One of the key issues in the technology development os directional solidification (DS) for multicrystalline silicon ingots for photovoltaic applications is to control the impurities distribution in the mould, which can influence important parameters related to the solar cell efficiency.It is well known that melt convection plays an important role in the impurities distribution. Therefore, a better control of melt convection will improve the quality and yield of the solidification process by influencing the interface shape and dopant and impurities distribution in a beneficial way. Based on the idea of melt stirring from Electromagnetic Czochralski method [1] and on a similar idea with one electrode for DS [2], a new configuration with two electrodes  is proposed for a DS melt placed in a vertical low intensity magnetic field. The two electrodes are in contact with the free melt surface and an electrical DC current passes through the electrodes.

In order to show the potential of this method, both numerical and experimental investigations have been carried out. The experimental set-up consists of a square-shaped crucible (7x7x7cm³), which contains a GaInSn alloy placed in a vertical magnetic field(fig.1). This eutectic alloy is suitable for such room temperature model experiments, because it is liquid above 11 degrees Celsius. The electrical current passes the melt through two electrodes (fig.1). The melt velocity is measured using an Ultrasound Doppler Velocimeter.Fig.1. Geometry of melt and the electromagnetic set-up  

STHAMAS3D software was used to performed numerical simulations. The melt is modeled as a Boussinesq fluid and the transient Navier–Stokes equations are solved simultaneously with the transient heat equation. The influence of the steady magnetic field on the melt flow is considered by the Lorentz force density in the Navier-Stokes equations. For the calculation of the Lorentz force, the electric current density is computed using a scalar electrical potential, which is obtained by solving  an additional equation.

It was found that even a small magnetic field (10 mT) and an electrical current in the electrodes of maximum 10 A can produce a significant stirring effect. The rotation rate increases with the increase of the intensity of electrical current and the flow structure depends on the electrodes position. The numerical results for the melt convection (intensity and flow patterns) support the experimental findings.

The main contribution of this idea is to provide some additional growth parameters (like intensity of electrical current and position of the electrodes) easy to be adjusted in order to control the melt flow and interface shape. It is also clear that the electrodes design is a crucial issue before to apply this idea in a real growth facility, mainly because the electrodes should passed the hot zone and can bring more impurities in the melt. 

The results prove that a combination of a low intensity vertical magnetic field and electrical current (EMF) has the potential to control the melt flow and interface shape and can lead to a homogeneous impurities distribution  through the melt stirring effect.

[1]  W.Wang, et.al., Jpn.J.Appl.Phys. Vol 39(2000), pp.372-377

[2] C. Tanasie, D. Vizman, J. Friedrich, J. Cryst. Growth 318 (2011) 293-297

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1. Numerical study of the influence of the melt rotation on the impurities diffusion in a directional solidification process of multicrystalline silicon
2. Effects of Crucible Coating on the Quality of Multi-crystalline Silicon Grown by a Bridgman Technique
3. Study of resistivity and lifetime profiles in highly polluted multi-crystalline silicon grown in a graphite crucible by a Bridgman technique
4. Solidification of multicrystalline silicon - phase field studies of micro-structures