Recently, the team of Professor Wang Jinliang from the School of Chemistry and Chemical Engineering of Beijing Institute of Technology has synthesized a series of A-DA'DA-type non-fullerene small molecule receptor materials (S-YSS-Cl, A-WSSe-Cl and S-WSeSe-Cl) for the first time), by gradually increasing the number of selenophene rings combined with a synergistic strategy of asymmetric molecular framework, the morphology and device performance parameters of the selenophene polymer solar cell film were precisely adjusted and optimized, and an efficiency of up to 17.51% of photoelectric conversion was obtained. The related results are titled "Synergistic Strategy of Manipulating the Number of Selenophene Units and Dissymmetric Central Core of Small Molecular Receptors Enables Polymer Solar Cells with 17.5% Efficiency" and published in the top international chemistry journal "Angewandte Chemie International Edition" (2021, 60, 19241-19252). Doctoral student Yang Can of the School of Chemistry and Chemical Engineering is the first author of the paper, Professor Wang Jinliang and Special Researcher An Qiaoshi of the School of Chemistry and Chemical Engineering are the co-corresponding authors, and Beijing Institute of Technology is the only corresponding institute. The collaborators of the paper also include Professor Zhang Shaowen from the School of Chemistry and Chemical Engineering and Professor Han Young Woo from Korea University.

Figure 1 The structure-activity relationship between the molecular structure, crystal packing and solar cell performance of selenophene receptor materials

Environmental pollution and energy crisis are two major problems facing the world today. The development and utilization of high-efficiency clean energy is a major scientific issue that needs to be resolved urgently in the national energy strategy. With its advantages of light weight, high mechanical flexibility, translucency, and easy roll-to-roll printing, polymer solar cells have attracted widespread attention in the field of high-efficiency clean energy materials in recent years. The non-fullerene small molecule receptor material can better absorb long waves and optimize the chemical structure, which enhances the balance of device parameters, thereby significantly improving the photoelectric conversion efficiency of polymer solar cells. Compared with thiophene-based small molecule receptor materials, receptor materials containing selenophene and its derivatives usually show the advantages of absorption spectrum red shift and strong charge transfer ability, making it more possible to achieve higher performance. However, the synthesis of materials poses certain challenges. Previously, there are relatively few research reports based on high-efficiency selenophene receptor material systems. At the same time, how to develop a new type of receptor material by analyzing molecular chemical structure, aggregation morphological characteristics (such as single crystal molecular accumulation and phase separation), understand its relationship with the performance of macroscopic devices, prepare high-efficiency polymer solar cells is a key scientific problem that researchers have been paying attention to and committed to solving.

In response to the above-mentioned basic problems and major needs, in recent years, Professor Wang Jinliang’s team has combined with domestic and foreign research groups to adopt simple and efficient synthesis strategies to develop a variety of high-efficiency organic small molecules for receptor material systems and battery devices, having achieved a series of progress in the polyfluorinated wide band gap DAD type small molecule donor material system and its application in ternary devices (J. Am. Chem. Soc. 2016, 138, 7687; Adv. Funct. Mater. 2016, 26, 1803; Joule, 2019, 3, 846; Adv. Funct. Mater. 2015, 25, 3514, etc.), ADA-type thienoselenophene or benzodiselenophene photovoltaic small molecule receptor material system (ACS Energy Lett., 2018, 3, 2967, etc.), high-efficiency small-molecule solar cell devices (Environ. Sci. 2021, 14, 3945, etc.), prediction and screening of high-efficiency receptor materials based on machine learning (Environ. Sci. 2021, 14, 90; J. Mater. Chem. A, 2021, September, 15684, etc.)

Figure 2 The device structure of a polymer solar cell (left); the comparison of the results of photovoltaic devices based on the asymmetric structure of a single selenophene ring A-WSSe-Cl and other reported selenophene receptor materials (right).

In order to achieve a more efficient selenophene receptor material system, Professor Wang Jinliang's team recently synthesized a series of symmetrical or asymmetric A-DA'D-A small molecule receptor materials (S-YSS-Cl, A-WSSe-Cl and S-WSeSe-Cl) with different numbers of selenophene rings (0, 1 and 2) (see Figure 1). The team systematically studied the synergistic effect and structure-activity relationship of the spectral absorption, single crystal stacking, photovoltaic performance and the morphology of the blend film of this new type of selenophene receptor molecule. The research results show that from S-YSS-Cl to A-WSSe-Cl to S-WSeSe-Cl, the optical band gap of the pure phase film gradually narrows and the electron mobility gradually increases. Complicated single crystal culture and structural analysis studies have shown that the increase in the number of selenophene substitutions leads to increasingly stronger π-π interactions between molecules. In addition, due to the existence of additional non covalent intermolecular weak interaction between s ∙ n, compared with thiophene analogue S-YSS-Cl, the crystals of A-WSSe-Cl and S-WSeSe-Cl contain more ordered and efficient three-dimensional interpenetrating charge transport channels. In addition, when mixed with common polymer donor material PM6, A-WSSe-Cl and PM6 blend films based on a single selenophene ring show the best intermolecular stacking surface and the most favorable three-dimensional nanofibrous interpenetrating network film morphology, resulting in the highest and most balanced charge mobility. The device results show that the photoelectric conversion efficiency based on the asymmetric molecular structure A-WSSe-Cl is as high as 17.51%, which is significantly better than the device performance based on the symmetric molecular structure S-YSS-Cl (the photoelectric conversion efficiency is 16.73%) and S-WSeSe-Cl (The photoelectric conversion efficiency is 16.01%). It is worth noting that the 17.51% photoelectric conversion efficiency is one of the highest performance of polymer solar binary battery devices reported so far based on small molecule receptor materials substituted with selenophene rings (see Figure 2). These results indicate that asymmetric small molecule receptors based on selenophene ring substitution have great application potential in polymer solar cells. Precisely modulating the number of selenophene ring substitutions combined with the coordinated strategy of asymmetric molecular skeletons can effectively solve the problem of balance between the parameters of the binary battery device and achieve a breakthrough in photoelectric conversion efficiency. All these provide a new strategy for the synthesis of a more efficient A-DA’D-A type non-fullerene small molecule receptor material system.

Link to the full text of the article: https://doi.org/10.1002/anie.202104766

The above-mentioned research work has been supported by the National Natural Science Foundation of China, the National Overseas High-level Talents Youth Program, the National Key Research and Development Program, the Beijing Institute of Technology Special Young Scholar Program, and the Beijing Key Laboratory of Photoelectric Conversion Materials. The BIT Analysis and Testing center provided support for the basic characterization of materials and the testing platform for organic thin-film optoelectronic devices.

In addition, Professor Wang Jinliang's team also reported other types of high-efficiency polymer solar cell photoactive layer material systems containing selenophene rings this year, such as high-efficiency benzodiselephene small molecule receptor materials based on two-dimensional conjugated side chain modification (J. mater. Chem. A, 2021, 9, 15665), thienoselenophene small molecule receptor materials based on monochlorinated regioisomerization (J. Mater. Chem. C, 2021, 9, 1923), small molecule materials containing selenophene isomers (ACS Appl. Mater. Interfaces, 2021, 10.1021/acsami.1c12028) that combine skeletal isomerization and core region isomerization strategies, and benzodiselenephene high efficiency polymer donor material system based on two-dimensional conjugated side chain modification (ChemSusChem, 2021, 10.1002 / CSSC. 202101232). Fir more progress, please visit website of Wang Jinliang’s team. https://cce.bit.edu.cn/kyjgjktz/wjlktz/index.htm