“We’re trying to use elastic strains to produce unprecedented properties,” says Ju Li, an MIT professor and corresponding author of a paper describing the new solar-funnel concept that was published this week in the journal Nature Photonics.
In the experiment, a stretched sheet of ultra-thin film material is poked down at its centre 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 centre. 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 the sun’s energy.
According to MIT, this manipulation of strain in materials is part of a whole new field of research, elastic engineering, and is potentially significant for solar PV applications.
The MIT team used computer modelling 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.
Unlike graphene, another prominent thin-film material, MoS2 is a natural semiconductor, and it has a crucial characteristic, known as a bandgap, that allows it to be made into solar cells or integrated circuits. But unlike silicon, which is used most commonly in solar cells, placing MoS2 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 colours of light.
In a conventional 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,” said Xiaofeng Qian, the paper’s co-author. “It’s very inefficient.”
But in the solar funnel the electronic characteristics of the material “leads them to the collection site [at the film’s centre], which should be more efficient for charge collection,” he adds.
More laboratory tests will follow to confirm this hypothesis.
The elastic strain engineering field has been opened up by recent scientific developments, including the development of nanostructured materials, such as carbon nanotubes and MoS2, that are capable of retaining large amounts of elastic strain indefinitely, as well as the development of the atomic force microscope and next-generation nanomechanical instruments, which impose force in a controlled manner.
Elastic strain engineering has already been put to commercial use in computing, with IBM and Intel achieving a 50% improvement in electron velocity by putting 1% elastic strain on nanoscale silicon channels in transistors.
“People knew for a long time that by applying high pressure, you can induce huge changes in material properties,” said Li, adding that recent work has shown that controlling strain in different directions, such as shear and tension, can yield an “enormous” variety of properties.