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Institute for Physical
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Iowa State University
2156 Gilman Hall
Ames, IA 50011-3110

Trap the Light Fantastic

IPRT researchers propose and test an innovative technology for trapping more light in solar cells to improve their performance.

Solar cells, which convert sunlight into electricity, are a promising alternative energy source. Indeed, in a recent survey by R&D magazine, 74 percent of respondents cited solar photovoltaic energy as the number one area where the United States should focus its energy research and development efforts.

A primary reason more research and development is needed is that a typical solar cell only converts about 20 percent of the light energy that falls upon it into electricity.  Even increasing that conversion rate by a small amount can vastly increase the usefulness of solar cells as an alternative energy source.

New and Improved Light Trapping

One way to boost their efficiency is through “light trapping,” essentially scattering incoming light to increase the chances that the cell will absorb the photons and thus generate more electricity. Today, back reflectors in solar cells are nothing more than metallic coatings that act to reflect and scatter light.  “Such reflectors are just not very good,” explains Rana Biswas, a physicist at the Catron Center for Solar Energy Research, Microelectronics Research Center and the U.S. Department of Energy’s Ames Laboratory.  “You lose three to seven percent of the light each time the light bounces off of it.”

Biswas is leading a team that has demonstrated a more sophisticated approach to light trapping to significantly increase the efficiency of solar cells.  The technology replaces reflective coatings with “photonic crystals” optimized for solar cells.  “It’s quite conceivable that you can increase efficiency 10 to 20 percent by capturing hard-to-collect photons,” Biswas says. The team includes Biswas, who is also an Iowa State University adjunct associate professor of physics and astronomy and electrical and computer engineering; and Dayu Zhou, a graduate student in electrical and computer engineering.

Photonic crystals are creating a revolution by manipulating and guiding light in novel ways.  Just as electronic bandgaps prevent electrons within a certain energy range from passing through a semiconductor, photonic crystals create photonic bandgaps that confine light of certain wavelengths.

Finding the Best

The photonic crystal designed by the team for use in solar cells is essentially a tiny lattice of silicon cylinders.  By using computer simulations, the researchers determined the optimal geometry of the photonic crystal — including thickness, lattice spacing and cylinder radius — to trap the most light and produce the highest amount of electricity from the cell. “The advantage of doing the simulation is that you can run all of these cases and find which one works best,” Biswas says.  The simulations are quite complex, so the researchers ran them on Iowa State University’s Blue Gene/L IBM supercomputer.  When installed, the system was the 73rd fastest computer in the world.

The research applies to any thin-film, amorphous silicon solar cell.  The photonic crystals are especially adept at trapping longer wavelength photons in the red and infrared regions, which is critical for enhancing cell efficiencies. And, it works better in thin solar cells, where it’s more difficult to trap light. Thin-film solar cells use less silicon, which is getting more expensive.

While determining the theoretical best design of the photonic crystals was challenging enough, the researchers have already been able to produce a preliminary experimental prototype at IPRT’s Microelectronics Research Center.  Built with assistance from Tony Barsic, Ben Curtin, Kyle Meyer, all undergraduates in electical and computer engineering, the prototype demonstrated that the theory was correct and that the use of photonic crystals can indeed improve the efficiency of solar cells.  With Vik Dalal, MRC director, Biswas and his team are now developing a complete and more complex solar cell using these concepts.

Much of this effort builds on the research into the fundamental science and application of photonic crystals at the Ames Laboratory.  For example, Biswas and his fellow researchers recently developed a photonic crystal “add-drop” filter that could eliminate electrical components from optical transmission links and guarantee virtually flawless data reception to end users of the Internet and other fiber-based telecommunications systems.