It's been the dream of visionaries for decades: Sunlight converted into direct current to use for recharging emission-free electric cars.

Photovoltaic cells offer that capability today, but now researchers claim to have found a way to improve the efficiency of that process greatly, using a device called a "rectenna."

This combines the functions of an antenna and a rectifier diode to convert light directly into direct current.

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Rectennas have been around for about 40 years, but only operating at short wavelengths--sometimes as short as 10 microns.

A new design of rectenna that uses carbon nanotubes with tiny rectifiers fabricated into them could form the basis for a more efficient way of harvesting solar energy, according to the researchers that created it.

This version--created by researchers at the Georgia Institute of Technology--is the first to operate at optical wavelengths, according to

In the devices, carbon nanotubes act as antennas to capture light from the sun and other sources.

As the waves of light hit these antennas, they create an oscillating charge that moves through built-in rectifiers. They switch on and off, creating a small direct current.

In theory, billions of these devices in an array could yield significant amounts of current, although the ones developed so far have an efficiency of less than 1 percent.

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But Baratunde Cola--associate professor at Georgia Tech's George W. Woodruff School of Mechanical Engineering and leader of the research team--believes they are only a proof of concept, and that greater things lay ahead for the technology.

"We could ultimately make solar cells that are twice as efficient at a cost that is 10 times lower," he said.

The research is supported by the Defense Advanced Research Projects Agency (DARPA), Space and Naval Warfare (SPAWAR) Systems Center, and the Army Research Office (ARO).

Photovoltaic solar power field at Volkswagen plant in Chattanooga, Tennessee

Photovoltaic solar power field at Volkswagen plant in Chattanooga, Tennessee

So far, the rectennas have been fabricated on rigid substrates, but the goal is reportedly to use a foil or other material that would produce flexible solar cells or photodetectors.

Cola believes his team can boost efficiency to up to 40 percent through improvements such as changing materials, opening up the carbon nanotubes to allow multiple conduction channels, and reducing resistance.

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He claims a rectenna with commercial potential could be available within a year.

As with all promising research, though, it's important not to automatically take encouraging lab results for guaranteed commercial viability.

[hat tip: Hugh K. Crawford]


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