Microcrystalline silicon solar cells prepared by 13.56 MHz PECVD at high growth rates: Solar cell and material propertie
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Microcrystalline silicon solar cells prepared by 13.56 MHz PECVD at high growth rates: Solar cell and material properties Tobias Roschek, Bernd Rech, Wolfhard Beyer, Peter Werner1, Felix Edelman2, Albert Chack2, Raoul Weil2, Robert Beserman2 Institut für Photovoltaik, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany 1 Max-Planck-Institut für Mikrostrukturphysik, D-06120 Halle, Germany 2 Solid State Institute, Technion - Israel Institute of Technology, Haifa 32000, Israel ABSTRACT Microcrystalline silicon (µc-Si:H) solar cells were prepared in a wide range of deposition parameters using 13.56 MHz plasma-enhanced chemical vapour deposition (PECVD). The best µc-Si:H solar cells were prepared close to the transition to amorphous silicon (a-Si:H) growth at very high deposition pressures (~10 Torr) showing solar cell efficiencies up to 8.0 % at a deposition rate of 5 Å/s. Investigations of the solar cells were performed by Raman spectroscopy and transmission electron microscopy (TEM). TEM measurements revealed similar structural properties with similar high crystalline volume fractions for these cells although they showed distinctly different efficiencies. However, an increased amorphous volume fraction was detected by Raman spectroscopy for the low efficiency cells prepared at low deposition pressures. This result is attributed to an increased ion bombardment at low pressures. INTRODUCTION Microcrystalline silicon (µc-Si:H) solar cells applied in a-Si:H/µc-Si:H tandem cells lead to high stable efficiencies [1,2,3,4]. For sufficient absorption of red and infrared light µc-Si:H i-layer thicknesses of more than one µm are required, and hence high deposition rates are an essential prerequisite for an industrial application. To achieve this, the very high frequency (VHF) PECVD technique has been widely used [1,2,3]. However, these high frequencies make an up-scaling to production size (~1 m2) difficult. For this reason one favours conventional RF (13.56 MHz) excitation frequency being also more compatible with existing deposition equipment. High rate deposition of µc-Si:H films in a high pressure regime at 13.56 MHz has been reported by Guo et al. [5]. First results on highly efficient µc-Si:H solar cells prepared at high deposition rates using 13.56 MHz and high pressures have been reported [6,7]. The material properties of microcrystalline silicon films and solar cells prepared by VHF were intensively studied during the past years by Raman and TEM measurements [8,9] focusing on the transition between microcrystalline and amorphous growth. Our development of µc-Si:H solar cells using 13.56 MHz was empirically performed by varying the i-layer deposition parameters in the p-i-n solar cell structures. In this paper we present a study of solar cells prepared using 13.56 MHz excitation frequency and high RF power PRF to achieve high deposition rates. The deposition pressure pdep was varied between 1 and 11 Torr. For each deposition pressure the hydrogen dilution at i-layer growth was varied to find the transition between am
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