Theoretical insights into fine-tuning π-bridges in polymer donors for high-performance organic solar cells: molecular...

The key to enhancing the power conversion efficiency (PCE) of organic solar cells (OSCs) lies in developing high-performance polymer donors. As the central structural unit, the π-bridge critically governs optoelectronic properties, elucidating its structure-property relationship is essential for rational molecular design. In this work, PffBT-2T and PffBT-4T polymers (differing in π-bridge length) were selected as reference molecules. Two novel polymer donors (PffBT-2T-fu and PffBT-4T-fu) were rationally designed by substituting the π-bridges in reference molecules with furan units. Based on density functional theory (DFT) and time-dependent DFT (TD-DFT), we systematically elucidated the synergistic regulatory mechanisms of π-bridge extension and furan substitution on molecular planarity, structural stability, optical absorption properties, charge transport characteristics, and the charge separation rate (K_CS) at the donor/acceptor (D/A) interface. Computational results reveal that PffBT-4T-fu exhibits the most favorable charge transport properties and K_CS, thereby validating the efficacy of this structural modulation strategy. Furthermore, the impact of an external electric field (F_ext) on the photovoltaic performance of the D:A blends was further investigated. Results demonstrate that applying an appropriate F_ext markedly accelerates K_CS, thereby increasing the short-circuit current density (J_SC) of OSCs. This work not only establishes a structure-property relationship related to the π-bridge, but also identifies F_ext as an effective auxiliary strategy, offering mechanistic insights and theoretical guidance for designing high-performance OSCs.