Direct production process of low cost solar-grade silicon feedstock for multi-crystalline solar cells

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In this study, the new process was developed for low cost silicon feed stock for multi-crystalline solar cells. Low cost metallurgical-grade silicon (MG-Si) was the raw material for the process. Modified electron beam (EB) and plasma arc (PA) remelting techniqued were applied for the elimination of impurities. Continuous solidification was employed to remove transition metals.Button melting techniquePlasma arc of Ar-H_2O mixture was applied to eliminate boron by forming boron oxide or hydroxide vapor. Boron content reached about 1ppmw within 3.6 K seconds at 0.19 to 0.39vol. %H_2O.When H_2O content was increased to 1.24vol. % boron content was lowered to below 1 ppm within 900sec.The impurities such as carbon, phosphorus, calcium and aluminum in MG-Si were removed by EB treatment under 10^<-2> Pa for thirty minutes. Ninety percent of carbon was eliminated and the content reduced to 15ppmw, 75% of aluminum was eliminated and the content decreased to 470ppmw, 89% of calcium was removed and the content decreased to 150ppmw, and 93% of phosphorus was removed and the content decreased to 3ppmw at 5.0kW.Unidirectional continuous casting of siliconContinuous casting of silicon in the vacuum was performed. Transition metals such as iron and titanium were removed by segregation. The iron content leached to less than 1/30 for the ingot length of more than 50% of the ingot height at the casting rates of 2mm/min. This method will be one of the silicon source for solar grade silicon.