Removal of Boron and Phosphorus from Silicon Using CaO-SiO 2 -Na 2 O-Al 2 O 3 Flux
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THE use of fossil fuels has been associated to the release of greenhouse gases, leading to global warming. As a consequence the need for renewable and clean energy has increased. Among various renewable energy sources such as biomass, wind, and hydro power, solar energy is most abundant, noting that Sun’s energy received by earth in 1 minute, if fully harvested, can supply the world’s energy demand of an entire year.[1] However, the high manufacturing costs of photovoltaic cells have hampered the widespread use of solar energy even with the government subsidies. Today’s US residential solar electricity price ranges from 29 to 64 cents/ kWh, which is approximately 3 to 6 times larger than the fossil fuel or nuclear energy based electricity.[2,3] One of the major cost items of the photovoltaic cells is the cost of high purity silicon, accounting for one-third of the finished module cost.[4,5] In recent years considerable research has been conducted on refining of metallurgical grade silicon (MG-Si, purity >96 pct) to solar grade silicon (SoG-Si), with the ultimate goal of reducing the production cost. Metallurgical refining routes have drawn the most attention due to the low operational cost. The reduction of the impurities in silicon to a certain level has to be achieved for a high efficiency solar cell, since the MARK LI, Ph.D. Candidate, TORSTEIN UTIGARD, Late Professor, and MANSOOR BARATI, Professor, are with the Department of Material Science and Engineering, University of Toronto, Toronto, ON M5S 3E4, Canada. Contact e-mail: mark.li@ mail.utoronto.ca Manuscript submitted September 3, 2013. Article published online January 9, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
impurities dictate the device performance. Among various elements, Al, As, B, and P are considered most detrimental due to the doping effect they have on Si. Directional solidification as a well-established process can remove those impurities with low segregation coefficients, but has little effect on boron and phosphorous that possess segregation coefficients substantially above any other impurity.[6,7] Vacuum refining, an alternative metallurgical purification method may be used for removing elements with high vapor pressure, such as P and Ca.[8] However, long processing time and the need for extra low vacuum levels make this method unattractive. Solvent refining is another possible method of removing boron and phosphorous. By alloying silicon with other metals, such as Al, Fe, and Cu which have higher affinity for impurities in liquid phase, the segregation coefficients of boron and phosphorous would be decreased.[9–24] Morita and his colleagues[9,13–24] conducted many studies on employing Al and Sn as solvent to improve the purity of silicon. During solidification, pure primary silicon phase in dendrite form precipitates out first, and impurities remain in the eutectic structure. However, the method may not be effective as a one-step purification technique as the data show that the segregation of impurities between the eutectic and Si is not large enough to
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