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It should be noted that initiators and impurities derived from them some-
times form carrier-trapping centers degrading carrier transport even in pho-
topolymerized thin films.
As mentioned in Sect. 1, low carrier mobility and specific dependence on
temperature and electric field have been recognized as phenomena caused by
the large disorder of the semiconductive polymers, so some investigations to
reduce disorder have been attempted. Ikeda et al. looked at the introduction
of mesogenic groups into side chain type photoconductive polymers [95, 96].
As shown in Fig. 16, they synthesized random copolymers
14, 15
containing
cyanobiphenyl or phenyl benzoate moieties, which are typical mesogens, as
well as carbazole dimer units with radical polymerization. The synthesized
polymers exhibited a nematic phase in a wide temperature range including
room temperature. Thin films were fabricated by the solution cast method on
substrates.
Fig. 16
Molecular structure of liquid-crystalline copolymers
Carrier transport characteristics were studied by the time-of-flight tech-
nique. Only hole transport was observed and its mobility was on the order
of 10
-7
cm
2
V
-1
s
-1
at room temperature, which was comparable to that of
conventional non-mesogenic polyvinylcarbazole derivatives. It strongly de-
pended upon electric field and temperature, which is similar to conventional
amorphous semiconductive polymers. In this study, introduced mesogenic
groups induced a nematic phase. However, the concentration of carbazole
chromophores that were associated with carrier transport was reduced by the
introduction of mesogenic groups, and the disorders were not greatly reduced
because of low molecular ordering in the nematic phase.
In 1993, a research group at the University of Bayreuth confirmed fast elec-
tronic transport in the discotic columnar phase of hexaalkoxytriphenylene
using the time-of-flight method [80], which made a strong impact on liquid
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