The impeded film-formation kinetics with ameliorated carrier trap density to construct efficient as-cast organic solar cells through short multi-branched alkylthio chains strategy

In the quest for cost-effective commercialization of non-fullerene acceptor (NFA) organic solar cells (OSCs), the as-cast devices with no additives or additional treatments show significant potential. However, the accelerated aggregation rate of NFAs often results in moderate crystallinity and poor carrier transport, leading to high trap densities that reduce power conversion efficiency (PCE). To address these challenges, we introduced two new NFAs BTP-S-C12 and BTP-S-DMO, which featured outer alkylthio chains and were synthesized using palladium-catalyzed coupling method. BTP-S-DMO, equipped with short multi-branched alkylthio chains (SMBA), demonstrated superior solubility and crystallinity compared to BTP-S-C12, which has longer linear alkylthio chains. Despite having nearly identical light absorption ranges, BTP-S-DMO exhibited ameliorated molecular packing. In situ spectroscopic characterizations showed that this SMBA approach effectively slowed down the aggregation kinetics of NFA molecules, promoting better packing and a favorable face-on orientation. The well-defined hierarchical morphology in the D18:BTP-S-DMO blend film ultimately resulted in excellent charge transport and lower trap density. This improvement led to an impressive PCE of 18.4% for D18:BTP-S-DMO-based as-cast binary OSCs. Further enhancement was achieved by the ternary strategy that blending D18:BTP-S-DMO:L8-BO together, which can reach a remarkable PCE of 19.0%, one of the highest reported for as-cast OSCs. This study not only presents a straightforward approach to controlling NFA molecular aggregation rate by tuning solubility and crystallinity but also offers significant potential for enhancing photovoltaic performance and advancing the commercialization of OSCs.