Numerical and experimental study of silicon crystallisation by Kyropoulos process for photovoltaic applications

Résumé

Kyropoulos technique is a top seeding process with the potential to produce high quality silicon ingots for photovoltaic applications. The crystal grows inside the melt in an unconfined low stress growth environment. A 3D numerical model of the full process, including heat transfer fluid dynamics and solidification, was created to develop a working experimental prototype of the process for silicon. The process had a square crucible heated by three heaters (top, bottom, side).Numerical simulation explained the complex coupled effect of radiation and melt flow on the solidification process. The study showed that the shape of the crystal is dependent on the melt flow. The growth was found to be sensitive to symmetry loss due to a strong coupled effect between melt flow and emissivity change of silicon during solidification. Control of heating parameters and homogenization of the melt flow allowed a consistent symmetric crystal growth. As a consequence, symmetric silicon crystals were obtained experimentally and numerically. Numerical investigation showed the possibility to control thermally, by heat diffusion and convection, the shape of the horizontal cross section to obtain a square ingot.The developed experimental prototype succeeded to grow monocrystalline silicon crystals. The process stabilized faceted growth in the <111> direction. The faceted monocrystalline crystals had a square horizontal cross section.

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