Synthesis and solution-processed field effect transistors of liquid crystalline oligothiophenes

Article abstract of DOI:10.1246/cl.2007.708

An organic field-effect transistor (OFET), showing a mobility 0.015 cm ² V^-1 s^-1 and an on/off ratio 10⁴, which are as high as those of a dry-processed OFET of oligothiophenes, has been built by wet process usin

Full text of DOI:10.1246/cl.2007.708

708
Chemistry Letters Vol.36, No.6 (2007)
Synthesis and Solution-processed Field Effect Transistors
of Liquid Crystalline Oligothiophenes
Minoru Ashizawa,ꢀ1 Reizo Kato,ꢀ1 Yoichi Takanishi,2 and Hideo Takezoe2
1Condensed Molecular Materials Lab., RIKEN, 2-1 Hirosawa, Wako 351-0198
2Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552
(Received February 26, 2007; CL-070214; E-mail: mashizawa@riken.jp)
H
TMS
An organic field-effect transistor (OFET), showing a
Pd(PPh3
)
4, CuI
mobility 0.015 cm2 Vꢁ1 sꢁ1 and an on/off ratio 104, which are
as high as those of a dry-processed OFET of oligothiophenes,
has been built by wet process using a solution of a newly synthe-
sized quaterthiophenes-base liquid crystalline molecule.
ArX
S
Ar
TMS
KOH
ArX =
C6H13
TMS
H
I
for 1
Br for 2
C6H13
S
I
S
There has been growing interest in organic thin–film transis-
tors (OTFTs) over the last several years due to their potential for
the fabrication process of large area, low cost, and flexible
devices.1 For industrial use, solution-processed methods such
as spin-coating, inkjet printing, and screen printing are highly
desirable. Soluble ꢀ-systems have been widely studied for this
purpose. For example, regioregular head-to-tail poly(3-hexyl-
thiophene) (P3HT) show the marked charge carrier mobilities.2
The requirement to improve TFT performance is to form the
highly crystalline domains in the thin films obtained from solu-
tion, in which the molecules self-organize and make cofacially
stacked ꢀ-frameworks. The resulting close ꢀ-stacks facilitate
charge carrier transport. Soluble liquid crystalline (LC) semicon-
ductors exhibiting excellent TFT performance have potentials to
enhance device performance by manipulating its film morpholo-
gies through LC phases.3 However, it is noted that LC molecules
in themselves would have strong tendency of self-organization
with highly organized fluidity from their concentrated solutions,
resulting in the crystalline domains of their thin films without LC
phase assistance.
In this paper, we focus on synthesis and simple drop-casted
FET of liquid crystalline quaterthiophenes 1 and 2, which consist
of a central quaterthiophene core with ethyne spacers connecting
hexyl chain containing two benzene or thiophene rings. A central
quaterthiophene (4T) core keeps molecular linearity at two
ꢁ-positions of the outside thiophene rings in comparison with
odd number rings of oligothiophene core.4 This linearity is
thought to be useful for face-to-face intermolecular ꢀ-stacking,
making ꢀ-conduction paths.
Ar
S
S
Pd(PPh3
)
4, CuI
THF, Et3
N
C6H13
S
S
n-BuLi
CuCl2
S
S
C6H13
1
2
19%
S
S
C6H13
S
S
C6H13
S
S
18%
Scheme 1.
orbital) levels of the materials (ꢁ5:28 eV for 1 and ꢁ5:26 eV
for 2) are determined using the onset positions of the oxidation
processes according to the previous report.7 The thermal proper-
ties of 1 and 2 were examined by differential scanning calorim-
etry (DSC) and polarized optical microscopy (POM). Figure 1
illustrates the heating and cooling DSC scans of 1. 1 and 2
showed LC phase characteristics with two endothermic peaks
of 1 and three endothermic peaks of 2: crystalline-to-smectic
C phase at ꢂ170 ꢃC and smectic C phase-to-nematic phase with
partial decomposition at ꢂ245 ꢃC for 1, and crystalline-to-
unknown smectic phase (smectic X) at ꢂ140 ꢃC, smectic X-to-
smectic C phase at ꢂ190 ꢃC, and smectic C phase-to-nematic
phase with partial decomposition at ꢂ210 ꢃC for 2 (POM images
of smectic C phase of 1 and 2 are shown in Figure S1). The
replacement of the phenyl rings of 1 with the thienyl rings gives
The present compounds 1 and 2 were prepared as shown
in Scheme 1 (yield: 19% for 1 and 18% for 2 by four steps).5
The synthesis involves the two-step Sonogashira cross-coupling
reactions.6 A central quaterthiophene core was constructed at the
final dimerization step by lithiation followed by CuCl2
oxidation, to avoid difficulties due to poor solubility of quater-
thiophene unit. Both 1 and 2 are moderately soluble in common
organic solvents.
On the cyclic voltammetry measurements, 1 and 2 showed a
similar quasireversible oxidation wave: anodic peak potential,
Epa ¼ 0:68 V for 1 and 0.67 V for 2 (vs Ag/AgNO3 in PhCN
with 0.1 M n-Bu4NPF6, grassy carbon working electrode, and
scan rate 100 mV sꢁ1). The HOMO (highest occupied molecular
cooling
heating
Crystalline
150
Smectic C
Nematic
partially
decomposed
50
100
200
250 300
Temperature /°C
Figure 1. Heating and cooling DSC traces at 5 ꢃC/min for 1.
Copyright ꢀ 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.6 (2007)
709
rise to a more ordered smectic phase (smectic X) of 2. This phase
could not be distinguished from smectic C only by POM images,
but this smectic X phase appears to come from the difference in
the orientation of end-capped hexyl chains.
Thin film of 1, obtained by drop-casting on a SiO2 substrate
from a CS2 solution followed by annealing below crystal-LC
phase-transition temperature (150 ꢃC), showed multiple X-ray
diffraction patterns (Figure S2).5 The most significant Bragg
mismatch (ꢂ0:3 eV) with the work function of gold metal,
resulting in large contact resistance. The FET mobility estimated
from the saturation region is 1:5 ꢄ 10ꢁ2 cm2 Vꢁ1 sꢁ1 (at VD
¼
ꢁ40 V, W ¼ 10000 mm, L ¼ 20 mm) with an on/off ratio of
104. On the other hand, devices of 2 showed only linear region
of output characteristics under an identical condition with 1.
The estimated linear mobility is 4:3 ꢄ 10ꢁ4 cm2 Vꢁ1 sꢁ1 (at
VD ¼ ꢁ40 V, W ¼ 5000 mm, L ¼ 10 mm) with an on/off ratio
of 102. The present FET performance is comparable to those
of thermally grown oligothiophene thin films. The solution proc-
ess dramatically decrease device fabrication time in comparison
with vacuum deposition, and is suitable for industrial applica-
tion. It should be noted that the reported devices upon herein
were fabricated by simple drop-casting and annealing under nor-
mal atmospheric conditions. Self-organization (molecular order)
of the present compounds from various concentrated solutions
should be investigated to improve film morphology for the
enhancement of the FET performance. Further manipulation in
or above LC phase temperature to improve the FET performance
is also expected.
˚
diffraction peaks (d-spacing of 20.6 A) up to the ninth order were
recorded, indicating well-defined molecular layers, but multiple
patterns imply the co-existence of grains with different molecu-
lar orientations. These orientations would be associated with
self-organization corresponding to the layered structure of smec-
˚
tic C phase. The d-spacing of 20.6 A corresponds to half of the
˚
molecular length (ca. 40 A) of 1. Therefore, the molecules are
largely tilted (ca. 60ꢃ) from the normal direction to the substrate.
Thin film transistors were fabricated by drop casting from
CS2 or toluene solutions of 1 and 2 onto bottom gate, bottom
contact HMDS (Hexamethyldisilazane)-treated SiO2 substrates
with photographically patterned gold source and drain elec-
trodes. Devices were annealed at 100 ꢃC under air (below the
LC phase temperature) prior to measurement (SEM images are
shown in Figure S3). All the devices exhibited p-channel behav-
ior with current modulation. The FET characteristics of thin
film of 1 from CS2 solution are shown in Figure 2. The output
characteristics show typical linear and saturated regions together
with nonohmic behavior at low VD region, probably due to
In summary, the new solution-processable LC quaterthio-
phenes 1 and 2 have been prepared. We have demonstrated that
TFTs made by even simple drop-casting film of 1 and 2 show
competing FET properties to dry-processed oligothiophene
devices. The ordered film structure of 1 without the LC phase
assistance indicates that the LC molecules are attractive candi-
dates applicable for the low-cost fabrication technique like the
inkjet printing.
V
G
This work was partially supported by a Grant-In-Aid for
Scientific Research (No. 16GS0219) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
-30
-25
-20
-15
-10
-5
-40 V
(a)
-30 V
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0
-10
-20
-30
-40
VG / V
Figure 2. Output (a) and transfer (b) characteristics of a TFT
fabricated with 1 as a semiconductor channel layer.
Published on the web (Advance View) April 28, 2007; doi:10.1246/cl.2007.708