Dephosphorization of Metallurgical-Grade Silicon by Electromagnetic Levitation
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THE development of solar cells with high-efficiency, low-cost and stable characteristics has attracted increasing attention within the photovoltaic energy conversion community. Crystalline silicon can be used as the raw material for producing solar cells, but the production cost is an important consideration. Metallurgical-grade silicon (MG-Si) has been used as a basis for the generation of solar-grade silicon (SOG-Si), providing competitive performance at a lower cost.[1] However, the phosphorus impurity in MG-Si is especially difficult to remove. The resistivity of silicon will be adversely affected by free electrons from phosphorus and the minority carrier lifetime will also be reduced, negatively affecting the performance of solar cells.[2] For this reason, the content of phosphorus should be limited to a maximum of 0.3 9 10-4 (wt pct).[3] The addition of iron to MG-Si can facilitate the removal of phosphorus by increasing the phosphorus activity. Additionally, when the melt temperature is lower than 1573 K (the melting point of 75 pctFe-Si), iron reduces the solid solubility of
QI JIANG and GUIFANG ZHANG are with the Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, P.R. China. Contact e-mail: [email protected] YINDONG YANG and ALEXANDER MCLEAN are with the Department of Materials Science and Engineering, University of Toronto, Toronto, ON M5S 3E4, Canada. Manuscript submitted June 5, 2020; accepted October 17, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
phosphorus, and the impurity remains in the melt phase, thus promoting the precipitation of refined silicon crystals.[4,5] A number of studies have been conducted on the dephosphorization of MG-Si. Safarian et al.[6] utilized vacuum induction melting to remove phosphorus from silicon. Zheng et al.[7] achieved phosphorus removal by an induction stirring method and discussed the kinetic aspects. Xie et al.[8] investigated the mass transfer behavior of phosphorus from liquid slag to solid slag. Shi et al.[9] and Suzuki et al.[10] studied the mass transfer factors affecting the removal of phosphorus from molten silicon using electron beam melting. Hence, the kinetic models for dephosphorization of metallurgical silicon are mostly based on vacuum induction melting, electromagnetic stirring and electron beam melting. The rate-controlling steps in most of these dephosphorization models include both chemical reaction or evaporation at the surface and mass transfer in the gas phase. However only a few studies have reported the utilization of electromagnetic levitation (EML) to study the dephosphorization of MG-Si. Electromagnetic levitation is a container-less process that eliminates contact between the liquid metal and refractories thus avoiding contamination from crucible materials. Also, the levitated droplet is well-mixed by strong electromagnetic stirring[11] which promotes good kinetic conditions, while the sample’s spherical geometry facilitates numerical calculations. The relationship between va
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