In a quest to find a new way to capture even more of sunlight's energy for generating electricity, researchers at MIT have come up with a cool new concept called a solar energy funnel. The proposal is based on the idea of creating elastic strain in a super thin semiconductor material, which would change the properties of the material and allow it to harness more of the spectrum.
MIT news explains:
In this case, the “funnel” is a metaphor: 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 household funnel. And yet, as it happens, the material actually does assume the shape of a funnel: It is 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.
The researchers have created computer models of elastic strain applied to molybdenum disulfide (MoS2), which is a semiconductor material and can be made into a film just one molecule thick. In the models, the elastic strain changed with the distance from the funnel's center, which meant that the electrons' potential energy also changed with distance from the center.
MIT News says MoS2 "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."
Another benefit is that, unlike in a traditional organic solar cell where the electron-hole pairs are diffused throughout the material after being generated by photons, the solar energy funnel actually leads the pairs to the collection site in the center, which should be a far more efficient way to gather charge and produce electricity.
The researchers plant to test the effect in a laboratory setting next to confirm that the material responds the same way as the models predict.