All Carbon Solar Cells Will Mean Cheap and Flexible
Solar Panels
Flexible photovoltaics made of
carbon promise low cost and durability, if their performance can be improved.
Carbon solar cells promise to be
inexpensive, printable, flexible, and tough enough to withstand extreme
conditions.
Using a grab bag of novel
nanomaterials, researchers at Stanford University have built the first
all-carbon solar cells. Their carbon photovoltaics don’t produce much
electricity, but as the technology is perfected, all-carbon cells could be
inexpensive, printable, flexible, and tough enough to withstand extreme
environments and weather.
The goal is not to replace solar
cells made from silicon and other inorganic materials, says Zhenan
Bao,
professor of chemical engineering at
Stanford University, who led the work. Rather, it is to fill new niches.
“Carbon is one of the most abundant elements on earth, and it is versatile,”
Bao says.
Carbon is remarkably
tough—atom-thick graphene and long, thin carbon nanotubes are two of the
strongest materials ever tested. So carbon photovoltaics might be sprayed on
the sides of buildings, or rolled up and taken into the desert. Various forms
of carbon can be printed to make thin, flexible, transparent, and even
stretchable electronics.
Thanks to its versatility, carbon in
one form or another was used to make each solar-cell component. The three main
parts—a nanotube cathode and a graphene anode sandwiching an active layer made
of nanotubes and buckyballs—were all made by printing or evaporating from inks.
Making the cathode work was the
trickiest part, says Bao—researchers have had a hard time making carbon nanomaterials
that collect electrons. The Stanford researchers solved the problem by picking
the right flavor of nanotubes and giving them a chemical treatment. This work
is described in the journal ACS
Nano.
The all-carbon photovoltaics convert
less than 1 percent of the energy in light into electricity (by comparison, a
silicon solar cell converts around 20 percent of light into electricity).
However, Bao says that her group worked mostly with off-the-shelf materials,
with just a bit of tuning. She attributes part of the problem to the roughness
of the carbon films, which trips up traveling charges, and says it should be
possible to smooth them out by working on the processing methods.
Carbon nanomaterials “are still
relatively new materials,” says Bao. “There’s a lot of research on how to
control their properties and how to use them.”
IBM Yorktown researcher and 2011 MIT
Technology Review young innovator Fengnian Xia,
who is not involved in the work, agrees, saying that the solar cells need
better-quality starting materials and processes. “The idea is great, and this
is a good first demonstration, but it’s not ready for realistic applications,”
he says.
Other groups are focused on making
better carbon materials for the active layers of photovoltaics. According to
theoretical calculations by Jeffrey Grossman at MIT, carbon solar cells should be able to reach 13
percent conversion efficiency.
For carbon solar cells to be
commercially viable, says Shenqiang Ren,
assistant professor of chemistry at the University of Kansas, their efficiency
must cross 10 percent. Ren’s lab set the conversion-efficiency record for
carbon solar cells (equipped with conventional metal electrodes) at 1.3 percent
this September, in work that appeared in ACS Nano. That’s
about how well the first polymer solar cells performed, he notes.
Ren is working with computational
materials scientists, including Grossman, to design better carbon photovoltaics
by picking the right kinds of carbon nanomaterials. With this guidance, Ren
says, his lab has already made carbon solar cells that convert 5 percent of
light energy into electricity, and he expects to go higher still.
- By Katherine Bourzac on November 15, 2012
http://www.technologyreview.com/news/506901/all-carbon-solar-cells-will-mean-cheap-and-flexible-solar-panels/
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