An unfused-ring acceptor enabling ∼12% efficiency for layer-by-layer organic solar cells

Cost-effective acceptors and device fabrication processes are decisive factors for commercializing organic solar cells (OSCs). Herein, a simple unfused-ring electron acceptor (ThPF-4F) cored with 3,4-bis(3-fluoro-4-methylphenyl)thiophene has been developed for layer-by-layer (LBL) high-efficiency OSCs. PBDB-T was selected as the donor and a non-orthogonal solvent system was used to fabricate the device. LBL OSCs (PBDB-T/ThPF-4F) exhibit a better vertical gradient distribution of a donor and an acceptor in the active layers than bulk-heterojunction (BHJ) ones (PBDB-T:ThPF-4F) as revealed by film-depth-dependent light absorption spectroscopy. The charge transport was thus facilitated with reduced recombination losses during charge transfer. LBL OSCs deliver a maximum power conversion efficiency (PCE) of 11.83% with a higher short-circuit current and fill factor in comparison to the optimized BHJ counterparts (11.10%). Importantly, the PCE of LBL OSCs is further improved to 11.97% by adding 10 wt% PC71BM in the PBDB-T layer. This work demonstrates the integration of the rational molecular design and facile device processing as a promising strategy to develop high-performance cost-effective OSCs.