The structure of graphene, a flexible material made of carbon atoms arranged in a layer just one atom thick, is represented in this diagram. Graphic: Christine Daniloff |
A promising approach for making solar
cells that are inexpensive, lightweight, and flexible is to use organic
compounds instead of silicon. But one stubborn problem has slowed the
development of such cells: Researchers have had a hard time coming up with
appropriate materials for the electrodes to carry the current to and from the
cells. Specifically, it has been hard to make electrodes using materials that
can match the organic cells’ flexibility, transparency and low cost.
The standard material used so far for
these electrodes is indium-tin-oxide, or ITO. But indium is expensive and
relatively rare, so the search has been on for a suitable replacement. Now, a
team of MIT researchers has come up with a practical way of using a possible
substitute made from inexpensive and ubiquitous carbon. The proposed material is
graphene.
An analysis of how to use graphene as an
electrode for such solar cells was published in Nanotechnology, in a paper by MIT professors Jing Kong and
Vladimir Bulovi? along with two of their students and a postdoctoral
researcher.
Graphene is transparent, so that
electrodes made from it can be applied to the transparent organic solar cells
without blocking any of the incoming light. In addition, it is flexible, like
the organic solar cells themselves, so it could be part of installations that
require the panel to follow the contours of a structure, such as a patterned
roof. ITO, by contrast, is stiff and brittle.
The biggest problem with getting
graphene to work as an electrode for organic solar cells has been getting the
material to adhere to the panel. Graphene repels water, so typical procedures
for producing an electrode on the surface by depositing the material from a
solution won’t work.
The team tried a variety of approaches
to alter the surface properties of the cell or to use solutions other than
water to deposit the carbon on the surface, but none of these performed well,
Kong says. But then they found that “doping” the surface changed the way it
behaved, and allowed the graphene to bond tightly. As a bonus, it turned out
the doping also improved the material’s electrical conductivity.
While the specific characteristics of
the graphene electrode differ from those of the ITO it would replace, its
overall performance in a solar cell is very similar, Kong says. And the
flexibility and light weight of organic solar cells with graphene electrodes
could open up a variety of different applications that would not be possible
with today’s conventional silicon-based solar panels, she says. For example,
because of their transparency they could be applied directly to windows without
blocking the view, and they could be applied to irregular wall or rooftop
surfaces. In addition, they could be stacked on top of other solar panels,
increasing the amount of power generated from a given area. And they could even
be folded or rolled up for easy transportation.
While this research looked at how to
adapt graphene to replace one of the two electrodes on a solar panel, Kong and
her co-workers are now trying to adapt it to the other electrode as well. In
addition, widespread use of this technology will require new techniques for
large-scale manufacturing of graphene. The ongoing work has been funded by the Eni-MIT Alliance Solar
Frontiers Center
and an NSF research fellowship.
Peter Peumans, an assistant professor of
electrical engineering at Stanford
Univ., who was not
involved in this study, says organic solar cells will probably become practical
only with the development of transparent electrode technology that is both
cheaper and more robust than conventional metal oxides. Other materials are
being studied as possible substitutes, he says, but this work represents “very
important progress” toward making graphene a credible replacement transparent
electrode.
“Other groups had already shown that
graphene exhibits good combinations of transparency and sheet resistance, but
no one was able to achieve a performance with graphene electrodes that matches
that of devices on conventional metal oxide (ITO) electrodes,” Peumans says.
“This work is a substantial push toward making graphene a leading candidate.”
