X‐Ray and Electron Spectroscopy of the CdS/(Ag,Cu)(In,Ga)Se₂ Interface With RbF Treatment

Abstract The chemical and electronic structure of the CdS/(Ag,Cu)(In,Ga)Se 2 (CdS/ACIGSe) interface for thin‐film solar cells, involving an absorber with a bulk [Ag]/([Ag]+[Cu]) (AAC) ratio of 0.06, a state‐of‐the‐art RbF post‐deposition treatment (PDT), and a chemical‐bath deposited CdS buffer layer, is studied. To gain a detailed and depth‐resolved picture of the CdS/ACIGSe interface, synchrotron‐ and laboratory‐based hard X‐ray, soft X‐ray, and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and X‐ray emission spectroscopy are combined. Compared to the bulk of the absorber, a Cu‐ and Ga‐poor ACIGSe surface is found, with a slightly increased AAC ratio. Strong evidence of a Rb–In–Se species (possibly with some Ag) at the absorber surface is compiled, with a corresponding band gap of 2.79 ± 0.12 eV. This finding is in clear contrast to comparable Ag‐free Cu(In,Ga)Se 2 absorbers with RbF‐PDT. The Rb–In–Se surface species is not removed by the (wet‐chemical) CdS deposition process, while some Se diffuses into the CdS layer and segregates at its surface. The CdS buffer layer shows a band gap of 2.48 ± 0.12 eV, and a cliff (≈ −0.4 eV) is determined in the conduction band alignment at the interface between the Rb–In–Se species and the CdS buffer.