Typical solar cells made of silicon miss out on a wide swath of energy shining from the sun.
But according to calculations made by scientists at the
Massachusetts Institute of Technology and China's Peking University and Xi'an
Jiaotong University, poking a sheet of material just a molecule thin changes the material's atomic structure and improves its ability to harvest a broader spectrum of sunlight.
Conventional solar panels made of crystalline silicon are most sensitive to wavelengths of sunlight in the red end of the
visible range or the near-infrared. Panels made of amorphous silicon are more sensitive to wavelengths of light in the blue range.
But the sun's peak wavelength is in the green part of the spectrum. Photons (light particles) from that wavelength of light do the best job at hitting atoms inside solar panels and knocking out the electrons that ultimately generate an electric current. If solar panels could be tuned to harvest a larger spectrum of sunlight, they'd generate more electricity and be more efficient.
In this week's issue Nature Photonics, MIT professor Ji Lu and his colleagues have proposed a radical idea that turns a very thin sheet of material into a kind of “solar energy funnel” that takes advantage of elastic strain. The material is molybdenum disulfide, which is not typically made into a solar panel but has been experimented with as a
semiconductor material for transistors. It's also in a certain class of substances called 'ultrastrength materials,' which can be stretched out of shape for long periods of time without breaking.
The technique involves poking the sheet of molybdenum disulfide with a microscopic needle. The pressure from the needle causes an elastic strain that not only takes on the shape of a funnel but increases in intensity toward the center.
Like silicon, molybdenum disulfide releases electrons when hit by sunlight. Stretching the material
into a funnel shape varies its atomic structure from the edge to the center, and allows different parts of the sheet to respond to
photons from different wavelengths of sunlight.
This means a single sheet of material can actually work harder to collect more energy from the sun. In addition, because the sheet is funnel-shaped, the charged particles will tend to
gather at the bottom of it — moved there by electrostatic energy and not gravity. Having the electric charges all end up in one
place is a lot more efficient than having them simply bouncing randomly
around the sheet.
All this sounds good, but it hasn't been confirmed by
real-world experiments; the calculations are all mathematical models. But the
principle has been used before. IBM and Intel have both done experiments with elastic
strains in silicon channels in transistors with some success.
Credit: MIT News/ Yan Liang