PORTLAND, Ore.–Now before you get all excited by the headline, which is not click-bait according to the researchers, a new kind of nanoscale rectenna (half antenna and half rectifier) can convert solar and infrared into electricity, plus be tuned to nearly any other frequency as a detector. The invention was made at Georgia Tech (Atlanta) and peer-reviewed in today’s issue of Nature Nanotechnology.
Right now efficiency is only one percent, but in the paper (DOI: 10.1038/nnano.2015.220) professor Baratunde Cola and colleagues at the Georgia Institute of Technology (Georgia Tech, Atlanta) convincingly argue that they can achieve 40 percent broad spectrum efficiency (double that of silicon and more even than multi-junction gallium arsenide) at a one-tenth of the cost of conventional solar cells (and with an upper limit of 90 percent efficiency for single wavelength conversion).
video: https://youtu.be/kMdYVLtBtoY
Professor Baratunde Cola explains and demonstrates how optical rectennas are going to change the world by making cheap solar-to-electricity converters out of carbon nanotubes whose end had been turned into a metal-insulators-metal tunnel diode.
(Source: Georgia Tech, used with permission)
Based on a theory put forth over 50-years ago the rectenna idea was before its time, but now technology has finally caught-up to the concept, which is well suited for mass production, according to Cola. It works by growing fields of carbon nanotubes vertically, the length of which roughly matches the wavelength of the energy source (one micron for solar), capping the carbon nanotubes with an insulating dielectric (aluminum oxide on the tethered end of the nanotube bundles), then growing a low-work function metal (calcium/aluminum) on the dielectric and voila–a rectenna with a two electron-volt potential that collects sunlight and converts it to direct current (DC).
“Our process uses three simple steps: grow a large array of nanotube bundles vertically; coat one end with dielectric; then deposit another layer of metal,” Cola told EE Times. “In effect we are using one end of the nanotube as a part of a super-fast metal-insulator-metal tunnel diode, making mass production potentially very inexpensive up to 10-times cheaper than crystalline silicon cells.”
The structure resembles a metal-insulator-metal capacitor of a few attofarads, but by reducing area of the capacitor “plate” (each nanotube bundle is just 10-to-20 microns in diameter), the high electrical field concentration at the end of the nanotube, and the low work function of the metal, makes the device behave like a peta-hertz tunnel diode excited by solar energy emitting electrons in femtosecond bursts.
For Cola’s proof of concept, his group grew multi-nanotube bundles of about 8-to-10 microns in diameter, which they then scaled up to thousands of side-by-side bundles measuring about an inch square which produced microwatts of output power from the sun.
For commercialization, billions or even trillions of carbon-nanotube bundles could be grown side-by-side, ramping up the power output into the megaWatt range, after optimization for higher efficiency.
“We still have a lot of work to do to lower contact resistance which will improve the impedance match between the antenna and diode, thus raising efficiency,” Cola told us.”Our proof-of-concept was tuned to the near-infrared. We used infrared-, solar- and green laser-light and got efficiencies of less than one percent, but what was key to our demo was we showed our computer model matched our experimental results, giving us the confidence that we can improve the efficiency up to 40 percent in just a few years.”
For the future, Cola’s group has a three tiered goal–first develop sensor applications that don’t require high efficiencies, second to get the efficiency to 20 percent for harvesting waste heat in the infrared spectrum, then start replacing standard solar cells with 40 percent efficient panels in the visible spectrum. The team is also seeking suitable flexible substrates for applications that require bending.
“Now we are at a point where engineers in industry, can reach out and give us more application conditions,” Cola told us. “One thing we can do that semiconductors can’t is operate at ultra-high temperatures–they never have to be cooled to work well. In fact, they work better at temperatures up to 400-to-500 degrees Celsius.”
Cola estimates that 10-to-20 research groups worldwide are working on retennas, but his is the only one to change the paradigm to make vertical instead of planar devices, which makes all the difference.
Other researchers contributing to the work included Asha Sharma, Virendra Singh and Thomas Bougher.
Funding was provided by the Defense Advanced Research Projects Agency (DARPA), the Space and Naval Warfare (SPAWAR) Systems Center and the Army Research Office (ARO).
— R. Colin Johnson, Advanced Technology Editor, EE Times 
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I just wanted to point out the list of research funding agencies at the end of this excellent article: DARPA (Pentagon), SPAWAR (Navy), and ARO (Army). The incredible value to our long-term economy of federal and DoD funded university and small-business is often underappreciated in these days when bashing anything federal has become a byword for some folks. Yet the truth is that it's often the federal and DoD funding in basic research that often triggers some of the highest value innovation in the private sector, where (as planned!) those research investments get transferred over to private groups that have the skills to make them into real products with real impacts.
This technology is still early, but this well-planned strategy of one of our finest universities promises to deliver a truly world-changing approach to renewable energy in the long term. The implications of that simple cannot be underestimated, and it is the kind of result that comes from well-planned, judicious use of public and private partnerships that pay careful attention to where the true strengths reside on both sides of that partnership.