First device that directly converts light to electricity

Researchers, including those of Indian-origin, have developed the first optical rectenna, a device that can directly convert light 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, 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.
Billions of rectennas in an array can produce significant current, though the efficiency of the devices demonstrated so far remains below one per cent.

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 at Georgia Institute of Technology in US.

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.
Using metallic multiwall carbon nanotubes and nanoscale fabrication techniques, Cola and collaborators Asha Sharma, Virendra Singh and Thomas Bougher at Georgia Institute of Technology constructed devices that utilise 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 5 to 77 degrees Celsius.
"Based on what others have done and what the theory is showing us, I believe that these devices could get to greater than 40 per cent efficiency," Cola said.
The study appears in the journal Nature Nanotechnology.