Study on the Morphology Evolution and Purification of Electrorefined Silicon
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THE photovoltaic industry is going through rapid growth,[1–3] and most commercial photovoltaic power generation is produced by crystalline silicon solar cells. For silicon solar cells to remain the mainstay for the photovoltaic industry, it is necessary to develop a lowcost solar grade (SoG) silicon feedstock. The most desirable approach to produce SoG silicon is to upgrade metallurgical grade (MG) silicon by refining impurities, especially B and P, in the molten state followed by directional solidification.[4,5] However, the segregation coefficients of B and P are 0.8 and 0.35, respectively, and are much greater than most of the other impurities, as Figure 1 shows. Therefore, B and P cannot be removed by directional solidification efficiently. Molten salt electrorefining, which has found such extensive application in light, refractory, and rare metal extraction and refining, is yet another technique that, in principle, can be used to extract and purify silicon.[6] The electrowinning of silicon was first reported in 1854,[7] but there have been relatively few systematic studies reported in the literature. The most detailed investigation to date has probably been that of Monnier YAN-QING LAI and JIE LI, Professors, MING JIA, JIAN-FENG YAN, JI-GUANG YI, and ZHI-GANG WANG, Doctoral Students, ZHONG-LIANG TIAN, Vice Professor, TMS Member, and YE-XIANG LIU, Professor, Academician of the Chinese Academy of Engineering, TMS Member, are with the School of Metallurgical Science and Engineering, Central South University, ChangSha, Hunan 410083, People’s Republic of China. Contact e-mail: jiamingsunmoon@ yahoo. com.cn Manuscript submitted April 16, 2009. Article published online January 28, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
et al.,[8,9] who showed that silicon of 99.99 pct purity could be produced in a two-stage winning and refining process. Their deposits were, however, apparently powdery and incoherent and required subsequent separation from entrapped solvent. Cohen et al.[10–12] also showed that single-crystal epitaxial silicon layers could be electrodeposited from solutions of K2SiF6 in a KF-LiF eutectic system. However, coherent and pure silicon layer can only be achieved with very low current density in the range of 4 to 12 mA/cm2; then, silicon layer as thick as 3 mm will be deposited over 4 days and the energy cost will be much higher than the economic level. This work presents a process to remove impurities from MG silicon by three-layer electrolysis.[13] Different from the prior works, we selected light metal with high purity as our cathode and alloying the MG silicon with copper as anode. Meanwhile, application of extremely high current density is also possible due to the large and fresh reactive surface area of liquid cathodic metal. Such advantages obviously improve the rate of deposition and current efficiency, which is a key point to reach commercial development.[14] The morphology and composition of the polycrystalline silicon produced and their dependence on the electrodeposition conditions are discussed in
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