Keun Cho

Fluorinated benzothiadiazole and indacenodithieno[3,2-b]thiophene based regioregular-conjugated copolymers for ambipolar organic field-effect transistors and inverters

We report the synthesis and characterization of a series of three soluble polymeric hybrids in benzothiadiazole–indacenodithieno[3,2-b]thiophene based ladder-type polymers (BT–IDTT) containing the acceptor unit 3,6-dithien-2-yl-2,5-dialkylpyrrolo[3,4-c]pyrrole-1,4-dione (DTDPP), and donor moieties 7,8-bithienyl benzo[1,2-b:4,5-b′]di-thiophene (BDTT) and 4,8-bis(5-ethylhexylselenophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDTSe) in BT–IDTT–DTDPP (P1), BT–IDTT–BDTT (P2) and BT–IDTT–BDTSe (P3) copolymers to tune the strength of ambipolar charge-carrier transport properties for organic field-effect transistors (OFETs) and ambipolar complementary inverters. A narrow band gap of 1.4–1.7 eV was achieved by the careful selection of the electron donor–acceptor unit and strong packing of those polymers in the film state. In particular, a very narrow bandgap of 1.38 eV and well-balanced ambipolar transistor characteristics were achieved by replacing benzodithiophene in P2 and P3 with diketopyrrolopyrrole in P1. Optimized top gate bottom contact OFETs with P1 polymer showed electron and hole mobilities of 0.015 and 0.007 cm2 V−1 s−1 respectively, and inverter gain of ∼14 using poly(methyl methacrylate) gate dielectric. Interestingly, hole transport properties were improved to a mobility of 0.1 cm2 V−1 s−1 in all OFETs with the BT–IDTT ladder-type polymer using fluorinated P(VDF-TrFE) dielectric, which can be ascribed to the increased hole accumulation at the semiconductor–dielectric interface.