Numerical study of the Bridgman directional solidification process of photovoltaic silicon ingot using power control technique

This work is concerned with the improvement of silicon photovoltaic solar cells using electromagnetic stirring in prior processing by controlling the hydrodynamics of the silicon bath during ingot solidification. The vertical Bridgman directional solidification furnace called VB2 was numerically investigated using a controlled power-down technique under forced convection generated by a travelling magnetic field, i.e. a full 3D numerical model of the VB2 furnace has been developed using the Comsol MultiphysicsTM software. Enthalpy formulation based on fixed-grid techniques is used for the phase-change phenomena considering magnetohydrodynamic forced convection created by a Lorentz force generated by a specific cylindrical Bitter coil. The model provides the temperature distribution in the entire furnace as well as specific details regarding the solidification process (i.e. solidification fraction and solid-liquid interface shape), flow patterns and melt velocities. The obtained numerical results show good consistency with experimental measurements and analysis, on the one hand, and can be considered as useful orientation to improve the solidification process, on the other hand. It is shown that the travelling magnetic field acting in the melt drives a moderately turbulent flow which has a significant impact on the solid-liquid interface in terms of morphology and symmetry during silicon solidification and, consequently, on the tight control of the crystallization process that can affect the silicon crystal morphology and impurity distributions. Keywords: Electromagnetic stirring, polycrystalline photovoltaic silicon, solid-liquid interface, travelling magnetic field, vertical Bridgman directional solidification. Table 1, Figs 5, Refs 13.