Flexible Thin-Film CZTS Solar Cell based on an Electroplated Metallic Precursor Deposited on a Molybdenum/Glass Coated Stainless Steel Foil

The thin-film solar cells have recently become economically competitive to crystalline silicon and a much higher fraction of these cells are expected in the future. Especially, CZTS (copper, zinc, tin and sulfur) is a promising absorber for thin-film solar cells. This study is focused on the development of a process for fabrication of flexible CZTS thin-film solar cells based on an electroplated copper/zinc/tin (CuZnSn) precursor deposited on stainless steel foil with molybdenum back contact layer and glass barrier layer. The most challenging part is to develop a reproduceable process for electroplating on the thin molybdenum coating, since the molybdenum requires etching to remove the natural oxide layer and such a treatment is depending on the unknown thickness if the oxide layer. If too little is removed there will be poor adhesion to the subsequently deposited precursor, if too much is removed (i.e. also molybdenum metal is etched away) the electric contact and conductivity of the molybdenum electrode is compromised. The problem has been solved by an anodizing treatment of the foils. In our previous work of fabrication of CuZnSn precursors on molybdenum foil substrate, a two-step electroplating process of tin and CuZn alloy was found to be the most feasible [1]. However, the pure tin (Sn) top layer gives rise to formation of secondary phases during the sulfurization process. By introducing an additional step of copper electroplating at the top, homogenous CZTS formation on the Mo/Ti/glass coated stainless foil substrate has been obtained.