Full-Dimensional Grain Boundary Stress Release for Flexible Perovskite Indoor Photovoltaics

6H-Intermediate Phase Enabled Slow Crystal Growth of Tin Halide Perovskites for Indoor Photovoltaics

The rapid expansion of Big Data and Internet of Things (IoT) has driven significant advancements in indoor photovoltaics (IPVs), which provide power to wireless IoT devices. Tin halide perovskites (THPs) have garnered significant attention for IPVs due to their excellent optoelectronic properties without the environmental risks of lead exposure. However, THPs face challenges in controlling their fast crystallization process. Here, we introduce a novel approach to precisely control the crystallization kinetics of FASnI2Br perovskite via the formation of the 6H-intermediate phase, supported by the mesomeric (+M) interaction effect of 4-aminopyridine hydrochloride (4APCl) in the perovskite precursor. The grazing-incidence wide-angle X-ray scattering measurements indicated the formation of 6H-intermediate phase for the FASnI2Br-4APCl perovskite during the crystallization process. The in situ ultraviolet-visible absorption spectroscopy during the spin coating and annealing process confirmed the reduction of crystal growth rate after the 6H-intermediate phase formation. Thus, high-quality perovskite films were obtained with reduced defects. The resulting IPVs achieved an efficiency of 21.55 % under indoor illumination at 1000 lux, exceeding all types of lead-free perovskite IPVs.

Interface Engineering with Polymer Interlayers: Achieving Uniform and Stable Perovskite Thick Films for X-ray Detection

Interface stress is a critical challenge limiting the optoelectronic performance and stability of perovskite-based X-ray detectors, often causing severe issues such as surface roughness, cracking, and film detachment during fabrication. To address this, polymer interlayers have been employed as effective stress-buffering layers at the substrate/perovskite interface. However, the influence of the polymer layer properties on the growth and quality of the perovskite films has received limited attention. In this study, we systematically explore the processability, mechanical properties, solubility, thermal stability, and adsorption strength with perovskite materials of four polymers - poly(vinylidene fluoride) (PVDF), poly(vinylpyrrolidone) (PVP), polyimide (PI), and poly(l-lactic acid) (PLLA) - using a combination of experimental and theoretical methods, emphasizing the role of polymer molecular characteristics in enhancing CsPbI2Br film quality. Among them, the CsPbI2Br thick film with a PI buffer layer demonstrates the most uniform and dense morphology, along with strong adhesion to the substrate, maintaining integrity even under a 50 g load. The resulting perovskite X-ray detectors exhibit excellent uniformity and long-term stability over 30 days of continuous operation. Moreover, the integration of the PI/CsPbI2Br thick film with a 64 × 64 thin-film transistor (TFT) enabled high-resolution X-ray imaging. This study provides valuable insights into selecting polymer interlayers to further optimize perovskite thick film-based X-ray detectors.

Enhancing Efficiency and Stability of Inverted Flexible Perovskite Solar Cells via Multi-Functionalized Molecular Design

Inverted flexible perovskite solar cells (f-PSCs) are promising candidates for mechanical photovoltaic applications due to their ease of preparation, lightweight, and portability. However, the weak interface connections, residual strain, and the nonradiative recombination loss among adjacent layers are critical challenges that restrict f-PSCs development. To address these issues, a functionalized molecule with multiple hydrogen bond acceptors, 4-Carboxyphenylboronic acid (4-BBA), is designed in the perovskite precursor for modulating perovskite crystallization, which achieves uniform and stress-relaxation perovskite film and forms a robust bridging structure anchored at the buried interface. Theoretical calculation and experimental results show that the C═O group passivates Pb2+ with I- vacancy defect through Lewis acid-base interactions, reducing trap-assisted recombination. Furthermore, the designed 4-BBA is preferentially deposited at the buried layer interface between the perovskite and substrate, forming hydrogen bonds with the self-assembled monolayer via B─OH bonds, creating a mechanically stable bridge between the layers. As a result, the power conversion efficiency of the champion f-PSC reached 25.30% (25.13% certified). And the f-PSC open-circuit voltage set a record of 1.21V. Importantly, the unencapsulated f-PSC using 4-BBA retains 95.3% of its original performance after 5000 cycles at a bending radius of 10mm, demonstrating extraordinary bending stability.

Toughened Interface by Engineering the Side Group of Conjugated Polymers to Stabilize Flexible Perovskite Solar Modules

Perovskite solar cells (PSCs) have attracted considerable attention due to their high power conversion efficiency (PCE), cost-effective manufacturing processes, as well as the potential flexibility. However, a significant challenge to the commercial applications of PSCs is their mechanical reliability. In this work, three naphthalene diimide polymers with distinct donor units are chosen to reduce surface trap states and enhance the long-term stability and mechanical reliability of photovoltaic devices. The champion rigid PSCs incorporating conjugated polymers achieved a 373% increase of adhesion toughness at the interface, with a champion PCE of 25.5% for a 0.16 cm2 single cell and 22.3% for a 30.9 cm2 module and retain 97% of the initial efficiency after 2000 h of continuous light soaking. Especially, the flexible PSCs exhibited improved mechanical stability, achieving a champion PCE of 24.8% for a 0.16 cm2 single cell and 20.3% for a 27.9 cm2 module, maintaining 95% of the initial efficiency after 5,000 bending cycles. This study highlights the potential of interfacial conjugated polymer in enhancing the efficiency and stability of PSCs.

Indoor light energy harvesting perovskite solar cells: from device physics to AI-driven strategies

The rapid advancement of indoor perovskite solar cells (IPSCs) stems from the growing demand for sustainable energy solutions and the proliferation of internet of things (IoT) devices. With tunable bandgaps and superior light absorption properties, perovskites efficiently harvest energy from artificial light sources like LEDs and fluorescent lamps, positioning IPSCs as a promising solution for powering smart homes, sensor networks, and portable electronics. In this review, we introduce recent research that highlights advancements in material optimization under low-light conditions, such as tailoring wide-bandgap perovskites to match indoor light spectra and minimizing defects to enhance stability. Notably, our review explores the integration of artificial intelligence (AI) and machine learning (ML), which are transforming IPSC development by facilitating efficient material discovery, optimizing device architectures, and uncovering degradation mechanisms. These advancements are driving the realization of sustainable indoor energy solutions for interconnected smart technologies.

Functionally Graded Oxide Scale on (Hf,Zr,Ti)B2 Coating with Exceptional Ablation Resistance Induced by Unique Ti Dissolving

Multicomponent Ti-containing ultra-high temperature ceramics (UHTCs) have emerged as more promising ablation-resistant materials than typical UHTCs for applications above 2000 °C. However, the underlying mechanism of Ti improving the ablation performance is still obscure. Here, (Hf,Zr,Ti)B2 coatings are fabricated by supersonic atmospheric plasma spraying, and the effects of Ti content on the ablation performance under an oxyacetylene flame are investigated. The (Hf0.45Zr0.45Ti0.10)B2 coating shows superior ablation resistance and cycling reliability at ≈2200°C. A functionally graded oxide scale comprising an outer dense layer and an underlying fine granular layer formed. The former is a better oxygen barrier owing to fewer cracks and the latter has high strain tolerance due to finer grain size. The uniform dissolving of ≈4 mol% Ti in the inner layer results in grain refinement via sluggish diffusion and thus stress release. For the outer layer, Ti segregation at the nanoscale leads to a metastable cubic (Hf,Zr,Ti)O2 and local severe lattice distortion, inhibiting the propagation of cracks. Ti ions' unique dissolving in the oxide scale enables a strong oxygen diffusion barrier with high strain tolerance, which is responsible for superior performance. This study provides new insights into the ablation behavior of Ti-containing multicomponent UHTCs.

Boosting mechanical durability under high humidity by bioinspired multisite polymer for high-efficiency flexible perovskite solar cells

Flexible perovskite solar cells (FPSCs) with high stability in moist air are required for their practical applications. However, the poor mechanical stability under high humidity air remains a critical challenge for flexible perovskite devices. Herein, inspired by the exceptional wet adhesion of mussels via dopamine groups, we propose a multidentate-cross-linking strategy, which combine multibranched structure and adequate dopamine anchor sites in three-dimensional hyperbranched polymer to directly chelate perovskite materials in multiple directions, therefore construct a vertical scaffold across the bulk of perovskite films from the bottom to the top interfaces, intimately bind to the perovskite grains and substrates with a strong adhesion ability, and enhance mechanical durability under high humidity. Consequently, the modified rigid PSCs achieve superior PCE up to 25.92%, while flexible PSCs exhibit a PCE of 24.43% and maintain 94.1% of initial PCE after 10,000 bending cycles with a bending radius of 3 mm under exposed to 65% humidity.

Methylthio Substituent in SAM Constructing Regulatory Bridge with Photovoltaic Perovskites

Inverted (p-i-n) perovskite solar cells (PSCs) have experienced remarkable advancements in recent years, which is largely attributed to the development of novel hole-transport layer (HTL) self-assembled monolayer (SAM) materials. Methoxy (MeO-) groups are typically introduced into SAM materials to enhance their wettability and effectively passivate the perovskite buried interface. However, MeO-based SAM materials exhibit a mismatch in highest occupied molecular orbital (HOMO) levels with perovskite layer due to the strong electron-donating capability of methoxy group. In this work, we introduced a methylthio (MeS-) substituent that is superior to methoxy as a highly versatile self-assembled molecular design strategy. As a soft base, sulfur atom forms a stronger Pb-S bond than oxygen. Additionally, within the CbzPh series of SAM materials, MeS-CbzPh demonstrates a more optimal HOMO level and enhanced hole transport properties. Consequently, the MeS-CbzPh HTL based device achieved an impressive power conversion efficiency (PCE) of 26.01 % and demonstrated high stability, retaining 93.3 % efficiency after 1000 hours of maximum power point tracking (MPPT). Moreover, in comparison with the commonly used 4PACz-based SAM molecular series, MeS-4PACz also exhibited the best performance among its peers. Our work provides valuable insights for the molecular design of SAM materials, offering a highly versatile functional substituent group.

Interfacial Field-Effect Enabling High-Performance Perovskite Photovoltaics

Currently, the power conversion efficiency (PCE) of inverted perovskite solar cells (PSCs) is still limited by reduced open-circuit voltage (VOC), due to defect-induced charge recombination. Most studies focus on defect passivation and improving carrier transport through introducing passivating molecules or macroscopic physical fields. Herein, to mitigate energy level mismatch and recombination losses induced by interface defects, an interface electric-field passivation is introduced, employing the ordered arrangement of the dipole molecule benzenesulfonyl chloride (BC). An enhanced VOC is achieved without the introduction of an external physical field, owing to the interfacial dipole field effect and chemical passivation by BC. Subsequently, an inverted device with a PCE of 25.41% is obtained, alongside exceptional stability, retaining 95% of the initial efficiency after 1157 h. This work demonstrates the effective dipole-induced interfacial field-effect passivation in inverted PSCs and contributes to further advancements in the efficiency and stability of inverted devices.

Visible Light-Triggered Self-Welding Perovskite Solar Cells and Modules

Flexible perovskite solar cells (F-PSCs) are highly promising for both stationary and mobile applications because of their advantageous features, including mechanical flexibility, their lightweight and thin nature, and cost-effectiveness. However, a number of drawbacks, such as mechanical instability, make their practical application difficult. Here, self-welding dynamic diselenide that is triggered by visible light into the structure of F-PSCs to improve their long-term stability by repairing cracks and defects in the absorber layer is incorporated. The diselenide confers the flexibility and self-welding properties to the Cs0.05MA0.05FA0.9PbI3 perovskite layer, enabling optimized F-PSC devices to achieve a power conversion efficiency of 24.85% while retaining ca. 92% of their initial efficiency after undergoing 15 000 bending cycles at a curvature radius of 3 mm. The corresponding flexible large-scale module with an active area of 15.82 cm2 achieved a record PCE of 21.65%.

Surficial Homogenic Effect Enables Highly Stable Deep-Blue Perovskite Light-Emitting Diodes

The device performance of deep-blue perovskite light-emitting diodes (PeLEDs) is primarily constrained by low external quantum efficiency (EQE) especially poor operational stability. Herein, we develop a facile strategy to improve deep-blue emission through rational interface engineering. We innovatively reported the novel electron transport material, 4,6-Tris(4-(diphenylphosphoryl)phenyl)-1,3,5-triazine (P-POT2T), and utilized a sequential wet-dry deposition method to form the homogenic gradient interface between electron transport layer (ETL) and perovskite surface. Unlike previous reports that achieved carrier injection balance by inserting new interlayers, our strategy not only passivated uncoordinated Pb2+ in the perovskite via P=O functional groups but also reduced interfacial carrier recombination without introducing new interfaces. Additionally, this strategy enhanced the interface contact between the perovskite and ETL, significantly boosting device stability. Consequently, the fabricated deep-blue PeLEDs delivered an EQE exceeding 5 % (@ 460 nm) with an exceptional halftime extended to 31.3 minutes. This straightforward approach offers a new strategy to realize highly efficient especially stable PeLEDs.

A multifunctional phenylphosphinamide additive for stable flexible inverted perovskite solar cells

The multifunctional phenylphosphinamide additive is used in flexible inverted perovskite solar cells to release tensile strain and increase the toughness of the perovskite film, achieving enhanced device efficiency and stability.

Sustainable Recycling of Selenium-Based Optoelectronic Devices

Selenium (Se), the world's oldest optoelectronic material, has been widely applied in various optoelectronic devices such as commercial X-ray flat-panel detectors and photovoltaics. However, despite the rare and widely-dispersed nature of Se element, a sustainable recycling of Se and other valuable materials from spent Se-based devices has not been developed so far. Here a sustainable strategy is reported that makes use of the significantly higher vapor pressure of volatile Se compared to other functional layers to recycle all of them from end-of-life Se-based devices through a closed-space evaporation process, utilizing Se photovoltaic devices as a case study. This strategy results in high recycling yields of ≈ 98% for Se and 100% for other functional materials including valuable gold electrodes and glass/FTO/TiO2 substrates. The refabricated photovoltaic devices based on these recycled materials achieve an efficiency of 12.33% under 1000-lux indoor illumination, comparable to devices fabricated using commercially sourced materials and surpassing the current indoor photovoltaic industry standard of amorphous silicon cells.

Photochemical Shield Enabling Highly Efficient Perovskite Photovoltaics

Oxygen is difficult to be physically removed. Oxygen will be excited by light to form free radicals which further attack the lattice of perovskite. The stabilization of α-FAPbI3 against δ-FAPbI3 is the key to optimize perovskite solar cells. Herein, the simple molecule, benzaldehyde (BAH) is adopted. The photochemical shield will be established in perovskite layer. Moreover, heterogeneous nucleation induced by BAH enhances the crystallization of α-FAPbI3. Consequently, the stability of device is improved significantly. The target device maintains 95% of original power conversion efficiency after 1500 h under air conditions and light-emitting diode light. The power conversion efficiency increases from 23.21% of pristine device to 24.82% of target device.

Mechanical Durability and Flexibility in Perovskite Photovoltaics: Advancements and Applications

The remarkable progress in perovskite solar cell (PSC) technology has witnessed a remarkable leap in efficiency within the past decade. As this technology continues to mature, flexible PSCs (F-PSCs) are emerging as pivotal components for a wide array of applications, spanning from powering portable electronics and wearable devices to integrating seamlessly into electronic textiles and large-scale industrial roofing. F-PSCs characterized by their lightweight, mechanical flexibility, and adaptability for cost-effective roll-to-roll manufacturing, hold immense commercial potential. However, the persistent concerns regarding the overall stability and mechanical robustness of these devices loom large. This comprehensive review delves into recent strides made in enhancing the mechanical stability of F-PSCs. It covers a spectrum of crucial aspects, encompassing perovskite material optimization, precise crystal grain regulation, film quality enhancement, strategic interface engineering, innovational developed flexible transparent electrodes, judicious substrate selection, and the integration of various functional layers. By collating and analyzing these dedicated research endeavors, this review illuminates the current landscape of progress in addressing the challenges surrounding mechanical stability. Furthermore, it provides valuable insights into the persistent obstacles and bottlenecks that demand attention and innovative solutions in the field of F-PSCs.

Electronically Manipulated Molecular Strategy Enabling Highly Efficient Tin Perovskite Photovoltaics

Buried interface modification can effectively improve the compatibility between interfaces. Given the distinct interface selections in perovskite solar cells (PSCs), the applicability of a singular modification material remains limited. Consequently, in response to this challenge, we devised a tailored molecular strategy based on the electronic effects of specific functional groups. Therefore, we prepared three distinct silane coupling agents, and due to the varying inductive effects of these functional groups, the electronic distribution and molecular dipole moments of the coupling agents are correspondingly altered. Among them, trimethoxy (3,3,3-trifluoropropyl)-silane (F3 -TMOS), which possesses electron-withdrawing groups, generates a molecular dipole moment directed toward the hole transport layer (HTL). This approach changes the work function of the HTL, optimizes the energy level alignment, reduces the open-circuit voltage loss, and facilitates carrier transport. Furthermore, through the buffering effect of the coupling agent, the interface strain and lattice distortion caused by annealing the perovskite are reduced, enhancing the stability of the tin-based perovskite. Encouragingly, tin PSCs treated with F3 -TMOS achieved a champion efficiency of 14.67 %. This strategy provides an expedient avenue for the design of buried interface modification materials, enabling precise molecular adjustments in accordance with distinct interfacial contexts to ameliorate mismatched energetics and enhance carrier dynamics.

In Situ Formation of 2D Perovskite Seeding for Record-Efficiency Indoor Perovskite Photovoltaic Devices

With 40% efficiency under room light intensity, perovskite solar cells (PSCs) will be promising power supplies for low-light applications, particularly for Internet of Things (IoT) devices and indoor electronics, shall they become commercialized. Herein, β-alaninamide hydrochloride (AHC) is utilized to spontaneously form a layer of 2D perovskite nucleation seeds for improved film uniformity, crystallization quality, and solar cell performance. It is found that the AHC addition indeed improves film quality as demonstrated by better uniformity, lower trap density, smaller lattice stress, and, as a result, a 10-fold increase in charge carrier lifetime. Consequently, not only does the small-area (0.09 cm2 ) PSCs achieve a power conversion efficiency of 42.12%, the large-area cells (1.00 cm2 , and 2.56 cm2 ) attain efficiency as high as 40.93%, and 40.07% respectively. All of these are the highest efficiency values for indoor photovoltaic cells with similar sizes, and more importantly, they represent the smallest efficiency loss due to area scale-up. This work provides a new method to fabricate high-performance indoor PSCs (i-PSCs) for IoT devices with great potential in large-area printing technology.

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.

Dual-Interface Modulation with Covalent Organic Framework Enables Efficient and Durable Perovskite Solar Cells

Dual-interface modulation including buried interface as well as the top surface has recently been proven to be crucial for obtaining high photovoltaic performance in lead halide perovskite solar cells (PSCs). Herein, for the first time, the strategy of using functional covalent organic frameworks (COFs), namely HS-COFs for dual-interface modulation, is reported to further understand its intrinsic mechanisms in optimizing the bottom and top surfaces. Specifically, the buried HS-COFs layer can enhance the resistance against ultraviolet radiation, and more importantly, release the tensile strain, which is beneficial for enhancing device stability and improving the order of perovskite crystal growth. Furthermore, the detailed characterization results reveal that the HS-COFs on the top surface can effectively passivate the surface defects and suppress non-radiation recombination, as well as optimize the crystallization and growth of the perovskite film. Benefiting from the synergistic effects, the dual-interface modified devices deliver champion efficiencies of 24.26% and 21.30% for 0.0725 cm2 and 1 cm2 -sized devices, respectively. Moreover, they retain 88% and 84% of their initial efficiencies after aging for 2000 h under the ambient conditions (25 °C, relative humidity: 35-45%) and a nitrogen atmosphere with heating at 65 °C, respectively.

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.

Progress in Surface Modification of SnO2 Electron Transport Layers for Stable Perovskite Solar Cells

The photovoltaic (PV) performance of perovskite solar cells (PSCs) has rapidly advanced in the recent years; yet, the stability issue remains one of the last-mile challenges on the road to commercialization. Charge transport layers and their interfaces with perovskites stand for critical tuning knobs that determine the device stability of PSCs. This review focuses on the effects of modification of SnO2 electron transport layers (ETLs) on the interfacial physicochemical properties and stability of PSC devices. In detail, the intrinsic defects, surface hydroxyls, and nonuniform morphology of SnO2 will negatively impact its interfacial physicochemical properties, thus degrading the device stability of PSCs. To tackle these existing issues, three modification approaches, such as surface morphology control, surface physicochemical modifications, and surface composite-structure design, are categorized. Lastly, future perspectives in further promoting the stability of PSCs from SnO2 ETLs are raised based on the currently unresolved issues from both material and device levels.

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.

Perspectives for the conversion of perovskite indoor photovoltaics into IoT reality

As a competitive candidate for powering low-power terminals in Internet of Things (IoT) systems, indoor photovoltaic (IPV) technology has attracted much attention due to its effective power output under indoor light illumination. One such emerging photovoltaic technology, perovskite cell, has become a hot topic in the field of IPVs due to its outstanding theoretical performance limits and low manufacturing costs. However, several elusive issues remain limiting their applications. In this review, the challenges for perovskite IPVs are discussed in view of the bandgap tailoring to match indoor light spectra and the defect trapping regulation throughout the devices. Then, we summarize up-to-date perovskite cells, highlighting advanced strategies such as bandgap engineering, film engineering and interface engineering to enhance indoor performance. The investigation of indoor applications of large and flexible perovskite cells and integrated devices powered by perovskite cells is exhibited. Finally, the perspectives for the perovskite IPV field are provided to help facilitate the further improvement of indoor performance.

Recent Strategies for High-Performing Indoor Perovskite Photovoltaics

The development of digital technology has made our lives more advanced as a society familiar with the Internet of Things (IoT). Solar cells are among the most promising candidates for power supply in IoT sensors. Perovskite photovoltaics (PPVs), which have already attained 25% and 40% power conversion efficiencies for outdoor and indoor light, respectively, are the best candidates for self-powered IoT system integration. In this review, we discuss recent research progress on PPVs under indoor light conditions, with a focus on device engineering to achieve high-performance indoor PPVs (Id-PPVs), including bandgap optimization and defect management. Finally, we discuss the challenges of Id-PPVs development and its interpretation as a potential research direction in the field.

2.11 - Accurate characterization of indoor photovoltaic performance

Ambient energy harvesting has great potential to contribute to sustainable development and address growing environmental challenges. Converting waste energy from energy-intensive processes and systems (e.g. combustion engines and furnaces) is crucial to reducing their environmental impact and achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications that can improve our quality of life (e.g. smart homes, smart cities, smart manufacturing, and smart healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and radiofrequency wireless power transfer. By bringing together the perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive insights into recent advances and present challenges in the field. Additionally, the Roadmap analyses the key performance metrics of these technologies in relation to their ultimate energy conversion limits. Building on these insights, the Roadmap outlines promising directions for future research to fully harness the potential of energy harvesting materials for green energy anytime, anywhere.

Indoor photovoltaics awaken the world's first solar cells

Selenium (Se) solar cells were the world's first solid-state photovoltaics reported in 1883, opening the modern photovoltaics. However, its wide bandgap (~1.9 eV) limits sunlight harvesting. Here, we revisit the world's oldest but long-ignored photovoltaic material with the emergence of indoor photovoltaics (IPVs); the absorption spectrum of Se perfectly matches the emission spectra of commonly used indoor light sources in the 400 to 700 nm range. We find that the widely used Te adhesion layer also passivates defects at the nonbonded Se/TiO₂ interface. By optimizing the Te coverage from 6.9 to 70.4%, the resulting Se cells exhibit an efficiency of 15.1% under 1000 lux indoor illumination and show no efficiency loss after 1000 hours of continuous indoor illumination without encapsulation, outperforming the present IPV industry standard of amorphous silicon cells in both efficiency and stability. We further fabricate Se modules (6.75 cm²) that produce 232.6 μW output power under indoor illumination, powering a radio-frequency identification-based localization tag.

Light-Triggered Sustainable Defect-Passivation for Stable Perovskite Photovoltaics

The generation of photoinduced defects and freely moving halogen ions is dynamically updated in real time. Accordingly, most reported strategies are static and short-term, which make their improvements in photostability very limited. Therefore, seeking new passivation strategies to match the dynamic characteristics of defect generation is very urgent. Without newly generated defects, a passivation molecule should exist in the configuration that would not become the initiation sites for defect generation. With newly generated defects, the passivation molecule should transfer into the other configuration that possesses the passivation sites. Herein, a classical photoisomeric molecule, spiropyran, is adopted, whose pre- and post-isomeric forms meet the requirements for two different configurations, to realize the state transition once the photoinduced defects appear during subsequent operation and dynamic capture for continuous renewal of defects. Consequently, spiropyrans work as light-triggered and self-healing sustainable passivation sites to realize continuous defect repair. The target devices retain 93% and 99% of their initial power conversion efficiencies after 456 h aging under ultraviolet illumination and 1200 h aging under full-spectrum illumination, respectively. This work provides a novel concept of sustainable passivation strategy to realize continuous defect-passivation and film-healing in perovskite photovoltaics.

Strain Release and Defect Passivation in Formamidinium-Dominated Perovskite via a Novel in-Plane Thermal Gradient Assisted Crystallization Strategy

It is essential to release annealing induced strain during the crystallization process to realize efficient and stable perovskite solar cells (PSCs), which does not seem achievable using the conventional annealing process. Here we report a novel and facile thermal gradient assisted crystallization strategy by simply introducing a slant angle between the preheated hot plate and the substrate. A distinct crystallization sequence resulted along the in-plane direction pointing from the hot side to the cool side, which effectively reduced the crystallization rate, controlled the perovskite grain growth, and released the in-plane tensile strain. Moreover, this strategy enabled uniform strain distribution in the vertical direction and assisted in reducing the defects and aligning the energy bands. The corresponding device demonstrated champion power conversion efficiencies (PCEs) of 23.70% and 21.04% on the rigid and flexible substrates, respectively. These highly stable rigid devices retained 97% of the initial PCE after 1097 h of storage and more than 80% of the initial PCE after 1000 h of continuous operation at the maximum power point. This novel strategy opens a simple and effective avenue to improve the quality of perovskite films and photovoltaic devices via strain modulation and defect passivation.

Dual-Interface-Reinforced Flexible Perovskite Solar Cells for Enhanced Performance and Mechanical Reliability

Two key interfaces in flexible perovskite solar cells (f-PSCs) are mechanically reinforced simultaneously: one between the electron-transport layer (ETL) and the 3D metal-halide perovskite (MHP) thin film using self-assembled monolayer (SAM), and the other between the 3D-MHP thin film and the hole-transport layer (HTL) using an in situ grown low-dimensional (LD) MHP capping layer. The interfacial mechanical properties are measured and modeled. This rational interface engineering results in the enhancement of not only the mechanical properties of both interfaces but also their optoelectronic properties holistically. As a result, the new class of dual-interface-reinforced f-PSCs has an unprecedented combination of the following three important performance parameters: high power-conversion efficiency (PCE) of 21.03% (with reduced hysteresis), improved operational stability of 1000 h T₉₀ (duration at 90% initial PCE retained), and enhanced mechanical reliability of 10 000 cycles n₈₈ (number of bending cycles at 88% initial PCE retained). The scientific underpinnings of these synergistic enhancements are elucidated.

Overcoming Perovskite Corrosion and De-Doping Through Chemical Binding of Halogen Bonds Toward Efficient and Stable Perovskite Solar Cells

4-tert-butylpyridine (TBP) is an indispensable additive for the hole transport layer in highly efficient perovskite solar cells (PSCs), while it can induce corrosion decomposition of perovskites and de-doping effect of spiro-OMeTAD, which present huge challenge for the stability of PSCs. Herein, halogen bonds provided by 1,4-diiodotetrafluorobenzene (1,4-DITFB) are employed to bond with TBP, simultaneously preventing perovskite decomposition and eliminating de-doping effect of oxidized spiro-OMeTAD. Various characterizations have proved strong chemical interaction forms between 1,4-DITFB and TBP. With the incorporation of halogen bonds, perovskite film can maintain initial morphology, crystal structure, and light absorbance; meanwhile, the spiro-OMeTAD film shows a relatively stable conductivity with good charge transport property. Accordingly, the device with TBP complex exhibits significantly enhanced stability in N₂ atmosphere or humidity environment. Furthermore, a champion power conversion efficiency of 23.03% is obtained since perovskite is no longer damaged by TBP during device preparation. This strategy overcomes the shortcomings of TBP in n-i-p PSCs community and enhances the application potential of spiro-OMeTAD in fabricating efficient and stable PSCs.