Fullerene/Non-fullerene Alloy for High-Performance All-Small-Molecule Organic Solar Cells

Organic solar cells (OSCs) that contain small molecules only were prepared with FG1 as the donor, a narrow band gap non-fullerene acceptor MPU4, and a wide band gap PC71BM. The OSCs based on optimized FG1:MPU4 (1:1.2) and FG1:PC71BM (1:1.5) active layers, respectively, gave power conversion efficiencies (PCEs) of 11.18% with a short circuit current (JSC) of 19.54 mA/cm2, open circuit voltage (VOC) of 0.97 V, and fill factor (FF) of 0.59, and 6.62% with a JSC of 12.50 mA/cm2, VOC of 0.84 V, and FF of 0.63%, respectively. A PCE of 13.26% was obtained from the optimized ternary FG1:PC71BM:MPU4 (1:0.3:0.9) OSCs and this arises because of the boost in a JSC of 21.91 mA/cm2 and FF of 0.68. The VOC of the ternary OSCs (0.89 V) lies between those for the OSCs based on FG1:MPU4 and FG1:PC71BM, which indicates the formation of an alloy of the two acceptors. The increase in JSC and FF in the ternary OSCs may result from the efficient energy transfer from PC71BM to MPU4 as well as more charge-transfer donor/acceptor interfaces, enhanced charge carrier mobilities resulting in better adjusted charge transport, and lower bimolecular and trap-assisted recombination. The appropriate phase separation, increased crystallinity, and reduced π–π stacking distance in the ternary active layer are consistent with the enhancement in the FF for OSCs based on a ternary active layer. The results of this work suggest the merging of the fullerene acceptor into the non-fullerene acceptor to form a fullerene/non-fullerene acceptor alloy, and this may be a viable approach to obtain high-performance OSCs.