We report on the surface-induced molecular order and the optical anisotropy of a pentacene thin film on a photoaligning polymeric gate insulator as a function of the annealing temperature (Ta). We used the poly(vinyl cinnamate) (PVCi) as a gate insulator for organic thin film transistors (OTFTs). The PVCi is commonly used a photo-polymer to align liquid crystal molecules. The polymer insulator, annealed at a certain temperature lower than the boiling temperature (Tb) of the solvent, has a higher density of functional groups than that annealed at a temperature higher than Tb. As a consequence, the pentacene molecules on the insulator annealed below Tb show a larger retardation value shown as Fig. 1. It means that the pentacene molecules in Fig. 1(a) are more ordered than in Fig. 1(b). Pentacene molecules are rod-shaped like liquid crystal molecules so that they can be ordered on the insulator layer. It was found that the functional group density of the insuating polymers strongly affects the ordering and the grain size of pentacene molecules evaporated on the insulator layer although no difference in the macroscopic properties such as the surface roughness and the contact angle was observed. When Ta < Tb, the optical anisotropy of the pentacene film increases and the grain size becomes large. This results in better performances of the OTFTs.

This work was supported in part by Samsung SDI-Seoul National University Display Innovation Program and the Ministry of Science and Technology of Korea through the 21st Century Frontier Research and Development Program at the Information Display Center.