Overall Distribution of Rubidium in Highly Efficient Cu(In,Ga)Se₂ Solar Cells

Thin-film solar cells based on Cu(In,Ga)Se₂ (CIGS) absorbers have achieved conversion efficiencies close to 23%. Such a high performance could be reached by incorporating heavy alkali elements into the CIGS absorber using an alkali fluoride post-deposition treatment (PDT). In order to improve the understanding of the effect of the PDT, we investigated a highly efficient CIGS solar cell whose absorber was subjected to a RbF-PDT. By applying synchrotron-based X-ray fluorescence analysis in combination with scanning transmission electron microscopy and electron backscatter diffraction to a cross-sectional lamella of the whole device, we were able to correlate the local composition of the absorber with its microstructure. The incorporated Rb accumulates at grain boundaries, with a random misorientation of the adjacent grains, at the p-n junction, and at the interface between the absorber and the MoSe₂ layer. The accumulation of Rb at the grain boundaries is accompanied by a reduced Cu concentration and slightly increased In and Se concentrations. Additionally, variations in the local composition of the absorber at the p-n junction indicate the formation of a secondary phase, which exhibits a laterally inhomogeneous distribution. The improved solar cell performance due to RbF-PDT can thus be expected to originate from a favorable modification of the back contact interface, the random grain boundaries, the p-n junction, or a combination of these effects.