Synthesis and photovoltaic properties of conjugated copolymers with benzo[1,2-b:4,5-b′]dithiophene and thiadiazolo[3,4-c]pyridine moieties

A series of main-chain and side-chain type D–A copolymers containing benzo[1,2-b:4,5-b′]dithiophene (BDT) and thienylbenzo[1,2-b:4,5-b′]dithiophene (BDTT) electron-donating units and an electron-withdrawing unit based on thiadiazolo[3,4-c]pyridine, PBDT-DTPyT, PBDT-T-DTPyT, and PBDTT-T-DTPyT were synthesized for polymer solar cells (PSCs). The copolymers exhibit good thermal stability, strong absorption in the visible region, and relatively lower HOMO energy levels from −5.22 to −5.38 eV. The results indicate thiadiazolo[3,4-c]pyridine led to enhanced π–π stacking of the polymer film owing to its stronger polarity. PBDTT-T-DTPyT exhibited a high hole mobility of 1.2 × 10−2 cm2 V−1 S−1. The interaction of the thiadiazolo[3,4-c]pyridine in the polymers and the hole extraction layer led to a non-ohmic contact at the interface. The conventional and inverted bulk heterojunction polymer solar cells based on PBDTT-T-DTPyT as the electron donor and (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) as the electron acceptor exhibited power conversion efficiencies of 1.68% and 2.08%, respectively.