Solar breakthroughs are relatively commonplace. However, typically they are iterative -- small increases by a percent or two in efficiency. Researchers at the Rensselaer Polytechnic Institute have invented a new solar cell that is anything but iterative as it blows away past offerings by a large margin; something RPI calls a "game-changer" for the solar business.
Against relatively cheap coal power, solar -- like nuclear and wind -- has struggled to compete from a purely economic standpoint. Worse yet, it trails wind and nuclear in terms of how close it is to being cost competitive. The light at the end of the tunnel is that solar have shown the highest gains in efficiency of any alternative energy source, making its future look very bright.
The new RPI solar cell is a normal cell covered in a special anti-reflective coating which traps sunlight from nearly every angle and part of the spectrum. The new cell is near perfect; it absorbs 96.21 percent of the sunlight shined on it, while a normal cell could only absorb 67.4 percent. That 43 percent efficiency boost, coupled with mass production, if properly implemented could place solar on the verge of competing unsubsidized with coal power, at last.
Shawn-Yu Lin, professor of physics at Rensselaer and a member of the university’s Future Chips Constellation describes the breakthrough, stating, "To get maximum efficiency when converting solar power into electricity, you want a solar panel that can absorb nearly every single photon of light, regardless of the sun’s position in the sky. Our new antireflective coating makes this possible."
Most materials have a mixture of light absorbing (anti-reflective) and light reflecting properties, depending on the angle and wavelength of light. For example, eyeglasses allow light to pass through on direct angles, but begin to reflect light at sharper angles. Solar panels in their current form operate with similar mixed character. In order to improve efficiency, mechanical components must be added to turn to panel to face the sun. This system entails significant cost and loss of energy efficiency, as well as a great maintenance burden.
With Professor Lin's discovery, the world's first cost-efficient static solar arrays could be produced. No matter what angle the sun was at, nearly all sunlight would be absorbed and converted to power. Professor Lin describes, "At the beginning of the project, we asked ‘would it be possible to create a single antireflective structure that can work from all angles?’ Then we attacked the problem from a fundamental perspective, tested and fine-tuned our theory, and created a working device."
Rensselaer physics graduate student Mei-Ling Kuo helped Professor Lin investigate various antireflective coatings. Their eventual choice was a nanomaterial, consisting of several fine anti-reflective sheets. Normal antireflective coatings consist of one sheet, which absorbs light at a specific wavelength. By stacking seven separate layers into a composite coat, they were able to absorb nearly the entire spectrum. Furthermore, the staggered nature of the layers "bent" the flow of sunlight to a favorable angle, trapping it in the coating. This means that if light manages to reflect off a lower layer, it will be sent back down by the upper layers.
Each layer was made from a special nanomaterial consisting of silicon dioxide and titanium dioxide nanorods positioned at an oblique angle. The material was grown through standard chemical vapor deposition techniques, and could be applied to the manufacturing of most standard solar cells, including III-V multi-junction and cadmium telluride cells.
On a microscopic level the nanomaterial looks like a forest of tiny, densely packed trees. Each layer is 50 nm to 100 nm thick.
The team hopes to bring their technology quickly to market, as it will require little in the way of manufacturing line changes. The research is detailed in the paper "Realization of a Near Perfect Antireflection Coating for Silicon Solar Energy", published in the journal Optics Letters.
Besides Lin and Kuo, the other researchers listed as co-authors on the paper were E. Fred Schubert, Wellfleet Senior Constellation Professor of Future Chips at Rensselaer; Research Assistant Professor Jong Kyu Kim; physics graduate student David Poxson; and electrical engineering graduate student Frank Mont.
The research was funded with the help of funding from the U.S. Department of Energy’s Office of Basic Energy Sciences, as well as the U.S. Air Force Office of Scientific Research.