Reported Efficiency Improvement in Optical Rectennas

Researchers have reported a two-fold efficiency enhancement in devices and a switch to diode materials that are stable in air. The same team previously announced the first optical rectenna in 2015. The rectennas directly convert electromagnetic fields at optical frequencies to electrical current. Now with these latest improvements, the rectennas could operate low-power devices, for example temperature sensors.

Georgia Tech researchers have developed a new higher efficiency rectenna design. Here, the device’s ability to convert blue light to electricity is tested. (Image credit: Christopher Moore, Georgia Tech)

According to the researchers, their device design, which is a combination of a diode rectifier and carbon nanotube antenna, could contend with traditional photovoltaic technologies for generating electricity from various sources, including sunlight. In addition, the technology that is utilized in the rectennas could directly transform thermal energy into electrical current.

This work takes a significant leap forward in both fundamental understanding and practical efficiency for the optical rectenna device. It opens up this technology to many more researchers who can join forces with us to advance the optical rectenna technology to help power a range of applications, including space flight.

Baratunde Cola, Associate Professor at George W. Woodruff School of Mechanical Engineering

The study was reported in the Advanced Electronic Materials journal on January 26. The U.S. Army Research Office under the Young Investigator Program and the National Science Foundation supported the work.

Optical rectennas work by coupling the electromagnetic field of the light to an antenna; in this case, an array of multiwall carbon nanotubes is used that have open ends. An oscillation is created in the antenna by the electromagnetic field which eventually produces an alternating electron flow. As soon as the flow of electrons reaches a peak at one end of the antenna, the diode traps the electrons by closing and then opens again to capture the subsequent oscillation, producing a current flow.

It is important that the switching takes place at terahertz frequencies in order match the light. The junction between the diode and antenna should provide marginal resistance to the electrons flowing through it while open and at the same time should prevent leakage while closed.

The name of the game is maximizing the number of electrons that get excited in the carbon nanotube, and then having a switch that is fast enough to capture them at their peak. The faster you switch, the more electrons you can catch on one side of the oscillation.

Baratunde Cola, Associate Professor at George W. Woodruff School of Mechanical Engineering

In order to offer a low work function, i.e., ease of electron flow, the team first employed calcium as the metal in their oxide insulator - metal diode junction. However, calcium tends to break down quickly in air, which means during the time of operation the device needs to be encapsulated and manufactured in a glovebox. That made the optical rectenna not only impractical for most applications, but also difficult to develop.

Therefore, Cola, Research Engineer Thomas Bougher and NSF Graduate Research Fellow Erik Anderson used aluminum in the place of calcium and attempted a range of oxide materials on the carbon nanotubes before finally settling on a bilayer material containing hafnium dioxide (HfO2) and alumina (Al2O3). The combination coating fabricated for the carbon nanotube junction, developed via an atomic deposition process, offers the required quantum mechanical electron tunneling properties by engineering the electronic properties of the oxide rather than the metals. This makes it possible to use air stable metals with higher work functions than calcium.

When this new combination was used to fabricate rectennas, they continued to be functional as long as a year. Cola said that other metal oxides could also be utilized.

Further, the team designed the slope of the hill down in which the electrons fall during the tunneling process. That also made it possible to boost the efficiency, and enables the use of a wide range of oxide materials. With this novel design, the asymmetry of the diodes was also increased which considerably boosted the efficiency.

By working with the oxide electron affinity, we were able to increase the asymmetry by more than ten-fold, making this diode design more attractive. That's really where we got the efficiency gain in this new version of the device.

Baratunde Cola, Associate Professor at George W. Woodruff School of Mechanical Engineering

From a theoretical standpoint, optical rectennas could contend with photovoltaic materials for changing sunlight into electricity. PV materials work by using a different principle, wherein electrons from the atoms of specific materials are knocked down by photons. These electrons are then converted into electrical current.

Bougher and Cola reported the first optical rectenna in September 2015 in the journal Nature Nanotechnology. While this device had been proposed hypothetically for over four decades, it was never demonstrated.

The previous version of the device reported in the journal generated power at microvolt levels. Currently, the rectenna generates power in the millivolt range with conversion efficiency going from 10 (-5) to 10 (-3). While this is still extremely low, it is a substantial gain.

Though there still is room for significant improvement, this puts the voltage in the range where you could see optical rectennas operating low-power sensors. There are a lot of device geometry steps you could take to do something useful with the optical rectenna today in voltage-driven devices that don't require significant current.

Baratunde Cola, Associate Professor at George W. Woodruff School of Mechanical Engineering

According to Cola, the rectennas could prove handy for powering internet of things devices, particularly if they can be used for producing electricity from scavenged thermal energy. With regards to changing heat into electric current, the principle is the same as for light – that is capturing oscillations in a field with the broadband carbon nanotube antenna.

"People have been excited about thermoelectric generators, but there are many limitations on getting a system that works effectively," he said. "We believe that the rectenna technology will be the best approach for harvesting heat economically."

In upcoming work, the researchers hope to optimize the operation of the antenna, and enhance their hypothetical understanding of the workings of the rectenna, thus enabling more optimization. Cola believes that the devices have the potential to speed up space travel, generating power for electric thrusters that will effectively boost the spacecraft.

"Our end game is to see carbon nanotube optical rectennas working on Mars and in the spacecraft that takes us to Mars," he said.

This work was the Young Investigator Program agreement W911NF-13-1-0491 and the National Science Foundation Graduate Research Fellowship program under grant DGE-1650044. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the sponsoring organizations.

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