Effect of Heat Transfer during the Vacuum Directional Solidification Process on the Crystal Quality of Multicrystalline Silicon

Abstract

Directional solidification (DS) is the most popular technique for massive production of multicrystalline silicon (mc-Si) in the solar industry. Constant improvement of the quality of silicon ingot production remains a research focus. In this work, the temperature distribution, thermal stresses, and melt–crystal (m/c) interface during the DS process with different pulling-down rates were studied by transient numerical simulation and verified by experiment. The results show that the thermal stresses and interface shape during crystal growth play an equally important role in the control of crystal quality, requiring an appropriate pulling-down rate to achieve thermal conditions in the furnace that provide an ideal temperature field in the silicon with lower thermal stresses and a suitable growth interface. Based on these results, an mc-Si ingot grown at 10 μm/s in a pilot-scale DS process had a larger grain size, vertical columnar structure, fewer defects, and a longer minority-carrier lifetime above 3 μs. This suggests that improvement of the quality of mc-Si ingots for solar cells requires comprehensive consideration of the effect of the thermal field conditions on the thermal stresses and grain orientation in the solidification process.