Numerical and experimental study of vacuum directional solidification purification process for SoG-Si in metallurgical route

• The effect of pulling rate on directional solidification purification is studied.
• The simulation results show a satisfactory agreement with the experiments.
• The silicon ingot for 10 μm/s meets the crystal morphology requirement of SoG-Si.
• A new route for metallurgical production of solar wafers is proposed.
• Silicon grown by new metallurgical route will have large-size columnar grain growth.

A transient global simulation was carried out to investigate the effect of pulling-down rate on the temperature distribution, melt convection and melt/crystal (m–c) interface in vacuum directional solidification purification process for SoG–Si in metallurgical route. Simulation results show that pulling-down rate has little effect on flow behavior of melt. The isotherms change in the bottom area of silicon for 10 μm/s pulling-down rate is larger than that of 5 μm/s, and it may lead to higher thermal tress in this part of the silicon ingot. The m–c interface is less convex to the crystal and its variation is smaller for 10 μm/s, it further contributes to desirable crystal growth. We found that silicon ingots produced by vacuum directional solidification purification with pulling-down rate of 10 μm/s can meet the crystal morphology requirement of solar grade silicon (SoG-Si). So we proposed a new route for metallurgical production of solar wafers which combining with removal of impurities and ingot casting process into a process. Preparation of multi-crystalline silicon (mc-Si) by the directional solidification process will have large-size columnar grain growth and produce the solar wafers with appropriate technological parameters for directional solidification process.