Ubiquitous silicon plays key role in solar revolution

	Michael Stavola, left, in the lab with his former graduate student, E Elinor Chen.

Silicon, one of the earth’s most common and versatile materials, is leading a new revolution in solar energy, says Michael Stavola, professor of physics.
The second most plentiful element in the earth’s crust (after oxygen), silicon is used in glass, cement and ceramics, and in a variety of synthetic plastic substances. It is also the key ingredient in the microchip that has paved the way for the revolution in microelectronics.
Increasingly, silicon is playing a critical role in solar cell technology. The silicon solar cell industry, says Stavola, is growing by more than 40 percent per year. Silicon is used in more than 90 percent of all solar cells for power modules; thin films composed of compounds of other semiconducting materials make up most of the rest.
Solar cell manufacturers typically use a wafer made of multicrystalline silicon that is cheaper than the single-crystal silicon used in microchips. This keeps costs low, but with a catch: Multicrystalline silicon has more defects, and greater amounts of carbon and other impurities, than does single-crystal silicon. These defects, which include grain boundaries and dangling bonds, reduce the ability of the solar cell to generate electricity.
Stavola has spent 25 years investigating defects in semiconductors and the ability of hydrogen to passivate, or neutralize, those defects. His goal is to answer fundamental questions about the behavior of hydrogen in semiconductors, particularly in silicon solar-cell materials.
“There is a whole layer of basic science questions associated with the effect of hydrogen on the production of industrial solar cells that people do not understand,” he says.
By improving the understanding of hydrogen’s role in passivating semiconductor defects, Stavola hopes to help neutralize one of the major obstacles to the wider use of solar energy—its price tag.
“It’s easy to make solar cells; the technology dates back to the 1950s,” he says. “The issue is cost. The bottom line is that you have to be able to make solar cells as cheap to use as coal is.”
Serendipitous team formation
As an experimental physicist, Stavola conducts vibrational spectroscopy experiments with a Fourier Transform Infrared Spectrometer. The vibrations of atoms offer clues to the atomic structure and chemistry of materials.
Stavola works closely with Beall Fowler, professor emeritus of physics and a theoretician who calculates the atomic structures and vibrational properties of material defects.
“It’s a wonderful partnership because our skills are complementary,” Stavola says. “Beall interprets the results of my experiments and comes up with ideas for a new set of tests. We go back and forth until we confirm whether our theories are correct or not.”
Stavola and Fowler studied in the same research group at the University of Rochester, but 15 years apart. They met in the late 1970s when Stavola was a graduate student and Fowler, already on the Lehigh faculty, returned to his alma mater to give a talk. They discovered their common interest in semiconductor defects and stayed in touch until 1989, when Stavola joined the Lehigh faculty.
In the early 1990s, Stavola met Ajeet Rohatgi, who earned a Ph.D. in metallurgy and materials science from Lehigh in 1977 and is founder of the University Center for Excellence in Photovoltaics, one of the nation’s leading solar energy research centers, at the Georgia Institute of Technology. Their initial encounter occurred at an international workshop on solar cells sponsored by the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL), an annual event Stavola has attended for more than 15 years.
Today, Rohatgi’s group provides Stavola’s students with silicon test samples that have been processed using the methods that industry uses to fabricate solar cells. Stavola also has ties with researchers at the Technical University of Dresden in Germany, Texas Tech University, the University of Rome, the University of Pittsburgh at Johnstown, and American Capital Energy. He has received funding from the National Science Foundation, from NREL, and from a consortium of solar cell companies.
A low-cost strategy
When a silicon solar cell is fabricated, a thin anti-reflection coating is added to facilitate the penetration and absorption of light into the silicon. This coating, which contains 20 percent hydrogen, is annealed when the solar cell is processed to make its metal contacts. During this process, the hydrogen diffuses from the coating into the silicon.
Inside the solar cell, defects in the silicon undermine the generation of electricity by causing electron-hole pairs that have been generated by exposure to sunlight to recombine. The hydrogen mitigates these defects, prolonging the lifetimes of the electron-hole pairs and restoring their contribution to the electric current.
“The manufacturers of solar cells are able to make hydrogen passivate the defects in the multicrystalline silicon, but without understanding what exactly is happening,” Stavola says.
Stavola’s team is the first to observe and characterize the hydrogen that is introduced into silicon solar cells during fabrication. The group has also determined the concentration and penetration depth of the hydrogen and compared these criteria for various solar-cell processing methods.
Other questions remain unanswered. Which defects are passivated by the hydrogen? What effect do the defect reactions have on the silicon? How can these reactions be modeled to enable fabricators to optimize all the steps in the hydrogenation process?
“There are a lot of ways to get hydrogen to enter solar cell materials,” Stavola says. “What is needed is a low-cost strategy that works in conjunction with the processing of the cell.”
Stavola began his studies of the effect of hydrogen on semiconducting materials in the 1980s at Bell Laboratories. In addition to silicon, the materials he has investigated include gallium-arsenide, gallium-nitride, semiconducting-oxides and gallium-arsenide-nitride.
“Every semiconducting material has an interesting hydrogen story. I work on a wide variety of materials, each of which has different hydrogen defect physics,” he says. “Solar cell technology is a natural fit for me.”
--Kurt Pfitzer