Improvements in the efficiencies of organic solar cells will eventually make them competitive with traditional silicon-based solar cells and, hopefully, ultimately with fossil fuels.

Drawn together by the force of
nature, but pulled apart by the force of man—it sounds like the setting for a
love story, but it is also a basic description of how scientists have begun to
make more efficient organic solar cells.

At the atomic level, organic
solar cells function like the feuding families in Romeo and Juliet.
There’s a strong natural attraction between the positive and negative charges
that a photon generates after it strikes the cell, but in order to capture the
energy, these charges need to be kept separate.

When these charges are still
bound together, they are known to scientists as an exciton. “The real question
that this work tries to answer is how to design a material that will make
splitting the exciton require less energy,” said senior chemist Lin
Chen of the U.S. Department of Energy’s (DOE) Argonne National Laboratory.

Excitons can be thought of as a
sort of “quasiparticle,” Chen said, because they exhibit certain unique
behaviors. When the two charged regions of the exciton—the electron and a
region known as a “hole”—are close together, they are difficult to pry apart.

When energy is added to the system, however, the charges begin
to separate, rendering the electrons and holes completely free and eventually
allowing for the possibility of generating current and extracting electricity.

“The closer the hole and the electron regions are inside an
exciton, the more likely they are to recombine without generating electricity,”
Chen said. “But if they are already ‘pre-separated,’ or polarized, the more
likely they are to escape from this potential trap and become effective charge
carriers.”

In the new experiment, Chen and her colleagues examined how four
different molecules in the polymer layer in the middle of a solar cell
generated different exciton dynamics. They discovered that more heavily
polarized excitons yielded more efficient polymer-based solar cells.

“If the conventional exciton, right after it is generated,
contains the hole and electron in almost the same location, these new materials
are generating an exciton that is much more polarized at the beginning,” Chen
said. Currently, the collaborative team is exploring new materials for
high-efficiency organic solar cells based on these findings.

Organic solar cells still have a ways to go to get close to the
efficiency of their inorganic, silicon-based competitors, but they remain much
more attractive from a cost perspective. Further research into the electronic
dynamics of organic photovoltaics is essential to improving their efficiency
and thus making solar power cost-competitive with conventional energy sources,
Chen said.

The work has been published in
the Journal of the American Chemical Society.

Source: Argonne National Laboratory