Molecular engineering of (E)-1,2-bis(3-cyanothiophene-2-yl)ethene-based polymeric semiconductors for unipolar n-channel field-effect transistors

Abstract

The exploration of unipolar n-channel semiconductors plays an important role in the advance of organic complementary inverters and complementary logic circuits. Based on the conventional donor–acceptor type conjugated copolymers, the further incorporation of electron-withdrawing cyano groups would lead to π-electron depletion along the conjugated skeleton. Thus, the resulting copolymers possess much lower frontier orbital energy levels, which not only facilitates electron injection but also blocks hole accumulation effectively. In this work, we decorated (E)-1,2-di(thiophen-2-yl)ethene at the 3-position of thiophene rings with strong electron-withdrawing cyano groups. The resulting compound of (E)-1,2-bis(3-cyanothiophene-2-yl)ethene was further functionalized and copolymerized with isoindigo (IID), diazaisoindigo (AIID), and fluorinated isoindigo (FIID) units, affording copolymers of PIID-DiCNTVT, PAIID-DiCNTVT, and PFIID-DiCNTVT, respectively. The electrochemical and ultraviolet photoelectron spectroscopy characterizations reveal that all the resulting copolymers show low energy levels of the highest occupied molecular orbital, which blocks hole injection effectively. Field-effect transistor characterizations further confirmed that this series of polymers behave exclusively as unipolar electron-transporting materials. Compared with devices based on PIID-DiCNTVT and PFIID-DiCNTVT, PAIID-DiCNTVT-based devices exhibit the highest electron mobility of 1.58 cm² V⁻¹ s⁻¹, exceeding the semiconducting performance of amorphous silicon.