Smart Materials to Empowering Perovskite Solar Cells with Self‐Healing Capability

Inspired by nature, intelligent self‐healing materials have recently been exploited also in the field of photovoltaics to mimic natural systems achieving self‐repairing. The past decade has witnessed perovskite solar cells (PSCs) skyrocketing to a certified power conversion efficiency of 26.1%. However, their intrinsic instability, when exposing to moisture, high temperature, and continuous illumination, hampers their commercial development for a long‐term use in ambient operating conditions. Therefore, the use of smart self‐healing materials, based on self‐assembling properties and dynamic interactions, empowers PSCs with self‐recovery abilities to reinforce their pivotal role as efficient photovoltaic devices and encourage their exploitation in the market. Herein, the current progress in self‐healing perovskite materials with a special focus on self‐recovery after moisture exposure or mechanical damage with the aim to provide a valuable insight for research on this topic to accelerate the PSC commercialization process is highlighted.


Introduction
"Self-healing" ability is the capability of some specific natural biomaterial and organs to self-repair.Mimicking nature, some intelligent synthetic materials can self-assemble thanks to dynamic interactions and have recently found application also to the field of photovoltaics. [1]The challenge in this case is to join amorphous and fluidic matters (self-healing polymers) to ordered crystalline or semicrystalline materials (semiconductors).
In the past decade, collaboration between academic research and industry have produced a dramatically fast improvement in perovskite solar cell (PSC) [2] technology in terms of efficiency, processing methods, and partially also for what concerns stability.Currently, a certified power conversion efficiency (PCE) of 26.1% has been obtained, reaching the 26.5% record efficiency of silicon heterojunction solar cells. [2]mpared to silicon-based solar cell technology, PSCs have the advantage of relatively simple solution processability and lower cost processing technology. [3]owever, their degradation within a few months under ambient operating conditions is not tolerable for commercial development of such types of devices. [4]mong several strategies to delay degradation, surface passivation and encapsulation approaches have been predominantly pursued so far. [5,6]Reviewing all types of passivation techniques developed up to now is beyond the scope of this work, but the reader may find detailed information on the subject in other remarkable and comprehensive reviews available on the topic. [7,8]However, in some cases, the bulk passivation approach has yielded inferior device performances. [9]In contrast, interface defect passivation has not always ended up with stability enhancement, even when device efficiencies have been improved. [10]herefore, if perovskite degradation is unavoidable for highly efficient devices, exploiting a degradation process that can be controlled and reversed seems a promising approach to insure a prolonged stability in ambient condition. [11]erovskite is a material with general formula ABX 3 (these perovskites are also known as 3D perovskites). [12]The first mineral, discovered in 1839 in Russia, was CaTiO 3 and belonged to the family of inorganic perovskites. [13]Speaking of the hybrid perovskite, we talk about a subclass of this family of perovskites in which A is an organic cation, generally methylammonium ), B a metal cation, typically lead (Pb 2þ ) or tin (Sn 2þ ), and X a halogen. [12,13]The 3D structure is made of connected octahedra in which the halide is at the vertices, while at the center is positioned the metal (B), and in the interstices, formed between the octahedra united by vertices, is placed the organic cation (A). [12,13]erovskite material itself shows intrinsic self-healing properties within crystal grains, as for example phase transitions, reversible ionic migration, and decomposition-recrystallization processes. [14]Of particular interest is perovskite capability to photocurrent self-healing thanks to light-activated trap states dissipation once the device is rested in the dark after light-induced degradation. [15]Nie et al. studied the photodegradation effect and the self-healing mechanism in MAPbI 3 -based thin films as well as in solar cells, related to the mid-gap states activated by light. [15][18] In this respect, Scheme 1 shows the charge carrier population of metastable trap states activated by light and the self-recovery process in the dark.
Indeed, devices with recoverable performance through dayand-night cycles can extend the device lifetime. [19]However, perovskite cannot heal defects at the grain boundaries or cure large mechanical failures, or decomposition due to water ingress in the device. [20]Thus, an additive-assisted self-healing procedure, obtained by the incorporation of self-healable materials, could promote the cure of defects, moisture-induced decomposition, and mechanical fractures (Figure 1). [14,21]n this review, current progress in self-healing perovskite materials will be discussed.After surveying the major reasons for device degradation as the necessary background for the understanding of the subject, a section of this article will be dedicated to examining advanced smart self-assembling materials that can be applied to photovoltaics.Then, a special focus on self-recovery after moisture exposure or mechanical damage due to several bending cycles will be given by discussing recent relevant examples of self-healable perovskite materials, trying to uncover their mechanism.Finally, a summary as well as future challenges and prospects of self-healing PSCs will be discussed with the general aim to provide a useful understanding for pushing research on this topic and promoting the PSC commercialization process.

Degradation in PSCs
As anticipated earlier, in this section of the review, the degradation of perovskites is discussed as a useful background needed to understand the general subject of self-healing perovskites and stimulate the design of devices that minimize irreversible degradation.However, in-depth reviewing of all types of degradation mechanisms is beyond the scope of this work and detailed information on the subject may be found elsewhere. [4,20]heme 1. Schematic representation of photocurrent degradation and self-healing mechanism.Valence and conduction bands (VB and CB) are sketched for different situations: photodegradation, recovery in dark and after self-healing.Dotted lines indicate metastable trap.Arrows show that photo-generated carrier can populate those trap states or relax to steady state in dark.Reproduced with permission. [15]Copyright 2016, Nature.In general, stability of PSC can be affected by several factors: temperature (T), [22] ambient relative humidity (RH), [23][24][25] voltage (V), [26] and illumination in particular by UV light. [23,27]Different strategies, such as encapsulation, [27,28] use of interlayer, [29,30] and changes in the halide composition of the perovskite material [31][32][33][34][35][36] have been used to overcome these problems, while several tests have been issued by the International Electrotechnical Commission to evaluate PSC stability. [37]SCs are often tested in insulated environments, where they are tested for individual stresses. [38]Thus, stability to heat, light, and humidity are tested separately, using dedicated certified tests such as thermal cycling, dump heat, and humidity freeze, to decipher experimental results as the consequence of a single stress type. [38]he factors leading to perovskite degradation are of two types: intrinsic and extrinsic ones. [39]Intrinsic factors are concerning the composition of the active material and involve displacements of elements present in the film, [39][40][41] due, for example, to thermal instability [22] or instability in a particular electromagnetic range as for example UV light. [23,27]These factors are decisive because the fact that the material, due to its intrinsic composition, is unstable at working temperatures and pressures renders any type of encapsulation technique ineffective.[25] Differently from intrinsic factors, extrinsic ones can be addressed through encapsulation strategies that aim to isolate the device from the external environment to prevent its deterioration and possible leakage of volatile components. [42,43]n this section, special emphasis will be given to particularly discuss methylammonium-based (MA-based) perovskite degradation as it is one of the principal types of perovskites integrated with organic macromolecules with healing capacity studied so far.

Thermal Instability
Understanding the reasons of MA-based perovskite thermal degradation is important to program self-healing PSCs that could reduce irreversible degradation.
One of the reasons for the PSC inability to maintain efficiency for a long period of time is due to the evaporation processes of the organic component of the perovskite that may occur even at relatively low temperatures such as 85 °C. [44]Often these components, given their volatile nature, escape from the perovskite film, favored by thermal stress and atmospheric agents (oxygen and humidity), leaving only the inorganic residues, which are not sufficient for photovoltaic conversion purposes. [45]This is due to the soft nature of CH 3 NH 3 PbI 3 (MAPbI 3 ) [46,47] characterized by a low enthalpy of formation (about 0.1 eV for unit formula); therefore, the barrier to switch from MAPbI 3 to its precursors, PbI 2 and MAI, is relatively low. [48][51][52][53] The thermal degradation of perovskites and the consequent formation of subproducts is summarize as a sketch in Figure 2a, while below are reported equations for the possible mechanisms of degradation obtained by studying MAPbI 3 decomposition only from a thermal viewpoint: Equation ( 1) is reversible, while reaction 2 is irreversible.However, as PbI 2 may degrade into Pb 0 and I 2 under UV-light illumination and volatile I 2 may escape; in that case, the first reaction should no longer be considered reversible. [54]lternative ways have also been suggested in which CH 3 I remains connected to Pb-I fragments leaving an unstable perovskite structure (CH 3 PbI 3 ) because of the inappropriate size of the CH 3 þ cation. [54] 3 NH 3 PbI 3 ðsÞ !NH 3 ðgÞ þ CH 3 PbI 3 ðsÞ (3) Finally, an additional reaction leading to the formation of alkenes and HI is also reported. [55]H 3 I ðsÞ !CnH 2 n ðsÞ þ n HI ðgÞ Although the predominance of one reaction over the other is strictly dependent on the environment, Equation ( 1) is kinetically limited by the strong C-N bond, this makes it dominant especially at low temperatures (between 55 and 85 °C). [44,53,56,57]owever, the decomposition pathway described by reaction 2, in which gas is release, is also relevant because, unlike reaction 1, it makes the system irreversible since reformation of methylammonium iodide is no longer possible. [20]Furthermore, the temperature at which degradation processes occur is one of the key problems regarding MA-based solar cells.There are conflicting results in this respect, as mass spectroscopy shows releases of MAI at temperatures of 60 and 180 °C. [58]In addition, it must be considered that since the degradation of MAPbI 3 is a desorption process; also the external pressure can greatly affect the rate of degradation and the release of components such as HI and CH 3 NH 2 . [56,59,60][63] Another important aspect to consider is the instability due the contact between perovskite and charge-transport materials.In particular, it has been reported how the contact between zinc oxide (ZnO), an electron transporting material used to replace titan dioxide (TiO 2 ), [64][65][66] and MAPbI 3 leads to a degradation during the annealing step. [67,68]In fact, the non-complete ZnO oxidation leads to the surface absorption of hydroxyl groups that, under annealing, trigger perovskite degradation with the formation of methylamine, water vapor, and lead iodide. [67,68]n addition, the relatively low isoelectric point of Zn 2þ leads to the deprotonation of MA with the consequent thermal decomposition. [67]

Humidity Exposure Instability
It is important to keep in mind that limited moisture and oxygen exposure during the annealing can help the crystallinity.This effect can suppress nonradiative recombination and therefore increase device performances. [69]Moreover, ambient RH exposure after fabrication may also improve performance even though it has the disadvantage of accelerating the degradation of the devices. [70]Thus, there should be fine consciousness on the role of moisture and oxygen to exploit the benefits, [71][72][73] while avoiding accelerated degradation.In fact, since MA salt is hygroscopic, it can absorb air moisture, therefore disrupting the perovskite lattice and fostering ion migration. [70]everal studies have shown how a device able to withstand for 15 months in nitrogen atmosphere can degrade in only 3 days in ambient environment at room temperature due to oxygen and water absorption and desorption of the organic component near the surface. [74,75]In fact, different molecules in the atmosphere can interact with perovskites, speeding up decomposition. [25]It is known that water is the main cause of degradation catalyzing Equation ( 1), [23] regardless of the presence of light (unlike O 2 whose degradation is related to lighting). [76]Spectroscopic measurements have shown that water molecules can take place in the space between MA þ and I À within the perovskite lattice even at RH below 10%. [77,78][81] A second proposed degradation mechanism considers the reversible formation of mono-and dihydrated species together with the consequent expansion of the material lattice, [82] when RH is greater than 70%. [79]It has also been shown that both hydrated phases can lead to irreversible degradation forming PbI 2 and other by-products, [25,70,79] including molecular H 2 and I 2 under illumination in the presence of oxygen. [23,24] As a summary, in Figure 2b, a schematic representation of the moisture-induced degradation of perovskites is shown.

Light-Oxygen Exposure Instability
Perovskite degradation that occurs under illumination and in the presence of oxygen is a very frequent phenomenon. [17]In fact, the photoexcited electron in the active material can react with O 2 forming the superoxide (O 2 À ) that in turn reacts with the CH 3 NH 3 þ cation forming PbI 2 , I 2 , and H 2 O leading to the formation of hydrated phases, and further degradation of perovskite, as discussed earlier (Figure 2c). [83,84]MA-based perovskites have been observed to degrade in the presence of light both in dry air and in ambient atmosphere. [60]However, no degradation was observed in samples kept in nitrogen under lighting and in dry air without lighting. [85]This indicates that the degradation process is activated by light, in presence of oxygen, and it is independent of the presence of water.Thus, in this case, the synergistic effect of light and oxygen is the main cause of the degradation of the PSC. [85][88][89] In fact, films containing PbI 2 residues are unstable, under lighting, even in an inert atmosphere, where the concentration of PbI 2 increases overtime. [90]One interesting thing is that this degradation occurs both with or without UV filter, accelerated by the presence of moisture for the reversible (but irreversible under lighting) formation of mono-and dihydrate phases. [25,79,91]It must be taken into account that also the methyl ammonium iodide degrades under illumination, in air, with the following reactions: [23,24] 2I À ↔ I 2 þ 2e À (10) where I 2 is generated by the photodissociation of PbI 2 together with Pb 0 .This results in the loss of methylamine, which has a boiling point of 17.5 °C, and HI.In addition, a role of I 2 in degradation under lighting has also been reported. [75]Considering that I 2 is one of the possible by-products of degradation, this aspect is very important.][94] The problem of ion migration is very important in perovskites with mixed composition (with different halogens and/or cations) due to segregation in domains characterized by the presence of a single halide. [17,18]Another interesting phenomenon is that exposing to 1 sun of illumination a PSC, the UV light caused degradation, is followed by a partial recovery of performance up to 70%, which is also achieved by leaving the sample in the dark for a few hours. [19]7][98][99][100]

Mechanical Damage
A different type of device degradation is the one consequent to mechanical damage such as lacerations, distortions, and cracks that can be caused during device assembling procedures or after several bending, in the case of flexible devices (Figure 2d). [101]urthermore, in the assembled devices, stress strain can be generated between the perovskite and the adjacent layers that is detrimental for photovoltaic efficiency. [62]Strategies should be designed to facilitating strain relaxation. [102]ince inorganic-organic halide perovskites are quite brittle and rigid, the perovskite film is the most fragile layer of the assembled cell. [101]evertheless, perovskites, differently from other inorganic semiconductors, present high ion mobility that can occurs both in the crystal lattice and at the grain boundaries. [16]In addition, the low energy crystal formation of perovskites suggests that they represent interesting types of self-healing materials by themselves. [103]ecovery of mechanical damage is of extreme importance when considering the preparation of efficient and long-lasting flexible PSCs (F-PSCs), which have recently attracted huge attention because of their application as portable and wearable electrical devices thanks to their low-cost processability and high power-per-weight. [101,102]Indeed, the "Achilles' heel" remains their poor mechanical stability. [11]In this respect, adding soft polymers at the grain boundaries can rise the flexibility of the perovskite layer itself and improve its self-healing capability. [11]ealing mechanical damage has predominantly been investigated in the polymer field.Generally, self-healing polymers are designed in a way that carry reversible dynamic covalent bonds (i.e., groups for Diels-Alder reaction or disulfide bond formation) or/and different types of dynamic noncovalent bonds, such as metal-ligand or electrostatic interactions, hydrogen bond, or Van der Waals forces.The healing process is based on the reformation of bonds that were broken after a mechanical stress.In some cases, self-healing polymers can self-repair extemporaneously through chemical reactions or the recovery of the noncovalent interactions without the action of any type of external stimuli.In other cases, the damaged polymers can self-heal only in the presence of external stimuli such as heat or light.Mechanical self-healing applied to PSCs will be explained in detail by discussing some relevant examples in the dedicated section of the review.
In summary, some key rules can be suggested to limit irreversible degradation of the perovskite material, while promoting its reformation: lowering the formation energy of the perovskite, capturing the gases that may form upon degradation to help the reformation of perovskite (i.e., implementing iodine caging strategies), and using the polymer-perovskite blend to tune the perovskite mechanical properties.
In the first case, lowering the formation energy of the perovskite will permit the reformation of the perovskite at low temperature, thus reducing heat degradation. [104]urthermore, since defects are known to speed up the degradation of the perovskite material and consequently of the devices, reducing their densities, guiding crystallization or applying passivation strategies, can decrease the degradation during daytime operation of the cells, so that the perovskite can effectively heal during the night. [7]erovskite grain boundaries with acidic function may be useful in the case of degradation pathways that consist of the deprotonation and evaporation of the small organic cations as well as the diffusion and oxidation of I À to I 2 .In fact, an acidic environment can favor the reduction of I 2 , while decreasing the deprotonation of MA.In this case, HI tolerant materials should be considered, since reduction in acidic environments promotes degradation through the formation of HI gas.Furthermore, carbonyl and sulfonyl groups could be considered to bind MA þ , while hydrogen-bonding donor groups can be used for I À .Moreover, such kind of functional groups are also useful to capture perovskite ions after water degradation. [105,106]or mechanical self-healing, polymers are an excellent choice to assist healing of broken perovskite grains. [11,107]Apart from direct incorporation of the polymers in the perovskite layer, the polymeric materials can also be added as a neighboring thin layer in the architecture of the solar cells. [108]he following sections of the review will be dedicated to discussing the basics of smart self-healing materials applied to photovoltaics and to presenting an overview of some relevant examples of self-healing PSCs to contrast the main degradation processes described in this unit.

Smart Self-Healing Materials for Photovoltaic Application
Some specific natural biomaterial and organs can self-repaired, and this capability is known as "self-healing" ability.Natureinspired, smart synthetic self-healing materials have been exploited to reproduce natural systems to achieve partial or complete self-restoring.
Smart self-healing materials have been developed and exploited in various research fields.For a comprehensive information related to this subject, we suggest considering other reviews that discuss the topic in detail. [109,110]This section of the review is dedicated to discussing self-healing materials that have been exploited in photovoltaic research. [111]The difficulty in this case is to combine amorphous and fluidic matters like polymers to ordered semiconductors.
In terms of self-healing mechanisms, two major categories are established: intrinsic, polymers with dynamic bonds, and extrinsic, with the local dispersion of healing agents as shown in Scheme 2.
In the intrinsic type of self-healing process (Scheme 2a), which is the most interesting one for our purpose, the healing mechanism is based on the formation, after breaking, of weak chemical bonds [112] within polymer chains in a macromolecular assembly process driven by reversible noncovalent type of bonds, including hydrogen or metal coordination bonding as well as hydrophobic, π-π stacking, or electrostatic interactions along with "host-guest" chemistry. [113]Also many types of dynamic covalent bonds can be exploited to induce self-assembling as for example: Diels-Alder reaction, imine, oxime, or acyl hydrazone chemistry, disulfide, boronic acid, or ester bonds as summarized in Scheme 3.
Upon damage, the restoration of the broken bond can occur either between different chains of the same polymer, or between two different macromolecules, or even between polymer chains and other molecules.Thus, allowing the realization of many kinds of functional self-healing matters.Depending on the kinetics of the supramolecular assembly, the healing process may occur within a few minutes or hours.This process is often triggered or speeded up by external stimuli such as heat, light, water, change in pH, or the presence of a catalyst.
As an example, Banerjee et al. [114] developed a polyisobutylene (PIB)-based polymer networks useful as coverage for photovoltaic devices.Tri-arm star PIB strains modified with low-molecular weight coumarin functions have been synthesized and the formation of a dynamic network (via photoinduced reactions) was achieved applying UV light.Furthermore, the self-healing process after occurrence of a cut was studied by atomic force microscopy (AFM).In their proposed mechanism, the healing mechanism is driven by breaking of the cyclobutane ring via photocleavage upon irradiation at 254 nm, allowing the polymer to flow and come closer.Once the coumarin functionalities came in proximity, reformation of the cyclobutane ring, illuminating at 365 nm, induced photodimerization again with the consequent formation of the cross-linked network, which finally turned into healing of the damage (Figure 3).
Different from the first category of intrinsic self-healing process, described so far, the extrinsic self-healing requires the in situ draining of healing agents, which are pre-embedded in the fabrication of the material as for example with the use of degradable microcapsules (Scheme 2b).Even though this is a promising approach as well to generate novel self-repairing materials, a detailed description of this type of self-healing mechanism is not included in our review because so far none of this type of self-healing materials has been applied to photovoltaics.For a general understanding of the subject readers, information may be found in another exhaustive work. [109]

Example of Self-Healing PSCs and Their Mechanism
In this section, a spotlight on self-recovery upon moisture exposure degradation as well as after mechanical damage is given by presenting a roundup of recent relevant examples of self-healable perovskite materials, trying to rationalize their self-healing mechanism as well.

Healing Moisture-Induced Degradation
In the following section, we will summarize some relevant examples of self-healing PSCs after exposure to moisture.
In a very interesting work, Zhao et al. [108] demonstrated the importance of using polyethylene glycol (PEG) as a cheap, long-chain, and hygroscopic polymer scaffold in the preparation of MAPbI 3 perovskite to prevent the degradation of spin-coated films as well as n-i-p cells.PEG was added into the perovskite precursor solution using the one-step spin-coating method.The increase stability and the self-healing ability of such devices toward moisture were demonstrated as shown in Figure 4a.Undoubtedly, the PEG-free perovskite film deteriorates irreversibly into PbI 2 .In contrast, PEG-MAPbI 3 film appeared yellow after some time of exposure to water vapor but turned black soon after removing the source of humidity.Importantly, also the current density-voltage ( J-V ) curves of PSCs show the PCE restoration to its initial value, demonstrating that both the PEG-MAPbI 3 films and the corresponding devices spontaneously healed themselves restoring the PCE after the water vapor remotion.Self-healing capability was attributed to the high hygroscopicity of the PEG polymer, which also exhibited a strong interaction with MAI as demonstrated by nuclear magnetic resonance (NMR) measurements. 1H-NMR spectra showed a chemical shift and a splitting of the signal related to the proton of the methylene group linked to the oxygen of the PEG, -[O-CH 2 -CH 2 ]n-, indicating a strong interaction between MA þ and O of the PEG chain.Figure 4b shows the self-healing mechanism (reversible degradation) of PEG-MAPbI 3 film.X-ray diffraction (XRD) analysis further demonstrated that perovskite film regenerated to its initial phase with comparable performance.
In their proposed mechanism, due to water ingress, perovskite hydrolysis into PbI 2 and MAI⋅H 2 O occurs.Then, moving the film away from the vapor source, water evaporates.Since PEG has a strong interaction with MAI, its evaporation is prevented so that held MAI can react with PbI 2 to form perovskite again in situ.
Encouraged by this work, other publications is related to the use of PEG as passivating agent followed up. [1,10,14,115]ifferently from the use of PEG, Niu et al. [116] added polyvinylpyrrolidone (PVP) to the methylammonium lead iodide Scheme 3. Schematic illustrative of some examples of the different types of dynamic bonds to drive the intrinsic type of self-healing process: a) noncovalent types of interactions and b) reversible covalent bonds.Reproduced with permission. [113]Copyright 2019, Wiley-VCH.
(MAPbI 3 ) perovskite precursor solution.This approach helped to control crystal growth allowing the formation of a compact perovskite film and cells with PCE of 20.32%, and furthermore, rendered the solar cells durable into wet environment with strong self-healing ability.When the devices were exposed to water vapor, PVP formed a barrier wrapping MAPbI 3 and blocking water ingress.At the same time, it inhibited the volatilization of MA in a sort of recyclable process of dissolution/recrystallization of perovskite.The improved efficiency and stability of the perovskite film is supposed to be related to the strong C=O … H-N hydrogen bonding interactions between PVP and MAPbI 3 , confirmed by NMR measurements.This strong interaction could slow down the crystallization process and reduced the trap states.Furthermore, the C=O … H-N hydrogen bonds at the grain surfaces increase the moisture stability suppressing the degradation of MA þ cations: only a small decay in device performances after 500 h of operation in high humidity (RH = 65% AE 5%) was observed followed by a rapid restoring ability after their removal from humid stress.
Using a different perspective, in a recent publication, Lalpour et al. [117] presented a work in which they modify conventional random porous TiO 2 electron-transport layer (ETL) using different layer of a copolymer coating.Uniform copolymertemplated TiO 2 (CT-TiO 2 ) ETLs (1, 2 and 3 layers) were obtained by dip coating with different rates.Interestingly, CT-TiO 2 promoted a self-healing effect in perovskite, probably due to the many hydroxyl groups (-OH groups) on TiO 2 surface after exposure of the cells to moisture.In fact, in their proposed mechanism for the self-healing, the hydroxyl group of the TiO 2 could probably bond to the N atom of the CH 3 NH 3 þ cation on one side and PbX 2 (X=halide atom) on the other, which helped to bring them closer and regenerate the perovskite composition as shown in Figure 5, similarly to what normally happens during the two-step perovskite synthesis.Furthermore, the instant self-healing mechanism can explain the fast regeneration observed.
Another interesting mechanism of self-healing was reported by Cheng et al. [118] In particular, by replacing the common metal electrode (Ag) used with indium tin oxide (ITO), the accumulation of ions at the interface between perovskite and ITO reaches saturation, avoiding the continuous degradation of perovskite. [118,119]In addition, in dark condition, the accumulated ions fill the anionic vacancies previously formed, restoring photovoltaic performance.b) proposed mechanism for the self-healing process.Reproduced with permission. [107]Copyright 2015, American Chemical Society.
Figure 5. a) Schematic representation of the mechanism for the self-healing perovskite on surface-templated TiO 2 .b) Images of the healing test of perovskite films at the beginning, upon water-spraying, and after 6 min.Reproduced with permission. [117]Copyright 2016, Nature.

Mechanical Self-Healing
Flexible devices are seriously required to satisfy the demand of next-generation optoelectronics, suitable as portable and wearable devices.In this respect, metal halide perovskites are proven to be appropriate materials for fabricating flexible photovoltaics.However, in addition to the poor crystallinity of perovskite on stretchable substrates, the fragility of crystals also affects their capability to endure several repeated mechanical bending and remains a critical challenge for the preparation of durable F-PSCs.Introducing self-healing capability is an interesting strategy if unavoidable cracks are formed.Therefore, in the last few years, researchers have started to investigate the self-healing ability of perovskite materials on bendable substrates from a mechanical perspective, seeking for key rules to design durable flexible devices.In this section, we will present an overview of some relevant examples of mechanical self-healing PSCs after exposure to several bending cycles or induced cracks by manually cutting the film. [119] far, healing mechanical damage has predominantly been studied in the polymer area.Efficient self-healing polymers need to find an equilibrium between high chain mobility and strong intermolecular forces or a smart engineering of the positioning, on the polymeric scaffold, of proper functional groups that allow the formation of reversible covalent bonds.In fact, as already discussed in the dedicated section of this review, the healing mechanism depend on the reformation of chemical bonds, which upon mechanical stress are broken, through intermolecular forces such as hydrogen bonds, metal-ligand or ion interactions or through reversible covalent bonds.Indeed, this process requires some displacement of the polymeric matrix to allow proximity of proper counterparts to favor the reformation of the bonds. [10]n interesting example of mechanical self-healing that exploits polyurethane (PU) to develop flexible and stretchable optoelectronics was presented by Meng et al. [21] who incorporated a thermal-responsive self-healing PU with dynamic oxime-carbamate bonds into the perovskite films, which helped Reproduced with permission. [122]Copyright 2021, American Chemical Society.
on one side to increase crystallinity and on the other to passivate perovskite film grain boundaries.In this way, stretchable PSCs displayed a stable efficiency of 19.15%.Moreover, such types of PSCs could maintain over 90% of their initial efficiency after 3000 h in air thanks to the self-healing PU scaffold and could release mechanical stress, thus repairing cracks.Devices recovered 88% of their original efficiency after 1000 cycles at 20% stretch.Furthermore, these devices showed high moisture stability thanks to the hydrophobic PU additive containing the dynamic covalent bond and the mechanical damage could be repaired at around 100 °C.Self-healing process was characterized by AFM and XRD analysis.
In a similar work Zhang et al. [120] have successfully demonstrated the self-healing of an all inorganic CsPbIBr 2 perovskite film, which is a type of perovskite that normally is more stable to temperature compared to MA-or FA-based perovskites, by incorporating a thermal-triggered PU.The defects created during the operation of assembling into a solar cell could be healed thanks to the rupture and reformation of the covalent disulfide bonds between adjacent chains.In fact, at high temperatures, self-healing of cracks within the perovskite film occurred, which in turn helped to restore 90% of the initial efficiency for the degraded PSC.The best inorganic PSC delivered a champion PCE of 10.61% and an excellent long-term stability under persistent light irradiation and high-temperature conditions.
Similarly, healing of broken perovskite grain boundary cracks by heat-activated approach was shown by Ge et al. [121] who also exploited a temperature-sensitive self-assembling PU to generate a self-healing framework for repairing cracks in perovskite crystal film upon thermal annealing (100 °C) without compromising the PCE.
In another example, Finkenauer et al. [11] presented the use of a mechanically resistant and self-healable thiourea-triethylene glycol (TUEG3) polymer with a glass transition temperature near  c) AFM images of cracks of pristine perovskite films before and after self-healing treatment (control).d,e) AFM images of cracks of polymer-perovskite films before and after self-healing process.Reproduced with permission. [123]Copyright 2022, Royal Society of Chemistry.
room temperature and capable to interact with perovskites.Thus, in this case, no temperature was needed to trigger the selfhealing process.TUEG3 was synthesized and incorporated into the perovskite thin film.TUEG3-MAPbI 3 hybrid composite thin films were fabricated adding quantities of TUEG3 to the perovskite precursor solution.The TUEG3 polymer in the active layer was found to heal cracks, while maintaining mechanical strength and good electronic properties of the composite film.
An interesting approach is to use moisture as the external trigger that activates the self-healing process.In this case, the role of water, which normally has a negative effect on the stability of perovskite, is strategically reversed.For example, in the case of hydrophilic polymers like poly(vinyl alcohol) (PVA), the polymeric material traps moisture inhibiting the harmful interaction with perovskite.In the work of Wang et al. [122] the PVA was added to FAPbI 3 to fabricate a self-healing flexible device.It was shown that PVA helped to stabilize the favorable black phase of FAPbI 3 and protected it from moisture.The composite films were subject to several mechanical stress showing a 20% decrease and 60% decrease in the response for the composite and pristine-FAPbI 3 films, respectively.In addition, the humid environment triggers an increase in the conductivity of the PVAmodified film.Therefore, the PVA in the perovskite bulk could repair the cracks after interacting with water inhibiting perovskite degradation.Figure 6 (AFM images) shows the crack reparation at the perovskite surface.Thus, such a self-healing process appears a promising approach for achieving durable and effective F-PSCs.
Using a different approach, Yang et al. [123] introduced a selfhealing system based on "Host-Guest" Chemistry into n-i-p type of F-PSCs on polyethylene naphthalate substrates.They also used moisture as a trigger for provoking the host-guest interaction between ß-cyclodextrin and adamantane groups and an in situ polymerization method.Furthermore, a spacer molecule for 2D/3D perovskite, methacryl guanidine hydrochloride, was also incorporated into the cross-linking polymer to further improve the efficiency of devices.Also in this work, AFM was utilized to probe in situ the self-healing process (Figure 7).
In summary, different examples of polymer-perovskite composites were shown and their mechanical self-healing was demonstrated.In most of the examples discussed here, protection from moister was also achieved and, in some cases, it was even the trigger for the mechanical self-repairing mechanism itself, providing a new promising avenue for durable flexible devices.

Conclusions and Future Perspectives
In conclusion, this work aimed to contribute as a comprehensive background for a rational design of fully self-healing PSCs, which may achieve both repairing of the active material and mechanical self-healing.In this respect, some key rules can be suggested to guide the design of future effective self-healing PSCs: 1) eliminating irreversible decay; 2) inserting all healable components through a fine choice of the self-healing agents; 3) preventing cracks and delamination thanks to release of strain formation and 4) providing proper healing of mechanical failures using polymers-perovskite composites.
A comprehensive understanding of the performance degradation mechanism is necessary so that irreversible degradation can be prevented via surface engineering.
Since the intrinsic self-healing of the perovskite material is efficacious within the crystal grains, perovskite films can be optimized in a way that large grain size and negligible boundaries can form.In addition, new self-healing agents should be designed to jointly lower the energy of the perovskite formation, increase resistance to moisture and passivate defects.Also, it must be found a good compromise between the ratio of the highly conductive crystalline perovskite part, essential for high-performance PSCs, and self-healing materials, which are normally amorphous and insulating (i.e., polymer matrices).
Furthermore, self-healing strategies should also be considered for other layers such as charge-transport layers and electrodes to also improve their resistance to heat, light, moisture, and mechanical failure.
Lastly, researchers in the PSC field are encouraged to pull inspiration not only from other photovoltaic areas but also from other materials science fields such as corrosion resistant materials or colloidal science.
With the use of specific smart materials and effective device architecture plan, high performant self-healing PSCs can be realized, which can elongate the lifetime of both flexible and rigid devices.

Figure 1 .
Figure 1.Schematic representation of the degradation and self-healing pathways of a PSC where TCO = transparent conductive oxide, ETL = electrontransport layer, and HTL = hole-transport layer.

Figure 2 .
Figure 2. Sketches of different types of perovskites degradation and summary of the consequent subproduct formation.a) Thermal-induced, b) moistureinduced, c) light-induced, and d) mechanical degradation.

Scheme 2 .
Scheme 2. Schematic representation of self-healing mechanism: a) intrinsic self-healing polymers with dynamic reversible bonds and b) extrinsic self-healing with the local dispersion of healing agents.

Figure 3 .
Figure 3. Sketches of a) reversible cross-linked network formation via photoinduced reaction of coumarin functionalized Tri-arm star PIB andb) proposed mechanism for the self-healing process.Reproduced with permission.[107]Copyright 2015, American Chemical Society.

Figure 4 .
Figure 4. a) Photographs of perovskite films with and without PEG showing color change evolution after water-spraying for 60 s and kept in ambient air in 45 s. b) Schematic representation of the proposed mechanisms for the self-healing properties in PSCs.Reproduced with permission.[108]Copyright 2016, Nature.

Figure 6 .
Figure 6.a) Mechanism of the self-healing process.b) AFM image of cracks on the perovskite film.c,d) Zoomed-in AFM images of cracks before and after the self-healing process.e,f ) Height profiles extracted from (c) and (d).Reproduced with permission.[122]Copyright 2021, American Chemical Society.

Figure 7 .
Figure 7. a) Schematic diagram of the self-healing process based on host-guest interaction.b,c) AFM images of cracks of pristine perovskite films before and after self-healing treatment (control).d,e) AFM images of cracks of polymer-perovskite films before and after self-healing process.Reproduced with permission.[123]Copyright 2022, Royal Society of Chemistry.