Influence of crystal growth rate on the thermal field in a directional solidification process for silicon solar cells

A global model of heat transfer, including the melt convection, the argon flow, the thermal conduction, the thermal radiation and phase change in a directional solidification furnace for multi-crystalline silicon ingot for solar cells was developed. Basing on the model, transient global simulations were carried out to study the influence of crystal growth rate on the global thermal field, the melt convection and the melt-crystal interface shape in a directional solidification process. The growth rate was controlled by the heater power supplying history in different processes. Three processes, namely, the slow-cooling process, the fast-cooling process and the medium-cooling process, were numerically investigated and compared. It was found that the growth rate significantly influences the global thermal field, the silicon melt flow pattern and the melt-crystal interface shape in a directional solidification process. If the growth rate is not appropriate, the solidification process may not proceed smoothly and high thermal stress may be induced in the grown crystal. The numerical results indicate the importance of appropriate design of the process parameters to control the thermal field in a solidification furnace for high quality silicon ingots.