Fluctuation analysis of an organic semiconductor-insulator interface

The space-charge region of an organic semiconductor (OS)-insulator interface is probed by analyzing the spontaneous, thermally driven drain current fluctuations of a field-effect transistor in which the OS forms the gate electrode. This so called "excess drain current noise" is the outcome of local fluctuations of the Fermi level, resulting from stochastic exchange of electrons between traps near the Fermi level. The power spectral density of this noise is characteristic of a Lorentzian process with a distribution of time constants, which is attributed to the disorder in the OS film. Furthermore, this disorder leads to local inhomogeneity of the work function in the film and a finite correlation length of the work function fluctuations. The measurement of work function noise is only possible within a correlation length of the OS-insulator interface. Through systematic variation of gate voltage, primary doping and secondary doping levels, the correlation length, disorder, and the trapping/de-trapping time constant are examined on polyaniline as a representative OS. A model is proposed for local work function variations and spontaneous charge-carrier fluctuations within polyaniline films with consequences for organic electronics using organic semiconductors.