Image: Matt Klug, Biomolecular Materials Group
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.
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 in Nature
Nanotechnology, is based on findings that carbon nanotubes can
enhance the efficiency of electron collection from a solar cell’s surface.
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
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.
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% from 8%—almost a one-third improvement.
improvement takes place even though the viruses and the nanotubes make up only
0.1% 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.
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,”
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
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.
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.
Kamat, a professor of chemistry and biochemistry at Notre Dame Univ. 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.”
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
thinks the process could well lead to a viable commercial product: “Dye-sensitized solar cells have already been commercialized in Japan, Korea
he says. If the addition of carbon nanotubes via the virus process can improve
their efficiency, “the industry is likely to adopt such processes.”
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.
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
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.