Research collaboration creates an ultra-high solar energy conversion system

Researchers from the University of Illinois at Urbana-Champaign, in collaboration with AMI Research and Development, LLC (AMI), have developed advanced technology supporting an ultra-high—greater than 65 percent—solar energy conversion system. With the current emphasis on the fast-track development of clean-energy projects to combat global warming, these advancements provide a head start towards marketable solutions.

“Although there has been steady improvement in the efficiency of solar cells over the last few decades, the cost per watt of installed solar energy systems has not crossed the threshold needed to attract consumers to use solar energy as a cost effective alternative to fossil fuels,” explained Milton Feng, a professor of electrical and computer engineering at Illinois. “Even with the declining cost of silicon and the near theoretical performance of multi-junction photovoltaic cells, the efficiency needed to allow the dollars per watt threshold to be crossed is likely to be greater than 50 percent.”

The new design—developed under a four-year, $400,000 grant from AMI—is founded on antenna array principles, whereby a wedge prism serves as a continuous phased array coupler to a waveguide. The result is phase-coherent concentration in the waveguide to enable input angular dependence, coupling of the evanescent field from the prism to the waveguide, and gap-dependent theoretical monochromatic efficiencies of up to 96 percent. Using this coupling mechanism as a spectrally broadband, full-aperture, coherent, light concentrator is unique in functionality and application as detailed in two recent patents.

Illinois researchers used advanced photolithography and thin-film deposition techniques to fabricate a tunable, multi-index layered silicon oxynitride (SiON) wideband waveguide and a selective area solar cell. The AMI-University of Illinois team has also completed the preliminary design of the coherent solar concentrator that provides a low concentration open architecture for spatially separated band gap optimized rectennas. Simulation results and supporting lab measurements confirm achievable optical efficiencies greater than 80 percent. Work is proceeding with the development of fabricated rectennas—pairing a bowtie antenna with a metal-insulator-metal (MIM) tunnel diode to convert electromagnetic radiation into DC current.

“The rectennas can be fabricated on bare silicon wafers, greatly reducing cost compared to a conventional photovoltaic cell,” said Ardy Winoto, a graduate student in electrical and computer engineering and the project’s primary researcher. “The idea is to fabricate an array of rectennas that resonate at different wavelengths to cover the entire solar radiation spectrum.”

“When integrated together, the solar cell should yield the ultra-high efficiencies of greater than 65 percent,” Feng said. The higher efficiency has the added advantage of lower installation costs, plus, the device works at room temperature, an important trait for commercialization.

Following the recent International Climate Summit in Paris, the focus has intensified for technologies that permit better use of clean energy from wind and solar.

“Our research seems to fit well,” Feng remarked. “We have already been working on high efficiency green energy device conversion for four years, earning two patents in the process. It appears that the international community now has the will to turn such research into viable, marketable solutions. It is a very exciting time!”

Milton Feng is the Holonyak Chair Professor in the Department of Electrical and Computer Engineering, and a research professor at the Micro and Nanotechnology Lab where most of the component fabrication was achieved.
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