Breakthrough nano-technology solar cell achieves 18.2% efficiency, eliminates need for anti-reflection layer

October 15, 2012, National Renewable Energy Laboratory

(—Scientists at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have produced solar cells using nanotechnology techniques at an efficiency – 18.2%—that is competitive. The breakthrough should be a major step toward helping lower the cost of solar energy.

NREL tailored a nanostructured while ensuring that the light-generated electricity can still be collected efficiently from the solar cell. 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. 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 of the surface to reach record solar cell efficiencies for this 'black silicon' material.

The paper, "An 18.2%-efficient black-silicon solar cell achieved through control of carrier recombination in nanostructures" by NREL's Jihun Oh, Hao-Chih Yuan, and Howard Branz, currently appears on Nature Nanotechnology's website.

Typically, solar cell manufacturers must add an extra anti-reflection layer, or two, to their cells, which boosts costs significantly.

NREL previously had demonstrated that their nanostructures reflected less light than the best anti-reflection layers of a solar cell. But until now, they hadn't been able to achieve overall efficiency with their black that could approach the best marks for other silicon cells.

Oh, Yuan, and Branz, first had to determine why the increased of the dramatically reduced the collection of electricity and hurt the voltage and current of the cells.

Their experiments demonstrated that the high-surface area, and especially a process called , 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.

This scientific understanding enabled them to suppress Auger 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.

The Energy Department funded the research grant through the American Recovery and Reinvestment Act.

Branz, the grant's principal investigator, said, "This work can have a big impact on both conventional and emerging solar cell based on nanowires and nanospheres. For the first time it shows that really great can be made from nanostructured semiconductors."

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."

"Now we have a clear study that shows how optimizing the surface area and the doping together can give better efficiency," Yuan said. "The surface area and the doping concentration near the surface affect nano-structured solar-cell performance."

First author, Oh, an NREL Postdoctoral Fellow said the NREL study "clearly shows that the right combination of a carefully nano-structured surface and good processing can reduce the cost while cutting unwanted reflection of sunlight."

Explore further: Nanostructures improve solar cell efficiency

More information: … /nnano.2012.166.html

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1 / 5 (1) Oct 15, 2012
Cost and durability, anyone?

It may work fine in the laboratory, but what happens when you use this coating in the field? Will it be degraded by weather and temperature cycles? And, perhaps most importantly, what does this coating cost?
not rated yet Oct 15, 2012
And, perhaps most importantly, what does this coating cost?

It is not coating. It is like drilling millions of holes onto the top surface of that silicon. So it is a process step, or actually several: deposit silver, immerse into solvent, clean up etc. Of course it has it`s own cost. Durability should be the same as with standard silicon surface with the caveat that it is probably very hard to clean the actual silicon surface, but any silicon cell is going to be placed under glass anyway, so this is not changing anything from current panels.
not rated yet Oct 15, 2012

Of course the word "coating" should be interpreted losely! To the subject matter..

I do not know of anyone using a naked silicon surface of anything. Silicon solar cells are covered by a glass to protect it. So, can you cover this surface with a glass? The reason I ask, it that as soon as you use an adhesive you add a layer with a different refractive index, which could affect the reflectivity.
About the cost. Solar cell manufacturers are racing to costs per Watt of the order of 1 USD per W. This equates to a surface cost of app. ~ 100-200 USD per suqare meter. If the new process steps add, say, 20 USD per m2, you are already nullifying the economic net gain. Consider that this cost is roughly equal to 1.5 USD per 30-inch wafer...which is not a whole lot.
not rated yet Oct 15, 2012
The biggest issue and the real question to be raised is how does this effect solar cells already on the market
not rated yet Oct 17, 2012
We must develop Solar to Hydrogen that is more efficient then using fossil fuel. So I say keep up the good work.
not rated yet Oct 18, 2012
This has got to be the virtual ultimate dead flat black. They are on to something! Let them work on it. One fella or two put it right that this will have to be under a protective layer of glass or something. Hard to clean will become nearly uncleanable as nanosization proceeds to limits of smallness with increased fragility so wide spectrum admittance and durable and flexible yet strong thin cover is imperative for commercial success. This keeping in mind plausible manufacturability.

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