Numerical simulation and validity of the surface photovoltage method in amorphous silicon with a Schottky contact

Abstract

The steady-state surface photovoltage technique (SPV) is widely used to evaluate minority photocarriers diffusion length for the characterization of photovoltaic device performance. This contact technique shows the difficulty to evaluate the true minority photocarriers diffusion length (L p ) of hydrogenated amorphous silicon (a-Si:H) even though the space charge region is reduced. In this work, we propose a formula to predict more accurately minority diffusion length L p depending on apparent diffusion length (L app ) and space charge width (W). This formula is deduced by Moore's analytical solution in relation to other forms. All transport phenomena taken into account, a complete one-dimensional numerical simulation of the SPV on an Au/a-Si:H Schottky structure is developed. The apparent diffusion length L app and the space charge width W are determined using SPV simulation. The standard density of states (DOS) of the a-Si:H is used. Poisson's equation and the two continuity equations for charge carriers are numerically solved. The SPV assumptions involving W are explored for a set of parameters of the a-Si:H DOS. This numerical simulation shows a linear dependence of L app up on the valence band tail energy, which agrees well with the experimental results. The L p values obtained by the new formula are compared to L app and discussed for a typical DOS parameters of a-Si:H.