VOC enhancement of thienobenzofuran and benzotriazole backboned photovoltaic polymer by side chain sulfuration or fluoridation

Abstract

In this work, four wide-bandgap (WBG) conjugated polymers based on asymmetrical building block thieno [2,3f]benzofuran (TBF) and fluorobenzotriazole (BZ), are synthesized and applied as donor materials in polymer solar cells (PSCs). The effect of conjugated side chains (alkylthiothiophenyl, alkylthiophenyl, alkylphenyl and fluoroalkoxyphenyl) on the optical absorption, electrochemical and photovoltaic properties are investigated in detail. These polymers exhibit similar properties, strong absorption from 300 to 650 nm with wide bandgaps ranging from 1.87 to 1.96 eV, excellent thermal stability, the high and balanced carrier mobility. Notably, the open-circuit voltages (VOC) of PTSDO-BZ increases by 0.16 V compared with PTDO-BZ. Similarly, larger VOC changes have also occurred in PPEH-BZ and PPFOEH-BZ comparation. The enhanced VOC can be ascribed to the low-lying highest occupied molecular orbital (HOMO) energy level by incorporating sulfur and fluorine substituents on the conjugated side chains. The as-cast devices of PTSDO-BZ:ITIC exhibit the highest power conversion efficiency (PCE) of 8.66%, with a VOC of 0.88 V, a short-circuit current density (JSC) of 15.06 mA cm−2, and a fill factor (FF) of 65.12%. The results indicate that the modification of the side chains of the TBF unit can effectively lower the HOMO energy levels and thereby obtaining high VOC.