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.”
Additional collaborators on this research are: Htay Hlaing of Broookhaven Lab and Stony Brook University, Won Ho Jo of Seoul National University, and Denis Andrienko and Kurt Kremer of the Max Planck Institute for Polymer Research.
This research was funded by the DOE Office of Science, Konarka Technologies, the Energy Laboratory Research and Development Initiative at Brookhaven Lab, the German Research Foundation, and the German Federal Ministry of Education and Research. The National Synchrotron Light Source at Brookhaven is also supported by the DOE Office of Science.
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