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January 2012

Polymer Chains Could Help Electrons Go the Distance

Improvements in energy transfer length provide insight into increasing the efficiency of organic solar cells

Jessica Morrison

Fluorescence images of single polymer chains before solvent vapor annealing or SVA, during the annealing process as aggregates start to form and as small aggregates after annealing.

Imagine your cell phone reliably powered by sunlight. To make this a reality, energy from the sunlight must be absorbed by a semiconducting material and separated into charges. Using a new research technique, scientists at the Charge Separation and Transfer Energy Frontier Research Center are developing a better understanding of organic polymer materials that behave like semiconductors.

Unlike traditional semiconductors, materials that conduct electricity, organic polymers are easy to prepare for mass production through techniques like printing on flexible surfaces. Understanding the properties that control energy transfer in these materials could go a long way in increasing the efficiency and lowering the cost of solar cells.

Earlier this year, CST researchers introduced solvent vapor annealing in an article in Angewandte Chemie as a new method for affecting the arrangement of single conjugated polymer chains, a type of semiconducting polymer. The CST researchers expanded this technique to create polymer chain aggregates, facilitating study into the property changes that occur as single polymer chains transition to a bulk polymer film.

“We really want to know how you go from a single chain to a film,” says David Vanden Bout, senior author on a second paper from the CST that appeared in Nature Materials. “There are a lot of differences between one individual polymer chain and many of them packed together in a film.”

One of those differences is energy transfer length—the distance that energy migrates before finding an interface between two different materials in a photovoltaic system. The charge separation that occurs at this interface is positively affected by an increase in energy transfer length. The farther energy can travel, the more likely it is to reach the interface and be separated into useful charges. This makes the energy transfer length of a material an important consideration in photovoltaic material development.

For bulk conjugated polymer films, the energy transfer length is 5 nanometers on average. In single conjugated polymer chains and aggregates, the energy transfer lengths are much longer, at 25 to 30 nanometers on average. This surprising result, that aggregates behave more like a single polymer chain than a polymer film, is leading the CST research team to question:“What is limiting the energy transfer in bulk films?” asks Vanden Bout. “Are bulk films so disordered that we don't see these effects? How do we make films that behave like our aggregates?”

These questions are important considerations as CST researchers continue to seek the answers that could someday change the way we charge our electronic devices.

More Information

Vogelsang J, J Brazard, T Adachi, JC Bolinger and PF Barbara. 2011. "Watching the Annealing Process One Polymer Chain at a Time." Angewandte Chemie International Edition 50(10):1-6. DOI: 10.1002/anie.201007084.

Vogelsang J, T Adachi, J Brazard, DA Vanden Bout and PF Barbara. 2011. "Self-Assembly of Highly Ordered Conjugated Polymer Aggregates with Long-Range Energy Transfer." Nature Materials 10:942-946. DOI: 10.1038/nmat3127

Acknowledgments

This work was funded by the CST, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. J Vogelsang was also funded partially by a fellowship from the German Research Foundation.

About the author(s):

The Efficiency of the Few Outshines the Many

Polymer chain aggregates move electrons faster than films in solar cells

Understanding the properties of organic polymers as photovoltaic materials could lead to a greener way to charge our cell phones: the sun. Photo credit: Brandon Morrison

Charging your cell phone or other devices with sunlight could be achieved with less expensive polymer solar cells, which use a carbon-based film to turn sunlight into electricity. These cells require relatively long distance electron transfer within the polymer film. The film is composed of long-chain molecules, called polymers. While single polymer chains efficiently transport electrons, the polymer films do not. Scientists applied solvent vapor annealing and microscopy to delve into this transport difference. With the annealing process, the team created a well-organized structure from tens of polymer molecules. This aggregate transferred electrons 5 to 6 times farther than the films. This work could lead to a precise process for designing efficient polymer solar modules. This work was supported by the Charge Separation and Transfer Energy Frontier Research Center, led by University of Texas at Austin.

Written by Jessica Morrison and Kristin Manke

More Information

Vogelsang J, J Brazard, T Adachi, JC Bolinger and PF Barbara. 2011. "Watching the Annealing Process One Polymer Chain at a Time." Angewandte Chemie International Edition 50(10):1-6. DOI: 10.1002/anie.201007084.

Vogelsang J, T Adachi, J Brazard, DA Vanden Bout and PF Barbara. 2011. "Self-Assembly of Highly Ordered Conjugated Polymer Aggregates with Long-Range Energy Transfer." Nature Materials 10:942-946. DOI: 10.1038/nmat3127

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.