Microscopic folds increase the power output and durability of solar cells. Credit: Frank Wojciechowski
their cue from the humble leaf, researchers have used microscopic folds
on the surface of photovoltaic material to significantly increase the
power output of flexible, low-cost solar cells.
The team, led by scientists from Princeton University, reported online April 22 in the journal Nature Photonics
that the folds resulted in a 47% increase in electricity generation.
Yueh-Lin (Lynn) Loo, the principal investigator, said the finely
calibrated folds on the surface of the panels channel light waves and
increase the photovoltaic material’s exposure to light.
a flat surface, the light either is absorbed or it bounces back,” said
Loo, a professor of chemical and biological engineering at Princeton.
“By adding these curves, we create a kind of wave guide. And that leads
to a greater chance of the light’s being absorbed.”
research team’s work involves photovoltaic systems made of relatively
cheap plastic. Current solar panels are typically made of silicon, which
is both more brittle and more expensive than plastics. So far, plastic
panels have not been practical for widespread use because their energy
production has been too low. But researchers have been working to
increase that efficiency with the goal of creating a cheap, tough and
flexible source of solar power.
researchers can increase the plastic panels’ efficiency, the material
could produce power from an array of surfaces from inserts in window
panels to overlays on exterior walls or backpacks.
“It is flexible, bendable, light weight and low cost,” Loo said.
most cases, researchers have focused on increasing the efficiency of
the plastic photovoltaic material itself. Recent developments have been
promising: a team from UCLA recently announced a system with a 10.6% efficiency. That approaches the 10 to 15% level seen as the target for commercial development.
said the folding method promises to increase those numbers. Because the
technique works with most types of plastic photovoltaic materials, it
should provide a boost to efficiency across the board.
is a very simple process that you can use with any material,” she said.
“We have tested it with other polymers and it works as well.”
Jong Bok Kim, a postdoctoral researcher in chemical and biological engineering and the paper’s lead author, explained in the Nature Photonics
paper that the folds on the surface of the panels channel light waves
through the material in much the same way that canals guide water
through farmland. By curving the light through the material, the
researchers essentially trap the light inside the photovoltaic material
for a longer time, which leads to greater absorption of light and
generation of energy.
expected that it would increase the photocurrent because the folded
surface is quite similar to the morphology of leaves, a natural system
with high light harvesting efficiency,” said Kim, a postdoctoral
researcher in chemical and biological engineering. “However, when I
actually constructed solar cells on top of the folded surface, its
effect was better than my expectations.”
the technique results in an overall increase in efficiency, the results
were particularly significant at the red side of the light spectrum,
which has the longest wavelengths of visible light. The efficiency of
conventional solar panels drops off radically as light’s wavelength
increases, and almost no light is absorbed as the spectrum approaches
the infrared. But the folding technique increased absorption at this end
of the spectrum by roughly 600%, the researchers found.
you look at the solar spectrum, there is a lot of sunlight out there
that we are wasting,” Loo said. “This is a way to increase efficiency.”
research team created the folded surface in Howard Stone’s laboratory
in the mechanical and aerospace engineering department by carefully
curing a layer of liquid photographic adhesive with ultraviolet light.
By controlling how fast different sections of the adhesive cured, the
team was able to introduce stresses in the material and generate ripples
in the surface. The shallower ripples were classified as wrinkles and
the deeper ones are called folds. The team found that a surface
containing a combination of wrinkles and folds produced the best
the math underlying the process is complex, the actual production is
straightforward. Loo said it would be quite practical for industrial
hinges on the fact that you can reproduce the wrinkles and folds,” Loo
said. “By controlling the stresses, we can introduce more or fewer
wrinkles and folds.”
benefit of the process is that it increases the durability of the solar
panels by relieving mechanical stresses from bending. The researchers
found the panels with folded surfaces were able to retain their
effectiveness after bending. A standard plastic panel’s energy
production would be diminished by 70 percent after undergoing bending.
said the researchers drew their inspiration from leaves. Seemingly a
simple object, the leaf is a miracle of natural engineering. Its green
surface is perfectly constructed to bend and control light to ensure
that a maximum amount of solar energy is absorbed to create energy and
nutrients for the tree. Recent work by Pilnam Kim, a postdoctoral
researcher in Stone’s lab, provided insight into how these microscopic
structures could be applied to synthetic devices.
you look at leaves very closely, they are not smooth, they have these
sorts of structures,” said Loo, who is deputy director of Princeton’s
Andlinger Center for Energy and the Environment. “We’d like to mimic
this geometric effect in synthetic, man-made light-harvesting systems.”
addition to Loo, the researchers included: Howard Stone, the Donald R.
Dixon ’69 and Elizabeth W. Dixon Professor in Mechanical and Aerospace
Engineering at Princeton, Jason Fleischer, an associate professor of
electrical engineering at Princeton, Jong Bok Kim, Pilnam Kim, and
Nicolas Pégard, a graduate student in Princeton’s electrical engineering
department; Soog Ju Oh, and Cherie Kagan, of the University of
Pennsylvania. Support for the research was provided by the Office of
Naval Research, the National Science Foundation, including those through
the Princeton Center for Complex Materials, the Air Force Office of
Scientific Research, and the Curtin-Stafford Fund at Princeton.
Wrinkles and deep folds as photonic structures in photovoltaics
Source: Princeton University