Drastic effect of alkylation position in structurally simple benzobisthiazole–terthiophene copolymers on the performance of nonfullerene organic photovoltaics

Abstract The development of cost-effective, high-performance polymer donors is essential for advancing the large-scale commercialization of organic photovoltaic cells. Herein, we report photovoltaic performance of 2 structurally simple benzobisthiazole (BBTz)–terthiophene copolymers, PDBTz3 and PDBTz4, in which the alkyl groups are introduced at the outer and inner β-positions of the thiophene rings flanking the BBTz core, respectively. Notably, when blended with the nonfullerene acceptor Y12, PDBTz3 showed almost no photovoltaic effect, while PDBTz4 showed a power conversion efficiency of 10.2%. A comprehensive investigation, in combination with optical, morphological, and computational analysis, revealed that this subtle variation in alkylation position drastically affects backbone planarity, molecular packing, and donor–acceptor interfacial energetics. In polymer:Y12 blend film, PDBTz3 adopts a highly twisted backbone conformation, leading to an unfavorable energy level alignment and inefficient hole transfer, while PDBTz4 maintains a coplanar backbone, enabling a favorable energy level alignment and efficient hole transfer. Consequently, PDBTz4 showed superior photovoltaic performance compared to PDBTz3. These findings highlight the critical role of alkylation position engineering in polymer design and provide a useful strategy for developing cost-effective, high-performance materials for next-generation organic photovoltaic cells.