Recently, Professor He Feng's team in the Department of Chemistry of Southern University of Science and Technology has achieved fruitful research results in the field of three-dimensional network receptor design and synthesis, and has published many papers in internationally renowned journals such as Angewandte Chemie, CCS Chemistry and Journal of Materials Chemistry A. Based on his contribution to the direction of three-dimensional network receptors, He Feng was invited to write a review paper in the famous energy and materials journal Advanced Energy Materials, which systematically summarized the single crystal structure analysis of three-dimensional network receptors and the performance research in organic solar cell devices in recent years, and promoted the development of three-dimensional network receptors in the field of organic solar cells.
At present, organic solar cells based on the hybrid preparation of conjugated polymer semiconductors and organic small molecule acceptors have made significant progress in the synthesis of materials and device processes. Considering that the preparation of highly efficient organic solar cells generally uses highly toxic halogenated aromatic hydrocarbon solvents, the research group takes health and environmental protection as the starting point to develop low-toxicity or even non-toxic non-halogenated aromatic hydrocarbon solvents to prepare organic solar cells. Based on the highly effective receptor BTIC-CF3-γ (Joule 2020, 4, 688-700) containing trifluoromethyl, the new high-performance receptor material BTIC-2Cl-γCF3 was synthesized through asymmetric strategies and combined with the unique properties and advantages of chlorine substitution, which can be fully dissolved (even halogen-free solvents) in most solvents to meet environmentally friendly device preparation requirements. In addition, the three-dimensional network structure of BTIC-2Cl-γCF3 is conducive to more efficient charge transmission and improves the photoelectric conversion performance of the device. When toluene is used as a processing solvent, the device efficiency reaches 16.31%, and the ternary efficiency is as high as 17.12%, while the translucent battery transmittance is 24.45%, and the efficiency is as high as 13.06%. These results show that BTIC-2Cl-γCF3 is a promising material in translucent photovoltaic-building integration products. The results were published in the form of a research paper in the international flagship journal of chemistry, Angewandte Chemie International Edition (10.1002/anie.202013053). Chen Hui, research assistant professor of the research group, Lai Hanjian, a doctoral student, and Chen Ziyi, a senior research scholar in the Department of Chemistry, are the first authors of the work and the corresponding author He Feng.
Figure 1. Crystal accumulation of BTIC-2Cl-γCF3 and toluene processing of organic solar device properties
As an extension of chlorine substitution, the research group simultaneously designed and synthesized two isomerized brominated acceptor materials, BTIC-2Br-β and BTIC-2Br-γ. The single crystal structure analysis results show that BTIC-2Br-γ has good planarity, and its 3D network structure is conducive to intermolecular accumulation and charge transfer. Organic solar cells based on BTIC-2Br-γ system obtained a photoelectric conversion efficiency of 16.52%, which is one of the highest conversion efficiencies of bromine replacing non-Fuller small molecule materials. The results show that bromine atom substitution is also a very efficient method for preparing high-performance non-fullerene small molecule receptors. The above results were published in CCS Chemistry, the flagship journal of the Chinese Chemical Society, and the first authors of the work were Doctoral Student Wang Huan, Research Assistant Professor Han Liang and Graduate Student Liu Tao, as well as South China University of Technology Postdoctoral Fellow Zhou Jiadong and Corresponding Author He Feng.
Figure 2. Chemical structural formula and photovoltaic properties of the acceptor material and its BTIC-2Br-γ crystal accumulation
In addition, on the basis of the previous work (iScience 2019, 17, 302), the research group synthesized three different positions of chlorine-substituted small molecule receptor materials, and found that the introduction of different chlorine substituent terminal groups had a very large impact on the performance of the final device of the receptor, and its energy conversion efficiency was between 7.39% and 15.04%. Compared with the traditional linear π-π stacking of conjugated molecules, this three-dimensional stacking configuration is conducive to more efficient electron jumping between molecules, thereby optimizing the photoelectric conversion performance of materials. The results were published in the Journal of Materials Chemistry A, the first author of the research paper is Mo Daize, a graduate doctoral student of the research group, the co-first authors are Chen Hui and Zhou Jiadong, and the corresponding author is Professor He Feng.
Based on his work on organic solar cell crystals, Feng He's team was invited to publish a review in the international materials journal Advanced Energy Materials. This review summarizes most of the receptor materials with definitive crystal data, discusses the relationship between the structural design, stacking patterns, and properties of molecules. In addition, the review proposes the concept of "three-dimensional network accumulation", in which electrons can be transported along the π-π stack in three directions of x y z, thereby increasing the mobility of the material and further improving its photoelectric properties. In addition, the article also discusses some of the problems related to single crystals encountered by acceptor materials, emphasizing how to understand and adjust the intermolecular interaction forces and aggregation states through existing single crystal information, so as to achieve more efficient directional synthesis and design of acceptor materials.
At present, single crystal resolution has been applied to explore PDI types, A-D-A types, A-D-A-D-A types, and non-thick ring acceptor materials. The team discussed the effect of isomers of the PDI type on molecular accumulation, which in turn affects their device performance. In the A-D-A type, the team found that chlorine-substituted receptors have a regular J-aggregation behavior due to multiple, powerful Cl-S interactions between molecules. In addition, the team proposed the concept of three-dimensional network accumulation in chlorine-substituted isomer systems, which caused the transition of molecules from two-dimensional linear accumulation to three-dimensional accumulation due to the different positions of chlorine substitution in end groups. In the system of large atomic bromine substitution, the accumulation behavior of three-dimensional networks has also been found. In addition, the increase in the number of halogen substitutions in the end groups will significantly improve the electron coupling between molecules, and the increase in the length of the alkyl chain on the nucleus will make the molecules have a more regular aggregation behavior, thereby improving performance. In the recently emerging high-efficiency A-D-A-D-A type receptor materials, the research group for the first time solved the single crystal structure of the trifluoromethyl-substituted acceptor molecule, and found that there is not only J aggregation between the end groups and end groups, but also the accumulation between the nuclei and the nuclei, forming H aggregates, so the accumulation between the molecules of the A-D-A-D-A system is formed by the synergy of H/J aggregation. In addition, due to the introduction of N atoms in the A-D-A-D-A system, the non-covalent bond interaction forces between molecules have changed, resulting in some new interaction forces such as F∙∙∙N, N∙∙∙O and N∙∙∙S. The synergy of multiple molecular interactions and H/J aggregation gives such molecules a three-dimensional network structure, which facilitates the transfer of charge between molecules.
He Feng's team believes that exploring the microscopic accumulation information of optoelectronic materials will have a further understanding of the properties of materials, and by regulating the aggregation state of molecules and the interaction between molecules, it can provide new ideas for the development of new and efficient photovoltaic materials. The paper was published online in the international material journal Advanced Energy Materials, the first author is Lai Hanjian, and the corresponding author is He Feng.
The above research has been supported by the National Natural Science Foundation of China, Shenzhen Municipal Science and Technology Innovation Commission, Shenzhen Nobel Prize Scientist Laboratory Project, Guangdong Provincial Key Laboratory of Catalysis, Guangdong Provincial Key Area Research and Development Project and other departments, and thanks to the Analysis and Testing Center of Southern University of Science and Technology for the strong support of the research group project.
Source: Southern University of Science and Technology