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For Solar-Grade Silicon - How Many "Ns" Are Enough?
by Staff Writers
Easton PA (SPX) Jun 23, 2011

Electronics grade silicon (currently produced using the Siemens purification process) has to be 9N. Solar grade silicon doesn't.

Recognizing that the solar energy industry has been too dependent on the expensive but standard Siemens methodology for refining silicon, RSI Silicon Products decided to reinvent the way solar-grade silicon is made.

RSI has developed a proprietary process to manufacture solar-grade silicon with a purity of 99.9999+ percent, or "six nines" (i.e., 1 part impurity per million). The silicon ends up at 99.99999+ percent pure (i.e., "seven nines," or less than 1 part impurity per 10 million) when made into wafers.

Silicon is commonly classified according to its purity level. Its cost is inversely proportional to its contained impurity. "Dirty," or less pure silicon, is quite inexpensive. Highly pure silicon is the most expensive.

Ironically, no one really knows what purity they need because they've only used the Siemens process. But the Siemens process has high capital costs and high production costs; it also uses highly toxic chemicals.

Given current economic realities, the solar industry is finally asking, How pure is pure enough?

Silicon comes in three grades:

+ Metallurgical grade silicon (MG-Si) - 98 percent pure

+ Solar grade silicon (SG-Si) - 99.9999, or 6N ("six nines") pure

+ Electronic grade silicon (EG-Si) - 99.9999999, or 9N ("nine nines) pure

Electronics grade silicon (currently produced using the Siemens purification process) has to be 9N. Solar grade silicon doesn't.

Yet until recently the Siemens process had been the only commercially available source of silicon for the solar industry.

Now, RSI Silicon's innovative methodology, which incorporates proven processes from other industries, offers solar energy producers an affordable alternative to the Siemens process. RSI Silicon's low-energy process is clean, uses renewable feedstock, and is nearly carbon-neutral.

RSI Silicon made one of its breakthroughs right in the first step of the wafering process, which is called "directional solidification" (DS). For example, using the Siemens process, DS actually adds impurities to silicon - which starts DS at 9N and finishes DS at 7N to 8N purity.

By contrast, the silicon turned out by the "ChemArc" method recently patented by RSI Silicon improves silicon purity from 6N before DS to 7N+ after DS. The secret is that ChemArc removes key impurities ahead of DS - thus enabling DS to act as an extra purification step.

The "upgraded metallurgical grade" (UMG) method and other low-cost methods have been unable to remove enough of the key impurities ahead of DS, and, as a result, DS under those conditions cannot act as a major additional purification step.

Consequently, the remaining impurities just naturally "DS-out."

The ChemArc method takes advantage of this fact. Since the DS step is required in the manufacture of silicon wafers, it does not add costs or steps.

The upshot of RSI Silicon's new method is that it delivers silicon for solar-grade applications in commercially viable quantities at a fraction of the cost of silicon manufactured using the old method.

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Siemens And The North Carolina Solar Center Announce Solar Exchange East
Atlanta GA (SPX) Jun 23, 2011
More than 130 solar industry representatives recently gathered for Solar Exchange West, hosted by Siemens, University of California, Los Angeles, Clean Tech LA and the LA Business Council. The event was held at the California Nanosystems Institute at UCLA in Los Angeles - bringing together industry and academia to collaborate and innovate in the field of solar technology. Participants incl ... read more

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