Ladder-type tetra-p-phenylene-based copolymers for efficient polymer solar cells with open-circuit voltages approaching 1.1 V

Abstract

Side-chain engineering of polymer backbones can induce subtle variations in polymer properties, resulting in a significant impact on their photovoltaic performance. In this work, four ladder-type tetra-p-phenylene containing copolymers with different alkyl side chains (P3FTBT1, P3FTBT1F, P3FTBT8O6 and P3FTBT1O6) were designed and synthesized. These copolymers have large bandgaps (∼2.0 eV) and deep-lying highest occupied molecular orbital (HOMO) energy levels (from −5.44 eV to −5.53 eV). The substitution of two hexyl groups with two methyl groups on the ladder-type tetra-p-phenylene unit afforded polymer P3FTBT1 which exhibits an enhanced power conversion efficiency (PCE) of 5.39%. Incorporation of fluorine into the benzo[c][1,2,5]thiadiazole (BT) unit gave polymer P3FTBT1F which exhibits a PCE of 4.50% with an open circuit voltage (V_oc) of 1.09 V. By introducing two alkoxy groups to the BT unit, P3FTBT1O6 was synthesized, and it exhibits a PCE of 5.73% with a V_oc of 1.02 V. The results suggest that the ladder-type tetra-p-phenylene is an excellent building block to construct donor–acceptor copolymers with high PCEs and large V_ocs.