Alternating Copolymers of Thiophene-Flanked Thiazoloisoindigo and Thiophene-Flanked Benzothiadiazole for High-Performance Ambipolar Organic Field-Effect Transistors

Research on conjugated polymers with planar backbone has boosted the development of organic filed-effect transistors (OFETs) with high charge carrier mobility. In general, conjugated polymers with linear coplanar backbones are preferred for efficient charge transport, while those with twisted backbones are disfavored. Herein, two new ambipolar polymers with the same backbone (consisting of electron-deficient thiophene-flanked thiazoloisoindigo (TzII) unit and thiophene-flanked benzothiadiazole (BT)) but with/without hexyl sidechains on thiophene rings in BT units, namely P(TzII-BT) and P(TzII-BT)-C6, were synthesized and their charge transport properties were compared. Theoretical calculations predict that P(TzII-BT)-C6 has a moderately twisted backbone compared with P(TzII-BT) due to the steric hindrance caused by the sidechain, which is confirmed by UV–vis analysis. However, the OFET devices based on P(TzII-BT)-C6 exhibit higher average hole and electron mobilities (5.43 and 1.70 cm2 V−1 s−1) than those based on P(TzII-BT) (4.64 and 1.28 cm2 V−1 s−1). Grazing incidence wide angle X-ray scattering (GIWAXS) study shows that P(TzII-BT)-C6 is more crystalline than P(TzII-BT), and atomic force microscopy (AFM) study shows that it also has a smoother surface, and both factors contribute to its higher mobility. Overall, the comparative study demonstrates that the copolymerization of thiophene-flanked TzII and BT is effective to synthesize ambipolar polymers with high charge carrier mobilities, and a moderately twisted backbone caused by the sidechain may lead to higher crystallinity and better surface morphology and thus is beneficial for charge transport, which is an important factor to be considered in the design of conjugated polymer for high-performance OFETs.