Published on October 14th, 2012 | by Zachary Shahan
Black Silicon Solar Cells Get Efficiency Boosts (Two of Them)
Here are two recent news stories on black silicon solar cells getting efficiency boosts from German and US scientists, courtesy Solar Love:
While conventional solar cells can absorb a good portion of the Sun’s light, they completely miss the boat on getting anything from the infrared spectrum. But black silicon solar cells are actually designed exactly for this. A group of researchers from the Fraunhofer-Gesellschaft institute in Germany have recently succeeded in doubling solar cell efficiency of black silicon solar cells!
Infrared radiation makes up about 25% of the solar spectrum. Black silicon can absorb almost all of this, and then turn it into electricity. So, there’s quite a bit of potential to improve the efficiency of solar panels by using black silicon.
What the heck is black silicon, you ask? “Black silicon is obtained by irradiating conventional silicon under sulfur atmosphere with a femtosecond laser,” explains Dr. Stefan Kontermann, group manager of the Fraunhofer Project Group Fiber optic sensor systems at the Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut.”The surface is roughened, installed individual sulfur atoms in the silicon lattice and the material is black.” (Translations from German courtesy of Google Translate.)
So, how did the researchers boost black silicon solar cell efficiency? (Warning: science speak coming.) “We achieved this by having changed the shape of the laser pulse, with which we irradiate the silicon,” says Kontermann.
Here’s more from the Fraunhofer-Gesellschaft press release: “This allowed the scientists to solve a problem of the black silicon: While the infrared light hitting normal silicon does not have enough energy to raise the electrons in the ‘conduction band’ and thus bring in the current cycle, ie convert it into electricity, the sulfur added to the black silicon creates a sort of intermediate step here.”
However, this intermediate step also allows electrons to jump the wrong way, resulting in lost electricity. But the researchers’ modification of the laser pulse solves this problem a bit by guiding more electrons in the right direction.
Where to go next?
The researchers’ next step is to evaluate how different shapes of laser pulses affect energy levels of the sulphur. And, eventually, the goal to create a system of algorithms that will be able to automatically identify how the laser pulse should be modified in order to achieve maximum efficiency.
The end goal is, of course, to merge the black silicon solar cells the researchers develop with existing commercial technology in order to bring a leading solar power product to market. According to the researchers, their intention is to add black silicon solar cells to conventional solar cells, creating a tendem solar cell, and thus boosting overall solar cell efficiency by about 1%.
“Furthermore, the scientists are planning a spin-off: In this company they want to market the laser system for manufacturers to expand their existing solar cell lines. They would then be able to produce the black silicon itself and build it into the cells.”
Yeah, this is all a bit over my head, but sounds interesting, and useful. Let’s hope so!
And, apparently, the research is rather highly esteemed, as this project has won an award from the “365 Places in the Land of Ideas” competition. An award ceremony is being held tomorrow (or later today for some of you), October 11, in Goslar.
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.