Mo/Cu(In, Ga)Se ₂ back interface chemical and optical properties for ultrathin CIGSe solar cells

Chemical and optical properties of the interface between a coevaporated Cu(In,Ga)Se ₂ (CIGSe) absorber thin film and the Mo back contact are investigated with the objective to reduce markedly the thickness of CIGSe layers from two microns down to about 100 nm. First a mechanical lift off technique allowed to separate Mo and CIGSe layers and perform X-ray photoelectron spectroscopy (XPS) and elipsometry studies on as prepared surfaces. On the Mo side small amounts of In and Ga are observed together with the formation of an MoSe ₂ layer. There is no evidence of the presence of Cu. On the opposite CIGSe side a clear depletion of Cu together with an enrichment of Ga is evidenced. There is no evidence of Mo. Optical reflectivity of the interface CIGSe/Mo is studied by ellipsometry showing a low reflectivity of the interface attributed to the formation of MoSe ₂ layer. The enhance light absorption in ultrathin absorbers using alternative, highly reflective back contacts are finally discussed.