Tunable thin-film crystalline structures and field-effect mobility of oligofluorene-thiophene derivatives

Air-stable p-type semiconducting oligofluorene-thiophene derivatives were vacuum-deposited on octadecyltriethoxysilane-treated SiO2/Si substrates. Effects of end-substituents and substrate deposition temperature (T-D) on molecular orientation, crystalline morphologies, and structures in these thin films were investigated by two-dimensional grazing incidence X-ray diffraction and atomic force microscopy, and those results were correlated with charge mobility in top-contacted devices. Crystalline morphologies of the first monolayer thin film in direct contact with the dielectric surface, influenced by T(D)s (25, 90, and 140 degrees C) and end-substituted groups (hydrogen, hexyl, and dodecyl), could be categorized as dendrite, compact disk, and single-crystal-like layered grains. The results of grazing incidence X-ray diffraction strongly support that molecular orientation in the films can be finely tuned through controlling substrate, T-D, and molecular architecture, resulting in high air stability and field-effect mobility in a top-contacted electrode of organic thin film transistors.