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 MoSe2 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 MoSe2 layer. The enhance light absorption in ultrathin absorbers using alternative, highly reflective back contacts are finally discussed. (C) 2011 Elsevier B. V. All rights reserved.