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Solar gear promises higher conversion efficiency

Posted: 31 Mar 2009 ?? ?Print Version ?Bookmark and Share

Keywords:solar panel? wafer manufacturing? solar cell efficiency?

Solaicx has developed its own manufacturing process for wafers used in solar panels. The result, according to the solar cell ingot manufacturer, is much higher solar conversion efficiencies.

"Everybody else grows silicon ingots for the solar industry using production equipment that was designed for the semiconductor industry," said John Sedgwick, Solaicx co-founder. "What we did was design unique, proprietary equipment that solves the wants and needs of solar industry."

Solaicx claimed that it is often approached about licensing its ingot-growing process but has so far declined, sticking with its business model as the exclusive manufacture of its ingots. The ingots are cut into wafers for solar cells.

Most silicon ingots are manufactured using growers designed for the chip industry, where the cost of the semiconductor wafers derived from that ingot is insignificant compared with the cost of chips. Hence, chipmakers do not optimize productivity or the cost of growing silicon ingots.

'Back to basics'
For solar, a wafer is essentially single, large PV diode. Consequently, wafer cost is a huge part of the end product. To meet those needs, Solaicx optimized its manufacturing facility to grow only ingots for the solar industry.

"We went back to basics and designed a machine, which removed every cost from the ingot growing process that was not related to the solar industry," said Sedgwick.

The major problem solved by Solaicx was ingot uniformity. Semiconductor ingot growers, when used for making solar cell wafers, have a resistivity that changes from one end of the ingot to the other. As a result, wafers cut from the ingot must be sorted based on their ability to convert sunlight into electricity. Solaicx claims its ingots can be sawed into wafers that uniformly produce solar cells of the highest possible efficiency.

In a traditional IC ingot grower, silicon and its dopants are melted in the bottom crucible. A seed is then lowered into the grower so that it just touches the molten mixture. As the molten material crystallizes on the bottom of the seed, it is slowly pulled upward. After about 12hrs, a pure, single-crystal ingot several meters long and either 6 inches or 8 inches in diameter is produced. The ingot is then cut into 100?m- to 300?m-thick wafers, the major component of a solar cell.

The problem with this method for the solar industry is that as the ingot is grown the silicon-to-dopant ratio changes. This causes the resistivity of the resulting wafers to increase from one end of the ingot to the other.

Solaicx claims it solved the problem by constantly feeding silicon and dopant into the crucible as the ingot grows, thereby maintaining a constant ratio throughout the entire growing process. Hence, wafers cut from its ingots have a more consistent resistivity that boosts solar efficiency.

"The level of impurities ordinarily gets higher as you pull an ingot out of the molten silicon," said Sedgwick. "But with our process, we are continuously feeding fresh silicon and dopant into the crucible under computer control to keep their ratio the same throughout the entire run."

- R. Colin Johnson
EE Times





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