Polymer Incorporates Alq3 for Organic LEDs

Daniel S. Burgess

As the market for alternative display technologies grows, aluminum tris(8-hydroxyquinoline), or Alq₃, continues to enjoy the attention of researchers for its excellent optical properties. Unfortunately, this small-molecule organic LED material, while inexpensive itself, must be deposited in vacuum conditions, which increases the complexity and cost of production.

Assistant professor Marcus Weck and graduate student Amy Meyers of Georgia Institute of Technology in Atlanta think that they may have found a solution to Alq₃'s problems by combining the best aspects of small-molecule and polymeric organic LEDs. They are incorporating the material into a polymer, which should enable it to be deposited employing low-cost solution-processing techniques such as spin coating or ink-jet printing. The Alq₃-functionalized polymers also should be suitable for the fabrication of flexible displays with long-term mechanical stability.

Researchers have developed a functional monomer containing aluminum tris(8-hydroxyquinoline), enabling them to bond the small-molecule organic LED material to a polymer backbone. The polymer promises to be compatible with inexpensive solution-processing manufacturing techniques for the fabrication of displays. Courtesy of Georgia Institute of Technology.

Weck explained that, as polymer scientists, they considered looking at what polymeric materials would offer the desired optical properties for organic LEDs, but decided that it would be better if the polymer were instead an "innocent bystander" with no influence on the optoelectronic properties of the final material. Working with Alq₃ was a natural choice, he said, because the material is well understood and because the team could draw from the more-than-a-decade of established research into manipulating its optical properties.

In the process, the researchers synthesize an Alq₃-functionalized monomer based on norbornene and employ ring-opening metathesis to polymerize the material, a reaction that takes approximately 12 hours. The solubility of the material in a mixture of chloroform and trifluoroacetic acid is improved by copolymerization with 5-nonylnorbornene.

The absorption and emission spectra of the copolymer in solution are virtually identical to those of Alq₃ alone, and research is under way by a group of physicists at the University of Arizona in Tucson to determine whether spin-coating the material into thin films adversely affects properties such as the quantum yield, brightness and fluorescence lifetime. Weck said he was afraid that the deposition process would lead to quenching but that the initial reports include no such effects. He noted that industry feedback would add welcome perspective on the practical potential of the material.

He and Meyers continue to work to simplify the synthesis process and expect to produce the next generation of the material within six months. They also are investigating means of shifting the wavelength of its emission.

As they wait to learn how well their prototype material works in a real-life situation, Weck is cautiously optimistic. "It is best to emphasize that we are in the early stages of an ongoing process," he said. "But we think we are on the right track."