XRF and high resolution TEM studies of Cu at the back contact in sputtered CdS/CdTe solar cells

We have used the MR-CAT beam-line of the Advanced Photon Source (APS) at Argonne National Laboratory to study Cu x-ray fluorescence (XRF) in CdS/CdTe/Au structures on fused silica without transparent conductive oxide (TCO). After a “peel-off” process that separates the back contact at the CdTe/Cu interface both parts were studied by XRF. In order to locate the Cu spatially, we prepared TEM cross-sections and performed analytical electron microscopy at the Electron Microbeam Analysis Laboratory (EMAL) at the University of Michigan, to obtain high-resolution electron microscopy (HREM) imaging with a point-to-point resolution of 1.7 Å together with fine-probe energy-dispersive x-ray spectroscopy (EDX). We found direct evidence of an interfacial Cu-rich layer between CdTe and Au with thickness of ∼ 4 Å and relatively high concentration of Cu in the Au layer. Based on previous EXAFS studies we suggest that a copper-oxide layer is formed at the CdTe/Au interface which might serve to inhibit diffusion of copper into the CdTe layer.