Rubidium Fluoride Post-Deposition Treatment: Impact on the Chemical Structure of the Cu(In,Ga)Se₂ Surface and CdS/Cu(In,Ga)Se₂ Interface in Thin-Film Solar Cells

Abstract

We present a detailed characterization of the chemical structure of the Cu(In,Ga)Se 2 thin-film surface and the CdS/Cu(In,Ga)Se 2 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. 1 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)Se 2 interface by the chemical bath deposition of the CdS buffer.

Surface and Interface Properties in Thin‐Film Solar Cells: Using Soft X‐rays and Electrons to Unravel the Electronic and Chemical Structure

“…This is in accordance with other studies observing such a reduction in Cu content at the CIGSSe surface after KF PDT, but in contrast to studies suggesting a complete KF PDT‐induced removal of Cu from the surface . Besides these changes of the surface stoichiometry, K or (in the ZSW case) Rb is observed on the absorber surface after PDT, while a reduction of Na occurs (see Figure and refs. ).…”

“…The XPS survey spectra of ZSW absorber samples before and after RbF PDT are presented in Figure . The peak intensities of the Ga, In, and Se lines remain constant after PDT, and a quantification shows no changes in the Ga/(Ga+In) (GGI) ratio . This is different from the effect of the KF PDT on the Empa absorber surface, where the data suggest an almost complete removal of Ga from the surface, and from the Stion (industrial) line, where the KF PDT leads to a substantial surface cleaning and a significant increase in the intensity of all Ga XPS and Auger lines after PDT …”

“…Over the last few years, we have studied the influence of the alkali PDT for samples from different partners, both in research as well as industrial environments . While some general effects can be observed, we find that the effect of the alkali PDT differs for different manufacturers, which is likely caused by different initial absorber compositions and/or preparation procedures.…”

“…Besides these changes of the surface stoichiometry, K or (in the ZSW case) Rb is observed on the absorber surface after PDT, while a reduction of Na occurs (see Figure and refs. ).…”

Heavy Alkali Treatment of Cu(In,Ga)Se2Solar Cells: Surface versus Bulk Effects

“…The presented measurements indicate that RbF and CsF‐PDT result in an accumulation of the alkali element at the surface of the absorber, in a similar manner as after KF‐PDT . It should be noted that differences in the exact surface chemistry of KF and of RbF treated absorbers have been observed . The accumulation of potassium near the surface has been interpreted as the formation of a K‐In(Ga)‐Se compound, sometimes labeled the “KIS layer.” The formation of a novel compound is supported by the observation that the surface bandgap of KF‐treated low‐temperature absorbers is considerably higher than that of untreated ones …”