Structural details of photovoltaic material revealed: The bilayer polymer backbone motif (3D image) is derived from the X-ray scattering pattern (background) obtained at beamline X9 of NSLS. In the 3D image, the yellow region denotes the paired backbones and the blue region denotes the liquid-like side chains. Image: Brookhaven National Laboratory |
Detailed studies of one of the best-performing organic
photovoltaic materials reveal an unusual bilayer lamellar structure that may
help explain the material’s superior performance at converting sunlight to
electricity and guide the synthesis of new materials with even better
properties. The research, published in Nature Communications, was conducted by
scientists at the U.S. Department of Energy’s (DOE) Brookhaven National
Laboratory, in collaboration with researchers from Stony
Brook University,
Seoul National
University in Korea, the Max Planck Institute for Polymer
Research in Germany,
and Konarka Technologies.
The material, known by the handle PCDTBT, is an example of a “polycarbazole conjugated polymer,” a molecule composed of a chain-like carbon
backbone with alkyl side chains. Its ability to move electrons around—both “donating”
and “accepting” them—makes it among the best organic photovoltaic materials
currently in use, able to convert sunlight to electricity with efficiency as
high as 7.2% in organic solar cells.
“Despite the fact that this material has been extensively
studied, no one has reported detailed structural features to provide a basis
for its superior performance,” said Brookhaven physicist Benjamin Ocko, who led
the current research. “Understanding why this material performs so well will
help scientists harness its essential attributes to engineer new materials for
a wide range of applications, including displays, solid-state lighting, transistors,
and improved solar cells,” he said.
To probe the molecular structure, the team exposed thin films of
PCDTBT to intense beams of X-rays at Brookhaven’s National Synchrotron Light
Source (NSLS) using a high-resolution X-ray scattering technique. Unlike
previous studies, which used less-intense X-rays, these studies revealed the
formation of a crystalline-like phase at elevated temperatures. Furthermore,
the patterns produced by the diffracted X-rays indicate that the structure is
comprised of layers of conjugated backbone pairs, a pattern quite different
from the single backbone constructions observed in all other organic
photovoltaic materials studied to date.
Xinhui Lu, the paper’s lead author, noted that by analyzing the
scattering patterns, they discovered undulations along the polymer’s backbone,
and how the undulations in neighboring backbones are shifted with respect to
each other. By carrying out molecular modeling simulations, the authors were
able to predict which polymer backbone configuration would be most stable.
In a conjugated polymer, the backbone provides the path for
electrical conductivity and the alkyl side chains, similar to simple oils,
provide the solubility required for processing. Though necessary, these side
chains interfere with the polymer’s electrical performance. PCDTBT is novel,
the scientists say, since it is predominately composed of the backbone with
little alkyl material. “Similar to oil and water, the polymer’s conjugated
backbone pairs ‘phase separate’ from their alkyl side chains and this gives
rise to the bilayer structure,” said David Germack, one of the paper’s
coauthors. It is this structural motif that likely contributes to the
material’s excellent electrical properties, and this understanding could guide
the design of new organic solar materials.
“While we have significant in-house expertise in synthetic
chemistry and organic solar device fabrication, we lack the in-depth structural
characterization tools available at Brookhaven Lab,” said Jeff Peet, a senior
scientist at Konarka Technologies, a world leader in the development and
commercialization of organic solar cells. “These kinds of tools and
collaborative studies with research partners at Brookhaven can elucidate very
subtle differences between materials, giving us critical insights into how we
should design our next generation of solar cell materials.”
