Phases, morphology, and diffusion in $CuInxGa1−xSe2$ thin films Available to Purchase

$CuInxGa1−xSe2$ thin films, with various $Ga/(Ga+In)$ ratios, suitable for solar cells were processed by selenizing stacked Cu, Ga, and In precursor layers in a $H2Se$ reactor in the temperature range of 400–500 °C. Cu/Ga/In and Cu/In/Ga precursors were obtained by sequential sputtering of the elemental layers. The Cu/Ga/In and Cu/In/Ga precursors, and the selenized films were characterized by scanning electron microscopy, x-ray diffraction, energy dispersive spectroscopy, and Auger electron spectroscopy. The precursors contained only binary and elemental phases in the as-deposited condition and after annealing. The selenized films had a nonuniform distribution of Ga and In. The surface of the selenized films were In rich, while the Mo/film interface in these films was Ga rich. The selenized films with $Ga/(Ga+In)$ ratios greater than 0.25 contain graded Ga and In compositions, and the selenized films with $Ga/(Ga+In)$ ratios less than 0.6 contain a phase-separated mixture of $CuInSe2$ and $CuGaSe2$ with the $CuInSe2$ near the surface and the $CuGaSe2$ near the Mo/film interface. Single phase, homogeneous $CuInxGa1−xSe2$ films were obtained by annealing the as-selenized films in argon in the temperature range of 500–600 °C for 60 min. Interdiffusion of In and Ga between the $CuGaSe2$ and the $CuInSe2$ phases was found to be responsible for the homogenization process. This homogenization process does not occur in the presence of a selenium atmosphere. Diffusion measurements yielded similar interdiffusion coefficients for Ga and In. The annealing temperature and time to effect homogenization depends on the $Ga/(Ga+In)$ ratio of the absorber films. Films with lower $Ga/(Ga+In)$ ratios require a homogenization temperature of 600 °C or more and films with higher $Ga/(Ga+In)$ ratios homogenize at a lower temperature of 400–500 °C, for an annealing time of 60 min.