Diffusion in polycrystalline thin-film layers was investigated. Bilayer structures of CuInSe2 on CuGaSe2, or vice versa, were fabricated by using Cu-rich and Cu-poor processes. In each case, Mo-coated soda-lime glass - with or without a Na barrier - was used. The bilayers were analyzed using secondary ion mass spectrometry, X-ray diffraction, scanning electron microscopy and transmission electron microscopy with energy-dispersive X-ray spectroscopy. It was found that grain-boundary diffusion was not significantly higher than the diffusion within grains. Diffusion was suggested to proceed mainly via vacant metal sites in the lattice. In Na-free films, greater diffusion into the bottom layers, as compared to films with Na, occurred in all cases. This was attributed to the presence of a larger number of vacancies; which facilitated In and Ga diffusion in the Na-free films.

Diffusion of Indium and Gallium in Cu(In,Ga)Se2 Thin Film Solar Cells. O.Lundberg, J.Lu, A.Rockett, M.Edoff, L.Stolt: Journal of Physics and Chemistry of Solids, 2003, 64[9-10], 1499-504