Low-dimensional metal halide perovskites have emerged as promising alternatives to the traditional three-dimensional (3D) components, due to their greater structural tunability and environmental stability. Dion-Jacobson (DJ) phase two-dimensional (2D) perovskites, which are formed by incorporating bulky organic diammonium cations into inorganic frameworks that comprises a symmetrically layered array, have recently attracted increasing research interest. The structure-property characteristics of DJ phase perovskites endow them with a unique combination of photovoltaic efficiency and stability, which has led to their impressive employment in perovskite solar cells (PSCs). Here, we review the achievements that have been made to date in the exploitation of DJ phase perovskites in photovoltaic applications. We summarize the various ligand designs, optimization strategies and applications of DJ phase PSCs, and examine the current understanding of the mechanisms underlying their functional behavior. Finally, we discuss the remaining bottlenecks and future outlook for these promising materials, and possible development directions of further commercial processes.

223. "Materials, Photophysics and Device Engineering of Perovskite Light-Emitting Diodes"

Ziming Chen, Zhenchao Li, Tom Hopper, Artem A Bakulin, Hin-Lap Yip

Reports on Progress in Physics,  2021, https://doi.org/10.1088/1361-6633/abefba

Here we provide a comprehensive review of a newly developed lighting technology based on metal halide perovskites (i.e. perovskite light-emitting diodes) encompassing the research endeavours into materials, photophysics and device engineering. At the outset we survey the basic perovskite structures and their various dimensions (namely three-, two- and zero-dimensional perovskites), and demonstrate how the compositional engineering of these structures affects the perovskite light-emitting properties. Next, we turn to the physics underpinning photo- and electroluminescence in these materials through their connection to the fundamental excited states, energy/charge transport processes and radiative and non-radiative decay mechanisms. In the remainder of the review, we focus on the engineering of perovskite light-emitting diodes, including the history of their development as well as an extensive analysis of contemporary strategies for boosting device performance. Key concepts include balancing the electron/hole injection, suppression of parasitic carrier losses, improvement of the photoluminescence quantum yield and enhancement of the light extraction. Overall, this review reflects the current paradigm for perovskite lighting, and is intended to serve as a foundation to materials and device scientists newly working in this field.

222. "High‐Performance Semi‐Transparent Organic Photovoltaic Devices via Improving Absorbing Selectivity"

Yaokai Li, Chengliang He, Lijian Zuo, Feng Zhao, Lingling Zhan, Xin Li, Ruoxi Xia, Hin‐Lap Yip, Chang‐Zhi Li, Xu Liu, Hongzheng Chen

Advanced Energy Materials.,  2021, 11, 2003408

High‐performance organic semi‐transparent photovoltaic (ST‐OPV) devices are achieved by improving the light‐absorbing selectivity, that is, the light‐absorbing capability in invisible regions and light transmission in the visible region. Systematic optimization, including developing a numerical method for photo‐active layer screening, interface engineering, and optical manipulation, enables high‐performance ST‐OPVs with the best light utilization efficiency of 4.1%, ranking among the highest for ST‐OPVs.

221. "Synthesis and photovoltaic performance of a non-fullerene acceptor comprising siloxane-terminated alkoxyl side chain"

 Zhuhao Wu, Rihang Qiu, Haiying Jiang, Qian Wang, Yinchu Chen, Haizhen Liu, Shenkun Xie, Hin-Lap Yip, Lianjie Zhang, Junwu Chen

Organic Electronics.,  2021, 106087

As an effective molecular modification strategy, side chain engineering has been widely used in promoting the photovoltaic performance of non-fullerene acceptors. Herein, a novel non-fullerene small molecular acceptor i-IEOSi-4F comprising siloxane-terminated alkoxyl side chain was successfully designed and synthesized. The molecule shows an optical band gap of 1.53 eV, with large extinction coefficient of 2.36 × 105 M−1 cm−1 in solution. Two fluorobenzotriazole based polymers J52 and PBZ-2Si with the same backbone units but different side chains were employed as the donor to construct the active layers that all can demonstrate suitable energy levels and complementary absorptions with i-IEOSi-4F. Relative to J52 only bearing alkyl side chain, PBZ-2Si with siloxane-terminated side chain could induce more balanced carrier transports and more favorable morphology, leading to a higher power conversion efficiency (PCE) of 12.66% with a good fill factor of 71.45%. The efficiency is 21% higher than that of 10.46% for the J52 based devices. Our results not only indicate that siloxane-terminated alkoxyl side chain is valuable for efficient non-fullerene acceptors, but also demonstrate that siloxane-terminated side chain on both polymer donor and small molecular acceptor is a useful combination to realize more efficient polymer solar cells.

220. "Monolithic perovskite/organic tandem solar cells: Developments, prospects, and challenges"

Yue-Min Xie, Qifan Xue, Qin Yao, Shenkun Xie, Tianqi Niu, Hin-Lap Yip

Nano Select.,  2021, https://doi.org/10.1002/nano.202000287

The recent progress made in perovskite‐based monolithic perovskite/organic tandem devices and perovskite/bulk‐heterojunction devices is systematically reviewed in terms of the sub‐cell and interconnecting layer properties. Specifically, the shortcomings in terms of the device photovoltaic parameters (PCE, FF, Jsc, and Voc) of perovskite/organic tandem devices compared with all‐perovskite tandem devices are systematically demonstrated.

219. "Utilization of Trapped Optical Modes for White Perovskite Light-Emitting Diodes with Efficiency over 12%"

Ziming Chen, Zhenchao Li, Zhen Chen, Ruoxi Xia, Guangruixing Zou, Linghao Chu, Shi-Jian Su, Junbiao Peng, Hin-Lap Yip, Yong Cao

Joule.,  2021, DOI:https://doi.org/10.1016/j.joule.2020.12.008

The inferior light extraction efficiency (LEE), which is generally less than 20%, based on optical modeling, and the difficulty in achieving white emission are the two main challenges in the metal-halide-perovskite light-emitting diode (PeLED) field. Herein, we report a simple and efficient approach to construct high-performance white PeLEDs with much-enhanced LEE by coupling a blue PeLED with a layer of red perovskite nanocrystal (PeNC) down-converter through a rationally designed multilayer semitransparent electrode (LiF/Al/Ag/LiF). The red PeNC layer allows the extraction of the trapped waveguide mode and surface plasmon polariton mode in a blue PeLED and converts them to red emission, resulting in over 50% LEE improvement. Simultaneously, the complementary emission spectrum of blue photons and down-converting red photons contributes to a white PeLED with a high external quantum efficiency and luminance of more than 12% and approximately 2,000 cd m −2, respectively, which represent state-of-the-art results in this field.

218. "D-A-π-A-D-type Dopant-free Hole Transport Material for Low-Cost, Efficient, and Stable Perovskite Solar Cells"

Tianqi Niu, Weiya Zhu, Yiheng Zhang, Qifan Xue, Xuechen Jiao, Zijie Wang, Yue-Min Xie, Ping Li, Runfeng Chen, Fei Huang, Yuan Li, Hin-Lap Yip, Yong Cao

Joule.,  2021, DOI:https://doi.org/10.1016/j.joule.2020.12.003

The development of low-cost and efficient hole transport materials (HTMs) is important for the commercialization of perovskite solar cells (PSCs). Comparing with the widely studied D-A-D and D-π-D linear-type small molecule HTMs, DTB-FL with a D-A-π-A-D molecular design is proposed, featuring facile synthesis and excellent optoelectronic properties. Moreover, the HTM with efficient surface passivation effects and proper energy level alignment at the hole extraction interface effectively inhibits recombination loss and improves the charge collection property. As a result, the champion efficiencies of 21.5% and 19.6% for active areas of 0.09 and 1.0 cm 2, respectively, with superior operational stability are achieved by using DTB-FL HTM. In addition, DTB-FL can also be used as efficient HTM for all-inorganic PSCs, producing an impressive PCE of 17.0% with a high V oc of 1.30 V. These results underscore the promising potential of the D-A-π-A-D molecular design in preparing low-cost dopant-free HTMs toward stable and efficient PSCs.

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