Electromagnetic stirring and retention to improve segregation in silicon for photovoltaics

The segregation of impurities was investigated during the crystallization of upgraded metallurgical grade silicon (UMG Si). An experimental solidification system has been developed, including a strong electromagnetic stirring in order to mix the melt, to increase the effective segregation coefficient.Our crucible has a slit-like opening at the bottom of a face, to be adapted to ribbon pulling for future developments of a crystallization system for the photovoltaic industry. The electromagnetic system allowed retaining the liquid silicon inside the crucible during the crystallization of an ingot (thickness ∼5 cm), or make it flow out after the partial crystallization of a bottom layer (thickness ∼2 mm).Starting from UMG Si of known composition, the chemical analysis of the impurity concentrations in that multicrystalline material shows an efficient purification, despite the rather high crystallization speed (estimated between 9 and 20 cm/h). Phosphorous concentration was reduced from 6 to 1.7 ppm during the segregation, which is difficult to achieve for this impurity, which has a segregation coefficient close to unity (k0=0.3).

► Experimental electromagnetic system to improve impurity segregation in UMG-Si. ► Layer of silicon crystallized under strong electromagnetic stirring (∼0.2 m/s). ► Efficient segregation of impurities obtained for high crystallization rate of 20 cm/h. ► Retaining liquid silicon to flow across the slit thanks to magnetic forces.