The chemical environment about Cd atoms in Cd chemical bath treated CuInSe₂ and CuGaSe₂

Recent reports in the literature have concluded that chemical bath deposited Cd (CBD-Cd) has beneficial effects on Cu(In, Ga)Se₂ solar cell performance. Before detailed mechanisms explaining why this is so can be proposed, a key question that must be answered is where in the lattice the Cd atoms reside. We have used near-edge X-ray absorption fine structure (XANES) to selectively investigate the local atomic environment about Cd atoms in CBD-Cd treated polycrystalline CuInSe₂ and CuGaSe₂ films. CBD-Cd treated CuInSe₂ and CuGaSe₂ layers grown on Mo-coated glass substrates using the three-stage method were excited by synchrotron radiation at the Cd L₃-edge (3537 eV) and the resulting XANES spectra were recorded. First-principles calculation using the code feff 8.10 show that the XANES spectra are consistent with Cd atoms being located in cation sites in the CuInSe₂ matrix. From the current data it is not possible to definitively state to what extent Cd resides on the group I or III sites, but it is clear that there is a contribution from both, not from only Cu sites as has been previously speculated in the literature. This is also found to be consistent with predictions for ion-exchange reactions which show that Cd is more likely to substitute for (surface) In than Cu.