Frequency-dependent complex conductivity of an organic thin-film transistor

We measure the complex impedance between source/drain electrodes and the gate electrode of a pentacene thin-film transistor (TFT) at frequencies 50 Hz < omega/2pi < 20 kHz. Modeling the TFT as a distributed RC network (RC transmission line), we find that the data cannot be explained by a model including only a real, frequency-independent sheet conductivity. Instead, we use the RC transmission line model to extract the frequency-dependent complex sheet conductivity sigma(omega) = sigma'(omega) + jsigma"(omega) of the pentacene film. At high frequencies, sigma(omega) increases with frequency, sigma'(omega) and sigma"(omega) become similar in magnitude, and the on/off ratio is significantly reduced.