Cu out-diffusion in kesterites—A transmission electron microscopy specimen preparation artifact

Solar cells based on Cu2ZnSn(S,Se)4 absorber layers have received a growing amount of interest. Typically a Mo(S,Se)2 layer is formed at the Cu2ZnSn(S,Se)4/Mo interface during processing. Transmission electron microscopy (TEM) analyses showed the presence of Cu in the Mo(S,Se)2 which was thought to cause secondary phase formation at the back contact. However, preparing TEM samples can induce artifacts leading to false conclusions. It is therefore of great importance to identify such artifacts. In this work, we show that the Cu presence in the Mo(S,Se)2 stems solely from TEM sample preparation and does not occur as part of the synthesis process.