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Thinner than paper light-absorption materials developed at Stanford

Thinner than paper light-absorption materials developed at Stanford
These four wafers contain the thinnest light-absorber layer ever built.

A wafer of material thousands of times thinner than paper could lower the cost and improve the efficiency of solar cells.

Researchers from Stanford University have built an efficient absorber of visible light out of nanosized materials.

When trying to cut the costs of solar, most scientists look to either enhance the efficiency of the materials used or to use less expensive materials. The Stanford team managed to do both.

They created a way to reduce the thickness of the cell without compromising – and possibly improving – its ability to absorb and convert light.

“Our results show that it is possible for an extremely thin layer of material to absorb almost 100 percent of incident light of a specific wavelength, “said Stacey Bent, a professor of chemical engineering at Stanford.

Key to the nanomaterial’s light-absorption properties are tiny nanodots of gold. The thin wafers are dotted with trillions of gold nanodots about 14 nanometers tall and 17 nanometers wide. These nanodots can be tuned to absorb the different spectrums of light.

“Much like a guitar string, which has a resonance frequency that changes when you tune it, metal particles have a resonance frequency that can be fine-tuned to absorb a particular wavelength of light,” explained postdoctoral scholar Carl Hagglund.

The entire visible light spectrum is made up of diffrent waves of light. These waves vary in length, such as violet light waves that are 400 nanometers long compared to red waves that are 700 nanometers long.

Mr. Hagglund and his colleagues were able to tune the gold nanodots used in these experiments to absorb reddish-orange light waves about 600 nanometers long.

The team then used a technique called block-copolymer lithography to fabricate wafers filled with their specially tuned gold nanodots. Each wafer contained about 520 billion nanodots per square inch. A thin-film coating was then applied to the top of the wafers using atomic layer deposition.

“It’s a very attractive technique because you can coat the particles uniformly and control the thickness of the film down to the atomic level. That allowed us to tune the system simply by changing the thickness of the coating around the dots,” explained Mr. Hagglund.

When exposed to light, the nanodots alone were able to absorb 93 percent of the reddish-orange light while the coated nanodot studded wafers absorbed 99 percent.

“The volume of each dot is equivalent to a layer of gold just 1.6 nanometers thick, making it the thinnest absorber of visible light on record – about 1,000 times thinner than commercially available thin film solar cell absorbers," said Mr. Hagglund.

The next step for the Stanford team is to demonstrate that the technology can be used in actual solar cells. They are working to build prototypes of structure that use this ultrathin material. They are also considering looking into other, even cheaper materials to make up their nanodots.

Other researchers on the project include Engineering Professor Mark Brongersma and former postdoctoral scholars Isabell Thomann and Han-Bo-Ram Lee from Stanford; and Gabriel Zeltzer and Ricardo Ruiz of Hitachi Global Storage Technologies in San Jose, Calif.

The research was supported by the Stanford Center on Nanotrsucturing for Efficient Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy. Additional support was provided by the Marcus and Amalia Wallenberg Foundation. – EcoSeed Staff

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