Chen Weijie

In article number 1703236, Fengyu Li, Yanlin Song, and co-workers report hysteresis-free, flexible, and large-scale perovskite solar cells with recorded photoelectric conversion efficiencies of 12.3% for a 1 cm² single chip and 8.4% for a 24 cm² solar module. This is the first time that a wearable solar power source that can supply power for multifunction electronic devices with a variety of body movements is fabricated practically.

In article number 1703546, Kilwon Cho and co-workers demonstrate that PCBM molecules chemically passivate the grain boundaries of organometal halide perovskite crystals. The chemical passivation prevents halogens at the crystal boundaries from exiting from the crystal lattice, and thereby retards thermal degradation of the perovskite crystals. The tunability of perovskite grain boundaries by additives is an advance toward the practical use of organometal halide perovskite solar cells.

Charge extraction rate in solar cells made of blends of electron donating/accepting organic semiconductors is typically slow due to their low charge carrier mobility. This sets a limit on the active layer thickness and has hindered the industrialization of organic solar cells (OSCs). Herein, charge transport and recombination properties of an efficient polymer (NT812):fullerene blend are investigated. This system delivers power conversion efficiency of >9% even when the junction thickness is as large as 800 nm. Experimental results indicate that this material system exhibits exceptionally low bimolecular recombination constant, 800 times smaller than the diffusion-controlled electron and hole encounter rate. Comparing theoretical results based on a recently introduced modified Shockley model for fill factor, and experiments, clarifies that charge collection is nearly ideal in these solar cells even when the thickness is several hundreds of nanometer. This is the first realization of high-efficiency Shockley-type organic solar cells with junction thicknesses suitable for scaling up.

Strongly suppressed recombination is observed in a polymer:fullerene system resulting in solar cell power conversion efficiencies as high as 9% at a junction thickness of 800 nm. Results indicate that solar cell devices made of this material system with thicknesses as large as 300 nm can exhibit Shockley-type behavior, i.e., the fill factor is unaffected by bimolecular recombination.

In article number 1701164, Wallace C.H. Choy and co-workers demonstrate an all-solution-processed switchable interconnecting layer (ICL) for both inverted and normal tandem organic solar cells (OSCs). This strategy shifts the views from conventionally complicated tunneling junction ICL where both electron/hole transport layers play several different roles towards simplified ICL where electron/hole transport layers play distinct decoupled role, advancing ICL for more adaptable tandem OSCs.

Inherent poor stability of perovskite nanocrystals (NCs) is the main impediment preventing broad applications of the materials. Here, TiO₂ shell coated CsPbBr₃ core/shell NCs are synthesized through the encapsulation of colloidal CsPbBr₃ NCs with titanium precursor, followed by calcination at 300 °C. The nearly monodispersed CsPbBr₃/TiO₂ core/shell NCs show excellent water stability for at least three months with the size, structure, morphology, and optical properties remaining identical, which represent the most water-stable inorganic shell passivated perovskite NCs reported to date. In addition, TiO₂ shell coating can effectively suppress anion exchange and photodegradation, therefore dramatically improving the chemical stability and photostability of the core CsPbBr₃ NCs. More importantly, photoluminescence and (photo)electrochemical characterizations exhibit increased charge separation efficiency due to the electrical conductivity of the TiO₂ shell, hence leading to an improved photoelectric activity in water. This study opens new possibilities for optoelectronic and photocatalytic applications of perovskites-based NCs in aqueous phase.

TiO₂ shell coated CsPbBr₃ core/shell nanocrystals are successfully constructed, resulting in excellent water, photo and thermal stability. TiO₂ shell coating effectively increases charge separation efficiency, hence leading to an improved photoelectric activity in water.

Organic-inorganic halide perovskite materials have become a shining star in the photovoltaic field due to their unique properties, such as high absorption coefficient, optimal bandgap, and high defect tolerance, which also lead to the breathtaking increase in power conversion efficiency from 3.8% to over 22% in just seven years. Although the highest efficiency was obtained from the TiO₂ mesoporous structure, there are increasing studies focusing on the planar structure device due to its processibility for large-scale production. In particular, the planar p-i-n structure has attracted increasing attention on account of its tremendous advantages in, among other things, eliminating hysteresis alongside a competitive certified efficiency of over 20%. Crucial for the device performance enhancement has been the interface engineering for the past few years, especially for such planar p-i-n devices. The interface engineering aims to optimize device properties, such as charge transfer, defect passivation, band alignment, etc. Herein, recent progress on the interface engineering of planar p-i-n structure devices is reviewed. This review is mainly focused on the interface design between each layer in p-i-n structure devices, as well as grain boundaries, which are the interfaces between polycrystalline perovskite domains. Promising research directions are also suggested for further improvements.

Interface engineering has been widely practiced on planar p-i-n perovskite solar cells since it brings about significant improvement in both device performance and stability. Recent progress is reviewed about the engineering of each interface of such devices and its effects, including defect passivation, accelerated charge transfer, enhanced stabilities, etc., which are key parameters for the photovoltaic devices.

Perovskite solar cells (PSCs) have developed rapidly over the past few years, and the power conversion efficiency of PSCs has exceeded 20%. Such high performance can be attributed to the unique properties of perovskite materials, such as high absorption over the visible range and long diffusion length. Due to the different diffusion lengths of holes and electrons, electron transporting materials (ETMs) used in PSCs play a critical role in PSCs performance. As an alternative to TiO₂ ETM, ZnO materials have similar physical properties to TiO₂ but with much higher electron mobility. In addition, there are many simple and facile methods to fabricate ZnO nanomaterials with low cost and energy consumption. This review focuses on recent developments in the use of ZnO ETM for PSCs. The fabrication methods of ZnO materials are briefly introduced. The influence of different ZnO ETMs on performance of PSCs is then reviewed. The limitations of ZnO ETM-based PSCs and some solutions to these challenges are also discussed. The review provides a systematic and comprehensive understanding of the influence of different ZnO ETMs on PSCs performance and potentially motivates further development of PSCs by extending the knowledge of ZnO-based PSCs to TiO₂-based PSCs.

Progress in perovskite solar cells based on ZnO electron-transport materials of different morphologies and their fabrication methods is summarized. The influence of the ZnO morphology and fabrication process on the performance of perovskite solar cells are reviewed and highlighted. Moreover, the issues of ZnO materials, and some solutions and strategies to promote the performance of solar cells, are introduced.

A novel wide-bandgap conjugated copolymer based on an imide-functionalized benzotriazole building block containing a siloxane-terminated side-chain is developed. This copolymer is successfully used to fabricate highly efficient all-polymer solar cells (all-PSCs) processed at room temperature with the green-solvent 2-methyl-tetrahydrofuran. When paired with a naphthalene diimide-based polymer electron-acceptor, the all-PSC exhibits a maximum power conversion efficiency (PCE) of 10.1%, which is the highest value so far reported for an all-PSC. Of particular interest is that the PCE remains 9.4% after thermal annealing at 80 °C for 24 h. The resulting high efficiency is attributed to a combination of high and balanced bulky charge carrier mobility, favorable face-on orientation, and high crystallinity. These observations indicate that the resulting copolymer can be a promising candidate toward high-performance all-PSCs for practical applications.

A novel wide-bandgap conjugated copolymer PTzBI-Si based on an imide-functionalized benzotriazole unit containing a siloxane-terminated side-chain is developed and used to fabricate all-polymer solar cells (all-PSCs). When processed with a green solvent 2-methyl-tetrahydrofuran, the all-PSC exhibits a power conversion efficiency of 10.1%, which represents the highest efficiency ever reported for all-PSCs.