Cyanobenzene-Modified Quinoxaline-Based Acceptors with Optimal Excitonic Behavior Enable Efficient Organic Solar Cells.

Cyanogroup (-CN) is a strongly electron-withdrawing and highly polar functional group; therefore, cyanation has been extensively utilized to optimize the terminal groups of high-performance small-molecule acceptors (SMAs) in organic solar cells (OSCs). Herein, by regulating the cyanobenzene substitution in the central core for the first time, four novel SMAs are synthesized, named phCN-F, phCN-Cl, 2phCN-F, and 2phCN-Cl. Theoretical and experimental analyses have shown that asymmetric and symmetric cyanobenzene-substitution of the central core, coupled with selective terminal groups, can significantly affect the intrinsic excitonic properties of the molecule. Blends based on asymmetric cyanobenzene-substitution molecules possess tighter molecular packing and more suitable phase separation to facilitate exciton dissociation, charge transport, and charge extraction. The optimal device performance of the phCN-F-based OSC reaches 20.16%, which is significantly higher than that of symmetrically substituted OSCs. Furthermore, devices prepared based on phCN-F maintain over 90% of their initial efficiency after being heated at 85°C for 3000 h, demonstrating excellent thermal stability. This study elucidates the potential mechanisms for optimizing device performance through asymmetric cyanobenzene-substitution of the central core, providing valuable insights for the further design of record-breaking SMAs.