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  • Photon funnel makes the most of sun’s rays

Photonics Spectra
Feb 2013
A solar “funnel” exploits materials under elastic strain and could provide a new way of harnessing photons for electricity by capturing a wider spectrum of solar energy.

“We’re trying to use elastic strains to produce unprecedented properties,” said Ju Li, an MIT professor and the lead author of the paper describing the new concept. Manipulating a material’s strain could open a whole new field of research.

The “funnel” in this case is a metaphor, however: Electronic forces create the funneling effect, not gravity as in a household 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. As the process occurs, the material – a stretched sheet of vanishingly thin material, pushed down at a center point with a microscopic needle – actually assumes a curved shape similar to a funnel.

The pressure from the needle creates an elastic strain, which increases toward the needle point. Because of the variation in the strain, the atomic structure is changed just enough to tune different sections to different wavelengths of light. This makes it possible to make use of not only visible light, but also some of the invisible spectrum, which accounts for much of sunlight’s energy.


A broad-spectrum solar energy funnel developed by engineers at MIT takes advantage of materials under elastic strain to capture a wider spectrum of solar energy.


The solar funnel uses precisely controlled elastic strain, which corresponds to stretched atomic bands, to govern electrons’ potential in the material. The team used computer modeling to determine the effects of the strain on a thin layer of molybdenum disulfide, a natural semiconductor that can form a film just a single molecule in thickness. Its bandgap enables it to be formed into solar cells. But unlike silicon, now used in most solar cells, putting the film under strain in the solar energy funnel causes its bandgap to vary across the surface, so that different parts respond to different colors of light.

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 as with the electron in a hydrogen atom, except this “artificial atom” is much larger in size and is two-dimensional.

The funnel could lead to better charge collection, the researchers say. 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 offer more efficient charge collection.

“People knew for a long time that by applying high pressure, you can induce huge changes in material properties,” Li said. But more recent work has shown that controlling strain in different directions, such as shear and tension, can yield an enormous variety of properties.

The researchers hope to soon conduct lab experiments to confirm the effect.


GLOSSARY
bandgap
In a semiconductor material, the minimum energy necessary for an electron to transfer from the valence band into the conduction band, where it moves more freely.
exciton
A moving, electrically neutral, excited condition of holes and electrons in a crystal. One example is a weakly bound electron-hole pair. When such a pair recombines, with the electron "falling" into the hole, the energy yielded is the bandgap decreased by the binding energy of the pair.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
shear
Image distortion that occurs when the axes of the original image are not perpendicular in the resulting image, making the resulting image appear slanted. Shear can be caused by movement of the original image during scanning or misaligment of the X and Y scanners.
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