A stochastic model for Cu(InGa)(SeS)2 absorber growth during selenization/sulfization

The optical and electronic properties of Cu(InGa)(SeS)₂ absorbers are functions of the gallium and sulfur fractions, defined as the mole fractions Ga/(In+Ga) and S/(Se+S). During absorber growth, through-film composition gradients arise, and these gradients affect the photovoltaic performance of the solar cell. However, there has been little effort in quantitative prediction of the through-film composition. In this work, we present a stochastic model to simulate film growth during selenization and sulfization processes. Our goal is to predict the through-film composition of a Cu(InGa)(SeS)₂ absorber film, especially the gallium and sulfur gradients. Our model can predict the steep gradient in gallium that is observed during the selenization reaction and the gallium homogenization that occurs with the sulfurization reaction. Although the model requires a large number of parameters, we show how they can be estimated from simplifying assumptions and related to physical properties. We believe that this method is the first attempt to model explicitly the through-film composition of a Cu(InGa)(SeS)₂ produced via selenization.