The discovery of a revolutionarily different way to generate electricity from sunlight has been made by researchers at MIT. The new technology, which is essentially a solar energy funnel, is able to use a much broader spectrum of sunlight's energy than conventional solar does, by utilizing materials under elastic strain. “We're trying to use elastic strains to produce unprecedented properties,” says Ju Li, an MIT professor and the lead author of a paper describing the new concept.
The 'funnel' in this case is a metaphor, though — it is electronic forces creating the funneling effect, not gravity as in a literal funnel. “Electrons and their counterparts, holes — which are split off from atoms by the energy of photons — are driven to the center of the structure by electronic forces.” But, interestingly, as the process occurs, the material actually assumes a funnel shape. The material is a stretched sheet of “vanishingly thin” material, pushed down at a center point with a microscopic needle, producing a curved shape similar to a funnel.
The pressure from the needle creates an elastic strain that increases toward the needle point. Because of the variation in the strain, the atomic structure is changed to the point where different sections are 'tuned' to different wavelengths of light. The majority of the energy in sunlight is invisible.
The material used is a thin layer of molybdenum disulfide, which is a semiconductor that can form a film just a single molecule in thickness. And it possesses a 'crucial characteristic' called bandgap, which allows it to be formed into solar cells. But unlike the material used in most solar cells, silicon, “putting the film under strain in the 'solar energy funnel' causes its bandgap to vary across the surface, so that different parts of it respond to different colors of light,” the MIT press release notes.
“It turns out that the elastic strain, and therefore the change that is induced in electrons' potential energy, changes with their distance from the funnel's center — much like the electron in a hydrogen atom, except this 'artificial atom' is much larger in size and is two-dimensional.”
The funnel will also lead to better charge collection, the researchers think. In typical solar cells, the excitons randomly move throughout the material after they've been generated by photons. But in the funnel, the characteristics of the material direct them to the collection site at the center, which should lead to more efficient charge collection.
“People knew for a long time that by applying high pressure, you can induce huge changes in material properties,” Li says. But more recent work has shown that controlling strain in different directions, such as shear and tension, can yield an enormous variety of properties.
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