Cu depletion on Cu(In,Ga)Se-2 surfaces investigated by chemical engineering: An x-ray photoelectron spectroscopy approach

Photovoltaic cells based on CIGS [Cu(In,Ga)Se-2] absorber technology are among the most efficient thin film solar cells and already an industrial reality. Room for improvement is still possible in the manufacturing process to approach the theoretical ultimate efficiency. This not only requires an optimal absorber material but also the control of the CIGS interface chemistry, especially at the front side with the buffer layer which represents one of the main challenges. In this paper, thanks to x-ray photoelectron spectroscopy (XPS) analysis, the CIGS surface chemical composition is studied after acid (HCl) and basic (KCN) samples dipping. Both are regularly employed to prepare CIGS surfaces. XPS monitoring of the surface composition evolution under air aging at an ambient atmosphere and over a period of 120 days is presented, bringing fundamental information about the surface oxidation trends. If the HCl treatment gives a remarkable deoxidation state for the CIGS surface, it also yields a slightly Se enriched surface indicating the presence of a Cu2-xSe binary side phase, which is totally removed, as expected, by the KCN process. The present comparative study based on intentional air aging of starting HCl and KCN treated surfaces sheds light on the reorganization mechanism of this I-III-VI quaternary compound toward oxidation of clean CIGS surfaces, in ambient conditions. The oxidation process occurs concomitantly with an Na migration toward the surface, with soda-lime glass at the back contact, acting as a nonlimiting supply, asking the question of a surface mechanistic correlation during the CIGS surface oxidation.