Directional solidification is the most important technique used in obtaining multicrystalline silicon for photovoltaic applications. The properties of multicrystalline silicon are correlated to the occurrence of C, O, N and their precipitates. The presence of C and N in the melt can lead to the formation of dislocations during the crystal growth process, which can act as recombination centers for electric carriers, affecting in a negative way the efficiency of the solar cells. On the other hand, the formation of SiC and Si3N4 precipitates will lead to cracks during the wafer sawing of the ingot.

An important source of impurities in a multicrystalline silicon ingot obtained by a directional solidification process is the crucible and its coating.

Previous studies [1] showed that natural convection speeds-up the transport of impurities originating form the crucible into the melt and that this effect is stronger for higher values of the diffusion coefficients. It is also expected that forced convection will affect the diffusion of impurities in the silicon melt.

In order to improve the impurity segregation and get a more homogenous distribution of dopants in the melt it’s necessary to improve the melt convection, especially near the solid-liquid interface.  Nowadays, in order to enhance the melt convection various methods are used. It was shown [2] that mechanical stirring can be a good option for improving the level of mixing in the melt even for small values of the stirrer’s rotation rates. Another option, for gaining a better control of the melt, would be the use of a special type of electromagnetic field (a combination between a vertical magnetic field and an electric current) [3, 4].

However, forced convection can also have a negative effect on the directional solidification process of multicrystalline silicon by increasing the crucible dissolution rate and therefore, increasing the quantity of impurities in the melt and in the crystal. It also affects the cohesion of the crucible coating leading to the release of dopants in the melt which can act as nucleation sites near the solid-liquid boundary and leading to the formation of mechanical stresses that can be important if they are embedded into the crystal. Therefore it is important to study the role played by the forced convection on the impurities diffusion.

This paper is focused on investigating the influence of the melt rotation obtained by enhanced convection on the diffusion of various impurities usually found in a multicrystalline silicon ingot.

It was shown that the transport of impurities is stronger for higher values of the diffusion coefficients. 

Bibliography

[1] A. Popescu, D. Vizman – Numerical study of the influence of melt convection on the crucible dissolution rate in a silicon directional solidification process, International Journal of Heat and Mass Transfer 54 (2011), 5540

[2] S. Dumitrica, D. Vizman, J.-P. Garandet, A. Popescu – Numerical studies on a type of mechanical stirring in directional solidification method of multicrystalline silicon for photovoltaic applications, Journal of Crystal Growth 360 (2012), 76

[3] C. Tanasie, D. Vizman, J. Friedrich – Numerical study of the influence of different types of magnetic fields on the interface shape in directional solidification of multi-crystalline silicon ingots, Journal of Crystal Growth 318 (2011), 293

[4] D. Vizman, C. Tanasie – Novel method for melt flow control in unidirectional solidification of multi-crystalline silicon, Journal of Crystal Growth 372 (2013), 1

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