Chen Weijie

Herein, the major optical losses of monolithic perovskite/silicon tandem solar cells (TSCs) are summarized, analyzing material selection and device design to date. Relevant strategies and challenges in monolithic perovskite/silicon TSCs are highlighted, comprising bandgap engineering of perovskites and light trapping methods, providing guidance for further improvement of tandem devices.

Perovskite/silicon tandem solar cells (TSCs), especially two‐terminal, with a record efficiency of 28% already realized, present great potential as low‐cost and efficient substitutes for dominant silicon photovoltaics. Achieving efficiencies exceeding 30% is quite realistic, as indicated by extensive optical simulations. Super light management in monolithic perovskite/silicon TSCs is one of the prerequisites to make this a reality. In this Review, various forms of optical losses, such as reflection loss, parasitic absorption, and current mismatch, are analyzed systematically to provide a better understanding of the performance of perovskite/silicon TSCs. Particularly, a simple refractive index matching rule derived from the Fresnel equation is proposed as a basis for material selection and device design. Meanwhile, an overview of the current strategies and challenges in monolithic perovskite/silicon TSCs is provided, comprising bandgap engineering of perovskites and light trapping methods, aiming to provide guidance for further improvement of tandem devices.

Stability and toxicity are bottlenecks for halide perovskite solar cells despite their remarkable efficiency. Double halide perovskites with heterovalent metal cations pave a way for lead‐free‐based devices for enhanced stability. This Review summarizes the theoretical and experimental progress of lead‐free double perovskite. The issues, challenges, applications, and future prospects are integrated to provide a full picture.

Perovskite solar cells (PSCs) have achieved a high power conversion efficiency (PCE) with a credible certified value over 25%. More efforts have been devoted to the development of stable and ecofriendly perovskite materials. Lead‐free double perovskites (LFDPs) are a noteworthy choice as a photoactive layer because of their favorable photovoltaic (PV) properties, intrinsic chemical stability, and environmental friendliness. This Review presents various LFDP materials whose structural stability and optoelectronic properties are predicted by theoretical calculations. The synthesis and experimental properties of LFDPs and their applications in PSCs and optoelectronics in pursuing high performance, low toxicity, and functional stability are also reviewed. Perovskites active layers are critical for PSCs, and their appropriate properties are responsible for achieving a high PCE. On the other side, the stability of PSCs under working conditions is a critical requirement for their practical applications. Defect‐ordered perovskites are also presented to provide another outlook on lead‐free perovskite‐based PVs. The introduction and interest toward LFDP in PSCs can represent a viable solution to the toxicity issue, stimulate further research, and bring a real impact to future PV technologies.

Recent case studies demonstrate how probing of local heterogeneities and ensemble averaged properties of perovskites by surface science techniques can help build connections between material properties and perovskite solar cell (PSC) performance. How the generation/healing of electronic defects within the semiconductor band‐gap influences PSC efficiency, lifetime, as well reproducibility is also the central focus of this review article.

Abstract

ABX₃ type metal halide perovskite solar cells (PSCs) have shown efficiencies over 25%, rocketing toward their theoretical limit. To gain the full potential of PSCs relies on the understanding of the device working mechanisms and recombination, the material quality, and the match of energy levels in the device stacks. In this review, the importance of designing PSCs from the viewpoint of surface/interface science studies is presented. For this purpose, recent case studies are discussed to demonstrate how probing of local heterogeneities (e.g., grains, grain boundaries, atomic structure, etc.) in perovskites by surface science techniques can help correlate material properties and PSC device performance. At the solar cell device level with active areas larger than millimeter scale, the ensemble average measurement techniques can characterize the overall average properties of perovskite films as well as their adjacent layers and provide clues to understand better the solar cell parameters. How generation and healing of electronic defects in perovskite films limit the device efficiency, reproducibility, and stability, and induce the time‐dependent transient behavior in the current‐voltage curves are also the central focus of this review. On the basis of these studies, strategies to further improve efficiency and stability, as well as reducing hysteresis are presented.

A series of high‐performance perylene diimide based molecular acceptors, namely, TPP‐PDI, TPO‐PDI, and TPS‐PDI, are smartly designed for efficient nonfullerene polymer solar cells. Combined with the optimization of the blend morphology through supramolecular molecular lock effect, the champion power conversion efficiency of 11.01% is realized in TPO‐PDI‐based devices.

Abstract

A series of perylene diimide (PDI) derivatives, TPP‐PDI, TPO‐PDI, and TPS‐PDI, are developed for nonfullerene polymer solar cells (NF‐PSCs) by flaking three PDI skeletons around 3D central cores with different configurations and electronic states, such as triphenylphosphine (TPP), triphenylphosphine monoxide (TPO), and triphenylphosphine sulfide (TPS). These small‐molecule acceptors have a “three‐wing propeller” structure due to their similar backbones. By changing the electron density of phosphorus atoms through oxidation and sulfuration, the “folding‐back” strength is decreased, resulting in a less twisted molecular conformation. The stronger electron‐withdrawing ability of the oxygen atom affords TPO‐PDI the least twisted conformation, which enhances the crystallinity of this complex. NF‐PSCs based on PTTEA:TPO‐PDI exhibit a high power conversion efficiency (PCE) of 8.65%. Ultimately, the joint “molecular lock” effect arising from OH⋅⋅⋅F and OH⋅⋅⋅OP supramolecular interactions is achieved by introducing 4,4′‐biphenol as an additive, which successfully promotes fibril‐like phase separation and blend morphology optimization to generate the highest PCE of 11.01%, which is currently the highest value recorded for NF‐PSCs based on PDI acceptors.

Replacing a 2,2′,7,7′‐tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9,9′‐spirobifluorene hole transporting layer with new alternatives such as poly(5,5‐didecyl‐5H‐1,8‐dithia‐as‐indacenone‐alt‐thieno[3,2‐b]thiophene) in mesoscopic perovskite solar cells reduces the fabrication cost and improves the operational stability without sacrificing the efficiency.

Abstract

Stability is the main challenge in the field of organic–inorganic perovskite solar cells (PSCs). Finding low‐cost and stable hole transporting layer (HTL) is an effective strategy to address this issue. Here, a new donor polymer, poly(5,5‐didecyl‐5H‐1,8‐dithia‐as‐indacenone‐alt‐thieno[3,2‐b]thiophene) (PDTITT), is synthesized and employed as an HTL in PSCs, which has a suitable band alignment with respect to the double‐A cation perovskite film. Using PDTITT, the hole extraction in PSCs is greatly improved as compared to commonly used HTLs such as 2,2′,7,7′‐tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9,9′‐spirobifluorene (spiro‐OMeTAD), addressing the hysteresis issue. After careful optimization, an efficient PSC is achieved based on mesoscopic TiO₂ electron transporting layer with a maximum power conversion efficiency (PCE) of 18.42% based on PDTITT HTL, which is comparable with spiro‐OMeTAD‐based PSC (19.21%). Since spiro‐based PSCs suffer from stability issue, the operational stability in the PSC with PDTITT HTL is studied. It is found that the device with PDTITT retains 88% of its initial PCE value after 200 h under illumination, which is better than the spiro‐based PSC (54%).

The OH…I⁻ hydrogen bonding interactions between poly(vinyl alcohol) (PVA) and FASnI₃ have the effects of introducing nucleation sites, slowing down crystal growth, directing the crystal orientation, reducing the trap states, and suppressing the migration of the ions. By adding PVA, the FASnI₃–PVA perovskite solar cells attain improved power conversion efficiency and stability.

Abstract

Tin‐based perovskites with narrow bandgaps and high charge‐carrier mobilities are promising candidates for the preparation of efficient lead‐free perovskite solar cells (PSCs). However, the crystalline rate of tin‐based perovskites is much faster, leading to abundant trap states and much lower open‐circuit voltage (V _oc). Here, hydrogen bonding is introduced to retard the crystalline rate of the FASnI₃ perovskite. By adding poly(vinyl alcohol) (PVA), the OH…I⁻ hydrogen bonding interactions between PVA and FASnI₃ have the effects of introducing nucleation sites, slowing down the crystal growth, directing the crystal orientation, reducing the trap states, and suppressing the migration of the iodide ions. In the presence of the PVA additive, the FASnI₃–PVA PSCs attain higher power conversion efficiency of 8.9% under a reverse scan with significantly improved V _oc from 0.55 to 0.63 V, which is one of the highest V _oc values for FASnI₃‐based PSCs. More importantly, the FASnI₃–PVA PSCs exhibit striking long‐term stability, with no decay in efficiency after 400 h of operation at the maximum power point. This approach, which makes use of the OH…I⁻ hydrogen bonding interactions between PVA and FASnI₃, is generally applicable for improving the efficiency and stability of the FASnI₃‐based PSCs.

The elastic modulus of 3D perovskite is very close to that of human bones and the elastic modulus of 2D perovskite with long chains is close to that of cartilage. By reconstructing a crystal lattice with different A cations at the surface of perovskite films, a nature “bone‐joint” configuration is built in perovskite, which provides a cushioning effect to external stresses.

Abstract

To improve the photovoltaic performance (both efficiency and stability) in hybrid organic–inorganic halide perovskite solar cells, perovskite lattice distortion is investigated with regards to residual stress (and strain) in the polycrystalline thin films. It is revealed that residual stress is concentrated at the surface of the as‐prepared film, and an efficient method is further developed to release this interfacial stress by A site cation alloying. This results in lattice reconstruction at the surface of polycrystalline thin films, which in turn results in low elastic modulus. Thus, a “bone‐joint” configuration is constructed within the interface between the absorber and the carrier transport layer, which improves device performance substantially. The resultant photovoltaic devices exhibit an efficiency of 21.48% with good humidity stability and improved resistance against thermal cycling.

The spintronic properties of different hybrid organic–inorganic perovskites (HOIPs) are studied in spin valve devices, including spin diffusion length and spin lifetime, as well as the impact of the chemical components on these properties. This study aims at demonstrating promising spintronic applications of HOIPs, and providing a clear path for engineering spintronic devices based on HOIPs.

Abstract

The hybrid organic–inorganic perovskites (HOIPs) form a new class of semiconductors which show promising optoelectronic device applications. Remarkably, the optoelectronic properties of HOIP are tunable by changing the chemical components of their building blocks. Recently, the HOIP spintronic properties and their applications in spintronic devices have attracted substantial interest. Here the impact of the chemical component diversity in HOIPs on their spintronic properties is studied. Spin valve devices based on HOIPs with different organic cations and halogen atoms are fabricated. The spin diffusion length is obtained in the various HOIPs by measuring the giant magnetoresistance (GMR) response in spin valve devices with different perovskite interlayer thicknesses. In addition spin lifetime is also measured from the Hanle response. It is found that the spintronic properties of HOIPs are mainly determined by the halogen atoms, rather than the organic cations. The study provides a clear avenue for engineering spintronic devices based on HOIPs.

A high power conversion efficiency of 11.76%, the best efficiency for all‐polymer solar cells, is achieved by printing fabrication based on PTzBI‐Si:N2200 processing with 2‐methyltetrahydrofuran. A Multi‐length‐scaled morphology is found in the bulk heterojunctions, which ensures fast transfer of carriers and facilitates exciton separation, and boosts carrier mobility and current density, thus improving the device performance.

Abstract

All‐polymer solar cells (all‐PSCs) exhibit excellent stability and readily tunable ink viscosity, and are therefore especially suitable for printing preparation of large‐scale devices. At present, the efficiency of state‐of‐the‐art all‐PSCs fabricated by the spin‐coating method has exceeded 11%, laying the foundation for the preparation and practical utilization of printed devices. A high power conversion efficiency (PCE) of 11.76% is achieved based on PTzBI‐Si:N2200 all‐PSCs processing with 2‐methyltetrahydrofuran (MTHF, an environmentally friendly solvent) and preparation of active layers by slot die printing, which is the top efficient for all‐PSCs. Conversely, the PCE of devices processed by high‐boiling point chlorobenzene is less than 2%. Through the study of film formation kinetics, volatile solvents can freeze the morphology in a short time, and a more rigid conformation with strong intermolecular interaction combined with the solubility limit of PTzBI‐Si and N2200 in MTHF results in the formation of a fibril network in the bulk heterojunction. The multilength scaled morphology ensures fast transfer of carriers and facilitates exciton separation, which boosts carrier mobility and current density, thus improving the device performance. These results are of great significance for large‐scale printing fabrication of high‐efficiency all‐PSCs in the future.

A general approach for lab‐to‐manufacturing translation is developed to achieve high‐performance flexible organic solar modules without obvious efficiency loss. The shear impulse during the coating/printing process is applied to control the morphology evolution of the bulk heterojunction layer for both fullerene and nonfullerene acceptor systems. A quantitative transformation factor of shear impulse between slot‐die printing and spin‐coating is detected.

Abstract

The blossoming of organic solar cells (OSCs) has triggered enormous commercial applications, due to their high‐efficiency, light weight, and flexibility. However, the lab‐to‐manufacturing translation of the praisable performance from lab‐scale devices to industrial‐scale modules is still the Achilles' heel of OSCs. In fact, it is urgent to explore the mechanism of morphological evolution in the bulk heterojunction (BHJ) with different coating/printing methods. Here, a general approach to upscale flexible organic photovoltaics to module scale without obvious efficiency loss is demonstrated. The shear impulse during the coating/printing process is first applied to control the morphology evolution of the BHJ layer for both fullerene and nonfullerene acceptor systems. A quantitative transformation factor of shear impulse between slot‐die printing and spin‐coating is detected. Compelling results of morphological evolution, molecular stacking, and coarse‐grained molecular simulation verify the validity of the impulse translation. Accordingly, the efficiency of flexible devices via slot‐die printing achieves 9.10% for PTB7‐Th:PC₇₁BM and 9.77% for PBDB‐T:ITIC based on 1.04 cm² . Furthermore, 15 cm² flexible modules with effective efficiency up to 7.58% (PTB7‐Th:PC₇₁BM) and 8.90% (PBDB‐T:ITIC) are demonstrated with satisfying mechanical flexibility and operating stability. More importantly, this work outlines the shear impulse translation for organic printing electronics.

Application of the proposed slow post‐annealing for layered 2D perovskite solar cells based on BA₂MA₃Pb₄I₁₃ photo‐absorber leads to a favorable alignment on the multi‐perovskite phases and resultant champion power conversion efficiency to 17.26%, showing simultaneously enhanced open‐circuit voltage and short‐circuit current.

Abstract

Layered Ruddlesden–Popper (RP) phase (2D) halide perovskites have attracted tremendous attention due to the wide tunability on their optoelectronic properties and excellent robustness in photovoltaic devices. However, charge extraction/transport and ultimate power conversion efficiency (PCE) in 2D perovskite solar cells (PSCs) are still limited by the non‐eliminable quantum well effect. Here, a slow post‐annealing (SPA) process is proposed for BA₂MA₃Pb₄I₁₃ (n = 4) 2D PSCs by which a champion PCE of 17.26% is achieved with simultaneously enhanced open‐circuit voltage, short‐circuit current, and fill factor. Investigation with optical spectroscopy coupled with structural analyses indicates that enhanced crystal orientation and favorable alignment on the multiple perovskite phases (from the 2D phase near bottom to quasi‐3D phase near top regions) is obtained with SPA treatment, which promotes carrier transport/extraction and suppresses Shockley–Read–Hall charge recombination in the solar cell. As far as it is known, the reported PCE is so far the highest efficiency in RP phase 2D PSCs based on butylamine (BA) spacers (n = 4). The SPA‐processed devices exhibit a satisfactory stability with <4.5% degradation after 2000 h under N₂ environment without encapsulation. The demonstrated process strategy offers a promising route to push forward the performance in 2D PSCs toward realistic photovoltaic applications.

Organic photovoltaic (OPV) cells promise to have a good photovoltaic performance under the indoor light environment. Via optimizing the active layers, 1 cm² OPV cells are fabricated and a top power conversion efficiency of 22% under 1000 lux illumination is demonstrated.

Abstract

Organic photovoltaic (OPV) technologies have the advantages of fabricating larger‐area and light‐weight solar panels on flexible substrates by low‐cost roll‐to‐toll production. Recently, OPV cells have achieved many significant advances with power conversion efficiency (PCE) increasing rapidly. However, large‐scale solar farms using OPV modules still face great challenges, such as device stability. Herein, the applications of OPV cells in indoor light environments are studied. Via optimizing the active layers to have a good match with the indoor light source, 1 cm² OPV cells are fabricated and a top PCE of 22% under 1000 lux light‐emitting diode (2700 K) illumination is demonstrated. In this work, the light intensities are measured carefully. Incorporated with the external quantum efficiency and photon flux spectrum, the integral current densities of the cells are calculated to confirm the reliability of the photovoltaic measurement. In addition, the devices show much better stability under continuous indoor light illumination. The results suggest that designing wide‐bandgap active materials to meet the requirements for the indoor OPV cells has a great potential in achieving higher photovoltaic performance.

A cosensitization strategy with use of dual heavy‐metal‐free NIR absorption Zn–Cu–In–Se and Zn–Cu–In–S quantum dots (QDs) as cosensitizers is applied to control the light‐absorption, electron‐injection, and charge‐recombination processes simultaneously in QD‐sensitized solar cells (QDSCs). An average power conversion efficiency of 13.18% and a new certified efficiency record of 12.98% are obtained for environmentally friendly QDSCs under AM 1.5G 1 sun irradiation.

Abstract

Generally, high light‐harvesting efficiency, electron‐injection efficiency, and charge‐collection efficiency are the prerequisites for high‐efficiency quantum‐dot‐sensitized solar cells (QDSCs). However, it is fairly difficult for a single QD sensitizer to meet these three requirements simultaneously. It is demonstrated that these parameters can be felicitously balanced by a cosensitization strategy through the adoption of environmental‐friendly Zn–Cu–In–Se and Zn–Cu–In–S dual QD sensitizers with cascade energy structure. Experimental results indicate that: i) the combination of the dual QDs can improve the light‐harvesting capability of the cells, especially in the visible light window; ii) the cosensitization approach can facilitate electron injection, benefitting from the cascade energy structure of the two QD sensitizers employed; iii) the charge‐collection efficiency can be remarkably enhanced by the suppressed charge‐recombination process due to the improved QD coverage on TiO₂. Consequently, this cosensitization strategy delivers a new certified efficiency record of 12.98% for liquid‐junction QDSCs under AM 1.5G 1 sun irradiation. Moreover, the constructed cells exhibit good stability in a high‐humidity environment.

An effective method is proposed for excellent power conversion efficiency (PCE) with high V _oc in polymer solar cells (PSCs) by introducing a weak electron‐deficient thiophene‐based IC terminal group into thieno[3,2‐b]thiophene central core‐based small molecule acceptors. An excellent PCE of 13.11% with V _oc of 0.88 V is obtained, which is the highest reported for A–D–A‐type nonfullerene acceptors containing the central thieno[3,2‐b]thiophene unit with sp³ hybridized carbon‐bridged cyclopentadiene fragments in binary PSCs.

A pair of pure regioisomeric acceptor–donor–acceptor (A–D–A) typed nonfullerene small molecule acceptors (NF‐SMAs) (4TTIC and 4TTIC‐Cl), containing a central thieno[3,2‐b]thiophene‐sp³ hybridized “carbon‐bridge”‐based fused ring core unit and thiophene‐based IC or chlorinated thiophene‐based IC are synthesized for polymer solar cells (PSCs). Compared with 4TTIC, 4TTIC‐Cl not only achieves a red‐shifted absorption spectra and lower energy levels but also enhancement of molecular packing and crystallinity. The 4TTIC‐Cl‐based blend films display higher and more balanced charge carrier mobilities, more favorable morphology, and more efficient exciton dissociation in comparison with the 4TTIC‐based blend film. The optimized devices based on PBDB‐ST:4TTIC‐Cl deliver an impressively high power conversion efficiency (PCE) of 13.11% and fill factor of 74%, much higher than that of the PBDB‐ST:4TTIC‐based devices. Moreover, a small energy loss of ≈0.54 eV and a decent V _oc of 0.88 V are simultaneously achieved for PBDB‐ST:4TTIC‐Cl‐based devices. Noticeably, the PCE of 13.11% is the highest reported value for NF‐SMAs containing the central thieno[3,2‐b]thiophene unit with sp³ hybridized carbon‐bridged cyclopentadiene fragments in binary PSCs. This study proves that introduction of less electron‐deficient thiophene‐based IC terminal group into thieno[3,2‐b]thiophene central core‐based SMAs is a very effective method for making high V _oc and excellent PCE simultaneously.

Publication date: 18 December 2019

Source: Joule, Volume 3, Issue 12

Author(s): Haiyan Chen, Dingqin Hu, Qianguang Yang, Jie Gao, Jiehao Fu, Ke Yang, Hao He, Shanshan Chen, Zhipeng Kan, Tainan Duan, Changduk Yang, Jianyong Ouyang, Zeyun Xiao, Kuan Sun, Shirong Lu

Context & Scale

Summary

Graphical Abstract

Herein, a facile method is provided to fabricate the CsPbIBr₂ inorganic perovskite solar cells under low temperatures. The ZnO electron transport layer modification and band‐alignment engineering contribute to the outstanding power conversion efficiency of 10.16%, representing the highest efficiency for CsPbIBr₂ when the fabrication temperature is lower than 160 °C.

Recently, the thermally stable and facilely fabricated inorganic CsPbIBr₂ perovskite solar cells (PSCs) have attracted tremendous attention where the electron transport layer (ETL) is vital. However, the typical sintering temperature for the widely used electron transport material, that is, TiO₂, is more than 400 °C, elevating the cost and hindering the application. Due to high electron mobility and low fabrication temperature, ZnO becomes a desirable alternative for TiO₂, as the ETL in CsPbIBr₂ PSCs, albeit with low open‐circuit voltage (V _oc). Herein, this work introduces a trace of NH₄Cl to the sol–gel‐derived ZnO precursor to decrease the work function of the ZnO film, tune the surface morphology of the perovskite film, and thus suppress the density of trap states in the CsPbIBr₂ films. Consequently, full‐coverage and pure‐phase CsPbIBr₂ films consisting of micron‐size and high‐crystallinity grains are obtained. More importantly, for the optimal NH₄Cl‐modified ZnO, a shining improvement in V _oc from 1.08 to 1.27 V boosts the champion CsPbIBr₂ PSCs to obtain a power conversion efficiency of 10.16%, which is the highest value reported among pure‐CsPbIBr₂ PSCs under a low fabrication temperature of 160 °C. In addition, the NH₄Cl‐modified ZnO ETL reduces the severe hysteresis and increases the device stability significantly.

A stable quantum dot (QD) ink is reported by using ammonium iodide for the liquid‐state ligand exchange, and improved photovoltaic performance of QD solar cell is obtained by using the ink for the deposition of QD solid film. Experimental studies and theoretical calculations reveal that the enhanced photovoltaic performance is attributed to the improved passivation on the QD surface.

Abstract

Liquid‐state ligand exchange provides an efficient approach to passivate a quantum dot (QD) surface with small binding species and achieve a QD ink toward scalable QD solar cell (QDSC) production. Herein, experimental studies and theoretical simulations are combined to establish the physical principles of QD surface properties induced charge carrier recombination and collection in QDSCs. Ammonium iodide (AI) is used to thoroughly replace the native oleic acid ligand on the PbS QD surface forming a concentrated QD ink, which has high stability of more than 30 d. The ink can be directly applied for the preparation of a thick QD solid film using a single deposition step method and the QD solid film shows better characteristics compared with that of the film prepared with the traditional PbX₂ (X = I or Br) post‐treated QD ink. Infrared light‐absorbing QDSC devices are fabricated using the PbS‐AI QD ink and the devices give a higher photovoltaic performance compared with the devices fabricated with the traditional PbS‐PbX₂ QD ink. The improved photovoltaic performance in PbS‐AI‐based QDSC is attributed to diminished charge carrier recombination induced by the sub‐bandgap traps in QDs. A theoretical simulation is carried out to atomically link the relationship of QDSC device function with the QD surface properties.

A cosensitization strategy with use of dual heavy‐metal‐free NIR absorption Zn–Cu–In–Se and Zn–Cu–In–S quantum dots (QDs) as cosensitizers is applied to control the light‐absorption, electron‐injection, and charge‐recombination processes simultaneously in QD‐sensitized solar cells (QDSCs). An average power conversion efficiency of 13.18% and a new certified efficiency record of 12.98% are obtained for environmentally friendly QDSCs under AM 1.5G 1 sun irradiation.

Abstract

Generally, high light‐harvesting efficiency, electron‐injection efficiency, and charge‐collection efficiency are the prerequisites for high‐efficiency quantum‐dot‐sensitized solar cells (QDSCs). However, it is fairly difficult for a single QD sensitizer to meet these three requirements simultaneously. It is demonstrated that these parameters can be felicitously balanced by a cosensitization strategy through the adoption of environmental‐friendly Zn–Cu–In–Se and Zn–Cu–In–S dual QD sensitizers with cascade energy structure. Experimental results indicate that: i) the combination of the dual QDs can improve the light‐harvesting capability of the cells, especially in the visible light window; ii) the cosensitization approach can facilitate electron injection, benefitting from the cascade energy structure of the two QD sensitizers employed; iii) the charge‐collection efficiency can be remarkably enhanced by the suppressed charge‐recombination process due to the improved QD coverage on TiO₂. Consequently, this cosensitization strategy delivers a new certified efficiency record of 12.98% for liquid‐junction QDSCs under AM 1.5G 1 sun irradiation. Moreover, the constructed cells exhibit good stability in a high‐humidity environment.

Organic solar cells are a promising low‐carbon technology for electricity generation. Recently, such cells have reached the milestone of 17% power conversion efficiency. Herein, the key players behind this recent surge in efficiency are discussed. Novel organic photovoltaic materials and device architectures are critically reviewed. Non‐fullerene donors and acceptors dramatically increase device efficiency.

Organic solar cells (OSCs) are one of the most promising low‐carbon technologies for the generation of electricity. It is blessed with a relatively lower installation time and cost, light weight, semitransparent nature, and suitability for roll‐to‐roll printing process. In the past, critics of OSCs were concerned about its limited efficiency compared with other contemporary photovoltaic (PV) technologies. However, in the past few years, researchers in this field have made sufficient progress in terms of high performance, and OSC efficiency has witnessed significant growth. Today, a large number of OSCs are demonstrating >10% efficiency, recently reaching the milestone of 17%. The boost in efficiency is crucial for the successful commercialization of OSC. Herein, the recent advancements in OSC are highlighted to analyze the key players working behind the surge in its efficiency. The contributions of novel organic photovoltaics materials and their morphology as well as novel device architectures are discussed. Finally, the major challenges facing the upscaling and commercialization of OSCs are addressed.

Asymmetrically ultrastretchable wafer‐scale monocrystalline silicon solar cells with world record ultrastretchability (95%) and efficiency (19%) are fabricated using a laser‐patterning based corrugation technique applied on commercial solar cells with interdigitated‐back‐contacts. The stretchability is achieved by using a biocompatible elastomer (Ecoflex) as a stretchable encapsulant and by orthogonally aligning the active corrugated silicon islands to the applied tensile stress.

Abstract

Stretchable solar cells are of growing interest due their key role in realizing many applications such as wearables and biomedical devices. Ultrastretchability, high energy‐efficiency, biocompatibility, and mechanical resilience are essential characteristics of such energy harvesting devices. Here, the development of wafer‐scale monocrystalline silicon solar cells with world‐record ultrastretchability (95%) and efficiency (19%) is demonstrated using a laser‐patterning based corrugation technique. The demonstrated approach transforms interdigitated back contacts (IBC) based rigid solar cells into mechanically reliable but ultrastretchable cells with negligible degradation in the electric performance in terms of current density, open‐circuit voltage, and fill factor. The corrugation method is based on the creation of alternating grooves resulting in silicon islands with different shapes. The stretchability is achieved by orthogonally aligning the active silicon islands to the applied tensile stress and using a biocompatible elastomer (Ecoflex) as a stretchable substrate. The resulting mechanics ensure that the brittle silicon areas do not experience significant mechanical stresses upon asymmetrical stretching. Different patterns are studied including linear, diamond, and triangular patterns, each of which results in a different stretchability and loss of active silicon area. Finally, finite element method based simulation is conducted to study the generated deformation in the different patterned solar cells.

By precisely controlling the film properties of electron transport layer SnO₂, flexible perovskite solar cells with a structure of indium tin oxide/SnO₂/FA_0.945MA_0.025Cs_0.03Pb(I_0.975Br_0.025)₃/Spiro‐OMeTAD/Ag gives a power conversion efficiency of 19.51% and a steady output of 19.01%. The flexible devices present excellent bending resistance and long‐term stability.

Abstract

Flexible perovskite solar cells (f‐PSCs) have attracted great attention due to their promising commercial prospects. However, the performance of f‐PSCs is generally worse than that of their rigid counterparts. Herein, it is found that the unsatisfactory performance of planar heterojunction (PHJ) f‐PSCs can be attributed to the undesirable morphology of electron transport layer (ETL), which results from the rough surface of the flexible substrate. Precise control over the thickness and morphology of ETL tin dioxide (SnO₂) not only reduces the reflectance of the indium tin oxide (ITO) on polyethylene 2,6‐naphthalate (PEN) substrate and enhances photon collection, but also decreases the trap‐state densities of perovskite films and the charge transfer resistance, leading to a great enhancement of device performance. Consequently, the f‐PSCs, with a structure of PEN/ITO/SnO₂/perovskite/Spiro‐OMeTAD/Ag, exhibit a power conversion efficiency (PCE) up to 19.51% and a steady output of 19.01%. Furthermore, the f‐PSCs show a robust bending resistance and maintain about 95% of initial PCE after 6000 bending cycles at a bending radius of 8 mm, and they present an outstanding long‐term stability and retain about 90% of the initial performance after >1000 h storage in air (10% relative humidity) without encapsulation.

High pressure is applied to investigate the optical response and structure evolution of a 2D double halide perovskite. (BA)₄AgBiBr₈ shows pressure‐induced emission accompanied with a crystal structure change, indicating potential applications in pressure sensing, information storage and trademark security.

Abstract

Two‐dimensional (2D) halide perovskites have attracted significant attention due to their compositional flexibility and electronic diversity. Understanding the structure–property relationships in 2D double perovskites is essential for their development for optoelectronic applications. In this work, we observed the emergence of pressure‐induced emission (PIE) at 2.5 GPa with a broad emission band and large Stokes shift from initially nonfluorescent (BA)₄AgBiBr₈ (BA=CH₃(CH₂)₃NH₃ ⁺). The emission intensity increased significantly upon further compression up to 8.2 GPa. Moreover, the band gap narrowed from the starting 2.61 eV to 2.19 eV at 25.0 GPa accompanied by a color change from light yellow to dark yellow. Analysis of combined in situ high‐pressure photoluminescence, absorption, and angle‐dispersive X‐ray diffraction data indicates that the observed PIE can be attributed to the emission from self‐trapped excitons. This coincides with [AgBr₆]⁵⁻ and [BiBr₆]³⁻ inter‐octahedral tilting which cause a structural phase transition. High‐pressure study on (BA)₄AgBiBr₈ sheds light on the relationship between the structure and optical properties that may improve the material's potential applications in the fields of pressure sensing, information storage and trademark security.

By precisely controlling the film properties of electron transport layer SnO₂, flexible perovskite solar cells with a structure of indium tin oxide/SnO₂/FA_0.945MA_0.025Cs_0.03Pb(I_0.975Br_0.025)₃/Spiro‐OMeTAD/Ag gives a power conversion efficiency of 19.51% and a steady output of 19.01%. The flexible devices present excellent bending resistance and long‐term stability.

Abstract

Flexible perovskite solar cells (f‐PSCs) have attracted great attention due to their promising commercial prospects. However, the performance of f‐PSCs is generally worse than that of their rigid counterparts. Herein, it is found that the unsatisfactory performance of planar heterojunction (PHJ) f‐PSCs can be attributed to the undesirable morphology of electron transport layer (ETL), which results from the rough surface of the flexible substrate. Precise control over the thickness and morphology of ETL tin dioxide (SnO₂) not only reduces the reflectance of the indium tin oxide (ITO) on polyethylene 2,6‐naphthalate (PEN) substrate and enhances photon collection, but also decreases the trap‐state densities of perovskite films and the charge transfer resistance, leading to a great enhancement of device performance. Consequently, the f‐PSCs, with a structure of PEN/ITO/SnO₂/perovskite/Spiro‐OMeTAD/Ag, exhibit a power conversion efficiency (PCE) up to 19.51% and a steady output of 19.01%. Furthermore, the f‐PSCs show a robust bending resistance and maintain about 95% of initial PCE after 6000 bending cycles at a bending radius of 8 mm, and they present an outstanding long‐term stability and retain about 90% of the initial performance after >1000 h storage in air (10% relative humidity) without encapsulation.

The elastic modulus of 3D perovskite is very close to that of human bones and the elastic modulus of 2D perovskite with long chains is close to that of cartilage. By reconstructing a crystal lattice with different A cations at the surface of perovskite films, a nature “bone‐joint” configuration is built in perovskite, which provides a cushioning effect to external stresses.

Abstract

To improve the photovoltaic performance (both efficiency and stability) in hybrid organic–inorganic halide perovskite solar cells, perovskite lattice distortion is investigated with regards to residual stress (and strain) in the polycrystalline thin films. It is revealed that residual stress is concentrated at the surface of the as‐prepared film, and an efficient method is further developed to release this interfacial stress by A site cation alloying. This results in lattice reconstruction at the surface of polycrystalline thin films, which in turn results in low elastic modulus. Thus, a “bone‐joint” configuration is constructed within the interface between the absorber and the carrier transport layer, which improves device performance substantially. The resultant photovoltaic devices exhibit an efficiency of 21.48% with good humidity stability and improved resistance against thermal cycling.