University of Connecticut (UC) engineering professor Brian Willis has developed a novel fabrication process called selective area atomic layer deposition (ALD) to fabricate a working solar power antenna device. Solar power antennas are incredibly small nanosized antenna arrays that are theoretically capable of harvesting more than 70 percent of the sun’s electromagnetic radiation and simultaneously converting it into usable electric power.
We’ve seen this technology before back in January of 2008 when nano technology was first applied working mostly in the infrared range up to 80% efficiency. It hasn’t made big news since as the power developed runs at extreme alternating current frequencies, some ten thousand billion times a second.
The nano-antennas – known as “rectennas” because of their ability to both absorb and rectify solar energy from alternating current to direct current – must be capable of operating at the speed of visible light and be built in such a way that their core pair of electrodes is a mere 1 or 2 nanometers apart, a distance of approximately one millionth of a millimeter, or 30,000 times smaller than the diameter of human hair.
Willis’ effort developed the novel ALD process while teaching at the University of Delaware, and patented the technique in 2011.
In the new design one of the two interior electrodes must have a sharp tip, similar to the point of a triangle. To function the tip of the triangular electrode must be within one or two nanometers of the opposite electrode, something similar to holding the point of a needle to the plane of a wall. Before the advent of ALD, existing lithographic fabrication techniques had been unable to create such a small space within a working electrical diode. Using sophisticated electronic equipment such as electron guns, the closest scientists could get was about 10 times the required separation. Through atomic layer deposition, Willis has shown he is able to precisely coat the tip of the rectenna with layers of individual copper atoms until a gap of about 1.5 nanometers is achieved. The process is self-limiting and stops at the 1.5 nanometer separation.
The size of the gap is critical because it creates an ultra-fast tunnel junction between the rectenna’s two electrodes, allowing a maximum transfer of electricity. The nanosized gap gives energized electrons on the rectenna just enough time to tunnel to the opposite electrode before their electrical current reverses and they try to go back. The triangular tip of the rectenna makes it hard for the electrons to reverse direction, thus capturing the energy and rectifying it to a unidirectional current.
This information suggests the output would be a very fast-pulsed direct current rather than the incredibly fast earlier alternating current design.
Because of the rectennas are incredibly small and fast tunnel diodes they are capable of converting solar radiation in the infrared region up through the extremely fast and short wavelengths of visible light. The rectenna devices don’t rely on a band gap and may be tuned to harvest light over the whole solar spectrum, creating maximum efficiency.
Willis and a team of scientists from Penn State Altoona along with SciTech Associates Holdings Inc., a private research and development company based in State College, Pa., recently received a $650,000, three-year grant from the National Science Foundation to fabricate rectennas and search for ways to maximize their performance.
Willis said, “This new technology could get us over the hump and make solar energy cost-competitive with fossil fuels. This is brand new technology, a whole new train of thought.”
The Penn State Altoona research team led by physics professor Darin Zimmerman, with fellow physics professors Gary Weisel and Brock Weiss serving as co-investigators, has been exploring the theoretical side of rectennas for more than a decade. SciTech Associates Holdings is a creature of Penn State emeritus physics professors Paul Cutler and Nicholas Miskovsky.
Zimmerman said, “The solar power conversion device under development by this collaboration of two universities and an industry subcontractor has the potential to revolutionize green solar power technology by increasing efficiencies, reducing costs, and providing new economic opportunities. Until the advent of selective atomic layer deposition, it has not been possible to fabricate practical and reproducible rectenna arrays that can harness solar energy from the infrared through the visible. ALD is a vitally important processing step, making the creation of these devices possible. Ultimately, the fabrication, characterization, and modeling of the proposed rectenna arrays will lead to increased understanding of the physical processes underlying these devices, with the promise of greatly increasing the efficiency of solar power conversion technology.”
The atomic layer deposition process should be favored by science and industry because it is simple, easily reproducible, and scalable for mass production. Willis said the chemical process is already used by companies such as Intel for microelectronics, and is particularly applicable for precise, homogenous coatings for nanostructures, nanowires, nanotubes, and for use in the next generation of high-performing semi-conductors and transistors.
Willis added the method being used to fabricate rectennas also could be applied to other areas, including enhancing current photovoltaics (the conversion of photo energy to electrical energy), thermoelectrics, infrared sensing and imaging, and chemical sensors.
Over the next year, Willis and his collaborators in Pennsylvania plan to build prototype rectennas and begin testing their efficiency. Willis compares the process to tuning in a station on a radio.
“We’ve already made a first version of the device. Now we’re looking for ways to modify the rectenna so it tunes into frequencies better. I compare it to the days when televisions relied on rabbit ear antennas for reception. Everything was a static blur until you moved the antenna around and saw the ghost of an image. Then you kept moving it around until the image was clearer. That’s what we’re looking for, that ghost of an image. Once we have that, we can work on making it more robust and repeatable,” Willis explained.
Willis says finding that magic point where a rectenna picks up maximum solar energy and rectifies it into electrical power will be the champagne-popping, “ah-ha” moment of the project.
“To capture the visible light frequencies, the rectenna have to get smaller than anything we’ve ever made before, so we’re really pushing the limits of what we can do,” Willis said, “And the tunnel junctions have to operate at the speed of visible light, so we’re pushing down to these really high speeds to the point where the question becomes ‘Can these devices really function at this level?’ Theoretically we know it is possible, but we won’t know for sure until we make and test this device.”
Yes, Willis is quite correct. One expects it will likely work and quite well. But the first problem is going to be cost. The technology now is producing chips valued at hundreds and sometimes thousands of dollars per square foot and higher.
The second potential problem is power management – what one suspects stalled the amazing work back in 2008 – how to get the power into a workable form. That matter isn’t mentioned in the press release nor is there a study published.
This is one of the technologies that offer the highest and grandest hopes. It also offers some of the highest and most extraordinary technological and fabrication challenges ever seen at the nano level.
We’re watching, hopeful, expectant and wishing the best of luck and progress.