Influence of Mo Back-Contact Oxidation on Properties of CIGSe2 Thin Film Solar Cells on Glass Substrates

Abstract

Copper indium gallium diselenide (CIGSe) solar cells grown on glass substrates have reached an efficiency of 20.3%. Their industrial production is becoming increasingly relevant. While various deposition techniques for the fabrication of the absorber are used by different groups and corporations, molybdenum (Mo) has become the back contact material of choice. Oxidation of the bare Mo layer prior to absorber deposition is a phenomenon that is generally hard to control or to avoid. Since the incorporation of sodium (Na) into the absorber layer is commonly achieved by diffusion from a glass substrate through the Mo layer, oxidation of the back contact will influence the diffusion, and thus the availability of Na during the CIGSe growth process. In order to investigate this effect, Na containing glass substrates with Mo layers in different stages of oxidation have been prepared using a damp heat treatment. The samples were coated with CIGSe by physical vapor deposition in a multistage co-evaporation process. The CIGSe/Mo-interface is investigated by Raman spectroscopy and secondary ion mass spectroscopy, using a lift-off technique. The damp heat treatment led to the formation of an oxide layer (presumably MoO2) and an increase of the sodium content in the grown absorber layers.