Charge transport in polycrystalline oligothiophene thin film transistors

Vacuum evaporated films of sexithiophene (6T) and octithiophene (8T) are polycrystalline, with a grain size that depends on the temperature of the substrate on which they are deposited. Thin film transistors were made with 6T and 8T films of various grain sizes. Data are interpreted according to a model where charge transport is limited by charge trapping and release at states located within grain boundaries. It is shown that the mobility increases with grain size. Moreover, a sharp contrast occurs between small grains, where the mobility is thermally activated, and large grains, where the mobility becomes practically temperature independent. The latter feature is ascribed to tunnelling through the energy barrier that forms at grain boundaries.