Cu K-edge X-ray fine structure changes in CdTe with CdCl2 processing

We have used the MR-CAT beamline of the Advanced Photon Source at Argonne National Laboratory to study the fine structure in the Cu K-edge X-ray absorption in 3 μm thick polycrystalline films of CdTe on fused silica. About 4 nm of evaporated Cu is diffused either with or without prior vapor CdCl2 treatments in dry air. Cu absorption is monitored through the Cu Kα fluorescence using a 13-element Ge detector. The absorption fine structure indicated predominantly Cu2Te when Cu was diffused into the as-deposited CdTe film but a CuxO environment when Cu was diffused after the vapor CdCl2 treatment. We believe most of the diffused Cu decorates grain boundaries as oxides, consistent with the low doping densities typically observed in CdTe solar cells. The significance for grain boundary passivation will be discussed.