Research performed in the centre shows how phosphorous removal through vaccum refining can be done with about 10 times less power consumption
Phosphorus (P) is one of the most detrimental impurities for the efficiency photovoltaic Si cells and its concentration must be fully controlled in solar silicon production. Phosphorus has a high segregation coefficient and hence cannot be removed from Si by directional solidification technique. Therefore, additional dedicated refining techniques are required for a complete removal of P from Si, and among all of them, vacuum refining is the most efficient method.
This method has been under research for 3 decades and recently is industrialised and it has been under research for more than 10 years at NTNU. The most recent results from this refining process are now published in the journal Vacuum with the title Vacuum refining of Silicon at ultra-high temperatures. In this paper the authors show when the process temperature exceeds 1.25 times the melting point of Si, the rate of process suddenly increases. The authors have discussed the kinetics of the process and they have developed a new theory for the evaporation of P from liquid Si, which bases on the decomposition of silicon phosphides at the melt surface. The power consumption of the vacuum refining process is discussed, and it is shown that refining at ultra – high temperatures reduces the power consumption about 10 times. Results of this research are promising in terms of providing high quality solar grade Si for PV applications with low costs and power consumption.
Picture: Arman H Kermani is looking though the sight window of silicon refining furnace at Norwegian lab for Si-based solar cell technology, NTNU. Photograph by: Harald Philipson.