Solar cells with perfect mix of plastics

U. WASHINGTON (US)—A recent discovery may help scientists pinpoint the right combination of materials and conditions needed to develop the ultimate solar cells—those that are inexpensive to produce and highly efficient at converting light to electricity.

Researchers the world over are working to develop organic solar cells made from low-cost plastics that will transform at least 10 percent of the sunlight that they absorb into usable electricity. A major obstacle is coaxing these carbon-based materials to reliably form the proper structure at the nanoscale (tinier than 2 millionths of an inch).

A research team headed by David Ginger, a University of Washington associate professor of chemistry, has found a way to make images of tiny bubbles and channels, roughly 10,000 times smaller than a human hair, inside plastic solar cells. These bubbles and channels form within the polymers as they are being created in a baking process, called annealing, that is used to improve the materials’ 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 that will lead to a better understanding of which materials created under which conditions are most likely to meet the 10 percent efficiency goal.

As researchers approach that threshold, nanostructured plastic solar cells could be put into use on a broad scale, he says. As a start, they could be incorporated into purses or backpacks to charge cellular phones or mp3 players, but eventually they could make in important contribution to the electrical power supply.

Most researchers make plastic solar cells by blending two materials together in a thin film, then baking them to improve their performance. In the process, bubbles and channels form much as they would in a cake batter. The bubbles and channels affect how well the cell converts light into electricity and how much of the electric current actually gets to the wires leading out of the cell. The number of bubbles 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. Some models used to guide development of plastic solar cells even ignore the structure issues and assume that blending the two materials into a film for solar cells will produce a smooth and uniform substance. That assumption can make it difficult to understand just how much efficiency can be engineered into a polymer, Ginger said.

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.

Ginger notes that the polymer tested is not likely to reach the 10 percent efficiency threshold. But the results, he says, will be a useful guide to show which new combinations of materials and at what baking time and temperature could form bubbles and channels in a way that the resulting polymer might meet the standard.

Making solar cells more efficient is crucial to making them cost effective, he explains. And if costs can be brought low enough, solar cells could offset the need for more coal-generated electricity in years to come.

“The solution to the energy problem is going to be a mix,” he says, “but in the long term solar power is going to be the biggest part of that mix.”

Ginger is the lead author of a paper documenting the work, published online July 9 by the American Chemical Society journal Nano Letters and scheduled for a future print edition. The research was funded by the National Science Foundation and the U.S. Department of Energy.

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