Harvest more energy from photons to improve solar cells and li-fi
The new approach is based on the finding that unexpected quantum effects increase the number of charge carriers, known as 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 finding is reported 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,” said Fang. The strength of the effect 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 this was something that was overlooked,” said Fang,
Previous experiments showing elevated production of electrons in such materials had been chalked up to defects in the materials. But Fang says those explanations “were not enough to explain why we observed such broadband absorption over such a thin layer” of material. But, Fang said, the team’s experiments back the newfound 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 aluminum 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, suggested Fang, because the materials involved are already widely used at industrial scale. “The oxide materials are exactly the kind people use for making better transistors,” said Fang and might now be harnessed to produce better solar cells and superfast photodetectors.
“The addition of a dielectric layer is surprisingly effective” at improving the efficiency of light harnessing, said Fang. And because solar cells based on this principle would be thin, Fang reckoned they would use less material than conventional silicon cells.
Because of their broadband responsiveness, Fang said, such systems also respond much faster to incoming light: “We could receive or detect signals as a shorter pulse than current photodetectors can pick up," explained Fang. This could even lead to new ‘li-fi’ systems, suggested Fand – using light to send and receive high-speed data.
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