Importance of the functional group density of a polymeric gate insulator for organic thin-film-transistors

We describe the importance of the functional group density of polymeric insulators (PI) for organic thin-film-transistors (OTFTs) in terms of the insulator processing temperature and the exposure of a linearly polarized ultraviolet (LPUV) light. The PI layers processed at lower temperatures than the boiling temperature (T_b) of the solvent have higher densities of functional groups than those processed above T_b. The carrier mobility in the pentacene OTFT processed below T_b increases at least by a factor of three with maintaining other electrical properties such as the threshold voltage and the current on/off ratio. Our results suggest that the preferential alignment of the pentacene molecules is not the main physical mechanism for the mobility enhancement. From the mobility anisotropy resulting from the polarization of the LPUV, the packing density of the pentacene molecules on the PI laver, dictated primarily by the density of functional groups, is found to play a critical role on the magnitude of the mobility.