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U of Washington studies bubbles in plastic solar cells to boost efficiency

David Ginger, a University of Washington associate
professor of chemistry, displays the tiny probe used to
record photocurrents in carbon-based solar cells.
Image Courtesy of the University of Washintgon

A research team at the University of Washington has found a way to improve the performance of plastic solar cells by inserting tiny bubbles and channels inside them.

The research team, headed by associate professor of chemistry David Ginger, has found a way to make images of tiny bubbles and channels—around 10,000 times smaller than human hair— form within the polymers of a plastic solar cell that can improve the material’s performance.

The researchers are able to measure directly how much current each tiny bubble and channel carries, thus developing an understanding of exactly how a solar cell converts light into electricity. Ginger believes this will lead to a better understanding of which materials created under which conditions are most likely to meet an efficiency goal like 10%.

Most plastic solar cells, also called organic solar cells, nowadays are made by blending two materials together to form a thin film that is baked to improve their performance. In the process, bubbles and channels form which affect how well the cell converts light into electricity. The number of bubble and channels and their configuration can be altered by how much heat is applied and for how long.

The exact structure of the bubbles and channels is critical to the solar cell's performance, but the relationship between baking time, bubble size, channel connectivity and efficiency has been difficult to understand.

For the current research, the scientists worked with a blend of polythiophene and fullerene, model materials considered basic to organic solar cell research because their response to forces such as heating can be readily extrapolated to other materials. The materials were baked together at different temperatures and for different lengths of time.

They were then tested using a small tool called an atomic force microscope to make a nanoscale image of the solar cell.
It records the channels and bubbles that were created as the material was formed and can help scientists determine quickly whether these are sufficient for the cell to achieve 10% efficiency.

A paper documenting the work was published online July 9 by the American Chemical Society journal Nano Letters and scheduled for a future print edition. Co-authors are Liam Pingree and Obadiah Reid of the University of Washington. The research was funded by the National Science Foundation and the US Department of Energy.

- Katrice R. Jalbuena

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