Ternary Polymer Solar Cells with High Open Circuit Voltage containing Fullerene and New Thieno[3',2',6,7][1]Benzothieno[3,2‐b]Thieno[3,2‐g][1]Benzothiophene‐based Non‐fullerene Small Molecule Acceptor

Herein, the synthesis of a new medium bandgap nonfullerene acceptor DBTBT‐IC consisting of di‐benzothieno [3,2‐b][1]‐benzothiophene DBTBT as the central donor unit and IC as terminal acceptor units and its use as the acceptor for the fabrication of single binary and ternary polymer solar cells is reported. DBTBT‐IC exhibits a medium optical bandgap of about 1.65 eV. When paired with the wide bandgap‐conjugated polymer PDTNIT as donor, the polymer solar cells based on the optimized PDTNIT: DBTBT‐IC active layer realized a power conversion efficiency of 12.34% (short‐circuit current = 8.06 mA cm −2 , open‐circuit voltage = 1.12 V, and fill factor (FF) = 0.62) which is higher than that for the PDTNIT:PC 71 BM counterpart, i.e., 9.08 % (short‐circuit current=14.58 mA cm −2 , open‐circuit voltage = 0.93 V, and FF = 0.67). When a small amount of PC 71 BM was introduced into the host PDTNIT: DBTBT‐IC binary layer, the polymer solar cell based on the optimized PDTNIT:PC 71 BM: DBTBT‐IC (1:0.3:0.9) ternary active layer attained an excellent power conversion efficiency of about 15.92 %, mainly due to the increase in short‐circuit current and FF. The increase in the short‐circuit current may be associated with the broader absorption profile of the ternary active layer as compared with the binary counterparts and more efficient exciton utilization, due the partial energy transfer from PC 71 BM to DBTBT‐IC .