The crystalline and morphological structures of polymer semiconducting films were controlled by selecting appropriate thermal properties of the polymeric chains, thereby improving polymer field-effect transistor (FET) performances. Poly(dioctyl-quaterthiophene-dioctyl-bithiazole) (PDQDB), comprising 5,5'-bithiazole and oligothiophene rings, was used as the basis for the polymer semiconductor studies. The Tg and Tm values of the thin-film state, rather than those of the bulk polymer state, were important in this study. A PDQDB film with a Tg of 101 °C in the thin-film state showed the highest maximum and average μFET values of 0.194 and 0.141 cm2 V-1 s-1, respectively, in an FET device at a post-processing temperature of 100 °C. On the other hand, relatively low average μFET values of 0.115, 0.098, and 0.079 cm2 V-1 s-1 were observed in FET devices prepared from PDQDB films with Tg values of 130, 165, and 180 °C, respectively, despite the dramatic increase in film crystallinity. With the variations in μFET, what we have noticed is that the standard deviations of the measured μFET values varied with the Tg values: 36.0% for the Tg = 165 °C film and 51.1% for the Tg = 180 °C film, indicating that the organic field-effect transistors performances were not uniform. These results were closely related to nano- and microscale nonuniformity in the PDQDB film structure in the presence of excessively activated grain structures. These variations were correlated with the crystalline and morphological structures of the PDQDB films prepared under various processing conditions.
Keywords: field-effect mobility; glass-transition temperature; melting temperature; polymer semiconductor; structural control; thermal post-process; thin-film state.