Metal particles have been used as a catalyst in this process because etching is accelerated near metal particles. At first, gold was the metal of choice but that was never going to work in mass production so scientists found a way to switch to silver particles - much cheaper at around $20 per troy ounce but still not cost-effective enough for mass use, even in small amounts, when it comes to even a small, but typical for solar, 100MW facility.
Researchers from Natcore Technology Inc. have successfully used copper as a catalyst. Copper costs about 20¢ per troy ounce, 1/100th of the cost of silver. The chemical stew that makes it possible is a mix of copper nitrate, phosphorous acid, hydrogen fluoride and water. When applied to a silicon wafer, the phosphorus acid reduces the copper ions to copper nanoparticles. The nanoparticles aid in removing electrons from the silicon wafer's surface, thereby oxidizing it. The oxidized silicon is dissolved by the hydrogen fluoride, resulting in a process that forges inverted pyramid-shaped structures into the silicon.
The result of fine-tuning the process is a black silicon layer with features as small as 590 nanometers (billionths of a meter) that reflect less than one percent of light. By comparison, a clean, un-etched silicon wafer reflects nearly 40 percent of light.
"There are still some challenges to overcome," says Andrew Barron, a Natcore co-founder and a professor at Rice University. "The spikes would still require a coating to protect them from the elements, and we're working on ways to shorten the process needed to perform the etching in the lab. We also need to completely remove the copper catalyst in order to extend the life of the solar cell. But this method is far more practical than previous methods."
"This is another step in our push to bring down the cost of solar energy and to make it cost-competitive with energy derived from conventional sources," says Chuck Provini, Natcore's president and CEO. "By switching from silver to copper, we'd lower the cost of producing a solar cell by a fraction of a penny. But over the course of a 100 MW facility, that's a saving of more than $100,000."
Published in Journal of Materials Chemistry A. SOURCE Natcore Technology Inc.