Determination of the density of localized states in disordered semiconductors using transient photocurrent spectroscopy with a finite-width light pulse excitation

An expression for the density of states (DOS) distribution in disordered semiconductors based on transient photocurrent (TPC) spectroscopy is described, which includes the integration of the light pulse width parameter. Whereas the original interpretation of TPC spectroscopy assumes a time delta-function for the carrier generation rate, the present theory treats the more realistic case of the photo-response to a light pulse of a finite pulse width, covering the rise and decay time ranges. A pulse-width-dependent DOS expression is derived and evaluated in comparison with the original TPC spectroscopy, by combined simulation and experiment in the case of a-Si:H at 300 K for a pulse width of 5 mu s. It is shown that at a given temperature the modified TPC spectroscopy enables states at shallower energies to be probed with improved accuracy and resolution. The spectroscopy limitation related to saturation of states by an excessively high photogeneration rate is demonstrated.