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Less Expensive Displays: New Technique Allows Polymer Processing of a Key
Solid-state Fluorescent Material
by John Toon
By chemically attaching a difficult-to-process solid-state fluorescent
material to a universal polymer backbone, researchers at the Georgia Institute
of Technology have built what may be a foundation for a new generation of
optoelectronic display devices based on inexpensive organic light-emitting
diodes (OLEDs).
Until now, the aluminum tris (8-hydroxyquinoline) (Alq3) material
– which is used as the emission and electron transport layer in organic
light-emitting diodes – had to be deposited under high vacuum conditions, which
requires costly equipment. Attaching it to a polymer backbone allows the
material to be applied using solution processes – simple spin-coating methods
already widely used for applying thin films of materials. "This could have a significant impact for industry because it would make the
manufacture of organic light-emitting diodes much easier," said
Marcus Weck, an
assistant professor in Georgia Tech's
School of Chemistry and Biochemistry. "You can do this on a lab bench
without million-dollar equipment. Being able to spin coat these organic systems
could allow production of large surfaces suitable for displays."
Details of the work were presented March 27th at the 225th American Chemical
Society National Meeting in New Orleans, LA. Sponsored by the
National Science Foundation and the
Office of Naval Research, the research
has also been published in the journal Macromolecules. Because they are based on polymers, organic light-emitting diodes produced
with the new technique could offer another significant advantage – physical
flexibility. That would allow production of displays that are less prone to
damage and that can operate in shapes and forms not possible with current
technology. Using the polymer poly(norbornene) as a backbone, Weck and graduate student
Amy Meyers designed a functional monomer containing Alq3, also known
as aluminum tris (8-hydroxyquinoline). The Alq3 was covalently bonded
to the poly(norbornene) backbone, which was selected because it can be
polymerized by ring-opening metathesis, a method that tolerates many functional
groups. Though the prototype material shows great potential, Weck cautions that much
work remains to be done before the new material finds its way into laptop
computers and other display systems. "From a scientific standpoint, this is a milestone, but there is a lot of
optimization and evaluation that must be done," he said. "We've shown that we
can change the polymer backbone and that we can change the connection of our Alq3
to the polymer." Though the pure polymer has limited solubility, the researchers hope to
improve that as part of an on-going optimization process. The optical properties
of the new material appear equivalent to the conventionally-produced material,
but the details are still under study. Weck believes the trade-offs between
easier processing and optical performance will ultimately be positive. As part of optimizing the chemistry, Weck and Meyers are adjusting the
chemistry to provide emissions of different colors that would be necessary if
the material is to be used in flat-panel displays. The material's yellow-green
luminescence can be shifted with chemical additions or introduction of optically
inactive spacer molecules. "We want to produce a polymer system that would provide whatever color was
needed," Weck said. "The goal would be to create a 'Lego-like' system in which
you put different components together to get the output you need. We would
provide a polymer backbone with an aluminum center, and then add more units to
shift the wavelength." The Georgia Tech researchers are working with scientists at the University of
Arizona to assess how well the new material would work in OLEDs. If long-term
testing shows the new polymer has the desired stability and other properties, it
could help open up new applications for OLEDs. "One of the issues that has held back the market is this vacuum deposition
requirement," said Weck. "Most polymeric LEDs are difficult to make and
optimize. Our system would be straightforward and could be very interesting to
industry." Earlier efforts to improve the processing properties of Alq3 have
involved mixing it with or doping it into a polymer. Neither of those strategies
has worked well. The Alq3 system is the first demonstration of a technique Weck
hopes will allow his research group to build many new types of polymers using
modular scaffolds programmed to attract building blocks of small molecules. Weak
and easily reversed chemical interactions would self-assemble those molecules to
form complex structures with predictable physical and chemical properties. In the natural world, self-assembly techniques produce thousands of varied
life forms -- bacteria to human beings -- based on a relatively small set of
amino acids and nucleosides combined in different ways. By emulating this
natural system, he hopes to simplify the synthesis of new materials for
light-emitting diodes, optical storage materials, biosensors, drug-delivery
materials and other applications. "The goal is to simplify the synthesis of designer polymers via self-assembly using combinatorial chemistry," Weck explained. "Our group is taking design lessons from Nature by incorporating into one system several of these weak interactions to get a degree of complexity that is difficult to achieve otherwise. We believe we now have the basic proof of principle to show that we will be able to address this problem." RESEARCH NEWS &
PUBLICATIONS OFFICE MEDIA RELATIONS CONTACTS: TECHNICAL CONTACT: Marcus Weck
(404-385-1796); Fax: (404-894-7452); 
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