Precise Control over Polymer Semiconducting Films by Tuning the Thermal Behavior of the Thin-Film State's Crystalline...

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 T_g and T_m values of the thin-film state, rather than those of the bulk polymer state, were important in this study. A PDQDB film with a T_g of 101 °C in the thin-film state showed the highest maximum and average μ_FET 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 μ_FET values of 0.115, 0.098, and 0.079 cm² V^-1 s^-1 were observed in FET devices prepared from PDQDB films with T_g values of 130, 165, and 180 °C, respectively, despite the dramatic increase in film crystallinity. With the variations in μ_FET, what we have noticed is that the standard deviations of the measured μ_FET values varied with the T_g values: 36.0% for the T_g = 165 °C film and 51.1% for the T_g = 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.