Ending group modulation of asymmetric non-fullerene acceptors enables efficient green solvent processed organic solar cells

Despite the progress made in non-fullerene acceptor (NFA) based organic solar cells (OSCs), it is quite chal-lenging to achieve efficient green solvent processed (GSP) OSCs. Herein, we design and synthesize three asymmetric NFAs named BTP-SO-BrF, BTP-SO-1F, and BTP-SO-2F with alkoxy/alkythio branched chains on the core unit but distinct ending groups (BrF-IC, 1F-IC, and 2F-IC), respectively. The asymmetric alkoxy/alkylthio branched chains help to enhance the solubility of these NFAs and compatibility with the PM6 polymer donor in non-halogen solvents. Furthermore, the ending group modulation studies unveil that PM6:BTP-SO-2F blend film exhibits shorter intermolecular stacking distance, more favorable fiber-like phase separation, and more balanced carrier mobilities than the other two NFAs. Consequently, optimal efficiency of 17.6% is achieved in the BTP-SO-2F-based devices with an open-circuit voltage of 0.909 V, a short-circuit current density of 24.54 mA cm-2, and an excellent fill factor of 0.789, compared to those of BTP-SO-BrF (15.5%) and BTP-SO-1F-based (16.2%) de-vices, which is the highest value reported for GSP-OSCs based on asymmetric NFA. Our studies reveal that the halogenation of ending groups for NFAs with asymmetric branched chains is an effective strategy to tune the morphology for high-efficiency and eco-friendly GSP-OSCs.