Ju Li, an MIT is a professor and corresponding author of a paper proposing a “solar energy funnel” that takes advantage of materials under elastic strain.

The MIT idea tries to answer a quest to harness a broader spectrum of sunlight’s energy to produce electricity.  Li said, “We’re trying to use elastic strains to produce unprecedented properties.”

Elastic Strain Photon Funnel. Click image for the largest view.

The paper published in the journal Nature Photonics, describes the idea in a metaphor as a new solar 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, not by gravity as in a common funnel. And yet, as it happens, the material actually does assume the shape of a funnel: It becomes a stretched sheet of vanishingly thin material, poked down at its center by a microscopic needle that indents the surface and produces a curved, funnel-like shape.

The pressure exerted by the needle imparts elastic strain, which increases toward the sheet’s center. The varying strain changes the atomic structure just enough to “tune” different sections to different wavelengths of light – including not just visible light, but also some of the invisible spectrum, which accounts for much of sunlight’s energy.

This is a lot like a finger pushing in on a sheet of say, stretched flat balloon material, putting a strain on the material as the shape is enforced.

Strain is defined as the pushing or pulling of a material into a different shape and it can be either elastic or inelastic. Xiaofeng Qian, a postdoc in MIT’s Department of Nuclear Science and Engineering who was a co-author of the paper, explains that elastic strain corresponds to stretched atomic bonds, while inelastic, or plastic, strain corresponds to broken or switched atomic bonds. A spring that is stretched and released is an example of elastic strain, whereas a piece of crumpled tinfoil is a case of plastic strain.

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.

The MIT team found 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.

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.

Qian explains in the solar funnel 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.”  This compares to today’s organic solar cell where the electron-hole pair, called an exciton, moves randomly through the material after being generated by photons that limits the capacity for energy production. “It’s a diffusion process,” Qian says, “and it’s very inefficient.”

The work is an evolutionary step forward convergence of lessons from four trends.  Li said the convergence, “has opened up this elastic strain engineering field recently.”

The trends are 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.

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

Elastic strain is already in the commercial sector.  IBM and Intel achieved a 50 percent improvement in velocity of electrons simply by imparting a 1 percent elastic strain on nanoscale silicon channels in transistors.

The idea has great potential.  Solar collection needs concentration by all and any means to gather energy at lower costs. One hopes the lab experiments confirm the work and offer ways to broaden the understanding.

Qian sums up that the solar funnel material electronic characteristics “leads them to the collection site [at the film's center], which should be more efficient for charge collection.”

More solar efficiency is a very good thing.


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