Researchers Develop Technique to Use 'Dirty' Silicon

0 Comments | Renewable Energy Today, August 19, 2005

The University of California, Berkeley recently announced that a research team led by UC Berkeley engineers has developed a new technique to handle metal defects in low-grade silicon, an advance the university said could "dramatically" reduce the cost of solar cells.

According to UC Berkeley, the growth of the semiconductor and solar cell industries has put increasing pressure on "relatively limited" supplies of the refined, highly purified form of silicon used to make nearly 90 percent of the world's solar cells, driving up the price of the material.

UC Berkeley noted that attempts so far to use the more abundant and cheaper form of silicon, which is laden with metal impurities and defects, have failed since solar cells made from this material do not perform as well, with manufacturing techniques used to remove impurities proving expensive.

"We have proposed a new approach to the use of dirty silicon," said UC Berkeley professor of materials science and engineering and principal research project investigator Eicke Weber. "Instead of taking the impurities out, we can leave them in but manipulate them in a way that reduces their detrimental impact on the solar cell efficiency."

UC Berkeley said the research team analyzed how metal contaminants in silicon respond to different types of processing using "highly sensitive" synchrotron X-ray microprobes capable of detecting metal clusters as small as 30 nanometers.

The researchers found that the nano-sized defects scattered throughout the silicon limited the average distance electrons were able to travel before losing their energy.

"We found that one way of managing these nano-sized metal defects is to round them up into large groups so that they are less disruptive to the electrons," said Tonio Buonassisi, a Ph.D. student in the UC Berkeley graduate group in applied science and technology.

Additionally, the research team discovered that it was able to manipulate the distribution of the metal impurities by varying the cooling rate of the silicon.

"Using this cooling technique, we were able to improve the distance electrons could travel by a factor of four compared with dirty silicon that had been left unaltered," said Buonassisi. "Although this is still not as efficient as ultrapure silicon, it is the proof of principle that poor-quality silicon can be easily improved."

Contact: UC Berkeley, website http://www.berkeley.edu.

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