Thursday, October 01, 2009 at 10:55:22 AM - by Jeanne Roberts

Phoenix, Arizona-based RoseStreet Labs Energy announced the prototype of a solar cell incorporating gallium-nitride (GaN) in silicon on Sept. 28 that promises to achieve efficiencies of 25 to 30 percent, though no independent confirmation has been provided.

This is in comparison to most solar cells, whose average efficiency falls in the 15- to 17- percent range. RoseStreet says it expects to start commercial production in the fall of 2010, and anticipates that when the GaN cells are market-ready they will perform as described, converting 25 to 30 percent of the sunlight that falls on them into electricity.

Currently, the most efficient solar cell on the market is made by California solar manufacturer SunPower, whose quoted efficiencies of 22 percent are due primarily to its back-contact technology, which eliminates gridlines on the front of the cell that block sunlight. SunPower also uses front texturing, localized contact points, cell thicknesses in the 150-micron range and a backside mirror surface to deliver solar electricity at about 6 grams of silicon per watt.

The only other use of nitride we’ve come across is Fraunhofer ISE’s thin-film prototype n-type silicon solar cell with efficiencies in excess of 23.4 percent which uses silicon nitride in the base layer.

The advantage of GaN is that it captures light from a broader spectrum than pure silicon, say RoseStreet developers. Gallium-nitride is also commonly used in light-emitting diodes (LEDs), not only because of its ability to capture visible short-wavelength and UV light but also because of its high heat tolerance. Gallium-nitride has a band-gap (solar spectrum receptivity) of 3.4 electron volts, or eV, as compared to silicon at 1.11. The only higher-rated element is diamond, at 5.5, and (for future researchers) aluminum nitride, at 6.3.

The development of RoseStreet’s technology comes via its 2005 licensing agreement with Berkeley, California-based Lawrence Berkeley National Laboratory, which is in turn affiliated with the U.S. Department of Energy, and its 2005 commercialization agreement with Ithaca, New York-based Cornell University.

Because RoseStreet's solar cells don’t require silicon, the company is working on coupling the GaN with other materials like amorphous (allotropic) silicon or glass as a deposition layer. The company plans to make and sell solar cells, but will likely outsource production. The cost of the product is likely to be about $1.50 per watt by 2014, according to CEO Bob Forcier, which makes it a fair if not exactly competitive price, at least in the current marketplace.

The question in everyone’s mind lately is; how many solar wannabes will opt for cheaper over more efficient, not realizing that the price is the same in the long run?

RoseStreet plans to offer Series A stock to venture capitalists in 1Q 2010, to provide the seed money for commercial production, but no further details have been made available.