S-to-O Atom Swapping Unlocks a Promising Difuranphthalimide-Based Polymer Donor with Efficiency over 20%

The pursuit of high-performance wide-bandgap polymer (WBG) donors remains a pivotal challenge for advancing nonfullerene organic solar cells (OSCs). Herein, we address this challenge via creating a novel electron-deficient building block, difuranylphthalimide (DFI), via a strategic sulfur-to-oxygen atom swap in dithienophthalimide (DTI). This atomic substitution engenders a polymer, PDFI, with distinctly advantageous properties derived from the furan moieties: a deeper highest occupied molecular orbital (HOMO) energy level, enhanced backbone planarity and crystallinity, and superior miscibility with state-of-the-art nonfullerene acceptors. These characteristics collectively foster optimized blend morphology with tighter π-π stacking, more efficient charge transport, and significantly suppressed nonradiative energy loss. Consequently, binary OSCs based on PDFI and the acceptor BTP-eC9 achieve an impressive power conversion efficiency (PCE) of 19.17%, which is the highest value achieved by furan-containing donor materials in OSCs to date. Furthermore, incorporating PDFI as a third component into the PM6:L8-BO system yields a ternary device with a PCE exceeding 20%. This work not only introduces PDFI as a top-performing nonhalogenated polymer donor but also establishes the incorporation of furan into electron-deficient units as a transformative strategy for developing high-efficiency, sustainable organic photovoltaic materials.