Efficient Synthesis of High-Performance Wide-Bandgap Polymers Based on Difluoronaphthodithiophene: Fluorination Position Impact on Photovoltaic Performance

In this study, two acceptor units, 4,7-difluoronaphtho[2,1-b:3,4-b′]dithiophene (4-DFT) and 5,6-difluoronaphtho[2,1-b:3,4-b′]dithiophene (5-DFT), were synthesized with high yield (>80%) for the first time. Utilizing these units, two wide-bandgap (WBG) donor–acceptor (D–A) copolymers, o-PDFNT and p-PDFNT, were successfully constructed. The optical bandgaps of these copolymers were determined to be 2.04 and 2.05 eV, respectively. The study revealed that the fluorination position on the polymer backbone significantly impacts molecular packing and charge transport properties. Specifically, p-PDFNT with fluorination at the 5,6-position exhibits closer π–π stacking and higher crystallinity compared to o-PDFNT. When blended with BTP-eC9-4F, p-PDFNT-based organic solar cells (OSCs) demonstrated a power conversion efficiency (PCE) of 16.24%, whereas OSCs based on o-PDFNT showed a relatively lower PCE of 13.56%. This indicates the potential of 4-DFT as an electron-deficient unit for the construction of efficient donor materials. Furthermore, the research highlights the crucial role of altering the fluorination position in controlling molecular aggregation, ultimately benefiting the enhancement of the photovoltaic performance in OSCs.