Selenophene‐fused Perylene Diimide‐Based Cathode Interlayer Enables 19 % Efficiency Binary Organic Solar Cells via Stimulative Charge Extraction

Abstract The cathode interlayer is crucial for the development of organic solar cells (OSCs), but the research on simple and efficient interlayer materials is lagging behind. Here, a donor‐acceptor (D–A) typed selenophene‐fused perylene diimide (PDI) derivative (SePDI3) is developed as cathode interlayer material (CIM) for OSCs, and a non‐fused PDI derivative (PDI3) is used as the control CIM for comparison. Compared to PDI3, SePDI3 shows a stronger self‐doping effect and better crystallinity, resulting in better charge transport ability. Furthermore, the interaction between SePDI3 and L8‐BO can form an efficient extraction channel, leading to superior charge extraction behavior. Finally, benefitting from significantly enhanced charge transport and extraction capacity, the SePDI3‐based device displays a champion PCE of 19.04 % with an ultrahigh fill factor of 81.65 % for binary OSCs based on PM6 : L8‐BO active layer, which is one of the top efficiencies reported to date in binary OSCs based novel CIMs. Our work prescribes a facile and effective fusion strategy to develop high‐efficiency CIMs for OSCs.