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Superefficient, Cost-Effective Solar Cell Breaks Conversion Records

Dec 8, 2006 David Biello Scientific American

A tiny solar cell doubles the efficiency of common photovoltaics' conversion of sunlight to electricity by capturing the energy from a broader spectrum of light.

Nature Image: germanium wafer manufacture
Image: COURTESY OF SPECTROLAB
SOLAR EFFICIENCY:  New solar cells capture more of the energy in sunlight by layering semiconducting material on top of germanium wafers, pictured here.
A tiny chip similar to the solar cells carried by many satellites and other spacecraft today--including the surprisingly long-lived Mars Rovers--has shattered previous records for maximum efficiency in producing electricity from sunlight. "This is the photovoltaic equivalent of the four-minute mile," affirms Larry Kazmerski, director of the Department of Energy's National Center for Photovoltaics in Colorado. "This is a disruptive technology that eventually could provide us, at least in the Southwest, with cost-competitive electricity fairly quickly."

The specifics: a germanium wafer is spun at high speeds and subjected to various gases that encourage the growth of layers of semiconducting material such as gallium arsenide. "We have somewhere between 20 and 30 layers of semiconductor material," explains David Lillington, president of Spectrolab, Inc., which developed the new cell. The resulting layers in one single solar device respond to different spectra of light. The top layer, for example, captures the energy of blue light while the middle layer absorbs green and the bottom uses red. Such triple-junction solar cells are specially tuned to work with concentrated light, in this case the wattage of 240 suns.

The resulting efficiency nearly doubles that of standard silicon solar cells, which hover at 22 percent. That gain requires, however, the use of light-concentrating devices, such as miniature plastic lenses and mirrors. The new solar cell achieved 40.7 percent efficiency under such concentrated light at the testing center at the National Renewable Energy Laboratory in Colorado. One cell of just 0.26685 square centimeter (or roughly 0.04 square inch) pumped out 2.6 watts of electricity when bathed at the maximum light concentration. "Every five minutes the spectrum of the sun changes," Kazmerski explains. So tests are conducted "under a simulator where everything stays constant."

Nature Image: graph of solar efficiencies
Image: COURTESY OF LARRY KAZMERSKI
HERE COMES THE SUN:  Various solar technologies continue to push upward the efficiency of their conversion of sunlight to electricity. Click on the picture to see the full graph.
Even though installed cells would require concentrators, the fact that fewer cells can produce the same amount of power--and that similar cells are already widely produced--means this system could potentially generate electricity in the range of 8 to 10 cents per kilowatt-hour--roughly equal to consumer electricity prices today. "Eventually it's hoped that it will come down enough to rival the price of traditional energy although that is future tense," notes Dave Garlick, a spokesman for Spectrolab's parent company Boeing.

"This could mean a significant increase in the penetration of solar power," says Clark Gellings, vice president of innovation at the Electric Power Research Institute, a think tank. "The real barrier was the cost of the device." Many utilities have already invested in solar, including giant solar farms in Portugal and China. In the U.S., Xcel Energy plans to build a relatively modest eight-megawatt solar power plant in Colorado next year using similar concentrated solar cells as well as other technology. "This is not a technology that is 10 years away," Kazmerski adds. "This is a technology that we are going to see out working next year."

Nevertheless, the record-breaking solar cells are at least 12 months away from full-scale manufacturing, Lillington says. "Before we put this new cell into production it needs to go through a qualification process to make sure it can withstand the rigors of the environment." Of course, its Martian peers have lasted 28 months in that harsh, alien environment.

And the triple-junction solar cell may not hold the efficiency record for long. "We are also looking at four-, five-, even six-junction solar cells," Lillington notes. "There are at least three or four different approaches to take the efficiency into the 45 percent range." And that means the price of energy harvested directly from the sun will continue to drop.

OVER VIEW



Updated: 2003/07/28