November 26, 2012
Harnessing Power By Funneling Solar Energy
Michael Harper for redOrbit.com — Your Universe Online
The concept is a simple one, borrowing from some very elementary principles. For instance, everyone knows the best way to capture and contain the most of an element or substance is with a funnel. These cones make it easier to put oil in our cars or fill small glass shakers from a larger bag of sugar.
This new concept places highly specialized materials under “elastic strain,” which allows the materials to first capture the sunlight and then collect it at the center.
“We´re trying to use elastic strains to produce unprecedented properties,” explains Ju Li, a professor at MIT and author of the paper which describes the research behind this solar funnel idea. This paper has been published in the journal Nature Photonics.
The common funnel used in auto repair or food service works thanks to gravity: The conical tube gathers the material downward to a smaller, central hole, all while gravity guides it along its intended path.
The solar funnel, on the other hand, works by electronic forces. As explained by the research authors, electrons and holes are guided to the center of the material under elastic strain by electronic forces. Though they refer to a funnel only as a metaphor, the resulting shape does resemble a household funnel.
To create this funnel, Li and team worked with a material which can be used to form a film only one molecule (or 6 angstroms) thick. This material, known as molybdenum disulfide (MoS2), is also a natural semiconductor and can be used to make solar cells or integrated circuits.
As it stands, silicon is often used to make solar cells. The problem with silicon cells, however, is the way in which the electron-hole pairs (called excitons) move within the material. These excitons move randomly throughout the silicon once generated by photons, says Xiaofeng Qian, a postdoctoral student in MIT´s Department of Nuclear Science and Engineering and co-author of the paper.
“It´s a diffusion process, and it´s very inefficient,” says Qian in the statement.
In their computer model of the solar funnel process, the center of this MoS2 film is indented with a needle, placing it under elastic strain. When the material is placed under this strain, it undergoes a characteristic change, causing certain parts of the material to respond to different colors of light. These different colors of light are then gathered at the film´s center, making it easier to collect.
According to Li, this research is only possible thanks to the convergence of 4 trends in field of elastic strain engineering.
The development of nanomaterials, such as carbon nanotubes and, more specifically, MoS2, have provided the kinds of materials and data needed to conduct this solar funnel research. These nanomaterials have shown they are capable of retaining large amounts of this elastic strain for indefinite periods of time, making them perfect candidates for this kind of technology, once it´s built.
The development of atomic force microscopes and nanomechanical instruments have also provided Li, Qian, and the rest of the MIT team with the tools needed to apply this strain with controlled accuracy. The advancements in electron microscopy and synchrotron facilities have also allowed Li and colleagues to accurately measure the amount of elastic strain being generated, while new electronic-structure calculation methods have provided the MIT team with a way to predict the way elastic strain will affect the materials involved.
“People knew for a long time that by applying high pressure, you can induce huge changes in material properties,” said Li, adding that only recently have studies shown the different ways properties are changed when this strain is introduced.