Called a rectenna – short for “rectifying antenna” – the device is made of common aluminum, silicon, and silicon dioxide (or glass) using standard processes from the semiconductor integrated circuit (IC) industry. The device has no moving parts, and uses direct conversion of infrared (IR) radiation to generate power.
“We have developed a new method for essentially recovering energy from waste heat,” says Paul Davids, a physicist and the principal investigator for the study. “Car engines produce a lot of heat and that heat is just waste, right? So imagine if you could convert that engine heat into electrical power for a hybrid car. This is the first step in that direction, but much more work needs to be done.”
In the short term, say the researchers, they are looking to make a compact infrared power supply – perhaps to replace radioisotope thermoelectric generators (RTGs), which are used for application such as powering sensors for space missions that don’t get enough direct sunlight to power solar panels.
The infrared rectenna developed by the researchers is about 1/8 x 1/8 inch, and metallically shiny with an aluminum top and silicon bottom. Its aluminum top is etched with stripes roughly 20 times smaller than the width of a human hair – a pattern too small to be seen by eye, but which serves as an antenna to catch infrared radiation.
Between the device’s aluminum top and silicon bottom is a very thin layer – about 20 silicon atoms thick, or 16,000 times thinner than a human hair – of silicon dioxide. The patterned and etched aluminum antenna channels the infrared radiation into this thin layer.
The infrared radiation trapped in the silicon dioxide creates very fast electrical oscillations – about 50 trillion times a second – which pushes electrons back and forth between the aluminum and the silicon in an asymmetric manner. This process, called rectification, generates net direct electrical current.
Because the infrared rectenna is made with the same processes used by the IC industry, it’s readily scalable, although not without challenges, say the researchers.
“There’s immense complexity under the hood and the devices require all kinds of processing tricks to build them.” says Rob Jarecki, the fabrication engineer who led process development. One of the biggest fabrication challenges, says Jarecki, was inserting small amounts of other elements into the silicon – or “doping” it – so that it would reflect infrared light like a metal.
The version of the infrared rectenna reported in a paper on the research produces eight nanowatts of power per square centimeter from a specialized heat lamp at 840°. So it would require a sheet of infrared rectennas slightly larger than a standard piece of paper to power a typical solar-powered calculator, which requires about five microwatts.
So going forward, the researchers are looking for ways to make the infrared rectenna more efficient. These include potentially making the rectenna’s top pattern two-dimensional x’s instead of one-dimensional stripes, in order to absorb infrared light over all polarizations; redesigning the rectifying layer to be a full-wave rectifier instead of the current half-wave rectifier; and making the infrared rectenna on a thinner silicon wafer to minimize power loss due to resistance.
Researcher Paul Davids says he thinks that within five years, the infrared rectenna may be a good alternative to RTGs for compact power supplies.
“We’ve been whittling away at the problem and now we’re beginning to get to the point where we’re seeing relatively large gains in power conversion, and I think that there’s a path forward as an alternative to thermoelectrics,” he says. “It feels good to get to this point. It would be great if we could scale it up and change the world.”