Analysis of Zinc Compound Buffer Layers in Cu(In, Ga)(S, Se) 2 Thin Film Solar Cells by Synchrotron-Based Soft X-Ray Spe
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Analysis of Zinc Compound Buffer Layers in Cu(In,Ga)(S,Se)2 Thin Film Solar Cells by Synchrotron-Based Soft X-Ray Spectroscopy I. Lauermann, M. Bär, A. Ennaoui, U. Fiedeler, Ch-H. Fischer, A. Grimm, I. Kötschau, M. Ch. Lux-Steiner, J. Reichardt, B. R. Sankapal, S. Siebentritt, S. Sokoll Hahn-Meitner-Institut, Berlin, Germany L. Weinhardt, O. Fuchs, C. Heske Experimentelle Physik II, Universität Würzburg, Germany C. Jung, W. Gudat BESSY, Berlin, Germany F. Karg, T. P. Niesen Shell Solar GmbH, München, Germany ABSTRACT Zinc-based buffer layers like ZnSe, ZnS, or wet-chemically deposited ZnO on Cu(In,Ga)(S,Se)2 absorber materials (CIGSSe) have yielded thin film solar cell efficiencies comparable to or even higher than standard CdS/CIGSSe cells. However, little is known about surface and interface properties of these novel buffer layers. In this contribution we characterize the specific chemical environment at the absorber/buffer-interface using X-ray Emission Spectroscopy (XES) and Photoelectron Spectroscopy (PES) in a complementary way. Evidence of intermixing and chemical reactions is found for different buffer materials and deposition methods.
INTRODUCTION Chalcopyrite thin film solar cells consist of a stack of different layers with specific functions. A typical structure is shown in Figure 1. So far, commercially available CIGSSe-cells require an n-type CdS-buffer layer between the p-type absorber and the n-type window material. The CdS-layer is usually formed using a chemical bath deposition (CBD) process. Because cadmium is toxic, replacement of CdS as buffer material has been aimed at by many research groups. Furthermore, the wet-chemical CBD process is difficult to integrate into an in-line production of CIGSSe modules and produces large amounts of toxic waste. It should preferably be replaced with “dry” methods like evaporation, CVD, or sputtering, or, at least, by wet-chemical processes with reusable solutions. Zinc chalcogenides like ZnS (deposited by CBD), ZnSe (deposited by chemical vapor deposition from metalorganic compounds, MOCVD) or ZnO (deposited by Ion Layer Gas Reaction, ILGAR) have been used as replacements for CdS and have yielded thin film solar cell efficiencies comparable to or even higher than those of standard CdS/CIGSSe cells [1-3]. However, the exact composition of these films as well as the possible presence of intermixing or chemical reactions at the buffer/absorber interface are, to a large extent, unknown. Furthermore, accelerated aging tests have revealed a reduced stability of CIGSSe cells with such buffer materials compared to standard cells. To study these buried interfaces, analytical methods are needed which yield depth information in a non-destructive way. Earlier work Zinc oxide window by Heske et al. [4,5,6,] has shown that X-ray 500nm emission spectroscopy (XES) is a valuable tool for Buffer 20 nm these examinations, in particular in combination with Absorber 1500 nm surface sensitive Photoelectron Spectroscopy (PES). Molybdenum back In XES, X-rays from a third-gene
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