Stanford scientists build the first all-carbon solar cell.

Stanford scientists build the first all-carbon solar cell.

Stanford University scientists have built the first solar cell made
entirely of carbon, a promising alternative to the expensive materials
used in photovoltaic devices today. The results are published in today‘s online edition of the journal ACS Nano.
“Carbon has the potential to deliver high performance at a low cost,” said study senior author Zhenan Bao,
a professor of chemical engineering at Stanford.  “To the best of our
knowledge, this is the first demonstration of a working solar cell that
has all of the components made of carbon. This study builds on previous
work done in our lab.”
Unlike rigid silicon solar panels that adorn many rooftops,
Stanford’s thin film prototype is made of carbon materials that can be
coated from solution. “Perhaps in the future we can look at alternative
markets where flexible carbon solar cells are coated on the surface of
buildings, on windows or on cars to generate electricity,” Bao said.
The coating technique also has the potential to reduce manufacturing
costs, said Stanford graduate student Michael Vosgueritchian, co-lead
author of the study with postdoctoral researcher Marc Ramuz.
“Processing silicon-based solar cells requires a lot of steps,”
Vosgueritchian explained. “But our entire device can be built using
simple coating methods that don’t require expensive tools and machines.”

Carbon nanomaterials
The Bao group’s experimental solar cell consists of a photoactive
layer, which absorbs sunlight, sandwiched between two electrodes.  In a
typical thin film solar cell, the electrodes are made of conductive
metals and indium tin oxide (ITO). “Materials like indium are scarce and
becoming more expensive as the demand for solar cells, touchscreen
panels and other electronic devices grows,” Bao said.  ”Carbon, on the
other hand, is low cost and Earth-abundant.”

For the study, Bao and her colleagues replaced the silver and ITO
used in conventional electrodes with graphene – sheets of carbon that
are one atom thick –and single-walled carbon nanotubes that are 10,000
times narrower than a human hair. “Carbon nanotubes have extraordinary
electrical conductivity and light-absorption properties,” Bao said.  
For the active layer, the scientists used material made of carbon
nanotubes and “buckyballs” – soccer ball-shaped carbon molecules just
one nanometer in diameter.  The research team recently filed a patent
for the entire device.
“Every component in our solar cell, from top to bottom, is made of
carbon materials,” Vosgueritchian said. “Other groups have reported
making all-carbon solar cells, but they were referring to just the
active layer in the middle, not the electrodes.”
One drawback of the all-carbon prototype is that it primarily absorbs
near-infrared wavelengths of light, contributing to a laboratory
efficiency of less than 1 percent – much lower than commercially
available solar cells.  ”We clearly have a long way to go on
efficiency,” Bao said.  “But with better materials and better processing
techniques, we expect that the efficiency will go up quite
dramatically.”

Improving efficiency
The Stanford team is looking at a variety of ways to
improve efficiency. “Roughness can short-circuit the device and make it
hard to collect the current,” Bao said. “We have to figure out how to
make each layer very smooth by stacking the nanomaterials really well.”
The researchers are also experimenting with carbon nanomaterials that
can absorb more light in a broader range of wavelengths, including the
visible spectrum.
“Materials made of carbon are very robust,” Bao said. “They remain
stable in air temperatures of nearly 1,100 degrees Fahrenheit.”
The ability of carbon solar cells to out-perform conventional devices
under extreme conditions could overcome the need for greater
efficiency, according to Vosgueritchian. “We believe that all-carbon
solar cells could be used in extreme environments, such as at high
temperatures or at high physical stress,” he said. “But obviously we
want the highest efficiency possible and are working on ways to improve
our device.”
“Photovoltaics will definitely be a very important source of power
that we will tap into in the future,” Bao said. “We have a lot of
available sunlight. We’ve got to figure out some way to use this natural
resource that is given to us.”
Other authors of the study are Peng Wei of Stanford and Chenggong Wang and Yongli Gao of the University of Rochester Department of Physics and Astronomy. The research was funded by the Global Climate and Energy Project at Stanford and the Air Force Office for Scientific Research.
Mark Shwartz is a communications/energy writer at the Precourt Institute for Energy at Stanford University.

Media Contact

Zhenan Bao, Chemical Engineering: (650) 723-2419, [email protected]
Michael Vosgueritchian, Chemical Engineering: (650) 725-8495, [email protected]
Mark Shwartz, Precourt Institute for Energy: (650) 723-9296, [email protected]

Worldwide Tech & Science