Wednesday, 7 August 2013

Regulating electron 'spin' may be key to making organic solar cells competitive

Regulating electron 'spin' may be
key to making organic solar cells
This is the laser set-up used to to
make the actual measurements
reported in the paper. Credit: Dr.
Akshay Rao
Organic solar cells that convert light
to electricity using carbon-based
molecules have shown promise as a
versatile energy source but have not
been able to match the efficiency of
their silicon-based counterparts.
Now, researchers have discovered a
synthetic, high-performance polymer
that behaves differently from other
tested materials and could make
inexpensive, highly efficient organic
solar panels a reality.
The polymer, created at the
University of Washington and tested
at the University of Cambridge in
England, appears to improve
efficiency by wringing electrical
current from pathways that, in other
materials, cause a loss of electrical
charge .
"In most cases you are generating
charge but you have to out-compete
all the areas of loss that keep you
from delivering the electricity from
the cell to the device you are trying
to power," said Cody Schlenker, a
postdoctoral researcher in the
laboratory of David Ginger, a UW
chemistry professor.
"These materials can be printed like
newspaper and manufactured into
rolls of film like plastic wrap, so they
could have a significant
manufacturing cost advantage over
traditional materials like silicon,"
Ginger said.
Schlenker and Ginger are co-authors
of a paper analyzing the new
material, published online Aug. 7 in
Nature . The lead authors are Akshay
Rao and Richard Friend of Cambridge,
who with Cambridge researchers
Philip Chow and Simon Gelinas did
sensitive measurements that
confirmed the properties of the
polymer. The material was created in
the lab of co-author Alex Jen, a UW
professor of materials science and
Organic solar cells change color
briefly as they convert light to
electricity, similar to how some
prescription glasses darken when
exposed to sunlight and become
clear indoors. The researchers used a
technique called photo-induced
absorption spectroscopy to measure
the color changes as "fingerprints" to
study pathways that devices use to
convert sunlight to electricity.
The same technique also pinpoints
"dead-end" pathways that do not
produce electricity, which are present
in most organic materials used for
solar cells and limit power
production. UW scientists were
surprised when their polymer
appeared to have few dead ends, but
they needed more sensitive
measurements to be sure.
Cambridge researchers had seen
hints of the same kind of behavior in
similar materials that could be used
in organic solar cells .
"They were seeing some of the same
features that we were seeing,
features that everyone said you
shouldn't be able to see," Schlenker
A vial holds a solution that contains
the UW-developed polymer "ink" that
can be printed to make the solar
cells. Credit: Yeechi Chen/University
of Washington
At a scientific meeting in Italy last
year, the two groups began
discussing the apparent surprising
properties of the UW-created
polymer, composed of carbon,
hydrogen, sulfur and nitrogen atoms.
The Cambridge researchers used
lasers to probe the polymer and saw
clear evidence of the behavior that
had only been hinted at in other
materials they had studied.
They found that the apparent lack of
electrical dead ends in the new
polymer is related to a quantum
mechanical property of electrons
called "spin." Essentially, with certain
spin configurations the material can
"rescue" electrical charges from what
otherwise would be energy-losing
Currently, organic solar cells can
achieve as much as 12 percent
efficiency in turning light into
electricity, compared with 20 to 25
percent for silicon-based cells.
Schlenker believes design concepts
based on the new material will help
to significantly close the gap between
these two types of solar cell.
Organic materials are semi-
transparent and tunable to any color,
and their flexibility and ease of
production mean that achieving
greater efficiency in changing light to
electricity could make them cheaper
and easier to deploy than the silicon-
based cells.
The carbon-based molecules in the
organic polymers are similar to
molecules already found in car
paints, some clothing dye and the
pigment in plant chlorophyll. Organic
dyes could be incorporated into ink
and printed on materials such as
shingles, siding or window frames.
Current materials are relatively low
cost and recyclable. Work to extend
their lifespan beyond five to seven
years and to find ways to replace
them relatively easily could make
them a feasible option for a home or
business, Schlenker said.
Solar cells now provide less than 0.2
percent of power used in the United
States, but improving efficiency and
finding ways to incorporate them into
building materials is one way to
make them cost-effective.
"You have to go in the direction of
adding no cost to the material you
already are planning to deploy,"
Schlenker said.
More information: The role of spin
in the kinetic control of
recombination in organic
photovoltaics, Nature , DOI: 10.1038/
Provided by University of Washington

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