X-ray absorption fine structure study of aging behavior of oxidized copper in CdTe films

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 2 μm thick polycrystalline films of CdTe on fused silica. 4 nm of evaporated Cu is diffused either with or without prior vapor CdCl₂ treatments in dry air. The phase-uncorrected radial distribution function inferred from the absorption fine structure indicates predominantly Cu₂Te when Cu is diffused into the as-deposited CdTe film but indicates a Cu₂O environment when Cu is 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. This Cu₂O likely plays a role in grain-boundary passivation. We also found that the chemical environment around Cu atoms in both CdTe and real cells can change with light soaking. This instability of Cu₂O in sputtered CdTe could contribute to cell degradation.