High Efficiency Thin Film Solar Cells with Intrinsic Microcrystalline Silicon Prepared by Hot Wire CVD
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High Efficiency Thin Film Solar Cells with Intrinsic Microcrystalline Silicon Prepared by Hot Wire CVD S. Klein, F. Finger, R. Carius, B. Rech, L. Houben1, M. Luysberg1, M. Stutzmann2 Institut für Photovoltaik, Forschungszentrum Jülich, D-52425 Jülich, Germany 1 Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 Jülich, Germany 2 Walter Schottky Institut, TU München, D-85748 Garching, Germany ABSTRACT Thin film microcrystalline silicon solar cells were prepared with intrinsic absorber layers by Hot Wire CVD at various silane concentrations and substrate temperatures. Independently from the substrate temperature, a maximum efficiency is observed close to the transition to amorphous growth, i.e. the best cells already show considerable amorphous volume fractions. A detailed analysis of the thickness dependence of the solar cell parameters in the dark and under illumination indicate a high electronic quality of the i-layer material. Solar cells with very high open circuit voltages Voc up to 600mV in combination with fill factors above 70% and high short circuit current densities jsc of 22mA/cm2 were obtained, yielding efficiencies above 9%. The highest efficiency of 9.4% was achieved in solar cells of 1.4µm and 1.8µm thickness. These cells with high Voc have considerable amorphous volume fractions in the i-layer, leading to a reduced absorption in the infrared wavelength region. INTRODUCTION The application of microcrystalline silicon (µc-Si:H) absorber layers in combination with amorphous silicon (a-Si:H) in tandem cell structures promises high conversion efficiencies of 14% and above [1]. In the past few years much progress regarding the performance of µc-Si:H material and solar cells has been made [1-3]. But in view of the necessity for high deposition rates on large areas, further progress in the deposition techniques is required. In this context, socalled hot wire (HW) or catalytic (CAT) chemical vapour deposition (CVD) [4, 5] has attracted much attention due to its very high deposition rates for µc-Si:H and the facility of up-scaling [6]. Until recently [7], the quality of the µc-Si:H material prepared by HW-CVD for use in solar cells was considered inferior to material prepared by PECVD. While the crystalline volume fractions are high and grain sizes are much larger than those of PECVD material, the defect densities are much higher, the material is porous and subject to in-diffusion of atmospheric gases, and the resulting solar cell efficiencies are low [8]. In a previous investigation we have identified the substrate temperature (TS) as a key factor for controlling the µc-Si:H material quality in the HWCVD process [9]. In particular it was found that high filament temperatures (Tf ), used to achieve an efficiant process gas decomposition and, therefore, high deposition rates, generally lead to considerable additional substrate heating. It was concluded that for substrate temperatures above 300°C the enhanced hydrogen desorption during the growth process results in insufficient grain boundary pa
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