Chemistry Reference
In-Depth Information
FIGURE 9.16
Examples of shape-persistent phosphorescent materials. (a) 1,3,5-Phenelyne-
based green phosphor
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[60]. (b) Carbazole-based phosphor
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[61].
that during charge injection, the spin of the charge cannot be controlled. When
electron and hole combine in a molecule, statistically the ratio of singlet to triplet
excitons is 1:3. Because fluorescent materials can only utilize the singlet excitons, at
least 75% of the excitons are wasted. This is why much interest has been paid to
phosphorescent materials that utilize triplet excitons as the emitter [23].
By synthesizing conjugated, dendritic ligands and coordinating themwith suitable
metal ions such as Ir, shape-persistent phosphorescent dendrimers can be constructed.
For example, Figure 9.16a shows the chemical structure of a phosphorescent DLED
material, reported by Burn and coworkers [60]. This green emissive dendrimer, when
blended in a host and used in a bilayer device, achieved a brightness of 400 cd/m
2
at
4.5 V, with a corresponding EQE of 16% and a power efficiency of 40 lm/W. This is
very close to the theoretical limit of efficiency of 20% based on an outcoupling of a
fifth of the light generated in the device.
Similar to fluorescent dendrimers discussed earlier, dendrons with electron- or
hole-transporting ability could be attached to the periphery of the ligand, to
enhance charge transport. For example, Wang and coworkers reported a series
of Ir-containing dendrimers with structures shown in Figure 9.16b [61]. Undoped
devices using a neat emissive dendrimer film of
showed excellent performance
with a peak luminous efficiency of 45.7 cd/A (13.4%), high luminance and high
current density (180,000 cd/m
2
, 360mA/cm
2
at 12V). More examples could be
found in a recent review by Burn et al. [62]
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9.2.3 Outlook
The development of DLEDs has matured to the stage where they are widely
recognized as the third class of materials in OLEDs. Device efficiencies have
