Science Magazine: Solar Cells Gear Up to Go Somewhere Under the Rainbow

Science 20 June 2008:
Vol. 320. no. 5883, p. 1585
DOI: 10.1126/science.320.5883.1585

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NANO SCIENCE AND TECHNOLOGY INSTITUTE NANOTECH 2008:

Solar Cells Gear Up to Go Somewhere Under the Rainbow

Robert F. Service

Today's solar cells do a fair job of converting visible light into electricity, but they ignore lower energy infrared (IR) photons, or heat, which don't have enough energy to generate electricity in semiconductors. At the meeting, researchers from the Idaho National Laboratory (INL) in Idaho Falls reported harvesting IR photons with arrays of antennas akin to those on televisions and in cell phones, a first step toward solar cells that convert heat to electricity. If the approach pans out, it could lead to solar cells capable of generating electricity after sunset and using the waste heat from industrial plants.

"It's certainly an intriguing idea," says Michael Naughton, a physicist at Boston College in Chestnut Hill, Massachusetts, whose group has built related antennas. But he notes that converting the energy from the collected IR light to electricity will require a separate set of advances. Says Naughton: "Either it has no chance of working, or it will be fantastic."

The notion of using antennas to capture electromagnetic waves and then convert that energy to electricity is decades old. In 1964, William Brown, an engineer at the U.S. aerospace company Raytheon, demonstrated a flying helicopter that absorbed microwaves and converted their energy to DC power to run a small engine. At the heart of the helicopter's success was a two-part device called a "rectenna": a microwave-absorbing antenna combined with a "rectifier" that converts the microwave energy to electricity. More recent are proposals to transmit microwave energy to Earth from arrays of solar collectors in space.

Several years ago, researchers led by Steven Novack at INL set out to capture and convert IR light, which has a wavelength two to five orders of magnitude shorter than microwaves. That meant the size of each antenna needed to be in the micrometer scale with numerous features in the nanometer range. To capture enough IR photons, Novack and his colleagues needed arrays with millions of the antennas side by side. The good news was that instead of having to use exotic semiconductor alloys to capture the light, they could do so by patterning gold in square spiral structures. Novack's team worked out a way to stamp out millions of gold spiral arrays on either silicon or cheap, flexible plastics. At the meeting, Novack reported that the arrays on silicon capture some 80% of the IR photons that hit them, whereas those on plastic manage a respectable 40% to 50%.

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