Synergistic direct and indirect central unit fluoridation of non-fullerene acceptor enables high-efficiency organic solar cells

The fluorination strategy for non-fullerene acceptors (NFAs), involving direct and indirect fluorination such as trifluoromethylation, has been established as an effective method to improve the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Although end-group fluorination has achieved significant success, the fluorination of central units in NFAs has received limited exploration owing to the lack of substitution sites in conventional Y-series acceptors. To systematically investigate the effect of different central unit fluorination strategies on molecular stacking and subsequently the performance of OSCs, we utilized recently developed phenazine (CH)-series acceptors to design a series of NFAs, namely CH-F, CH-CF, and CH-FCF, which are featured with central unit mono-fluorination (CH-F), mono-trifluoromethylation (CH-CF), and synergistic fluorination and trifluoromethylation (CH-FCF), respectively. Among them, CH-FCF-based blend films (PM6/CH-FCF) demonstrate strengthened molecular interactions and crystallinity, an excellent fibrillar network morphology and bettered charge generation/transport performance. Consequently, the binary OSCs based on CH-FCF achieved the best PCE of 18.41 %, which outperforms its counterparts based on CH-F (17.34 %) and CH-CF (17.62 %). These results emphasize the significance of synergistically regulating the central unit of small molecule acceptors through different fluorination strategies to manipulate molecular packing and thus enhance the photovoltaic performance of OSCs.