Annealing-Induced Effects on the Chemical Structure of the In2S3/CuIn(S,Se)2 Thin-Film Solar Cell Interface

We have investigated the impact of heat treatments on the chemical structure of the In2S3/CuIn(S,Se)2 thin-film solar cell interface using X-ray photoelectron and soft X-ray emission spectroscopy. As-grown, we find the formation of a sulfur-poor (indium-rich) In2S3 surface, an abrupt interface, and sulfur atoms in both In2S3 and CuIn(S,Se)2 chemical environments (as expected for an abrupt interface and a thin overlayer). After a heat treatment at 200 °C to simulate subsequent process steps, a strong copper and sodium diffusion into the In2S3 layer is observed. This diffusion extends throughout the layer, indicating the formation of a copper–indium–sulfide phase.