Homogeneity of single phase Cu(In,Ga)Se2 produced by selenisation of metal precursors: An optical investigation

Two-stage processes involving the selenisation of metallic precursor layers are among the most promising techniques for the formation of chalcopyrite-based solar cell absorber layers on a commercial scale. In this paper, the homogeneity of Cu(In0.75Ga0.25)Se2 prepared by a new two-stage technique [V. Alberts, Semicond. Sci. Technol., 19 (2004) 65.], which involves the selenisation of sputtered CuIn0.75Ga0.25 precursor films in steps designed to control the reaction rates of the binary selenide phases and to prevent the formation of the more stable CuGaSe2 phase, is studied. Photoluminescence spectroscopy, optical absorption measurements and X-ray diffraction measurements confirm that layers grown by a traditional process, which involves a single selenisation step, contain separate quaternary phases: gallium-rich phases are found closer to the substrate, while gallium-poor phases reside near the front surface. Layers produced by the novel process do not show this grading. A line appearing at ∼ 0.8 eV is ascribed to NaIII, which results from the out-diffusion of Na from the glass substrate.