Self‐healing polymers in rigid and flexible perovskite photovoltaics

Abstract

Over the past 10 years, perovskite solar cell (PSC) device technologies have advanced remarkably and exhibited a notable increase in efficiency. Additionally, significant innovation approaches have improved the stability related to heat, light, and moisture of PSC devices. Despite these developments in PSCs, the instability of PSCs is a pressing problem and an urgent matter to overcome for practical application. Recently, polymers have been suggested suggestion has been presented to solve the instability issues of PSCs and increase the photovoltaic parameters of devices. Here, first, the fundamental chemical bond types of self‐healing polymers are presented. Then, a comprehensive presentation of the ability of self‐healing polymers in rigid and flexible PSCs to enhance the various physical, mechanical, and optoelectronic properties is presented. Furthermore, valuable insights and innovative solutions for perovskite‐based optoelectronics with self‐healing polymers are provided, offering guidance for future optoelectronic applications.

INTRODUCTION

The newest technological developments have significantly highlighted the growing importance of optoelectronic and photonics devices. However, traditional optoelectronic devices are generally fabricated on rigid silicon and/or germanium-based substrates that fail to meet the market demand for flexible and wearable technology systems.^1,2 On the other hand, a novel generation of optoelectronic devices fabricated with innovative and mechanically flexible materials and substrates is needed.^3–5

To date, different kinds of innovative semiconductor materials and active components have been identified and employed in a new generation of optoelectronic devices. For instance, perovskite materials are among the most fascinating semiconductors among rising materials for optoelectronic and energy device materials due to their excellent light harvesting efficiency, suitable band gap, low-cost solution processing, outstanding electron diffusion length, and high carrier mobility.^6–8 These unprecedented perovskite properties bring out the development of novel generations of optoelectronic devices with outstanding performance. Manufactured perovskite devices are manufactured at a lower cost and have shown enhanced performance compared with traditional devices.^9–11 Owing to their rapidly increasing power conversion efficiency (PCE), which is currently comparable to that of commercially available silicon photovoltaics, PSCs have transformed the entire solar cell industry. Owing mainly to encapsulation techniques and stability tests specifically designed for practical applications, the stability of PSCs has advanced significantly. However, drawbacks prevent future device materials maturations, such as perovskite degradation pathways against the environment, and optoelectronic mechanical and electrical failure, which still need to be overcome.

Mechanical, physical, or chemical damage caused by impact, fall, wear, failure reaction, or cutting may alter the optoelectronic performance and function of perovskite-based devices, especially solar cells, resulting in the need to supply new devices. In addition to physical or mechanical damage, external conditions such as luminosity, humidity, temperature, and oxygen can negatively affect all behavior of PSCs, ultimately causing irreversible device performance degradation.^12–14 In addition, perovskite-based thin films contain numerous grain boundaries and defects, resulting in the loss of different photovoltaic performances of PSCs. Uncontrolled perovskite grain boundary engineering creates a channel for ion migration originating from adjacent layers as well as the diffusion of humidity and oxygen atoms. Moreover, the exposure of perovskite grain boundaries to harsh external conditions or mechanical impact triggers crack formation, which rapidly spreads throughout the whole film. As a result, the propagation of mechanical failure results in severe electrical failure with rapid deterioration of the device's electrical and mechanical function.^15,16 To such an extent, any failure is very stringent to return to its original state through self-healing, resulting in poor physical and mechanical stability in devices. Various universal strategies such as interface optimization, crack-filling operations, substitution/doping engineering, and composition, have been systematically applied to suppress failure caused by grain boundary defects in the plethora of perovskite-based devices.^17–20 As a result of these applied strategies, the popularity of perovskite-based devices has tremendously increased with the maturation of subsequent technology systems with sustained high performance, but each approach fails to overcome the drawbacks of current PSCs neatly. Recently, the self-healing strategy, defined as a straightforward and effective approach to repairing a damaged system, has also been applied for perovskite grain boundary and surface defect passivation in perovskite films.^21,22 Self-healing refers to the restoration of various physical, mechanical, and optoelectronic properties at damaged sites to sustain device durability against unwanted external damage, rapid motion, or harsh environmental conditions.^23–25 Among various self-healing agents, self-healing polymers are exhibited for their unique properties. Self-healing polymer materials/agents are intelligently designed so that they meet the increasingly rigid and flexible PSC modules thanks to critical advantages such as low cost, soft/light properties, and room temperature processability.^26–28 In general, the current self-healing mechanisms are divided into two categories, extrinsic and intrinsic, depending on whether active healing functions are incorporated within the polymer chains or outside. To achieve self-healing, extrinsic self-healing requires the incorporation of active agents, usually monomers or polymerization initiators, to repair damaged regions of the polymer matrix. Intrinsic self-healing repairs the damaged area via polymeric dynamic covalent or hydrogen bonds at the damaged area.^29–31

Self-healing polymers have been employed in a comprehensive range of applications, such as insulating polymers, semiconducting polymers, or materials embedded with brittle active components. The development of physically and mechanically stable PSC devices using self-healing polymers mixed with perovskite is a great opportunity. The use of self-healing polymers as doping/additive or surface passivation materials was introduced to engineer the perovskite layer through the distribution of self-healing polymers at the surface of grain the boundaries of perovskite films.^32,33 They effectively passivate the grain boundary defects while contributing to satisfactory bendability through crack formation suppression and crack repair at the grain boundaries. Additionally, specific classes of self-healing polymers having hydrophobic properties may protect the surface of perovskite and assist in repelling moisture and oxygen diffusion for environmental long-term stability.^34,35 In the case of the incorporation of a self-healing polymer into a perovskite precursor solution, a polymer having a long chain with numerous polar carbonyl groups could form a Lewis-coordination complex by bonding with the hydrogen interactions of the methylammonium cation (MA⁺) of the perovskite.^36,37 Herewith, interactions in-between perovskite and self-healing polymers may effectively promote the self-healing behavior of PSC devices, which has great potential in enhancing the robustness and prolonging devices' lifetime.³⁸ Moreover, the unique properties of self-healing polymers could not only improve the crystalline quality but also suppress ion migration, resulting in high-performance perovskite-based optoelectronic devices.^39,40 Furthermore, self-healing polymers act as a protective scaffold in the perovskite structure to prevent mechanical stress accumulation from damage in the structure after multiple bending/stretching, charge-trapping, and polaron formation.^41,42

In the last few years, self-healing polymers in PSC devices have been widely researched for their ability to improve healing efficiency, prolong durability, and improve the mechanical performance of next-generation PSCs. Therefore, a detailed evaluation of self-healing polymers in PSCs will profoundly contribute to developments in this field. The first part of the review article summarizes the fundamental chemical bond types of self-healing polymers. In Section 2, a thorough evaluation of the device structure and the use of self-healing polymers in rigid PSCs are conducted. We subsequently summarize recent progress in the use of self-healing polymers in flexible PSCs. Finally, we provide an overall summary and discussion of the future development directions of self-healing polymers in PSCs and the potential challenges ahead.

EVOLUTION OF SELF-HEALING POLYMERS DEVELOPMENT

The theory of self-healing emerged between the 1950s and 1990s.^43,44 This behavior is accomplished through the interfacial molecular diffusion of polymer chains across a crack. Researchers have reported that self-healing efficiency is related mainly to polymer diffusion. They formulated equations to define self-healing efficiency, demonstrating that the values are influenced by the ratio of fracture stress, strain, and toughness of the healed sample to those of the original sample. In 1996, dry published pioneering research that marked significant advancements in extrinsic self-healing polymers.⁴⁴ His work focused on self-healing polymers and led to the categorization of these materials into two main types: extrinsic and intrinsic. Extrinsic self-healing polymers generally depend on embedded capsules or vessels containing healing agents. When cracks develop and rupture the capsules, the healing agents are released into the damaged areas, initiating chemical reactions that repair the cracks. However, the healing process ceases once the healing agents are depleted. Consequently, this method typically results in limited healing cycles, complex sample preparation, and reduced mechanical behavior owing to the incorporation of external agents.⁴⁵ In contrast, intrinsic self-healing polymers utilize reversible chemistry, including supramolecular interactions or dynamic covalent bonds, enabling the reformation of bonds after they break. The discovery of the first reversible supramolecular interaction in intrinsic self-healing polymers emerged from Fall's master's thesis, which demonstrated that the ion-interaction induced self-healing properties in ionomer. In 2002, Chen introduced a novel self-healing polymer featuring a catalyst-free healing process, which theoretically allows for indefinite repetition through Diels–Alder (D-A) chemical reactions.⁴⁶ Since that initial discovery, significant advancements have been carried out in the progress of intrinsic self-healing polymers, resulting in numerous breakthroughs. These polymers are primarily created using two types of reversible chemistry. The first type involves reversible covalent bonds, including D-A reactions,⁴⁶ boron ester bonds,⁴⁷ acylhydrazones,⁴⁸ alkoxyamines,⁴⁹ disulfide bonds,⁵⁰ and siloxanes.⁵¹ The second type consists of supramolecular interactions, such as ionic interactions, hydrogen bonds, metal–ligand interactions, host–guest interactions (HGI), key-and-lock, and so forth.^52–58 Reversible covalent bonds are distinct due to their comparatively lower bond energies compared to their irreversible counterparts, granting them dynamic characteristics. Generally, supramolecular interactions result in even weaker association strengths than reversible covalent bonds. Figure 1A,B illustrates the evolution of self-healing polymers, showing the progress timeline of intrinsic self-healing polymers.

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The solution-processing method for thin films permits the integration of external doping during film formation. Polymeric doping progresses are frequently incorporated into perovskite-based devices to impart repair capabilities.^59,60 The flexibility of these materials allows for easy modification of their functionalities, promoting different types of reversible self-healing mechanisms. The exceptional self-healing properties of these polymers are largely attributed to their entangled chain structures, which facilitate interactions among the functional groups within their repeating units. Polymeric doping agents are incorporated within the perovskite grain boundaries/surfaces to passivate defects and protect the layer against external environmental conditions (Figure 1C). The type of self-healing mechanism used dictates whether the recovery process will be reversible or irreversible, thus distinguishing it from mere passivation or protective barriers. The functional groups provided by these self-healing materials are essential for their effects, enhancing the material's ability. The properties of polymeric-based doping agents are significantly influenced by their different bonding behavior. Chemical bonds provide strength because they involve strong interactions between polymer chains through direct bonding, but it often takes a lot of time and energy to initiate the healing process. However, intermolecular forces such as hydrogen bonds, ion–dipole interactions, and proton–proton stacking provide the basis for physical bonds. These weaker bonds make the initiation of the healing process easier. Detailed explanations of each bonding type will be provided in the following sections.

OVERVIEW OF SELF-HEALING POLYMERS

The self-healing polymers require careful consideration of the interplay between thermodynamics, transient relationships, and kinetics at the molecular level. These materials encompass various physical and chemical mechanisms that contribute to their self-healing properties.^61–65 Physically, molecular mobility, permeability, flexibility, and intermolecular chain diffusion are essential for self-healing polymers in the vicinity of the injured surface. These properties can facilitate the movement and rearrangement of polymer segments, allowing the material to repair itself effectively. The capability of polymer chains to diffuse and re-establish connectivity is important in restoring the material's integrity. Chemically, self-healing polymers leverage various interactions and reactions to enable the healing process. Reversible chemical reactions that are initiated by outside stimuli are provided by dynamic covalent bonds. These bonds can break and reform, allowing the material to undergo multiple healing cycles. Non-bonding interactions, including hydrogen bonding, ionic interactions, metal–ligand complexation, hydrophobic interactions, and van der Waals forces, contribute to the healing efficiency of the polymer. These weaker interactions can undergo association and dissociation, facilitating the reassembly of polymer segments and restoring the material's properties. By understanding self-healing polymers' physical and chemical aspects, researchers can tailor their properties and design strategies to create materials with enhanced healing efficiency, prolonged durability, and improved mechanical performance. In this section, we will explain the fundamental types of chemical bonding of self-healing polymers.

Disulfide bonds

The vulcanization of polymers using sulfur is a well-established and commercially successful method for creating reversible sulfur cross-links in elastomeric materials. As a result, extensive research has focused on dynamic covalent disulfide (SS) bonds based on thiol/disulfide exchange reactions.⁶⁶ The disulfide bonds have been found to undergo metathesis exchange reactions involving the disruption and reformation of neighboring SS bonds through free radicals or ionic intermediates. SS bonds' reversibility allows reduction reactions to form two thiol groups, which can be oxidized to regenerate the disulfide bonds (Figure 2A). The compatibility of disulfide chemistry with polymeric networks makes it highly advantageous for self-healing applications. Generally, disulfide bonds can be categorized as aliphatic or aromatic based on their chemical structures, thereby providing temperature-reversible self-healing in low Tg gel networks. Above the Tg, these locked bonds can be easily activated, granting the elastomers effective self-healing and reprocessing capabilities (Figure 2B).⁶⁸ However, the system still needs the external heat stimulus to achieve self-healing capability. Kim et al. synthesized PTMEG by reacting PTMEG with various DMs. They then introduced disulfide-based for chain extension, resulting in polyurethanes with aromatic disulfide bonds in the hard segment (Figure 2C). The study demonstrated that the sparse domains of the isophorone diisocyanate-based hard segment exhibited the highest efficiency in exchange reactions with aromatic disulfides. This enabled efficient self-healing at room temperature.⁶⁷ However, could be improved the high cost of aromatic disulfide monomers hinders industrial production. Additionally, these materials often exhibit a non-transparent appearance, limiting their potential applications.

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Self-healing materials get inspired from nature and employ the principles of supramolecular chemistry, which involves using non-covalent, transient bonds to create reversible networks. These interactions can produce robust, mechanically dynamic systems, which is desirable for building self-healing polymers, despite their lower strength than covalent bonds. In contrast to covalent bonding, polymer networks connected by supramolecular interactions can undergo reversible remodeling, transitioning from low-density and high-free-volume states to solid-like and elastic networks. This flexibility in network design is advantageous for developing self-healing polymers. Weaker supramolecular connections can renew because they are dynamic; they will first dissociate when a mechanical force is applied. This dynamic behavior allows for the self-healing of the material, as the broken bonds can reform and restore the integrity of the polymer network.

Hydrogen-bonding

Hydrogen bonding (H-bonding) is a specific type of non-covalent interaction. H-bonds exhibit directionality and significantly impact the properties of materials, including their mechanical strength, viscoelastic behavior, and self-healing capabilities. The strength of an H-bond is determined by the donor-acceptor distance, with shorter distances generally indicating stronger and more covalent interactions.⁷¹ H-bonds based on the donor-acceptor distance: strong H-bonds have distances of 2.2–2.5 Å and are predominantly covalent, moderate H-bonds have distances of 2.5–3.2 Å and are predominantly electrostatic, and weakly electrostatic H-bonds have distances of 3.2–4.0 Å. The corresponding energies for these H-bonds are around 40-14, 15-4, and less than 4 kcal mol⁻¹, respectively. Therefore, by leveraging the strength, self-healing polymers can be developed with compatible physical/mechanical strength, making them attractive for various applications where resilience and durability are desired.

Binder et al.⁶⁹ reported that the introduction of directional associative end groups with hydrogen bonds in supramolecular polyisobutylenes offers a means to design the mechanical behavior. The functionalization of high-segmental-mobility polyisobutylenes with end groups that can engage in triple hydrogen bonding, such as thymine and 2,6-diaminotriaine, enables the formation of supramolecular networks that exhibit rubber-like behavior (Figure 2D). The triple hydrogen bonds formed between the thymine and 2,6-diaminotriaine groups provide strong and directional interactions, thus contributing to excellent mechanical properties. These hydrogen bonds can undergo dynamic association and dissociation, regulating the mechanical behavior of the material. This includes the ability to undergo self-healing, where the material can recover its integrity and mechanical strength after being damaged or fractured. On the other hand, the mechanical properties of ureidopyrimidinone derivatives, such as Young's modulus and tensile strength, strongly depend on temperature due to the dissociation of hydrogen bonds. This temperature sensitivity arises from the dynamic nature of hydrogen bonding, where the bonds break and reform as temperature changes. Bao et al.⁶⁵ designed the polysiloxane elastomers can achieve tunable mechanical properties by introducing a mixture of self-healing ability hydrogen bonds (Figure 2E). Isophorone bis urea units could take the shape of a maximum of isophorone bis urea units caused by the presence of isophorone urea units. This difference in bonding capability affects the overall strength and stability of the hydrogen-bonded network in the material. By carefully designing the molecular structure and composition of the materials, it is possible to control the strength and dynamics of hydrogen bonding, thereby achieving the desired mechanical properties. This understanding provides the relationship between hydrogen bonding and mechanical for various applications.

π–π stacking interactions

Recent advancements in supramolecular polymer chemistry have focused primarily on harnessing hydrogen bonding interactions for self-healing materials. Indeed, any reversible supramolecular interaction has the potential to be utilized for imparting self-healing properties in materials. Colquhoun and colleagues⁷⁰ reported the combination of pyrene (a π-electron donor) and chain-folding copolyimide (an acceptor of π electrons) end-capped chains with various spacers, as shown in Figure 2F. In this system, the self-healing ability relies on the adjustment of the spacer and composition to tune Tg. The π–π interactions are disrupted upon heating, causing the pyrenyl end-capped chains and copolyimide detachment. The pyrenyl end-capped chains flow into the damaged area because of a flexible spacer in them. As a result, the damaged region is repaired thanks to the π–π stacking. Following this, Burattini et al.⁵³ highlight the importance of aromatic π–π stacking interactions and complementary hydrogen bonding in achieving self-healing properties in elastomeric supramolecular polymer blends. The self-healing elastomeric supramolecular polymer blend was developed by combining a chain-folding polyimide with telechelic polyurethane with pyrenyl end groups. The compatibility of the blend was achieved through aromatic π–π stacking interactions. Moreover, computational modeling was employed to understand the underlying mechanisms of self-healing behavior.

Ionic interactions

Ions have a significant function in various self-healing processes and can also influence the behavior of polymer networks. In biological systems, molecular motors rely on chemical processes leading to macromolecular segment swelling and shrinkage, resulting in motion. The imbalance between osmotic and entropic forces, which results from the energy stored between negatively charged filaments and electrostatic repulsions, frequently drives these processes. Furthermore, ions in polymer systems can influence the material's behavior and properties. Electrostatic interactions between ions and polymer chains can affect the material's conformation, stability, and mechanical response. By harnessing these ionic interactions, self-healing capabilities can be introduced, allowing the material to repair damage or recover its original properties. Ward et al.⁷² developed poly(ethylene-co-methacrylic acid) and polyethylene-g-poly(hexyl methacrylate) are examples of polymers that exhibit self-repairing properties in Figure 3A. The healing mechanism involves generating heat due to friction during the projectile impact. This heat causes localized melting of the polymer, leading to the diffusion and fusion of the melted surfaces, restoring its mechanical properties. Furthermore, ICs can also enable self-healing in materials where macromolecules carry opposite charges. Lapitsky et al.⁷³ reported that electrostatic interactions between macromolecules carrying opposite charges can lead to the formation of neutrally charged polyelectrolyte complexes, such as the addition of complexing polyamines with phosphate-bearing multivalent anions can disrupt the ICs and enhance chain mobility, thereby exhibiting its toughness and self-healing abilities of ionic gels (Figure 3B). Another type of ion–dipole interaction is observed between ionic liquids and fluorinated polymers. Wang et al.²⁴ observed that the ion–dipole interaction between the ionic liquid and the polymer allows for reversible bonding and enables the polymer to repair itself when damaged (Figure 3C). In the typical fluorinated polymer system, PDF-HFP possesses a perfect content of HFP. Unique characteristics of fluorinated-polymer enable it to possess self-healing behavior at low temperatures.

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Host–guest interactions

The host–guest chemistry and host–guest interactions have an important function in the design of self-healing materials. These interactions involve the specific association of two structurally distinct yet size-compatible macromolecular entities through multiple dynamic weak interactions, including hydrogen bonding, π–π interactions, and van der Waals forces. These offer a means to achieve spatially oriented bonding between polymer segments with compatible dimensions. Various macrocyclic host molecules are utilized in host–guest interactions. By designing the structure of both the host/guest molecules, self-healing efficiency can be fine-tuned based on complexation dynamics. Harada et al.⁷⁴ developed that the β-cyclodextrin and adamantine can be incorporated into the side chains of water-soluble polymer backbones, enabling multipoint host–guest interactions. By adjusting the concentration and ratio of the host–guest moieties, the mechanical properties of the hydrogel can be tailored to meet specific requirements (Figure 3D). Scherman et al.⁷⁵ presented that the host molecule of cucurbit[8]uril can be used in self-healing materials. In the presence of CB[8] and polymerizable guest molecules, such as PBM and acrylamide, in situ polymerization can occur, forming a self-healable polymer. These ruptured bonds can reform and trigger self-healing, allowing the material to recover its integrity and mechanical properties (Figure 3E). Overall, host–guest interactions, mediated by molecules such as β-cyclodextrin and cucurbit[8]uril, provide a versatile strategy for designing self-healing materials with tunable properties and the ability to recover from damage.

Comparison of type of self-healing materials

We have categorized self-healing materials into different categories based on two distinct types of chemical connections (Table 1).

TABLE 1 Comparison of type of self-healing materials based on mechanism, advantages, and disadvantages.

Type of self-healing materials	Mechanism	Advantages	Disadvantages
Diels–Alder reactions	Reversible Diels–Alder reactions	Multiple healing cycles, high thermal stability, strong bond strength	Requires specific temperature conditions, a slower healing process
Boron ester bonds	Reversible ester formation and cleavage with boronic acids	Efficient healing under mild conditions, good mechanical properties	Sensitivity to moisture, potential degradation in humid environments
Acylhydrazones	Reversible condensation between aldehydes/ketones and hydrazides	Self-healing at room temperature, tunable mechanical properties	Sensitive to pH changes, slower healing process, possible hydrolytic instability
Trithiocarbonates	Reversible cleavage and reformation of trithiocarbonate bonds	Good thermal stability, efficient healing, chemically robust	Requires specific chemical conditions, potential toxicity, limited to certain applications
Alkoxyamines	Reversible cleavage and reformation of C–ON bonds	High efficiency, self-healing under mild conditions	Requires specific chemical environment, and limited mechanical strength
Disulfide bonds	Dynamic exchange of disulfide bonds	Heals at room temperature, good flexibility, simple chemistry	Limited mechanical strength, requires specific conditions for optimal healing
Siloxanes	Reversible formation and cleavage of Si–O bonds	Excellent thermal and chemical stability, efficient healing with high flexibility	Requires moisture or catalysts, slower healing process
Ionic interactions	Electrostatic interactions between charged groups	Strong interactions, good mechanical properties, fast healing	Sensitive to humidity, potentially limited healing cycles
Hydrogen bonds	Attractive force between hydrogen and electronegative atoms	Heals at room temperature, easy to form and break, abundant in nature	Weaker mechanical properties, lower healing efficiency, sensitivity to environmental conditions
π–π stacking	Attractive interactions between aromatic rings	Easy to activate and reversible healing ability, stable under various conditions	Slow healing kinetics and limited mechanical strength, dependent on aromatic content
Metal–ligand interactions	Coordination bonds form when metal ions and ligands interact	High specificity and tunable properties	Sensitive to environmental conditions and potential for metal ion leaching
Host–guest interactions	Inclusion of guest molecule within the host molecule cavity	Highly specific and reversible healing ability	Complex synthesis and limited mechanical strength
Key-and-lock interactions	Complementary binding between two molecules	High specificity and efficient healing	Limited to specific molecular pairs and potential for limited mechanical properties

Summary:

• Covalent bonds

  • Advantages: Self-healing materials based on covalent bonds, such as DA reactions, boron ester bonds, and acylhydrazones, offer high mechanical strength, multiple healing cycles, and robustness owing to strong bond formation.
  • Disadvantages: These materials often require specific awakening to activate the healing properties.

• Non-covalent bonds

  • Advantages: Materials based on non-covalent bonds, such as ionic interactions, hydrogen bonds, and π–π stacking, benefit from easier initiation of the healing process due to weaker intermolecular forces. They typically heal at room temperature and are highly flexible in design.
  • Disadvantages: Compared with covalently bonded materials, these materials generally have weaker mechanical properties. The healing efficiency of these materials might be relatively low, and they may be sensitive to environmental conditions such as humidity and temperature.

OVERVIEW OF PEROVSKITE SOLAR CELLS

Device architectural and basic components

Owing to their superb benefits such as perfect efficiency, low production costs, and lightweight, PSCs have attracted widespread research interest in solar cell research markets. In a short time, the PCE of PSCs in the laboratory has risen rapidly from ~4% to over 26%. In 2023, Zhang et al.⁷⁶ presented a PCE of 25.4%. In the same year, Liang et al.⁷⁷ broke through the 26% barrier with the additive strategy using the 1-(phenylsulfonyl)pyrrole molecule. As for 2024, Zheng et al.⁷⁸ achieved 26.15% by introducing the piperazinium diiodide molecule as a surface modifier in inverted PSCs. In another study, Chen et al.⁷⁹ declared a certified PCE of 26.15 and 0.05 cm⁻² in inverted PSCs. As far as we know, the record efficiency was announced by Liu et al.⁸⁰ by using a self-healing molecule, and the efficiency was announced as a certified PCE of 26.54%.

The term “perovskite” refers to a large group of compounds that have a similar crystal structure, such as the calcium titanium oxide (CaTiO₃) mineral.⁸¹ Use of the organic–inorganic perovskite materials in PSC applications, the basic crystal structure of a component is an ABX₃, where A is an organic and/or inorganic cation such as methyl ammonium (MA) or formamidinium (FA) or cesium (Cs⁺), B is a divalent metal cation (Pb²⁺ or Sn²⁺), and X is a halide anion (Cl⁻, Br⁻, or I⁻).^82,83 Figure 4A,B depict crystal structure and anions/cations of perovskite materials.

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As for the architectural structure of PSCs, PSCs are designed based on two primary architectural devices known as regular (n–i–p) and inverted (p–i–n) structures depending on which transport (electron/hole) materials, as shown in Figure 4C,D. Both (n–i–p) and (p–i–n) structures devices consist of five different layers, which are (I) the transparent conducting oxide (e.g., fluorine-doped tin oxide (FTO) or indium tin oxide (ITO), (ii) the electron transport layer (ETL), (iii) perovskite-based absorbing layer materials, (iv) the hole transport layer (HTL), and (v) the metal electrode such as gold (Au), silver (Ag), aluminum (Al) and non-metal electrode such as carbon).⁸⁴ The main differences between these architectures in PSCs are the position of the transport materials. In an n–i–p structure, ETL is grown before the absorber layer; incoming light initially passes from ETL to the perovskite layer and then passes HTL. On the contrary, in a p–i–n structure, HTL is grown before the absorber layer and incoming light initially passes from HTL to the perovskite layer and then passes ETL. In an n–i–p structure, the most common ETL and HTL materials are titanium dioxide (TiO₂) and 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9-9′-spirobifluorene (spiro-OMeTAD), respectively. TiO₂ ETL materials require annealing (>450°C) at high temperatures and for a long time. On the other hand, tin oxide (SnO₂) has superior electron mobility, enhanced stability when exposed to UV light, and reduced processing temperatures, which makes it a highly promising alternative. In addition, SnO₂ exhibits excellent band alignment with perovskite absorbers and possesses greater electron mobility. In the last few years, impressive works have been done for PSC applications of SnO₂ ETL materials.^85,86 Yuan et al.⁸⁷ presented very effective electron transport materials for PSCs by employing low-temperature solution-processed SnO₂ nanocrystals coated with amorphous NbO_x. The fabricated devices resulted in a significant PCE of 24.01% with enhanced stability. In another work, Yoo et al.⁸⁸ fine-tuned the composition and film coverage of the SnO₂ ETL by altering the conditions of chemical bath deposition. Afterward, they separated the passivation technique from the use of chloride-based additives. These additives can enhance the formation of perovskite crystal grains and result in a certified PCE of 25.2%. Min et al.⁸⁹ established a cohesive contact between a thin film made of perovskite and an electrode made of Cl-doped SnO₂. The cohesive interface improved the extraction and movement of charges from the perovskite layer, while simultaneously decreasing interfacial defects. The PCE has successfully reached a value of 25.5% certified PCE.^90,91 On the other hand, in a p–i–n structure, the most common HTL and ETL materials of the device are nickel oxide (NiO)/poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), respectively. While NiO HTL material requires a high annealing temperature, PEDOT:PSS HTL material is acidic, damaging the perovskite layers. In recent years, appreciated approaches have been presented in developing novel and effective ETMs and HTMs to create alternatives for the reference materials mentioned above.^92–94 The conduction and valence band values of commonly used ETL materials, perovskite materials, and HTL materials in PSC applications are collected from literature and summarized in Figure 4E.^95–99

Merits and shortcomings of PSCs

In the last decade, there has been substantial advancement in perovskite-based photovoltaic device technologies and the extensive implementation of photovoltaic systems. PSCs are currently generating significant interest for commercialization thanks to their superior features perfect PCE outputs, low-cost fabrication process, and easy production steps compared to commercial solar devices. Nevertheless, the performance and long-term stability of these devices have been hindered by trap states that arise from ionic vacancies, poor interfaces, and grain boundaries. It is necessary to identify and measure faults in PSCs to tackle these problems and reduce the negative impact caused by these imperfections.^100–103 Significant methods for finding new materials and altering the current layer have been created by lowering the rate of recombination within the cell, resulting in stable and highly efficient cells. More stable multi-cation structures have been observed recently, with the addition of other cations like rubidium (Rb⁺), cesium (Cs⁺), and formamidinium (FA⁺) to the structure.¹⁰⁴ Similar to this, by varying the amounts of Br⁻, Cl⁻ and I⁻ halide in the perovskite structure, structures with multiple halides in different combinations can be obtained.^105,106

In addition to the drawbacks associated with the perovskite layer, various other obstacles hinder the actualization of the commercialization potential of PSCs. Without accounting for cost, the energy levels, and mobility parameters of the charge transfer material are additional factors limiting cell efficiency. While TiO₂ is typically used as the ETL material in high-efficiency cells in conventional architecture, efforts to improve efficiency values through surface modification and doping have recently acquired relevance in improving conductivity.¹⁰⁷ Poor UV stability, which is one of the key elements influencing the cell's performance under operating conditions, is another issue brought on by TiO₂. As mentioned above, SnO₂-based ETL materials are a great alternative to TiO₂ ETL materials.

The organic-based HTL materials like spiro-OMeTAD and PTAA are employed in high-efficiency n–i–p structures. Some salt additives (LiTFSI and tBP dopants) are used to improve the performance of devices; however, these ions penetrate the perovskite layer under operating conditions and cause degradation.¹⁰⁸ Many strategies have been discovered to stop this type of ion migration from causing the perovskite layer to deteriorate. One of these strategies is the design of additive-free HTM materials. Cheng et al.¹⁰⁹ designed a dopant-free HTL material called BDT-C8-3O, which exhibits asymmetry. This material can attain a high level of crystallinity, even when produced using environmentally friendly solvents. The n–i–p pero-devices achieved record PCEs of 24.11% (certified 23.82%). More importantly, the devices also showed exceptional operational and high-temperature stabilities, with retention of over 84% and 79.5% of their initial efficiency over 2000 h, respectively. Very recently, Liu et al.¹¹⁰ created a cost-effective small molecule called BCzSPA, which is based on a helical orthogonal core. They have successfully used BCzSPA as a dopant-free HTM in high-performance n–i–p PSCs. The resulting photovoltaic solar cells attained a promising device efficiency of over 25.4%. These cells have also demonstrated perfect stabilities under high-temperature tests. Making HTL-free perovskite cells is one of these techniques. As yet, the efficiency recorded for charge transfer layer-free solar cells is still far below the commercialization target, despite significant advances in this field.¹¹¹ Preventing high mobility Li ion penetration is another strategy to eliminate the adverse effects caused by organic HTL.

On the one hand, compared to rigid photovoltaics, flexible PSCs have unique advantages such as arbitrary-shaped shaping, roll-to-roll manufacturing, high power-per-weight, and flexibility. Large-area flexible PSCs can be mass-produced easily and efficiently using the roll-to-roll deposition technique. It provides advantages, including high production capacity, economical operation, and optimal material utilization. The evaporation of solvents and film transformations, which are frequently completed in ovens, determine the roll-to-roll deposition rate in solution phase chemistries for flexible PSCs.^112,113 Creating shortcuts to shorten the time needed for these procedures can improve flexible PSCs' cost-effectiveness even more. Flexible and lightweight PSCs can be produced quickly and cheaply using the roll-to-roll method, which makes them ideal for a range of wearable devices, portable power packs, and BIPV.¹¹⁴ The development of roll-to-roll coating or printing technology has thus made it possible to move flexible photovoltaics from the lab to fabricate large-area devices. It is essential to realize low roughness and large-scale production of consecutive functional layers to enable the roll-to-roll manufacture of flexible PSCs.

Ion migration induced self-healing performance in perovskite materials

Perovskite materials exhibit unique properties such as high ionic mobility and relatively low formation energy, facilitating reversible perovskite decomposition and reconstruction, which contributes to their intrinsic self-healing capabilities.^115,116 These self-healing properties are particularly evident within crystal grains through processes like phase transitions, reversible ionic migration, and decomposition–recrystallization. Nie et al.³⁹ investigated the synergistic effects between photodegradation and the self-healing mechanism in organometallic halide perovskite solar cells. Under illumination, trap states form, confining free carriers and altering the orientation of organic cations, which facilitates further trap formation. In darkness, the system reverts to its original configuration, restoring the free-carrier concentration. This process is attributed to light-activated metastable states that form charge regions under light and dissipate in darkness (Figure 5A). Furthermore, they also showed that adjusting the operating conditions of the solar cell can regulate the degradation rates (Figure 5B). The solar cell device demonstrates the ability to self-heal and can undergo multiple cycles of 1-Sun illumination and dark periods, suggesting that dark restoration may help mitigate photocurrent degradation. Additionally, the results also show that reducing a temperature to 0°C under continuous illumination, even up to 4-Sun intensity, can greatly enhance device efficiency. This is evidenced by the absence of photocurrent degradation, unlike devices operating at higher temperatures (Figure 5C).

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Domanski et al.¹¹⁹ developed a theory about the dynamics of ions and vacancies examining halogen ion migration based on the device biasing and employing time-of-flight secondary ion mass spectrometry coupled with depth profiling. Their results showed that halide ions moved approximately 100 times faster than their positive counterparts, supporting the rapid mobility of halide ions and aligning with the previous findings that the J_SC value was returned to normal when the device was placed in darkness. The decomposition process happens in two separate stages, with the initial stage (regime I) exhibiting reversible performance degradation and the J_SC value restored up to 90% of its original performance after the devices are placed in darkness. In contrast, the second phase (Regime II) suggests irreversible transformations, characterized by permanent degradation due to ongoing deterioration and complete conversion to PbI₂. Nan et al.¹²⁰ demonstrated that forward relaxation (trapping) and backward relaxation (de-trapping) are in a competitive relationship, determining both photocurrent degradation and recovery, thereby achieving self-healing behavior during the photocurrent degradation process. They choose VPb and Vi to elucidate the varying timescales of ion migration based on the defect-trapped exciton theory and DFT calculations. Their analysis revealed that the trapping energy of VPb is 38 meV, which is higher than the 26 meV trapping energy of Vi. This indicates that halide vacancies are not only easier to trap but also exhibit the faster self-healing of halide vacancies.

The reversibility of ion migration explains the self-healing behavior observed during the light–dark cycle. An ions separate within the perovskite lattices and move forward to the interface during light illumination. Furthermore, the interaction strength between the migrating ions and the functional layers determines the self-healing properties of the perovskites. Specifically, the saturation phenomenon occurs at the interface between the perovskite and ITO until the reaction between the I− ions and the metal electrode is exhausted (Figure 5D,E).¹¹⁷ In the dark, ions that have predominantly gathered at this interface will migrate back to their original vacant sites due to the concentration gradient difference within the perovskite layer. The degree of recovery in darkness is influenced by the aging efficiency, displaying complete reversibility of degradation during the initial stages. Post-aging efficiency retained 80% of its original value. This phenomenon is attributed to the formation of metastable defects and the reversible ion movement under light exposure. Liu et al.¹²¹ declared that light exposure induced partial self-healing of the perovskite material after more than 8 h. During light–dark cycles, ion migration not only facilitated the self-healing process but also aided in defect elimination, as observed in the mitigation of deep-level defects in CsPbI_3−xBr_x through thermal admittance spectroscopy. When a fresh film was stored under low-humidity ambient conditions for 168 h, the deep-level defects were reduced by a factor of 100. The decrease in deep-level defects was attributed to stress relaxation from ion migration as well as polar water molecules accelerated the elimination of interstitials.

Gao et al.¹¹⁸ presented a self-healing mechanism initiated by a chemical process. Under light radiation, MAPbI₃ initially decomposes into intermediates such as PbI₂ and CH₃NH₃I. This is followed by the formation of volatile compounds like CH₃I, HI, and NH₃, along with produced results including molecular hydrogen (H₂), iodine (I₂), and metal lead (Pb), as depicted in Figure 5F. The photodegradation process, coupled with the formation of defects or ion migrations, creates light-sensitive transient states within the MAPbI₃ layer. These transient states capture light-generated carriers for extended durations, leading to hysteresis in PSCs and diminishing their photovoltaic efficiency. The self-healing mechanism during photodegradation involves the initial trapping of carriers in metastable states within the valence and conduction bands of the MAPbI₃ active layer, followed by partial relaxation of these carriers back to their respective bands. The accumulation of metastable trapped states under initial illumination and their dissipation under continued illumination, causes the perovskite solar cells to degrade and subsequently recover. The device's performance hinges on the equilibrium of these processes. As the chemical and crystal structure of MAPbI₃ is slowly restored, this recovery should decelerate the photodegradation in PSCs and allow for partial recuperation.

These findings demonstrate that perovskite initially decomposes under light exposure but reconstitutes with continued illumination. This process is facilitated by substantial transfer layers that retain decomposed molecules, enabling a chemical reversal triggered by light exposure. Other scientific studies, such as those involving high-energy proton¹²² and gamma-ray irradiation¹²³ also provide evidence for this self-healing phenomenon.

Overall, the self-healing behavior seen in perovskite materials results from the facile movement of ions within the lattice. These mobile ions return to energetically favorable positions, thereby effectively repairing defects or displacements triggered by external influences like light illumination, radiation, or varying environmental conditions. This inherent self-healing capability emphasizes the importance of preventing complete decomposition of the perovskite structure. The main issue lies in the degradation-prone nature of organic cations and halide anions. Ideal systems are those that can either trap these ions within the lattice or replenish them, thus ensuring the perovskite structure's integrity is maintained.

The reasons for perovskite degradation

Modern technologies, such as integrated circuits and optoelectronic devices, are using novel thin-film materials. This requires production processes that allow for precise control over film characteristics like as thickness, conformity, composition, and crystal structure. This advancement has enabled the utilization of various types of materials for applications such as integrated photonics, high-efficiency photovoltaics, light-emitting diodes, and photodetectors. However, external factors such as light illumination, moisture, heat, and oxygen can have a detrimental impact on the efficiency and reliability of PSCs, leading to permanent deterioration of device performance. In addition, perovskite-based thin films are characterized by a high number of grain boundaries and imperfections, which result in the degradation of various photovoltaic properties in PSCs. The unregulated manipulation of perovskite grain boundaries facilitates the formation of a pathway for the movement of ions from neighboring layers, as well as the passage of moisture and oxygen atoms.

Like other semiconductors, perovskites undergo an excited state upon light absorption. This particular state has been demonstrated to have enhanced ionic mobility as a result of the formation of vacancies in iodide. It has been hypothesized that this is caused by the oxidation of iodide atoms by holes created by light. The conversion of iodide to iodine ends up in a diminish in the size of ions, enabling them to exit the lattice and create an interstitial and vacancy.^124,125 Figure 6B depicts the reaction scheme employed to assess the degrading impact of the CH₃NH₃PbI₃ layer under light exposure in the absence of encapsulation.¹²⁶ CH₃NH₃PbI₃ perovskite can change to PbI₂ with losing CH₃NH₂ and HI. Although TiO₂-based PSCs have the highest photovoltaic efficiency, the TiO₂ layers are extremely susceptible to degradation from UV radiation, even under inert conditions. On the titania surface, rapid recombination occurs as a result of oxygen desorption caused by photoinduction. When perovskite photoactive layers are exposed to both light and oxygen, they deteriorate quickly. Bryant et al.¹²⁷ investigated the impact of light (white LEDs with a UV filter) and oxygen on the stability of CH₃NH₃PbI₃. Researchers conducted experiments to study the aging process of MAPbI₃ in various atmospheres, both in the absence of light and under light conditions using a UV blocking filter (Figure 6C,F). Figure 6C,D display the absorption properties of CH₃NH₃PbI₃ films after being aged for 48 h in different controlled environments in the dark under with dry air, while Figure 6E,F depict the film exposure to light and oxygen for just 48 h (with a relative humidity of approximately 48%). The perovskite samples aged in N₂ with light and dry air in the dark show no signs of deterioration (Figure 6C,D). However, the CH₃NH₃PbI₃ films undergo deterioration when exposed to dry air with light and ambient air with light. Furthermore, Figure 6E,F clearly demonstrate that after being exposed to light and oxygen for only 48 h, there is a significant shift in the absorption onset from 780 to 520 nm, accompanied by a noticeable change in color from dark brown to yellow. These observations are in line with the deterioration of the perovskite crystal structure and the resulting presence of PbI₂ in the degraded films.

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Halide perovskite materials are widely recognized for their susceptibility to humid environmental conditions. Perovskite is influenced by moisture and solvent vapor. While it is possible to prevent degradation caused by polar solvents by controlling the processing conditions and employing orthogonal solvents to deposit consecutive layers, it is inevitable that the completed devices will be subjected to humid environments when they are put into use.¹³⁰ Li et al.¹³¹ employed in situ XRD to demonstrate the creation of a monohydrate when a MAPbI₃ film was subjected to 80% humidity. Figure 6G demonstrates that during the early stage of 0–5.5 h, there was no substantial presence of the hydrate phase. Only the MAPbI₃ structure with the (110) plane was detected. By extending the duration of exposure to 80% humidity, the monohydrate MAPbI₃·H₂O with space group P21/m was produced, which corresponds to the (001) and (100) crystal planes. Hybrid perovskites are not only sensitive to oxygen and light but also to temperature. It has been observed that at temperatures as low as 85°C in inert conditions, MAPbI₃ degrades into PbI₂.¹²⁸ This degradation may be visualized by the conductive atomic force microscopy (C-AFM) images displayed in Figure 6H,I. Figure 6H represents the pure state of the perovskite, while Figure 6I represents the AFM image of the film waiting at 85°C for 24 h. It is evident that the places in the topography image where grainy features are present form distinct dark regions in the current maps after degradation. Consequently, these regions do not contribute to the electrical current. In another work, Stone et al.¹²⁹ employed in situ x-ray-assisted analysis and in situ XRD to see the conversion from the initial state to the MAPbI₃ phase. Figure 6J,K display the composite plots of in situ XRD and example XRD scans of a perovskite film created using one-step spin-coating. The in situ annealing process was conducted at 100°C for 5, 30, and 70 min, respectively. Between 20 and 70 min, there is a consistent decrease in the precursor phase, which corresponds to the increase in the perovskite phase.

SELF-HEALING POLYMERS IN PEROVSKITE SOLAR CELLS

Perovskite solar cell technologies have risen in popularity due to amazing PCE of over 26%, which exceeds the PCEs of different solar cell devices such as multi-crystalline Si of 22.3% and thin film cells of CIGS-based cells of 22.6% as well as CdTe-based thin-film SCs of 22.1%.¹³² More significantly, perovskite materials with soft and light superiorities can be deposited on different rigid and flexible substrates, resulting in application in many living fields such as for soft electronic devices, wearable products, and portable electronic devices. In this section, we will comprehensively evaluate perovskite-based solar cells on rigid conductive substrates and flexible substrates benefiting from self-healing polymers with unique properties.

Self-healing polymers in rigid perovskite solar cells

PSCs are generally fabricated on ITO or FTO rigid substrates. ITO and FTO glasses are ideal substrate materials for high-performance devices because of their good optical transmittance and satisfactory electrical conductivity and heat resistance properties. These substrates allow the use of the most effective ETL materials, such as TiO₂ and ZnO, which require high annealing temperatures to crystallize and adhere to the surface of the structure of PSCs. In addition, rigid substrates positively affect the growth dynamics of perovskite, allowing it to grow in high crystal quality and large grain size. Because of these reasons, the most conspicuous results for PSC devices have been achieved on the rigid substrates.

Although perovskite photovoltaic technology has developed rather quickly in comparison to its counterparts, it has several grand challenges in industrialization such as grain boundary/surface defects, undercoordinated elemental ions, non-radiative recombination losses, and photocurrent hysteresis.^133,134 At grain boundary and surface defects are some of the exciting material characteristics that are decisive factors in the photovoltaic performance and long-term stability performances of perovskite-based optoelectronic systems and block further progress toward reaching the theoretical power conversion efficiencies.

Self-healing polymers are a great opportunity to improve rigid perovskite-based solar cells' physical and optoelectronic properties. The use of self-healing polymers as surface passivation layers and/or incorporation materials and/or antisolvent agents was introduced to engineer the perovskite layer through the distribution of self-healing polymers at the surface of the grain boundaries of perovskite films. Self-healing polymers effectively passivate the grain boundary defects by contributing to the repair of cracks at grain boundaries. Herewith, they have great potential in PSC applications. Wanting to benefit from this potential, Zhang et al.¹³⁵ employed the self-healing polystyrene (PS) polymer in the architecture of the triple-cation-based solar devices, as shown in Figure 7A. AFM morphology image confirmed that the polymer has covered the perovskite surface and been filled to the grain defects. Based on the AFM morphology (Figure 7B), the PS-5 (5 mg mL^–1 PS additives) film had a height difference of just 5–10 nm between the grain center and the boundary, compared to a 20–30 nm difference for the control film (PS-0), displaying decreased grain boundary depth and width in/on the perovskite films. The PbI₂ phase appears rapidly after the heat application for the control perovskite film, as shown in (Figure 7C). With time, the intensity of the PbI₂ increases continuously, while the decomposition rate of the PS-5 film significantly decreases, and the PbI₂ phase does not emerge up to the first 50 min. In addition to temperature stability, moisture stability performance was tested on boiling water for 15 s. The bare reference perovskite film (no PS) immediately degrades into the PbI₂ phase within 1 s, as shown in Figure 7D. On the other hand, there is no obvious color change in PS-5 film. After the films were kept in boiling water for 15 s, the control film turned completely yellow and degraded, while the perovskite film with PS-5 still showed high strength. On the other hand, the PS-0 and PS-5 films have completely transformed into yellow after 60 s. Surprisingly, after keeping degraded films for a time, the yellow PS-5 perovskite film can partly transform into dark color (Figure 7E). The PS polymer provides excellent self-healing ability to perovskite films. To observe the effects of the PS polymers on the morphology, AFM and SEM analyses of the films kept at 160°C for 120 min were performed. The AFM and SEM analyses revealed that the perovskite film without the PS polymer had a morphology that transformed into the typical microstructure of PbI₂. In contrast, the film with PS polymer had an appearance similar to the standard perovskite grain structure. Additionally, a device having PS polymer protects its 73% efficiency under high humidity of 80%, while about 45% efficiency is kept for the control cell (Figure 7F). Niu et al.¹³⁷ presented a polyvinylpyrrolidone (PVP) capped methylammonium lead iodide (MAPbI₃) perovskite film for controlling the nucleation/growth of perovskites and endow the devices with self-healing ability in a moisture environment. They emphasized that PVP could interact with the ANH₂ groups of the MA⁺, which could heal themselves and resist the humidity of the anchored methylammonium iodide molecules. In addition, PVP could slow down crystal growth and solid diffusion during heating. The fabricated devices with 6 mg mL^–1 PVP achieved the best PCE of 20.32%, a good FF of 77.24%, and lower trap density (control device efficiency of 18.47%). Owing to the effect of PVP, the PCE retained more than 90% of the initial value after 500 h under 65% ± 5% RH conditions. Additionally, after being kept in a water vapor environment for 1 min, the produced with the PVP device regained 80% of its initial image, while no such effect was observed in the control film. Fan et al.¹³⁸ demonstrated that the 2-(1H-pyrazol-1-yl)pyridine (PZPY) into the 3D lead halide perovskite scaffold to obtain remarkable thermodynamic self-repairing capabilities of perovskite films. PSCs containing PZTS polymer presented a more flexible structure and self-healing capability; that is, the corresponding device retained over 90% of the initial device performance at 85°C and 55% relative humidity. Very recently, Zhao et al.¹³⁶ investigated the impact of pHEMA (poly(2-hydroxyethyl methacrylate)) on enhancing the photovoltaic performance and durability of methylammonium lead iodide and establish a method for the self-repair of the perovskite-polymer composite after being exposed to varying levels of humidity. Figure 7H,I illustrate the beneficial impact of pHEMA molecule on the performance of PSC devices. The best device, including 5 wt% of pHEMA, reached a champion PCE of 17.8%, whereas the device without pHEMA achieved a PCE of 16.5%. Moreover, devices including pHEMA are seen to maintain 95.4% of their optimal efficiency after being exposed to 35% relative humidity for 1500 h, in contrast to the 68.5% efficiency achieved by the pure MAPI device.

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When perovskite-based devices are used commercially, lead leakage is most frequently caused by the natural diffusion of lead that occurs when the perovskite layer is exposed to the environment. This diffusion of lead is not limited to the device's attenuation of stability. For this, Jiang et al.¹³⁹ created a realistic simulation to determine the mechanical damage of the devices. According to this work, devices with epoxy encapsulation reduced Pb leakage compared to the other encapsulation methods, which is related to their greater mechanical strength and self-healing abilities.

In another work, initially, Zheng et al.¹⁴⁰ proposed the self-healing properties of a fullerene derivatized polyurethane (C₆₀-PU) film that had a physical crack by hand scribing with a blade. The optical microscope photographs of the fracture healing process of the C₆₀-PU film are displayed in Figure 8A. As represented in Figure 8A, the deep cracks were repaired in 15 min under heat treatment at 80°C. The thin film cracked due to an external force associated with the dissolution of hydrogen and disulfide bonds, among other intermolecular covalent bonds. These broken disulfide bonds reconnect via cross-linking with the nearby chain when heated (Figure 8B). As a result, with heat stimulation, the disulfide exchange reaction could readily mend the film fracture. By incorporating the C₆₀-PU polymer into the structure, partial grain growth occurred compared with that of the reference sample, as shown in Figure 8C,D. As a result, C₆₀-PU treatment not only improves the efficiency of 21.36% of the device (Figure 8E) but also presents good operational stability in ambient conditions (25 ± 5°C and 40% relative humidity). Sun et al.¹⁴¹ reported that polyvinyl alcohol (PVA), an inexpensive and widely accessible water-soluble addition, increased the photovoltaic performance and long-term humidity tolerance of MAPbI₃ (Figure 8F). The ideal amount of PVA can be added to produce a film with longer carrier lifetimes, reduced series resistance, and compact crystal grains. This film has a PCE of 17.4%, which is 11.6% higher than the cell without the PVA additive (Figure 8G). Most importantly, even after 30 days in an environment with high humidity (around 90% RH), the PCE of the unencapsulated devices retains more than 90% of its initial level, indicating their more excellent stability and humidity tolerance (Figure 8H). A versatile CLPU is created by chemically bonding polyurethane to passivate surface imperfections in perovskite, hence reducing non-radiative recombination.¹⁴⁴ Additionally, it can efficiently prevent moisture from infiltrating. The implementation of this approach successfully produces PSCs with outstanding consistency, resulting in a PCE of 23.14%. Furthermore, the unencapsulated devices exhibit improved durability at 35% ± 5% RH for approximately 3000 h and under 65% ± 5% RH for over 700 h. Jo et al.¹⁴⁵ suggested using a polymethyl methacrylate/sodium borate salt (PMMA/borax) combination as an antisolvent to boost rigid PSC performance and stability. They anticipate that adding an alkali salt to the polymer/perovskite mixture may be a workable passivating technique to maximize the long-term stability of devices. Thus, after being treated with PMMA/borax, the devices showed a notable increase in efficiency, going from 20.93% to 22.05%. Additionally, they maintained 80% of their initial performance for 1084 h at 22°C and 50% relative humidity. Zhao et al.¹⁴⁶ also obtained similar results in the novel polymer scaffold architecture.

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All-inorganic perovskite materials are potentially being used in large-scale applications.^147,148 However, the large-scale commercial deployment of all-inorganic perovskite solar cells still needs to be improved by a few issues. There are still technical issues with large-area all-inorganic perovskite film preparation, and the material solution's stability and homogeneity must be resolved. To solve these problems, self-healing polymers can be produced via various methods and strategies. While it has been demonstrated that the addition of self-healing polymer strategies can effectively enhance the photovoltaic performance of organic–inorganic mixed perovskite PSCs, this strategy is rarely documented in all inorganic PSCs. One of these rare studies was conducted by Zhang et al.¹⁴² They designed a polyurethane (PU) polymer that has self-healing properties by being thermally triggered. PU polymer contains hydrogen and dynamic chemical bonds, which rearrange the SS bond that happens upon thermal-triggered, resulting in the disulfide-exchange reaction recovering the crack by a simple heat treatment, as schematically shown in Figure 8I. To determine the interaction between PU polymer and inorganic perovskite, XPS spectra have been conducted (Figure 8J), PU-modified inorganic CsPbIBr₂ perovskite film centered at 138.6 and 143.5 eV for Pb 4f_7/2 and Pb 4f_5/2, to move toward lower binding energies. This displays that there is a chemical interaction between the O, N, or S group of PU polymer and the CsPbIBr₂ perovskite, providing defective Pb²⁺ receives lone-pair electron donations from O, N, or S atoms, which allows for the decrease of detrimental defects.^149,150 Finally, a champion PCE of 10.61% with a J_SC of 11.58 mA cm⁻² is obtained for the best PSC at 0.10 mg mL^–1 PU ratio, which is much higher than 8.61% for the pristine cell, as shown in Figure 8K. The operational SPO stability of PSC under continuous one-sun irradiation at a temperature of 25°C and 50% humidity is displayed in Figure 8L. It is evident that after 28 h, the unencapsulated inorganic CsPbIBr₂ PSC with PU maintains 56% of its original PCE, whereas the control device's PCE approaches zero. In a recent study, a self-polymeric monomer of N-(hydroxymethyl) acrylamide (HAM) polymer was used in the CsPbBr₃ inorganic perovskite structure.¹⁴³ The precise mechanism of perovskite crystallization aided by HAM monomer additives' self-polymerization technique is depicted in Figure 8M. Authors argued that the HAM monomer could operate as a Lewis base to mix with CsBr via the CO⋯Cs interaction because of the lone electron pairs from the oxygen atom of the CO group. Also, the NH groups can act as hydrogen bond donors and interact with the OH groups in the CsBr aqueous solution through NH⋯O hydrogen bonding. The authors preferred to confirm these claims with the FTIR and NMR spectra, as shown in Figure 8N,O. Figure 8P describes the charge transfer process's specific band parameters and schematic diagram. The VBM of the CsPbBr₃ film after HAM doping is observed to be shifted upward, which is attributed to the charge extraction and transport. Consequently, the fabricated carbon-based HAM-incorporated CsPbBr₃ PSCs without hole transport layers achieve a state-of-the-art PCE of 9.05%, which is significantly greater than the reference device's 6.50% efficiency. Table 2 summarizes the effects of self-healing polymers on device performance, stability conditions, and device healing efficiency in rigid PSC devices.

TABLE 2 Names of polymer additives, photovoltaic parameters, stability conditions, healing efficiency in rigid PSC device architectures.

Self-healing polymers	Devices	JSC (mA/cm2)	VOC (V)	FF	PCE (%)	Stability conditions	Healing efficacy (%)	Reference
PS	Control	23.34	1.06	77.60	19.20	30°C and 80% humidity	≥45	135
Doping	23.56	1.09	78.70	20.20	≥73
PVP	Control	22.75	1.06	76.75	18.47	65% ± 5% RH	≥50	137
Doping	23.95	1.10	77.24	20.32	≥90
PZPY	Control	Unk.	Unk.	Unk.	Unk.	85°C and 55% RH	~0	138
Doping	~21.70	1.08	77	18.10	≥90
C60-PU	Control	22.38	1.11	81.18	20.12	25 ± 5°C 40% RH	≥30	140
Doping	22.44	1.15	82.66	21.36	≥60
PVA	Control	22.13	0.99	71.21	15.60	85%–95% RH	~0 ≥90	141
Doping	22.76	1.02	74.73	17.41
PMMA	Control	23.85	1.17	75.00	20.93	22°C and 50% RH	~32 ≥80	145
Doping	23.76	1.19	78.00	22.05
PU	Control	10.98	1.25	70.20	8.61	25°C 40% RH	~0 ≥56	142
Doping	11.58	1.29	70.86	11.61
HAM	Control	6.69	1.32	73.61	6.50	30 days air atmosphere at 85% RH	≥77.6	143
Doping	7.54	1.47	81.76	9.05	≥92.2
pHEMA	Control	22.2	1.03	72.3	16.50	1500 h in 35% RH	≥68.5	136
Doping	22.6	1.04	75.7	17.80	≥95.4
502 ADHESIVE	Control	22.52	1.08	71.27	17.30	1224 h 45% RH 30°C	≥60	174
Doping	23.52	1.08	79.21	20.12	≥95
PEG	Control	Unk.	Unk.	Unk.	Unk.	70% RH dark environment	~0	146
Doping	22.5	0.98	72	16	≥65

Self-healing polymers in flexible perovskite solar cells

Wearable power sources, shaped electronic devices, and portable electronics are in high demand due to the electronics industry's rapid development of new technology. Therefore, flexible PSCs have advantageous characteristics, including flexibility, portability, lightweight, and compatibility with curved surfaces. More importantly, continuous roll-to-roll technology can produce flexible solar cells in large quantities. This is an important benefit compared to the fabrication conditions for rigid devices, which is linked to slower manufacturing speeds, more infrastructure requirements, and higher costs. Additionally, because rigid solar cells are more significant and significantly heavier, their storage and transportation costs are higher. On the other hand, flexible solar cells are more lightweight, thin, and flexible during installation, storage, and transportation, which significantly lower all associated expenses. This is especially true as flexible solar cells are frequently put in different desired places.⁴¹

The potential for flexible and stretchable PSCs has attracted great interest in academia and industry as next-generation real applications for prominent long-term operating life. In the solar sectors, flexible PSCs are more suitable for real-life applications than rigid-based PSCs. However, perovskite-based device flexibility must increase the photovoltaic performance to compete with solid cells obtained with rigid PSCs. The main reasons for the relatively lower photovoltaic performance of flexible PSCs compared with rigid counterparts can be explained as follows. (i) Low performance of flexible PSCs would result from neighboring ETL/HTL transport layers and absorber layers becoming of the high roughness and high deformation ratio of polymer substrates.¹⁵¹ (ii) The undesired transmittance and high sheet resistance of flexible substrates cause low photovoltaic parameters (especially FF and J_SC) of devices.¹⁵² (iii) The low-annealing temperature required by flexible substrates causes low crystal quality perovskite films having numerous grain boundaries, which results in low charge transport properties of devices.¹⁵³ To solve these problems of flexible PSCs, self-healing polymers are a perfect candidate because self-healing polymers not only prevent damaged sites but also make strong electrical and mechanical behaviors of flexible devices by acting as a scaffold/recoverer. Meng and colleagues¹⁵⁴ proposed a strategy that combines a self-healing polymer (polyurethane (s-PU)) with perovskite. The s-PU self-healing polymer had dynamic covalent and the thermally reversible covalent polymer (Figure 9A), displayed mechanical self-renewing properties under 100°C temperature as shown systematically in Figure 9C. When s-PU self-healing polymer is included in the perovskite structure, perovskite films' average grain size grew from 600 nm to 1.5 mm. (Figure 9D). The relatively high intensity of the (110) lattice planes of perovskite with s-PU polymer becoming sharper and stronger was revealed by the GIWAXS analysis, which confirmed the enhanced crystallinity (Figure 9E,F). As a reflection of grain size, the device with s-PU polymer presented a stabilized efficiency of 19.15%, while the control device gave 14.7% efficiency (Figure 9H). To confirm the self-healing behavior of the absorber layers with/without s-PU polymer, the AFM characterization system has been used. An annealing process may cure stretching-induced cracks (Figure 9G), illustrating the ability of s-PU to self-repair at grain boundaries. To validate that PSCs had a stretchable function, the mechanical stability was examined. After stretching for 200 cycles, PSCs' normalized average PCE ranged between 1% and 20% in stretch ratio. (Figure 7I). Despite 20% stretching, the cell with s-PU retains 81% of its initial value, indicating an excellent stretching tolerance. In contrast, the control cell allows a noticeable drop to 60% PCE. More importantly, thermal annealing is followed by verification of the self-healing mechanism. Following a 10-min thermal annealing process at 100°C, the PCE recovered to 93% of its initial value. Then, 1000 continuous cycles of stretching tests using 20% stretches were carried out (Figure 9J). As a result, device performance can recover to 34%–88% of its initial value in these test conditions.

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Theoretically, the interactions of self-healing polymers with perovskite crystal systems have been examined in detail in different studies. For example, Zhang et al.¹⁵⁶ demonstrated by using density functional theory (DFT) that pyridine units in polysiloxane as an innovative self-healing polymer acted as Lewis bases and formed a chelation adduct with PbI₂ (−1.601 eV) in the precursor, exhibiting strong intermolecular Pb²⁺ coordination interactions. Consequently, the production of FAPbI₃ would be restricted by the greater energy barrier created by the abundant Pb²⁺-polysiloxane chelation interactions. Similarly, Zhang and colleagues¹⁵⁵ were initially hypothesized, and DFT analysis was used to thoroughly validate the mechanism of interactions between dynamic 2,6-pyridine carboxamide (PDCA) polymer and perovskite (Figure 9K). The dynamic PDCA polymers can initially function as Lewis bases to produce a tri-chelation adduct with PbI₂ in the precursor because of the strong intermolecular Pb²⁺–N_amido, I–N_pyridyl, and Pb²⁺–O_amido coordination interactions. Because of the adequate PDCA Pb²⁺ tri-chelation interaction, there will be a greater energy barrier when reacting with formamidinium iodide (FAI) to form the perovskite crystals. This is confirmed by computation results. Moreover, experimental results verified the strong interaction between PDCA and Pb²⁺, as seen in the shift of XPS peaks in the Pb 4f_7/2 and 4f_5/2 (Figure 9L). The champion PCE of the PDCA-doped device shows excellent performance with 19.50% (Figure 9M), while the reference cell value is comparable to those achieved based on 16.40%. Additionally, the photovoltaic performance of the PSC was examined at 20% strain (Figure 9N). The devices fabricated with PDCA self-healing polymer demonstrated exceptional self-healing capabilities and stability at room temperature because cracks at the GBs healed by self-healing properties with plenty of hydrogen bonds. In conclusion, enhanced stretching stability and self-healing opportunities via self-healing polymers were presented to expand the applications of PSCs to portable and flexible electronics fields. In 2024, Wang et al.¹⁵⁷ designed a cross-linkable elastomer ADP having an electrostatic dynamic bond, which can undergo in situ cross-linking and coordinate with [PbI₆]⁴⁻ in order to control the crystallization process of perovskite. These elastomers connected to the perovskite grain boundaries can release any lingering tensile strains and mechanical stresses. This results in improved stability and flexibility of the perovskite-based devices. This can be due to the enhanced mechanical strength resulting from the ADP molecules and grain boundaries of perovskite. In addition, they conducted peak force quantitative nanomechanical mapping (PFQNM) to assess the mechanical characteristics of the films. The average DMT modulus of the pristine perovskite and perovskite/ADP film is calculated to be 6.46 and 4.14 GPa, respectively. Perovskite films that contain cross-linked elastomers on the grain boundaries are expected to exhibit enhanced flexibility compared to perovskite/ADP films.

Flexible PSCs are a great alternative to traditional solar cells in portable power sources. Nevertheless, owing to the disadvantages of perovskite mentioned above, and the ability to form grain boundary cracks in flexible perovskite films, their physical and mechanical behaviors are still insufficient to meet real-life applications. Very recently, to deal with this matter, a self-healing 5-(1,2-dithiolan-3-yl) pentanehydrazide hydroiodide (TA-NI) containing dynamic covalent disulfide bonds, H-bonds, and ammonium is designed by Chen et al.¹⁵⁸ The cross-linked TA-NI demonstrates dynamic self-healing capabilities and elastomeric characteristics at room temperature. In addition, TA-NI can cross-link simultaneously with the growth of the perovskite film. It is attached to the perovskite grain boundaries to the grain boundary like “ligaments” and spontaneously repairs undesirable in the perovskite film caused by mechanical stress. In photovoltaic performance results for devices, perfect PCE values of 23.84% and 21.66% are obtained for 0.062 and 1.004 cm², respectively. The resulting flexible pero-SCs show promising improvements in efficiency. More importantly, the flexible devices demonstrated improved overall stabilities with over 90% of both 20 000 bending cycles and 1248 h of operational stability. The ability of self-healing polymers to repair devices at low temperatures makes them attractive for real-life applications. For this, ingeniously, Xue et al.¹⁵⁹ selected environmentally friendly polyurethane polymer and introduced it into perovskite film through a two-step blade coating method (Figure 10A). The self-repairing mechanism schemes film with polyurethane polymer are displayed in Figure 10B. The polyurethane is uniformly dispersed along the perovskite film's grain boundaries due to its high molecular weight and high thermodynamic properties up to 250°C. In severe bending deformation, it can partially release the mechanical tension. The devices will repair themselves under real environmental conditions. The temperature, stress, and strain relationship diagrams in two and three dimensions during the polyurethane shape memory process are shown in Figure 10C, while device configuration with a PEN/ITO/SnO₂/Perovskite/Spiro-OMeTAD/Au configuration is displayed in Figure 10D. The energy level diagram of the device is shown in Figure 10E. A more suitable energy band diagram was obtained with the polyurethane effect, which greatly reduces the energy loss of the perovskite photovoltaic device. As a result, the PCE of PSCs based on polyurethane self-healing polymer is 21.3%, especially with a V_OC of up to 1.18 V (Figure 10F). The flexible PSCs' mechanical recovery characteristics and bending stability are examined to further verify the flexibility performance. The micron-level cracks in the perovskite layer with polyurethane are visible in Figure 10G upon bending. Filling the cracks is filamentous polyurethane. The polyurethane can self-repair the fractured perovskite film after heating it at 37°C for 30 min (Figure 10H). The device's PCE drops to 6.14% after 3000 cycles of bending under an 8 mm bending radius, as Figure 10K illustrates. The device's initial efficiency is 21.31%. Remarkably, heat-treating at 37°C for 30 min can recover the PCE of the related device to 19.26%. The simulation findings displayed in Figure 10I demonstrated the presence of noticeable unrepairable cracks at a displacement of 0.61 μm in the control perovskite layer. On the other hand, the polyurethane-added film can maintain the filament-like connection at the displacement of 3 μm in Figure 10J. Chu et al.¹⁶⁰ fabricated the flexible solar cells with a self-healing polymer-based composite (Figure 10L,M), They cut the device at the metal contact area to test the recovery performance of the produced solar cells (Figure 10N). The digital image in Figure 10N indicated that the self-healing polymer composite filled the damaged site after cutting the Au contact. Before and after cutting, the electrical resistance of the film was measured to test its self-healing ability. As a result, as Figure 10O illustrates, the electrical channels in the Au contact were nearly entirely recovered. The resistance measured between the active area and the Au contact before severing the Au contact was 0.82 Ω. After cutting the Au contact and the device breaking down, the value climbed dramatically to 1.73 × 10¹⁰ Ω without the composite layer. However, when the composite layer was used, the conductivity was fully recovered within 1 min. The resistance was measured to be 1.47 Ω after healing. The recovery ratio histogram for the 16 devices in Figure 10P showed a high rate of healing performance with an average value of 99% reproducibility.

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Han et al.¹⁶¹ synthesized the two types of self-healing polymers coined PU-MCU and PU-IU with different cyclic linkages (Figure 11A) to control grain growth, defect passivation mechanism, device stability, and mechanical energy distribution. The morphological analysis revealed that the self-healing polymers adduct not only the improved growth of perovskite film improve the carrier lifetime but also assist the carrier lifetime (Figure 11B). In photovoltaic results, high crystallinity with appeased defects reflects good device performance, and PCEs with over 23% efficiency were produced from the polymer-modified-PSCs (Figure 11C). When metal halide perovskite films are stretched, the films display self-recover behaviors by rebuilding hydrogen bonds following mechanical relaxation (Figure 11D). They conducted a nano-indentation test technique for polymer-modified metal halide perovskite films to demonstrate how the polymer modification affected the mechanical properties of perovskite films (Figure 11E). The film that incorporated polymer displayed non-linear properties, which could originate from the stretching of a cross-linked network in the polymer-perovskite composite scenario rather than the mechanical behavior of pure perovskite films. Also, the PU-IU film dissipated 1040.88 ± 223.80 pJ of energy during the testing cycles, while the bare film dissipated 200.26 ± 22.78 pJ (Figure 11F). The reference and polymer-based devices were put through a bending test on flexible PSCs with a curvature radius of 1 mm to evaluate their self-recovery capacity. On the test's 1000th and 2000th bending cycles, the devices were heated to a mild temperature of 100°C. (Figure 11G). The efficiency of the PSC with the PU-IU device was recovered to approximately 93% of its initial PCE by heating for the 1000th cycle. However, the control device's device efficiency was only recovered to approximately 42% of its initial PCE. In Figure 11H,I, the confocal PL mapping shows the creation of cracks and their recovery by the hybrid cross-linked network in the polymer-doped perovskite film. Deviations in the PL's wavelength, intensity, and FWHM significantly increased around the crack that developed in the control MHP film after 1000 bending cycles. In a recent study, to further improve the bending resistance of stretchable PSCs, Gong et al.¹⁶² introduced self-healing polyurethane (PU) with a bionic high-speed patterned meniscus coating technique as illustrated in Figure 11J. They contended that stress destruction-induced perovskite cracks can be repaired and perovskite crystal quality improved by orientated PU with self-healing characteristics. The corresponding large-area flexible PSCs have PCEs of 18.71% (1.01 cm²) and 16.74% (9 cm²), respectively, as illustrated in Figure 11L. After 20% stretching, the flexible PSCs made using the self-healing polymer maintain 88% of the original PCE, demonstrating remarkable mechanical stability. On the other hand, there is a noticeable degradation in the normalized PCE of PSCs for the reference samples, which is 49% lower than its initial PCE (Figure 11M). As explained earlier, an efficient method to improve the durability and address the negative consequences of lead leakage in PSCs is to incorporate a well-designed lead trapping or sedimentation layer into the encapsulation architecture. Modified polyurethane adhesive (PUA) was developed to implement a practical encapsulation for lead leakage in flexible PSCs.¹⁶³ The PUA film can be applied to a metal electrode, resulting in a modest increase in efficiency from 23.96% to 24.15%. The PUA film exhibits excellent adherence to the flexible substrate, and the initial efficiency of the flexible large-area device (17.2%) that is enclosed by PUA remains at 92.6% over duration of over 1820 h. (Table 3 summarizes the effects of self-healing polymers on device performance, stability conditions, and device healing efficiency in flexible PSC devices).

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TABLE 3 Names of polymer additives, photovoltaic parameters, stability conditions, healing efficiency in flexible PSC device architectures.

Self-healing polymers	Devices	JSC (mA/cm2)	VOC (V)	FF	PCE	Stability conditions	Healing efficacy (%)	Reference
s-PU	Control	19.27	1.00	76.31	14.74	3000 h 40% RH	≥59	154
s-PU-doped	22.34	1.09	78.65	19.15	≥90
PDCA	Control	19.69	1.12	78	16.40 ± 0.15	3000 h 20% RH	≥61	155
Doping	21.72	1.12	79	19.50	≥82
TA-NI	Control	Unk.	1.12	77.02	21.71	45–55°C 1248 h 20 000 bending cycles	≥67	158
TA-NI-Doping	25.21	1.16	82.00	23.84	≥90
SMPU	Control	23.72	1.11	0.72	19.20	3000 h in the N2 glovebox condition	≥70	159
Doping	24.21	1.18	0.74	21.33	≥89
PU-IU	Control	24.64	1.104	75.95	20.65	1000th cycle	≥42	161
Doping	25.34	1.154	79.50	23.25	≥93
PU	Control	22.10	1.15	73.04	18.56	20% stretching	≥50	162
PDAI-Doping	23.16	1.15	75.25	20.04	≥88
PAT	Control	20.10	1.10	76.02	17.51	2000 h 80% RH in the dark	≥65	156
Doping	21.66	1.12	77.02	19.58	≥83

Self-healing polymers in both rigid and flexible perovskite solar cells

When self-healing polymer materials are grafted into perovskite films such that they have the potential to heal themselves, their unfavorable stability characteristics can turn them into helpful catalysts to repair broken devices. It is anticipated that doing so will greatly extend the device's lifespan. As mentioned in the sections above, self-healing polymers are used in rigid and flexible PSCs. In addition, recent reports have shown that these SHP polymers have started to be used in two device architectures simultaneously. For example, Lan et al.¹⁶⁴ synthesized the self-healing PUDS having disulfide bonds employed in both rigid and flexible solar cells simultaneously. The broken disulfide bonds can readily produce free radicals as the temperature rises over TS. When the temperature is lowered to achieve self-healing, sulfur in the free radical state can create disulfide bonds with the closest free radical. The schematic diagram of PUDS polymer and the self-healing mechanism of devices are illustrated in Figure 12A,B. The manual fracture with a millimeter-scaled width rapidly heals over 10 min of heat treatment at 80°C, as illustrated in Figure 12C. The best outputs of ~17.2% and ~20.3% on flexible and rigid substrates are obtained, respectively. Consequently, it also attains over 87% of the initial PCE during thermal annealing at 80 C for self-healing (Figure 12D,E). These self-healing polymers increase the efficiency of PSCs and change the macroscopic mechanical properties and strength of the films. To test this hypothesis, Finkenauer et al.¹⁶⁵ implanted the thiourea-trimethylene glycol polymer (TUEG3) polymer into MAPbI₃-based perovskite. Using nano-indentation, the composite films' mechanical characteristics were examined to determine their hardness and Young's modulus. The moduli were derived from the slope of the first unloading curve (Figure 12G). The average modulus of the 0.2% film was 18.8 GPa, which was somewhat higher than that of the 2.1% and 0% films (Figure 12H). The 0.2% and 2.1% films' comparatively higher Young's moduli imply reduced dislocation densities and significant interactions between the organic and inorganic moieties. The perovskite grain formation is less constrained at lower polymer mass percentages, and the tiny polymer chains serve as nucleation sites, which may result in a higher perovskite macro packing density and modulus. For a similar purpose, here, Ge et al.¹⁶⁶ designed and employed a functional polymeric adhesive (called AD-23) in rigid/flexible devices (Figure 12I) to improve mechanical stability and enhance device performance through a dynamic thermal self-healing mechanism. Due to the favorable mechanical effect, AD-23's chemical structure also facilitates better perovskite-PTAA interaction. Due to its amphiphilicity, adding AD-23 to a perovskite solution significantly increases its riveting property on hydrophobic HTL. Additionally, the uniform perovskite film undoubtedly provides for the uniform reaction of cationic salt solutions, which is advantageous to the devices. PSCs with this AD-23 self-healing polymer attain power conversion efficiencies (PCEs) of over 20% on flexible substrates and nearly 22% on rigid substrates (Figure 12J,K). PSC with AD-23 modification demonstrated damping and a slight recovery; under these harsh conditions, the device's efficiency retained more than 80% of its initial state after more than 260 h of storage. Under the identical testing settings, however, after 25 h, only 20% of the reference device's initial PCE remained (Figure 12L). It explains this arresting difference between the devices by the effect of the supramolecular connection between the perovskite and the functional groups in AD-23, as well as the healing function of AD-23. Yang et al.¹⁶⁷ prepared a terpolymer (TPP) prepared a terpolymer (TPP) to construct host–guest interaction between self-healing polymer and Cs_0.05(MA_0.11FA_0.89)_0.95Pb(I_0.97Br_0.03)₃ precursor. The ¹H NMR spectrum of mixed TPP and PbI₂ was recorded to identify the interaction between the two compounds. As seen in Figure 12M, An intense NH/I hydrogen bonding interaction between the guanidine group in MAGH and octahedral [PbI₆]⁴⁻ was detected by observing the split and shifted proton signal of the C(NH₂)⁺ group in MAGH. The hydrogen bond between the guanidine group of TPP and [PbI₆]₄⁻ octahedra has also been validated by FTIR spectra. The carrier dynamics throughout the perovskite films were examined through the use of PL spectroscopy. The shifting in Figure 12N demonstrated the expected suppression of Pb⁰ deep energy level defects created by halide perovskite vacancies.¹⁶⁸ Perovskite films' PL spectra in Figure 12O show that, when the amount of TPP polymer gradually increases, the PL intensity shows a rising and dropping trend. This could be due to the conflict between the insulating effect and defect passivation.¹⁶⁹ TRPL also confirmed these changes. To determine the ideal TPP concentration, 20 independent devices were produced, and it is shown in Figure 12P that the optimum rate is 0.5 mg mL^–1 TPP. A champion PCE of nearly 23% was achieved, with a J_SC of 24.36 mA cm⁻², V_OC of 1.17 V, and good FF of 81%, while the reference device gave a PCE of 20.81%, with a J_SC of 23.95 mA cm⁻², V_OC of 1.15 V, and FF of 75.68%, as displayed in Figure 12Q. On the other hand, this efficiency value for the optimum flexible cell was determined as 20.46%. Also, to instruct the self-healing process by the host–guest interaction between TPP and perovskite, after cracking, the perovskite films were kept for 4 h at 60%–70% RH. In SEM images, it was evident that the fissures in TPP-modified had healed, whereas TPP-0 showed no change. Furthermore, there has been a significant improvement in mechanical stability. Specifically, cracks at the perovskite grain boundaries, generated by 4000 bending cycles at a 6.25 mm bending radius, were effectively self-healed, resulting in a PCE recovery to 85% of its initial PCE thanks to host–guest interaction effects.

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One of the secrets of long-lasting device production is obtaining perfect perovskite films with fewer grain boundaries as well as low defect density. Recently, to effectively control the crystallization, Xue et al.¹⁷⁰ provided poly(ethylene glycol) methacrylate (PEGDMA) during the growth process of perovskite films that may chemically attach at the grain boundaries. XRD patterns in Figure 13A showed the reference and modified-perovskite films. It is understood that this situation is observed in the XRD patterns in Figure 13A. In this case, it is seen that films with high crystal quality and low defect density are obtained with the PEGDMA effect. The absorber layers with/without PEGDMA have average double exponential fitting lifetimes of ~1140 and 560 ns, respectively (Figure 13B). These findings also show that non-radiative recombination can be inhibited by the PEGDMA modification Figure 13C. The hole trap density for PSCs with and without PEGDMA is determined to be 5.38 × 10¹⁵ and 6.32 × 10¹⁵ cm⁻³, respectively, following the production of ITO/PTAA/perovskite/spiro-OMeTAD/Au. Defects in the matching perovskite films can be efficiently reduced by adding PEGDMA. Figure 13D,E shows the top-view SEM images. The control perovskite film has grains that are roughly 510 nm in size. As the PEGDMA content rose (0.25, 0.5, 1, 2 mg mL^–1), the grain size of the PEGDMA-containing films dramatically increased compared to the control films. Grain size at its maximum is around 1200 nm when 0.5 mg mL^–1 PEGDMA is added. Overall, the rigid and flexible PSCs with PEGDMA achieved 23.15% and 21.41% (PCE) with minimal hysteresis (Figure 13F). Additionally, the flexible PSCs show good mechanical stability and flexibility, holding onto over 91% of the initial value even after 5000 bending cycles at a 3 mm radius (Figure 13G). Even with perfect advancements in PSCs, grain boundary cracks are still unavoidable and can compromise the stability and dependability of the device by creating a tunnel for ions, oxygen, and water penetration. A self-healing technique using dynamic polymers having hydrogen bond host–guest interaction and dynamic disulfide bond presents to address this issue. This strategy not only repairs grain boundary cracks but also releases mechanical stress. Yang et al.¹⁷¹ introduced polyurethane (DSSP-PPU) with disulfide bond exchange and multiple H-bonds to release residual stresses (Figure 13H). Only the DSSP-PPU-doped films generated cracks had self-healed after being stored at room temperature for 1 h, as in situ, as seen by AFM in Figure 13I. Moreover, after 6000 bending cycles (r = 7 mm), the device based on the DSSP-PPU-doped perovskite film maintained 93% of its initial PCE, as displayed in Figure 13J. These “ligaments” modulated solar cell devices preserved approximately 90% of their initial PCE after 8000 bending cycles in total by repeatedly repeating these bending and healing cycles. As a result, the DSSP-PPU elastomers acting as “ligaments” could increase the perovskite film's mechanical resilience while enabling the cracks to self-heal.

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Very recently, a cross-linking network integrating naturally polymerizable LA molecules has been designed to increase the mechanical stability and solar efficiency of rigid and flexible perovskite devices.¹⁷² Through dynamic disulfide and hydrogen connections, the LA inside can self-assemble into ring-opening polymers. The thermally induced ring-opening polymerization of LA is illustrated in Figure 13K, along with a schematic representation of the defect passivation mechanism and interface optimization The disulfide exchange process, partial chelation, and hydrogen bonding should be the sources of the self-healing mechanism.¹⁷³ The intense contact and dynamic interactions between Poly(LA) and the perovskite components align with these reconfigurable bonds. Figure 13L shows the self-healing feature schematically. The intermolecular hydrogen bonds, SS bonds, and other interactions are destroyed by mechanical force, which causes visible delamination or fissures in the perovskite layer and the p/n interface. This characteristic causes the dynamic rearrangement of SS bonds, which includes the rupture of SS bonds in the initial polymer chain and subsequent cross-linking with nearby chains and the reconstruction of hydrogen bonds. In addition to the inherent brittleness of the perovskite films, the residual lattice strain within the films is a significant contributing factor to these fissures. The scattering peaks for all perovskite films, as seen in Figure 13N, progressively move to smaller 2θ, signifying an increase in the crystal plane distance d₍₀₁₂₎ and the films' ability to withstand the tensile strain. The perovskite control film exhibited evidence of being significantly subjected to tensile strain, as evidenced by its relatively sizeable negative slope. In contrast, the slope of the perovskite-Poly(LA) film decreases even more, suggesting a significant release of micro- and residual strain. Using dynamic polymer networks with self-healing processes offers a viable way to develop flexible PSCs that are not only very resilient, flexible, and self-repairing but also incredibly durable. The consequent self-healing Poly(LA)-containing PSCs gifted a champion PCE of 19.03% with J_SC of 23.78 mA cm⁻² and 22.43% with J_SC of 24.55 mA cm⁻², respectively, as shown in Figure 13P,Q. The impressive J_SC value of the rigid device was also confirmed by EQE analysis, as shown in Figure 13R. (Table 4 summarizes the effects of self-healing polymers on device performance, stability conditions, and device healing efficiency in both rigid and flexible PSC devices).

TABLE 4 Names of polymer additives, photovoltaic parameters, stability conditions, as well as healing efficiency in both rigid and flexible PSC device architectures.

Self-healing polymers	Devices	JSC (mA/cm2)	VOC (V)	FF	PCE	Stability conditions	Healing efficacy (%)	Reference
PUDS	Flexible	21.91	1.13	69.47	17.19	4000 bending cycles with a radius of 3 mm	≥84	164
Rigid	22.88	1.14	78.21	20.30	Unk.	Unk.
TUEG3	Flexible	20.04	0.99	69.0	13.64	3000 bending cycles with a radius of 5 mm	≥80	165
Rigid	20.61	1.10	76.7	17.42	85°C	Unk.
AD-23	Flexible	23.59	1.07	81.22	20.50	280 h 85°C 85%–93% RH	≥80	166
Rigid	24.61	1.13	79.07	21.99	Unk.
TPP	Flexible	22.87	1.14	78.31	20.46	4000 bending cycles with a radius of 6.25 mm	≥85	167
Rigid	24.36	1.17	80.95	23.06	(RH) of 20%–30%	≥99
PEGDMA	Flexible	24.27	1.15	77	21.41	5000 bending cycles with a radius of 5 mm	≥86	170
Rigid	24.94	1.16	80	23.15	Unk.	Unk.
DSSP-PPU	Flexible	23.49	1.17	80.84	22.24	8000 bending cycles with a radius of 2 mm	≥90	171
Rigid	24.63	1.18	81.73	23.73	1300 h 20%–30% RH	≥100
Poly(LA)	Flexible	23.78	1.06	76.00	19.03	1000 bending cycles with a radius of 5 mm	≥80	172
Rigid	24.55	1.16	78.58	22.43	Unk.	Unk.
Ab-WPU	Flexible	22.90	1.14	80.21	21.27	1000 bending cycles with a radius of 6 mm	≥96	175
Rigid	25.41	1.17	81.26	24.20	70 h 85% RH	≥65

CONCLUSIONS AND FUTURE PERSPECTIVES

In summary, first, we provide the fundamental chemical bond types of self-healing polymers for PSC applications. Recent years have reviewed many self-healing polymer types, their unique mechanisms, and how self-healing performance affects perovskite-based solar cells. There is still a requirement for comprehension because purported self-healing performance is currently mainly assessed based on the enhanced performance of the PSC devices. Although there have been some highly encouraging advances so far, self-healing polymers in PSCs still need development before they can be widely used in real-world conditions. Therefore, we list a few of these alternative perspectives to provide more effective PSC devices further:

• As seen in PSC applications, self-healing polymers are generally designed as dynamic covalent disulfide bonds and H-bonds interactions. Apart from this, self-healing polymers can be designed to have bond interactions with the metal–ligand coordination bonds Schiff base bonds, and boronic ester bonds to prevent oxidation or reduction of the component ions, resist moisture permeation, passivate defect states, and collectively lower the perovskite formation energy, all of which increase the yield of perovskite devices recovery.
• For long-live PSCs, it can be ideal for developing more specific multifunctional (phosphonic acid, thiol, and sulfonate functional groups) self-healing polymer with high carrier mobility to capture perovskite ions after moisture degradation and improve the J_SC of PSCs.
• Future studies could concentrate on creating high-flexibility self-healing materials because the materials used for flexible and wearable photovoltaic systems currently do not meet high flexibility requirements. Due to a mismatch in the lattice constants and thermal expansion coefficients between the perovskite and surrounding layers, stress–strain is unavoidable in PSCs. It has a substantial impact on device performance. It is essential to develop strategies that prevent residual strain from forming and encourage safe pathways for strain relaxation to treat strain-related performance degradation.
• Most self-healing polymers are triggered by relatively high temperature that gains self-healing properties. Self-healing polymers can be designed to be triggered at lower temperatures, incredibly close to room temperature. In addition, self-healing polymers that can recover under different triggers (moisture or impact) other than temperature can be developed.
• While the integration of self-healing polymers can significantly enhance the longevity and durability of perovskite solar cells, it is crucial to address their potential impact on device efficiency. Here, we outline several strategies and advancements to mitigate this issue and optimize the balance between self-healing capabilities and electrical performance: First, the new type of self-healing polymers should aim to simultaneously reduce the energy needed for perovskite formation, prevent moisture penetration, passivate defect states, and inhibit the oxidation/reduction of constituent ions, thus improving perovskite recovery efficiency. One effective strategy is to design polymers that are both intrinsically conductive and self-healing, leveraging advanced polymer chemistry. For example, conjugated polymers with dynamic covalent bonds can offer excellent electrical conductivity while retaining self-healing properties, thereby eliminating the need for additional conductive fillers, simplifying material processing, and enhancing device performance. Second, auxiliary self-healing strategies can further improve the self-healing properties of PSCs by addressing issues related to the perovskite precursor solution and crystallization kinetics. Optimizing the preparation process can help maintain the chemical characteristics and behavior of the perovskite precursor solution in its colloidal state. Additionally, introducing molecular self-assembly and synthesizing an intrinsically conductive polymeric material can enhance charge transfer efficiency, thereby maintaining the photovoltaic devices' performance without sacrificing PCE. For example, researchers should consider extending the self-healing capabilities in the electron or hole transport layer, such as using phosphate groups to capture the perovskite ions from the hole transport layers. This functionality offers a promising approach for integrating self-healing materials into charge-transporting layers. Overall, conducting a comprehensive analysis of the trade-offs between durability and efficiency is crucial. By evaluating the overall efficiency of the solar cell throughout its operational life and comparing the long-term cost savings from reduced maintenance and extended lifespan against potential initial efficiency loss.
• Making large-area or flexible PSCs requires mechanical failure, such as bending and fracturing. In the presence of these stimuli, all other layers and interfaces should remain stable, and the driving conditions of the healing agents should be easily attained. Therefore, self-healing polymers should also be used for other layers, including electrodes, substrates, and charge transport layers, in perovskite device architectures to resist physical and mechanical failure in addition to additive/doping functions.
• Different kinds of and for photodetectors employee self-healing materials such as metals, ceramics, and nano/micro-based composites can be produced as alternatives to self-healing polymers for employees in PSCs, LEDS, photodetectors, and other device applications.

ACKNOWLEDGMENTS

The authors extend their appreciation to the Deanship of Research and Graduate Studies at King Khalid University for funding this work through Large Research Project under grant number (R.G.P.2/491/45).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

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