Removing Phosphorus from Molten Silicon: A Thermodynamic Evaluation of Distillation

Within the framework of purification of molten silicon for solar cells, phosphorus is one of few impurities that remains problematic. The present purpose is to develop a new method that leads to a final phosphorus concentration around 0.5 wppm. We have first focused on the main thermodynamic parameters that control the process: relevant thermodynamic parameters are taken from literature, such as activities, partial pressures and interaction coefficients. According to our results, distillation of phosphorus under vacuum occurs at high temperatures. Yet, the residual oxygen present in the vacuum is a hindrance to the distillation, since it evaporates in the form of silicon oxides. (C) 2015 The Electrochemical Society. All rights reserved.