Real-time study of Ga diffusion processes during the formation of Cu (In,Ga)Se2: The role of Cu and Na content

We study by means of real time X-ray diffraction the effect of the Cu and Na content on the diffusion of Ga during the formation of Cu(In,Ga)Se-2 films for solar cell applications. We analyze the diffraction data recorded during the annealing of stacks of different compositional ratios. A model for the film formation is suggested, which relies on two distinct steps: accumulation of Ga near the Mo back contact and In-Ga-interdiffusion. The process of Ga-acumulation near the back contact is stronger for the films containing Na. The interdiffusion step starts at about 750 K and is strongest for films with low Na content. We observe that Cu-Se strongly enhances the interdiffusion when using a barrier to prevent Na diffusion from the glass substrate. Microstructural characterization of films with different copper content shows that the steepest Ga-depth-profiles are obtained for a [Cu]/([In]+[Ga]) ratio of about 1. (C) 2013 Elsevier B.V. All rights reserved.