Isomeric Alkylthiophene Side-Chain Engineering of a Small-Molecule Acceptor for Efficient Organic Solar Cells

Side-chain engineering is widely used to design materials to improve photovoltaic performance for organic solar cells. In this work, three isomeric acceptors (namely, BTP-Th1, BTP-Th3, and BTP-Th6) bearing varying alkylthiophene side chains are developed to elucidate the influence of thiophene substitution positions on their physicochemical properties and photovoltaic performance. A comparative analysis shows that BTP-Th1 with a longer alkyl chain length between thiophene and conjugated backbone exhibits a more planar molecular conformation, enhanced crystallinity, and higher electron mobility than BTP-Th3 and BTP-Th6. By using PM6 as a polymer donor, the device based on PM6:BTP-Th1 demonstrates enhanced exciton dissociation, improved and more balanced carrier mobility, and reduced nonradiative recombination, achieving a power conversion efficiency (PCE) of 15.12%, which is higher than that of BTP-Th3- (PCE = 14.79%) and BTP-Th6-based (PCE = 13.30%) devices.