New insight into the modulated photocurrent technique using 2D full numerical simulations

Abstract

The modulated photocurrent technique has been used for a few decades now to provide information on the localized density of states (DOS) in semiconductor thin films. In previous studies, a simplified analytical treatment has been performed in 1D to derive equations that one has been using to reconstruct the DOS located near the majority carrier band. In these former studies, all the physical quantities were considered as constant within the sample depth, however, we know that it is not the case especially in the presence of a strong gradient of photogeneration in some materials. In this paper, for the first time, we show results of 2D simulations of the modulated photocurrent (MPC) experiment in order to study the effect of vertical photogeneration inhomogeneity on the MPC and to discuss the validity of the 1D assumption in more realistic cases. Thanks to these 2D simulations, new insight is gained in the understanding of the MPC experiment. Indeed, we show that we can explain and reproduce some of the experimental behaviors that the 1D theory failed to predict. Hydrogenated amorphous silicon (a-Si:H) is taken as a reference material for demonstration purposes, but this 2D approach can also be extended to other semiconductor materials and other types of inhomogeneities.