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Investigation of Chalcopyrit thin film solar cells

On a substrate (typically glass or foil) a molybdenum (Mo) back contact is deposited. The heterojunction within the solar cell is formed by a polycrystalline p-type CIGSe absorber layer (2 µm), a thin n-type CdS buffer layer (50 nm) and an n-type transpar

Thin film solar cells based on chalcopyrites, such as copper indium gallium diselenide Cu(In1-X,GaX)Se2 (CIGSe) with 0 < X < 1, are multi-layer systems. The figure above shows a scanning electron microscope (SEM) micrograph of a solar cell cross section. The semiconductor band gap is determined by the ratio of the matrix elements In to Ga. This ratio can be varied through the thickness of the absorber layer and the gradient of composition affects the photovoltaic properties of CIGSe thin films. Hence, the composition depth profile has to be optimized in order to obtain maximum efficiencies in solar cell applications.   To determine the elemental depth profile of CIGSe thin films, destructive and non-destructive techniques can be applied. For an accurate determination of the composition, most of the methods need standards that are more or less close to the composition of the measured sample. One non-destructive method which does not require well characterized standards for calibration purposes is reference-free X-ray fluorescence (XRF) analysis.   Synchrotron radiation based X-ray fluorescence analysis under grazing incidence conditions (GIXR) provides non-destructive access to the compositional depth profile of the CIGSe thin films. The detection of fluorescence photons in combination with well-characterized instrumentation allows for reference-free quantitative analysis, which does not require standard samples for calibration purposes. Based on an analytical description of the physical processes using fundamental parameters, fluorescence line intensities of the specimen can be calculated. For reliable quantification of measured fluorescence intensities of the specimen, absolute count rates have to be determined, which implies the knowledge of all experimental and instrumental parameters of the measurement set up [1].   The general suitability of the method for determining depth gradients in CIGSe thin films was already demonstrated by both calculations and reference-free XRF investigations using the instrumentation of the Physikalisch-Technische Bundesanstalt (PTB) at BESSY II [2].     Further investigations will be made in close collaboration with the Institute Technology of the Helmholtz-Zentrum Berlin and the PTB-Group Röntgenspektrometrie. Point of contact: Christian Herzog Relevant publications: Streeck, C; Brunken, S; Gerlach, M; Herzog, C; Honicke, P; Kaufmann, CA; Lubeck, J; Pollakowski, B; Unterumsberger, R; Weber, A; Beckhoff, B; Kanngiesser, B; Schock, HW; Mainz, R, Appl. Phys. Lett., 2013, 103(11), 113904

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