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

Abstract

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 oligothio-phene 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 degrees C in the thin-film state showed the highest maximum and average mu(FET) values of 0.194 and 0.141 cm(2) V-1 s(-1), respectively, in an FET device at a post-processing temperature of 100 degrees C. On the other hand, relatively low average mu(FET) values of 0.115, 0.098, and 0.079 cm(2) V-1 s(-1) were observed in FET devices prepared from PDQDB films with Tg values of 130, 165, and 180 degrees C, respectively, despite the dramatic increase in film crystallinity. With the variations in mu(FET), what we have noticed is that the standard deviations of the measured mu(FET) values varied with the T-g values: 36.0% for the T-g = 165 degrees C film and 51.1% for the T-g = 180 degrees 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.