Phases, morphology, and diffusion in CuInxGa1-xSe2 thin films

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 degrees 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 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 degrees C or more and films with higher Ga/(Ga+In) ratios homogenize at a lower temperature of 400-500 degrees C, for an annealing time of 60 min.