Electrochemical etching of molybdenum for shunt removal in thin film solar cells

High yield and reproducible production is a major challenge in up-scaling thin film Cu(In,Ga)Se-2 (CIGS) solar cells to large area roll-to-roll industrial manufacturing. Pinholes enabling Ohmic contact between the ZnO:Al front-contact and Mo back contact of the CIGS cell create electrical shunts that are detrimental to the output of the cell and production yield. This paper describes a self-limiting electrochemical etching method to reduce shunts by dissolving Mo exposed through pinholes in a CIGS cell. Anodic polarisation measurements show that Mo oxidizes at a high rate in alkaline solutions. At pH 14 the current density was sufficient to allow fast Mo oxidation. However, at this pH the Mo film is initially converted to a MoO2 film that retards further oxidative dissolution. Addition of K3Fe(CN)(6) as oxidising agent accelerates the MoO2 film dissolution resulting in complete Mo film removal in a few minutes. Standard and shunted CIGS cells treated with the Mo etching solution showed a reduction in the number of low shunt resistant areas. In shunted CIGS cells an increase in cell conversion efficiency from 3 to 5.9 % was also found.