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Organic Light Emitting Diodes (OLED)
A/P Terence Wong Kin Shun
Organic light emitting devices are like the conventional
inorganic LEDs except that the active
layer are made from molecular thin films composed
mainly of carbon and hydrogen. This emerging optoelectronics
technology began in 1987 with the first small
molecule OLED developed at Eastman Kodak and received
a further boost with the observation of electroluminescence
in thin film conjugated polymers at Cambridge
University.
The most promising area of application for organic
electroluminescent materials is in information displays,
especially large area emissive displays and displays on
curved or flexible surfaces, which have been gaining in
importance in recent years as a result of the growth
of portable wireless devices. OLED provides a viable
medium for a sheet of light on a nonplanar substrate,
something that inorganic LED cannot achieve.
All OLEDs are based on a capacitor type structure
(fig.1). The active organic layers are sandwiched between
two electrodes one of which is transparent. Before a bias
is applied, there are no carriers within the organic layer.
The carriers must be injected from the electrodes. Within
the organic layer, the carriers-electrons and holes - meet
and form entities called excitons. When these excitons
decay, a photon is emitted.
Organic layer
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glass
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Cathode
Figure 1: Schematic diagram of an OLED
Research on OLEDs at the PhRC began in 1995. The
initial effort has focused on the synthesis of molecular
complexes and conjugated polymers and the study of
their optical properties. The conjugated polymer studied
was the homopolymer polythiophene and its alkylsubstituted
derivatives. The structural, thermal and thermomechanical
properties of these polymers were studied.
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In addition, the direct patterning of the polymer films by
an ultraviolet laser was demonstrated. Discrete OLED devices
were fabricated in-house using these materials (see
Figure 2).
Figure 2: Discrete OLED with patterned emission area. Emission
color is yellow
We have worked on improving the efficiency of the devices
by incorporating charge transport layers. We have
studied the use of a mixed organic/inorganic solvent to
electropolymerize polybithiophene at low voltage, and of
an electrochemically deposited polybithiophene layer to
enhance the performance of an OLED. Chemical synthesis
of rare earth metal chelate complexes and oligomers
emitting in the blue and violet part of the spectrum, and
synthesis of conjugated polymer blends and copolymers
are other ongoing works. At present, we are collaborating
with industry to build better devices and the end goal is a
multicolor panel with possibly an active matrix driver.
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Figure 3: Examples of conjugated oligomers synthesised
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Did You Know? In 1964, William Bennett invented the argon-ion
laser at Yale University. In 2000 his failing eyesight was corrected with
retinal surgery using an argon-ion laser.