A benzo[1,2-b:4,5-b′]dithiophene-based copolymer with deep HOMO level for efficient polymer solar cells

A new D-A conjugated polymer containing a benzo[1,2-b:4,5-b′]dithiophene donor unit and bisthienyl-s-tetrazine acceptor unit, PBDT-TTz, was synthesized by Stille cross-coupling polymerization and was characterized by 1H NMR, elemental analysis and GPC. The copolymer shows good thermal stability with decomposition temperature of 276 °C and broad absorption band from 350 to 700 nm. Cyclic voltammetric measurement shows that the HOMO energy level of the copolymer is deep-lying (−5.42 eV), which implies that the copolymer has good stability in the air and assures a higher open circuit potential when it is applied in photovoltaic cells. Bulk-heterojunction polymer solar cells were fabricated using PBDT-TTz blended PC71BM as the active layer. The power conversion efficiency of the PSC is 3.32% under 100 mW/cm2 AM 1.5 illumination, indicating that PBDT-TTz is a potential candidate for polymer solar cells.