This angled view shows an aluminum oxide template (gray) with a regular array of 60-nm holes filled with an organic semiconductor (purple). This nanoconfined semiconducting polymer produces twice the amount of electricity for a given amount of absorbed sunlight as compared to the same material spread as a thin film. Image: Brookhaven National Laboratory
Sometimes a change in surroundings makes all the difference.
That’s the approach a group of researchers at the U.S. Department of Energy’s
(DOE) Brookhaven National Laboratory has used to improve the electricity output
of a semiconductor material used in polymer-based solar cells. By confining the
light-absorbing/charge-separating material within nanometer-scale pores,
instead of using it in continuous thin-film sheets, the scientists enhanced the
material’s electrical conductivity by more than 500 times—and produced solar
cells with twice the electricity output for the same amount of absorbed
The advance doesn’t improve overall solar cell efficiency,
however, because the nano-confined light-absorbing material doesn’t cover as
much area as in the thin-film format. But the research suggests that such
nanoscale restructuring, described in Applied
Physics Letters and ACS Nano,
might eventually achieve that goal, and make polymer-based solar cells—potentially
manufactured as inexpensively as plastics—more competitive in the marketplace.
“Judged by their physical properties, organic semiconductors
should be more efficient at converting sunlight to electricity than they are,”
says Charles Black, group leader for electronic materials at Brookhaven’s
Center for Functional Nanomaterials (CFN). “One of the goals of our research is
to understand why—and to devise new solar cell architectures to improve them.”
The team, including materials scientist Jonathan Allen and
other Brookhaven collaborators, focused its attention on a common blend of
organic materials used in polymer-based solar devices. The blend is typically
used as a thin-film sheet between two conducting metal electrodes. Incident
sunlight “excites” electrons in the organic semiconductor and produces electron-hole
pairs. The blend helps separate these pairs and sends the electron to one
electrode and the hole to the other in order to produce electricity.
“While the organic blend is very efficient at splitting
electron-hole pairs, it’s not so good at transporting them out,” Allen says.
To understand why, the scientists partnered with Kevin Yager
(CFN) and Ben Ocko (Condensed Matter Physics Department) to examine the
material using bright beams of X-rays at Brookhaven’s National Synchrotron
Light Source (NSLS). By measuring the angles at which X-rays diffracted around
molecules composing the organic film, the scientists determined that the
molecules packed in such a way to enhance electrical conductivity in the plane
of the film, but not perpendicular to it.
“Solar cells need electrons and holes to move in the
out-of-plane direction—that is, toward the conductors designed to collect the
current,” says Allen. “So we decided to try turning the material on its side by
confining it to spaces more elongated in the out-of-plane direction.”
Charles Black (standing) and Kevin Yager, coauthors on this research, probing the properties of organic solar materials at the CFN. Photo: Brookhaven National Laboratory
Back at the CFN, the scientists created a template of tiny
holes measuring 60 nm in diameter and about 110 nm deep. Instead of generating
each hole one by one, they used an electrochemical method to self-assemble
arrays of billions of pores per square centimeter in a piece of aluminum foil.
“Flowing an electric current in an electrolyte will oxidize
the aluminum in a particular way, creating regular arrays of nanometer-scale
holes,” says Allen. “We can do this over large areas without the need for any
specialized tools,” he adds.
Then the scientists filled the holes with the polymer blend,
added metal conductor layers on top and bottom, and measured the current
produced while illuminating the device with artificial sunlight.
“The out-of-plane conductivity of the material confined to
the holes is 500 times better than when it is formed as a thin film,” says
But the scientists were surprised upon measuring the crystal
structure of the nanoconfined material at the NSLS. Instead of seeing crystals
aligned in the out-of-plane direction—as if the material had been turned on its
side—they saw no alignment to the crystal arrangement whatsoever.
“We didn’t expect such a highly conducting material to be
randomly oriented, but it is,” says Ocko.
The team believes the orientation of crystal stacking in a
thin film arrangement actively blocks the out-of-plane movement of electrical
current. However, confining the same material within nanopores inhibits such
long-range crystal alignment, thus removing the roadblocks to out-of-plane
Solar cells made from the reconfigured material produce
twice as much electricity for the same amount of absorbed sunlight, compared to
an unconfined, continuous film. While this is a substantial improvement in the
material performance, the overall solar device efficiency remains unimproved
because the confining nanostructured template occupies valuable space. And that
space is not generating electricity.
The team is exploring ways to minimize that wasted space to
better leverage the improved material performance. They also plan to
investigate whether the nano-containment approach could provide similar
benefits for other organic solar materials.
“We’re excited about the research directions these results
suggest,” says Black. “They play to the strengths of the CFN, making great use
of Brookhaven’s capabilities to correlate a material’s structure to its
electrical function—and ultimately to improve its performance.”