Engineers at the Georgia Institute of Technology have developed a new form of technology that combines two well-established technologies — a rectifier and an antenna — to create a device capable of converting light directly into DC current.
The technology, referred to as an “optical rectenna” was created using nanometer-scale components. Should it prove scalable, it could allow photodetectors to operate without the need for cooling systems, better serve energy harvesters that convert waste heat to electricity, and provide the means to more efficiently capture solar energy.
Part antenna, part rectifier diode, the image above is the world’s first optical rectenna.
The optical rectenna is based on multiwall carbon nanotubes and small rectifiers fabricated onto them. The nanotubes serve as antennas, capturing light from the sun or other sources; as the light waves hit the antennas, they create an oscillating charge that moves through the rectifier devices attached to them. As this happens, the rectifiers switch on and off at record petahertz speeds which, in turn, creates a small direct current.
Putting this solution into real world perspective — if there were, say, billions of rectennas in an array, the device could produce a significant current. That’s still a way’s off though as the current device has demonstrated an efficiency of less than one percent so far (the researchers are hoping to boost output via optimization techniques, with the belief the device will be ready for commercialization within a year).
“We could ultimately make solar cells that are twice as efficient at a cost that is ten times lower, and that is to me an opportunity to change the world in a very big way” said Baratunde Cola, an associate professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech. “As a robust, high-temperature detector, these rectennas could be a completely disruptive technology if we can get to one percent efficiency. If we can get to higher efficiencies, we could apply it to energy conversion technologies and solar energy capture.”
Now, for those familiar with their technology history, you’ll note rectennas were actually first developed in the 1960s and 1970s. These devices were able to operate at wavelengths as short as ten microns, but for more than four decades, researchers have been unable to make a device that can work with optical wavelengths.
“The physics and the scientific concepts have been out there,” said Cola. “Now was the perfect time to try some new things and make a device work, thanks to advances in fabrication technology.”
Creating the rectennas begins with growing forests of vertically-aligned carbon nanotubes on a conductive substrate. By way of atomic layer chemical vapor deposition, the nanotubes are then coated with an aluminum oxide material for insulation. When that step’s complete, physical vapor deposition is used to deposit optically-transparent thin layers of calcium followed by aluminum metals atop the nanotube forest. The resulting disparity of work functions between the nanotubes and calcium results in a potential of roughly two electron volts — just enough to drive the electrons out of the nanotube antennas when they’re excited by light.
When in actual operation, the oscillating waves of light pass through the transparent calcium-aluminum electrode to interact with the nanotubes. The metal-insulator-metal junctions at the tips of the nanotubes serve as rectifiers, switching off and on at femtosecond intervals; this allows electrons generated by the antenna to flow one way into the top electrode. Super-low capacitance (a few attofarads, for those curious) allows the 10-nanometer diameter diode to operate at these impressive frequencies.
“A rectenna is basically an antenna coupled to a diode, but when you move into the optical spectrum, that usually means a nanoscale antenna coupled to a metal-insulator-metal diode,” Cola explained. “The closer you can get the antenna to the diode, the more efficient it is. So the ideal structure uses the antenna as one of the metals in the diode — which is the structure we made.”
A long series of tests conducted by Cola and his team verified measurements of both current and voltage. This confirmed the existence of rectenna functions which, to this point, had only been predicted theoretically.
Worth pointing out about the tests: the devices were able to operate across a range of temperatures from 5 to 77 degrees Celsius.
While the rectennas fabricated by the Georgia Tech team are currently grown on rigid substrates, the goal is to one day grow them on foil (or other material) that would allow for the production of flexible solar cells or photodetectors. They also hope to improve its efficiency by better selecting the device’s source materials, opening the nanotubes a bit more (so as to allow for multiple conduction channels), and reducing the resistance in the structures themselves.
“We think we can reduce the resistance by several orders of magnitude just by improving the fabrication of our device structures,” he said. “Based on what others have done and what the theory is showing us, I believe that these devices could get to greater than 40 percent efficiency.”
Read the published report, aptly entitled “A carbon nanotube optical rectenna”.
Via the Georgia Institute of Technology
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