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BIT’s progress in the field of polymer solar cells acceptor materials based on hetero-dihalogenated terminals

News resource & Photographer: School of Chemistry and Chemical Engineering

Editor: News Agency of BIT

Translator: Gao Xinyuan, News Agency of BIT

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Beijing Institute of Technology, February 23rd, 2022: Recently, the Beijing Institute of Technology (BIT) team has made progress in the field of polymer solar cells acceptor materials based on hetero-dihalogenated terminals. The related research results were entitled “Non-fullerene acceptors with hetero-dihalogenated terminals induce significant difference in single crystallography and enable binary organic solar cells with 17.5% efficiency”, published in the top international energy journal Energy & Environmental Science (2022,15,320-333). Master student Wang Lai from the School of Chemistry and Chemical Engineering is the first author of the paper. Professor Wang Jinliang from the School of Chemistry and Chemical Engineering and special researcher An Qiaoshi are the co-corresponding authors, and BIT is the only communication unit.

Environmental pollution and energy crisis are the major problems facing the world today. The development and utilization of high-efficiency clean energy is one of the critical scientific issues that need to be solved urgently in National Energy Plan. Polymer solar cells have attracted extensive attention in the field of high-efficiency clean energy materials in recent years due to its light weight, high mechanical flexibility, translucency, roll-to-roll printable and other advantages. Novel non-fullerene acceptors (NFAs) with A-D-A or A-DA'D-A structures dominate the improvement of polymer solar cells owing to their strong and broad absorption and easy to chemically modify. There are many methods to adjust the intermolecular packing, absorption spectra and film morphologies of NFAs currently. Among them, the strategy of utilizing two different types of halogenated terminal groups and asymmetric molecular backbones is considered to be a simple but effective strategy to change the absorption range and optimize the energy level, leading to better aggregate morphology and device performance. Due to the reversibility of condensation reactions, these asymmetric receptors often require relatively complex synthesis and purification procedures to remove by-products. However, directly adopting the terminal modification strategy with hetero-dihalogened atom substitution and the symmetric molecular skeleton strategy is expected to avoid the complicated purification procedures and further improve the performance of photovoltaic devices, but the acceptor materials based on hetero-dihalogenated end groups are rarely reported. At the same time, how to regulate the molecular chemical structure and aggregation morphological characteristics and understand the relationship between them and device performance through novel terminal groups and single crystal molecular packing patterns, and then develop high-efficiency new acceptor materials, is also one of the key scientific problems that has been concerned and devoted to solve in the field of polymer solar cells.

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Fig.1 (a) Molecular structure of three acceptor materials; (b) Comparison of absorption spectra of thin films; (c) Comparison of HOMO and LUMO energy level diagrams of molecular frontier orbitals; (d) J-V curves of the device; (e) Statistical comparison chart of binary battery performance between the acceptor molecule Y-BO-FCl and other reported acceptor materials modified with various isohalogen end groups.

On the basis of the previous research on the regulation of the end group structure of A-D-A-type small molecule materials (Adv. Funct. Mater. 2022, 32,2108289; ACS Energy Lett.,2018, 3, 2967; J. Mater. Chem. A, 2020, 8, 4856; J. Mater. Chem. C,2021, 9, 1923-1935, etc.), in order to obtain high-performance hetero-dihalogenated end groups acceptor molecular material systems, the team of Prof. Wang Jinliang from School of Chemistry and Chemical Engineering of BIT have recently synthesized a series of novel hetero-dihalogenated end groups (FCl-IC, FBr-IC, ClBr-IC) and the corresponding A-DA'DA-type fused-ring acceptor materials (Y-BO-FCl, Y-BO-FBr, Y-BO-ClBr) through a synergistic strategy of regulating the types of end-group halogens and incorporating hetero-dihalogenated end-groups. The synergistic effect and structure-activity relationship of these novel hetero-dihalogenated end groups on the thin film spectral absorption, single crystal packing, photovoltaic performance and morphology of the blended films of the modified acceptor molecular materials were systematically investigated. The frontier molecular orbital energy levels (HOMO and LUMO) of all fluorinated acceptors (Y-BO-FCl and Y-BO-FBr) are slightly reduced compared to Y-BO-ClBr. The team also obtained, for the first time, detailed X-ray single-crystal diffraction data of the acceptor molecular system modified with hetero-dihalogenated end groups. Corresponding single-crystal analytical study shows that fluorine-substituted terminal groups can significantly alter the crystal system and intermolecular packing patterns and distances of the modified acceptor molecules. In addition, combined with DFT theoretical calculation and analysis, compared with the other two acceptor materials, the acceptor molecule Y-BO-FCl modified with fluorochloroisohalogenated(?) end groups exhibits the optimal molecular skeleton geometric plane structure and the smallest intermolecular stacking distance, the largest intermolecular π−π electron coupling and the most ordered three-dimensional molecular packing network, which help to improve the crystallinity of the Y-BO-FCl molecule and enhance the charge transport ability in multiple directions of its thin film state.

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Fig. 2 Acceptor molecular materials modified by three hetero-dihalogenated end groups. a) Y-BO-FCl; b) Y-BO-FBr; c) Y-BO-ClBr molecular structure, intermolecular crystal packing mode and distance, intermolecular interaction coupling quantitative parameters comparison.

In addition, after using two-dimensional grazing incidence X-ray diffraction technology, atomic force microscopy characterization technology, transmission electron microscopy characterization technology, etc. to analyze the aggregate morphology of the donor-acceptor blend film, it was found that when mixed with the common polymer donor material PM6, compared with PM6: Y-BO-FBr and PM6: Y-BO-ClBr blend films, PM6: Y-BO-FCl blend films have the largest crystal coherence length and the strongest face-on crystal orientation trend, the best Nanofibrous interpenetrating network structure and optimal phase separation size. Finally, the PM6:Y-BO-FCl-based polymer solar cell device achieved a photoelectric energy conversion efficiency of 17.52%, which was significantly higher than that of the PM6:Y-BO-FBr and PM6:Y-BO-ClBr-based devices (energy conversion efficiencies are 16.47% and 13.61%, respectively). In-depth study of the device physical process shows that among the three material systems, the high performance of PM6:Y-BO-FCl-based devices is mainly attributed to the lowest charge recombination, the largest and most balanced charge mobility, and the most excellent blending morphology. This is the highest performance of binary battery devices based on various isohalogen end-modified symmetric or asymmetric acceptor materials so far reported. This systematic study shows that the strategy of introducing fluorine or chloro hetero-dihalogenated end groups is one of the effective ways to enhance the crystalline intermolecular packing and film morphology of acceptor materials and achieve excellent photovoltaic performance, which has huge application potential in polymer solar cells. In addition, considering the diversity of replaceable atoms in the end group, this work further confirms that the optimized end group strategy has a positive effect on the improvement of battery performance by local asymmetric halogenation of the end group of the acceptor material, which is of great significance for design of subsequent high-performance photovoltaic materials.

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Fig. 3 The structure-activity relationship between the molecular structure, crystal packing difference and solar cell performance of acceptor molecular materials modified by three hetero-dihalogenated end groups

During the revision of the paper, the team of Professor Cao Xiaoyu of Xiamen University, the team of Researcher Zhu Xiaozhang of the Institute of Chemistry of Chinese Academy of Sciences, and the team of Professor Feng Xiao of the School of Chemistry and Chemical Engineering greatly assisted. The research work was funded by the General Program of the National Natural Science Foundation of China, the National Overseas High-level Young Talents Program, the BIT Teli Young Fellow Recruitment Program and other projects, as well as being supported by the Beijing Key Laboratory of Photoelectric Conversion Materials and the organic thin film optoelectronic device test platform of BIT Analysis and Testing Center and the BL14B1 line station of Shanghai Synchrotron Radiation Facility Center.


Paper link: https://doi.org/10.1039/D1EE01832A