Cheaper, better solar cell is full of holes
A silver wafer reflects the face of NREL research scientist Hao-Chih Yuan, before the wafer is washed with a mix of acids. The acids etch holes, absorbing light and turning the wafer black. Credit: Dennis Schroeder
A new low-cost etching technique developed at the U.S. Department of Energy's National Renewable Energy Laboratory can put a trillion holes in a silicon wafer the size of a compact disc.
As the tiny holes deepen, they make the silvery-gray silicon appear darker and darker until it becomes almost pure black and able to absorb nearly all colors of light the sun throws at it.
At room temperature, the black silicon wafer can be made in about three minutes. At 100 degrees F, it can be made in less than a minute.
The breakthrough by NREL scientists likely will lead to lower-cost solar cells that are nonetheless more efficient than the ones used on rooftops and in solar arrays today.
R&D Magazine recently awarded the NREL team one of its R&D 100 awards for Black Silicon Nanocatalytic Wet-Chemical Etch. Called "the Oscars of Invention," the R&D 100 awards recognize the most significant scientific breakthroughs of the year.
Howard Branz, the principal investigator for the project, said his team got interested in late 2006 after he heard a talk by a scientist from the Technical University of Munich. The scientist described how his team had created black silicon by laying down a thin gold layer using a vacuum deposition technique. Quickly, NREL senior scientist Qi Wang and senior engineer Scott Ward gave it a try.
"We always ride on the shoulders of others," Branz said. "We started by replicating the Munich experiment."
Packets of Light, Golden Holes
Think of light as coming in little packets. Each packet is a photon, which potentially can be changed into an electron for solar energy. If the photon bounces off the surface of a solar cell, that's energy lost. Some of the light normally bounces off when it hits an object, but a 'black silicon' wafer will absorb all the light that hits it.
The human eye perceives the wafer as black because almost no sunlight reflects back to the retina. And that is because the trillion holes in the wafer's surface do a much better job of absorbing the wavelengths of light than a solid surface does.
It's roughly the same reason that ceiling tiles with holes in them absorb sound better than ceiling tiles without holes. Scientists by the late 19th century had already done experiments to show that what works for absorbing sound also works for absorbing light.
The team from Munich used evaporation techniques that require expensive vacuum pumps to lay down a very thin layer of gold, perhaps 10 atoms thick, Branz said. When a mixture of hydrogen peroxide and hydrofluoric acid was poured on the thin gold layer, nanoparticles of gold bored into the smooth surface of the wafer, making billions of holes.
The NREL team knew right away that the vacuum pumps and evaporative equipment needed to deposit the gold were too costly to become commercially viable.
NREL's Goal: Simplify the Process, Lower the Cost
"Our thinking was that if the goal is to make it cheaper, we want to avoid vacuum deposition completely," Branz said.
In a string of outside-the-box insights combined with some serendipity, Branz and colleagues Scott Ward, Vern Yost and Anna Duda greatly simplified that process.
Rather than laying the gold with vacuums and pumps, why not just spray it on? Ward suggested.
Rather than layering the gold and then adding the acidic mixture, why not mix it all together from the outset? Dada suggested.
In combination, those two suggestions yielded even better results.
The scientists put a suspended solution of gold nanoparticles, called colloidal gold, on the silicon surface, and let the water evaporate overnight to leave just the gold, which then etched into the wafer. The wafer turned nearly as black as with the evaporated gold.
