Diffusion of Zn in solar-grade Cu(In,Ga)Se2 and single-crystal CuInSe2 thin films

The diffusion behaviour of Zn in polycrystalline CIGSe is found to be very similar to that in monocrystalline CISe, which indicates that grain boundaries only play a minor role as segregation sites and fast-transport pathways. The Zn diffusion coefficient shows a slightly stronger temperature dependence than Cd in CIGSe while its values are lower by about one order of magnitude. Surprisingly, the diffusion profiles exhibit peculiar shapes with a second maximum near the CI(G)Se-substrate interface. Zn may be relevant to CIGS technology as it is a major component of the transparent oxide layer. In addition, Zn may penetrate into the active zone during solar-cell processing when the CdS buffer layer is replaced by ZnS. The present results were obtained from diffusion experiments with the radiotracer Zn-65 using lamp-oven annealing followed by ion-beam sputter-sectioning. We discuss possible implications with regard to diffusion mechanisms and site occupancy.