This is really quite
unbelievable. We are harnessing a virus
to properly manufacture at the nanometer level.
Imagine me giving you an resultant product and asking you to explain how
it was made?
Obviously we have a brave new
protocol for tackling nano engineering.
We are still a long way to
developing a really attractive solar manufacturing protocol that is
economically satisfying, but I think I am now glimpsing the road forward for
the first time. I have grown weary of
incremental improvements while climbing an exponential curve. This method may allow us to do really nifty
assembly other that what is traditional photo masking manufacture.
Solar power goes viral
by David L. Chandler
for MIT News Office
In this diagram, the M13 virus consists of a strand of DNA (the
figure-8 coil on the right) attached to a bundle of proteins called peptides -
the virus coat proteins (the corkscrew shapes in the center) which attach to
the carbon nanotubes(gray
cylinders) and hold them in place. A coating of titanium dioxide (yellow spheres)
attached to dye molecules (pink spheres) surrounds the bundle. More of the
viruses with their coatings are scattered across the background. Image: Matt
Klug, Biomolecular Materials Group
Researchers at MIT have found a way to make significant improvements to the power-conversion efficiency of solar cells by enlisting the services of tiny viruses to perform detailed assembly work at the microscopic level.
In a solar cell, sunlight hits a light-harvesting material, causing it
to release electrons
that can be harnessed to produce an electric current. The new MIT research,
published online this week in the journal Nature Nanotechnology,
is based on findings that carbon nanotubes -
microscopic, hollow cylinders of pure carbon - can enhance the efficiency of
electron collection from a solar cell's surface.
Previous attempts to use the nanotubes, however, had been thwarted by
two problems.
First, the making of carbon nanotubes generally produces a mix of two
types, some of which act as semiconductors (sometimes allowing an electric
current to flow, sometimes not) or metals (which act like wires, allowing
current to flow easily).
The new research, for the first time, showed that the effects of these
two types tend to be different, because the semiconducting nanotubes can
enhance the performance of solar cells, but the metallic ones have the opposite
effect. Second, nanotubes tend to clump together, which reduces their
effectiveness.
And that's where viruses come to the rescue. Graduate students Xiangnan
Dang and Hyunjung Yi - working with Angela Belcher, the W. M. Keck Professor of
Energy, and several other researchers - found that a genetically engineered
version of a virus called M13, which normally infects bacteria, can be used to
control the arrangement of the nanotubes on a surface, keeping the tubes
separate so they can't short out the circuits, and keeping the tubes apart so
they don't clump.
The system the researchers tested used a type of solar cell known as
dye-sensitized solar cells, a lightweight and inexpensive type where the active
layer is composed of titanium dioxide, rather than the silicon used in
conventional solar cells. But the same technique could be applied to other
types as well, including quantum-dot and organic solar cells, the researchers
say. In their tests, adding the virus-built structures enhanced the power
conversion efficiency to 10.6 percent from 8 percent - almost a one-third
improvement.
This dramatic improvement takes place even though the viruses and the
nanotubes make up only 0.1 percent by weight of the finished cell. "A
little biology goes a long way," Belcher says. With further work, the
researchers think they can ramp up the efficiency even further.
The viruses are used to help improve one particular step in the process
of converting sunlight to electricity. In a solar cell, the first step is for
the energy of the light to knock electrons loose from the solar-cell material
(usually silicon); then, those electrons need to be funneled toward a collector,
from which they can form a current that flows to charge a battery or power a
device.
After that, they return to the original material, where the cycle can
start again. The new system is intended to enhance the efficiency of the second
step, helping the electrons find their way: Adding the carbon nanotubes to the
cell "provides a more direct path to the current collector," Belcher
says.
The viruses actually perform two different functions in this process.
First, they possess short proteins called peptides that can bind tightly to the
carbon nanotubes, holding them in place and keeping them separated from each
other. Each virus can hold five to 10 nanotubes, each of which is held firmly
in place by about 300 of the virus's peptide molecules.
In addition, the virus was engineered to produce a coating of titanium
dioxide (TiO2), a key ingredient for dye-sensitized solar cells, over each of
the nanotubes, putting the titanium dioxide in close proximity to the wire-like
nanotubes that carry the electrons.
The two functions are carried out in succession by the same virus,
whose activity is "switched" from one function to the next by
changing the acidity of its environment. This switching feature is an important
new capability that has been demonstrated for the first time in this research,
Belcher says.
In addition, the viruses make the nanotubes soluble in water, which
makes it possible to incorporate the nanotubes into the solar cell using a
water-based process that works at room temperature.
Prashant Kamat, a professor of chemistry and biochemistry at Notre Dame
University who has done extensive work on dye-sensitized solar cells, says that
while others have attempted to use carbon nanotubes to improve solar cell
efficiency, "the improvements observed in earlier studies were
marginal," while the improvements by the MIT team using the virus assembly
method are "impressive."
"It is likely that the virus template assembly has enabled the
researchers to establish a better contact between the TiO2 nanoparticles and
carbon nanotubes. Such close contact with TiO2 nanoparticles is essential to
drive away the photo-generated electrons quickly and transport it efficiently
to the collecting electrode surface."
Kamat thinks the process could well lead to a viable commercial
product: "Dye-sensitized solar cells have already been commercialized in Japan , Korea
and Taiwan
Belcher and her colleagues have previously used differently engineered
versions of the same virus to enhance the performance of batteries and other
devices, but the method used to enhance solar cell performance is quite
different, she says.
Because the process would just add one simple step to a standard
solar-cell manufacturing process, it should be quite easy to adapt existing
production facilities and thus should be possible to implement relatively
rapidly, Belcher says.
The research team also included Paula Hammond, the Bayer Professor of
Chemical Engineering; Michael Strano, the Charles (1951) and Hilda Roddey
Career Development Associate Professor of Chemical Engineering; and four other
graduate students and postdoctoral researchers. The work was funded by the Italian
company Eni, through the MIT Energy Initiative's Solar Futures Program.
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