A New ‘Spin’ on Organic Solar Cells

Wednesday, August 7, 2013 @ 04:08 PM gHale


Organic solar cells that convert light to electricity using carbon-based molecules have shown promise as a versatile energy source, but have not been able to match the efficiency of their silicon-based counterparts.

That may soon change as researchers found a synthetic, high-performance polymer that behaves differently from other tested materials and could make inexpensive, highly efficient organic solar panels a reality.

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The polymer, created at the University of Washington (UW) and tested at the University of Cambridge in England, can improve efficiency by wringing electrical current from pathways that, in other materials, cause a loss of electrical charge.

“In most cases you are generating charge but you have to out-compete all the areas of loss that keep you from delivering the electricity from the cell to the device you are trying to power,” said Cody Schlenker, a postdoctoral researcher in the laboratory of David Ginger, a UW chemistry professor.

“These materials can be printed like newspaper and manufactured into rolls of film like plastic wrap, so they could have a significant manufacturing cost advantage over traditional materials like silicon,” Ginger said. Schlenker and Ginger are co-authors of a paper analyzing the new material.

Organic solar cells change color briefly as they convert light to electricity, similar to how some prescription glasses darken when exposed to sunlight and become clear indoors. The researchers used a technique called photo-induced absorption spectroscopy to measure the color changes as “fingerprints” to study pathways that devices use to convert sunlight to electricity.

The same technique also pinpoints “dead-end” pathways that do not produce electricity, which are present in most organic materials used for solar cells and limit power production. UW scientists found their polymer appeared to have few dead ends, but they needed more sensitive measurements to be sure.

Cambridge researchers had seen hints of the same kind of behavior in similar materials for organic solar cells.

“They were seeing some of the same features that we were seeing, features that everyone said you shouldn’t be able to see,” Schlenker said.

At a scientific meeting in Italy last year, the two groups began discussing the apparent surprising properties of the UW-created polymer, composed of carbon, hydrogen, sulfur and nitrogen atoms. The Cambridge researchers used lasers to probe the polymer and saw clear evidence of the behavior they found hints of in other materials they studied.

They found the apparent lack of electrical dead ends in the new polymer related to a quantum mechanical property of electrons called “spin.” Essentially, with certain spin configurations the material can “rescue” electrical charges from what otherwise would be energy-losing pathways.

Currently, organic solar cells can achieve as much as 12 percent efficiency in turning light into electricity, compared with 20 to 25 percent for silicon-based cells. Schlenker said design concepts based on the new material will help to significantly close the gap between these two types of solar cells.

Organic materials are semi-transparent and tunable to any color, and their flexibility and ease of production mean that achieving greater efficiency in changing light to electricity could make them cheaper and easier to deploy than the silicon-based cells.

The carbon-based molecules in the organic polymers are similar to molecules already found in car paints, some clothing dye and the pigment in plant chlorophyll. Organic dyes could incorporate into ink and end up printed on materials such as shingles, siding or window frames.

Current materials are relatively low cost and recyclable. Work to extend their lifespan beyond five to seven years and to find ways to replace them relatively easily could make them a feasible option for a home or business, Schlenker said.

Solar cells now provide less than 0.2 percent of power used in the United States, but improving efficiency and finding ways to incorporate them into building materials is one way to make them cost-effective.

“You have to go in the direction of adding no cost to the material you already are planning to deploy,” Schlenker said.



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