SUSTech Department of Chemistry Professor Feng He’s team made crucial breakthroughs in designing and synthesizing high-performance non-fullerene acceptors for the development of organic solar cells. Their findings were published in Angewandte Chemie, CCS Chemistry, Journal of Materials Chemistry A, and Advanced Energy Materials. In the paper published in Advanced Energy Materials, the team systematically summarized the crystal structure analysis and performance research of 3D network acceptors in organic solar cell devices in recent years, which greatly promotes the development of 3D network acceptors in OSCs.

Organic solar cells (OSCs) based on conjugated polymer semiconductors and organic small molecule acceptors have seen remarkable advances in material synthesis and device technology. Considering that highly toxic halogenated aromatic solvents were widely used in the fabrication of efficient OSCs, the research group developed low toxicity or even non-toxic non-halogenated aromatic solvents to fabricate OSCs for health and environmental protection. Based on an efficient acceptor containing trifluoromethyl (Lai et al, Joule 2020, 4, 688-700), a high-performance acceptor material, BTIC-2Cl-CF3, was synthesized by asymmetric strategy and combined with the unique properties and advantages of chlorine substitution. The molecule can be fully dissolved in most solvents (even halogen-free solvents), which enables it to meet the requirement of eco-friendliness in device fabrication. In addition, the 3D network structure of BTIC-2Cl-CF3 improves the efficiency of charge transfer and photoelectric conversion performance of the device. When toluene is used as a processing solvent, the efficiency of the device reaches 16.31%, the ternary efficiency reaches 17.12%, the transmittance of 15 nm Ag electrode semi-transparent cell is 24.45%, and the efficiency is 13.06%. All of these indicate that BTIC-2Cl-CF3 is a promising material in semitransparent building integrated photovoltaic products. The results have been published in the flagship journal Angewandte Chemie International Edition (10.1002/ Anie.202013053). Research Assistant professor Hui Chen, doctoral candidate Hanjian Lai, and Doctor Ziyi Chen from the Department of Chemistry, are the first authors of this work, and the corresponding author is Professor Feng He.

Figure 1. Crystal packing of BTIC-2Cl-CF3 and performance of organic solar cell devices processed with toluene

Halogenation is an effective strategy to tune the absorption spectrum and energy level of the semiconductor materials. The research team designed and synthesized two isomeric acceptors (BTIC-2Br-β and BTIC-2Br-γ) with bromine in different positions on the end groups. Crystallographic analysis of a single crystal of BTIC-2Br-γ shows that that the whole molecular backbone presents a plane structure. Multiple intermolecular interactions such as Br∙∙∙π and CN∙∙∙H existing in the solid state allows BTIC-2Br-γ to form a 3D network packing structure, which is beneficial to charge transfer and transport. With the polymer PBDB-TF as the donor, BTIC-2Br-γ-based solar cells exhibit an outstanding PCE of 16.52%, which is the highest recorded value among brominated acceptors, while the BTIC-2Br-β-based device shows only a relatively low PCE of 8.11%. The results reveal that the isomerism strategy plays an effective role in obtaining the high performance non-fullerene small molecule acceptors. The work has been published in CCS Chemistry. The authors include doctoral candidate Huan Wang, Research Assistant Professor Liang Han, postgraduate Jiadong Zhou, and Doctor of South China University of Technology Tao Liu and the corresponding author is Professor Feng He from SUSTech.

Figure 2. The chemical structures and photovoltaic properties of the acceptors and the crystal packing of BTIC-2Br-γ.

Also, based on the team’s previous work (Lai et al, iScience 2019, 17, 302; Lai et al, J. Phys. Chem. Lett. 2019, 10, 4737), they also synthesized three non-fullerenes acceptors with different position of the Cl atom of the end groups. It is found that the introduction of different chlorine substituted end groups would have great effects on the photovoltaic performance of the device, and the power conversion efficiency of them varies from 7.39% to 15.04%. When compared with the traditional linear π-π stacking conjugated molecules, the three-dimensional stacking configuration is more effective for electronic hopping transfer among the molecules, thereby optimizing the photoelectric conversion performance of the resultant materials. This work has been published in the Journal of Materials Chemistry A. The first author is the graduated Ph.D. student Daize Mo, the co-first authors are Research Assistant Professor Hui Chen of Feng’s Group and the postdoctoral fellow Jiadong Zhou (South China University of Technology), and the corresponding author is Professor Feng He.

Based on their work on organic solar cell crystals, Professor Feng He’s team was invited to publish relevant reviews in the international materials journal Advanced Energy Materials. In order to further promote OSC development, it is necessary to understand the packing information at the atomic level to help develop acceptor systems with superior performance. The packing arrangements and intermolecular interactions of these acceptors in the solid state, observed by single-crystal X-ray crystallography, are often used to design materials with expected physicochemical properties. In this review, the chemical structures of acceptors revealed by single-crystal X-ray crystallography are summarized, and the relationship between structural design, packing arrangement, and device properties are discussed. Besides, the concept of“3D network packing” in acceptor systems is proposed, which offers better charge transfer properties in reported chlorinated, fluorinated, brominated, and trifluoromethylated systems. Some current issues related to single-crystal studies in OSCs are also discussed, with an emphasis on the significance of developing acceptors by understanding and adjusting the aggregation states and intermolecular interactions of materials by single-crystal analysis.

The paper also discussed the applications of single-crystal X-ray diffraction techniques to OSCs in the packing information at atomic levels to understand the electron transport behavior and provide an insight into the relationship between structures and properties. For instance: 1) the electron transport is greatly influenced by intermolecular interactions and π∙∙∙π stacking in acceptors, 2) more ordered J-aggregates are found in chlorinated acceptor compared its fluorinated and hydrogenated analogues due to the enhanced intermolecular interactions, 3) it is proved that the isomerization has a great effect on molecular packing arrangements and photovoltaic performance, 4) when the molecular coplanarity is improved, the gradually ordered J-aggregate will be formed with the increase of the alkyl chain length substituted on the center core, 5) the electron coupling values can be effectively improved to some extent by increasing the halogenated numbers, 6) the enhanced electronic donation of core is not always work in ordered molecular packing, it is necessary to have an appropriate core size and terminal units to promote the construction of 3D networks for more effective electron transport, 7) the intermolecular interactions and aggregation states in A-DADA- type acceptors are significantly different from that in A-D-A-type acceptors, the coordinated H/J-aggregates leading to more electron transport channels, it might be an important reason for the high efficiency of A-DAD-A-type acceptors, 8) the 3D network packing can promote the electron transport in acceptors, which is similar to the isotropic transport in fullerene acceptors for favorable charge transfer, and the halogenation at γ-position on terminal units might be an efficient way for the construction of such structures. An understanding of 3D networks could provide an insight into the electron transport behaviors for the design of high-performance materials for next-generation organic solar cells. Single crystals explored in this contribution could help the researchers in organic optoelectronics to understand the charge carrier transportation processes in active layers. It also offers a guideline for the development of new generation materials with improved and balanced device performance.

Researchers believe that exploring the micro packing information of optoelectronic materials will further help understand the properties of materials, and provide new ideas for the development of new and efficient photovoltaic materials by regulating the aggregation states and intermolecular interactions. The related paper was published online in Advanced Energy Materials, DOI: 10.1002/aenm.202002678. Doctoral candidate Hanjian Lai is the first author of this work, and the corresponding author is Professor Feng He.