SLAC National Accelerator Laboratory
A new material design tested in experiments at the Department of
Energy's SLAC National Accelerator Laboratory could make low-cost solar panels
far more efficient by greatly enhancing their ability to collect the sun's
energy and release it as electricity.
A team of University of California, Los Angeles, scientists
found that by assembling the components of the panels to more closely resemble
the natural systems plants use to tap the sun's energy, it may be possible to
separate positive and negative charges in a stable way for up to several weeks
compared to just millionths of a second -- the current standard for many modern
solar panels.
The team's X-ray studies at SLAC's Stanford Synchrotron
Radiation Lightsource (SSRL), a DOE Office of Science User Facility, enabled
them to see, at a microscopic level, which material design has the most ideal
structure at the nanoscale for promoting this charge separation. The results
are published in the June 19 edition of Science.
Plastic Panels Provide Low-cost Alternative to Silicon
To capture energy from sunlight, conventional rooftop solar
cells use silicon, which can be expensive. Solar cells can also be made using
lower-cost materials like plastics, but plastic cells are far less efficient --
in large part because the separated positive and negative charges in the
material often recombine before they can become electrical energy.
"Modern plastic solar cells don't have well-defined
structures like plants do because we never knew how to make them before,"
Tolbert said. "But this new system pulls charges apart and keeps them
separated for days, or even weeks. Once you make the right structure, you can
vastly improve the retention of energy."
A Better Recipe for 'Spaghetti and Meatballs'
The UCLA-developed system is composed of strands of a polymer,
the building block of plastics, that absorb sunlight and pass electrons to a
fullerene, a spherical carbon molecule also known as a "buckyball."
The materials in these types of solar cells are typically
organized like a plate of cooked pasta -- a disorganized mass of long, skinny
polymer "spaghetti" with random fullerene "meatballs." But
this arrangement makes it difficult to get current out of the cell because the
electrons sometimes hop back to the polymer spaghetti and are lost.
The researchers figured out how to arrange the elements more
neatly -- small bundles of uncooked spaghetti with precisely placed meatballs.
Some fullerene meatballs are designed to sit inside the polymer spaghetti
bundles and others are forced to stay on the outside.
The fullerenes inside the structure take electrons from the
polymers and toss them to the outside fullerenes, which can effectively keep
the electrons separated from the polymer for weeks. A series of experiments at
SSRL and other studies confirmed the best arrangement of the polymer strands
and buckyballs.
Successes and Next Steps
"When the charges never come back together, it becomes
easier to get them out of the solar cell in the form of electricity," said
Benjamin Schwartz, a UCLA professor of chemistry and a co-author of the study.
"This is the first time such long charge lifetimes have been shown using
this type of material."
Researchers found that the materials self-assemble into this
ordered form when placed in close proximity. The new design is also more
environmentally friendly than current technology, because the materials can
assemble in water instead of more toxic organic solutions that are typically
used.
"Once you make the materials, you can dump them into water
and they assemble into the appropriate structure because of the way the
materials are designed," Schwartz said.
The researchers are now working on how to incorporate the
technology into actual solar cells, and Tolbert said the team is planning
follow-up research at SSRL.
The work was supported by the National Science Foundation and
the U.S. Department of Energy, and the DOE Office of Biological and
Environmental Research.
