Reducing impurities of multicrystalline silicon in a unidirectional solidification furnace for solar cells

Abstract

Multicrystalline silicon has now become the main material in the photovoltaic market because of its low production cost and the relative high conversion efficiency of solar cells made from this material. Effective control of carbon and oxygen impurities in the crystal is required for the production of a high-quality component. In order to reduce impurity levels in a unidirectional solidification furnace, it is proposed to add a crucible cover to the casting process. Results from numerical simulations indicate a marked reduction of carbon impurity in a multicrystalline silicon ingot. The effect of crucible cover material on impurities in multicrystalline silicon was also investigated. These results show that the carbon concentration within the silicon ingot can be reduced by 10 times if the cover is made from carbon and by 1,000 times if the cover is made from tungsten. These results show that an effective and economical method for designing a cover is to use carbon and deposit a thin layer of tungsten on it. Experimental tests have also been carried out by placing a tungsten cover above the crucible. Results indicated that the carbon impurity has significantly decreased; however, the measured carbon concentration in the crystal is larger than the theoretically predicted value, despite the use of a crucible cover. A theoretical analysis has shown that this difference is due to a reaction between the crucible and the graphite susceptor. Furthermore, global simulations have shown that this reaction has a marked effect on carbon and oxygen impurities. When the carbon activity on the surface of the graphite susceptor increases, both oxygen and carbon impurities in crystal increase rapidly. Therefore, the production of high-purity multicrystalline silicon can be improved by designing a crucible cover with a thin layer of tungsten on it and by setting a free space between the silica crucible and the graphite susceptor to prevent reaction between them.