Enhancement of ambipolar charge transport for quinoidal bithiophene-diazaisoindigo-based donor−acceptor copolymers via fluorine substitution strategies

Abstract

Developing ambipolar polymer semiconductors is one of the important difficulties and challenges in the research field of organic semiconductors and has significant practical significance for the development of organic electronics. In this study, we designed and synthesized four kinds of quinoidal bithiophene-diazaisoindigo (QBTAIID) building blocks through several reaction steps such as nucleophilic addition, reduction, and dehydrogenation. Thereafter, four donor−acceptor (D−A) QBTAIID-based copolymers P1, P1-2F, P2 and P2-2F were developed. The quinoidal characteristics of QBTAIID units and the existence of S···O=C and N···S intramolecular noncovalent interactions endowed the D−A copolymers with high conjugated backbone coplanarity. The proximity of the branch points of the alkyl chains to the main chains reduced the frontier molecular orbital energy levels and achieved a denser lamellar packing, thereby improving charge transport performances. Moreover, fluorine substitution not only brought about lowered frontier molecular orbital energy levels but also improved thin film crystallinity. As a result, the fluorinated D−A copolymer P1-2F afforded the highest ambipolar transport characteristic with high hole/electron mobilities of 0.77/1.44 cm^2 V^−1 s^−1, in comparison with those of the other copolymers P1, P2 and P2-2F. The study highlights the great potential of quinoidal polymers in the construction of high-performance ambipolar organic field−effect transistors.