Heterocycle substitution plays a vital role in the molecular design of ultra-narrow bandgap (ultra-NBG) non-fullerene acceptor (NFA) toward efficient organic solar cells (OSCs). However, how different heterocycles, i.e., benzotriazole (BTA), benzothiadiazole (BT), and benzoselenadiazole (BSe), impact the optoelectronic properties of asymmetric ultra-NBG NFAs remains unclear. Herein, three asymmetric NFAs based on BTA, BT, and BSe electron-deficient fused-ring core, denoted as BTP-N, BTP-S, and BTP-Se were designed and synthesized. All three NFAs show strong absorption from visible to near-infrared regions, corresponding to the ultra-NBG below 1.26 eV, much lower than that of Y6. The reasons why these NFAs performed differently are systematically investigated by comparing their optoelectronic and morphological properties. The three NFAs exhibited differences in their photovoltaic performance with device efficiencies of 14.20% achieved by single-junction binary PBDB-T: BTP-Se-based device, much higher than those of PBDB-T:BTP-N-and PBDB-T:BTP-S-based devices (11.97% and 12.56%, respectively). Furthermore, PBDB-T:BTP-Se-based device with the unique synergistic effect of vinylene pi-bridge and fused-benzoselenadiazole core enabled an ultrahigh short-circuit current density (J(sc)) of 28.66 mA cm(-2), which is the highest reported J(sc) value among binary NFA-based OSCs. Meanwhile, the exceeding 14% efficiency of PBDB-T:BTP-Se-based device is also one of the highest values in binary PBDB-Tbased OSCs. This work demonstrates the success of the fused-benzoselenadiazole strategy in the design of high-performance ultra-NBG asymmetric NFAs, which not only achieves a well-balanced trade-off between the open-circuit voltage and J(sc), but also helps optimize morphology for efficient charge dissociation, suppressed charge recombination, and balanced carrier mobility. It may provide a promising approach to constructing high efficiency near-infrared OSCs.
