Even at their most efficient, conventional solar panels are hard-pressed to make the grade. The silicon solar cells in widespread use today operate at an efficiency of less than 20 percent.
That’s why researchers have been focused on looking for new cell fabrication technologies and solar-collecting techniques that might up their efficiency of harnessing the sun’s electromagnetic radiation.
A research bonus would be finding a way to directly use solar radiation. Currently, it takes a separate mechanism to convert that captured energy into useable electricity in a commercial power grid.
The inefficiencies of silicon-based panels, plus the extra steps needed to make use of the stored rays, are a big reason why widespread adoption of solar power remains impractical.
But a new solar-energy-capturing technique being developed by University of Connecticut and Penn State Altoona researchers shows promise by using a novel fabrication technique that moves away from silicon.
Using nanosized antenna arrays instead of traditional silicon cells to capture the sun’s electromagnetic radiation, the researchers were able to harvest more than 70 percent of the solar power the device collected. At the same time, the antenna arrays were able to simultaneously convert solar rays into useable electric power.
These nanoantennas—also known as rectennas—absorb solar energy and rectify it from alternating to direct current. Currently, silicon solar panels generate power from only a small portion of the electromagnetic spectrum. But the research team says the new rectenna device would be able to maximize efficiency by harvesting visible and infrared light, something that hasn’t been done before.
“There’s a bit of nature inspiration behind it—that our eyes receive light—and the idea is extending devices at other frequencies down to the visible level,” says UConn engineering professor Brian Willis.
The promise of nanosized antenna arrays capable of harvesting the sun’s rays more productively has been talked about for years. In theory, nanoantennas would work because they were expected to have the ability to absorb and rectify solar energy from alternating to direct current at the speed of visible light. But the problem was how to construct these devices in such a way that their core pair of electrodes are a mere one or two nanometers apart—that’s a distance of about one millionth of a millimeter, or 30,000 times smaller than the diameter of human hair.
The gap could not be any wider than that because the space must create an ultra-fast tunnel junction between the rectenna’s two electrodes to maximize electricity transfer.
Advanced manufacturing process makes it possible
The potential breakthrough came in the form of a manufacturing approach using a chemical process called selective atomic layer deposition (ALD), which allows scientists to compose a working rectenna device. Before ALD, it was impossible to create a small enough space within a working electrical diode. With the new technique, thought, Willis was able to precisely coat the tip of the rectenna with layers of individual copper atoms until a gap of about 1.5 nanometers was achieved.
Willis says the exciting thing about using ALD for industries and scientists is its scalability for mass production. It is already in use in microelectronics manufacturing and as a coating technique for nanotubes and nanowires.
The researchers have already made a pilot version of the device and are now testing how to modify rectennas so they can better tune into frequencies. The team is hopeful that they will soon find that magic spot where the rectenna can reach peak solar energy absorption and rectify it into electrical power.
Willis says: “There’s a couple of ways we see this being implemented. One would be that it would supplement existing solar cells to make them more efficient. That would be the first step, but the next would be to have these as stand-alone devices. That would be an entirely different train of thought as this would be an entirely different kind of device rather than the semiconductor silicon ones that are what’s already out there.”
The solar grail, he says, is that this new technology could lower the high cost of harnessing solar energy and make it cost competitive with fossil fuels.
Top Image: Brian Willis holds a rectenna device. Courtesy Sean Flynn/UConn.