A new thieno-isoindigo derivative-based D–A polymer with very low bandgap for high-performance ambipolar organic thin-film transistors

A new thieno-isoindigo derivative, bis(5-oxothieno[3,2-b]pyrrole-6-ylidene)benzodifurandione (BTPBF) was synthesized by replacing the outer benzene ring of the isoindigo derivative (BIBDF) with thiophene. This was used for the first time as an acceptor to construct a donor–acceptor polymer (PBTPBF-BT) for organic thin-film transistors. The thermal stability, photophysical-, and electrochemical properties, microstructure, and transistor characteristics were also investigated. Compared to the isoindigo derivative-based polymer PBIBDF-BT, the thieno-isoindigo derivative-based polymer PBTPBF-BT had a much smaller bandgap (0.71 eV), a similar deep LUMO level, (−3.94 eV) and a higher HOMO level (−5.18 eV). Therefore, when Au metal was used as the electrode and the devices were tested under similar vacuum conditions, PBIBDF-BT exhibited n-type transport, whereas PBTPBF-BT exhibited ambipolar transport due to the deep LUMO and suitable HOMO levels. Highest mobilities of 0.45 and 0.22 cm2 V−1 s−1 were obtained for hole and electron, respectively. Under the air condition, the hole mobility of PBTPBF-BT significantly increased to 0.61 cm2 V−1 s−1 and electron mobility was maintained at 0.07 cm2 V−1 s−1. Overall, this study demonstrates that replacing the outer benzene with thiophene can effectively vary the polymer properties such as bandgap, energy levels and, as a result, tune the transport behavior.