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Material’s Structure Hints at More Efficient Solar Cell

UPTON, N.Y., April 27, 2012 — New details about the structure of one of the most efficient photovoltaic materials could inform the synthesis of new materials with even better properties.

PCDTBT, one of the best known organic photovoltaic materials, converts sunlight to electricity with a 7.2 percent efficiency, but the reasons why were a mystery.

“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 National Lab 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.”


Structural details of photovoltaic material revealed: The bilayer polymer backbone motif (3-D image) is derived from the x-ray scattering pattern (background) obtained at beam line X9 of NSLS. In the 3-D image, the yellow region denotes the paired backbones and the blue region, the liquidlike side chains.

Ocko's team took thin films of PCDTBT and exposed them to high-intensity x-rays from Brookhaven's National Synchrotron Light Source (NSLS). The scientists performed the work in collaboration with researchers at Stony Brook University, Seoul National University in Korea, Max Planck Institute for Polymer Research in Germany and Konarka Technologies Inc. of Lowell, Mass., an organic solar cell developer that helped fund the work.

Exposing it to such strong radiation revealed the crystallinelike phases that were formed when the film was heated. The x-rays also revealed that PCDTBT's structure was made up of layers of conjugated backbone pairs. This is very different from the single backbone structures observed in every other photovoltaic material studied to date.



Brookhaven Lab research team members (l-r) Xinhui Lu, Htay Hlaing, David Germack and Ben Ocko.

PCDTBT is a polycarbazole conjugated polymer, which is a molecule composed of a carbon backbone, with alkyl side chains. The backbone allows for electricity to be conducted along the path. Normally, in these kinds of polymers, the alkyl side chain gets in the way, but in PCDTBT, there is not enough alkyl to interfere, which allows the backbone and side chain to phase separate. This gives rise to the bilayered structure, and subsequently PCDTBT's ability to accept and donate electrons, which is what makes it such an efficient photovoltaic.

The research, also funded by the US Department of Energy's Office of Science, the Energy Laboratory Research and Development Initiative at Brookhaven Lab, the German Research Foundation, and the German Federal Ministry of Education and Research, was published in the April 24 issue of Nature Communications.



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