Highly sensitive thin film phototransistors based on a copolymer of benzodithiophene and diketopyrrolopyrrole

A new copolymer (P(DPP4T-co-BDT)) was synthesized by Stille coupling polymerization of 3,6-bis(5′-bromo-[2,2′-bithiophen]-5-yl)-2,5-bis(2-octyldodecyl)pyrrolo-[3,4-c]-pyrrole-1,4(2H,5H)-dione and 2,6-bis(trimethyltin)-4,8-dimethoxybenzo[1,2-b:3,4-b′]dithiophene. P(DPP4T-co-BDT) showed good solution processability, good thermal stability with decomposition temperature of >330 °C, and strong and broad absorption in the range of 500–900 nm. Field-effect transistors based on P(DPP4T-co-BDT) thin films exhibited a hole mobility of up to 0.047 cm2 V−1 s−1, an on/off current ratio of 106, and a threshold voltage of −5 V after thermal annealing at 200 °C. Thin film phototransistors based on P(DPP4T-co-BDT) exhibited a photoresponsivity of up to 4.0 × 103 A W−1 and a photocurrent/dark-current ratio of 6.8 × 105 under white light irradiation with a low light intensity (9.7 μW cm−2).