Biasing Spin Statistics: Research Explains How to Boost the Efficiency of Polymer Organic Light-emitting Diodes (OLEDs)

They are lightweight, flexible, easily tailored, operate on low voltages and can be deposited on large areas using simple techniques such as ink-jet printing or spin-coating.

By combining the electrical properties of metals and semiconductors with the mechanical properties of plastics, these materials are poised to provide a foundation for new generations of flexible displays for computers and other devices. Until recently, however, many researchers believed these light-emitting polymers were limited in their efficiency, able to convert no more than 25 percent of their energy into light.

But in a presentation made March 30 at the 227th national meeting of the American Chemical Society, researchers made the theoretical case that efficiency of the materials can be much higher. Based on theoretical calculations done by scientists at universities on three continents, the study should encourage researchers to pursue techniques that could improve efficiency of the polymer devices, said Jean-Luc Brédas, a professor in the School of Chemistry and Biochemistry at the Georgia Institute of Technology.

"These results are important in the sense that they lead to an understanding of why polymer LEDs can have an efficiency that goes beyond the 25 percent limit predicted on the basis of simple spin statistics," said Brédas, who also is part of a research team at the University of Mons-Hainaut in Belgium. "It's important to show that there are ways past this theoretical limit."

Polymer LEDs consist of a thin film (0.1 microns) of a polymer such as polyparaphenylene vinylene sandwiched between two electrodes. They are usually built on a transparent substrate which can be glass or flexible plastic.

When voltage is applied to the electrodes, the top electrode (cathode) injects electrons into the polymer film, while the bottom electrode (anode) injects positive charges, also known as holes. Those charges migrate along the polymer chains until they meet.

"We are very interested in understanding at the microscopic level how the electronic structure of the polymer and the way the chains are oriented to one another influence the mobility of those charges," Brédas said. "We are looking at these processes from a molecular standpoint with a chemical perspective, trying to describe transport as electron transfer reactions."

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