Donor-acceptor polymers with a regioregularly incorporated thieno[3,4-b]thiophene segment as a π-bridge for organic photovoltaic devices

A series of donor-acceptor (D-A) polymers with regioregularly incorporated a thieno[3,4-b]thiophene as a π-bridge between electron donor and electron acceptor segments were successfully synthesized for the first time. The optical, thermal, and electrochemical properties of the D-A polymers with the thieno[3,4-b]thiophene as a π-bridge were characterized by UV/vis spectroscopy, thermogravimetric analysis, and cyclic voltammetry measurements. They exhibited low energy bandgap because of extended π-conjugation length and increased electron density through conjugated polymer backbones. Moreover, they showed suitable HOMO and LUMO energy levels for photovoltaic applications. The bulk heterojunction organic photovoltaic devices based on the D-A polymers with the regioregularly incorporated thieno[3,4-b]thiophene segment exhibited about 10% higher power conversion efficiency (PCE) than the D-A polymer with thienyl segment as a π-bridge due to enhanced light harvesting ability and excellent nanoscale network. This study implies that the D-A polymers with the regioregularly incorporated thieno[3,4-b]thiophene segment as a π-bridge can achieve high PCE through structural modification of the polymers.