Diethynylbenzene-based liquid crystalline semiconductor for solution-processable organic thin-film transistors

Article abstract of DOI:10.1166/jnn.2010.2947

We report here the synthesis and characterization of novel diethynylbenzene-based liquid crystalline semiconductor (P1) for organic thin-film transistors (OTFTs), Compound P1 was synthesized by the Sonogashira coupling reaction between 2-bromo-5-(4-hexylt

Full text of DOI:10.1166/jnn.2010.2947

Copyright © 2010 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 10, 6800–6804, 2010
Diethynylbenzene-Based Liquid Crystalline
Semiconductor for Solution-Processable
Organic Thin-Film Transistors
1
2
2
3
4
Pramod Kandoth Madathil , Benoît Heinrich , Bertrand Donnio , Fabrice Mathevet , Jean-Louis Fave ,
2
3
5
1ꢀ∗
1ꢀ∗
Daniel Guillon , Andre-Jean Attias , Changjin Lee , Tae-Dong Kim , and Kwang-Sup Lee
1
Department of Advanced Materials, Hannam University, Daejeon, 305-701, Korea
2
IPCMS-CNRS, Strasbourg 67034, France
3
4
Laboratoire de Chimie des Polymère and Institut des NanoSciences de Paris, Université P. et M. Curie, Paris 75005, France
5
Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea
We report here the synthesis and characterization of novel diethynylbenzene-based liquid crystalline
semiconductor (P1) for organic thin-film transistors (OTFTs). Compound P1 was synthesized by
the Sonogashira coupling reaction between 2-bromo-5-(4-hexylthiophen-2-yl)thieno[3,2-b]thiophene
and 1,4-bis(dodecyloxy)-2,5-diethynylbenzene. Top contact OTFTs were fabricated by spin casting
with 2 wt% solution of P1 in chloroform and their best performance, which exhibited a hole mobility
−
5
2
of 4.5×10 cm /Vs, was showed after annealing of the films at liquid crystalline temperature. Time-
−
6
2
of-flight (TOF) mobility measured at liquid crystalline phase was observed to be 1.5×10 cm /Vs
for both pDo es il ti ivv ee r ea dn db ny e Pg ua tbi vl ies hc i an r gr i eT r es .c hT hn eo sl oe g rye st ou l: t sU inn i dv iec ar st ei tyt hoa ft St ho eu tl hi q eu ri dn Cc r ay sl i tf ao l rl inn ii at y helps to
improve the molecul aI Pr :p 1a c1 k7 i n. 1g 2 a1 n. 2d 8 e. 7n 7h aOn nc e: Wc hea dr g, e1 3m Jo ab ni lit 2y 0f 1o 6r P0 17 .: 4 T5 h: e4 s5 e advantages can be
applicable to design and c oC nos pt r yu rci gt hs ot :l uAt i mo n e- pr i rco ac ne s Ss ca ibel en tOi f iTc F PT u mb al i tse hr iea rl ss for electronic applications.
Keywords: Solution-Processing, Semiconductor, Liquid Crystal Materials, OTFTs.
1
. INTRODUCTION
We report herein the synthesis and characterization of a
new diethynylbenezene-based liquid crystalline semicon-
ductor (P1) for OTFT application. Hexyldithienothiophene
Solution-processable organic thin-film transistors (OTFTs)
have received much attention as potential alternatives to
conventional silicon transistors for large area, flexible and
low-cost electronic applications since these materials can
be used by various solution-processing techniques, such
units are introduced to the both ends of the molecule for
10ꢁ11
improving molecular packing and mobilities.
The hole
−5
2
mobility in the OTFT devices was 4.5×10 cm /Vs after
annealing at liquid-crystalline temperature. The molecular
orientation and time-of-flight (TOF) measurements of P1
were investigated in relation to the OTFT performance.
1
–6
as spin-coating, ink-jet printing etc. In order to utilize
organic materials for practical OTFT applications, it is
important that the materials should possess high mobilities
with good stability under air, moisture, and light exposure.
Among many organic semiconductors, side-chain-
substituted small molecules, which exhibit liquid crys-
talline phases at elevated temperatures, are of great interest
because of improved carrier mobility through the orien-
tation of the molecules.7 They consist of a rigid part,
usually a ꢀ-conjugated system, and a flexible part con-
taining long alkyl chains. The substitution pattern needs
to be designed carefully such that the side-chains provid-
ing adequate solubility and film-forming properties do not
interfere with the ability of the molecule to ꢀ-stack.
2. EXPERIMENTAL DETAILS
2.1. Materials
All commercially available materials were used as received
–9
12
unless otherwise specified. Thieno[3,2-b]thiophene (1),
1
3
(5-hexylthiophen-2-yl)tri-n-butylstannane (3), and 1,4-
diiodo-2,5-(dodecyloxy)benzene (5) were synthesized
according to reported literature procedures.
1
4
2,5-Dibromothieno[3,2-b]thiophene (2) Compound 1
(2 g, 14.3 mmol) was dissolved in dry DMF (20 mL) in a
Schlenk flask and a solution of N-bromosuccinimide (5 g,
30 mmol) in 40 mL dry DMF was added to the reaction
∗
Authors to whom correspondence should be addressed.
6
800
J. Nanosci. Nanotechnol. 2010, Vol. 10, No. 10
1533-4880/2010/10/6800/005
doi:10.1166/jnn.2010.2947

Madathil et al. Diethynylbenzene-Based Liquid Crystalline Semiconductor for Solution-Processable Organic Thin-Film Transistors
ꢀ
1
mixture at 0 C. The reaction mixture was stirred for 1 h
0.6 g of P1 (55%). H NMR (CDCl ꢂ: ꢃ 7.35 (s, 2H), 7.20
3
at room temperature and the reaction mixture was poured
into ice water, extracted with dichloromethane, dried over
(s, 2H), 7.03 (d, 2H), 6.98 (s, 2H), 6.70 (d, 2H), 4.02 (t,
4H), 2.80 (t, 4H), 1.85–0.85 (m, 68H).
MgSO and the solvent was evaporated. The crude product
4
was purified by column chromatography using petroleum
2
.2. General Instrumentation
ether as eluent to provide 2 g of compound 2 (94%).
1
1H NMR spectra were performed on a Varian Oxford
300 MHz spectrometer. Differential scanning calorimetry
(DSC) was conducted under nitrogen atmosphere on a TA
H NMR (CDCl ꢂ: ꢃ 7.17 (s, 2H).
3
2
- Bromo- 5- (4- hexylthiophen- 2- yl) thieno [3, 2-b]thio-
phene (4) Compound 4 was synthesized by Stille coupling
reaction. A two-necked round-bottomed flask was charged
with compound 2 (1 g, 3.4 mmol), 3 (1.8 g, 4.0 mmol),
and Pd(PPh ꢂ (0.12 g, 0.1 mmol). Dry toluene (40 mL)
ꢀ
Instrument 2100 with a ramping rate of 5 C/min. UV/vis
absorption spectra and photoluminescence (PL) spectra
were measured by a Shimadzu UV-2401 and LS-50B lumi-
nescence spectrophotometer, respectively. Transient pho-
tocurrents were measured by conventional TOF setup
using a nitrogen laser (l = 337 nm, sub-ns pulse duration,
3
4
was added and the mixture was refluxed under nitrogen
atmosphere for overnight. After cooling, the solution was
filtered and extracted with water. The organic layer was
1
0 ꢄJ/pulse) as the excitation source.
dried over MgSO and evaporated by rotary evaporator.
4
Compound 4 (0.9 g, 67%) was obtained after purifica-
tion by column chromatography using hexane as eluent.
2.3. OTFT Device Fabrication
1
H NMR (CDCl ꢂ: ꢃ 0.89 (t, 3H), 1.30–1.33 (m, 6H),
3
A layer of organic semiconductor was spin-coated onto
1
7
.66–1.68 (m, 2H), 2.79 (t, 2H), 6.69 (d, 1H), 7.0 (d, 1H),
.15 (s, 1H), 7.19 (s, 1H).
SiO surface of a heavily doped silicon wafer as the gate
2
electrode. The SiO surface was pretreated with octyl-
1
, 4- (2- Methyl- 3-butyn- 2-ol)-2, 5-didodecyloxybenzene
2
trichlorosilane (OTS). Top contact OTFTs were fabricated
by evaporating gold through a shadow mask to form source
and drain electrodes on the semiconducting thin films. The
device has 50 ꢄm of a channel length and 5 mm of a
(
(
6) To a 100 mL of three-necked flask, compound 5
0.25 g, 0.4 mmol), Pd(PPh ꢂ Cl (0.01 g, 0.02 mmol)
3
2
2
and copper iodide (0.01 g, 0.05 mmol) was added and
dried under vacuum. The flask was charged with nitrogen
gas and 50 mL of freshly distilled triethylamine and
channel width. The OTFT characteristics were measured
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IP: 117.121.28.77 On: Wed ,w 1i t3h Ja a Kn e2 i 0t h 1l e6 y0 47 2: 40 05 : s4 e5m iconductor characterization sys-
2
-methyl-3-butyn-2-ol (0.1 mL, 0.8 mmol) were added.
Copyright: American Scientific Publishers
ꢀ
tem under ambient condition. To achieve the optimal per-
The reaction mixture was heated at 90 C for overnight.
After evaporating solvent, the crude product was purified
by column chromatography using dichloromethane and
methanol (30:1) as eluent. The isolated product 6 (0.2 g,
8
3
formance, the different OTFT devices were annealed at 60,
ꢀ
8
0, 100, and 132 C for 30 min. The saturated field-effect
mobility was calculated from the amplification characteris-
tics by using the classical equations describing field-effect
transistors.
1
6%) was obtained. H NMR (CDCl ꢂ: ꢃ 6.85 (s, 2H),
3
.93 (t, 4H), 2.10 (s, 2H), 1.8–0.9 (m, 58H).
1
,4-Dithienyl-2,5-didodecyloxybenzene (7) The mixture
of 6 (0.1 g, 0.2 mmol), excess of NaOH (0.1 g, 2.5 mmol)
in 10 mL of toluene was refluxed for 24 hrs. After
the reaction, solvent was evaporated, washed with water,
and extracted with dichloromethane. The crude prod-
uct was purified by flash column chromatography using
3
. RESULTS AND DISCUSSION
A new liquid crystalline compound P1 was synthesized
using the procedure shown in the Scheme 1. Com-
pound 2 was prepared by bromination of compound 1 with
N-bromo-succinimide. Compound 4 was obtained by Stille
coupling reaction between compound 2 and compound 3.
dichloromethane as eluent and yielded 0.07 g of the com-
1
pound 7 (90%). H NMR (CDCl ꢂ: ꢃ 6.95 (s, 2H), 3.97
3
(t, 4H), 3.33 (s, 2H), 1.8–0.9 (m, 46H).
Bu
3
Sn
S
C
6
H
13
NBS
S
3
S
S
C6H13
S
Br
Br
Br
DMF
S
Pd(PPh3)4
Toluene
2
.1.1. Target Molecule P1
S
S
1
2
4
The compound 7 (0.26 g, 0.5 mmol) in diisopropylamine
12 mL) was added into the solution of compound 4 (0.4 g,
.0 mmol), Pd(PPh ꢂ Cl (0.08 g, 0.1 mmol) and CuI
OC12
H
25
OC12
H
25
OC12H
25
(
1
Pd(PPh3)2Cl , CuI
2
OH
2
-Methyl-3-butyn-2-ol
NaOH
I
I
H
H
3
2
2
Et
3
N
HO
Toluene
C
12
H
25
O
C
S
12
H
25
O
C
12 25
H O
(0.02 g, 0.1 mmol) in 40 mL of dried THF. After reflux-
5
6
7
ing for overnight, the reaction mixture was cooled to room
temperature, filtered, and extracted with dicholomethane.
OC12
H
25
S
S
Pd(PPh
3 2 2
) Cl , CuI
S
C6H13
4
+
7
6 13
C H
(i-Pr) NH, THF
2
S
S
The organic layer was dried over MgSO and evaporated.
4
12 25
C H O
P1
The crude product was purified by column chromatogra-
phy using hexane and chloroform (3:1) as eluent to obtain
Scheme 1. Synthetic route for P1.
J. Nanosci. Nanotechnol. 10, 6800–6804, 2010
6801

Diethynylbenzene-Based Liquid Crystalline Semiconductor for Solution-Processable Organic Thin-Film Transistors Madathil et al.
6x106
0
0
0
0
0
0
0
0
.14
.12
.10
.08
.06
.04
.02
.00
PL (Solution)
x106
5
4
3
2
UV (Solution)
x106
x106
x106
UV (Film)
1x106
0
3
00
350
400
450
500
550
Wavelength (nm)
Fig. 3. A Schleren texture image corresponding to nematic liquid crys-
talline phase for P1.
Fig. 1. Absorption and emission spectra of P1 in chloroform solution
and a thin film.
monotropic nematic liquid crystalline phase were observed
ꢀ
ꢀ
during cooling over a short range from 134 C to 129 C
(
dance with the result obtained by DSC.
Compound 7 was synthesized from 1,4-dihydroxybenzene
by a multi-step reaction as described in the experimental
section. Target compound P1 was synthesized by Sono-
gashira coupling reaction between compound 4 and 7.
Figure 1 shows the optical properties of P1. An absorp-
tion maximum at 425 nm was observed in chloroform
solution. P1 thin film exhibited a maximum absorption at
ꢀ
Fig. 3). At 127 C the P1 completely crystallized in accor-
To further study liquid crystallinity and orientation of
the molecule, X-ray diffraction measurement was per-
formed with different temperatures. The mesophase rang-
ing on few degrees only was uneasy to detect by X-ray
diffraction. We nevertheless observed a change of the
diffractogram on cooling from the isotropic liquid, just
before the transition into the crystalline phase as shown in
Figure 4.
4
42 and 472 nm. This large bathochromic shift might be
due to high degree of structural order of the P1 in the
solid state.16 Emission spectrum in chloroform solution
was showed maxima at 474 nm and 506 nm.
ꢀ
Delivered by Publishing Technology to: UT nh i ev eX r s- ri at yy op fa tSt eo r un t ho fe ri sn o Ct r oa pl i if co rl ni qi au id obtained at 145 C
Liquid crystalline behavior for the P1 was characterized
IP: 117.121.28.77 On: Wed, 13 Jan 2016 07:45:45
is shown in Figures 4(a and c). It shows two circular
Copyright: American Scientific Publishers
halos, one in the wide-angle region, reflecting the molten
by differential scanning calorimetry (DSC), polarized opti-
cal microscopy (POM), and small-angle X-ray diffraction
SA-XRD). DSC results reveal melting and liquid crys-
aliphatic chains in the isotropic state (dch = 4.6 Å), and
the other at small angle region (d = 18–20 Å). On cool-
ing, one could observe the fragmentation of both rings into
crescents on the equator of the diffractograms recorded
at 132 C in the mesophase temperature (Fig. 4(b)).
This clearly identifies the nematic phase formation for
(
talline features (Fig. 2). There was an endothermic peak at
ꢀ
1
17 C corresponding to solid-state rearrangement of the
ꢀ
material. Its isotropic melting peak was observed at 135 C.
On cooling, a small exothermic peak at 132 C and main
crystallization peak at 127 C were observed. In addition
solid-state rearrangement appeared clearly at 56 C.
ꢀ
ꢀ
ꢀ
ꢀ
(
a)
(b)
We could not observe any visible transition in the POM
ꢀ
ꢀ
images at 117 C during heating with 1 C/min of ramping
rate. However, clear schlieren textures corresponding to
1
27 °C
1
1
5
0
5
0
5
Cooling
56 °C
132 °C
(c)
(d)
6
00
00
00
00
00
00
5000
132 °C
45 °C
1
5
4
3
2
1
4
3
2
000
000
000
Heating
117 °C
–
135 °C
0
–
10
0
5
10 15 20 25 30
–180 –120 –60
0
60 120 180
20
40 60 80 100 120 140 160 180 200 220
Temperature(°C)
2θ (degree)
ψ (degree)
ꢀ
Fig. 4. X-ray diffraction patterns of isotropic liquid at 145 C (a) and
(c), mesophase at 132 C (b), and azimuthal integration for wide-angle
ꢀ
Fig. 2. DSC curve of P1 obtained at a cooling and heating rate of
C/min.
ꢀ
ꢀ
5
halo at 145 and 132 C (d).
6
802
J. Nanosci. Nanotechnol. 10, 6800–6804, 2010

Madathil et al. Diethynylbenzene-Based Liquid Crystalline Semiconductor for Solution-Processable Organic Thin-Film Transistors
the material which indicates lateral packing (ribbon for-
+
30 V
mation). Figure 4(d) shows azimuthal integrations (ꢅ-
integrations) of the wide-angle halo at 145 C and 132 C,
also indicating nematic phase of the P1.
–30 V
ꢀ
ꢀ
t=3.5 µs
t=3.5 µs
To investigate the charge transport properties of P1, top-
+
contact OTFTs were fabricated on heavily doped n -Si
wafers. Surface modification of SiO was done by treat-
2
ing with octyltrichlorosilane (OTS). 2 wt% of P1 solution
was spin-coated with 1000 rpm of spin rate for 2 min.
Patterned gold layers of thickness 100 nm were deposited
for source and drain electrodes. Although various anneal-
ing temperatures were utilized to induce orientation of the
molecule, most of devices did not show OTFT behavior
measured under ambient condition except the one annealed
T=132 °C
1000
10000
Time (µs)
ꢀ
at 132 C of mesophase temperature. The best value for
−
5
2
Fig. 6. Transient photocurrent of positive and negative carriers in
nematic phase.
hole mobility of P1 was found to be 4.5 × 10 cm /Vs.
The output (a) and transfer characteristics (b) of the OTFT
device are shown in Figure 5.
from double logarithmic plots. The charge-carrier transport
properties were characterized in the monotropic nematic
mesophase at 130 C on cooling. Transient photocurrents
The properties of charge-carrier transport were also
studied by time-of flight method TOF experiments. The
samples were prepared by filling P1 by capillary action
ꢀ
of positive and negative carriers for a ± 30 V applied bias
were recorded (Fig. 6). The TOF experiments revealed an
ambipolar charge transport of P1 having mobility of 1.5×
ꢀ
in the isotropic phases at about 145 C into liquid crys-
tal cells consisting of two indium tin oxide-coated glass
slides (ITO) separated by 4 ꢄm spacers. Transient pho-
tocurrents were measured by conventional TOF setup.
The Resulting transient photocurrents were recorded on
−6
2
1
0
cm /Vs for both positive and negative charge carriers.
4
. CONCLUSION
a digital oscilloscop eD ea nl i vd e tr rea dn s bi ty t Pi mu eb sl i swh ei nr eg dT ee t ec rhmn ion l eo dg y to: University of Southern California
IP: 117.121.28.77 On: Wed, 13 Jan 2016 07:45:45
We have successfully synthesized and characterized
Copyright: American Scientific Publishers
solution-processable organic semiconductor P1. The
nematic liquid crystalline phase of P1 was identified by
DSC, POM and XRD analysis. The best hole mobility
0
1
2
3
4
V
(a)
0 V
0 V
0 V
0 V
1
.5×10–8
−5
2
of 4.5 × 10 cm /Vs showed only for OTFT devices
annealed at liquid crystalline phase. The TOF measure-
ment exhibited ambipolar charge transport behavior with
–
8
9
1
.0×10
−
6
2
mobilities of 1.5 × 10 cm /Vs for both positive and
negative carriers at liquid crystalline phase. Although the
mobilities for P1 were low, they indicated that liquid crys-
talline phase can improve the molecular ordering resulting
in enhanced mobility.
–
5
.0×10
0.0
0
10
20
30
40
V (V)
d
Acknowledgment: This study was supported by a grant
from the Fundamental R&D Program for Core Technol-
ogy of Materials funded by the Ministry of Knowledge
Economy, Republic of Korea (M2007010004) and the
National Research Foundation (NRF ERC 2009-0063224).
Tae-Dong Kim acknowledges support from University
Research Program in Hannam University (2010).
–
–
–
–
–
–
–
–
9
9
9
9
9
9
9
9
8
7
6
5
4
3
2
1
.0×10
.0×10
.0×10
.0×10
.0×10
.0×10
.0×10
.0×10
(
b)
0.10
.09
0.08
0
0
0
.07
.06
0.05
.04
References and Notes
0
.0
0
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gate voltage (V ) characteristics of P1 at a drain voltage of −40 V.
d
d
G
d
G
J. Nanosci. Nanotechnol. 10, 6800–6804, 2010
6803

Diethynylbenzene-Based Liquid Crystalline Semiconductor for Solution-Processable Organic Thin-Film Transistors Madathil et al.
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1
Received: 31 December 2008. Accepted: 18 November 2009.
Delivered by Publishing Technology to: University of Southern California
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Copyright: American Scientific Publishers
6
804
J. Nanosci. Nanotechnol. 10, 6800–6804, 2010