Crystallization of Silicon by a Directional Solidification Method

Abstract

Abstract. This chapter introduces crystallization process of multicrystalline silicon by using a directional solidification method. Numerical analysis, which includes convective, conductive, and radiative heat transfers in the furnace is also introduced. Moreover, a model of impurity segregation is included in this chapter. A new model for three-dimensional (3D) global simulation of heat transfer in a unidirectional solidification furnace with square crucibles was also introduced. Directional Solidification Method: Strengths and WeaknessesMulticrystalline silicon (mc-Si) has a large demand of photovoltaics to overcome difficulty of the present green problem [1]. The directional solidification method is a key method for large-scale production of mc-Si in highly efficient solar cells. The maximum efficiency of the solar cell based on mc-Si is 18%. However, the use of commercially available wafers typically results in solar cell efficiency of about 16% in industrial solar cell processes.There are many problems that must be solved to achieve high efficiency. Mc-Si has many dislocations and grain boundaries that are introduced during the solidification process. Moreover, Mc-Si crystals are grown in a crucible, a process that degrades purity of the crystals due to their attachment to the crucible wall. Such defects and impurity can reduce the conversion efficiency of solar cells. Therefore, we should control distributions of dislocations, grain boundaries, and impurities during the solidification process. In the recent studies, quasi single crystal or multicrystalline with large grain size have been grown by using a modified directional solidification methods [2,3].The directional solidification process has several merits. Square-shaped crystalline silicon can be grown by using a square-shaped crucible. When round-shaped crystals grown by the Czochralski method are used as raw materials for square-shaped solar cells, a large amount of waste of silicon materials