The effect of the substrate on KF post deposition treatments of narrow gap Cu(In,Ga)Se2 absorber layers

Abstract

Alkali post deposition treatments (PDT) are the standard method to increase the efficiency of Cu(In,Ga)Se2 solar cells. In this study, the effects of potassium fluoride (KF) PDTs on narrow band gap Cu(In, Ga)Se2 (CIS) layers are investigated. The CIS layers were grown on substrates such as glass with alkali-barrier/Mo, glass/Mo, and glass/indium-doped tin oxide. It was found that the effect of the PDT depends on the substrates and that there are conditions under which KF-PDT is detrimental to solar cell performance. Time-of-flight secondary ion mass spectrometry measurements revealed limited ion exchange between Na and K, which caused inhibited diffusion of K into the absorber layer. Further opto-electrical characterization indicated increased recombination in the solar cell. Capacitance-voltage-frequency measurements combined with modelling revealed the formation of an interface defect that is limiting the open circuit voltage and reducing the fill factor. Our findings suggest that the lack of K diffusion into the absorber layer promotes the formation of defects at the surface. This study highlights the complex interaction between alkali coming from PDT and the alkali already present in the absorber layer.