Nantennas: A solar cell's new partner in crime

Solar cell using nantennas could convert sunlight into electricity at 95 percent efficiencyNantennas can convert infrared sunlight into electricity at 95 percent efficiency—but there’s this one little hitch, the electricity produced moves too fast, oscillating at trillions of times a second.

The electricity used in household appliances oscillates at 60 times per second (60 hertz). Putting that type of juice into a common household appliance would be like putting a 747 engine in a Pinto, and trying to drive it around a roller rink.

"Our overall goal is to collect and utilize as much solar energy as is theoretically possible and bring it to the commercial market in an inexpensive package that is accessible to everyone," said University of Missouri Chemical Engineering Department Associate Professor Patrick Pinhero. "If successful, this product will put us orders of magnitudes ahead of the current solar energy technologies we have available to us today."

The nantennas were first developed at the Idaho National Laboratory (INL) by a team of researchers, including Pinhero. But there was initially no way to slow the oscillation down to a usable level.

With a previous technology, called metal-insulator-metal diodes, said Garrett Moddel, a professor of electrical, computer and energy engineering at the University of Colorado, his team has the ability to slow down the oscillation.

“We have demonstrated these devices working at 30 terahertz several years ago," he said. "Thirty terahertz is infrared light. Now we need to move up by a factor of 10 to visible-light frequencies.”

But there was problem.

“There are some fundamental limits to MIM technology, which will make it very difficult to apply to solar cells operating in the visible part of the solar spectrum,” he said.

Enter a new method.

“Recently, we have been developing a new technology, called ‘geometric diodes,’ which is designed to overcome these difficulties. They are tiny arrowhead-shaped devices that allow electric charge carriers to move more freely in one direction than the other,” said Moddel, who patented the devices. “My lab is working on the ultra-high-frequency diodes to rectify the visible light frequency oscillations. The frequency is above 10 to the power 14 hertz, i.e., above 100 terahertz, which is a million times higher than TV broadcast frequencies.” 

But what does this have to do with solar?

The antenna device, called a rectenna, could be coupled with a conventional photovoltaic cell increasing the efficiency of such a combined device, Moddel said.

“For example, conventional solar cells could harvest the high-frequency part of the solar spectrum, where that technology works well, and rectenna solar cells could be used to harvest the low-frequency part, where that technology works best,” he said.

Now that the group has developed a way to extract electricity from the collected heat and sunlight using the special high-speed electrical circuitry, they are working to bring the product to commercialization with Dennis Slafer of MicroContinuum, Inc. through a newly formed company called RedWave Energy.

Editor's Note: On May, 23, we made a few changes to this story in order to clarify a few points that our source felt were missing. If you sent out a version of this article before 2:15 p.m. Mountain Standard Time, please send out this revised edition. We apologize for any inconveniences.