Mobility-independent doping in crystalline rubrene field-effect transistors

We report doping effects in an organic semiconductor, crystalline rubrene. Oxygen-related states are introduced (removed) by annealing in oxygen (vacuum), at an elevated temperature. Room temperature stability is found in the resulting effects: (1) about two orders of magnitude increase in carrier density at equilibrium, (2) significant modification of threshold voltages, and (3) an unchanged field-effect mobility in the on-current state. Density of states data are modeled as tunneling from the valence band in the channel region into deep-level acceptors in the adjacent region. These oxygen acceptors are the likely dopant species.