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Depth Resolved Temperature Effects on CdS and Zn(O,S) Buffer Layers on Cu(In,Ga)Se2 Absorbers
R. Wenisch, T. Kodalle, N. Maticiuc, Y. Wang, T. Bertram, H.A. Yetkin, J. Deumer, C.A. Kaufmann, R. Schlatmann, I. Lauermann
Perovskites and Other Non-Silicon Materials, MJs and Tandems
Subtopic: CI(G)S, CdTe and Related Thin Films
Event: 37th European Photovoltaic Solar Energy Conference and Exhibition
Session: 3BV.2.40
ISBN: 3-936338-73-6
0,00 EUR
Document(s): poster


Cu(In,Ga)Se2 (CIGSe) based photovoltaic cells have been demonstrated to be ideal bottom devices in tandem solar cells for enhanced power-conversion efficiency. However, the interface between the absorber and the standard CdS buffer is stable only up to about 200°C limiting the choice of materials and excluding further high-temperature processing steps for the top cell. Here, we investigate the buffer degradation mechanisms above 200°C, responsible for the deterioration of the CdS/CIGSe interface. In the same way, an alternative sputter-deposited Zn(O,S) buffer-layer and the effects of RbF post-deposition treatment (PDT) on both CIGSe/buffer interfaces are investigated. The study employs in-situ, synchrotron-based, hard X-ray photoelectron spectroscopy yielding depth resolved information on concentration and chemistry of contained elements. In the experiments, the temperature was slowly increased from 150°C to 350°C while simultaneously taking repeated measurements, which results in a quasi-continuous temperature resolution. The measurements covered photoemission signals from Cu, In, Ga, Se, Na, Cd, Zn, O and S spectral regions. While a complete quantitative analysis of the recorded data is challenging, correlations between different elemental signals represent an alternative approach to understanding composition and chemistry. In this way, degradation temperatures and mechanisms can be identified. While CdS decomposes with Cd dissolving in the CIGSe and S reacting with Ga, Zn(O,S) remains relatively stable to eventually form Zn(SO4) at high temperatures > 250°C. RbF-PDT in turn, stabilizes the Zn(S,O) buffers; sulfate formation was not observed in this case. The CdS buffers are also stabilized by RbF-PDT. Moreover, RbF-PDT-induced Na depletion and Na diffusion at elevated temperatures, as reported previously1, are confirmed in this study. The onset of Na diffusion is found to be delayed to higher temperatures for CIGSe absorbers with RbF-PDT.