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Researchers develop method of harvesting more energy from photons

05 November 2015

Researchers have found a way to boost significantly the energy that can be harnessed from sunlight, a finding that could lead to better solar cells or light detectors.

Illustration: Christine Daniloff/MIT

The new approach, developed by researchers at MIT and elsewhere, is based on the discovery that unexpected quantum effects increase the number of charge carriers - electrons and 'holes' - that are knocked loose when photons of light of different wavelengths strikes a metal surface coated with a special class of oxide materials known as high-index dielectrics. The photons generate what are known as surface plasmons — a cloud of oscillating electrons that has the same frequency as the absorbed photons.

The surprising finding is reported this week in the journal Physical Review Letters by authors including MIT’s Nicholas Fang, an associate professor of mechanical engineering, and postdoc Dafei Jin. The researchers used a sheet of silver coated with an oxide, which converts light energy into polarization of atoms at the interface.

“Our study reveals a surprising fact: Absorption of visible light is directly controlled by how deeply the electrons spill over the interface between the metal and the dielectric,” Fang says. The strength of the effect, he adds, depends directly on the dielectric constant of the material — a measure of how well it blocks the passage of electrical current and converts that energy into polarization. “In earlier studies,” Fang says, “this was something that was overlooked.”

Previous experiments showing elevated production of electrons in such materials had been chalked up to defects in the materials. "[Those explanations] were not enough to explain why we observed such broadband absorption over such a thin layer [of material]," says Fang. However, the team’s experiments back the new found quantum-based effects as an explanation for the strong interaction.

The team found that by varying the composition and thickness of the layer of dielectric materials (such as aluminium oxide, hafnium oxide, and titanium oxide) deposited on the metal surface, they could control how much energy was passed from incoming photons into generating pairs of electrons and holes in the metal — a measure of the system’s efficiency in capturing light’s energy. In addition, the system allowed a wide range of wavelengths, or colors, of light to be absorbed, they say.

The phenomenon should be relatively easy to harness for useful devices, Fang says, because the materials involved are already widely used at industrial scale. “The oxide materials are exactly the kind people use for making better transistors,” he says; these might now be harnessed to produce better solar cells and super-fast photodetectors.

According to Fang, the addition of a dielectric layer is surprisingly effective at improving the efficiency of light harnessing. And because solar cells based on this principle would be very thin,  they would use less material than conventional silicon cells.

Because of their broadband responsiveness, such systems also respond much faster to incoming light. It might be possible to receive or detect signals as a shorter pulse than is the case with current photodetectors - which might even lead to new 'li-fi' systems that use light to send and receive high-speed data.

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