New Designs and QC Light Up Solar Cells

In a field west of the NC State campus, a large array of solar panels looks up toward the sun. If it catches enough rays, the array can generate power for seven or eight households. Still, University researchers believe design improvements can juice these and other solar
panels to double—or even triple—the amount of electricity they produce.

The husband-wife research team of Drs. Salah Bedair and Nadia El-Masry, along with electrical engineering professor John Hauser, has already devised solar cells that have the potential to operate at more than twice the efficiency of other photovoltaic devices—40 percent versus 18 to 19 percent. With a three-year grant from the Department of Energy, they aim to raise the bar even further, to about 50 percent efficiency. Their secret is the multi-junction solar cell, a Dagwood sandwich-type device where layers of light-absorbing circuits are stacked atop one another. The researchers are now working to add a fourth layer to the device. “The more we can stack, the better,” says Bedair, a professor of electrical engineering. “But there are limitations to the materials.”

“We are establishing a global reputation as a leading research and academic institution.”

Each layer consists of a film of indium gallium arsenide (InGaAs). Tweaking the relative concentration of the three constituent elements allows the films to absorb different wavelengths of light. The three layers on the team’s current solar cells take in the blue and green end of the spectrum, Bedair says, and they hope the fourth layer can pick up more on the red and infrared end. But the stacking and the tweaking alter the crystalline structure of InGaAs, says El-Masry, a professor of materials engineering. So researchers must work at producing crystals with as few defects as possible since defects hinder the process of converting light energy to electricity. “There’s really no limit to the multi-junction concept,” she says, “if you know how to put it together.”

In the multi-junction solar cell, layers of light-absorbing circuits are stacked atop one another in a Dagwood sandwich-type device.

Reducing defects is also the goal of the Silicon Solar Consortium (SiSoC), a materials and processing research center led by NC State and funded by 14 member companies and the National Science Foundation. Researchers use advanced instrumentation to measure the structural, chemical, and electrical behavior of silicon-based solar cells, checking for defects and impurities that can degrade performance, says SiSoC Director George Rozgonyi, a professor of materials science and engineering. “We work with leading-edge companies to optimize crystal growth and device processing,” Rozgonyi says. “Solutions to problems often lie in applying fundamental materials science concepts to improving the quality of materials in solar cells, leading to better performance and more energy generation.”


Graduate student Oliver Luen (background) processes multi-junction solar cells developed by researchers Salah Bedair and Nadia El-Masry.

Dr. George Rozgonyi (right) discusses the characteristics of silicon with Ph.D. candidate Jay Bharathan.