Effect of Chlorine Substitution Position of End-group on Intermolecular Interaction and Photovoltaic Performance of Small Molecule Acceptors
The structure-property relationship of small molecular acceptors (SMAs) is a key issue in the molecular design of new-generation acceptor materials for further improving the device efficiencies of polymer solar cells (PSCs). Herein, three couples of SMAs isomers were synthesized, based on three central fused ring units and two IC isomer electron-withdrawing terminal units with chlorine substitution on different position of its benzene ring: Cl-1 with chlorine at the same side with C=O group of IC and Cl-2 with chlorine at the same side with CN groups of IC. Through systematical investigation, we found that the chlorine substitution position of the terminal groups has regular and significant influence on the molecular packing and photovoltaic performance of the SMAs. Molecular packing behavior of the SMAs is tightly related to and determined by the configuration of their terminal groups, no matter what central fused ring of the SMAs. Specifically, the Cl-1-based SMAs possess stronger crystallinity with long range ordering packing in its molecular plane direction, while the more abundant and stereoscopic π-π intermolecular interaction in the Cl-2-based SMAs promotes the molecules to form three-dimensional charge transporting channels and leads to their red-shifted absorption and higher electron mobilities. Therefore, the Cl-2-based PSCs exhibit higher power conversion efficiency (PCE) than that of the Cl-1-based devices, and the best PCE of a Cl-2 SMA-based PSC reached 16.42%. These results highlight the importance of the investigation of intermolecular interaction, packing and arrangement of the SMAs in the solid-state, which may provide a direct insight to explore the relationship between molecular structure and property of the photovoltaic materials. Moreover, we envision that if fragments like end group or side chains with more diverse molecular interaction are added in the design and synthesis of the SMAs, it may be beneficial to promoting the molecular π-π accumulation and further improving the molecular order, forming suitable molecular packing and morphology in blend films, finally affecting the efficiency of the PSCs.