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, <b>BTP-Th1</b>, <b>BTP-Th3</b>, and <b>BTP-Th6</b>) 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 <b>BTP-Th1</b> with a longer alkyl chain length between thiophene and conjugated backbone exhibits a more planar molecular conformation, enhanced crystallinity, and higher electron mobility than <b>BTP-Th3</b> and <b>BTP-Th6</b>. By using PM6 as a polymer donor, the device based on PM6:<b>BTP-Th1</b> 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 <b>BTP-Th3</b>- (PCE = 14.79%) and <b>BTP-Th6</b>-based (PCE = 13.30%) devices.