Thiophene π-Bridge Fluorination Enables Conformational Locking and Molecular Packing for High-Performance TPD-Based Nonfullerene Acceptors

Engineering the π-bridge of nonfullerene acceptors (NFAs) is an effective strategy for regulating their optoelectronic properties and molecular packing; however, the influence of thiophene π-bridge fluorination on charge transport and recombination remains insufficiently understood. Here, two thieno[3,4-c]pyrrole-4,6-dione (TPD)-based NFAs, TC8 and TFC8, incorporating thiophene π-bridges bearing alkyl-substituents and both alkyl- and fluorine substituents, respectively, were designed and synthesized to elucidate the effect of π-bridge fluorination on organic solar cell (OSC) performance. Fluorination gives rise to intramolecular S··F noncovalent interactions, inducing conformational locking that enhances intermolecular π–π stacking while modulating the optical absorption and frontier energy levels. Consequently, TFC8-based OSCs showed a power conversion efficiency (PCE) of 12.7%, more than twice that of TC8-based devices (5.8%), accompanied by improvements in the short-circuit current density (JSC) and fill factor (FF). Detailed device analyses revealed that fluorination of the π-bridge lowers both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, leading to more favorable energy-level alignment with the donor PM6 for efficient charge separation. Moreover, the PM6:TFC8 blend film exhibited enhanced and well-balanced charge transport, suppressed bimolecular and trap-assisted recombination, and improved molecular ordering, resulting in more efficient charge extraction and photovoltaic performance. This work demonstrates that thiophene π-bridge fluorination is an effective molecular design strategy for simultaneously regulating molecular packing, charge transport, and recombination, providing useful guidelines for the development of high-performance NFAs.