Black silicon has been around for so long, it's hard to remember it hasn't come out yet.
'Black silicon' refers to silicon wafers rutted with billions of tiny, nano-scale pits. The irregular surface effectively traps a larger percentage of photons by preventing light from being reflected, similar to how holes in ceiling tiles absorb sound. In theory, this gives solar cells made from black silicon a higher potential for efficiency because more of the sunlight that strikes them can be turned into electricity. Regular silicon solar cells are expected to max out at 25-percent efficiency.
Wafers and cells from black silicon look black, instead of a shimmery gray, because less light bounces off the surface and into your eye. (SunPower has been a leader in texturing surfaces for maximum efficiency.)
Sionyx, a spin-out from Harvard, revealed a technique for producing black silicon a few years ago with brief pulses of lasers and sulfur gases. Researchers in Munich have bored holes into silicon wafers with gold.
Late last week, NREL researchers discussed a cheaper way to produce black silicon using chloroauric acid instead of colloidal gold. The researchers initially were trying to refine the results from Munich, but instead stumbled on a cheaper way to produce black silicon. The NREL technique can also achieve results fairly quickly and without an expensive vacuum manufacturing environment.
"You take a beaker, put a silicon wafer in, pour in the chloroauric acid, pour in the hydrofluoric acid and hydrogen peroxide, and wait," said Howard Branz, the principal investigator, in a prepared statement.
Unwanted reflections on the NREL-produced black silicon were reduced to less than two percent, even better than the three percent to seven percent achieved in Munich.
Some of the next steps will include trying to figure out how to optimize the etching process. So far, the group has found that the best results occurred when the holes average around 500 nanometers deep with diameters slightly smaller than the smallest wavelength of light.
NREL, like all of the national labs, has been accused of failing to act aggressively to license its technology, but that has improved in recent years. If the results of this research can be amplified, black silicon could one day end up in roof-mounted solar panels.