Enhanced In Situ Separation of Boron at the Silicon Alloy Solidification Interface through Innovating the Impurity Chemical Reconstruction Approach for SoG-Si

Abstract

Removal of trace impurities such as boron (B) and phosphorus (P) has always been a challenge for the preparation of solar-grade silicon (SoG-Si). Chemical reconstruction has been thought to be an effective way to remove trace impurities by changing the phase structure and location of impurities. The traditional way of the chemical reconstruction of impurities at the grain boundary after solidification inevitably relies on the subsequent separation of crystalline Si and chemical reconstruction medium. Thus, we develop a new method for the chemical reconstruction of B at the front of the solidification interface of the Si alloy, which provides advantages for the in situ separation of the reconstructed impurity phase in the supergravity filtration separation process. The chemical reconstruction of B at the solidification front of the Si alloy has been realized by adding 25–75% of the liquation agent aluminum (Al) and about 10 000 ppm of the impurity modifier agent titanium (Ti) with the help of Thermo-Calc thermodynamic calculation and experimental verification. The influence mechanism of the supergravity field on the precipitation behavior of the chemically reconstructed phase TiB2 is revealed by the derived Maxwell–Stefan (MS) equation under the action of the supergravity field. A special filter-type combined crucible is skillfully designed and strictly controlled to cool the crucible containing the alloy melt at a constant temperature gradient under the supergravity field to achieve in situ separation of impurities at the solid–liquid interface of the Si alloy. The chemical reconstruction phase TiB2 is mainly retained on the surface of crystalline Si during the process of supergravity centrifugal filtration, which is more suitable for the deep removal of the B impurity and B content in Si drops below 1.5 ppm by simple pickling. In addition, the Si–Al–(Ti) alloy with a small amount of impurities obtained after centrifugal filtration separation can be further used for the chemical reconstruction of impurities, and the recycling of chemical reconstruction medium has been achieved. The present work proposes a new idea of chemical reconstruction of impurities at the front of alloy solidification and in situ separation, which expands the way of in situ separation of trace impurities at the solid–liquid interface of the alloy.