Diffusion of indium and gallium in Cu(In,Ga)Se₂ thin film solar cells

The diffusion of indium and gallium in polycrystalline thin film Cu(In,Ga)Se₂ layers has been investigated. Bilayer structures of CuInSe₂ on top of CuGaSe₂ and vice versa have been fabricated in both a Cu-rich and Cu-poor process (in relation to the ideal stoichiometry). In each process molybdenum coated soda-lime glass with and without a sodium barrier was used. These bilayers were analyzed with secondary ion mass spectrometry, X-ray diffraction, scanning electron microscope and transmission electron microscope equipped with energy dispersive X-ray spectroscopy. It was found that the grain boundary diffusion was not significantly higher than the diffusion inside the grains, also for Cu-rich layers. The diffusion is suggested to mainly proceed via vacant metal sites in the lattice structure. In sodium free films a higher diffusion into the bottom layers, compared to films with sodium, was seen in all cases. This observation was explained with a larger number of vacancies, that facilitates indium and gallium diffusion, in the sodium free films. The difference in diffusion between indium in CGS layers and gallium in CIS layers, in both Cu-rich and Cu-poor processes, was small for layers with sodium.