Grain size dependent mobility in polycrystalline organic field-effect transistors

Thin film transistors were made with vacuum evaporated sexithiophene and octithiophene. Scanning electron microscopy shows that the films are polycrystalline, with a grain size that increases when the temperature of the substrate is increased. The carrier mobility of the film increases with grain size. It is thermally activated for small grains, but becomes temperature independent for larger grains. These data are interpreted in terms of a model where charge transport is limited by trapping and thermal release at localized states located at grain boundaries.