Bipolar carrier transport in Tri-Substituted octyloxy-Truxene DLC

Article abstract of DOI:10.1080/15421406.2011.568891

In this study, bipolar charge carrier transport of 2,7,12- trioctyloxytruxene has been studied by a time-of-flight method. The substance studied exhibits a hexagonal columnar mesophase of which melting and clearing points are 118C and 133C, respectively. Ambipolar electronic carrier transports were observed in the mesophase. The drift mobility measurements reveal it is in the order of 10^-2cm² V^-1 s^-1 in the Col_h and it increases to 10^-1cm² V^-1 s^-1 at the columnar mesophase - metastable phase transition, indicating truxene is an interesting molecular core for mesophase semiconductors. Copyright Taylor & Francis Group, LLC.

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Bipolar Carrier Transport in Tri-
Substituted Octyloxy-Truxene DLC
a c
a
b
b
b
H. Monobe , C. Chen , K.-Q. Zhao , P. Hu , Y. Miyake
, A.
c
c
a
Fujii , M. Ozaki & Y. Shimizu
a
Research Institute for Ubiquitous Energy Devices, National Institute
of Advanced Industrial Science and Technology (AIST), Midorigaoka,
Ikeda, Osaka, Japan
b
College of Chemistry and Materials Science, Shichuan Normal
University, Chengdu, Shichuan, China
c
Graduate School of Engineering, Osaka University, Yamada-oka,
Suita, Osaka, Japan
Version of record first published: 30 Jun 2011.
To cite this article: H. Monobe, C. Chen, K.-Q. Zhao, P. Hu, Y. Miyake, A. Fujii, M. Ozaki & Y. Shimizu
(2011): Bipolar Carrier Transport in Tri-Substituted Octyloxy-Truxene DLC, Molecular Crystals and
Liquid Crystals, 545:1, 149/[1373]-155/[1379]
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Mol. Cryst. Liq. Cryst., Vol. 545: pp. 149=[1373]–155=[1379], 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.568891
Bipolar Carrier Transport in Tri-Substituted
Octyloxy-Truxene DLC
1
H. MONOBE, C. CHEN, K.-Q. ZHAO, P. HU,
2
2
2
1
Y. MIYAKE, A. FUJII, M. OZAKI, AND
,3
3
3
1
Y. SHIMIZU
1
Research Institute for Ubiquitous Energy Devices, National Institute of
Advanced Industrial Science and Technology (AIST), Midorigaoka,
Ikeda, Osaka, Japan
2
College of Chemistry and Materials Science, Shichuan Normal
University, Chengdu, Shichuan, China
3
Graduate School of Engineering, Osaka University, Yamada-oka, Suita,
Osaka, Japan
In this study, bipolar charge carrier transport of 2,7,12-trioctyloxytruxene has been
studied by a time-of-flight method. The substance studied exhibits a hexagonal
ꢀ
ꢀ
columnar mesophase of which melting and clearing points are 118 C and 133 C,
respectively. Ambipolar electronic carrier transports were observed in the
mesophase. The drift mobility measurements reveal it is in the order of 10 cm
ꢁ
2
2
ꢁ1
ꢁ1
ꢁ1
2
ꢁ1 ꢁ1
V
s
in the Col and it increases to 10 cm V s at the columnar mesophase -
h
metastable phase transition, indicating truxene is an interesting molecular core for meso-
phase semiconductors.
Keywords Charge carrier transport; discotic liquid crystals; mesopahse semicon-
ductor; truxene
1
. Introduction
Discotic liquid crystals (DLCs) have shown some interesting features for their elec-
tronic properties such as charge transport along the columnar axis as well as the
anisotropic property of conduction in the recent decade [1–13]. In fact, in highly
ꢁ
1
2
ꢁ1 ꢁ1
ordered columnar mesophases, fast carrier mobility (ꢂ10 cm V
s ) by elec-
tronic process of charge carrier transport was found which is comparable to those
of amorphous silicon [3–6]. Thus, the applications for organic semiconductors such
as one-dimensional conductors, photoconductors, molecular wires and fibers, light
emitting diodes and photovoltaic cells have also been exploited [14,15]. Furthermore,
DLCs are expected as a self-assembling molecular semiconductor ink to fabricate the
Address correspondence to H. Monobe, Research Institute for Ubiquitous Energy
Devices, National Institute of Advanced Industrial Science and Technology (AIST),
Midorigaoka, Ikeda, Osaka 563-8577, Japan. Tel.: þ81-72-7519149; Fax: þ81-72 7519628;
E-mail: monobe@ni.aist.go.jp
149=[1373]

1
50=[1374]
H. Monobe et al.
organic electronic devices by solution process. In particular, those are deeply inter-
esting materials in terms of printed electronics. In addition, the ambipolar charge
carrier transport is another important property as the organic semiconductors
[
16]. Liquid crystalline semiconductors are very effective in improving the present
device with their unique features of electrically inactive structural defects including
domain boundaries and fast bipolar charge carrier mobility.
Especially for discotic liquid crystalline semiconductors, molecules self-organize
to molecularly stacked columnar structure and these molecular order could be active
as 1D path for charge transport by electronic hopping process. Therefore, different
types of p-conjugated molecular core of DLC are exploited its charge carrier
mobility such as triphenylene [1–4,10–13], hexabenzocoronene [5], phthalocyanine
[
6], porphyrin [7], perylene [8], pyrene and carbazole [9] and those carrier mobility
ꢁ1 ꢁ1
ꢁ
4
ꢁ1
2
in the columnar mesophases are in the order of 10 to 10 cm V
other hand, truxene is another interesting core structure for DLCs as an organic
s . On the
ꢁ3
transistor and photoconductor while those charge carrier mobility are 10 and
ꢁ1 ꢁ1
ꢁ4
2
cm V
1
of 2,7,12-trioctyloxytruxene (in short Trx(OC8)3) by a time-of-flight (TOF) method
0
s , respectively [17,18]. Recently, we reported the fast hole mobility
ꢁ2
2
ꢁ1 ꢁ1
s in the hexagonal columnar
and its hole drift mobility is around 2 ꢃ 10 cm V
(
Col ) mesophase [19].
h
In the present report, charge carrier transport was investigated for a
truxene-based DLC, Trx(OC8)3, by TOF method to show that bipolar electronic
carrier transports in the mesophase. Temperature dependence of positive and nega-
tive charge carrier mobility were also reported correlated to the mesomorphism.
2
. Experimental Methods
A truxene-based DLC, Trx(OC8)3, has been synthesized through six steps and syn-
thetic routes is shown in Figure 1 [19]. The product was purified by column chroma-
tography and further purification by several repetition of recrystallization from the
mixture of ethanol and n-hexane.
The phase transition temperatures and enthalpies were detected by a Differential
Scanning Calorimeter (TA Instrument, 2920 MDSC) and the textures of the meso-
phase were observed by a polarizing microscope (POM) (Olympus, BX-51) equipped
with a hot stage (Mettler, FP80HT). Powder X-ray diffraction (XRD) studies were
Figure 1. Synthetic Route of 2,7,12-trioctyloxytruxene (in short Trx(OC8)3).

Bipolar Carrier Transport in Trialkoxy Truxene DLC
151=[1375]
carried out by using a Rigaku Geigerflex X-ray diffractometer (CuK ) with a custom
a
hotstage. The charge carrier mobility was measured by TOF method on a liquid
crystal film sandwiched between Indium-Tin-Oxide covered glass plates. The cell
thickness was approximately 15 mm, as determined by the interference pattern of
the cell in the UV–visible spectrum. The cell was capillary filled in under an inert
gas (Ar) after being degassed under vacuum by slowly cooling the sample from
the isotropic liquid (Iso) state. No surface treatment was used in this study. The cell
was set up in a hotstage equipped with a polarizing microscope and externally biased
by a stabilized DC power supply. A N laser (k ¼ 337 nm, 800 ps, u ¼ 1 mm) was used
2
for a pulsed light irradiation. A transient photocurrent was detected by a digital
oscilloscope (Hewlett Packard, HP54820A) with a variable-gain high-speed current
amplifier (FEMTO, DHPCA-100).
3
. Results and Discussion
.1. Phase Transition Behaviors
DSC curves of Trx(OC8)3 is shown in Figure 2. On cooling process (bottom) by
3
ꢀ
ꢁ1
0
between isotropic phase (I) and mesophase and crystallization at 131 C
.5 C min , there are two exothermic peaks corresponding to the transition
ꢀ
ꢁ
1
3.4 kJ mol ) and 87 C (7.3 kJ mol ), respectively. Following on heating process
ꢀ
ꢁ1
(
(
ꢀ
ꢁ1
middle) by 0.5 C min , Trx(OC8)3 shows two endothermic peaks corresponding
ꢁ1
ꢀ
to the transition crystal - mesophase and the isotropic melt at 118 C (37.9 kJ mol
ꢁ1
)
and 133 C (3.4kJmol ) in spite of the crystal - mesophase transition at temperature
ꢀ
ꢀ
ꢀ
ꢁ1
88 C on 2nd heating at scanning rate of 10 C min [19]. There was another broad exo-
thermic peak around 80 C (20.7kJmol ). This implies that after cooling process it might
be a metastable phase and forming more stable state causes an exothermic peak and 30 C
ꢀ
ꢁ1
ꢀ
ꢀ
ꢁ1
Figure 2. DSC curves of Trx(OC8)3 (heating and cooling rate; 0.5 C min ). (Figure appears
in color online.)

1
52=[1376]
H. Monobe et al.
higher shift of the phase transition temperature to the mesophase was observed. Indeed,
ꢁ1
ꢀ
ꢀ
the melting point of recrystallized sample was 118 C at a scanning rate of 5 C min . The
ꢀ
ꢁ1
optical texture observation by POM at 10 C min revealed that the phase transition at
ꢀ
ꢀ
88 C and 133 C which are the melting and clearing points, respectively. A typical
fan-shaped texture was seen for the meophase, indicating a Col mesophase is formed.
h
3
.2. XRD Analysis
ꢀ
XRD measurement for non-aligned sample was performed at 120 C in the meso-
phase. Figure 3 shows the XRD patterns of Trx(OC8)3. A set of reflections corre-
p
observed and this is an evidence that hexagonal arrays of columns exist. Additional
sponding to d , d and d20 of which ratio in distance is 1: 1= 3: 1=2 was
1
0
11
˚
˚
two reflections were observed at ca. 4.6 A (broad halo) and ca. 3.6 A (shoulder),
which are assigned to the average distance of the molten alkyl chains and intraco-
˚
lumnar order, respectively. Shoulder reflection peak near 3.6 A indicates the exist-
ence of the molecular stacking in intra columnar structure. The lattice parameter
˚
of the Col phase was calculated to be a ¼ 20.9 A and this value is almost 2=3 for
h
˚
the molecular diameter (ca. 31 A) with fully extended chains and this is less than that
˚
of corresponding six-chain analogue (ca. 25 A) [20]. This is reasonable that the
reduced number of periphelal chains causes dense packing of columns in Col struc-
h
ture because the volume of aklylchains is decreased. Trx(OC8)3 exhibits a Col
ꢀ
h
ꢀ
mesophase of which melting and clearing points are 118 C and 133 C, respectively.
The reduced number of substituted alkyl chains leads to the decrease of thermal stab-
ility of Col mesophase in comparison to the corresponding six-chain analogue
h
ꢀ
ꢀ
(melting: 86 C, clearing: >300 C) [20]. These results are consistent with the optical
textures observed on cooling from the Iso.
ꢀ
Figure 3. XRD pattern (powder) of Trx(OC8)3 at 120 C.

Bipolar Carrier Transport in Trialkoxy Truxene DLC
153=[1377]
ꢀ
Figure 4. Polarizing microphotographs of the sample cell for 15.4 mm-thick cell at 120 C (a)
before and (b) after rotation of the sample on the stage by 45 . (Figure appears in color
online.)
ꢀ
3
.3. Charge Carrier Transport Property
The transient photocurrent measurements have been performed by TOF technique
for the partly homeotropically aligned area (Fig. 4) to investigate the charge trans-
port along the columnar axis. Figure 5(a) and (b) show a typical transient photocur-
ꢀ
rent of Trx(OC8)3 (80 C, thickness 15.4 mm) for positive and negative carriers in the
mesophase under various electric fields, respectively. A clear fast transit (arrows in
Fig. 5) was observed which corresponded to the transport of positive and negative
charge carriers by electronic process. The drift mobility for hole and electron are
ꢁ
2
ꢁ1
2
ꢁ1 ꢁ1
almost same value and in the range of 5 ꢃ 10 ꢂ 1 ꢃ 10 cm V
s at a transit
time range of microseconds without electric field dependence.
Figure 6 shows temperature dependence of the charge carrier mobility for posi-
tive and negative charge carriers on cooling process. Bipolar carrier transports were
observed for Trx(OC8)3. The drift mobility measurements reveal it is in the order of
ꢁ
2
2
ꢁ1 ꢁ1
ꢁ1
2
ꢁ1 ꢁ1
1
0
cm V
s
Col - metastable phase transition and the value of mobility kept almost constant
in the Col meophase and it increases to 10 cm V s at the
h
h
within the same phase. This implies truxene is an interesting molecular core for liquid
Figure 5. Log-log plots of transient photocurrent for (a) positive and (b) negative carriers of
ꢀ
Trx(OC8)3 at 80 C on cooling process.

1
54=[1378]
H. Monobe et al.
Figure 6. Temperature dependence of drift charge carrier mobility of Trx(OC8)3 for positive
circle) and negative (square) carriers. (Figure appears in color online.)
(
crystalline semiconductors. However, in fact, in order to obtain the ambipolar elec-
tronic carrier transport, it was needed careful purification of compound by means of
repeated recrystallization. Furthermore, the hole drift mobility of the present sample
was faster than that of previously reported [19]. It was probably due to the purity of
the liquid crystalline sample used and the less impurities in the electric path in the
bulk sample leads to the fast electronic carrier mobility [21]. These results imply
purity of liquid crystalline semiconductor is one of the important issues to get fast
charge carrier mobility.
4
. Conclusions
A truxene DLC with three peripheral chains, Trx(OC8)3, exhibits a Col mesophase
h
ꢀ
ꢀ
of which melting and clearing points are 118 C and 133 C, respectively. TOF mea-
surements of charge carrier mobility in truxene DLC with three peripheral chains,
Trx(OC8)3, were carried out and bipolar electronic carrier transports in the meso-
phase and lower metastable phase were observed. The drift mobility for hole and
ꢁ
2
2
ꢁ1 ꢁ1
electron are almost same order and the value is 5 ꢃ 10 cm V
s
in the Colh
ꢁ1 ꢁ1
s at the Col - metastable phase
ꢁ1
2
mesophase and it increases to 1 ꢃ 10 cm V
h
transition. These results indicate truxene is an interesting molecular core for liquid
crystalline semiconductors.
Acknowledgment
This research was partly supported by Japanese Society for the Promotion of Science
(
(
JSPS) - the National Natural Science Foundation of China (NSFC) Joint Project
No. 50811140156).

Bipolar Carrier Transport in Trialkoxy Truxene DLC
155=[1379]
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