Energy & Power
Optical ‘rectenna’ converts light to DC current
By Anthony Capkun
October 2, 2015 – Using nanometre-scale components, researchers say they have demonstrated the first optical rectenna—a device that combines the functions of an antenna and a rectifier diode to convert light directly into DC current.
Based on multiwall carbon nanotubes and tiny rectifiers fabricated onto them, the optical rectennas could provide a new technology for photodetectors that would operate without the need for cooling; energy harvesters that would convert waste heat to electricity; and ultimately, for a new way to efficiently capture solar energy.
In the new devices, developed by engineers at the Georgia Institute of Technology, the carbon nanotubes act as antennas to capture light from the sun or other sources. As the waves of light hit the nanotube antennas, they create an oscillating charge that moves through rectifier devices attached to them. The rectifiers switch On/Off at petahertz speeds, creating a small direct current.
Billions of rectennas in an array can produce significant current, say researchers, though the efficiency of the devices demonstrated so far remains below 1%. They hope to boost that output through optimization techniques, and believe that a rectenna with commercial potential may be available within a year.
“We could ultimately make solar cells that are twice as efficient at a cost that is 10x lower, and that is, to me, an opportunity to change the world in a very big way,” said Baratunde Cola (photo), 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.”
“The physics and the scientific concepts have been out there,” said Cola, noting rectennas were developed in the 1960s and 1970s, but that it wasn’t until advances in fabrication technology that it was now “… the perfect time to try some new things and make a device work”.
Using metallic multiwall carbon nanotubes and nanoscale fabrication techniques, Cola and collaborators Asha Sharma, Virendra Singh and Thomas Bougher constructed devices that utilize the wave nature of light rather than its particle nature. They also used a long series of tests—and more than a thousand devices—to verify measurements of both current and voltage to confirm the existence of rectenna functions that had been predicted theoretically. The devices operated at a range of temperatures from 5ºC to 77ºC.
Fabricating the rectennas begins with growing ‘forests’ of vertically-aligned carbon nanotubes on a conductive substrate. Using atomic layer chemical vapour deposition, the nanotubes are coated with an aluminum oxide material to insulate them. Finally, physical vapour deposition is used to deposit optically transparent thin layers of calcium then aluminum metals atop the nanotube forest. The difference of work functions between the nanotubes and the calcium provides a potential of about two electron volts, enough to drive electrons out of the carbon nanotube antennas when they are excited by light.
In operation, oscillating waves of light pass through the transparent calcium-aluminum electrode and interact with the nanotubes. The metal-insulator-metal junctions at the nanotube tips serve as rectifiers switching On/Off at femtosecond intervals, allowing electrons generated by the antenna to flow one way into the top electrode. Ultra-low capacitance, on the order of a few attofarads, enables the 10-nanometre diameter diode to operate at these 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.”
The rectennas fabricated by Cola’s group are grown on rigid substrates, but the goal is to grow them on a foil or other material that would produce flexible solar cells or photodetectors.
Cola sees the rectennas built so far as simple proof of principle. He has ideas for how to improve the efficiency by changing the materials, opening the carbon nanotubes to allow multiple conduction channels, and reducing resistance in the structures.
“We think we can reduce the resistance by several orders of magnitude just by improving the fabrication of our device structures,” Cola 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% efficiency.”
Photos Rob Felt, Georgia Tech.