Fullerene-Free All-Small-Molecule Ternary Organic Solar Cells with Two Compatible Fullerene-Free Acceptors and a Coumarin Donor Enabling a Power Conversion Efficiency of 14.5%

Organic solar cells (OSCs) based on a ternary single bulk-heterojunction using either donor:donor:acceptor or donor:acceptor:acceptor with complementary absorption profiles and well-matched energy levels can effectively improve the power conversion efficiency (PCE). Herein, we report the fabrication of binary and ternary OSCs using a wide-band-gap donor molecule C2 in combination with two fullerene-free acceptors (FFAs), i.e., a medium-band-gap DBTBT-IC and a low-band-gap AQx-3. After incorporation of DBTBT-IC in the host C2:AQx-3 binary BHJ active layer, the ternary blend C2:DBTBT-IC:AQx (1.0:0.2:1.0) attains a PCE of 14.49%, which outperforms the respective binary devices, i.e., 10.02% for C2:DBTBT-IC and 10.86% for C2:AQx-3. The higher PCE of the ternary OSC may be related to the simultaneous increase in both the photocurrent (JSC) and the fill factor (FF). The enhancement in the JSC may be associated with the increase in the light-harvesting efficiency of the ternary blend, efficient utilization of the exciton via the energy transfer from DBTBT-IC to AQx-3, and more channels for the charge transfer in the ternary OSC. The higher FF for the ternary devices may be associated with the efficient charge transport and suppression of charge recombination due to appropriate nanoscale phase separation. Thus, the use of a small-molecule donor and two FFAs with complementary absorptions and band gaps may be a good strategy to fabricate efficient ternary OSCs.