Thermally activated sulfurization of In-Cu bilayers for CuInS<inf>2</inf> solar cells

Abstract

Sulfur vapor or H 2 S is often used for the fabrication of CuInS 2 thin film solar cells by sulfurizing Cu/In bimetallic layers at elevated temperature over 400 °C. In order to promote low temperature sulfurization and/or faster sulfurization, sulfur vapor was thermally activated before it reaches Cu/In metallic precursor layers for the formation of CuInS 2 thin films. The thermal activation of sulfur at over 600 °C dissociates high molecular weight sulfur molecules primarily comprised of S8 into smaller size molecules. Thermodynamic calculation predicted more spontaneous sulfurization process with S 2 molecules. However, experimental results indicated that more complex phase transformation process was occurring with the thermally activated sulfur molecules. While In-rich Cu/In bimetallic precursor layers only experienced minor compositional change in Cu/In ratio after sulfurization, Cu-rich films have lost significant amount of In resulting increased Cu/In ratio after sulfurization process. The compositional change of the Cu-rich bilayers was sensitive to the temperature at which sulfur molecules were activated (dissociated). In general, more indium loss was observed with a higher activation temperature. The Cu/In ratio was reduced down slightly below 1.0 by KCN etching of the sulfurized CuInS 2 films indicating the existence of CuS phase and also indium binary phase, both observed in XRD patterns.