Branched Terminal Alkyl Chain Enabling 15.7% Efficiency Oligothiophene Donor

Optimizing the alkyl chains in a terminal group is one of the most efficient strategies to enhance the power conversion efficiency (PCE) of all small molecule organic solar cells (ASM‐OSCs). Oligothiophene, as an easily synthesized small molecule (SM) donor with an explicit structure, can influence intermolecular packing, planarity, and crystallinity through optimization of the alkyl chain in the terminal group. Herein, two SMs, Tz6T‐P and Tz6T‐iP , are synthesized using the n‐propyl and isopropyl isomers, respectively, as alkyl chains in the terminal group. Compared with the linear chain (n‐propyl) of Tz6T‐P , the rigid branch chain (isopropyl) in Tz6T‐iP exhibits an almost complanate configuration in the ground state and higher crystallinity. Consequently, Tz6T‐iP features lower levels of bimolecular recombination and trap‐assisted recombination, higher ability for exciton dissociation and charge collection, and longer carrier lifetimes. Tz6T‐iP achieves a PCE of 15.7% for oligothiophene‐based ASM‐OSCs. These results provide innovative strategies, demonstrating that branching the terminal alkyl chain can adjust planarity, crystallinity, and separation morphology for the structural design of photovoltaic materials.