Dependence of the MoSe 2 Formation on the Mo Orientation and the Na Concentration for Cu(In,Ga)Se 2 Thin-Film Solar Cell
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Dependence of the MoSe2 Formation on the Mo Orientation and the Na Concentration for Cu(In,Ga)Se2 Thin-Film Solar Cells Daniel Abou-Ras1,2, Debashis Mukherji1, Gernot Kostorz1, David Brémaud2, Marc Kälin2, Dominik Rudmann2, Max Döbeli3, and Ayodhya N. Tiwari2,4 1 ETH Zürich, Institute of Applied Physics, 8093 Zürich, Switzerland 2 ETH Zürich, Thin Film Physics Group, Laboratory for Solid State Physics, Technoparkstrasse 1, 8005 Zürich, Switzerland 3 Paul Scherrer Institute, c/o ETH Zürich, Institute for Particle Physics, 8093 Zürich, Switzerland 4 Centre for Renewable Energy Systems and Technology, Department of Electronic and Electrical Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK
ABSTRACT The formation of MoSe2 has been studied on polycrystalline Mo layers and on Mo single crystals in dependence of the Mo orientation, the Na concentration, and also as a function of the Se source and the substrate temperatures. The Mo substrates were selenized by evaporation of Se. The samples were analyzed by means of X-ray diffraction, Rutherford backscattering spectrometry, elastic recoil detection analysis, and by conventional and high-resolution transmission electron microscopy. It was found that the crystal structure and orientation of the MoSe2 layer change with increasing substrate temperature. However, the texture of MoSe2 does not depend on the orientation of the Mo substrate. It was also found that the MoSe2 growth is significantly influenced by the Na concentration at substrate temperatures of 450°C and 580°C. INTRODUCTION Molybdenum is widely used as back contact material in Cu(In,Ga)Se2 (CIGS) thin-film solar cells in substrate configuration. MoSe2 has been reported to form between the Mo back contact and CIGS during the CIGS growth, leading to an improved electrical contact [1]. The back contact is therefore not only a layer of polycrystalline Mo but a MoSe2/Mo stack. However, Mo easily oxidizes [2], and the oxide is water-soluble. This leads to a long-term degradation of the solar cell performance. Thus, there is a need to study other back-contact concepts. E.g., a very thin, intentionally grown layer of MoSe2 may be applied to act as a buffer layer on, e.g., a transparent conducting oxide (TCO) back contact, prior to the CIGS deposition, so that a tunnel junction is formed, which facilitates a quasi-ohmic contact to the CIGS absorber. Recently, it has been reported [3,4] that solar cells on a TCO back contact with intentionally grown MoSe2 intermediate layer show a better photovoltaic performance than without the MoSe2 intermediate layer. To form MoSe2 on Mo layers by selenization, the Se atoms or molecules react first with the Mo surface atoms, and then the Se gas has to diffuse through the already formed MoSe2 layer to the MoSe2/Mo interface. A more detailed description of the MoSe2 formation is given in [3]. As an example, an interface between CIGS deposited at 580°C and Mo is shown in Fig. 1. A MoSe2 intermediate layer is revealed (formed during the deposition of CIG
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