Low bandgap copolymer of 1,4-diketopyrrolo[3,4-c]pyrrole and thieno[3,2-b]thiophene: Synthesis and applications in polymer solar cells and field-effect transistors

A new conjugated polymer, poly(3,6-dihexyl-thieno[3,2-b]thiophenyl-2,5-diyl-alt-2,5-di(2-ethylhexyl)-3,6-bis(thien-2,5-diyl)-pyrrolo[3,4-c]-pyrrole-1,4-dione) (DH-PTTDPP) was synthesized by Pd-catalyzed Stille-coupling method. The thermal, absorption, electrochemical, photovoltaic properties, and field-effect charge transport of the polymer were examined. The polymer is soluble in common organic solvents and possesses a broad absorption band covering from 500 nm to 900 nm with an optical bandgap of 1.33 eV. Field-effect transistors (FETs) based on DH-PTTDPP thin film showed a hole mobility of 2.5 × 10−3 cm2 V−1 s−1 with an on/off ratio of 105, which is among the high values for the conjugated polymers. Polymer solar cells (PSCs) based on DH-PTTDPP/PC70BM (1:1.5, w/w) demonstrated a power conversion efficiency of 1.90% under the illumination of AM 1.5, 100 mW cm−2. These results indicate that DH-PTTDPP is an attractive polymer semiconductor for the applications in FETs and PSCs.