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We report on the chemical structure of Cu(In,Ga)Se2(CIGSe) thin-film solar cell absorber surfaces and their interface with a sputter-deposited Ga2O3buffer. The CIGSe samples were exposed to a RbF postdeposition treatment and an ammonia-based rinsing step, as used in corresponding thin-film solar cells. For a detailed chemical analysis of the impact of these treatments, we employed laboratory-based X-ray photoelectron spectroscopy, X-ray-excited Auger electron spectroscopy, and synchrotron-based hard X-ray photoelectron spectroscopy. On the RbF-treated surface, we find both Rb and F, which are then partly (Rb) and completely (F) removed by the rinse. The rinse also removes Ga–F, Ga–O, and In–O surface bonds and reduces the Ga/(Ga + In) ratio at the CIGSe absorber surface. After Ga2O3deposition, we identify the formation of In oxides and the diffusion of Rb and small amounts of F into/onto the Ga2O3buffer layer but no indication of the formation of hydroxides.

We present a detailed characterization of the chemical structure of the Cu(In,Ga)Se2thin-film surface and the CdS/Cu(In,Ga)Se2interface, 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)Se2interface by the chemical bath deposition of the CdS buffer.

We report on the initial stages of CdS buffer layer formation on Cu(In,Ga)Se2(CIGSe) thin-film solar cell absorbers subjected to rubidium fluoride (RbF) postdeposition treatment (PDT). A detailed characterization of the CIGSe/CdS interface for different chemical bath deposition (CBD) times of the CdS layer is obtained from spatially resolved atomic and Kelvin probe force microscopy and laterally integrating X-ray spectroscopies. The observed spatial inhomogeneity in the interface’s structural, chemical, and electronic properties of samples undergoing up to 3 min of CBD treatments is indicative of a complex interface formation including an incomplete coverage and/or nonuniform composition of the buffer layer. It is expected that this result impacts solar cell performance, in particular when reducing the CdS layer thickness (e.g., in an attempt to increase the collection in the ultraviolet wavelength region). Our work provides important findings on the absorber/buffer interface formation and reveals the underlying mechanism for limitations in the reduction of the CdS thickness, even when an alkali PDT is applied to the CIGSe absorber.