Thiophene-thiazole based Semiconducting Copolymers for High Performance Polymer Field-effect Transistors.

We report a newly-synthesized donor (D) - acceptor (A) type semiconducting copolymer, consisting of thiophene as an electron-donating and thiazole as an electron-accepting unit (PQTBTz-TT-C8) for the active layer of the organic field-effect transistors (OFETs). Specifically, this study investigates the structure and electrical property relationships of PQTBTz-TT-C8 with comprehensive analyses on the charge-transporting properties corresponding to the spin rate of the spin-coater during the formation of the PQTBTz-TT-C8 film. The crystallinity of PQTBTz-TT-C8 films are examined with grazing incidence X-ray diffraction (GIXRD). Temperature-dependent transfer measurements of the OFETs are conducted to extract the density of states (DOS) and characterize the charge-transport properties. The comparative analyses on the charge-transports within the framework of the physical model, based on polaron hopping and Gaussian DOS, reveal that the pre-factors of both physical charge transport models are independent of the spin-coating condition for the films. For staggered structural transistors, however, the thickness of the PQTBTz-TT-C8 films, which strongly affect the series resistance along the charge transfer path in a vertical direction, is changed in accordance with the spin-coating rate. In other words, the spin-coating rate of the PQTBTz-TT-C8 films influence the thickness of the polymer films, yet any significant changes in the crystallinity of the film or electronic coupling between the neighboring molecules upon the spin-coating condition were barely noticeable. Since the PQTBTz-TT-C8 backbone chains inside the thin film are stacked up with the edge-on, the series resistances are changed according to the thickness of the film, and thus the performance of the device varies depending on the thickness.