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 (J_SC) of 19.54 mA/cm², open circuit voltage (V_OC) of 0.97 V, and fill factor (FF) of 0.59, and 6.62% with a J_SC of 12.50 mA/cm², V_OC 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 J_SC of 21.91 mA/cm² and FF of 0.68. The V_OC 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 J_SC 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.