Recently, the team of Professor Wang Jinliang from Beijing Institute of Technology has made important progress in building efficient organic solar cell receptor materials based on regionally regulated heterogeneous trihalogenated terminal groups. Relevant research results have been published in the top international chemical journalAngelwandte Chemie International Edition.It is entitled "Regioisomer Free Difluoro Monochloro Terminal based Hexa Catalogened Acceptor with Optimized Crystal Packaging for Efficient Binary Organic Solar Cells". Beijing University of Technology is the only author correspondence unit, Professor Wang Jinliang and Special Researcher An Qiaoshi from the School of Chemistry and Chemical Engineering are the corresponding authors of this paper, and Yan Lu, a master's degree candidate, and Zhang Heng, a doctoral candidate, are the co first authors of this paper.

With the "double carbon" goal proposed, a revolution in energy is imperative. As a clean energy technology, organic solar cells (OSCs) have attracted extensive attention due to their light, thin, soft and easy processing advantages. Thanks to the development of efficient polymer donor materials and new non fullerene receptor materials, the photoelectric conversion efficiency of organic solar cells has made an important breakthrough. In order to achieve the commercialization goal of OSCs, it is necessary to develop small molecule receptor materials (SMAs) with better performance and deeply explore the relationship between their molecular stacking and device performance. A-DA'D-A SMAs are a class of highly efficient receptor materials that are very popular in the field of OSCs at present. Many structural optimizations around these materials have promoted the further improvement of the efficiency of OSCs. Core engineering, side chain engineering and end group engineering are common means to regulate molecular stacking and improve photovoltaic performance. Among them, end group engineering is limited by limited modifiable sites and high synthesis difficulty. Changing the number or location of halogen atoms is one of the most commonly used strategies to adjust the molecular energy level and the morphology of donor acceptor blend films to obtain significant device performance. In addition, the introduction of different types of halogen atoms on the terminal group may lead to a huge difference in the performance of SMAs photovoltaic devices. However, the types of the same halogen terminal groups on the market are limited, so the heterohalogenated SMAs that introduce multiple halogens on the same terminal group obviously need to be developed and studied. At the same time, how to control the molecular properties and aggregation morphological characteristics and understand the relationship between them and device performance through novel heterohalogenated terminal groups and single crystal molecular stacking mode to achieve the creation of high efficiency receptor materials is also one of the key scientific issues in the field of organic solar cells.

Figure 1. Molecular structure and basic properties

(a) Chemical structure of materials;(b) Surface electrostatic potential (ESP); (c) Absorption spectrum of solution; (d) Absorption spectrum of the film; (e) Energy level distribution diagram of donor/recipient materials.

Based on the above mentioned key scientific issues, Professor Wang Jinliang's team has made early research on the synthesis of small molecule receptor materials regulated by terminal groups and the performance regulation of high-performance organic solar cells (Energy Environ. Sci., 2022,15, 320; Adv. Funct. Mater. 2022, 32, 2108289; ACS Energy Lett., 2018, 3, 2967; J. Mater. Chem. A, 2020, 8, 4856; J. Mater. Chem. A, 2019, 7, 11802; J. Mater. Chem. C, 2021, 9, 1923, etc.), two novel heterodihalogenated and heterotrihalogenated terminal groups with location determined fluorination and chlorination were successfully synthesized by the stepwise chlorination/fluorination strategy, and named o-FCl-IC and FClF-IC respectively. Subsequently, three kinds of SMAs (Y-Cl, Y-FCl and Y-FClF) without regional isomers based on the Y series central core were synthesized respectively, which have three different heterohalogenated terminal groups. The team systematically discussed the influence of step by step terminal heterohalogenation on the basic photoelectric properties, single crystal stacking behavior, film morphology and photovoltaic properties of SMAs. With the increasing of fluorine generation purpose in the heterohalogenated terminal group (from Y-Cl to Y-FCl and Y-FClF), the molecular absorption spectrum gradually shifts to red, and HOMO energy level and LUMO energy level decrease, at the same time, the electron mobility in the pure film is gradually increased.

Figure 2. Three dimensional stacking mode and π - π interaction information of Y-FCl (a) and Y-FClF (b) single crystals

The team obtained the single crystal structure of heterodihalogenated/heterotrihalogenated terminal acceptors Y-FCl and Y-FClF based on the region regularity for the first time. Crystallographic analysis and density functional theory (DFT) calculations show that, compared with Y-FCl, with the increase of the number of fluorine substitutions, Y-FClF neighboring molecules not only maintain the original crystal system, but also show better molecular planarity, shorter π - π stacking distance, more significant π - π electronic coupling and more orderly three-dimensional molecular stacking network, thus improving the charge transfer capability in multiple directions of its film state. In addition, the PM6: Y-FClF blend film shows the most orderly and significant face on molecular stacking orientation on the molecular stacking, shows the strongest crystallization tendency under proper phase separation, and has the highest and most balanced charge mobility among the three blend films. Therefore, the organic solar cell based on PM6: Y-FClF achieves a photoelectric conversion efficiency of 17.65%, which is significantly higher than the cell efficiency based on PM6: Y-FCl (16.00%) and PM6: Y-Cl (14.47%). More importantly, this efficiency is one of the highest in binary organic solar cells based on heterohalogenated small molecule receptors.

Figure. 3. (a) J-V curve of solar cell;(b) EQE external quantum efficiency diagram; (c) Carrier mobility histogram.

To sum up, this work shows that the combination of accurate control of the number of fluorine/chlorine atoms and the regionally regular hetero trihalogenation strategy is one of the cooperative methods to improve the SMAs intermolecular crystal stacking, optimize the morphology of the blend film and improve the photovoltaic performance of the device, which has great application potential in organic solar cells. It provides a new idea for the subsequent design of high-performance organic solar cell receptor materials.

Figure. 4. Structure activity relationship diagram between the molecular structure, crystal stacking and battery performance of the receptor molecular materials modified by three heterohalogenated terminal groups.

The above research work has been supported by the National Natural Science Foundation of China, the National Overseas High level Talents Youth Project, the Beijing Institute of Technology Special Young Scholars Program and other projects, as well as the Beijing Key Laboratory of Optoelectronic Conversion Materials and the Analysis and Test Center of Beijing University of Technology. Li Xiong's team from Beijing Business University, Cao Xiaoyu's team from Xiamen University, and Zhu Xiaozhang's team from Institute of Chemistry, Chinese Academy of Sciences gave strong support.

Brief introduction of the authors:

Wang Jinliang, professor and doctoral supervisor of the College of Chemistry and Chemical Engineering. He graduated from the School of Chemistry and Molecular Engineering of Peking University in 2008 and was selected into the National Overseas High Level Talent Support Plan in 2013. He is mainly engaged in the chemical research of organic and polymer photovoltaic energy conversion materials, and has carried out distinctive research work in the efficient synthesis of monodisperse polyfluoro photovoltaic donor materials and high-efficiency selenophene receptor materials, as well as device performance regulation. He has presided over and undertaken the National Natural Science Foundation of China, the National Overseas High level Talent Youth Program, the General Program of the Beijing Natural Science Foundation, the Beijing Institute of Technology's Distinguished Young Scholars Program and other topics. So far inJ Am. Chem. Soc.and other international high-level academic journals published more than 80 SCI papers, which were cited more than 6000 times. In 2016, he was awarded as the Excellent Master's Thesis Instructor of Beijing Institute of Technology, and in 2021, he was awarded as the Excellent Undergraduate Thesis Instructor of Beijing. At present, he is a member of the eighth editorial board of Journal of Beijing Institute of Technology (English Edition).

An Qiaoshi is a special researcher and doctoral supervisor of the School of Chemistry and Chemical Engineering of Beijing Institute of Technology. In April 2020, he joined the School of Chemistry and Chemical Engineering of Beijing Institute of Technology, mainly engaged in organic optoelectronic materials and devices. So far, he has been inEnergy Environmentas the first/corresponding author Sci.;Angew. Chem., Int. Ed.; ACS Energy Lett.; Adv. Funct. Mater.; Nano Energyand other international high-level journals published more than 30 SCI papers, including 9 ESI highly cited papers, which were cited more than 6000 times in total, and presided over projects such as the National Natural Science Foundation.