Modulated photocurrents in a sandwich-cell structure: II. Influence of bias illumination intensity and carrier diffusion

The influence of bias illumination level and carrier diffusion on the modulated photocurrents (MPCs), measured in amorphous solids in a sandwich electrode configuration, is investigated theoretically. Based on the multiple-trapping model the approximate formulae for MPCs, taking into account the mentioned physical factors, are derived. It is demonstrated that the absolute magnitude of the density of states can be determined from the MPC frequency spectra, measured at suitable bias illumination intensities. The carrier diffusion affects the MPCs solely for relatively high modulation frequencies and/or low applied voltages. The criterion for neglecting the diffusion effect is given. The corresponding frequency dependences of the photocurrent phase shift and amplitude as well as of the related quantities, calculated for the exponential trap distribution, are presented.