Tailoring small-molecule acceptors through asymmetric side-chain substitution for efficient organic solar cells

Side chain engineering of small-molecule acceptors (SMAs) is a promising strategy for improving device efficiency in organic solar cells (OSCs). This study investigates the parent SMAs of BT-BO and BT-TBO, along with the newly synthesized asymmetric SMA, BT-ASY, which features branched alkyl chains and thiophene side chains substituted at the β positions of the thiophene units, respectively. Despite exhibiting comparable optical and electrochemical properties, the PM6:BT-ASY-based device achieves a power conversion efficiency (PCE) of 18.08% representing a significant improvement over its symmetric counterparts. This enhancement is primarily attributed to improved charge mobility, extended carrier lifetimes, optimized molecular packing, and effective phase separation, as confirmed by grazing incidence wide-angle X-ray scattering measurements. Our findings highlight that asymmetric side-chain strategy enhances π-π stacking and electronic coupling, offering a simple yet effective approach to improving photovoltaic performance. This work underscores the potential of asymmetric structural modifications in SMAs for advancing OSC technology and renewable energy solutions.