Stress distribution in polycrystalline silicon thin film solar cells on glass measured by micro-Raman spectroscopy
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1024-A07-04
Stress distribution in polycrystalline silicon thin film solar cells on glass measured by micro-Raman spectroscopy George Sarau1,2, Michael Becker1,3, Andreas Berger1,3, Jens Schneider4, and Silke Christiansen1,3 1 Max-Planck-Institute of Microstructure Physics, Weinberg 2, Halle, 06120, Germany 2 Institute of Photonic Technology, Albert-Einstein Str. 9, Jena, 07745, Germany 3 Martin-Luther-University Halle-Wittenberg, Hoher Weg 8, Halle, 06109, Germany 4 CSG Solar AG, Sonnenallee 1-5, Thalheim, 06766, Germany ABSTRACT Micro-Raman spectroscopy is used to measure stress distributions in 1.5 µm thick polycrystalline silicon thin film solar cells on glass. These measurements are combined with transmission electron microscopy and atomic force microscopy to assign the measured local stresses to the sample’s microstructure. Expansion and contraction of the silicon lattice in the layer and the borosilicate glass substrate during the thermal processing of the solar cell as well as quartz beads of µm size that reside on the glass substrate for light-trapping purposes induce internal stresses that locally vary with structural features. While the thermal processing induces an average tensile stress in the silicon layer originating from the thermal mismatch between glass and silicon, the latter results in lateral stress gradients up to 208 + 12 MPa in the mapped area. INTRODUCTION Polycrystalline silicon thin film solar cells on glass represent a reliable alternative to standard silicon wafer-based technology. Crystalline silicon on glass (CSG) photovoltaic technology offers many advantages, including a lower manufacturing cost as it combines low material consumption and single-panel, large-scale substrate processing (1.38 m2 glass sheets) [1,2], a special interdigitated contacting scheme which reduces the influence of a shunt to a small region [3], and potential for high efficiency (10.4% for a CSG minimodule has already been shown) [4]. Besides electrical and optical properties, the knowledge of the spatial distribution and values of residual mechanical stresses are important for materials optimization. Stresses occur during thermal processing due to the thermal expansion mismatch between glass and silicon and induce the formation of extended lattice defects (dislocations, low-angle grain boundaries, cracks) that degrade solar cell performance. In particular, cracks influence long term reliability and price due to aesthetic considerations of customers. In the present work we study local internal stresses in 1.5 µm thick polycrystalline silicon thin film solar cells deposited onto glass using micro-Raman spectroscopy (µRS). In addition, microstructural and topological characterization is carried out by Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). EXPERIMENT The thin film silicon solar cells on glass substrates are provided by CSG Solar AG which is producing solar modules based on the CSG technology at its new production site in Thalheim, Germany [5]. To provide good light trapping, bor
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