Academy physicists probe mystery of 'black silicon'

Dr. Michael Shaffer describes a research project Oct. 9, 2009, to determine why "black silicon" -- a product from sulfur being added to silicon using lasers -- absorbs up to 500 times more light than regular silicon. The research, underway at the U.S. Air Force Academy in Colorado Springs, Colo., may decrease the cost per kilowatt-hour to produce solar energy. Dr. Shaffer is a contractor with MITRE Corporation. (U.S. Air Force photo/Rachel Boettcher)

Dr. Michael Shaffer describes a research project Oct. 9, 2009, to determine why "black silicon" -- a product from sulfur being added to silicon using lasers -- absorbs up to 500 times more light than regular silicon. The research, underway at the U.S. Air Force Academy in Colorado Springs, Colo., may decrease the cost per kilowatt-hour to produce solar energy. Dr. Shaffer is a contractor with MITRE Corporation. (U.S. Air Force photo/Rachel Boettcher)

A 200-milliJoule pulse laser fires in this timed exposure Oct. 9, 2009, at the U.S. Air Force Academy in Colorado Springs, Colo. The laser is used for a variety of experiments, including tests to determine how "pushing" sulfur into a silicon-based solar cell increases the cell's efficiency. (U.S. Air Force photo/Rachel Boettcher)

A 200-milliJoule pulse laser fires in this timed exposure Oct. 9, 2009, at the U.S. Air Force Academy in Colorado Springs, Colo. The laser is used for a variety of experiments, including tests to determine how "pushing" sulfur into a silicon-based solar cell increases the cell's efficiency. (U.S. Air Force photo/Rachel Boettcher)

The surface structure of black silicon, as imaged by an electron microscope, contains towers and valleys that dramatically increase the substance's surface area. Physicists theorize the structure contributes to black silicon's efficiency in absorbing light. (U.S. Air Force photo)

The surface structure of black silicon, as imaged by an electron microscope, contains towers and valleys that dramatically increase the substance's surface area. Physicists theorize the structure contributes to black silicon's efficiency in absorbing light. (U.S. Air Force photo)

U.S. AIR FORCE ACADEMY, Colo. -- Physicists here are researching new, more cost-efficient ways to create a substance that could make solar energy cheaper to produce.

Dr. Michael Shaffer and retired Lt. Col. Jody Mandeville are using nanosecond bursts from a 200-milliJoule pulsed laser in the Academy's Physics Department to produce black silicon, which is made by treating silicon in a sulfur hexafluoride atmosphere.

Nanosecond pulses are 1 million times longer than the femtosecond bursts currently used to produce black silicon, Dr. Shaffer said. If longer laser exposures produce similar results, black silicon may become much cheaper to manufacture.

"The whole laser system we're using is contained in two feet," Dr. Shaffer explained. "The femtosecond laser requires the whole back of the lab and is much more complex." It's also much more expensive to operate and maintain than the relatively inexpensive nanosecond laser used for black silicon research here.

The research aims to understand exactly what causes black silicon to absorb up to 20 times more light than regular silicon. After laser processing, the silicon's rough texture consists of many micrometer-sized cones, Harvard physicist Eric Mazur said in an Oct. 11, 2008, New York Times article. Dr. Mazur and his graduate students are credited with discovering black silicon.

"There's some debate as to the exact mechanism that causes the enhanced performance. The sulfur as well as the texturing seems to contribute to the improvements. We are trying to better understand what each does to increase light absorption," Dr. Mandeville said.

The Academy physicists also believe the sulfur is responsible for black silicon's absorption of infrared light -- a property not found in normal silicon, Dr. Shaffer said.

Even a dramatic increase in light absorption may only yield a 1- or 2-percent increase in solar cell efficiency, but every step forward counts, Dr. Mandeville said.

"Even if you can improve efficiency by only a couple of percent, that's a big effect because solar energy is a multi-billion-dollar industry," he said. "The big push is to get the energy cost per kilowatt-hour to below the cost of burning coal."

Dr. Shaffer is also involved with high-power alkali laser development that may eventually be applied in programs like the Airborne Laser, ground-based laser defense systems or countermeasure systems to defend aircraft from heat-seeking missiles. Dr. Shaffer holds a doctorate in atomic, molecular and optical physics from Old Dominion University for his work on the photoassociative spectroscopy of ultracold, metastable argon.

Dr. Mandeville's previous projects include research and development at Eglin Air Force Base, Fla., for laser radars. He holds a doctorate in optics and nanotechnology from the University of British Columbia.