Analysis of the modulated photocurrent experiment

Abstract

The analysis of the modulated photocurrent (MPC) experiment is based on the fact that empty gap states at a given energy, well above the respective quasi-Fermi level, are related to the phase shift and amplitude of the MPC of a specific frequency. This is used to extract the density-of-states (DOS) distribution of disordered semiconductors. However, only a limited region at higher frequencies of the experimental spectra corresponds usually to empty gap states. Moreover, the physics involved in the rest of the data is not quite clarified and thus cannot be used to extract any useful information about the DOS. In order to overcome this difficulty, simple theoretical relations of the real and imaginary terms of the MPC with specific gap states of any frequency are derived for the first time. The general expressions of the probing energy levels are presented suggesting that different gap states can be probed by changing not only the modulation frequency but alternatively the bias-light level also. The derived theoretical relations allow the determination of the energy profile of the trapping gap states that are at the quasi-Fermi level and above it, their capture coefficient, and the absolute magnitude of the recombination centers using all the experimental data. Also, by employing computer simulations, it is shown that contributions from different gap state distributions can be resolved directly from the experimental spectra without prior assumptions about the DOS.