Simulation of Symmetrically Doped Silicon Nanowire Solar Cells
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Simulation of Symmetrically Doped Silicon Nanowire Solar Cells Felix Voigt1,2, Thomas Stelzner1 and Silke H. Christiansen1,3 Institute of Photonic Technology, Jena, Germany. 2 Institute of Physics, University of Oldenburg, Germany. 3 Max Planck Institute for the Science of Light, Erlangen, Germany.
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ABSTRACT Silicon nanowire solar cells were simulated using the Silvaco TCAD software kit. For optimization of speed the simulations were performed in cylinder coordinates with cylindrical symmetry. Symmetric doping was assumed with a dopant density of 1018 cm-3 in the p-type core and inside the n-type shell. In the implementation a cathode contact was wrapped around the semiconductor nanorod and an anode was assumed at the bottom of the rod. Optimization of cell efficiency was performed with regard to the rod radius and the rod length. In both optimization processes clear maxima in efficiency were visible, resulting in an optimal radius of 66 nm with the pn junction at 43.5 nm and an optimal rod length of about 48 ȝm. The maximum of efficiency with respect to the rod radius is due to a decrease of short-circuit current density (Jsc) and an increase of open-circuit voltage (Uoc) with radius, while the maximum with respect to the rod length is explained by the combination of an increase of Jsc and a decrease of Uoc. Fill factors stay rather constant at values between 0.6 and 0.8. Further, the influence of a back surface field (BSF) layer was surveyed in simulations. Positioning the BSF next to the cathode contact considerably improved cell efficiency. In addition, simulations with a cathode contact on top of the nanowire structure were undertaken. No severe deterioration of cell performance with increasing radius was observed so far in this configuration. Hence, nanorods with much larger radii can be used for solar cells using this contact scheme. In comparison to simulations with wrapped cathode contacts, Jsc and Uoc and therefore efficiency is considerably improved. INTRODUCTION The aim of building solar cells from a nanostructured thin film compositions such as radially doped nanorods is to use low quality semiconductor material and achieve still reasonable cell efficiency [1]. One major benefit of the nanorod film is its high capability of light trapping [2]. In the proceedings papers [3, 4] calculations according to a device physics model and simulations for nanorod solar cells were presented and different types of contacting schemes were proposed. In this paper, we will extend these investigations on the influence of different contact configurations. Mainly, we will concentrate on the geometry of the cells and estimate the effect of rod radius and length variations on cell performance parameters. SIMULATIONS Simulations were undertaken within the TCAD software package of Silvaco, including the device simulator ATLAS. We used cylindrical symmetry of a silicon nanorod in order to reduce the simulation coordinate system to two dimensions. Each simulation was performed for one single rod, with given radius and length.
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