Black-silicon solar cells with 18.2% efficiency have been created by researchers from the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL). These don’t need extra anti-reflection layers (like typical solar cells do), which should help to significantly lower the cost of solar energy.
The specially designed nanostructured surface ensures that the light-generated electricity is still collectable in an efficient way from the solar cell, while still conferring the inherent advantages of a ‘black silicon‘ material.
“The researchers made nano-islands of silver on a silicon wafer and immersed it briefly in liquids to make billions of nano-sized holes in each square-inch of the silicon wafer surface,”an NREL news release states. “The holes and silicon walls are smaller than the light wavelengths hitting them, so the light doesn’t recognize any sudden change in density at the surface and, thus, don’t reflect back into the atmosphere as wasted energy. The researchers controlled the nanoshapes and the chemical composition of the surface to reach record solar cell efficiencies for this ‘black silicon’ material.”
Regularly, solar cell manufacturers need to add an additional anti-reflection layer, sometimes even more than one, to their cells; this raises the manufacturing costs considerably.
In previous research, NREL has shown that its nanostructures reflect significantly less light than even the best anti-reflection layers currently used. It had previously been unable to achieve overall conversion efficiencies that could challenge other solar cells with these ‘black-silicon’ solar cells, however.
To reach these efficiencies, the researchers first needed to resolve why having an increased surface area on the nanostructure-featuring solar cells drastically reduced the gathering of electricity current.
“Their experiments demonstrated that the high-surface area, and especially a process called Auger recombination, limit the collection of photons on most nanostructured solar cells. They concluded that this Auger recombination is caused when too many of the dopant impurities put in to make the cell work come through the nanostructured surface,” NREL states.
“This scientific understanding enabled them to suppress Auger recombination with lighter and shallower doping. Combining this lighter doping with slightly smoother nanoshapes, they can build an 18.2%-efficient solar cell that is black but responds nearly ideally to almost the entire solar spectrum.”
Branz added: “The next challenges are to translate these results to common industrial practice and then get the efficiency over 20%. After that, I hope to see these kinds of nanostructuring techniques used on far thinner cells to use less semiconductor material.”
The research was just published on the website of Nature Nanotechnology.