19.5% Efficiency organic solar cells enabled by a direct C-H arylation-derived wide-bandgap small-molecule guest donor

The incorporation of guest materials has been recognized as an effective strategy to further boost the photovoltaic performance. However, most guest materials introduced into ternary bulk heterojunctions are currently synthesized using multi-step Stille reactions, which typically suffer from poor atom-/step-economy, low cost-effectiveness and serious environmental problems. Consequently, guest materials synthesized through low-cost and eco-friendly tin-free approaches are highly required yet suffer from frequently poorly resolved issues. Herein, an asymmetric small-molecule guest donor oPhFO was designed and synthesized by a tin-free direct C-H activation strategy and incorporated into the classical PM6:BTP-ec9 host system. oPhFO exhibited a wide-bandgap and strong crystallinity, complementary absorption and cascade-like energy level alignment with the PM6:BTP-ec9 host system, which helps to improve the light-harvesting ability and open-circuit voltage. Moreover, oPhFO with a large dipole moment exhibited good miscibility with PM6, which finely regulated the pre-aggregation and crystallization kinetics of PM6 and BTP-ec9. This led to an optimized blend morphology and vertical phase separation, thereby achieving highly efficient exciton dissociation and charge transport. As a result, the PM6:oPhFO:BTP-ec9 ternary device achieved a champion power conversion efficiency of 19.5% with synchronously enhanced photovoltaic parameters. These results provide valuable guidelines for exploring tin-free guest materials while promoting the integration of green chemistry and clean energy.