Research explains how to boost efficiency of polymer organic light-emitting diodes

Biasing spin statistics

Organic light-emitting diodes (OLEDs) based on pi-conjugated polymers offer significant advantages over other display materials. 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 to be made March 30 at the 227th national meeting of the American Chemical Society, researchers make 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.”

When the charges meet, a two-step charge-recombination process takes place in which the opposite charges neutralize one another, producing an excited state (an exciton) in the polymer. The decay of that excited state is what can produce light.

During the charge-recombination process, the spin directions of the electrons involved can orient themselves into four possible combinations, each with an equal statistical likelihood. These orientations form excitons in two different patterns. The first pattern, known as a “singlet,” can have only one of the four possible combinations. The other, known as a “triplet,” can have three different combinations.

Though the formation of singlet and triplet excitons both result in neutralization of the charges, only the singlets – which according to spin statistics should be created in just 25 percent of the recombinations – produce light in pi-conjugated polymers. Theoretically, that means 75 percent of the charge recombinations are wasted.

“We really need these polymers to go beyond 25 percent for the devices to be more efficient,” Brédas said. “Our theoretical work is oriented at how we might have deviations in that statistical limit – how we can bias the spin statistics.”

By taking advantage of complex restrictions on the amount of energy that can be released by the materials during the recombination process, Brédas and his colleagues show theoretically that systems built from long polymer chains should be able to boost the percentage of light-emitting singlets to as high as 50 percent.

That should be possible, Brédas explained, because in long chains, triplets are believed to take much longer to convert into neutral excitons after they initially meet to form a loosely-bound “charge transfer state.” If another process – such as intersystem crossing or dissociation – intervenes before the internal conversion takes place, the loosely-bound positive and negative charges in a triplet charge transfer state may transform into a singlet. In contrast, the singlet charge transfer states decay quickly, allowing no time for other processes to intervene.

“The calculations we have done based on electron transfer theory show that in short chains, the rates (to go from the charge transfer state to the neutral exciton state) are very fast for both singlets and triplets,” he explained. “But when you get to longer chains, the rate of formation of singlets remains large, but the rate of formation of triplets slows considerably. This means that in longer chains, you can bias the spin statistics to produce more singlet neutral excitons than would be predicted.”

Beyond demonstrating that the efficiency limit could be broken, the paper also suggests avenues – such as re-ordering the polymers or making substitutions in their backbones – that could boost these efficiency improvements. Brédas is working with experimental researchers to pursue that goal, and to study other applications for the materials in photovoltaics.

Of note, alternative OLEDs are based on molecular materials and produced through a vapor deposition process that requires more complex processing. However, these molecular materials can incorporate heavy-metal atoms that allow triplet excitons to electroluminesce, theoretically allowing 100 percent efficiency.

At Georgia Tech, the research of Brédas has been supported by the U.S. National Science Foundation, the Office of Naval Research and the IBM Shared University Research Program. At the University of Mons-Hainaut, support for collaborators David Beljonne, Aijun Ye and Zhigang Shuai came from the European Commission IST Program, the Belgian Federal Services for Scientific, Technical and Cultural Affairs and the Belgian National Science Foundation. And at the Chinese Academy of Sciences, support for Zhigang Shuai came from the National Science Foundation of China and the Ministry of Science and Technology of China.

Beyond the ACS presentation, the work will be described in a paper accepted for publication in the journal Advanced Functional Materials.