Durable strategies for perovskite photovoltaics

Solar cells can convert solar energy into electricity, which is one of the renewable routes to respond to the fossil energy shortage and climate crisis all over the world. PVSK materials have been used in solar cells due to their ambipolar carrier transportation, high mobility, wide absorption, and good defect tolerance, which contribute to a rapid increase in device efficiency from 3.8% (2009) to 25.2% (2019).^1–6 This efficiency breakthrough over a decade represents the irrefutable potential of perovskite solar cells (PSCs) in substituting conventional silicon-based solar panels. In addition, the simple fabrication technique of PSCs, such as the low-temperature solution-processed method, doctor blading, and slot-die coating, also improves the commercialization possibility.^7–9 Nevertheless, it is indispensable to guarantee the stability of PSCs for lowering total inputs during the recycling process of industrialization.

The stability of PVSK photovoltaics is capable of being classified into structural firmness and environmental resistance. Structural firmness depends on the device structure and PVSK structure. The former refers to a suitable device architecture where the contiguous layers of the PVSK film with good intrinsic stability will not lead to corrosion of the active layer.^10,11 It has been reported that the acidity of PEDOT:PSS and acetonitrile that is the solvent of Li-TFSI in Spiro-OMeTAD can chemically corrode PVSK layers.^11–13 In addition, the PVSK structure is required to be in a stable dynamic range standardized with the tolerance factor (t) that is related to the sizes of composed ions in the PVSK material.¹⁴ For a practicable and stable structure in solar cells, the t value should be in the range of 0.8–1.¹⁵ Furthermore, firmness also depends on the film quality because if too much traps in the films, it may cause adverse carrier and ion transfer, influencing both the working performance and the lifetime.¹⁶ As for the environmental stability, it depends on the resistance to moisture, oxygen, light, temperature, and durability under various electrochemical situations. Thereinto, water molecules permeate into solar cells to dissolve PVSK through the hydration process producing lead iodide.¹⁷ Although low humidity has the tendency to control crystallization, dense moisture is still detrimental to long-term maintenance of the film phase and quality.^18,19 In particular, for organic–inorganic PVSKs, the ammonium salt with hygroscopicity may further decompose to aqueous hydrogen iodide, which obtains gaseous iodine with the participation of oxygen or through self-decomposition.¹⁷ Oxygen also plays a vital role in light-induced PVSK degradation, accepting photoelectrons to become superoxide that easily reacts with PVSK through deprotonation.^20–23 The product has water molecules resulting in a vicious cycle. Additionally, the common electron-transporting material, TiO₂, is unstable under UV light because of light-induced traps in this layer causing harmful electron flow, so that PVSK will resolve with the lack of electrons.¹⁰ In addition, devices are always warmed up after a period of illumination due to the nonradiative recombination occurring in the films or interfaces, which stresses the importance of trap reduction and makes us pay attention to another unstable impact factor, thermo. For the microcosmic PVSK structure, high temperature touches off the expansion of the PVSK crystal lattice, thereby creating a serious phase transformation.²⁴ In fact, thermal expansion arises in every layer of a device, causing physical volume enlargement and excited ion diffusions, which engender PVSK decomposition into a lead iodide solid disturbing the photovoltaic properties of the device.^25–27 Concerning the electrical or chemical environments in stable PSCs, different testing conditions may lead to serious hysteresis, and the inner ion migration is a type of changing electrical condition.^28,29 In addition, the acidity of PVSK and other layers is also important to device durability.^11,12,30 Observingly, structural firmness and environmental resistance are inseparable and complement each other. If the structure of PVSK is easily deformed, the ambient inferior factors will have a higher possibility of influencing the devices. Similarly, adverse environmental conditions ultimately cause the distortion or decomposition of the interior structure. That is to say, it is important to ensure both structural and environmental stability as there are a variety of solutions for different factors.

Here, we discuss the corrosion mechanisms under these various factors. Moreover, the keynote is focusing on the PVSK film and its proximal layers to organize corresponding feasible approaches on enhancing the durability of PSCs. It is hopeful to pave the way for the PVSK photovoltaic manufacture with long-term stability in diverse aspects.

The device architecture of PSCs can be divided into two types, inverted p-i-n and conventional n-i-p structures.³¹ Both of them contain electrodes for collecting charge carriers, hole-transporting and electron-transporting layers, and the PVSK layer for absorbing light.³¹ As a part of the device structure, it is significant to pay attention to the intrinsic stability of transporting layers. Too many traps or the incomplete coverage of the layers gives chances for moisture or ions to pass, inferior to the maintenance of the PVSK layer.³² 

Taking the common electron-transporting material, TiO₂, as an example, it is seen that it possesses a number of oxygen vacancies serving as an n-type semiconductor, able to be stabilized by outside oxygen.^10,33 However, during long-term working illumination, the layer can absorb UV light in the spectrum to produce photoelectron–hole pairs. The holes in the valence band are going to combine with the negative charge of oxygen vacancies, desorbing surface-adsorbed oxygen.¹⁰ At the same time, the residual uncombined sites have a high possibility to trap photoelectrons from the PVSK layer, not only reducing the photocurrent density but also causing the iodine ion to become iodine and further volatile hydrogen iodide.¹⁰ This UV-induced degradation existed in mesoporous and compact TiO₂ layers.^10,34 Inspired by solid-state dye sensitized solar cells, researchers raised core/shell structures to passivate the intrinsic oxygen vacancies in mesoporous TiO₂.³⁵ As shown in Fig. 1(a), the CdS shell suppressed the oxygen desorption, which decreased the trap sites in TiO₂ and prevented the direct contact of oxygen with the PVSK layer.³³ In addition, Al-doped TiO₂ was used in mesoporous PSCs where every Al³⁺ substituted two Ti⁵⁺ to eliminate the complexion with oxygen so that it can directly interact with oxygen vacancies without introducing trapping levels.³⁶ Similarly, Mg²⁺ mixed into ZnO, an electron-transporting material, occupied the zinc interstitial site for enhanced photostability.³⁷ With respect to planar heterojunction PSCs, Li et al. applied cesium bromide to modify the compact TiO₂ surface, getting rid of the activated intrinsic trap sites originating from oxygen dislodgement and less reduction in absorption ability of PVSK without decomposing the byproduct PbI₂ found.³⁴ Besides, fullerene has been used to modify or replace TiO₂, and plenty of metal compounds have potential to take the place of TiO₂ for better stability.^38,39 For example, ZnSe that could harvest more UV light served as an electron-transporting layer instead of TiO₂ in normal PSCs to abate the UV irradiation on the PVSK film and simultaneously avoid the excessive UV absorption to generate trap states in the TiO₂ layer.⁴⁰ Except for the thin film, the organized nanostructure also can replace TiO₂ in electron transfer such as an In₂S₃ nanoflake array.⁴¹ Actually, a branch in electron-transporting materials is a wide-bandgap metal oxide including ZnO, SnO₂, Nb₂O₅, WO₃, BaSnO₃, BaTiO₃, SrTiO₃, Zn₂SnO₄, and Zn₂Ti₃O₈.⁴² An example is PbZrTiO₃ ferroelectric oxide being an electron extraction thin layer, which created defect-dipoles to separate photocarriers and help in current directional transfer under a bias voltage and UV illumination, beneficial to the long-term device operation.⁴³ Although the choices are various, these oxides still should be guaranteed good intrinsic stability through doping or modification. The popular SnO₂ has been reported to have a better photostability than TiO₂ due to its less photocatalytic activity.⁴⁴ However, its trap states are still an unavoidable problem for high performance. As a result, Roose et al. have indicated that doping Ga can greatly reduce traps in SnO₂ and lower the recombination.⁴⁵ Ren et al. modified SnO₂ through chlorine to decrease trap density at the interface.⁴⁶ In addition to metal oxides, fullerene and its derivatives are potential electron-transporting materials in hysteresis-free PSCs because they possess the ability to passivate the trap states in PVSKs.⁴⁷ For example, Wang et al. proved PCBM/ICBA double fullerene layers decrease the traps in the PVSK.⁴⁸ In an overall consideration, the potential metal compounds possess a divalent anion in the VI array of the element periodic table and metal ions whose activity is not so intense, which may provide a direction for intending synthesis of novel electron-transporting materials.

As the neighbors of the PVSK active layer, the chemical conditions of transporting layers have great influence on device durability. In inverted PSCs, PEDOT:PSS is a universal hole-transporting material due to its facile processability, visible transparency, excellent conductivity, suitable energy band, and feasible flexibility.^49–51 However, the strong acidity of PEDOT:PSS can corrode the contacted PVSK layer and the top electrode, causing coarse PVSK morphology and destructive anode.¹¹ To deal with acidity, various alkalis are utilized for neutralization. Thereinto, introducing strong base additives such as NaOH, KOH, and ammonia has manifested effective pH modulation, but this violent neutralization process is adverse for charge carrier transportation and device efficiency.^52–55 Imidazole as a hydrosoluble mild base was employed to tune the pH value of PEDOT:PSS into neutrality and even slight base through different concentrations and extend the working span of PSCs when maintaining conductivity.^11,12 The frequently used hole-transporting material Spiro-OMeTAD has the ability to chemically shock the PVSK film, attributed to its usual additives, TBP and Li-TFSI.^13,56 The solvent of Li-TFSI, which is acetonitrile, is corrosive to PVSKs, and TBP can interact with PbI₂ to form complexes, both fade PVSK films.^13,56 Abandoning additives of Spiro-OMeTAD seems a reasonable way to this problem, but the conductivity drops a lot.⁵⁷ In other words, it is possible for devices based on dopant-free Spiro-OMeTAD to work for a longer time despite some efficiency sacrifice, and forasmuch, it is urgent to develop non-corrosive and non-hygroscopic additive materials for p-type doping. An attempt has been made in that two-dimensional MoS₂ was incorporated into Spiro-OMeTAD to immobilize Li⁺ for the sake of depressing the ion shift to the PVSK film and further inhibiting the degradation during deliquescence [Fig. 1(b)].⁵⁸ Luo et al. doped PTAA through hydrophobic LAD to replace Spiro-OMeTAD and obtained a longer air exposure duration.⁵⁹ Another viable solution to this chemical impact is inserting a blocking layer under the doped Spiro-OMeTAD. Li et al. used montmorillonite as a buffer layer forming possible hydrogen bonds with Spiro-OMeTAD, which impeded the direct contact between TBP and the PVSK film.⁵⁶ In addition, providing candidates for non-doped hole-transporting materials is of great significance in stability enhancement for both PSC architectures. Metal compounds such as the NiO_x nanofilm, Fe₂O₃ nanoparticles, and hydrophobic Cu₂O quantum dots have been introduced in PSCs as inorganic hole-transporting layers.^60–62 Moreover, a number of organic acceptors have been produced for stable PSCs. For instance, an inexpensive small molecule TS-CuPc with F4-TCNQ doped was capable of varying the pH value to 7.4, avoiding acidic PEDOT:PSS and pure basic TS-CuPc to cause chemical corrosion.⁶³ Besides, basic TS-CuPc doping increased the pH value of PEDOT:PSS, similar to a weak base reaction.⁶⁴ In addition, hydrophobicity is always an auxiliary property in the novel materials.^62,65–67 Leijtens et al. took two acceptors with a similar hydrophobic molecular structure, AS44 and EH44 doped by a silver salt instead of Li-TFSI, to avert the device decomposition under water ambience, and EH44 presented better resistance to water due to the largest contact angle and the most black color after water immersion for 1 min in Fig. 1(c).⁶⁶ Christians et al. further reported that triplet-cation PSCs based on SnO₂ (n-transporting layer) and EH44 (p-transporting layer) achieved 94% efficiency retention after operation for 1000 h under humidity ranging from 10% to 20%.⁶⁵ Besides Spiro-OMeTAD, PTAA gradually showed its advantages in high-efficiency PSCs, although its thermal stability still needs improvement. Kim et al. have used CuPC to take the place of PTAA, increasing the device thermal stress.⁶⁸ Gil et al. have incorporated CuCrO₂ nanoparticles into PTAA for enhanced thermal and light stability.⁶⁹ Therefore, it is serviceable for better durability to put efforts in ameliorating the intrinsic stability of transporting layers and eliminating the detriments under the electrical or chemical environment caused by the layers.

As an active layer, PVSKs play a principal role in device durability. Once the decomposition is triggered, the original light absorption process cannot be carried on and the decomposed ions will diffuse to deprave other layers and electrodes, thereby irreversible device degradation transpired. In addition, the bulk structure of PVSK also has a great influence on the film and device stabilities because the trap density in a single crystal is much smaller than that of the polycrystalline film.^70–72 However, because the deposition technique is complicated, this section focuses on intrinsic stability and environmental resistance of polycrystal PVSK layers to give feasible strategies and materials.

PVSK has an octahedral structure whose vertexes and cores are occupied by anion X and cation B, respectively.¹⁵ The cuboctahedral voids of this framework are inhabited by cation A, which makes a face-centered cubic lattice with a composition of ABX₃ for PVSK.¹⁵ Adjusting ions in the three sites is the primary purpose in composition engineering, able to further regulate physical properties of PVSKs, such as energy level distribution, transparency, absorbance, bandgap, and the most important stability.^73,74 Thereinto, anion X is closely relevant to photoelectric properties, especially the optical bandgap. Although it has been proposed that the incorporation of Br⁻ was beneficial to humidity obstruction of the devices, the regulated bandgap did not favor the absorption, so the device performance was poor.⁷⁵ In addition, current good selections of anions are mainly concentrated on halide ions, acetate ion, thiocyanate ion, and oxalate ion, and thus, its related stability improvement needs to be deepened.^16,76,77 The same as anion X, the choice of cation B is abstracted in Pb²⁺ and Sn²⁺ in consideration of device performance and environmental protection.¹⁵ Therefore, we mainly discuss cation A engineering for durable PSCs.

On the one hand, various ion composition contributes to distinction in thermal resistance for PVSK because the internal microscopic interaction force will be varied for different ions.⁷⁸ Commonly, cation A that can be inorganic or organic interacts with the octahedral framework (BX₆). The interaction is not only electrostatic force but also ionic or hydrogen bondings between cation A and anion X, which is always the halide ion.⁷⁹ That is to say, the inorganic cation (e.g., Cs⁺, Rb⁺) does not easily migrate under high temperature owing to the existence of strong ionic bonding, explaining why Cs⁺ incorporation into organic–inorganic hybridized PSCs is more stable under high temperature.⁸⁰ Although hybridized PSCs theoretically have poorer thermodynamic stability than inorganic PVSKs, different organic cation A brings discrepancy in the respective thermal stability. Comparing the common MA⁺ (CH₃NH₃⁺) and FA⁺ [CH(NH₂)₂⁺], the activation energy for thermal decomposition of FA-based PVSKs was higher than that of MA-based PVSKs.⁷⁸ Speculatively, if cation A can connect with the octahedron structure through a stronger force, a higher activation energy is required for ion migration, which hints enhanced thermal stability of PVSKs. Noticeably, ion migration takes effect during light-induced and moisture-induced degradation of PSCs, too. Lee et al. have demonstrated that Cs⁺ partial substitution in FAPbI₃ increases the interaction with I⁻, which stabilized the ions to avoid their diffusion evoked by light and moisture and prevent the decomposition of the PVSK film, so the absorptions were better for double-cation PVSKs [Fig. 2(a)].⁸¹ However, the operation condition of devices has an influence on light stability, so it is difficult to estimate the effect of cation engineering on device durability under illumination.⁸² 

Size effect during composition engineering has on the other hand huge impact on the intrinsic structure stability of PVSKs, which is usually defined by t.¹⁵ Generally, a higher value of t suggests a dynamic stable PVSK structure, and the value range from 0.8 to 1 is desired because it is nearly cubic lattice.¹⁵ Distortion will happen when t is more than 1 or less than 0.8 so that the structure will be hexagonal or orthorhombic.¹⁵ To take advantage of the size effect, many cations with larger radii have been introduced to site A for adjusting the inner structure stability. A typical instance is Gua⁺, which was introduced into the MAPbI₃ structure, swelling the crystal lattice to near cubic in Fig. 2(b) for a more stable PVSK in thermo.⁸³ In addition, EA⁺ and DMA⁺ have been used to better resist moisture.⁸⁴ In particular, it is possible for a three-dimensional PVSK to stratify into a two-dimensional (2D) structure through large cation A management, such as the insertion of PEA⁺ into a PVSK structure, which hindered the water permeation but depressed the device performance.⁸⁵ As a result, 2D PSCs possess higher stability while experiencing some efficiency loss due to a larger bandgap, so investigators have tried to make use of the merits of the 2D PVSK on a three-dimensional PVSK, which changes the optical bandgap and even contributes to marvelous moisture or thermal resistance. More details about 2D PVSK will be given in the following section about wide-bandgap materials. The important point needed to be paid attention is that the proceeding of composition engineering, however, tends to affect the choices of the ion size. For example, larger FA⁺ has been mixed in MAPbI₃, a tetragonal lattice, to obtain a double-cation PVSK for a cubic structure at a higher t value and better thermal stability as explained before, yet undesirable phase transformation still occurred.^86,87 Consequently, smaller cation Cs⁺ and Rb⁺ were added to occupy partial site A, which formed ionic bonding with halide anions to inhibit ion migration and enhanced the upper limit of allowable temperature, despite sacrificing some structural stability.^80,86 In other words, ion types and ratios are not complementary but a compromise in component engineering. Suitable selections can make the corresponding PSCs extensively stable.

During the fabrication process of PSCs, crystallization and external impurities have great possibility to introduce non-negligible defects and traps, which cause unfavorable trap states to make carrier recombination or act as ion migration channels, leading to negative impact on device performance.^16,77 Moreover, ions migrating to the interfaces will induce phase segregation, interface reaction, or electrode degradation, to attenuate not only intrinsic stability but also photothermal and electrical durability of PSCs.^20,82,88 In particular in the grain boundaries and surfaces of the PVSK film, a large quantity of defects and traps exist. In order to abate this phenomenon, several passivation methods have been proposed, which are generally accompanied by the presence of special bonding or interaction forces, covering covalent bonds, ionic bonds, hydrogen bonds, chelation, and so on. Covalent bonding is always involved with deactivation through organic Lewis acids or bases. Lee et al. employed non-volatile Lewis base urea into PVSK to increase the grain size and counteract the traps around the grain boundaries through the Lewis acid–base adduct approach according to the narrower distributed current signal for the PVSK film in Fig. 3(a), so that there was less space in the PVSK films for water osmosis leading to decomposition.⁸⁹ Ionic bonding involves ion doping and substitution, resembling the composition engineer on PVSKs. Wang et al. pretreated the bottom interface of the PVSK through potassium chloride whose ions diffused into the absorption layer to dispel interstitial sites and halide vacancies, for less trap formation.⁹⁰ Some organic materials are particularly equipped with both positive and negative functional groups, enabling passivation of different types of defects through ionic interactions. Figure 3(b) illustrates a sulfonic zwitterion interacting with the PVSK structure where the positive-charged quaternary ammonium group connected with the MA⁺ vacancy and the negative-charged sulfonic group is attracted by the I⁻ vacancy.⁹¹ Similarly, Wang et al. have applied sulfobetaine zwitterions to stabilize CsPbI₃ phase change.⁹² Wang et al. have used CHI to passivate the inorganic PVSK thin film for less traps.⁹³ Besides bonding through electron sharing or transfer, hydrogen bonding as a strong intermolecular interaction is widely engaged in passivation for stable PVSK photovoltaics.^94,95 Li et al. revealed that the –PO(OH)₂ and –NH₃⁺ terminal groups of a phosphonic acid ammonium cross-linking agent interacted with neighboring PVSK grains through O–H⋯I and N–H⋯I hydrogen bonding, respectively, assisting the PVSK filling the TiO₂ scaffold to obtain more uniform PVSK films and further uplift the moisture resistance for the films and devices.⁹⁵ It is obvious that –NH₃⁺ can use its hydrogen atom to connect with the halide anion, but we need to pay attention to the positive charge of this ion that possesses ionic nature to act as an acceptor for charge neutralization, which was to occupy the MA⁺ vacancies in this example.⁹⁵ Additionally, the connection between polyvalent metal ions and organics may contribute to coordination bonds such as chelation. For example, polyaspartic acid sodium with many –COO⁻ generated a strong chelating cooperation with Pb²⁺ to passivate surface traps originating from Pb²⁺ and acquire enlarged PVSK grains.⁹⁶ It cannot be ignored that most of passivation processes do not rely on one kind of single bonding but involve diversified interactions as we have told about –NH₃⁺ above. Another case was Cu:NiO doped FAMA-PVSK in which cross-linking and chemical bonding together facilitated the thermal and air stresses [Fig. 3(c)].⁹⁷ Besides, PVSK nanocrystals and quantum dots also can be directly applied in stable PSCs through assisted bonding force or charge coupling. For example, Xi et al. have proposed a ligand named FPEAI to interact between the CsPbI₃ nanocrystals to enhance the PVSK crystal lattice structure.⁹⁸ Ling et al. have taken GA⁺ as a surface matrix of CsPbI₃ quantum dots through the ligand exchange process to stabilize the cubic structure by interparticle electrical interactions.⁹⁹ Evidently, the passivation process can administer crystallization and durability of PSCs through a possible multiple bonding aggregation, which is complicated and needs overall consideration.

Moisture is one of the degradation factors for PSCs, and the most direct solution at present is the utilization of hydrophobic materials. They can resemble a cover to physically inhibit water infiltration in a macroscopic view and can make use of the hydrophobic molecular structure to chemically repulse outside moisture. In fact, these materials are mostly organic, and their hydrophobicity relies on special functional groups in the molecular structure.

The simplest one is the alkyl group shown in Fig. 4(a) where MAPbI₃ ink was doped by a small amount of bilateral alkylamine additive whose structure was NH₂–R–NH₂.¹⁰⁰ The R represented the alkyl chain for water repelling. Importantly, although nitrogen atom interacted with Pb²⁺ through coordination bonding to passivate the grain boundaries, the moisture resistance was partially in virtue of the alkyl group.¹⁰⁰ It is noteworthy to make a distinction between the passivation through special bonding discussed in the last section about perovskite passivation by special bonding and the hydrophobic function group. The purpose of the former is to reduce the defects and traps in PVSK or its interfaces for better intrinsic stability, whereas the latter focuses on outside assistance for enhancing the water inhibition ability of PVSKs or PSCs. The benzene ring is also hydrophobic. As an additive for MAPbI₃ precursor solution, butylated hydroxytoluene antioxidant that contains both the benzene ring and methyl groups increased the contact angle of the PVSK film to water by ∼15° when the doping concentration is 4.2 wt. %, inferring the enhanced restraint in water penetration.¹⁰¹ Besides single small molecules, a lot of polymers also possess good hydrophobicity and connect with PVSKs through cross-linking.^95,102 For instance, insulating polystyrene, a kind of plastic material, has been inserted between PVSK and transporting layers, which formed a tunneling contact in order to reduce nonradiative recombination and isolate water ambience.^102,103 In addition, several carbon networks are nice choices for moisture defense. A fullerene amine interlayer was added into a planar PSC below the metal electrode, which helped the devices exposed to 45% humidity for 20 days maintain more than 90% of the original efficiency value.¹⁰⁴ It is also obvious that the PVSK degraded to make the dropped water being yellow without the interlayer protection. In addition, a compact hydrophobic carbon layer has been used in PSCs free of the hole-transporting layer, which bated the moisture corrosion.¹⁰⁵ In one respect, it was doped by MAI to offer excess raw materials for PVSK reconstitution under decomposition temperature, promoting the thermal stability.¹⁰⁵ Like a carbon network, an S₈ ring has been reported to be cut as a bridge between two oleylamine molecules so that a long chain was propitious to impede water from the PVSK layer due to the increased contact angle for the treated film [Figs. 4(b) and 4(c)].¹⁰⁶ 

Besides doping and modification, it is practical to employ hydrophobic materials in device encapsulation. Common encapsulation methods such as package lid sealing through UV adhesives raise the technique difficulty, and the residual water vapor inside the encapsulating covering still can lead to gradual device breakdown, and forasmuch, the employment of hydrophobic materials concentrated on polymers constructs feasible defensive shelters to physically and chemically get rid of water.^107,108 There is an instance shown in Figs. 4(d) and 4(f) that a Teflon layer spinning-coated on the top of the PSCs presented no color change during water soaking, revealing suppressed moisture-induced PVSK degradation.¹⁰⁷ Therefore, there are a wide range of selections for hydrophobic materials, and their usable methods are diverse consisting of PVSK additives, insertion layer, or device encapsulation. Actually, hydrophobic materials always contain other special functional groups or atoms for interacting with the PVSK structure well, as some passivations mentioned in the last section about perovskite passivation by special bonding; thus, they may get the ability to adjust intrinsic or thermal stability of PVSKs.¹⁰³ In other words, stability improvements through hydrophobic materials are generally not unidirectional (only for moisture inhibition) but multi-directional and complementary. Specially, a hydrophilic polymer in a way improves moisture stability of PVSK films and devices. It kept water in itself so that moisture is difficult to permeate the layer, and its good adhesivity in enhancing the mechanical stability between the PVSK and transporting layers makes less space at the interface for moisture contact with PVSK films.¹⁰⁹ Hence, both hydrophobic and hydrophilic materials need to be treated dielectrically for their role in moisture stability improvement.

In PSCs, the application of wide-bandgap materials, which require higher energy to excite, has no effect on the visible light absorption of PVSK active layers. Moreover, the heterojunction constructed in contact with the PVSK layer effectively decreases the charge leakage during carrier extraction and transfer.¹¹⁰ Generally, it can be classified into two kinds, wide-bandgap metal compounds and low-dimensional PVSKs, which both possess the talent in polishing the stability of PVSK photovoltaics.

It is valuable to pay attention to the chemical stability of wide-bandgap metal compounds, which is originated from the low solubilities and high melting/boiling points.^42,110 In the manufacturing process of silicon-based solar cells, it is popular to passivate surfaces through silicon oxide, silicon nitride, or aluminum oxide for lower defect density and better silicon protection from degradation.^111–113 Similarly, a large number of reports about PSCs have demonstrated that wide-bandgap metal compounds have the capability of effectively boosting device stability when acting as interface layers or participating in PVSK formation. For instance, PbI₂ whose bandgap is ∼2.3 eV and water solubility is 0.756 g/l has been demonstrated that its excess quantum and uniform distribution in a PVSK precursor can fill the pinholes and grain boundaries formed during crystallization, which assists the intrinsic stability of PVSK films and shrinks the space for moisture to contact active layers.^114,115 However, the amount of PbI₂ should be curious due to the light-induced degradation in the PVSK film with too more PbI₂.^115,116 Yang et al. put forward more insoluble lead oxysalts [PbSO₄ and Pb₃(PO₄)₂] produced through reactions between organic salts and the PVSK material, to cover the PVSK films for decreased shallower trap-state density, and enhanced the defense to moisture.¹¹⁷ In addition, the PVSK single crystals protected by PbSO₄ did not change color during water immersion, while the control one turned to yellow, suggesting the lead oxysalts were useful in inhibiting moisture.¹¹⁷ Besides metal oxysalts, metal oxides are capable of being introduced into PSCs for longer durability. For example, Al₂O₃ modified the PVSK interface to improve the stability in humidity and under UV light.^117,119 As mentioned above, metal oxides have a broad application in transporting layers due to their excellent charge mobility and suitable matching of band levels, which not only helps the intrinsic stability of transporting films but also facilitates less traps and good hydrophobicity for the interfaces.

Another type of wide-bandgap material is low-dimensional PVSKs mainly referring to the 2D PVSK with a general formula of A′_mA_n−1B_nX_3n+1, where m equals 1 or 2 if spacer cation A′ is divalent or monovalent and n represents the number of inorganic octahedral layers.¹²⁰ 2D PVSKs have attracted a great deal of attention due to their outstanding environmental stability in virtue of the van der Waals interaction of hydrophobic spacer cations.¹²¹ In addition, the layered structure of 2D PVSKs impedes ion migration.¹²¹ Accordingly, it is of much significance to apply the material that is usually produced through the self-assembling of alkylammonium salt and metallic halides in 3D-PSCs. For example, the pentafluorophenylethylammonium cation (FEA⁺) was introduced to replace the original site A cation in the octahedral framework to obtain an ultrathin 2D PER film, (FEA)₂PbI₄, on the surface of the active layer through soaking the spin-coated 3D PVSK layer in the FEAI solution.¹²² The surface texture in the SEM image of the modified PVSK film indicated the formation of a 3D/layered structure, and its corresponding contact angle with water had an increment, stating the better moisture resistance.¹²² Besides immersion in solution to get a 3D/layered architecture, it also can be formed through simple precursor management or surface modification. Wang et al. tuned the ratio of BA⁺ in BA_x(FA_0.83Cs_0.17)_1−xPb(I_0.6Br_0.4)₃, and then a 2D-3D PVSK heterostructure was got by annealing, which improved the bandgap of the active layer to 1.61 eV, allowing a highly oriented PVSK layer to maintain 80% of efficiency for 1000 h in air.¹²³ Cho et al. used mixed solution of iBAI and FAI to generate a 2D film passivating the (FAPbI₃)_0.85(MAPbBr₃)_0.15 layer.¹²⁴ This mixed 2D–3D passivation treatment greatly enhanced the device stability at high humidity.¹²⁴ In addition to a 3D/layered structure, it is admissible to conglutinate crystal grains of the absorption film through 2D PVSKs. Figure 5(a) exhibits the PSC architecture based on 2D/3D FACsPbI₃ where 2D (PEA)₂PbI₄ within the PVSK film adheres to the cracks between the grains for assisting the phase stabilization.¹²⁵ Not only alkylammonium halides can participate in the generation of 2D PVSK, but also some non-halide salt gains the layered structure. HOOC(CH₂)₄NH₃I has been proposed to prepare a precursor with PbI₂, which served as an additive for MAPbI₃ precursor solution, obtaining a matched structure with an increased energy gap of 0.09 eV.¹²⁶ The lower conduction band of a 2D layer blocks the electron recombination, which was closely related to the light-induced charge carrier offset, so that more than 60% efficiency was kept after 300 h continuous illumination.¹²⁶ In unstable Sn-based PSCs, inorganic pseudohalogen NH₄SCN was employed to separate the nucleation and crystal growth processes during the growth of a 2D–quasi-2D–3D Sn-PVSK layer, which reduced the layer number of 2D PEA₂SnI₄ for enhanced air stability.¹²⁷ Hence, it is worth to deeply consider 2D PVSKs including halides or non-halides, organic or inorganic, in the further photovoltaic stability improvement. There are some investigations about one-dimensional (1D) PVSK in durable PSCs. For example, Gao et al. formed a 1D TAPbI₃ capping layer to isolate the PVSK layer from the air ambience, raising the environmental stability of the devices [Fig. 5(b)].¹²⁸ Zhang et al. reported to control the dimension of quadruple-cation PVSK through adjusting the amount of doping GA⁺, and they obtained a 1D/3D PVSK for overall facilitation on stability.¹²⁹ Hence, the combination of low-dimensional and 3D PVSKs is rather beneficial to the device lifespan.

Ionic liquids that are only composed by anions and cations generally refer to molten salts that are fluid around room temperature.¹³⁰ The irregular stacking of the ions caused by the asymmetry internal structure of ion liquids results in the low melting points, and because of the stronger electrostatic attraction between the ions, their volatilities are usually very low, which enables ion liquids engaged in crystallization control for PSCs by retarding the growth process of PVSKs during annealing for ampliative crystal grains and enhanced device performance.^130,131 High electrical conductivity is another attractive property for ion liquids, thus giving them chances to take the role of transporting layers or be the additives for transporting materials, which ameliorates the electrical property and arrests carrier recombination in PSCs.¹³² The most important for ion liquids is their exceptional thermal and electrochemical stability, because of which it is difficult to change the state in high temperature and different solutes.¹³⁰ As an example, ETI, an amphiphilic ion liquid, was merged into MAPbI₃ to gradient distribute for better thermal protection by inhibiting MA⁺ diffusion to adjoining layers, which achieved more than twice efficiency preservation for the doped device than that of the control when experiencing continuous illumination and heating at 60 °C for 700 h.¹³³ Bai et al. introduced BMIMBF₄ ion liquid into triplet-cation inverted PSCs to suppress the ion migration and enhance the stability under heat and light, which guaranteed the long-term working of the device.^134, Figure 6 gives the molecular structures for some examples of ionic liquids used as dopants in PSCs.¹³⁵ Furthermore, most ion liquids resist water rooted from the hydrophobic molecular structure of the organic ions such as benzimidazolium, pyrrolidinium, and triazolium.¹³⁵ This hydrophobicity enables to construct moisture shelter for PVSK films and devices. Wang et al. exposed the PSC based on fabricated hydrophobic ionic liquid to relative humidity of 57% for 40 days and nearly no efficiency decline.¹³⁶ In addition to doping, many researchers take ion liquids as a kind of interface modifier to do away with the surface traps.¹³⁷ In another aspect, simulating a hydrophobic membrane physically prevents moisture and oxygen, thereby improving the device duration. Particularly, it is allowable for ion liquids to be the solvent for PVSK precursors. For instance, MAI and PbI₂ were dissolved in an ion liquid MAFa by the ratio of 1:1, and the solution was spinning coated to directly form the MAPbI₃ film for perfect directional crystallization without intermediates, and then the PVSK degradation process was suppressed, which might come from the abiding nonvolatile ion liquid solvent or its inhibition to MA⁺ diffusion.¹³⁸ In general, environmental-friendly ionic liquids that can be innovated into PSCs through various methods are helpful in stability enhancement including heat, light, and water for PVSK photovoltaics.

In general, it is significant to focus on the PVSK block and its interfaces with the adjacent transporting layers, which can improve the intrinsic stability and device durability under different conditions including light, heat, humidity, and electrochemical environment. Exceptionally, encapsulation through hydrophobic materials for better durability focuses on the whole device rather than the only PVSK layer. Strategies for sustainable PSCs involve component engineering of PVSKs to enhance their intrinsic stability and passivation methods to reduce trap density for higher resistance on electrical aging induced by ion migration. In addition, the effective materials for better sustainability are diverse, containing hydrophobic materials, low-dimensional PVSKs, metal oxides, and ionic liquids, all of which can be employed in PVSK doping, interface modification, or transporting layer adjustment. Nevertheless, it is evident that most of these materials interact with PVSKs through both physical and chemical reactions rather than just a single force to shield the films and devices. For example, hydrophobic molecules often closely associate with the PVSK film through hydrogen bonds with the addition of a physical shelter.¹⁰⁰ Charged molecules’ passivation refers to electrostatic force and chemical ionic bonding.⁹⁵ Therefore, the protection mechanism research on stability improvement needs to be expanded in order to provide a good foundation for subsequent material selection. Moreover, it is necessary to strengthen the comprehensive resistance studies, although these materials can, respectively, inhibit the aging of PSCs caused by one or more external conditions. At present, ionic liquids with excellent conductivity seem to have a greater advantage among them. Their suppression ability on ion migration is obviously beneficial to thermal stability and retards light-induced degradation to some extent.^131,133 Simultaneously, they are easily modified to possess hydrophobicity for moisture filter, and the low volatility makes them an environmental-friendly material during adjusting crystallization, yet the major difficulty is the complicated synthesis and purification procedure.^130,136 Besides, 2D perovskite materials and passivation approaches are commonly effective methods, boosting stability and ensuring high efficiency.

From the future research perspective, PSC stability under pure oxygen is interesting since most of the PVSK decomposition involves the participation of oxygen.^20,139 If the contact of pure oxygen is hindered, the decomposition reaction could be inhibited to a certain extent for better moisture and light stability. Despite several investigations about antioxidants engaged in PSCs, none of them concentrated on only oxygen influence, so the inner mechanism without oxygen is also attractive.^101,140,141 Moreover, the antioxidant applications are mostly for PSCs with tin to impede the oxidation of Sn²⁺.¹⁴¹ In addition to the stability of PVSK and its adjacent transporting layers, the impact of metal electrodes on the durability of PSCs after a long operation period is equally considerable. It has been raised that the pinholes in the hole-transporting layer offer channels for iodine ion diffusion to oxidize silver, corroding the electrode.⁸⁸ Another view indicates that the electrode collects carriers to become metal ions, which can migrate to nether layers, inferior to the device.¹⁴² Guarnera et al. provided a thin porous Al₂O₃ buffer layer above the PVSK layer to avoid the shunting pathways.¹¹⁸ Hence, the electrode stability plays an influential role in durable PVSK photovoltaics. In addition, sputtering process control for smooth substrates that influences the morphology of uplayers and further affects the traps should be paid attention to. In addition, for thermal stability, thermal expansivity is an important point to focus, producing or selecting some “glue” materials that have the potential to solve the problem. Similarly, applying light-responsive materials to enhance the intrinsic and light stability of the devices is also a promising direction.

All above are performance persistence research on rigid PSCs, but PVSKs have developed flexible devices due to their soft nature that requires additional attention. In flexible photovoltaics, substrate stability and bending resistance are special to be in advertency. The former is related to the thermal limitation of flexible substrates during the preparation process, and the latter delves the ductility of every layer in a device, on which related attempts have been put, such as low-temperature fabrication, substrate enhancement, and stress and strain adjustment.^143–145 For example, Al-doped ZnO as an electron-transporting layer and t-BAC passivated PVSK jointly realized a low-temperature processed flexible CsPbI₂Br PSC.¹⁴³ Other studies are about the strain changes for different contacting layers to PVSKs.¹⁴⁵ Although researchers have some involvement in this direction, they are still deficient in targeted and fundamental investigations on flexible stability.

Finally, it is valuable to emphasize the standardization about stability characterizations of PVSK photovoltaics. Current durability measurements have been conducted under various conditions, so that they cannot be compared with each other and cannot give much guidance for the following researches. Moreover, standardization is fundamental for the commercialization of PSCs, to be on a par with conventional silicon-based photovoltaics. At present, only a small number of researchers have put forward their opinions on standardization.^146,147 Therefore, this standardization still needs more support from academia and industry for universal acknowledgment in the future.

The data that support the findings of this study are available within this article.

The authors acknowledge financial support from the Natural Science Foundation of China (Grant Nos. 61674109 and 51821002), the National Key R&D Program of China (Grant No. 2016YFA0202402), the Natural Science Foundation of Jiangsu Province (No. BK20170059), and the Open Fund of the State Key Laboratory of Integrated Optoelectronics (No. IOSKL2018KF07). This project was also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology, by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and by the“111” Project of the State Administration of Foreign Experts Affairs of China.