Alta Devices lifts curtain on high-efficiency solar cell

The key to boosting solar cell efficiency is capturing light emitted by solar cells themselves, according to stealthy solar start-up Alta Devices.

Low-profile solar start-up Alta Devices today will detail a technique it claims can deliver record solar cell efficiency, a step toward making solar cheaper than fossil fuels.

Executives from the Silicon Valley company, which has raised at least $72 million in funding, will present technical papers today at the IEEE's Photovoltaic Specialist Conference in Seattle on how it has set a new mark in converting sunlight to electricity. Among its investors is Bill Joy, a Kleiner, Perkins, Caufield & Byers partner and famed technologist.

In tests, engineers achieved 27.6 percent efficiency last year and later hit 28.2 percent efficiency, the company said today. Both results were verified by the National Renewable Energy Laboratory and are more than 1 point higher than previous efficiency records for single-junction solar cells.

Eli Yablonovitch, Alta Devices co-founder and a professor at the University of California at Berkeley, said that a solar cell itself can emit light itself and that capturing the light is a previously untapped method for boosting its performance. "It was not recognized that to maximize the voltage (to increase solar cell current), we needed the material to generate more photons inside the solar cell. Counter-intuitively, efficient light emission is the key for these high efficiencies," he said in a statement.

The solar cell material the company is working with is gallium arsenide (GaAs), the type often used in high-performance solar devices with several layers of semiconductors, such as those used on satellites or concentrating solar photovoltaic generators.

In a summary of its technical paper, Alta Devices said that it has made a thin-film GaAs cell that can be placed on a flexible substrate.

A summary of that paper and others indicate that the company is seeking to minimize the amount of expensive gallium arsenide used with a novel production method, which would reduce costs. The bottom layer of the solar cell would be made of "microwire" arrays embedded in another material and then made thinner with a "simple peel-off process," according to one paper summary.

The company, which was founded in 2007, did not indicate when it expects to have a commercial product. But executives said that the work it has done on capturing light emitted by solar cells can be applied by other researchers in the field who are struggling to push the limits of solar cell efficiency.

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