An unexpected change in polymer structure opens a new avenue in the search for improved solar cell efficiency

An unexpected change in polymer
structure opens a new avenue in
the search for improved solar cell
efficiency
Figure 1: A small change in polymer
structure changes how the polymer
chains stack together in a thin film,
resulting in a dramatic improvement
in solar cell efficiency. Credit: Itaru
Osaka, RIKEN Center for Emergent
Matter Science
Solar cells based on organic polymers
are of great interest because the
materials are both cheaper to make
and easier to process than those
used in traditional inorganic solar
cells. To date, however, the very best
power conversion efficiencies for
polymer solar cells remain below the
threshold for practical application.
Itaru Osaka from the Emergent
Molecular Function Research Group
at the RIKEN Center for Emergent
Matter Science and co-workers have
now serendipitously discovered that
changing the polymer's structure
results in a significant enhancement
of power conversion efficiency1.
When light energy is absorbed by the
polymer in a polymer solar cell,
electrons are excited to higher energy
levels to produce a high-energy
electron and a corresponding electron
'hole'. To convert the light energy
into electrical current, these electrons
and holes have to move through the
polymer to the electrodes before
they recombine and the energy is
lost. Much research has been
devoted to understanding how to
improve this conversion process.
Osaka and his colleagues had been
working with a particular type of
copolymer containing a repeating
naphthodithiophene–
naphthobisthiadiazole structure called
PNNT-DT. "PNNT-DT has very low
solubility," explains Osaka, "so we
were interested in attaching
additional alkyl side chains to the
polymer to improve its
processability." As expected, this
modification significantly improved
the solubility of the polymer, but
also significantly and unexpectedly
improved the power conversion
efficiencies of solar cells made with
the polymer.
In the solar cells, the polymer is
deposited as a thin film, and analysis
revealed that these new 'alkylated'
polymers were arranged so that the
polymer chains lay flat in stacks on
the surface rather than aligned
perpendicular to it. This causes the
charge carriers—electrons and holes
—to move perpendicular to the
surface rather than parallel,
improving the power conversion
efficiency (Fig. 1). "This unexpected
change in orientation produced solar
cells with an efficiency of up to 8.2%
compared with just 5.5% for the
unalkylated material," says Osaka.
Ultimately, Osaka and his co-workers
hope to exploit this dramatic
efficiency enhancement in other
polymers in order to move closer to
producing polymer solar cells that
can truly compete with the 15% or
greater efficiency of inorganic solar
cells. "We need a greater
understanding of why this switch in
polymer orientation occurs, and then
we need to apply it to other polymers
that can absorb a wider range of
visible light wavelengths," he says.
More information: Journal of the
American Chemical Society 135,
8834–8837 (2013). DOI: 10.1021/
ja404064m