Synchrotron-based spectroscopy for the characterization of surfaces and interfaces in chalcopyrite solar cells
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Synchrotron-based spectroscopy for the characterization of surfaces and interfaces in chalcopyrite solar cells Iver Lauermann, Paul Pistor, Immo Kötschau, and Marcus Bär Hahn-Meitner-Institut, Glienicker Str. 100, D-14109 Berlin, Germany ABSTRACT In this paper we describe synchrotron based, state-of-the-art spectroscopic methods for the analysis of surfaces and interfaces in thin film photovoltaic devices, their merits and their limitations. Using results obtained with the "CISSY" end station at the BESSY synchrotron in Berlin, Germany, we show how surface sensitive Synchrotron excited X-ray Photoelectron Spectroscopy (SXPS) and Soft X-ray Emission Spectroscopy (SXES ), which yields compositional and chemical depth information in the ten to hundred nm scale, have increased our knowledge of the chemistry of surfaces and buried interfaces of these systems. INTRODUCTION Chalcopyrite thin film s olar cells consist of a stack of layers as shown exemplarily in Figure 1: n- ZnO/i-ZnO/CdS/Cu(In,Ga)(S,Se) 2 /Mo/glass. The performance of the solar cell is determined by the properties of the materials used and their interfaces. The most crucial interfaces are those between p-absorber and n-buffer and between n-buffer and n-window material. These interfaces are especially complicated because to date in all industrial production lines for chalcopyrite solar cells a wet-chemical deposition method is utilized to produce the CdS-buffer layer. The solution used for this deposition can change the absorber surface and residuals of that solution can remain in the buffer layer. However, even interfaces that are not close to the pn-junction of the solar cell can crucially affect its performance, i.e. by the formation of additional junctions or highly resistive interface layers. Therefore, it is crucial to gain detailed information on the chemical, electronic and structural composition of layers and interfaces in thin film solar cells in order to understand and optimize the device. A multitude of methods have been used in the past to characterize thin film solar cells; an ove rview was recently given in [1]. In this paper the focus is on two methods for surface and interface characterization involving synchrotron radiation: x-ray photoelectron spectroscopy (SXPS) and soft x-ray emission spectroscopy (SXES). Two exa mples of appliFigure 1: SEM-image of a cross section of a chalcopyrite cations to thin film solar cell solar cell. The various layers are shown in different colors characterization are given. (online version) for clarity.
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X-ray photoelectron and Auger spectroscopy XPS is a widely used technique for the characterization of surfaces. It is based on the photoelectric effect, i.e. the emission of electrons from a sample upon irradiation with light, which was first explained by Einstein in 1905 [2]. The kinetic energy EKin of the ejected electron is linked with its binding energy EB and the energy of the exciting radiation hν via: EKin = hν- EB
(Eq. 1)
Standard set-ups for XPS use the radiation from x-ray sources
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