Controlling Polymer Donor Properties through Site-Specific Fluorination for High-Efficiency Organic Solar Cells

Fluorination is a widely adopted molecular engineering strategy in the development of high-performance polymer donors for organic solar cells (OSCs). In this study, two isomeric D−π–A-type conjugated polymers, PBT-BDT-F and PffBT-BDT, were designed by selectively introducing fluorine atoms onto either the donor (BDT) or acceptor (BT) units. Comprehensive experimental and theoretical investigations were conducted to understand how the position of the fluorine substitution influences polymer properties and device performance. UV–vis absorption and temperature-dependent spectral analysis revealed that PBT-BDT-F exhibits slightly weaker aggregation behavior in solution and solid states. Electrostatic potential (ESP) mapping and intermolecular interaction calculations demonstrated that PBT-BDT-F achieves better energetic compatibility and weaker donor–acceptor interactions with L8-BO, promoting optimal phase separation. Consequently, PBT-BDT-F-based devices achieved a significantly higher power conversion efficiency (PCE) of 13.2%, with an open-circuit voltage of 0.87 V and a short-circuit current density of 22.6 mA·cm–2, outperforming the PffBT-BDT-based devices (PCE = 10.7%). These results highlight the crucial role of the fluorination position in tailoring the optoelectronic properties, morphology, and charge dynamics of conjugated polymers, offering a rational design pathway for next-generation high-efficiency OSCs.