Model-based Quantitative Assessment of Crystallinity and Parasitic Absorption in Microcrystalline Silicon Solar Cells
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Model-based Quantitative Assessment of Crystallinity and Parasitic Absorption in Microcrystalline Silicon Solar Cells Thomas Lanz1, Corsin Battaglia2, Christophe Ballif2 and Beat Ruhstaller1 1 Zurich University of Applied Sciences, Institute of Computational Physics, 8401 Winterthur, Switzerland 2 Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin Film Electronics Laboratory, 2000 Neuchâtel, Switzerland ABSTRACT We investigate the influence of the crystallinity of the absorber layer and parasitic absorption in the doped layers and electrodes on the external quantum efficiency and reflection of microcrystalline silicon (ȝc-Si:H) solar cells. Using an optical light scattering model we systematically study variations in the crystallinity and validate a simple normalization procedure that allows assessing the gains that can be achieved by reducing the parasitic absorption. The optimization potential is demonstrated with solar cell samples with increased crystallinity and eliminated parasitic absorption. INTRODUCTION Thin-film silicon solar cells are a viable candidate for the large-scale deployment of photovoltaics as they may be manufactured at low cost and use abundant and non-toxic materials[1]. However, further efficiency enhancements on the cell level that may be transferred to the module level are necessary for the technology to become competitive with established crystalline silicon photovoltaics. In this contribution we investigate, experimentally and by means of numerical modeling, the gains that may be achieved in microcrystalline silicon solar cells by increasing the crystallinity of the absorber layer and reducing the parasitic absorption. The first one can be controlled by the deposition conditions, the second one is experimentally more difficult to realize. In an idealized solar cell, where the electrodes are non-absorbing and the doped layers omitted, current can no longer be extracted and experimentally the absorption in these cells can only be studied via the external reflection[2]. Computer modeling, however, allows for the calculation of the absorption in the intrinsic layer for different degrees of absorptivity in the doped layers and electrodes. We employ a numerical light-scattering algorithm, that we recently validated with external quantum efficiency spectra of ȝc-Si:H solar cells[3]. By reducing the parasitic absorption, both the absorption in the intrinsic layer and the external reflection increases. Recently, it was demonstrated experimentally, that in cells with non-absorbing electrodes, the parasitic absorption of the regular cell contributes proportionately to the reflection and the intrinsic absorption[2]. THEORY AND EXPERIMENT For the optical simulation of the solar cells we use our scalar light-scattering model[3]. The model integrates coherent and incoherent propagation of light and scattering at rough layer interfaces. We use the scalar scattering theory to determine the amount of scattered vs. specular
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