Differential in-depth characterization of co-evaporated Cu(In,Ga)Se_2 thin films for solar cell applications

摘要

In this paper we report an alternative approach to perform in-depth characterisation of Cu(In,Ga)Se_2 (CIGS) absorber layers. While usually groups stop their co-evaporation process at different points and analyse the precursor or intermediate phases, we perform in-depth analysis on the finished absorber layer as it will be used in the solar cell. A co-evaporated CIGS layer was cut to several samples, which then were chemically etched to different thicknesses. Compared to sputtering ablation techniques, this avoids the selective abrasion of atoms with different binding energies. The samples were analysed by Raman spectroscopy and X-ray diffraction. In-depth information is obtained by differentiating the signals of samples with different thicknesses after etching and a first order correction for absorption losses was executed. The Ga/(Ga + In) ratio extracted from X-ray diffraction measurements is in good agreement with the double gradient observed by glow discharge optical emission spectroscopy. A slight variation might indicate residual stress in the CIGS layer. A preferred (112) orientation across the whole film together with changing (220), (116) and (312) orientation preferences is reported and explained on the basis of the CIGS crystal structure. Raman signals attributed to ordered vacancy compounds are found throughout the whole sample thickness and not only close to the surface, as often reported in the literature.