Engineers at the Georgia Institute of Technology 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. It uses the wave nature of light (rather than its particle nature) to convert microwave energy into direct current electricity. If the device’s efficiency can be improved (and the researchers are confident that it can), the rectenna could be used to build solar cells that are twice as efficient and ten times cheaper than the ones we use today.
“As a robust, high-temperature detector, these rectennas could be a completely disruptive technology if we can get to one percent efficiency,” said researcher Baratunde Cola. “If we can get to higher efficiencies, we could apply it to energy conversion technologies and solar energy capture.”
To build the rectenna, the researchers first had to grow carbon nanotubes on a substrate. Then the nanotubes were coated with an electrical insulator (aluminum oxide) and a thin layer of calcium; and finally, aluminum metals to make a metal diode. “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.”
As waves of light hit the nanotube antennas, they create an oscillating charge that moves through rectifiers, which switch on and off at record high speeds, creating a small direct current. The researchers claim it is the only diode that is fast enough to open and close the gate at the speed of solar energy oscillating in an antenna.
Looking ahead, the team hopes to optimize the device to produce more current. Additionally, they would like to grow rectennas on foil so they could be used in flexible solar cells or photodetectors. The researchers believe that a commercially-viable rectenna could be available within a year.