Aromatic heterocycle 1,3,4-oxadiazole-substituted thieno[3,4-b]thiophene to build low-bandgap polymer for photovoltaic application

Electron-deficient heterocycle 1,3,4-oxadiazole is first introduced to the 2-position of thieno[3,4-b]thiophene (TT) to construct a new building block 2-(thieno[3,4-b]thiophen-2-yl)-5-(alkylthio)-1,3,4-oxadiazole (TTSO) with alkylthio chain. The polymer PBDT-TTSO based on TTSO and benzodithiophene (BDT) exhibits a deep lying highest occupied molecular orbital (HOMO) energy level of -5.32 eV and low-bandgap of 1.62 eV. The power conversion efficiency (PCE) of 5.86% is obtained with a relatively high V _OC of 0.74 V, a J _SC of 13.1 mA cm^-2, and FF of 60.5%. Furthermore, as S atom in thioether can be oxidized easily, the optoelectronic properties of PBDT-TTSO treated with different oxidants are preliminary investigated. Interestingly, the oxidation products still maintain high PCE with reduction less than 30%. This work demonstrates a new method to regulate HOMO energy levels by introducing electron-deficient aromatic heterocyclic moiety. The electron-deficient aromatic heterocycle 1,3,4-oxadiazole replaces the carbonyl group to investigate the optical, electrochemical, and photovoltaic properties of thieno[3,4-b]thiophene-based polymers.