16-07-2012, 02:32 PM
Progress with Light-Emitting Polymers
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Introduction
Polymeric electroluminescence (EL) is a phenomenon
based on amorphous, disordered organic semiconducting materials
that has stimulated technological activity across a wide
interdisciplinary reach within academic, industrial, and governmental
sectors. Efforts aimed at developing associated
technology have created a rare opportunity to develop highvalue
products and have contributed fundamental scientific
insight in this area, which is poised for commercial success.
Recent materials advances have endowed the concept of
ªplastic lightº with the optical, electrical, and mechanical
characteristics that truly make it disruptive technology within
the display and lighting industries in that it is compatible with
conventional device replacement and it offers new opportunities
for exploitation.
Historical Development
In the 1950s, Bernanose, who applied a high-voltage alternating
current (AC) field to crystalline thin films of acridine
orange and quinacrine,[1±4] first observed EL in organic material.
In 1960 two researchers at The Dow Chemical Company
also prepared AC-driven electroluminescent cells using doped
anthracene, a p-conjugated, three-ring, fused aromatic system.
This work resulted in the first patent on EL devices based on
polynuclear aromatic organic compounds.[5] Later in the decade,
Pope prepared a DC EL cell using single crystals of anthracene,
which was observed to fluoresce in the blue by applying
400 V across the specimen.
Device Structure
At present, the two-layer structure represents the preferred
embodiment for the commercial PLED device, and it is illustrated
in Figure 1. The hole-transporting layer (HTL) material
film is laminated over the transparent ITO anode structure,
over which is applied the emitting layer (EML) film. A cathode
possessing a low work function is deposited to provide
electron injection into the p* band of the EML. Afterwards,
the whole device may receive an encapsulation of some kind
to improve its durability.
Polymeric Luminescent Material Development
The design of luminescent materials for use in LED devices
is as critical to device performance as the process of constructing
the device itself. Processability, purity, thermal and oxidative
stability, color of emission, luminance efficiency, balance
of charge carrier mobility, and others are among many important
materials properties required for a system to be viable in
commercial LED device applications. Great strides have been
made toward the development of new polymeric materials for
luminescent applications.