'Black silicon' boosts solar cell efficiency

Rough surface means black silicon traps wide range of light frequencies, says company after decade of development
Alok Jha, green technology correspondent
guardian.co.uk,
Wednesday October 15 2008 17.38 BST

An ultra-sensitive form of the silicon used in most solar panels may soon help to harness the near limitless power of the sun. Thanks to an extremely rough surface, "black silicon" can absorb more light and can also trap a wider range of frequencies, including infra-red rays, that normally pass straight through standard silicon.
Eric Mazur, a physicist at Harvard University, discovered black silicon by accident in his laboratory in 1998 when one of his research team blasted normal silicon with a very short laser pulse. Almost a decade later, the company created to commercialise his work, SiOnyx, has announced the production of the first commercial-grade wafers.
While producing electricity from the sun's rays has enormous potential, the industry has been hampered by the high cost of silicon wafers. Research teams around the world have been hunting for ways to bring down costs by improving the efficiency of solar cells.
Mazur said that photovoltaic (PV) cells using black silicon would significantly increase the efficiency of modern panels, the majority of which only convert around 8% of the energy falling on them into electricity. The very best convert around 20%. He said that a black silicon wafer could approach the theoretical limit of converting around 30%-40% of the energy falling on it into electricity.
Daniel Davies, chief technology officer at the PV manufacturer Solarcentury, said there were two potential areas for development in solar cells: getting more out of conventional crystals of silicon and the high-volume manufacture of thin film PV.
"SiOnyx sounds like an interesting method of increasing the efficiency of conventional crystalline PV," he said. "If this can be easily integrated into the cell processing that already exists then the potential to increase the global manufacturing capacity with a relatively low level of intervention is very exciting."
Making textured surfaces on silicon is already possible, notes Keith Barnham, a physicist at Imperial College London who works on PV cells. "What's important is if this [technique] helps the light absorption so that you can get away with thinner cells. There has been a silicon shortage and the price is not coming down, so the less you use of it the better. With good light-trapping, which is what these can do, you can hope to get higher efficiency in a much thinner cell."
The rough surface of black silicon allows it absorb light from many different angles, producing an electrical response to light that is 500 times greater than normal silicon. Another advantage is that black silicon can absorb infra-red radiation, which makes up around a quarter of the energy coming from the sun and which normally passes through silicon panels. As well as PV cells, it could improve the performance of any device that uses silicon to detect low levels of light, such as night vision goggles, medical imaging equipment, surveillance satellites and even digital cameras.
"We've been completely stealth, there hasn't been anything published about the company since its founding. We wanted to make sure we could scale the technology into a commercial foundry," said Steve Saylor, chief executive of SiOnyx. "The recent success in doing that is why we've started to introduce the company."
More than $11m (£6.3m) of venture capital has been invested so far in the Massachusetts-based SiOnyx. Mazur said the most recent work has shown that additional cost of making black, as opposed to standard, silicon wafers was marginal. "We were in a standard silicon foundry and there was no modification required to it."
Greenpeace's chief scientist Doug Parr expressed a note of caution: "The challenge [for solar] is cheap mass production and a widespread application of the technology. Many apparent breakthroughs fall at this hurdle, but we hope that black silicon can get to the next stage."
Mazur said his discovery of black silicon was serendipitous: "We were doing research on the chemical reactions on metal surfaces and, on a hunch, I said let's look at semiconductors, without a clear plan."
One of his research team fired short pulses of laser light at a piece of silicon wafer in the presence of the gas sulphur hexafluoride. When Mazur examined the resulting black silicon under a high-powered microscope, he found the surface was covered in a forest of short spikes.