Modulated photocurrents in a sandwich-cell structure

Abstract

Approximate formulae describing modulated photocurrents in asandwich configuration of sample electrodes are derived. Thecarrier transport is described in terms of the multiple-trappingmodel. The formulae refer to arbitrary spatial distribution ofgenerated carriers. The case of exponential light absorption in thesample is considered in detail. It is shown that for limiting lowand high modulation frequencies, both the amplitude and the phaseshift of the photocurrent are almost independent of the lightabsorption coefficient. In the intermediate-frequency range, forthe case of surface carrier generation, the phase shift of thephotocurrent usually shows oscillations. This makes it possible toobtain the energetic density of states from a simple formula.Examples of calculated frequency dependencies of the photocurrentphase shift and amplitude are given for exponential and Gaussiantrap distributions as well as for a single trap level.