A 200-milliJoule pulse laser is being used at the Air Force Academy 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 is being used at the Air Force Academy to determine how “pushing” sulfur into a silicon-based solar cell increases the cell’s efficiency. U.S. Air Force photo/Rachel Boettcher

Academy seeking more effective way to change silicon

Researchers at the Air Force Academy are working to create a more efficient, cost-effective way of making black silicon — a material that is used to absorb light in solar photovoltaic cells, cameras and night vision goggles.
Black silicon is not typically found in nature; it is created when silicon is immersed in sulfur hexafluoride and hit with a high-intensity laser beam. Harvard researchers stumbled on the substance about a decade ago and licensed the patents for production to Massachusetts-based Sionyx Corp.
However, no one has discovered a way to efficiently create the substance in large quantities. That’s why the academy and its partner, Mitre Corp., are hoping to improve the process.
The joint project began six months ago, using a different laser to determine if they can increase the light spectrum absorbed by black silicon.
“We’re hoping the technology will allow the silicon to work better, to create more electricity, to absorb more light — even in the infrared,” said Jody Mandeville, a researcher with Mitre. “We’ve know that this happens for a while, for more than a decade, but we’re trying to understand why it happens and to see if we can make it happen in a more efficient way.”
Mandeville and academy scientist Michael Shaffer are using nanosecond bursts from a 200-milliJoule pulsed laser to produce black silicon.
“We think it might be the gas which is the element responsible for extending the spectral range of the black silicon beyond normal silicon,” Shaffer said. “When we blast the laser material inside the atmosphere, the sulfur hexafluoride is embedded in the surface.”
While Sionyx holds the license to commercialize the Harvard technology, academy researchers believe that a smaller, less intense laser can produce the same effects.
“We’re not challenging their findings,” Mandeville said. “But we want to better understand those claims to help improve on the process.”
Sionyx says that its system — shallow junction photonics — is a way past using expensive, often toxic materials. The company uses a femto-second laser that exposes the target to high intensity pulses as short as “one billionth of a millionth of a second.”
Air Force research is focusing on nanosecond bursts, which are 1 million times longer than the femto-second bursts used by Sionyx, Shaffer said.
If longer laser exposures produce similar results, black silicon could be cheaper to manufacture because the femto-second laser is more expensive to operate and maintain, he said.
“The whole laser system we’re using is contained in two feet,” Shaffer said. “The femto-second laser requires the whole back of the lab and is much more complex.”
The research will have important applications — no matter who wins the race to the final product. While the academy focuses on military applications, there also is a large commercial market.
The global market for silicon is $200 billion — with the market for silicon chips used in mobile phone cameras accounting for $7 billion.
For the military, the research could mean ways to cheaply move off the electric grid.
“It’s for energy security,” Shaffer said. “Investing in green energy and other sources will lead to more secure bases here and more secure forward operating bases in war zones. Green energy efficient is a dominant theme in the DoD, for similar reasons as the rest of the country.”
The academy has set the goal of creating its own energy by 2015, and the black silicon research will help accomplish that because of its applications in solar photovoltaic cells.
“But it can assist with other parts of the military mission as well,” Mandeville said. “Because we believe that black silicon can absorb infrared, it might be used to create cheaper, less complicated things like night vision goggles.”
However, neither the academy nor Mitre is able to take the technology beyond the laboratory and patent process. For that, they will need a private company, someone willing to invest in the technology and use it to create products such as digital cameras and solar photovoltaic cells.
And this might be the perfect time for such private investment, said Dr. Michael Larson, associate vice chancellor for research and El Pomar Chair of Engineering and Innovation at the University of Colorado at Colorado Springs.
Sionyx has raised $11 million in venture funding for its work.
“The government is placing a lot of money into research and development of green or alternative energies,” he said. “Everything from tax credits to the stimulus bill has an alternative energy component. But it’s also part of the culture now.”
Larson said that consumers are demanding more efficient and cheaper alternative energy products — and that smart companies will begin to invest in those products.
“It used to be that cost was the driving factor,” he said. “But now, sustainability is the buzz word — in many ways. People have shown that they will pay more if a product is sustainable and environmental.”
Which means the academy could be headed in the right direction. It is joining other research institutions following the government’s guidance to find other reliable energy sources.
“We don’t have a science program from the government,” he said. “But universities know that’s where the government’s focus is — so they are creating the research and development for these products.”