- We have developed a liquid crystal forming
helical structures, and confirmed that it exhibits hole (positive hole)
conductivity.
- The periodicity of the helical structure
can be modulated by an external field. Applications to circularly polarized
luminescence devices can be expected, and thereby a reduction in electric power
consumption for liquid crystal displays.
-
Moreover, applications to organic
semiconductor lasers are expected.
Synopsis
In the National Institute of Advanced
Industrial Science and Technology (Hiroyuki Yoshikawa, President), Masahiro
Funabashi (researcher), and Nobuyuki Tamaoki (group leader) of the Molecular
Smart System Group of the Nano Technology Institute (Hiroshi Yokoyama,
Director), have succeeded in the development of an organic semiconductor
exhibiting optical properties as cholesteric liquid crystals. Since it shows a
charge transport characteristic as a semiconductor, and also forms helical
structures, its application to circularly polarized luminescence devices and
organic semiconductor lasers is expected.
As cholesteric liquid crystals have helical
structures whose periodicities are comparable with the wavelengths of visible
light, they can reflect or confine circularly polarized light with specific
wavelengths. Thus, utilizing these properties, circularly polarized luminescence
and optically induced laser oscillation have been investigated. Cholesteric
liquid crystals are usually insulating materials, and hence to realize
electrically driven devices (Fig. 1) the development of conducting cholesteric
liquid crystals has been needed.
Figure 1. A schematic illustration of the application of a cholesteric
semiconductor.
It has been reported that liquid crystals in
which molecules are closely packed, like crystals, can exhibit the electron
conduction observed for semiconductors. However, for cholesteric liquid
crystals, which form liquid-like structures, ionic conduction has been observed,
but the hole or electron conduction usually detected for semiconductors has not
been observed until now.
In this work, AIST has succeeded in the
synthesis of a cholesteric liquid crystal, phenyl-quarter-thiophene derivative
(3-QTP-4Me-Ph05*). This liquid crystal exhibits hole conduction as a
semiconductor in a cholesteric liquid crystal phase. We also have synthesized
its dimer, and found that it has a cholesteric structure at room temperature and
can emit circularly polarized light by optical excitation.
The results obtained have been published in
the July issue of ChemPhysChem, a physico-chemical specialty journal.
Furthermore, they will be presented at The International Liquid Crystal Society
meeting held at Keystone, Colorado.
Figure 2. A cholesteric liquid crystal, phenyl-quarter-thiophene derivative,
3-QTP-4Me-Ph05*.
Background for
Research Work
Recently, organic semiconductors have been
considered to be useful to realize inexpensive and flexible opto-electronic
devices, and investigations regarding electroluminescence devices and lasers
using organic semiconductors have been actively carried out.
The introduction of light-wavelength scaled
superstructures is essential to create new optical functions for organic
semiconductors, and thus, for their introduction, micro-fabrication using
lithography has been applied. However, chiral (optical rotatory) liquid
crystalline molecules can spontaneously form superstructures with periodicities
comparable to light wavelengths.
Utilizing such periodical structures,
circularly polarized luminescence and laser oscillation have been investigated,
but the application to electroluminescence devices has been impossible, because
liquid crystals are usually insulators.
Circularly polarized electroluminescence
devices may be used as the back lights for liquid crystal displays. As polarized
light filters are not needed in this case, the energy loss due to the filters
can be halved, leading to a longer operating life and an enhancement of the
reliability of the displays. Moreover, using them together with circularly
polarized light transmitting films, high quality displays may be
expected.
History of Research
Work
Our research group has carried out
investigation of the relationship between molecular structures and their
physical properties, and the establishment of the guiding principle for
molecular design to aim at the application of liquid crystalline organic
semiconductors to practical opto-electronic devices.
Details of Research
Work
Liquid crystalline semiconductors have
carrier mobility (a parameter proportional to the magnitude of electric current)
comparable to that of molecular crystals. In addition, they have high solubility
to organic solvents, enabling thin film formation and device fabrication in the
solution process. Recently, it has been reported that a liquid crystal forming a
layered structure like a crystal exhibits a charge transfer mobility of
approximately 10-1 cm2/Vs, and its application to organic thin film transistors has been
examined.
However, as liquid crystalline materials
have fluidity, not only hole or electron conduction, but also ionic conduction
occurs. Until now, the liquid crystals which have exhibited electric conduction
as semiconductors have been only smectic (layered structure) liquid crystals and
discotic columnar (columnar structure) liquid crystals. For nematic and
cholesteric liquid crystals, with no crystal-like structures but a fluidity like
a liquid, only ionic conduction due to impurities has been observed, and hole or
electric conduction has not been observed.
If cholesteric liquid crystals with
periodicities comparable to visible light wavelengths have electronic
conductivity, the fabrication of circularly polarized electroluminescence
devices and organic semiconductor lasers can be realized. Moreover, as the
periodic structures of cholesteric liquid crystals can be modulated by external
electric fields, their application to luminescence devices with tunable emission
light-wavelength can be considered.
Cholesteric phase structures are equivalent
to twisted nematic phase structures. Thus, by introducing chirality to liquid
crystalline molecules exhibiting a nematic phase, the molecules enable forming a
cholesteric phase. We have designed liquid crystalline molecules having large
aspect ratios (length-to-width ratios) for the nematic phase formation and
further having a partial structure of a conducting polymer for high electric
conduction.
Actually, we have newly synthesized
phenylquarterthiophene derivatives with a long ð-electronic conjugated system
and with chiral alkyl side-chains, as shown in Figure 3. This substance shows
the cholesteric phase at temperatures between 162 and 83°C in the cooling
process.
Figure 3. The cholesteric liquid crystalline semiconductor we have newly
synthesized.
The positive carrier mobility in the
cholesteric phase of the synthesized liquid crystal showed a value of 2 x
10-4 cm2/Vs, which is 1-2 orders higher than those of the conventional
nematic and cholesteric liquid crystals with low molecular weight. Also, as
shown in Figure 4, the positive charge transfer mobility saturates with
increasing temperature, being different from ionic conduction dominated by
viscosity which monotonously decreases with increasing temperature. This
conduction is suggested to be the hole conduction observed for
semiconductors.
For negative charge transfers, two types of
carriers can be considered, the mobility of one carrier is 2 x
10-4 cm2/Vs, similar to the value of the positive charge conduction, and that
of the other carrier is 10-5 cm2/Vs, similar to those of the usual low molecular nematic liquid
crystals. The latter mobility increases monotonously with temperature, and its
activation energy agrees with that of the viscosity, indicating that the
conduction is ionic conduction dominated by viscosity.
Figure 4. Temperature dependence of charge transfer mobility.
Figure 5. An illustration of electron conduction in the cholesteric
phase.
The liquid crystalline semiconductor,
3-QTP-4Me-Ph05, does not maintain the cholesteric phase, but crystallizes at
room temperature. Furthermore, as its helical periodicity is longer than the
wavelengths of visible light, it exhibits no selective reflection in the visible
light region. Thus, we have synthesized a dimeric cholesteric semiconductor to
which binaphthyl groups are introduced as chiral parts; at room temperature,
this compound maintains the cholesteric phase, and further shows interference
colors, as it has a selective reflection band in the visible light
region.
Because of this property, we have observed
circularly polarized fluorescence when it is excited by ultra-violet light with
a wavelength at which the fluorescence spectrum and selective reflection band
overlap. Figure 6 shows circularly polarized fluorescence spectra, where the
circularly polarized light dichroic parameter is 1.4 at the maximum.
Figure 6a. Molecular structure of the dimeric cholesteric liquid crystal we
have synthesized.
Figure 6b. Circularly polarized light emission spectrum for the dimeric
cholesteric liquid crystal we have synthesized.
Future Prospects
We plan to further enhance charge carrier
mobility, and fabricate electroluminescence devices enabling circularly
polarized luminescence in the solution process. Also, we have aimed at optically
excited laser oscillation, and then the realization of electrically driven
organic semiconductor lasers. |