Effects of Interelectrode Spacing on the Properties of Microcrystalline Silicon Absorber and Solar Cells
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EFFECTS OF INTERELECTRODE SPACING ON THE PROPERTIES OF MICROCRYSTALLINE SILICON ABSORBER AND SOLAR CELLS Bill Nemeth1, Xiaodan Zhang2, Yanfa Yan3, and Qi Wang1 1
National Renewable Energy Laboratory, Golden, CO, 80401 Institute of Photo-electronic Thin Film Devices and Technology of Nankai University, Tianjin 300071, China 3 Department of Physics, University of Toledo, Toledo, OH. 2
ABSTRACT We study the effect of the spacing between electrodes in very high frequency plasma enhanced chemical vapor deposition on the properties of microcrystalline silicon films and their related n-i-p solar cells. We vary the spacing from 0.2 to 1.0 cm to deposit microcrystalline silicon at 67.8 MHz while maintaining other growth parameters. The spacing between the electrodes significantly changes the plasma conditions, which govern film precursor chemistry as well as introduce etching and ion bombardment to the film; thereby, influencing nucleation and growth of the microcrystalline Si films. The resulting films were characterized by UV-Vis spectrometry, atomic force microscopy, X-ray diffraction, and transmission electron microscopy. We found that deposition rate decreases, while surface roughness and short circuit current density increase with smaller spacing. INTRODUCTION Microcrystalline silicon (μc-Si) offers advantages compared to traditional amorphous silicon when used as the absorber in thin film solar cells due to less degradation and higher current density. However, the process window for device grade material is very narrow, and is primarily governed by silane radicals and atomic hydrogen found in the plasma. Three theories can be found to explain the mechanism of microcrystalline growth: the surface diffusion model, the selective etching model, and the chemical annealing model [1]. Interactions between electrons, ions, radicals, and molecules in the plasma and on the growing film determine film growth rate, surface morphology, crystallite size, and subsequent electrical and optical properties. The creation of these precursors is influenced by chamber and gas inlet configuration as well as typical growth conditions such as power, frequency, pressure, dilution, and flow rate. From the following equation (1), the residence time (a) can be calculated, which takes into account several of these variables including electrode area (A), interelectrode gap (H), temperature (T), pressure (p), and total flow rate (Ftot) [2].
(1) The residence time represents the amount of time that the precursor gas has to react and deposit within the confines of the electrodes under the applied plasma conditions. Various defects can be found in films resulting from ion bombardment, growing grain boundaries, and
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undesirable radical incorporation, which will limit the performance of the film, so careful control of growth parameters is crucial. We sought to try to understand the effect that a very narrow gap would have on resulting films as well as incorporating these films into functioning solar cells. EXPERIMENTAL Solar cells with a n-i-p s
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