The quest to harness a broader spectrum of sunlight's energy to produce electricity has taken a radically new turn, with the proposal of a 'solar energy funnel' that takes advantage of materials under elastic strain. "We're trying to use elastic strains to produce unprecedented properties," says Ju Li, an MIT professor. The new solar-funnel work uses precisely controlled elastic strain to govern electrons' potential in the material. The MIT team used computer modeling to determine the effects of the strain on a thin layer of molybdenum disulfide (MoS2), a material that can form a film just a single molecule (about six angstroms) thick.
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. In the future, the researchers hope to carry out laboratory experiments to confirm the effect.
Unlike graphene, another prominent thin-film material, MoS2 is a natural semiconductor: It has a crucial characteristic, known as a bandgap, that allows it to be made into solar cells or integrated circuits. But unlike silicon, now used in most solar cells, placing the film under strain in the "solar energy funnel" configuration causes its bandgap to vary across the surface, so that different parts of it respond to different colors of light.
In an organic solar cell, the electron-hole pair, called an exciton, moves randomly through the material after being generated by photons, limiting the capacity for energy production. "It's a diffusion process," Qian says, "and it's very inefficient."
But in the solar funnel, he adds, the electronic characteristics of the material "leads them to the collection site [at the film's center], which should be more efficient for charge collection."
The convergence of four trends, Li says, "has opened up this elastic strain engineering field recently": the development of nanostructured materials, such as carbon nanotubes and MoS2, that are capable of retaining large amounts of elastic strain indefinitely; the development of the atomic force microscope and next-generation nanomechanical instruments, which impose force in a controlled manner; electron microscopy and synchrotron facilities, needed to directly measure the elastic strain field; and electronic-structure calculation methods for predicting the effects of elastic strain on a material's physical and chemical properties.
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