Stretchable Perovskite Solar Cells with Recoverable Performance

Fully Stretchable Microbial Fuel Cell with 75% Stretchability

A decent stretchability is of paramount significance to operate microbial fuel cell (MFC) under mechanically dynamic conditions. However, it remains a grand challenge to fabricate fully stretchable MFC without compromising its power output. Here, using Shewanella oneidensis MR-1 (S. oneidensis) as the model electrogenic bacteria, the study demonstrates a fully stretchable MFC device that can operate with a stretchability of 75%. The design takes advantage of a stretchable and ion-conductive polyurethane membrane, which encapsulates the biohybrids composed of S. oneidensis and reduced graphene oxide (rGO) on the polydimethylsiloxane (PDMS) current collector for synchronous stretching. It is discovered that the "stretchable" living biohybrids can sustain an adaptive bio-current output under stretching/releasing stimulation. The design also employs a stretchable air cathode. The stabilized peak power density of the stretchable MFC follows an increasing trend with the applied strain, and reaches 5.0 ± 0.7, 5.9 ± 0.9, 6.2 ± 1.1, 6.6 ± 1.4 µW cm-2 at strains of 0%, 25%, 50%, and 75%, respectively (n = 3). At 75% strain, the stretchable MFC yields a maximum current output of 104 ± 27 µA cm-2 and an open-circuit voltage of 283 ± 30 mV (n = 3). The results provide insights to design stretchable MFCs to power the next-generation on-skin devices, soft robotics, and sustainable electronics.

Indoor Light Harvesting Perovskite Solar Cells on Conducting Oxide-Free Ultrathin Deformable Substrates

Perovskite solar cells (PSCs) are receiving renewed interest since they have reached high power conversion efficiency (PCE) and show potential for application not only on rigid and flexible substrates but also on mechanically deformable substrates for integration on nonplanar curvilinear surfaces. Here we demonstrate PSCs fabricated on transparent conducting oxide-free ultrathin polyethylene terephthalate substrates capable of efficiently harvesting indoor light even under compressive strain. Interface engineering with poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) improved the shunt resistance and band alignment at the perovskite-hole transport layer interface, which resulted in enhanced charge extraction, leading to 114% improvement in PCE from 5.57 to 11.91% under 500 lx indoor white LED (4000 K) illumination. The champion device exhibited a PCE of 18.37% under 250 lx cool white LED (4000 K) light. The maximum power output (P max) of the devices varied from 13.78 to 25.38 μW/cm2 by changing the indoor light illumination from 250 to 1000 lx, respectively. Moreover, the devices showed impressive performance even after mechanical deformation and retained 83 and 76% for 1 sun and indoor light, respectively, under 30% compressive strain. Our approach paves the way for fabrication of efficient indoor light harvesting PSCs on mechanically deformable substrates for integration on nonplanar surfaces prone to compressive strain.

Highly flexible and transparent colorless polyimide substrate sandwiched between plasma polymerized fluorocarbon and InGaTiO for high performance flexible perovskite solar cells

We integrated transparent antireflective coatings and transparent electrodes onto flexible colorless polyimide (CPI) substrates to fabricate high-performance flexible perovskite solar cells. Multifunctional PPFC/CPI/IGTO substrates were fabricated by sputtering the optimal plasma-polymerized fluorocarbon (PPFC) antireflective coating and InGaTiO (IGTO) electrode films on both sides of the CPI substrate. By applying PPFC with a low refractive index (1.38) as an antireflective coating, the transparency of the PPFC/CPI/IGTO substrate increased by an additional 1.2%. In addition, owing to the amorphous characteristics of the PPFC and IGTO layers, the PPFC/CPI/IGTO substrate showed constant sheet resistance and transmittance change even after 10,000 cycles during the bending tests. The flexible perovskite solar cells, fabricated on the PPFC/CPI/IGTO substrate, exhibited an increase in current density of 1.48 mA/cm2 after the deposition of the PPFC antireflective coating. These results confirmed that the PPFC/CPI/IGTO substrate was durable against high-temperature treatment, flexible, and exhibited excellent electrical characteristics. This enhanced the efficiency and durability of the flexible perovskite solar cells. Moreover, the hydrophobic PPFC layer allowed the self-cleaning of inflexible perovskite solar cells. Given these attributes, the PPFC/CPI/IGTO structure has been recognized as a good choice for multifunctional substrates of flexible perovskite solar cells, presenting the potential for enhancing performance.

Supramolecular Polyurethane "Ligaments" Enabling Room-Temperature Self-Healing Flexible Perovskite Solar Cells and Mini-Modules

Flexible perovskite solar cells (F-PSCs) have emerged as promising alternatives to conventional silicon solar cells for applications in portable and wearable electronics. However, the mechanical stability of inherently brittle perovskite, due to residual lattice stress and ductile fracture formation, poses significant challenges to the long-term photovoltaic performance and device lifetime. In this paper, to address this issue, a dynamic "ligament" composed of supramolecular poly(dimethylsiloxane) polyurethane (DSSP-PPU) is introduced into the grain boundaries of the PSCs, facilitating the release of residual stress and softening of the grain boundaries. Remarkably, this dynamic "ligament" exhibits excellent self-healing properties and enables the healing of cracks in perovskite films at room temperature. The obtained PSCs have achieved power conversion efficiencies of 23.73% and 22.24% for rigid substrates and flexible substrates, respectively, also 17.32% for flexible mini-modules. Notably, the F-PSCs retain nearly 80% of their initial efficiency even after subjecting the F-PSCs to 8000 bending cycles (r = 2 mm), which can further recover to almost 90% of the initial efficiency through the self-healing process. This remarkable improvement in device stability and longevity holds great promise for extending the overall lifetime of F-PSCs.

Self-Healing Behavior of the Metal Halide Perovskites and Photovoltaics

Perovskite solar cells have achieved rapid progress in the new-generation photovoltaic field, but the commercialization lags behind owing to the device stability issue under operational conditions. Ultimately, the instability issue is attributed to the soft lattice of ionic perovskite crystal. In brief, metal halide perovskite materials are susceptible to structural instability processes, including phase segregation, component loss, lattice distortion, and fatigue failure under harsh external stimuli such as high humidity, strong irradiation, wide thermal cycles, and large stress. Developing self-healing perovskites to further improve the unsatisfactory operational stability of their photoelectric devices under harsh stimuli has become a cutting-edge hotspot in this field. This self-healing behavior needs to be studied more comprehensively. Therefore, the self-healing behavior of the metal halide perovskites and photovoltaics is classified and summarized in this review. By discussing recent advances, underlying mechanisms, strategies, and existing challenges, this review provides perspectives on self-healing of perovskite solar cells in the future.

Strain Effects on Flexible Perovskite Solar Cells

Flexible perovskite solar cells (f-PSCs) as a promising power source have grabbed surging attention from academia and industry specialists by integrating with different wearable and portable electronics. With the development of low-temperature solution preparation technology and the application of different engineering strategies, the power conversion efficiency of f-PSCs has approached 24%. Due to the inherent properties and application scenarios of f-PSCs, the study of strain in these devices is recognized as one of the key factors in obtaining ideal devices and promoting commercialization. The strains mainly from the change of bond and lattice volume can promote phase transformation, induce decomposition of perovskite film, decrease mechanical stability, etc. However, the effect of strain on the performance of f-PSCs has not been systematically summarized yet. Herein, the sources of strain, evaluation methods, impacts on f-PSCs, and the engineering strategies to modulate strain are summarized. Furthermore, the problems and future challenges in this regard are raised, and solutions and outlooks are offered. This review is dedicated to summarizing and enhancing the research into the strain of f-PSCs to provide some new insights that can further improve the optoelectronic performance and stability of flexible devices.

Dynamic covalent polymer engineering for stable and self-healing perovskite solar cells

Perovskite films are susceptible to degradation during their service period due to their weak mechanical properties. Acylhydrazone-bonded waterborne polyurethane (Ab-WPU) was employed as dynamic covalent polymer engineering to develop self-healing perovskite solar cells (SHPSCs). Ab-WPU enhances the crystallinity of the perovskite film, passivates the defects of the perovskite film through functional groups, and demonstrates promising flexibility and mild temperature self-healing properties of SHPSCs. The champion efficiency of SHPSCs on rigid and flexible substrates reaches 24.2% and 21.27% respectively. The moisture and heat stability of devices were improved. After 1000 bending cycles, the Ab-WPU-modified flexible device can be restored to an efficiency of over 95% of its original efficiency by heating to 60 °C. This is because the dynamic acylhydrazone bond can be activated to repair perovskite film defects at a mild temperature of 60 °C as evidenced by in situ AFM studies. This strategy provides an effective pathway for dynamic self-healing materials in PSCs under operational conditions.

Ultra Robust and Highly Efficient Flexible Organic Solar Cells with Over 18 % Efficiency Realized by Incorporating a Linker Dimerized Acceptor

The wearable application of flexible organic solar cells (f-OSCs) necessitates high power conversion efficiency (PCE) and mechanical robustness. However, photoactive films based on efficient non-fullerene small molecule acceptors (NF-SMAs) are typically brittle, leading to poor mechanical stability in devices. In this study, we achieved a remarkable PCE of 18.06 % in f-OSCs while maintaining ultrahigh mechanical robustness (with a crack-onset strain (COS) of higher than 11 %) by incorporating a linker dimerized acceptor (DOY-TVT). Compared to binary blends, ternary systems exhibit reduced non-radiative recombination, suppressed crystallization and diffusion of NF-SMAs, and improved load distribution across the chain networks, enabling the dissipation of the load energy. Thus, the ternary f-OSCs developed in this study achieved, high PCE and stability, surpassing binary OSCs. Moreover, the developed f-OSCs retained 97 % of the initial PCE even after 3000 bending cycles, indicating excellent mechanical stability (9.1 % higher than binary systems). Furthermore, the rigid device with inverted structure based on the optimal active layer exhibited a substantial increase in efficiency retention, with 89.6 % after 865 h at 85 °C and 93 % after more than 1300 h of shelf storage at 25 °C. These findings highlight the potential of the linker oligomer acceptor for realizing high-performing f-OSCs with ultrahigh mechanical robustness.

Recoverable Flexible Perovskite Solar Cells for Next-Generation Portable Power Sources

Flexible perovskite solar cells (FPSCs) with excellent recoverability show a wide range of potential applications in portable power sources. The recoverability of FPSCs requires outstanding bendability of each functional layer, including the flexible substrates, electrodes, perovskite light absorbers, and charge transport materials. This review highlights the recent progress and practical applications of high-recoverability FPSCs, and illustrates the routes toward improvement of the recoverability and environmental stability through the choice of flexible substrates and the preparation of high-quality perovskite films, as well as the optimization of charge-selective contacts. In addition, we explore the intrinsic properties of each functional layer from the physical perspective and analyze how to select suitable functional layers. Additionally, some effective strategies are summarized, including material modification engineering of selective contacts, additives and interface engineering of interlayers, which can release mechanical stress and increase the power-conversion efficiency (PCE) and recoverability of the FPSCs. The challenges of making high-performance FPSCs with long-term stability and high recoverability are discussed. Finally, future applications and perspectives for FPSCs are discussed, aiming to promote more extensive commercialization processes for lightweight and durable FPSCs.

Advances on the Application of Wide Band-Gap Insulating Materials in Perovskite Solar Cells

In recent years, the development of perovskite solar cells (PSCs) is advancing rapidly with their recorded photoelectric conversion efficiency reaching 25.8%. However, for the commercialization of PSCs, it is also necessary to solve their stability issue. In order to improve the device performance, various additives and interface modification strategies have been proposed. While, in many cases, they can guarantee a significant increase in efficiency, but not ensure improved stability. Therefore, materials that improve the device efficiency and stability simultaneously are urgently needed. Some wide band-gap insulating materials with stable physical and chemical properties are promising alternative materials. In this review, the application of wide band-gap insulating materials in PSCs, including their preparation methods, working roles, and mechanisms are described, which will promote the commercial application of PSCs.

Progress and Challenges Toward Effective Flexible Perovskite Solar Cells

The demand for building-integrated photovoltaics and portable energy systems based on flexible photovoltaic technology such as perovskite embedded with exceptional flexibility and a superior power-to-mass ratio is enormous. The photoactive layer, i.e., the perovskite thin film, as a critical component of flexible perovskite solar cells (F-PSCs), still faces long-term stability issues when deformation occurs due to encountering temperature changes that also affect intrinsic rigidity. This literature investigation summarizes the main factors responsible for the rapid destruction of F-PSCs. We focus on long-term mechanical stability of F-PSCs together with the recent research protocols for improving this performance. Furthermore, we specify the progress in F-PSCs concerning precise design strategies of the functional layer to enhance the flexural endurance of perovskite films, such as internal stress engineering, grain boundary modification, self-healing strategy, and crystallization regulation. The existing challenges of oxygen-moisture stability and advanced encapsulation technologies of F-PSCs are also discussed. As concluding remarks, we propose our viewpoints on the large-scale commercial application of F-PSCs.

Perovskite Grain-Boundary Manipulation Using Room-Temperature Dynamic Self-Healing "Ligaments" for Developing Highly Stable Flexible Perovskite Solar Cells with 23.8% Efficiency

Flexible perovskite solar cells (pero-SCs) are the best candidates to complement traditional silicon SCs in portable power applications. However, their mechanical, operational, and ambient stabilities are still unable to meet the practical demands because of the natural brittleness, residual tensile strain, and high defect density along the perovskite grain boundaries. To overcome these issues, a cross-linkable monomer TA-NI with dynamic covalent disulfide bonds, H-bonds, and ammonium is carefully developed. The cross-linking acts as "ligaments" attached on the perovskite grain boundaries. These "ligaments" consisting of elastomers and 1D perovskites can not only passivate the grain boundaries and enhance moisture resistance but also release the residual tensile strain and mechanical stress in 3D perovskite films. More importantly, the elastomer can repair bending-induced mechanical cracks in the perovskite film because of dynamic self-healing characteristics. The resultant flexible pero-SCs exhibit promising improvements in efficiency, and record values (23.84% and 21.66%) are obtained for 0.062 and 1.004 cm²  devices; the flexible devices also show overall improved stabilities with T₉₀  >20 000 bending cycles, operational stability with T₉₀  >1248 h, and ambient stability (relative humidity = 30%) with T₉₀  >3000 h. This strategy paves a new way for the industrial-scale development of high-performance flexible pero-SCs.

Highly Efficient Flexible Perovskite Solar Cells through Pentylammonium Acetate Modification with Certified Efficiency of 23.35

Among the emerging photovoltaic technologies, rigid perovskite solar cells (PSCs) have made tremendous development owing to their exceptional power conversion efficiency (PCE) of up to 25.7%. However, the record PCE of flexible PSCs (≈22.4%) still lags far behind their rigid counterparts and their mechanical stabilities are also not satisfactory. Herein, through modifying the interface between perovskite and hole transport layer via pentylammonium acetate (PenAAc) molecule a highly efficient and stable flexible inverted PSC is reported. Through synthetic manipulation of anion and cation, it is shown that the PenA⁺ and Ac^- have strong chemical binding with both acceptor and donor defects of surface-terminating ends on perovskite films. The PenAAc-modified flexible PSCs achieve a record PCE of 23.68% (0.08 cm² , certified: 23.35%) with a high open-circuit voltage (V_OC ) of 1.17 V. Large-area devices (1.0 cm² ) also realized an exceptional PCE of 21.52%. Moreover, the fabricated devices show excellent stability under mechanical bending, with PCE remaining above 91% of the original PCE even after 5000 bends.

Recent Advances in Printable Flexible Optical Devices: From Printing Technology and Optimization Strategies to Perspectives

Recently, flexible optical devices have triggered booming developments in various research fields, including display equipment, sensors, energy conversion, and so on, due to their high compatibility, portability, and wearability. With the advantages of strong design ability, high precision, and high integration, printing technologies have been recognized as promising methods to realize flexible optical devices. In this Perspective, recent progress on printing strategies for fabricating flexible optical devices are introduced systematically. First, through adjusting the composition of inks, selecting flexible substrates, and controlling external stimulation, fabrication of flexible optical devices based on inkjet printing is illustrated. Then, flexible optical devices fabricated by template-induced printing, 3D printing, slot-die printing, and screen printing are summarized. Finally, prospects and future development directions based on printing technology for flexible optical devices are proposed.

Self-Healing Ability of Perovskites Observed via Photoluminescence Response on Nanoscale Local Forces and Mechanical Damage

The photoluminescence (PL) of metal halide perovskites can recover after light or current-induced degradation. This self-healing ability is tested by acting mechanically on MAPbI3 polycrystalline microcrystals by an atomic force microscope tip (applying force, scratching, and cutting) while monitoring the PL. Although strain and crystal damage induce strong PL quenching, the initial balance between radiative and nonradiative processes in the microcrystals is restored within a few minutes. The stepwise quenching-recovery cycles induced by the mechanical action is interpreted as a modulation of the PL blinking behavior. This study proposes that the dynamic equilibrium between active and inactive states of the metastable nonradiative recombination centers causing blinking is perturbed by strain. Reversible stochastic transformation of several nonradiative centers per microcrystal under application/release of the local stress can lead to the observed PL quenching and recovery. Fitting the experimental PL trajectories by a phenomenological model based on viscoelasticity provides a characteristic time of strain relaxation in MAPbI3 on the order of 10-100 s. The key role of metastable defect states in nonradiative losses and in the self-healing properties of perovskites is suggested.

Bio-Inspired Pangolin Design for Self-Healable Flexible Perovskite Light-Emitting Diodes

Despite tremendous developments in the luminescene performance of perovskite light-emitting diodes (PeLEDs), the brittle nature of perovskite crystals and their poor crystallinity on flexible substrates inevitably lead to inferior performance. Inspired by pangolins' combination of rigid scales and soft flesh, we propose a bionic structure design for self-healing flexible PeLEDs by employing a polymer-assisted crystal regulation method with a soft elastomer of diphenylmethane diisocyanate polyurethane (MDI-PU). The crystallinity and flexural strain resistance of such perovskite films on plastics with silver-nanowire-based flexible transparent electrodes are highly enhanced. The detrimental cracks induced during repeated deformation can be effectively self-healed under heat treatment via intramolecular/intermolecular hydrogen bonds with MDI-PU. Upon collective optimization of the perovskite films and device architecture, the blue-emitting flexible PeLEDs can achieve a record external quantum efficiency of 13.5% and high resistance to flexural strain, which retain 87.8 and 80.7% of their initial efficiency after repeated bending and twisting operations of 2000 cycles, respectively.

Modified Fabrication of Perovskite-Based Composites and Its Exploration in Printable Humidity Sensors

Organic perovskites are promising optoelectronic semiconductor materials with photoelectric applications. It is known that the luminescence of perovskites is highly sensitive to hydron molecules due to its low moisture resistance of crystal structure, indicating its potential application on humidity-sensing. Herein, a novel perovskite-based compound (PBC) with minimal defects was developed to promote the photoluminescence performance via optimization of the drying method and precursor constitutions. Perovskite materials with good structural integrity and enhanced fluorescence performance up to four times were obtained from supercritical drying. Moreover, the hydrophilic polymer matrix, polyethylene oxide (PEO), was added to obtain a composite of perovskite/PEO (PPC), introducing enhanced humidity sensitivity and solution processibility. These perovskite/PEO composites also exhibited long-term stability and manifold cycles of sensitivity to humidity owing to perovskite encapsulation by PEO. In addition, this precursor solution of perovskite-based composites could be fancily processed by multiple methods, including printing and handwriting, which demonstrates the potential and broaden the applications in architecture decoration, logos, trademarks, and double encryption of anti-fake combined with humidity.

Surface Chelation Enabled by Polymer-Doping for Self-Healable Perovskite Solar Cells

Polymer doping is an efficient approach to achieve self-healing perovskite solar cells. However, achieving high self-healing efficiency under moderate conditions remains challenging. Herein, an innovative self-healable polysiloxane (PAT) containing plenty of thiourea hydrogen bonds was designed and introduced into perovskite films. Abundant thiourea hydrogen bonds in PAT facilitated the self-healing of cracks at grain boundaries for damaged SPSCs. Importantly, the doped SPSCs demonstrated a champion efficiency of 19.58% with little hysteresis, almost rivalling those achieved in control atmosphere. Additionally, owing to the effective chelation by PAT and good level of thiourea hydrogen bonds, after 800 cycles of stretching, releasing and self-healing, the doped SPSCs retained 85% of their original IPCE. The self-healing characteristics were demonstrated in situ after stretching at 20% strain for 200 cycles. This strategy of pyridine-based supramolecular doping in SPSCs paves a promising way for achieving efficient and self-healable crystalline semiconductors.

A Bionic Interface to Suppress the Coffee-Ring Effect for Reliable and Flexible Perovskite Modules with a Near-90% Yield Rate

The inhomogeneity, poor interfacial contact, and pinholes caused by the coffee-ring effect severely affect the printing reliability of flexible perovskite solar cells (PSCs). Herein, inspired by the bio-glue of barnacles, a bionic interface layer (Bio-IL) of NiO_x /levodopa is introduced to suppress the coffee-ring effect during printing perovskite modules. The coordination effect of the sticky functional groups in Bio-IL can pin the three-phase contact line and restrain the transport of perovskite colloidal particles during the printing and evaporation process. Moreover, the sedimentation rate of perovskite precursor is accelerated due to the electrostatic attraction and rapid volatilization from an extraordinary wettability. The superhydrophilic Bio-IL affords an even spread over a large-area substrate, which boosts a complete and uniform liquid film for heterogeneous nucleation as well as crystallization. Perovskite films on different large-area substrates with negligible coffee-ring effect are printed. Consequently, inverted flexible PSCs and perovskite solar modules achieve a high efficiency of 21.08% and 16.87%, respectively. This strategy ensures a highly reliable reproducibility of printing PSCs with a near 90% yield rate.

Near 90% Transparent ITO-Based Flexible Electrode with Double-Sided Antireflection Layers for Highly Efficient Flexible Optoelectronic Devices

As a widely used substrate for flexible electronics, indium-tin oxide-based polymer electrodes (polymer-ITO electrodes) exhibit poorly visible light transmittance of less than 80%. The inferior transmittance for polymer-ITO electrodes severely limits the performance improvement of polymer-ITO based electronics. Here, a conceptually different approach of the double-sided antireflection coatings (DARCs) strategy is proposed to modulate both the air-polymer substrate interface and ITO-air interface refractive index gradient, to synergistically improve the transmittance of polymer-ITO electrodes. On the basis of SiO₂ nanoparticles antireflection layer on polymer substrate, a polymer-metal oxide composite antireflection film is fabricated on the ITO side. Resultantly, the transmittance of ITO-based flexible electrodes is successfully improved from 76.8% to 89.8%, which is the highest transmittance among the reported ITO-based flexible electrodes. Furthermore, the photoluminescence emission intensity of luminescent materials enveloped with the DARCs electrodes increases by 74% over that with reference electrodes, demonstrating the DARCs antireflection strategy can efficiently improve the performance of flexible optoelectronic devices. With DARCs electrode, the flexible perovskite solar cells exhibit an enhanced efficiency from 18.80% to 20.85%.

Mechanically and operationally stable flexible inverted perovskite solar cells with 20.32% efficiency by a simple oligomer cross-linking method

Due to their great potential in wearable and portable electronics, flexible perovskite solar cells (FPSCs) have been extensively studied. The major challenges in the practical applications of FPSCs are efficiency, operational stability, and mechanical stability. Herein, we developed a facile approach by incorporating a cross-linking oligomer of trimethylolpropane ethoxylate triacrylate (TET) into perovskite films to simultaneously enhance the power conversion efficiency (PCE) and stability of FPSCs. A PCE of 20.32% was achieved, which are among the best results for the inverted FPSCs. Both mechanical and environmental stabilities were improved for the TET-incorporated FPSCs. In particular, the PCE retained approximately 87% of its initial value after 20,000 bending cycles at a radius of 4 mm. The inverted FPSCs retained 85% of the initial PCE after 500 h storage at 85 °C and 90% after 900 h continuous one-sun illumination. A joint experiment-theory analysis ascribed the underlying mechanism to the reduced defect densities, improved crystallinity, and stability of the perovskite absorbers on flexible substrates caused by TET incorporation.

Recent Progress of Electrode Materials for Flexible Perovskite Solar Cells

Flexible perovskite solar cells (FPSCs) have attracted enormous interest in wearable and portable electronics due to their high power-per-weight and low cost. Flexible and efficient perovskite solar cells require the development of flexible electrodes compatible with the optoelectronic properties of perovskite. In this review, the recent progress of flexible electrodes used in FPSCs is comprehensively reviewed. The major features of flexible transparent electrodes, including transparent conductive oxides, conductive polymer, carbon nanomaterials and nanostructured metallic materials are systematically compared. And the corresponding modification strategies and device performance are summarized. Moreover, flexible opaque electrodes including metal films, opaque carbon materials and metal foils are critically assessed. Finally, the development directions and difficulties of flexible electrodes are given.

Recent Advances in Electronic Skins with Multiple-Stimuli-Responsive and Self-Healing Abilities

Wearable electronic skin (e-skin) has provided a revolutionized way to intelligently sense environmental stimuli, which shows prospective applications in health monitoring, artificial intelligence and prosthetics fields. Drawn inspiration from biological skins, developing e-skin with multiple stimuli perception and self-healing abilities not only enrich their bionic multifunctionality, but also greatly improve their sensory performance and functional stability. In this review, we highlight recent important developments in the material structure design strategy to imitate the fascinating functionalities of biological skins, including molecular synthesis, physical structure design, and special biomimicry engineering. Moreover, their specific structure-property relationships, multifunctional application, and existing challenges are also critically analyzed with representative examples. Furthermore, a summary and perspective on future directions and challenges of biomimetic electronic skins regarding function construction will be briefly discussed. We believe that this review will provide valuable guidance for readers to fabricate superior e-skin materials or devices with skin-like multifunctionalities and disparate characteristics.

Self-Healing Materials for Electronics Applications

Self-healing materials have been attracting the attention of the scientists over the past few decades because of their effectiveness in detecting damage and their autonomic healing response. Self-healing materials are an evolving and intriguing field of study that could lead to a substantial increase in the lifespan of materials, improve the reliability of materials, increase product safety, and lower product replacement costs. Within the past few years, various autonomic and non-autonomic self-healing systems have been developed using various approaches for a variety of applications. The inclusion of appropriate functionalities into these materials by various chemistries has enhanced their repair mechanisms activated by crack formation. This review article summarizes various self-healing techniques that are currently being explored and the associated chemistries that are involved in the preparation of self-healing composite materials. This paper further surveys the electronic applications of self-healing materials in the fields of energy harvesting devices, energy storage devices, and sensors. We expect this article to provide the reader with a far deeper understanding of self-healing materials and their healing mechanisms in various electronics applications.

Dynamic covalent polymers enabled by reversible isocyanate chemistry

Reversible isocyanate chemistry containing urethane, thiourethane, and urea bonds is valuable for designing dynamic covalent polymers to achieve promising applications in recycling, self-healing, shape morphing, 3D printing, and composites.

Thermal Dynamic Self-healing Supramolecular Dopant Towards Efficient and Stable Flexible Perovskite Solar Cells

Flexible perovskite solar cells draw great attention due to their likeable traits like low cost, portability, light-weight, et al. However, mechanical stability is still the weak point in their practical application. Herein, we prepared efficient FPSC with remarkable mechanical stability by dynamic thermal self-healing effect, which can be realized by the usage of supramolecular adhesive. The colloidal adhesive was obtained by random copolymerization of acrylamide and n-butyl acrylate, which is amphiphilic, has a proper glass transition temperature and high density of hydrogen bond donors and receptors, providing the possibility of thermal dynamic repair of stress damage in FPSCs. The adhesive also greatly improves the leveling property of the precursor solution on the hydrophobic poly[bis(4-phenyl)(2,4,6-trimethylphenyl)]amine (PTAA) surface. PSCs containing this adhesive achieves more than 20% power conversion efficiency (PCE) on flexible substrates and 21.99% PCE on rigid substrates (certified PCE of 21.27%), with improved electron mobility and reduced defect concentration.

Thermal-Triggered Dynamic Disulfide Bond Self-Heals Inorganic Perovskite Solar Cells

One great challenge for perovskite solar cells (PSCs) lies in their poor operational stability under harsh stimuli by humidity, heat, light, etc . Herein, a thermal-triggered self-healing polyurethane (PU) is tailored to simultaneously improve the efficiency and stability of inorganic CsPbIBr 2 PSC. The dynamic covalent disulfide bonds between adjacent molecule chains in PU at high temperatures self-heal the in-service formed defects within CsPbIBr 2 perovskite film. Finally, the best device free of encapsulation achieves a champion efficiency up to 10.61% and an excellent long-term stability in air atmosphere over 80 days and persistent heat attack (85 o C) over 35 days. Moreover, the photovoltaic performances are recovered by a simple heat treatment.

Soft Polymer-Organolead Halide Perovskite Films for Highly Stretchable and Durable Photodetectors with Pt-Au Nanochain-Based Electrodes

The rigid and brittle nature of methylammonium lead iodide (MAPbI₃) polycrystalline films limits their application in stretchable devices due to rapid deterioration in performance on cycling. By incorporation of polymer chains in the MAPbI₃ films, a strategy to alter the mechanical modulus and the viscoelastic nature of the films has been developed. Combining this with flexible nanochain electrodes, highly stretchable and stable perovskite devices have been fabricated. The resultant polymer-MAPbI₃ photodetector exhibits ultralow dark currents (∼10^-11 A) and high light switching ratios (∼10³) and maintains 75% of performance after 30 days. The viscoelastic nature and lower modulus of the polymer improve the energy dissipation in the polymer-MAPbI₃ devices; as a result, they maintain 52% of the device performance after 10000 stretching cycles at 50% strain. The difference in the mechanical behavior is clearly observed in the failure mode of the two films. While rapid catastrophic cracking is observed in MAPbI₃ films, the intensity and size of such crack formation are highly limited in polymer-MAPbI₃ films, which prevent their failure.

Interface chelation induced by pyridine-based polymer for efficient and durable air-processed perovskite solar cells

Polymer doping is a significant approach to precisely control nucleation and crystal growth of perovskites and enhance electronic quality in perovskite solar cells (PSC) prepared in air. Here, a brand-new self-healing polysiloxane (SHP) with dynamic 2,6-pyridinedicarboxamide (PDCA) coordination units and plenty of hydrogen bonds was designed and incorporated into perovskite films. PDCA units, showing strong intermolecular Pb 2+ -N amido , I - -N pyridyl , and Pb 2+ -O amido coordination interactions, were expected to enhance crystallinity and passivate the grain boundary. In addition, abundant hydrogen bonds in SHP afforded the self-healing of cracks at grain boundaries for fatigue PSCs. Significantly, the doped device demonstrated a champion efficiency of 19.50% with inconspicuous hysteresis, almost rivaling those achieved in control atmosphere. This strategy of heterocyclic-based macromolecular doping in PSCs will pave a way for realizing efficient and durable crystalline semiconductors.

CsPbBr3 Nanocrystal Induced Bilateral Interface Modification for Efficient Planar Perovskite Solar Cells

Organic-inorganic halide perovskite solar cells (PSCs) have drawn tremendous attention owing to their remarkable photovoltaic performance and simple preparation process. However, conventional wet-chemical synthesis methods inevitably create defects both in the bulk and at the interfaces of perovskites, leading to recombination of charge carriers and reduced stability. Herein, a bilateral interface modification to perovskites by doping room-temperature synthesized CsPbBr3 nanocrystals (CN) is reported. The ultrafast transient absorption measurement reveals that CN effectively suppresses the defect at the SnO2 /perovskite interface and boosts the interfacial electron transport. Meanwhile, the in situ Kelvin probe force microscopy and contact potential difference characterizations verify that the CN within the upper part of the perovskites enhances the built-in electric field, facilitating oriented migration of the carriers within the perovskite. Combining the superiorities of CN modifiers on both sides, the bilaterally modified CH3 NH3 PbI3 -based planar PSCs exhibit optimal power conversion efficiency exceeding 20% and improved device stability.

Pushing the Limits of Flexibility and Stretchability of Solar Cells: A Review

Emerging forms of soft, flexible, and stretchable electronics promise to revolutionize the electronics industries of the future offering radically new products that combine multiple functionalities, including power generation, with arbitrary form factor. For example, skin-like electronics promise to transform the human-machine-interface, but the softness of the skin is incompatible with traditional electronic components. To address this issue, new strategies toward soft and wearable electronic systems are currently being pursued, which also include stretchable photovoltaics as self-powering systems for use in autonomous and stretchable electronics of the future. Here recent developments in the field of stretchable photovoltaics are reviewed and their potential for various emerging applications are examined. Emphasis is placed on the different strategies to induce stretchability including extrinsic and intrinsic approaches. In the former case, engineering and patterning of the materials and devices are key elements while intrinsically stretchable systems rely on mechanically compliant materials such as elastomers and organic conjugated polymers. The result is a review article that provides a comprehensive summary of the progress to date in the field of stretchable solar cells from the nanoscale to macroscopic functional devices. The article is concluded by discussing the emerging trends and future developments.

Fabrication of Perovskite Solar Cells with Digital Control of Transparency by Inkjet Printing

Semitransparency is an attractive and important property in solar cells since it opens new possibilities in a variety of applications such as tandem cell configuration and building-integrated photovoltaics. Metal halide perovskite has the optimal properties to function as the light harvester in solar cells and can be made as a thin film, while its chemical composition can change its band gap. However, achieving high transparency usually compromises the solar cell's efficiency. Here we report on a unique approach to fabricating semitransparent perovskite solar cells that does not rely on their composition or their thickness. The approach is based on a scalable process, inkjet printing of arrays of transparent pillars, which are composed of inert photopolymerizable liquid compositions and are partly covered by the perovskite. This material can be printed at specific locations and array densities, thus providing a digital control of both the transparency and efficiency of the solar cells. The new semitransparent device structure shows 11.2% efficiency with 24% average transparency without a top metal contact. Further development including deposition of a transparent contact enabled the fabrication of fully semitransparent devices with an efficiency of 10.6% and average transparency of 19%.

Inkjet-Deposited Single-Wall Carbon Nanotube Micropatterns on Stretchable PDMS-Ag Substrate-Electrode Structures for Piezoresistive Strain Sensing

Printed piezoresistive strain sensors based on stretchable roll-to-roll screen-printed silver electrodes on polydimethylsiloxane substrates and inkjet-deposited single-wall carbon nanotube micropatterns are demonstrated in this work. With the optimization of surface wetting and inkjet printing parameters, well-defined microscopic line patterns of the nanotubes with a sheet resistance of <100 Ω/□ could be deposited between stretchable Ag electrodes on the plasma-treated substrate. The developed stretchable devices are highly sensitive to tensile strain with a gauge factor of up to 400 and a pressure sensitivity of ∼0.09 Pa^-1, respond to bending down to a radius of 1.5 mm, and are suitable for mounting on the skin to monitor and resolve various movements of the human body such as cardiac cycle, breathing, and finger flexing. This study indicates that inkjet deposition of nanomaterials can complement well other printing technologies to produce flexible and stretchable devices in a versatile manner.

Wearable Tin-Based Perovskite Solar Cells Achieved by a Crystallographic Size Effect

Tin-based perovskite solar cells (PSCs) demonstrate a potential application in wearable electronics due to its hypotoxicity. However, poor crystal quality is still the bottleneck for achieving high-performance flexible devices. In this work, graphite phase-C₃ N₄ (g-C₃ N₄ ) is applied into tin-based perovskite as a crystalline template, which delays crystallization via a size-effect and passivates defects simultaneously. The double hydrogen bond between g-C₃ N₄ and formamidine cation can optimize lattice matching and passivation. Moreover, the two-dimensional network structure of g-C₃ N₄ can fit on the crystals, resulting an enhanced hydrophobicity and oxidation resistance. Therefore, the flexible tin-based PSCs with g-C₃ N₄ realize a stabilized power conversion efficiency (PCE) of 8.56 % with negligible hysteresis. In addition, the PSCs can maintain 91 % of the initial PCE after 1000 h under N₂ environment and keep 92 % of their original PCE after 600 cycles at a curvature radius of 3 mm.

Moisture-Triggered Self-Healing Flexible Perovskite Photodetectors with Excellent Mechanical Stability

Flexible devices are urgently required to meet the demands of next-generation optoelectronic devices and metal halide perovskites are proven to be suitable materials for realizing flexible photovoltaic devices. However, the tolerance to moisture corrosion and repeated mechanical bending remains a critical challenge for flexible perovskite devices. Herein, a self-healing formamidinium lead iodide (FAPbI₃ ) film is fabricated to cure mechanical damage by absorbing moisture from the surrounding environment. A poly(vinyl alcohol) microscaffold is designed not only to stabilize the black phase of the FAPbI₃ film but also to endow it with self-healing ability in a humid environment. The photodetector based on a self-healing film exhibits a high responsivity of 11.3 A W^-1 and recovers to over 90% of the initial responsivity after the self-healing process. This work provides an effective self-healing strategy to stabilize the operation of flexible perovskite devices under normal high-humidity environmental conditions.