Regulating electron 'spin' may be key to making organic solar cells competitive

August 7, 2013, University of Washington
This is the laser set-up used to to make the actual measurements reported in the paper. Credit: Dr. Akshay Rao

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.

Now, researchers have discovered a synthetic, high-performance polymer that behaves differently from other tested materials and could make inexpensive, highly efficient organic solar panels a reality.

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

"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, published online Aug. 7 in Nature. The lead authors are Akshay Rao and Richard Friend of Cambridge, who with Cambridge researchers Philip Chow and Simon Gelinas did sensitive measurements that confirmed the properties of the polymer. The material was created in the lab of co-author Alex Jen, a UW professor of materials science and engineering.

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 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 were surprised when 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 that could be used in .

"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.

A vial holds a solution that contains the UW-developed polymer "ink" that can be printed to make the solar cells. Credit: Yeechi Chen/University of Washington

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 that had only been hinted at in other materials they had studied.

They found that the apparent lack of electrical dead ends in the new polymer is 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 can achieve as much as 12 percent efficiency in turning light into electricity, compared with 20 to 25 percent for silicon-based cells. Schlenker believes design concepts based on the new material will help to significantly close the gap between these two types of solar cell.

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 be incorporated into ink and 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 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.

Explore further: An unexpected change in polymer structure opens a new avenue in the search for improved solar cell efficiency

More information: The role of spin in the kinetic control of recombination in organic photovoltaics, Nature, DOI: 10.1038/nature12339

Related Stories

UCLA scientists double efficiency of novel solar cell

July 29, 2013

Nearly doubling the efficiency of a breakthrough photovoltaic cell they created last year, UCLA researchers have developed a two-layer, see-through solar film that could be placed on windows, sunroofs, smartphone displays ...

The fluorescent future of solar cells

May 9, 2013

( —For some solar cells, the future may be fluorescent. Scientists at Yale have improved the ability of a promising type of solar cell to absorb light and convert it into electrical power by adding a fluorescent ...

New technology to enable development of 4G solar cells

July 29, 2013

Professor Ravi Silva of the University of Surrey's Advanced Technology Institute has identified the range of combinations of organic and inorganic materials that will underpin new 4th generation solar cell technology – ...

Future looks bright for carbon nanotube solar cells

June 18, 2013

( —In an approach that could challenge silicon as the predominant photovoltaic cell material, University of Wisconsin-Madison materials engineers have developed an inexpensive solar cell that exploits carbon nanotubes ...

Recommended for you

Using organoids to understand how the brain wrinkles

February 20, 2018

A team of researchers working at the Weizmann Institute of Science has found that organoids can be used to better understand how the human brain wrinkles as it develops. In their paper published in the journal Nature Physics, ...

Pattern formation—the paradoxical role of turbulence

February 19, 2018

The formation of self-organizing molecular patterns in cells is a critical component of many biological processes. Researchers from Ludwig-Maximilians-Universitaet (LMU) in Munich have proposed a new theory to explain how ...

Bringing a hidden superconducting state to light

February 16, 2018

A team of scientists has detected a hidden state of electronic order in a layered material containing lanthanum, barium, copper, and oxygen (LBCO). When cooled to a certain temperature and with certain concentrations of barium, ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Aug 08, 2013
This article does not address the proposed "how" of regulating electron spin in this organic solar cell application as intimated in the caption above.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.