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

Abstract

${\mathrm{CuIn}}_x{\mathrm{Ga}}_{\text{1−x}}{\mathrm{Se}}_{\mathrm{2}}$ thin films, with various $\mathrm{Ga/(Ga+In)}$ ratios, suitable for solar cells were processed by selenizing stacked Cu, Ga, and In precursor layers in a ${\mathrm{H}}_{\mathrm{2}}\mathrm{Se}$ 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 $\mathrm{Ga/(Ga+In)}$ ratios greater than 0.25 contain graded Ga and In compositions, and the selenized films with $\mathrm{Ga/(Ga+In)}$ ratios less than 0.6 contain a phase-separated mixture of ${\mathrm{CuInSe}}_{\mathrm{2}}$ and ${\mathrm{CuGaSe}}_{\mathrm{2}}$ with the ${\mathrm{CuInSe}}_{\mathrm{2}}$ near the surface and the ${\mathrm{CuGaSe}}_{\mathrm{2}}$ near the Mo/film interface. Single phase, homogeneous ${\mathrm{CuIn}}_x{\mathrm{Ga}}_{\text{1−x}}{\mathrm{Se}}_{\mathrm{2}}$ 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 ${\mathrm{CuGaSe}}_{\mathrm{2}}$ and the ${\mathrm{CuInSe}}_{\mathrm{2}}$ 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 $\mathrm{Ga/(Ga+In)}$ ratio of the absorber films. Films with lower $\mathrm{Ga/(Ga+In)}$ ratios require a homogenization temperature of 600 °C or more and films with higher $\mathrm{Ga/(Ga+In)}$ ratios homogenize at a lower temperature of 400–500 °C, for an annealing time of 60 min.