Influence of side-chain substitution for thiophene-based conjugated polymers on the charge transport properties: Carboxylate ester group versus carboxamide group

Polythiophene derivatives (PTs) have attracted intense research interests because of their facile synthesis compared to other conjugated polymers. In this study, we investigate and compare the side-chain substitution effects of two electron-withdrawing side chains, the carboxylate ester and the carboxamide groups, for a thieno [3,2-b]thiophene (TT)-based PT. Comparative studies including optical and electrical characterizations, molecular simulation, morphology, and transistor performance, are conducted. The results show that the carboxylate ester side chain enables the backbone to adopt a centrosymmetric conformation and induces a push-pull effect on the backbone. As a result, its derived polymer (P1) possesses a higher crystallinity and a superior intra-charge transfer (ICT). Whereas, the carboxamide side chain imposes a larger torsion on the backbone. Besides, it brings in a strong intramolecular dipole moment (μmz) along the side chain direction. It results in more intense aggregation but with lower crystallinity. Finally, P1 (with carboxylate ester side chain) and P2 (with carboxamide side chain) deliver dramatically different hole mobility (μh) values (1.19 × 10−2 and 1.48 × 10−5 cm2 s−1V−1), showing ∼1000 times difference. In addition, owing to the strong electron-withdrawing ability of the carboxylate ester group, P1 owns deep-lying energy levels and this enables air-stable mobility. The μh value of the P1 device shows negligible changes when storing in air for 30 days. Collectively, our result shows that two electron-withdrawing side chains exhibit distinctly different dipole moment directions that results in distinct charge transport properties.