Benzothienoisoindigo-based polymers for efficient polymer solar cells with an open-circuit voltage of 0.96 V

Two conjugated polymers have been designed and synthesized by introducing benzothienoisoindigo as electron-withdrawing moieties on side chains. PBDTT-TBID based on alkylthienyl-substituted benzo[1,2-b:4,5-b’]dithiophene (BDT) derivative as electron-donating moieties possesses a low-lying highest occupied molecular orbital energy level of −5.38 eV, so its polymer solar cells (PSCs) shows a high open-circuit voltage (Voc) of 0.96 V, which is one of the highest Voc values among benzothienoisoindigo-based polymers. PBDT-TBID based on alkoxyl-substituted BDT derivative possesses better molecular coplanarity, stronger intramolecular charge transfer, stronger intermolecular π-π stacking, more suitable microphase separation with PC71BM, more effective exciton dissociation, and higher hole mobility than PBDTT-TBID, which lead to higher short-circuit current density (Jsc = 11.81 mA cm−2), fill factor (FF = 0.58) and power conversion efficiency (PCE = 5.87%) values than those based on PBDTT-TBID. The PCE value is still higher than those of the reported benzothienoisoindigo-based polymers though it lags behind those of recent non-fullerene PSCs.