David B. Joyce

A simple process to remove boron from metallurgical grade silicon

It is necessary to develop solar grade (SoG) silicon for the photovoltaic industry. A desirable approach is to upgrade metallurgical grade (MG) silicon. The most problematic impurities to remove from MG silicon are B and P. A simple process to remove B from MG silicon has been developed by refining MG silicon in the molten state followed by directional solidification. With this approach, B has been reduced to 0.3 ppma, P to < 10 ppma and all other impurities to <0.1 ppma using commercially available, as-received MG silicon. It remains to develop a similar P reduction process so that SoG silicon production from MG silicon can be commercialized. The B-removal process was applied to B overdoped electronic grade silicon, and the resulting material was used for crystal growth. Test solar cells of 12.5-13.4% (1 cm 2 ) efficiency were produced.

Production of low-cost solar grade (SoG) silicon feedstock

A simple refining process of blowing moist gases over molten silicon removed high segregation impurities, especially B and P, and reduced other impurities. Using this process with heavily B-doped (Hi-B) silicon scrap from the electronic industry will double the feedstock available to the PV industry short term. When the refining process is followed by directional solidification, it can be used for upgrading metallurgical grade (MG) silicon to produce solar grade (SoG) silicon. For the long term, total processing in an MG silicon production plant can result in

Upgrading metallurgical grade (MG) silicon for use as solar grade feedstock

10/kg SoG silicon feedstock.

Production of solar-grade silicon by refining of liquid metallurgical-grade silicon

The most attractive approach to upgrade metallurgical-grade (MG) silicon is to chemically refine impurities, especially boron (B) and phosphorus (P), in the molten state followed by directional solidification. The most problematic impurity is B as it has the highest segregation coefficient. A simple refining process of blowing gas through the melt was developed to reduce B in MG silicon effectively, and the process has been scaled up to charge sizes up to 150 kg. The B reduction has been maintained with increase in charge sizes and confirmed by using it on B-doped silicon at ten times the concentration. The simplicity of the refining process when adapted to a commercial MG silicon plant is expected to produce solar grade (SoG) silicon at low cost.

System and method for crystal growing

The growth of the photovoltaic industry will be limited by the availability of silicon feedstock. Currently, feedstock supplies are scraps and surplus from the microelectronics industry. A program has been initiated to produce solar grade (SoG) silicon by upgrading metallurgical grade (MG) silicon using pyrometallurgical techniques in the liquid state followed by directional solidification. This approach is based on earlier results when all impurities, including B and P, were reduced to <1 ppm level. It is intended to analyze the earlier data, undertake thermodynamic analysis and produce SoG silicon that can be extended to a commercial scale operation. The feasibility of this approach will be demonstrated in charge sizes up to 500 kg.