Now for something completely new in the quest to gather in more of sun’s energy as fuel for photovoltaic devices: The MIT solar funnel.
This “funnel” is at once theoretical, metaphorical and real, it seems. It has been demonstrated so far using computer modeling, not laboratory testing; it isn’t like your household funnel, in that it uses electronic forces, not gravity, to exert its concentrating power; and yet it actually does take on something of a shape of a funnel as it gathers in and puts photons to work.
“Elastic strain” is at the heart of the concept. This is the stretching of a material – placing it under strain – in order to shift the material’s properties, here in a way that enhances its ability to, first, respond to different colors of light in the spectrum, and then to collect the charge that is created.
The researchers say they believe that molybdenum disulfide (MoS2), “a material that can form a film just a single molecule (about six angstroms) thick,” is uniquely able to be engineered to perform this function.
MoS2 is a natural semiconductor, and what they researchers have done – again, theoretically and computationally – is poke a microscopic needle into a sheet of this untrathin material, creating what the researchers call “an inhomogenous strain field.”
Typically, the solar energy that is converted into a molecule called an exciton (a combination of an electron and a theoretical, matching “hole”) is left to inefficiently and randomly find its way in a material; with the solar funnel, the exciton is led to the collection site, at the film’s center.
“We’re trying to use elastic strains to produce unprecedented properties,” MIT’s Ju Li, author of a paper describing the new solar-funnel concept published this week in the journal Nature Photonics, said in a statement.
According to MIT, four trends have recently come together to open up the elastic strain engineering field:
- 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.