Chen Weijie

A stable intermediate‐state film is obtained by using teramethylene sulfoxide (TMSO), originating from the formation of stronger coordination bond between TMSO and all perovskite precursors, which extends the annealing window and promotes the formation of a high‐quality film with larger grains and textured surface. 21.14% efficiency is achieved attributable to the improvement of the long‐wavelength response and fill factor.

Abstract

As the power conversion efficiency (PCE) of perovskite solar cells (PSCs) is increased to as high over 25%, it becomes pre‐eminent to study a scalable process with wide processing window to fabricate large‐area uniform perovskite films with good light‐trapping performance. A stable and uniform intermediate‐state complex film is obtained by using tetramethylene sulfoxide (TMSO), which extends the annealing window to as long as 20 min, promotes the formation of a high‐quality perovskite film with larger grains (over 400 nm) and spontaneously forms the surface texture to result in an improved fill factor and open‐circuit voltage (V _oc). Moreover, the superior surface texture significantly increases the long‐wavelength response, leading to an improved short‐circuit current density (J _sc). As a result, the maximum PCE of 21.14% is achieved based on a simple planar cell structure without any interface passivation. Moreover, a large area module with active area of 6.75 cm² is assembled using the optimized TMSO process, showing efficiency as high as 16.57%. The study paves the way to the rational design of highly efficient PSCs for potential scaled‐up production.

An all‐inorganic mixed‐halide perovskite solar cell with a power conversion efficiency of 16.42% is realized by using a Cs₂CO₃‐doped ZnO electron transport layer, which ascribes to the interfacial energy level tuning for reducing ohmic loss at the contact and enlarging the built‐in potential. A high thermostability is simultaneously obtained via surface defect passivation for improving the CsPbI₂Br film against phase transformation.

Abstract

Inorganic mixed‐halide CsPbX₃‐based perovskite solar cells (PeSCs) are emerging as one of the most promising types of PeSCs on account of their thermostability compared to organic–inorganic hybrid counterparts. However, dissatisfactory device performance and high processing temperature impede their development for viable applications. Herein, a facile route is presented for tuning the energy levels and electrical properties of sol–gel‐derived ZnO electron transport material (ETM) via the doping of a classical alkali metal carbonate Cs₂CO₃. Compared to bare ZnO, Cs₂CO₃‐doped ZnO possesses more favorable interface energetics in contact with the CsPbI₂Br perovskite layer, which can reduce the ohmic loss to a negligible level. The optimized PeSCs achieve an improved open‐circuit voltage of 1.28 V, together with an increase in fill factor and short‐circuit current. The optimized power conversion efficiencies of 16.42% and 14.82% are realized on rigid glass substrate and flexible plastic substrate, respectively. A high thermostability can be simultaneously obtained via defect passivation at the Cs₂CO₃‐doped ZnO/CsPbI₂Br interface, and 81% of the initial efficiency is retained after aging for 200 h at 85 °C.

The influence of the synergetic effects of fluorination and chlorination on multiple temporal‐scale photocarrier dynamics is thoroughly studied based on a 2 × 2 matrix of organic solar cells consisting of PBDB‐T, PBDB‐T‐2Cl, ITIC, and IT4F. Both fluorination and chlorination play a positive role in exciton diffusion, exciton dissociation, charge transport, and collection, contributing to the improvement of photovoltaic performance.

Fluorination and chlorination have yielded a novel class of materials and achieved tremendous progress in enhancing photovoltaic efficiency in organic solar cells (OSCs). However, their effects on photocarrier dynamics remain elusive in these organic photovoltaic systems. Herein, a comprehensive study on the underlying mechanisms is conducted based on a 2 × 2 photovoltaic matrix, consisting of PBDB‐T, PBDB‐T‐2Cl, ITIC, and IT4F. Chlorination of donors enhances exciton migration and relaxation rates and promotes the extraction of polarons. The more efficient charge transfer and a larger proportion of long‐lived polarons are observed in fluorine‐containing acceptor‐based systems, which are in favor of charge generation in the actual devices. According to the enlarged dielectric constant in the PBDB‐T‐2Cl:IT4F blend, the improved exciton delocalization, the decreased exciton binding energy, and Coulomb capture radius are obtained relative to other three binary systems, which can increase charge separation efficiency and reduce the probability of bimolecular recombination. The simultaneous fluorination and chlorination can optimize molecular packing and nanoscale phase separation, facilitating effective exciton diffusion, exciton dissociation, and charge transport. These results highlight the important role of fluorination and chlorination on these fundamental mechanisms, possibly resulting in some new molecular design principles toward high‐performance OSCs.

Ternary organic solar cells (OSCs) based on PTQ10:Y6:PC₇₁BM demonstrate a higher power conversion efficiency of 16.07% in comparison with the binary OSCs based on PTQ10:Y6. Comprehensive morphology analysis of the blend films is conducted by grazing‐incidence wide‐angle X‐ray scattering, transmission electron microscopy, and photo‐induced force microscopy technologies to understand the morphology evolution during the device optimization.

Herein, PC₇₁BM is used as the third component (the second acceptor) to improve the photovoltaic performance of the organic solar cells (OSCs) based on a low‐cost polymer donor PTQ10 and a nonfullerene small‐molecule acceptor Y6. The ternary OSCs based on PTQ10:Y6:PC₇₁BM reach a higher power conversion efficiency (PCE) of 16.07% with enhanced short‐circuit current density and a better fill factor in comparison with the binary OSCs based on PTQ10:Y6, which is ascribed to the higher electron mobility, better charge extraction, and suppressed charge recombination of the ternary PSCs. The film morphology of the OSCs is studied by grazing‐incidence wide‐angle X‐ray scattering, photo‐induced force microscopy, and transmission electron microscopy, which reveals that with the treatment of additive (0.5% CN) and thermal annealing, the phase separation of Y6 is obviously enhanced, whereas no significant changes occur for that of PTQ10 component. Besides, the addition of PC₇₁BM slightly lowers the ratio of the face‐on orientation in the blend film and attenuate the aggregation of acceptor Y6. In addition, compared with the binary PTQ10:Y6 OSC, the ternary device based on PTQ10:Y6:PC₇₁BM shows better device stability, demonstrating a great potential for the practical application of ternary OSCs.

Low‐temperature black‐phase CsPbI₃ evolution processes are designed using chemical bond engineering for the fabrication of efficient and ambient‐air‐stable solar cells. After optimization, the low temperature (160 °C)‐annealed 3% polyvinylpyrrolidone device shows the highest efficiency of 10.0% and sustains ≈80% of its initial power conversion efficiency after 5 months of storage in ambient‐air conditions.

Cesium‐based fully inorganic black‐phase (BP) lead halide perovskites (such as α‐, β‐, and γ‐CsPbI₃) with excellent thermal stability and a decently high photovoltaic performance have attracted increasing attention. However, a below 200 °C fabrication process of the desirable BP CsPbI₃ has rarely been reported. Herein, the detailed crystal structure evolution of ambient‐air‐stable BP CsPbI₃ prepared under low temperature conditions is investigated by exploiting the strong coordination bonding between CO in polyvinylpyrrolidone (PVP) and Pb in CsPbI₃ and inflection effect of PVP under annealing. It is found that ambient‐air‐stable BP CsPbI₃ films are formed and the energy barrier for the long‐term stable BP CsPbI₃ formation is significantly reduced (the required annealing temperature is only 80 °C). After optimization, the highest power conversion efficiencies (PCEs) of ≈4.0% and 10.0% are recorded for the 3% PVP‐added devices with light absorbers annealed at 80 and 160 °C, respectively. More importantly, the 3% PVP device annealed at 160 °C maintains ≈80% of its original PCE after 5 months storage under ambient‐air conditions.

Flexible perovskite solar cells have attracted plenty of attention in both educational and business communities. Herein, the research background is summarized and technological advancement with regard to flexible substrates is evaluated. In addition, different stability tests with and without encapsulation are briefly examined. Last but not least, the upscaling issues and the material costs are also vividly discussed.

In recent years, the era of perovskites has experienced splendid development. Among perovskites, flexible perovskite solar cells (FPSCs) have received increasing attention due to their high efficiency, light weight, low cost, excellent flexibility, and low‐temperature solution processing ability. In the last decade, the power conversion efficiency of FPSCs has increased significantly from 2.62% to more than 20%. Herein, a succinct overview of the current endeavor to achieve low‐temperature FPSCs is provided. The recent developments, including flexible substrates, transparent conductive electrodes, perovskite absorbers, and device manufacturing methods, are vividly discussed. The strategies for enhancing the stability and flexibility of FPSCs are presented in terms of electrode materials, device encapsulation, and structural effects. Finally, the most encouraging and potential studies for the future of flexible PSCs are revealed.

High‐quality CsPbBr₃ films and efficient perovskite solar cells are fabricated through a two‐step method from all green solvents with the assistance of the Hansen solubility theory.

Toxic solvents used in the fabrication of perovskite solar cells are an obstacle for their commercialization. Replacing those toxic solvents with green solvents is very important for both ecological environment safety and the health of operators working in manufactory and labs. CsPbBr₃‐based solar cells have attracted increasing attention due to its high stability. Herein, high‐quality CsPbBr₃ films are prepared using all green solvents based on a two‐step spin‐coating method. In the first step, a green solvent system of polyethylene glycol (PEG) with the addition of γ‐butyrolactone is used for preparing PbBr₂ solutions by matching the Hansen solubility parameters (HSPs) between PbBr₂ and the mixed solvent system. By optimizing the HSPs and viscosity, a new complex of PbBr₂·(PEG) is formed by spin‐coating from the PbBr₂ solution, followed by acetic acid dropping while spinning. In the second step, green water is used to dissolve CsBr to prepare a high concentration CsBr/H₂O solution. High‐quality CsPbBr₃ films with full coverage are obtained by spin‐coating CsBr/H₂O solution onto the PbBr₂·(PEG) films after annealing. As a result, a solar cell with configuration of fluorine‐doped tin oxide/TiO₂/CsPbBr₃/carbon exhibits a power conversion efficiency of 8.11% due to its high‐quality harvest layer.

Chalcogenide perovskites (BaZrS₃, SrZrS₃, BaHfS₃, and SrHfS₃) synthesized by solid reaction processes exhibit an extraordinary strong light absorption of >10⁵ cm⁻¹ in the bandgap region. The superior optical characteristics originate from the unique band structures of the chalcogenide perovskites, making the materials ideal for the top cell in tandem solar cell devices.

All existing solar cell materials including hybrid perovskites show rather small absorption coefficient (α ) of ≈10⁴ cm⁻¹ in the bandgap (E _g) transition region. The weak band‐edge light absorption is an essential problem, limiting conversion efficiency particularly in a tandem solar cell. Herein, all distorted chalcogenide perovskites (BaZrS₃, SrZrS₃, BaHfS₃, and SrHfS₃) are found experimentally to exhibit extraordinary high α exceeding 10⁵ cm⁻¹ near E _g, indicating the highest band‐edge α among all known solar cell materials. The giant absorption in the E _g region, which is consistent with the first principles, arises from the intense p–d interband transition enabled by dense S 3p valence states. For solar cell application, low‐gap BaZrS₃ derivatives, Ba(Zr,Ti)S₃ and BaZr(S,Se)₃, are further synthesized. Among the possible candidates of top‐cell materials, an earth‐abundant and nontoxic Ba(Zr,Ti)S₃ alloy shows great potential, reaching a maximum potential efficiency exceeding 38% in a chalcogenide perovskite/crystalline Si tandem architecture.

The acid treatment of TiO₂ weakens the bonding of octahedral chains in anatase TiO₂, rendering the formation of amorphous TiO₂ buffer layer on the surface of anatase TiO₂. This amorphous TiO₂ buffer layer contains rich oxygen vacancies, which increase the donor density of TiO₂.

Abstract

The ability to effectively transfer photoexcited electrons and holes is an important endeavor toward achieving high‐efficiency solar energy conversion. Now, a simple yet robust acid‐treatment strategy is used to judiciously create an amorphous TiO₂ buffer layer intimately situated on the anatase TiO₂ surface as an electron‐transport layer (ETL) for efficient electron transport. The facile acid treatment is capable of weakening the bonding of zigzag octahedral chains in anatase TiO₂, thereby shortening staggered octahedron chains to form an amorphous buffer layer on the anatase TiO₂ surface. Such amorphous TiO₂‐coated ETL possesses an increased electron density owing to the presence of oxygen vacancies, leading to efficient electron transfer from perovskite to TiO₂. Compared to pristine TiO₂‐based devices, the perovskite solar cells (PSCs) with acid‐treated TiO₂ ETL exhibit an enhanced short‐circuit current and power conversion efficiency.

Electron Transport An acid‐treatment strategy was used to create an amorphous TiO₂ buffer layer situated on the anatase TiO₂ surface as an efficient electron‐transport layer, as described by Z. Lin, Z. L. Wang, Y. Yang, and co‐workers in their Research Article on https://doi.org/10.1002/anie.201910471page 1611.

Three hole transport materials (HTMs) based on a substituted triphenylamine moiety have been synthesized and employed in perowskite solar cells, reaching efficiencies of 19.4 %. Although all these HTMs show very similar chemical and physical properties, they provide different carrier recombination kinetics.

Abstract

Three hole transport materials (HTMs) based on a substituted triphenylamine moiety have been synthesized and successfully employed in triple‐cation mixed‐halide PSCs, reaching efficiencies of 19.4 %. The efficiencies, comparable to those obtained using spiro‐OMeTAD, point them out as promising candidates for easily attainable and cost‐effective alternatives for PSCs, given their facile synthesis from commercially available materials. Interestingly, although all these HTMs show similar chemical and physical properties, they provide different carrier recombination kinetics. Our results demonstrate that is feasible through the molecular design of the HTM to minimize carrier losses and, thus, increase the solar cell efficiencies.

Structurally different conjugation systems afford new small molecules of the SM‐axis for disparate functionality of third components in ternary organic solar cells. Systematic investigation of the SM‐axis series and host donor/acceptor materials for photoluminescence and microstructural properties reveals synergistic features of two major working models co‐existing in SM‐axis‐based ternary organic solar cells, thus achieving improved performance along variations in conjugated pathways.

Abstract

A family of the SM‐axis series based on benzo[1,​2‐​b:4,​5‐​b′]​dithiophene and 3‐ethylrhodanine (RD) units with structurally different π‐conjugation systems are synthesized as a means to understand the structure–property relationship of conjugated pathways in ternary non‐fullerene organic solar cells (NF‐OSCs) as a third component. The optical and electrochemical properties of the SM‐axis are highly sensitive both to the functionalized direction and to the number of RD groups. Enhanced power conversion efficiencies (PCEs) of over 11% in ternary devices are obtained by incorporating optimal SM‐X and SM‐Y contents from PBDB‐T:ITIC binary NF‐OSCs, while a slightly lower PCE is observed with the addition of SM‐XY. The results of in‐depth studies using various characterization techniques demonstrate that working mechanisms of SM‐axis‐based ternary NF‐OSCs are distinctly different from one another: an energy‐transfer mechanism with an alloy‐like model for SM‐X, a charge transfer with the same model for SM‐Y, and an energy transfer without such a structure for SM‐XY. As extension of the scope, a SM‐X‐based ternary NF‐OSC in the PM6:IT4F system also shows a greatly enhanced PCE of over 13%. The findings provide insights into the effects of conjugated pathways of organic semiconductors on mechanisms of ternary NF‐OSCs, advancing the understanding for synthetic chemists, materials engineers, and device physicists.

Fluorine and methyl dual functionalized MF1 and MF2 blended with PM7 show high charge mobilities and decent micromorphology and thus good thickness‐insensitive properties, and achieve over 11% power conversion efficiency (PCE) when the active layer thickness is over 400 nm (nearly 70% FF) and over 10% PCE when the active layer thickness is over 500 nm, recording the highest values for such thicknesses.

Abstract

Thickness‐insensitive small molecule acceptors (SMAs) are still a great challenge for developing thick‐film organic solar cells (OSCs) towards practical use. Herein, two SMAs, MF1 and MF2, are designed and synthesized by employing a bifunctional end group with fluorine and methyl moieties. Combined with fused‐ring cores with alkyl side chains, both MF1 and MF2 exhibit ordered π–π stacking and high charge carrier mobilities in neat and blend films. The champion devices based on PM7:MF1 and PM7:MF2 deliver high power conversion efficiencies (PCEs) of 12.4% and 13.7%, and high fill factors (FFs) of 78.3% and 74.5%, respectively. With increasing active layer thickness, the FFs of the OSCs decrease relatively slowly, demonstrating the preferrable properties of MF1 and MF2 in terms of their thickness insensitivity, especially for MF1. As a result, the two thick‐film OSCs achieve over 11% PCEs at an active layer thickness over 400 nm (an FF close to 70% for PM7:MF1) and over 10% PCEs when the thickness is increased up to 500 nm. These are the highest PCEs among OSCs with such active layer thicknesses to date. This work reveals a molecular design strategy by reasonably combining fluorine and methyl together to simultaneously enhance charge carrier mobilities and fine‐tune the morphology, which is beneficial to achieve high‐performance thick‐film OSCs.

A series of electron transporting chiral stereoisomers of naphthalene diimide crystalline materials having N‐substituted two chiral groups is rationally designed and synthesized for the simultaneous achievement of low‐temperature solution processability, high device performance, and long‐term temporal (and high‐temperature) device stability.

Abstract

A series of chiral stereoisomers of electron transporting materials with two chiral substituents is rationally designed and synthesized, and the influence of stereoisomerism on their physical and electronic properties is investigated to demonstrate highly efficient and stable perovskite solar cells (PSCs). Compared to mesomeric naphthalene diimide (NDI) derivatives, which have heterochiral side groups with centrosymmetric molecular packing of symmetric‐shaped conformers in the crystalline state, enantiomeric NDI derivatives have homochiral side groups that exhibit non‐centrosymmetric molecular packing of asymmetric‐shaped conformers in the crystalline state and exhibit better solution processability based on one order of magnitude higher solubility. A similar trend is observed in different rylene diimide stereoisomers based on larger semiconducting core perylene diimide. The PSCs based on NDI enantiomers with good film‐forming ability and a very high lowest phase transition temperature (T _lowest) of 321 °C exhibit a high and uniform average power conversion efficiency (PCE) of 19.067 ± 0.654%. These PSCs also have a high temporal device stability, with less than 10% degradation of the PCE at 100 °C for 1000 h without encapsulation. Therefore, chiral stereoisomer engineering of charge transporting materials is a potential approach to achieve high solution processability, excellent performance, and significant temporal stability in organic electronic devices.

A Lewis acid tris(pentafluorophenyl)borane and nonhygroscopic lithium salt (LiClO₄) codoping strategy is introduced to tailor C₆₀ and fabricate highly efficient inorganic CsPbI₂Br perovskite solar cells with reduced hysteresis. Consequently, square‐centimeter inorganic CsPbI₂Br perovskite solar cells yield a record power conversion efficiency (PCE) of 14.44%. In addition, the first inorganic perovskite solar module with an efficiency exceeding 12% is reported, using a self‐developed quasi‐curved heating method.

Abstract

Although inorganic perovskite solar cells (PSCs) are promising in thermal stability, their large open‐circuit voltage (V _OC) deficit and difficulty in large‐area preparation still limit their development toward commercialization. The present work tailors C₆₀ via a codoping strategy to construct an efficient electron‐transporting layer (ETL), leading to a significant improvement in V _OC of the inverted inorganic CsPbI₂Br PSC. Specifically, tris(pentafluorophenyl)borane (TPFPB) is introduced as a dopant to lower the lowest unoccupied molecular orbital (LUMO) level of the C₆₀ layer by forming a Lewis acidic adduct. The enlarged free energy difference provides a favorable enhancement in electron injection and thereby reduces charge recombination. Subsequently, a nonhygroscopic lithium salt (LiClO₄) is added to increase electron mobility and conductivity of the film, leading to a reduction in the device hysteresis and facilitating the fabrication of a large‐area device. Finally, the as‐optimized inorganic CsPbI₂Br PSCs gain a champion power conversion efficiency (PCE) of 15.19%, with a stabilized power output (SPO) of 14.21% (0.09 cm²). More importantly, this work also demonstrates a record PCE of 14.44% for large‐area inorganic CsPbI₂Br PSCs (1.0 cm²) and reports the first inorganic perovskite solar module with the excellent efficiency exceeding 12% (10.92 cm²) by a self‐developed quasi‐curved heating method.

Nature Photonics, Published online: 20 January 2020; doi:10.1038/s41566-019-0577-1

Nature Photonics, Published online: 20 January 2020; doi:10.1038/s41566-019-0573-5

Nature Energy, Published online: 20 January 2020; doi:10.1038/s41560-019-0535-7

Nature Energy, Published online: 20 January 2020; doi:10.1038/s41560-019-0538-4

Nature Energy, Published online: 22 January 2020; doi:10.1038/s41560-020-0552-6