Nucleation of Binary Cu-Se Phases and Electrochemical Cementation Mechanism on Polycrystalline Molybdenum Thin Films in Cu(II)-In(III)-Se(IV) Acidic Solutions

Abstract

The behavior of sputter deposited polycrystalline thin films of molybdenum covered glass plates, used as standard back contact for thin Cu(In, Ga)(S,Se)2 solar cells is examined while in contact with acidic aqueous solutions containing Cu(II), In(III) and Se(IV) species at open circuit potential. These solutions are used for the electrodeposition of CuInSe2 thin films. An electrochemical investigation was carried out, in combination with SEM-FEG observations, Raman spectroscopy and XPS surface analysis. It is shown that molybdenum undergoes an electrochemical cementation reaction. Its potential increases with immersion time. Immediately after immersion, small nuclei appear on the surface, which then grow to form a quasi-continuous layer after 600s. The chemical composition of the layer evolves with immersion time. No indium is incorporated. Cu-Se phases are formed whose composition changes from Se/Cu > 0.3 to a Se/Cu atomic ratio ~ 0.7. The final layer contains two phases, Umangite Cu3Se2 and CuSe.