Features of KF and NaF Postdeposition Treatments of Cu(In,Ga)Se2 Absorbers for High Efficiency Thin Film Solar Cells

The introduction of a KF postdeposition treatment (KF PDT) of Cu­(In,Ga)­Se2 (CIGS) thin films has led to the achievement of several consecutive new world record efficiencies up to 21.7% for the CIGS solar cell technology. The beneficial effect of the KF PDT on the photovoltaic parameters was observed by several groups in spite of differing growth methods of the CIGS layer. For CIGS evaporated at lower temperature on alkali-free, flexible plastic substrates, a postdeposition treatment to add Na was already successfully applied. However, with the introduction of additional KF under comparable conditions, distinctly different influences on the final absorber alkali content as well as surface properties are observed. In this work we discuss in more detail the intrinsically different role of both alkali-treatments by combining several microstructural and compositional analysis methods. The ion exchange of Na by K in the bulk of the absorber is carefully analyzed, and further evidence for the formation of a K-containing layer on the CIGS surface with increased surface reactivity is given. These results shall serve as a basis for the further understanding of the effects of alkali PDT on CIGS and help identify research needs to achieve even higher efficiencies.