Achieving Balanced Charge Transport and Favorable Blend Morphology in Non-Fullerene Solar Cells via Acceptor End Group Modification

Generally, the electron-withdrawing substitution on the end-capping group of the acceptor–donor–acceptor type small-molecule acceptor (SMA) narrows the optical bandgap, and the electron-donating group lifts the lowest unoccupied molecular orbital (LUMO) energy level of nonfullerene SMA, which increase the short-circuit current density (JSC) and open circuit voltage (VOC) of the organic solar cells (OSCs), respectively; however, their synergistic effect on the properties of SMA has remained elusive. Here, we first report a new end-capping group (EG), namely, 5-fluoro-6-methyl-3-dicycanovinylindan-1-one (CFDCI), that concurrently possesses an electron-withdrawing fluorine substitute and an electron-donating methyl group. A prototype SMA (namely, ITCF) based on CFDCI and its two control counterparts were prepared to fully understand the structure–property relationship that the new EG exerts on the resultant SMA. The ITCF demonstrated a moderately crystalline morphology in pristine film and more balanced charge transport properties as well as a reduced amount of bimolecular recombination in blend film in comparison with its counterparts. The ITCF-based devices demonstrated a high power conversion efficiency (PCE) of 13.25% with an outstanding fill factor (FF) of 78.8%, which significantly outperformed their counterparts. Our study provides an important strategy to judiciously tune the properties of the SMAs for improving the performance of the OSCs.