMS results showed that the BF 4 À was mainly located at the buried interface ( Figure 4a), while both the BMIM þ and Cl À existed throughout the bulk film (Figure 4b,c). The perovskite/NiO interface aggregated BMIM þ and BF 4 À ion pairs. According to previous reports, the BMIM þ and BF 4 À ion pairs accumulated in the perovskite/NiO interface would bring spontaneous dipolar polarization, resulting in well-matched work function and higher carrier mobility. [40] The PCE of devices preprocessed with BMIMBF 4 onto the NiO substrate increased from 17.3% to 17.9%, but the film stability was not obviously enhanced. Therefore, the BMIM þ distributed in perovskite films is responsible for the improved stability. The BMIM þ ions were too large to intercalate into the perovskite lattice and, hence, accumulated at grain boundaries, which passivated the defects and suppressed the ion migration ( Figure 4d). Du et al. employed the ILs EMIMHSO 4 and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF 4 ) as additives to CsPbI 3 perovskites. [10] The results of ToF-SIMS showed that the HSO 4 À is mainly distributed at the perovskite/TiO 2 interface, while the BF 4 À is mainly located at the perovskite/spiro-OMeTAD interface, where spiro-OMeTAD is 2,2 0 ,7,7 0 -tetrakis(N,N-di-p-methoxyphenylamine)9,9 0spirobifluorene. The EMIM þ was distributed throughout the bulk film and accumulated at both the top and bottom of the film. They suggested that the role of EMIM þ (electron donor) and HSO 4 À (electron acceptor) determined their different distributions. The HSO 4 À of the buried interface improved the band alignment ( Figure 4e) and was conducive to the charge transport, while the BF 4 À at the top interface induced an unfavorable energetic alignment interface ( Figure 4f ). Consequently, the PCE of EMIMHSO 4 -modified device achieved 20.01%, and maintained 95% of the initial PCE under an ambient condition with 25 AE 5% RH at 25 AE 5°C without encapsulation for 1000 h, which showed better performance and stability than the device modified by the EMIMBF 4 . Wei et al. used IL 1-ethyl-3-methylimidazolium trifluoroacetate (EMIMTFA) as the additive of MAPbI 3 . [38c] The ToF-SIMS results exhibited that the EMIM þ was distributed throughout the bulk film and mainly accumulated at the buried interface, while the TFA À was mainly located at the buried interface ( Figure 4g). Because the EMIM þ could interact with PbI 2 to form 1D perovskite of EMIMPbI 3 , the multilevel distribution of EMIM þ effectively passivated the bulk and interface defects. As a result, the PCE of EMIMTFA-modified device remarkably increased from 18.93% to 22.14% and retained 89% of the original PCE in ambient conditions with a humidity of %35% without encapsulation for 120 h. Xia   www.advancedsciencenews.com www.solar-rrl.com depth-profiling XPS measurement (Figure 4h). The depth-profiling XPS results (Figure 4i) revealed that the F 1s peak of ET þ gradually disappeared from the surface to the bulk of the perovskite layer, indicating a gradient distribution of ETI. The authors supposed that the gradient distribution was related to the large size and the strong amphiphilic character of the ETI, which was conducive to the migration of ETI to the air-dimethyl sulfoxide (DMSO) interface during crystallization. The ETI located at the top surface inhibited MA þ outdiffusion and prevented decomposition. As a result, the ETI-modified device preserved about 83% of the initial PCE in an argon atmosphere and %60°C under constant 100 mW cm À2 illumination without encapsulation for 700 h, which showed a better thermal and light stability than that of the reference device (only 49%). Lin   The symbol of " p " means the number of ions, and more " p " means more ions.
www.advancedsciencenews.com www.solar-rrl.com changes in device structures, charge transport materials, and perovskite components. The ILs accumulated at interfaces can influence the band alignment, resulting in positive or negative effects on the charge transport and the device performance.
The ILs located at perovskite grain boundaries usually play a role in passivating defects, suppressing ion migration and enhancing the perovskite stability. In brief, the positional distribution of ILs in devices is closely related to the performance and stability of PSCs.

Poly(ionic Liquid)s
The conventional ILs have some intrinsic disadvantages, such as easy aggregation and migration, limiting the further enhancement of the repeatability and operational stability of PSCs.  (Figure 5a). [42] The PEa played the role of 3D skeleton templates to further improve high-quality perovskite films, and, thus, increased the grain size from 300 to 600 nm. The repeating units of PEa contain carbonyl groups (C = O), which can firmly anchor PEa at grain boundaries and passivate the uncoordinated Pb 2þ in perovskite. As a result, the PEa-modified device showed significant performance improvements (from 18.16% to 21.47%) and retained over 92% of its initial PCE under continuous illumination at 70-75°C for 1,200 h. Yang et al. incorporated PILs poly-1-vinyl benzyl-triethylammonium chloride (PIL-Am) and poly-1-vinyl benzyl-3-methylimidazole chloride (PIL-Im) into perovskites (Figure 5b). [43] The DFT calculations showed that PIL-Am and PIL-Im expanded I À ion migration energy from 0.0 to 0.37 and 0.34 eV, respectively (Figure 5c). The enlarged migration energy suppressed the I À migration, which was attributed to the physical barrier (electrostatic interaction and steric hindrance) and chemical interaction from PILs. As a result, the device modified with PIL-Am was dramatically enhanced from 20.26% to 22.22%. Further, the PIL-Am-doped PSCs retained 80% of their initial PCE under AM 1.5 G illumination without encapsulation for nearly 1500 h. Gong et al. employed PILs poly-1-vinyl-3-methylimidazolium (PImIL) and poly-1-vinyl-3propyltrimethoxysilane imidazolium chloride (PImIL-SiO) as additives to Cs 0.05 FA 0.85 MA 0.10 Pb(I 0.97 Br 0.03 ) 3 . [44] The siloxane (Si-O) groups can undergo hydrolytic condensation with the permeated moisture to form a secondary protective barrier, which enhances the humidity stability of PSCs (Figure 5d). The imidazolium cations and Si-O groups of PImIL-SiO can firmly anchor grain boundaries and passivate defects, thus effectively suppressing the migration of I À ions and device degradation (Figure 5e). The grazing incidence X-ray diffraction (GIXRD) measurement showed that the PImIL-SiO could efficiently release the residual tensile strain in perovskite films (Figure 5f ). Benefiting from the abovementioned advantages, the PCE of PImIL-SiO-doped device increased from 20.63% to 22.46%. The PImIL-SiO-doped device maintained 87% of initial PCE at 85°C in N 2 for 250 h and retained about 85% of original PCE in the air with a 50-70% RH without encapsulation for 1100 h, respectively.
In short, compared with conventional ILs, PILs have three advantages that make their application in PSCs more promising. First, the PILs contain more chemical anchoring sites, which can passivate defects in perovskites more effectively. Second, the synergistic effect of the physical barrier and chemical interaction from PILs effectively suppresses ion migration. Third, PILs can efficiently release the residual tensile strain in perovskite, thus improving the humidity stability and the phase stability.

Ionic Liquids as Solvents for Perovskite Precursor Solution
Based on their special functional groups, such as -COOH and -NH 2 , ILs can effectively dissolve perovskites by forming Pb-O coordination and N-H···I hydrogen bonds. It has been demonstrated that replacing conventional solvents, such as DMF, DMSO, and N-methyl-2-pyrrolidone (NMP), with ILs can greatly enhance the quality of perovskite films. In the following section, the mechanism by which ILs act as solvents to improve the crystal quality will be discussed.
The IL MAAc, as an effective solvent, has been widely investigated in recent years. Öz et al. developed three protic ILs, namely, MAFa, MAAc, and methylammonium propionate (MAP), as solvents for solution processing of MAPbX 3 (X = I, Br). [45] Three protic ILs were blended with cosolvents, such as water, ethanol, isopropanol, and acetonitrile, and found that different combinations and ratios can change the preferred crystal orientation ( Figure 6a). As a result, the PCE of (MA 0. 15 (Figure 6b). [46] The grazing incidence wide-angle X-ray scattering (GIWAXS) showed that MAAc could make the preferred crystal orientation in multiple directions as additives due to the covalent binding of Pb 2þ to Ac À (Figure 6c À -, PbAc 2 À -, and PbCO 3 À -based devices achieved 14.48%, 19.21%, and 20.13%, respectively. [47] Wu et al. found that the excess lead halides (PbCl 2 , PbBr 2 , PbI 2 ) could form MAPbX 3-x Ac x (X = Cl, Br, I) nanocrystals with the MAAc, thus regulating perovskite growth. [48] As a result, the PbCl 2 -modified devices improved the PCE from 17.80% to 21.26%. Li et al. demonstrated that the MAAc can be physically adsorbed on the indium tin oxide (ITO) electrode and form an interfacial dipole layer, resulting in a well-matched work function and higher electron extraction ability (Figure 6f ). [49] Therefore, the PCE of electron transport layer (ETL)-free devices modified by MAAc achieved 21.08%.
Several researchers have shown that the perovskite film morphology depends on the size of micelles that form in the precursor solutions. Chao et al. found that the micellar size distribution was more uniform, and the colloid concentration was higher in MAAc than in DMF by dynamic light scattering (DLS), and, thus, www.advancedsciencenews.com www.solar-rrl.com more uniform and compact perovskite films were achieved (Figure 6g). [50] As a result, the MAPbI 3 -based device reached a PCE of 20.05%. Wang et al. and Sun et al. used methylammonium difluoroacetate (MA þ DFA À ) and acetonitrile (ACN) to reduce the Pb-O bond strength and further increase the micellar size, respectively. [51] Consequently, the PCE of both MA þ DFA Àand ACN-modified devices achieved over 21%. The interaction between carboxyl-based ILs and PbI 2 was further studied. Wang et al. investigated the local coordination environments of Pb in MAAc and DMF:DMSO by extended X-ray Figure 5. a) Chemical structure analysis. Reproduced with permission. [42] Copyright 2020, Royal Society of Chemistry. b) Chemical structure of PILs and the schematic of ion immobilization in perovskite by PILs. Reproduced with permission. [43] Copyright 2022, Wiley-VCH. c) I À migration pathways and migration energies as obtained from nudged elastic band (NEB) calculations for perovskite films: control, with PIL-Am, and with PIL-Im. The yellow trajectories are I À migration pathways. Reproduced with permission. [43] Copyright 2022, Wiley-VCH. d) Schematic illustration of the formed dual-resistance of ion migration and moisture erosion with hydrolytic crosslinking of siloxane. Reproduced with permission. [44] Copyright 2022, Chinese Chemical Society. e) Cross-sectional SEM images of the devices based on control and the corresponding PILs-modified perovskite layers before and after aging at 85°C for 3 days. Reproduced with permission. [44] Copyright 2022, Chinese Chemical Society. f ) GIXRD patterns with different instrumental ψ values (10°-50°) for control and PILs-modified perovskite, and the corresponding linear fit of 2θ-sin2ψ for control and PILs-modified perovskite films. Reproduced with permission. [44] Copyright 2022, Chinese Chemical Society.
www.advancedsciencenews.com www.solar-rrl.com absorption fine structure (EXAFS). [52] The EXAFS revealed that the Pb-O interaction in MAAc system was stronger than that in the DMF-DMSO system, resulting in the solution changing from yellow to colorless (Figure 6h).  Reproduced with permission. [46] Copyright 2020, The American Association for the Advancement of Science. c) 2D GIWAXS diffraction images of MAAc perovskites. Reproduced with permission. [46] Copyright 2020, The American Association for the Advancement of Science. d) 2D GIWAXS diffraction images of DMF perovskites. Reproduced with permission. [46] Copyright 2020, The American Association for the Advancement of Science. e) The diagram of MAAc in perovskite grain boundaries. Reproduced with permission. [46] Copyright 2020, The American Association for the Advancement of Science. f ) Device structure with MAAc dipole contact and diagram of energy levels. The representations of dipole are qualitative based on the hypothesized mode of interaction and the exact mode needs to be further evaluated. Reproduced with permission. [49] Copyright 2020, American Chemical Society. g) DLS spectra of MAAc and DMF perovskite precursor solution with different concentrations from 10 to 300 mg ml À1 . Reproduced with permission. [50] Copyright 2019, Elsevier. h) EXAFS spectra and fits in R-space at the Pb L 3 -edge of MAAc and DMF:DMSO with the magnitude (solid squares, blue) and real components (solid squares, pink) of the Fourier transform (FT). Reproduced with permission. [52] Copyright 2020, Wiley-VCH. i) Images of PbI 2 @MAFa and PbI 2 @DMF:DMSO solutions and schematic diagram of interactions in the solutions. Reproduced with permission. [53] Copyright 2021, The American Association for the Advancement of Science.
www.advancedsciencenews.com www.solar-rrl.com system to a corner sharing structure in the MAFa system, due to strong interactions between MAFa and PbI 2 in the forms of C=O···Pb chelation and N-H···I hydrogen bonds (Figure 6i). [53] The corner sharing structure notably reduced the formation energy barriers for FAI penetration into PbI 2 , thus the stable α-FAPbI 3 could be prepared regardless of RH (20-90%) and temperature (25-100°C) and the PCE of devices fabricated in ambient air exceeded 24%. Gu et al. employed a series of ILs solvents, namely, MAAc, MAP, and methylammonium isobutyrate (MAIB), to study the influence of steric hindrance on chemical interaction between ILs and PbI 2 in the solution. [54] The liquid-state 13 C NMR measurements showed that the relative shift of the carbon atom in the carboxylate group followed the trend of MAAc (Δδ = 0.452 ppm) > MAP (Δδ = 0.257 ppm) > MAIB (Δδ = 0.231 ppm), indicating that the interaction between the ILs and PbI 2 decreases with the increase of steric hindrance. The strong interaction between IL and PbI 2 can slow the crystallization process and improve the perovskite film quality, but it is not conducive to the solvent removal. Therefore, the MAP-based device achieved the best photovoltaic performance with a PCE of 20.56%. As discussed above, there are several reasons why ILs can improve the quality of perovskite films as solvents: 1) ILs can promote preferred crystal orientation due to covalent binding; 2) ILs can increase the size of micelles formed in precursor solutions due to weaker chemical interaction strength between ILs and perovskites, resulting in uniform and compact perovskite films; 3) ILs have high boiling points, allowing a small amount to remain in perovskite films and passivate defects.

Ionic Liquids for Interface Modification
For both n-i-p and p-i-n type PSCs, the high defect densities and poor interface contacts at the buried and top interfaces limit their performance. A series of ILs has been used as interface modifiers to improve the perovskite film quality, passivate defects, and enhance carrier transport.

Buried Interface
TiO 2 has been commonly applied as an ETL in n-i-p type PSCs, while the relatively low electron mobility and high electronic trap densities of TiO 2 hinder the further improvement of the device performance. Yang et al. used [BMIM]BF 4 to modify the TiO 2 / perovskite interface. [41b] The theoretical calculation showed that the [BF 4 ] À group preferred to bond to TiO 2 , while the [BMIM] þ group was more likely to adsorb on the perovskite surface ( Figure 7a). Therefore, the [BMIM]BF 4 would spontaneously form a surface dipole between the TiO 2 /perovskite interface (Figure 7b), thus aligning the work functions and promoting electron mobility. As a result, the electron mobility of [BMIM]BF 4 -modifed TiO 2 (1.36 Â 10 À3 cm 2 V À1 s À1 ) was four times higher than the reference TiO 2 (3.83 Â 10 À4 cm 2 V À1 s À1 ) and the hysteresis was eliminated ( Figure 7c) 6 , the energy alignment between TiO 2 and MAPbI 3 was improved and the electron extraction was enhanced, resulting in a significant increase in J sc (from 19.30 to 23.52 mA cm À2 ). Shi et al. found that precoating MAAc onto the TiO 2 layer could improve the CsPbIBr 2 film quality because the uniformly distributed MA þ would act as a nucleation center and reduce the formation potential barrier of the α-phase perovskite (Figure 7d). [55] As a result, the PCE of MAAc-modified devices was enhanced from 6.42% to 8.85%. Yin et al. employed IL 1-butyl-3-methylimidazole hexafluorophosphate (BMIMPF 6 ) to modify the TiO 2 /CsPbI 2 Br interface. [56] According to the AFM, the water contact angle, and SEM measurement results, the surface of BMIMPF 6 -modified TiO 2 was smoother and more hydrophobic, which facilitated the formation of complete perovskite films, inhibited the perovskite nucleation density, and increased the grain size from 0.73 to 1.88 μm (Figure 7e). Therefore, the PCE of BMIMPF 6modified device increased from 10.65% to 13.19%.
Compared with TiO 2 , SnO 2 has a higher electron mobility, a superior UV light stability, and a relatively lower processing temperature. Gao et al. incorporated IL 4-fluorophenylammonium tetrafluoroborate (FBABF 4 ) to further modulate the SnO 2 /perovskite interface. [57] The XPS and 1 H NMR measurements revealed that BF 4 À coordinated with SnO 2 and the FBA þ formed a hydrogen bond with FAI, which passivated uncoordinated Sn and suppressed FA þ migration (Figure 7f ). Consequently, the FBABF 4 -modified device achieved a PCE of 23.07% and retained 85% of the initial PCE with 35 AE 5% RH in the dark for 3000 h. Zhang et al. used the IL potassium tetrafluoroborate (KBF 4 ) to passivate the SnO 2 / perovskite interface. [58] BF 4 À can reduce hydroxyl (-OH) defects at the SnO 2 surface and K þ can diffuse into the perovskite grain boundaries and improve the crystallinity (Figure 7g). As a result, the KBF 4 -modified device reached a PCE of 22.90% and maintained 85% of the original efficiency under ambient conditions with a RH of 70 AE 10% for 1000 h. Peng et al. employed BMIMPF 6 to modify the SnO 2 /perovskite interface. [59] BMIMPF 6 increased the conduction band energy (E CBM ) of SnO 2 from À4.43 to À4.24 eV, which was closer to the E CBM of the perovskite material and, thus facilitated electron transport. In this way, the PCE of BMIMPF 6 -modified devices enhanced from 18.03% to 19.66%. Noel et al. introduced IL BMIMBF 4 into SnO 2 /perovskite interface. [60] Interestingly, they found that the BMIMBF 4 not only made the SnO 2 surface more n-type but also shifted the perovskite Fermi level toward the conduction band, which reduced the electron trap density in the perovskite. Therefore, the PCE of the BMIMBF 4modified device increased from 19.2% to 20.8%.
In addition to TiO 2 and SnO 2 ETLs, ILs have also been employed to modify other ETL materials, including ZnO, Nb 2 O 5 , and C 60 . [61] ILs have also been used to modify the hole transporting layer (HTL) in p-i-n type PSCs, especially poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). [62] Since the modification mechanism is similar to that discussed above, we will not repeat it here. In summary, ILs modify the buried interface by passivating defects, regulating the crystallization dynamics, and aligning the work functions, which effectively improve the performance and stability of PSCs.

Top Interface
Due to the polycrystalline nature of perovskite layers prepared by solution-processing methods, the top surface and grain  (EMIMSCN) to modify the top surface of (FA 0.92 MA 0.08 )PbI 3 perovskite film. [66] The DFT calculation results showed that both EMIM þ and SCN À could passivate the V I and Pb-I antisite defects of perovskite surfaces. Copyright 2016, Royal Society of Chemistry. d) Chemical structure of MAAc ILs and the preparation processes of the CsPbIBr 2 films with/without MAAc IL interfacial modification procedure. Reproduced with permission. [55] Copyright 2021, American Chemical Society. e) AFM images, water contact angle measurements, and top-view SEM images of pristine TiO 2 and TiO 2 /IL ETLs. Reproduced with permission. [56] Copyright 2021, American Chemical Society. f ) The schematic illustration of the chemical interaction mechanism of FBABF 4 with perovskite layer and SnO 2 layer. Reproduced with permission. [57] Copyright 2021, Elsevier. g) Schematic diagram of the mechanism of KBF 4 acting at the SnO 2 /perovskite interface. Reproduced with permission. [58] Copyright 2022, American Chemical Society.  Reproduced with permission. [67] Copyright 2021, Wiley-VCH. d) Schematic image of a CsPbI 2 Br PSC with the structure FTO/TiO 2 /CsPbI 2 Br(ILs)/spiro-OMeTAD/Au. Reproduced with permission. [68] Copyright 2021, Springer Nature. e) The relative interaction strengths of different anions with the I À vacancy at the surface of the perovskite. Reproduced with permission. [68] Copyright 2021, Springer Nature. f ) The binding energy of different anions with Pb-I antisite defect. Reproduced with permission. [68] Copyright 2021, Springer Nature. g) The binding energy of different ions with PbI 2 . Reproduced with permission. [68] Copyright 2021, Springer Nature. h) UPS of perovskite films with and without DMIMPF 6 treatment. Reproduced with permission. [69] Copyright 2020, Wiley-VCH. i) Energy band diagram of the perovskite device. Reproduced with permission. [69] Copyright 2020, Wiley-VCH. j) Illustration of interface modification via depositing an ionic liquid layer between CsPbI 3 and PCBM, and schematic representation of energy level alignment induced by the interfacial dipole. Reproduced with permission. [70] Copyright 2022, Elsevier. k) Scheme of the humidity stability test. Reproduced with permission. [72] Copyright 2021, American Chemical Society. l) Linear fit of 2θ-sin2ψ for the measured films.
Reproduced with permission. [74] Copyright 2022, Wiley-VCH. m) Stability of the control and [BMIM]Br-treated devices (the statistical data of each condition are collected from five devices). Reproduced with permission. [74] Copyright 2022, Wiley-VCH.  (Figure 8d). [68] The DFT calculation results showed that BF 4 À has the highest binding energy to I À vacancy and Pb-I antisite compared with PF 6 , Cl À , Br À , and I À (Figure 8e,f ). In addition, the binding energy of anions (BF 4 À and PF 6 À ) to PbI 2 were noticeably higher than that of the cations (PMIM þ , BMIM þ , HMIM þ , BMMIM þ , and HMMIM þ ) (Figure 8g). Therefore, the BF 4 À played a more important role in passivating defects of inorganic perovskite in comparison with imidazole cations due to the stronger interactions (Pb-F and Cs-F). Consequently, the PCE of BMMIMBF 4modified device showed the largest improvement from 15.62% to 17.02%.
ILs can both enhance the charge carrier transport and extraction between the top interface of n-i-p and p-i-n type PSCs. Wang et al. used IL 1,3-dimethyl-3-imidazolium hexafluorophosphate (DMIMPF 6 ) to modify the perovskite/spiro-OMeTAD (i-p) interface. [69] According to the ultraviolet photoelectron spectroscopy (UPS) measurement (Figure 8h), the work function of DMIMPF 6 -treated perovskite surface decreases from 4.32 to 3.87 eV, thus enhancing hole extraction and suppressing carrier recombination ( Figure 8i). Therefore, the PCE of DMIMPF 6modified device was improved from 21.09% to 23.25%. Wang et al. applied BMIMPF 6 to optimize the perovskite/ PCBM (i-n) interface. [70] The spontaneous dipole polarization of BMIMPF 6 at the perovskite/PCBM interface reduced the energy barrier and facilitated electron extraction (Figure 8j). As a result, the efficiency of the BMIMPF 6 -modified device was promoted from 8.6% to 13.21%.
ILs can react with the perovskite of the top surface to improve the efficiency and stability of PSCs. [71] Zhang et al. employed EMIMBF 4 to modify the top surface of MAPbI 3 . [72] After annealing at 100°C for 5 min, the top surface of MAPbI 3 was reconstructed by EMIMBF 4 and formed (EMIM) x MA 1Àx Pb[(BF 4 ) x I 1Àx ] 3 . The (EMIM) x MA 1Àx Pb[(BF 4 ) x I 1Àx ] 3 interlayer significantly enhanced the humidity stability of MAPbI 3 (Figure 8k). Zhuang et al. employed 1-ethyl-3-methylimidazolium-based ILs (EMIMX) (X = Cl, Br, I) to treat the top surface of perovskites. [73] It was verified that EMIMX could interact with PbI 2 to form a 1D passivation layer, which passivated the surface defects and enhanced the hydrophobicity. Therefore, the PCE of the EMIMBr-modified device increased from 18.06% to 20 (Figure 8l). As a result, the [BMIM]Br-treated device reached a PCE of 23.4% and exhibited good light stability maintaining over 80% of the initial PCE under 1 sun AM 1.5 G illumination with encapsulation for 1400 h (Figure 8m).
In brief, similar to the modification of the buried interface, ILs play the same role in the modification of the top interface, such as passivating defects and enhancing the charge carrier transport. It is worth noting that ILs can modulate the PbI 2 at the top interface and form stable complexes, which are conducive to the efficiency and stability of PSCs.

Ionic Liquids for Passivation of Charge Transport Layers
Due to their unique properties, ILs can be introduced into the charge transport layers (ETL or HTL) as dopants. They have an influence on both the charge transport layer's morphology and its electrical properties.

Electron Transport Layer
Due to their high ionic conductivity, ILs are candidate materials for passivating layers on conducting layers. Yang et al. used IL 1-benzyl-3-methylimidazolium chloride as the passivation material on top of flexible PSCs (Figure 9a). [75] The flexible device with the IL passivation reached a high PCE of 16.09%, attributed to the high electron mobility and suitable work function of the IL (Figure 9b). Cheng et al. developed IL hydroxyethylfunctionalized imidazolium iodide (BIPH-II) to treat the surface of FTO (Figure 9c). [76] The hydroxyl (-OH) end group of BIPH-II adsorbs onto the FTO surface through covalent self-assembly, resulting in a well-matched work function. The strong π-π stacking interaction between adjacent BIPH-II molecules was conducive to intramolecular electron transfer, thus improving the conductivity. As a result, the PCE of BIPH-II-based device was noticeably enhanced from 9.01% to 17.31% and the hysteresis was decreased.
ILs can also be applied in the preparation of electron transport materials, such as SnO 2 and TiO 2 . Huang et al incorporated IL tetramethylammonium hydroxide N(CH 3 ) 4 OH (TMAH) into SnO 2 nanoparticle suspensions to suppress the aggregation, thus facilitating the deposition of compact SnO 2 films. [77] The c-AFM measurements showed that TMAH significantly promoted the conductivity of the SnO 2 because N(CH 3 ) 4 þ played the role of a capping agent for oxygen vacancy passivation on the surface of nanoparticles. Consequently, the PCE of TMAH-based devices increased from 18.14% to 20.28%. Wang et al. used IL 1-carboxymethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide (HOOCMIMNTF 2 ) to fabricate TiO 2 -based ETL by the microwave technique at 70°C. [78] The steric hindrance of TiO 2 nanoparticles was increased due to the bidentate chelation between the carboxyl functional group of HOOCMIMNTF 2 and TiO 2 , resulting in a more compact and uniform TiO 2 film morphology. Therefore, the PCE of devices based on HOOCMIMNTF 2 -modified TiO 2 ETL enhanced from 16.64% to 19.55%.
Jian et al. introduced IL 1-dodecyl-3-methylimidazolium bromide ([C 12 mim]Br) into the hole transport materials, namely, conjugated polyelectrolytes of potassium poly[3-(4-carboxylatebutyl)thiophene (P3CT-K). [85] The [C 12 mim]Br with long alkyl chain acted as cationic surfactants and inhibited the aggregation behavior of P3CT-K, thus improving the morphology of P3CT-K films. As a result, the PCE of the device prepared using [C 12 mim] Br-modified P3CT-K increased from 16.21% to 18.81%. Zhang et al. used IL 1-ethyl-3-methylimidazole diethyl phosphate (EMDP) as a surfactant to obtain a stable and well-dispersed NiO x colloidal solution, thus leading to flatter and smoother Figure 9. a) The structure of the flexible PSCs with ss-IL as the ETM. Reproduced with permission. [75] Copyright 2016, Wiley-VCH. b) Energy-level diagram of the PSCs, exhibiting the collecting process of photogenerated charge carriers. Reproduced with permission. [75] Copyright 2016, Wiley-VCH. c) The device structure of the perovskite solar cell modified with BIPH-II. Reproduced with permission. [76] Copyright 2020, Wiley-VCH. d) Ball-and-stick model showing the oxidation process in spiro-OMeTAD. Reproduced with permission. [81] Copyright 2018, Elsevier. e) Contact angle (θ) measurements of perovskite and sample with FDT, with different BMPyTFSI dopant concentrations. Reproduced with permission. [82] Copyright 2020, American Chemical Society. In conclusion, first, ILs can be used to prepare inorganic CTL materials, and, thus, improving the CTL morphology. Second, ILs can act as p-type dopants to oxidize the HTL materials and enhance their conductivity. Finally, although the use of ILs as a single CTL is less reported, it is still a possible direction and merits further investigation.

Ionic Liquids for Perovskite Solar Modules
The PSCs have achieved high efficiency based on small-sized devices (%0.1 cm À2 ). However, due to the poor quality of large-area perovskite films, such as uneven coverage and chemical composition, low crystallinity, and high surface roughness, the device performance drops sharply after scaling up the device area. [86] According to the analyses in Section 2.1, ILs can suppress nucleation and delay crystal growth, resulting in reducing defects and improving film quality in small-sized devices. In the following, ILs applied in perovskite modules will be discussed.
Roll-to-roll (R2R) is a potentially low-cost method for fabricating flexible perovskite modules. However, device performance can be hindered by issues such as warpage, undulation, and small processing windows in R2R processes. Kang et al. designed an ionogel perovskite (IG-PVK) matrix by incorporating the IL 1-ethyl-3-methylimidazolium ethyl sulfate (EMIMEtSO 4 ) into a cross-linkable polymer network, taking into consideration the characteristics of the R2R process (Figure 10a). [87] The strong electron-donating ability of the O atoms in EMIMEtSO 4 improved the outer electron density of Pb, leading to a strong affinity between the perovskite grains and ionogel. The ionogel on the surface and grain boundaries slowed down the volatilization of the organic cations and reduced the corresponding tensile stress. Due to the well-designed grain boundary, an impressive efficiency of 21.76% was achieved in flexible PSCs, with improved operational and mechanical stability.
Spray-coating method has been applied to fabricate large-area solar modules, with merits of high solution utilization and low waste. Yang et al. employed IL BMIMBF 4 to improve the morphologies of large-area perovskite films based on the ambient ultrasonic spray deposition. Moreover, they used vacuumassisted extraction after spraying, which accelerated the solvent evaporation process and enhanced the crystallinity (Figure 10b). [88] As a result, the BMIMBF 4 -modified perovskite film (5 Â 10 cm 2 ) was cut into eight samples and achieved a consistent device performance with a PCE of 14.83 AE 0.62%.
Screen printing is another promising thin-film fabrication technique for PSC industrialization, which offers high pattern Figure 10. a) Schematic illustration of the fabrication of IG-PVK devices. Reproduced with permission. [87] Copyright 2022, Royal Society of Chemistry. b) Schematic illustration of ultrasonic spray coating of perovskite precursor solution with vacuum extraction. Reproduced with permission. [88] Copyright 2022, Royal Society of Chemistry. c) Schematic of the transfer/leveling procedure during the formation of perovskite thin films by means of the screenprinting process, and the schematic of the thermal annealing deposition of wet perovskite thin films. Reproduced with permission. [89] Copyright 2022, Springer Nature. d) The photovoltaic performance of perovskite solar modules by the spin-coating process. Reproduced with permission. [16] Copyright 2022, Elsevier.
www.advancedsciencenews.com www.solar-rrl.com flexibility, high productivity, and cost-effective production capability. One challenge in preparing perovskite films by screen printing is improving the viscosity of perovskite ink, which facilitates the ink cohesive and adhesive to the substrate and improves the printing quality. Chen et al. applied IL MAAc as solvents to increase the viscosity of perovskite ink, and achieved a smooth and uniform perovskite film surface (Figure 10c). [89] The high viscosity properties of MAAc suppress the volatilization of organic cations (MA þ /FA þ ) and the protic amine carboxylic acid of MAAc can stabilize the perovskite ink. As a consequence, the small module (16.37 cm À2 ) reached a PCE of 11.80%, and maintained 96.75% of the original efficiency over 300 h of operation at maximum power point. Spin-coating is the most employed technique in the fabrication of small-area devices, but it is not typically suitable for the solar modules because the quality of perovskite films drops rapidly as substrate size increases. Due to the significant crystallization regulation effect of ILs, it provides a possibility to prepare high-efficiency solar modules by spin-coating technique. Gao  Cl improved uniformity and grain morphology, attributing to the slower nucleation/crystallization of perovskite films. [16] As a result, the [C3CNmim]Cl-modified module exhibited a PCE of 20.89%, which was higher than that of the control module (18.54%) (Figure 10d). Wang et al. used the BMIMSCN to enhance the quality of large-area perovskite films prepared by spin-coating technique. [35] The crystallization process was regulated, attributing to the new Pb-N bonding forming between the BMIMSCN and Pb 2þ . Therefore, the smoother and more uniform film morphology was achieved, and the PCE of BMIMSCN-modified module (10 cm À2 ) dramatically increased from 17.1% to 20.4%.
In summary, the reports mentioned above demonstrate that ILs can still improve the quality of perovskite films even after scaling up their size. Moreover, ILs can be easily combined with various thin-film fabrication techniques, such as R2R, spray coating, and screen printing. Hence, we believe that using ILs will become a universal strategy in the fabrication of large-size solar modules.

Summary and Outlook
In summary, we have provided a comprehensive review of the application of ILs in PSCs. In the first part where ILs are considered as an additive in the perovskite layer, and showed that ILs could improve perovskite film quality through suppression of nucleation, and retarding crystal growth and recrystallization. The interaction between ILs and perovskites can be modulated by changing the backbone, spatial hindrance (alkyl chain length), and functional groups. Thereafter, we conclude that the distribution position of ILs determines its role in passivating defects, tuning work function, and inhibiting ion migration. To solve the shortcomings of conventional ILs, such as easy aggregation and migration, a promising class of candidate materials, namely, PILs, have been presented. In the section that covers ILs as solvents, it is shown that carboxyl-based ILs are the main choice and have multiple functions, including controlling crystallinity, passivation of defects, and regulation of the energy levels. In the interface modification part, the effect of ILs on the buried and top interfaces are, respectively, discussed. The ILs can tune the work function and enhance carrier transport in both the buried and top interfaces due to spontaneous dipolar polarization. For the buried interface, ILs can regulate the hydrophobicity of substrates, thus facilitating the formation of high-quality perovskite films. At the top interface, ILs can react with perovskite to form complexes, which can effectively improve the performance and stability of PSCs. In the charge transport layer (ETL or HTL) part, ILs are commonly applied as dopants to make the morphology and electrical properties of the charge transport layer better. At last, the ability of ILs to regulate crystallization is discussed after scaling up the area of perovskite films, and ILs can still effectively improve the quality of perovskite films.
Up to now, the application of ILs in PSCs has been widely studied, and more than 100 articles have been published on the subject. However, there are still some unsolved problems as follows: 1) Researchers have explained reasons why ILs can enlarge the perovskite grain size by suppressing nucleation, delaying crystal growth, and recrystallization. In many articles, ILs have been shown to influence preferred crystal orientation, but a mechanistic explanation is lacking. A clear understanding of the role played by ILs in preferred crystal orientation facilitates the preparation of high-quality perovskite films. 2) It is known that the spatial distribution position of ILs in PSCs determines their function, but what drives the different locations of ILs is still unclear. If the mechanism is understood, this will allow the design of ILs and/or processes to control the position and, thus, achieve the desired function.
We believe that applying ILs will reduce ion migration and provide a promising pathway to retard the degradation of perovskite absorber in realizing the commercialization of PSCs in the future.