Fine‐Tuning Alkyl Chains on Quinoxaline Nonfullerene Acceptors Enables High‐Efficiency Ternary Organic Solar Cells with Optimizing Molecular Stacking and Reducing Energy Loss

Material design of guest acceptor is always a big challenge for improving the efficiency of ternary organic solar cells (OSCs). Here, a pair of isomeric nonfullerene acceptors based on quinoxaline core, Qx–p‐C7H8O and Qx–m‐C7H8O, is designed and synthesized. By moving the alkoxy chain attached on side phenyl from meta‐position to para‐position, both π–π stacking distance and crystallinity are enhanced simultaneously. They obtain the uplifted lowest unoccupied molecular orbital level. Compared to Qx–m‐C7H8O, Qx–p‐C7H8O exhibits wider absorption spectrum and higher extinction coefficient. Using D18‐Cl:N3 as host materials, the addition of guest acceptor Qx–p‐C7H8O significantly improves the power conversion efficiency (PCE) from 17.61% to 18.49% because of higher open‐circuit voltage (0.875 V) and short‐circuit current density (27.85 mA cm−2). This can be attributed to the faster exciton dissociation, more balanced carrier mobility, fine fiber morphology, and lower energy loss in the ternary devices. However, Qx–m‐C7H8O‐based ternary device achieves relatively low PCE of 17.17% because this device shows extremely low electron mobility. The results indicate that molecular stacking, film morphology, etc., can be effectively modulated by fine‐tuning the side chains of guest materials, which may be an effective design rule for further improving the PCE of OSCs. Using D18‐Cl:N3 as host materials, the addition of guest acceptor Qx–p‐C7H8O significantly improves the power conversion efficiency (PCE) from 17.61% to 18.49% with optimizing molecular stacking and reducing energy loss. Qx–m‐C7H8O‐based ternary device achieves low PCE of 17.17%. Fine‐tuning the alkyl chain of the central unit can adjust the molecular packings, thus preparing high‐performance organic solar cells.