Chemistry Reference
In-Depth Information
tained films were studied by the time-of-flight technique. The tripheneylene
monomer exhibited a hole mobility of 6
10
-4
cm
2
V
-1
s
-1
×
in the columnar
10
-4
cm
2
V
-1
s
-1
when the
materials was mixed with the photo-initiator. The hole mobility decreased to
1
phase, and the hole mobility was retained at 5
×
10
-4
cm
2
V
-1
s
-1
at 80
◦
C after photopolymerization. Unlike monomeric
triphenylene derivatives, the hole mobility was temperature-dependent, being
1
×
10
-5
cm
2
V
-1
s
-1
at room temperature [99].
Application of triphenylene-based polymers as a hole transport layer in
electroluminescence devices was also studied. The triphenylene polyacrylate
was spun on ITO substrate, on which Alq
3
and Al cathode were vacuum-
deposited; a brightness of 1390 cd m
-2
was obtained at 4.5 V [100].
Because of the conformation change and defect formation during the pho-
topolymerization process, long coherence length of the columns in the colum-
nar phase was difficult to maintain. Therefore, the excellent carrier transport
characteristics in the columnar phases of monomeric triphenylene deriva-
tives could not be retained in the polymerized films.
×
3.3
Semiconductive Polymers with Nematic Phases
Various organic semiconductors have been used for electroluminescence de-
vices [101]. Electroluminescence devices are suitable for flat panel displays
and therefore they can be applied as backlights for liquid crystal displays,
in which images are displayed by combining linearly polarized light, an op-
tically anisotropic liquid crystal layer, and a polarizer. In order to generate
linearly polarized light, another polarizer is essential, and therefore 50%of
light absorbed by the polarizer is lost as heat [102]. Conventional organic
semiconductors are amorphous and thus light emitted from devices based on
amorphous organic semiconductors is not polarized. Polarized light-emitting
devices, without using a polarizer, can contribute to an increase in energy
performance. The films of liquid-crystalline semiconductors that have a uni-
axial molecular alignment are promising candidates for such polarized light-
emitting devices because their transition dipoles are uniaxially aligned in the
films (as shown in Fig. 18). In addition, polarized light-emitting displays have
a potential for 3D display [103].
It is easier to realize macroscopic uniaxial molecular alignment in a ne-
matic phase than in 1D columnar and 2D smectic systems. In the application
to electroluminescence devices emitting linearly polarized light, uniaxial mo-
lecular alignment is more important than good carrier transport properties.
Kelly, O'Neill and coworkers synthesized liquid crystals containing fluorene
moieties in central cores with a polymerizable group, the 1,4-pentadien-3-
yloxy moiety [104-106]. These liquid crystals have extended
-conjugated
systems that contribute to fast intermolecular charge transfer. A monomeric
liquid-crystalline semiconductor that has the same aromatic core structure
π
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