KF post-deposition treatment of industrial Cu(In, Ga)(S, Se)2 thin-film surfaces: Modifying the chemical and electronic structure

Abstract

The chemical and electronic structures of industrial chalcopyrite photovoltaic absorbers after KF post-deposition treatment (KF-PDT) are investigated using electron spectroscopies to probe the occupied and unoccupied electronic states. In contrast to a variety of recent publications on the impact of KF-PDT, this study focuses on industrial Cu(In,Ga)(S,Se)2 absorbers that also contain sulfur at the surface. We find that the KF-PDT removes surface adsorbates and oxides and also observe a change in the S/Se ratio. Furthermore, the KF-PDT leads to a Cu reduction at the surface but to a much lower degree than the strongly Cu-depleted or even Cu-free surfaces reported for (non-industrial) sulfur-free Cu(In,Ga)Se2 absorbers. The valence band maximum at the surface is found at a lower energy compared to the untreated absorber, and the conduction band minimum is found at a higher energy, overall revealing a widening of the bandgap in the surface region.

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

“…Different from the behavior of Ga, In, and Se, we find a strong reduction of the Cu 2p signal on the ZSW absorber surface after RbF PDT. 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 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. ).…”

Limitation of Current Transport across the Heterojunction in Cu(In,Ga)Se2Solar Cells Prepared with Alkali Fluoride Postdeposition Treatment

“…A VBM shift and/or bandgap widening at the absorber surface could reduce the interface hole concentration, and a reduced carrier recombination near the absorber surface has been assumed in previous studies . Moreover, the downward shift of the VBM was considered as an outcome of the change of surface composition of the CIGS after PDT due to Cu and Ga depletion at the CIGS surface . A Cu‐depleted surface facilitates the in‐diffusion of Cd into the copper vacancies ( V Cu ) during the chemical bath deposition (CBD) of the CdS buffer layer, as suggested in previous studies .…”