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We present a detailed characterization of the chemical structure of the Cu(In,Ga)Se2 thin-film surface and the CdS/Cu(In,Ga)Se2 interface, both with and without a RbF post-deposition treatment (RbF-PDT). For this purpose, X-ray photoelectron and Auger electron spectroscopy, as well as synchrotron-based soft X-ray emission spectroscopy have been employed. Although some similarities with the reported impacts of light-element alkali PDT (i.e., NaF- and KF-PDT) are found, we observe some distinct differences, which might be the reason for the further improved conversion efficiency with heavy-element alkali PDT. In particular, we find that the RbF-PDT reduces, but not fully removes, the copper content at the absorber surface and does not induce a significant change in the Ga/(Ga + In) ratio. Additionally, we observe an increased amount of indium and gallium oxides at the surface of the treated absorber. These oxides are partly (in the case of indium) and completely (in the case of gallium) removed from the CdS/Cu(In,Ga)Se2 interface by the chemical bath deposition of the CdS buffer.

Abstract: Doping an indium sulfide buffer layer with sodium is a promising route to replace the “state‐of‐the‐art” CdS buffer layer in chalcopyrite‐based thin‐film solar cells, as it achieves efficiencies as high as 17.9% for large‐area devices (30 cm × 30 cm). We report on the chemical and electronic structure of the InxSy:Na/CuIn(S,Se)2 (CISSe) interface for thin‐film solar cells by means of photoelectron, soft x‐ray emission, and inverse photoemission spectroscopy. For as‐deposited InxSy:Na buffer layers, we find a sulfur‐poor surface and, in comparison to undoped InxSy and the standard CdS buffer, derive a large electronic surface band gap of 2.60 ± 0.11 eV. The conduction band offset at the buffer/absorber interface is a spike of 0.32 ± 0.10 eV. After annealing at 200°C to simulate the thermal load of subsequent cell manufacturing processes, an additional diffusion of copper and selenium from the absorber towards the buffer layer surface is observed, leading to a distinct electronic surface band gap decrease of the InxSy:Na buffer layer (to 2.11 ± 0.11 eV). We speculate that the diffusion of absorber elements causes a band gap widening at the former absorber surface and that both effects lead to a reduction of the conduction band spike for the buried InxSy:Na/CISSe interface after annealing. [ABSTRACT FROM AUTHOR]