First optical rectenna could double solar cell efficiency

The first ‘optical rectenna’, a device that combines an antenna and a rectifier diode, has been developed with the capability to double the efficiency of solar cells.

Engineers at the Georgia Institute of Technology have been working on the project for six years and have finally unveiled the new device which is able to convert light directly into DC current.

Based on multiwall carbon nanotubes with tiny rectifiers (devices that convert current from AC to DC) fabricated onto them, the optical rectennas show promise in a number of areas.

The engineers said the new technology could allow for photodetectors that can 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.

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 and off at record high petahertz speeds, creating a small direct current.

Individually, the current produced is negligible, but if billions of rectennas were implanted into an array they would be capable of producing significant current.

Although the current prototype is less than one per cent efficient, the researchers hope to boost that output through optimisation 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 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 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 per cent efficiency. If we can get to higher efficiencies, we could apply it to energy conversion technologies and solar energy capture."

Developed in the 1960s and 1970s, rectennas have operated at wavelengths as short as ten microns, but for more than 40 years researchers have been attempting to make devices at optical wavelengths.

There were many challenges: making the antennas small enough to couple optical wavelengths and fabricating a matching rectifier diode small enough and able to operate fast enough to capture the electromagnetic wave oscillations.

"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."

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