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)Se2 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.