Chemistry Reference
In-Depth Information
9.2 SHAPE-PERSISTENT CONJUGATED DENDRIMERS
AS LIGHT-EMITTING MATERIALS
9.2.1 Introduction to OLEDs
OLEDs represent a relatively mature area of organic electronics. Since their discov-
ery [20], OLEDs have attractedenormous attentiondue to their applications in full color
flat-panel displays, solid-state lighting, andasbacklights for liquid-crystaldisplays [21].
Before outlining the general requirements in the molecular design of light-emitting
dendrimers, it may be necessary to briefly introduce how anOLEDworks. In a simplest
one-layer OLED device, active materials are sandwiched between two electrodes [22].
Electrons are injected into the lowest unoccupied molecular orbital (LUMO) of the
activematerial at the cathode.At the anode, holes are injected into thematerial's highest
occupied molecular orbital (HOMO). These positive and negative charge carriers then
move in opposite directions within the active layer under an applied voltage. When
coming across, they combine to generate excitons that emit light with wavelength
determined by the energy gap between the HOMO and LUMO. However, in a single
material, usually either hole transport or electron transport dominates, resulting in a
largewaste of the excessive charge and reduced device efficiency. Therefore, to balance
the charge transport, one or more layers are commonly incorporated between the
cathode and anode (Figure 9.2); each layer is optimized for hole transport, electron
transport, or light emitting [23]. The overall efficiency of an OLED device can be
described in terms of the external quantum efficiency (EQE), which is the number of
photons emitted by the device divided by the number of charge pairs injected. However,
other parameters, such as lifetime at working conditions, maximum brightness, and
color purity, are equally important in evaluating the performance of an OLED device.
The operationmechanismof an OLED device imposes several requirements on the
active materials. First of all, the dendrimer should have high luminescent quantum
yield (LQY) in the solid state. This requirement has several implications: first,
conjugated units with inherent high LQY should be incorporated into the dendritic
scaffold. Typical building blocks include 9,10-substituted anthracene [24] and
oligofluorene [25] among others. Second, interchromophore interactions should be
FIGURE 9.2
Illustration of the configuration of an OLED device: (a) a single layer device.
(b) A multilayer device with electron- and hole-transporting layers separated from the light-
emitting layer.
