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

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.

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.