An amorphous polythiophene as a binder material for organic thin-film transistor channel applications

Article abstract of DOI:10.1080/15421401003613675

Blending of crystalline low molecular-weight organic semiconductors, e.g., 6,13-bis (triisopropylsilylethynyl)pentacene (TIPS-Pentacene), and binder polymers has recently been known to be an effective way to achieve high performance organic thin-film transistors (OTFTs). However, only limited number of examples has been reported as binder materials for OTFT applications, e.g., poly(triarylamine), poly(-methylstyrene), and etc. In this work, an amorphous polythiophene derivative, poly(3-(2-ethylhexyl)thiophene) (P3EHT), was prepared by introducing 2-ethylhexyl group as a side-chain to the polythiophene. The P3EHT was synthesized by Grignard metathesis (GRIM) polymerization to give a molecular weight (Mn) of 4,100 (PDI=3.4), and it was readily soluble in common organic solvents, such as chloroform, tetrahydrofuran, toluene, and etc. An amorphous nature of the P3EHT was confirmed from differential scanning calorimetry (DSC) and ultraviolet-visible spectroscopy results. A mobility of 2.5×10^-4cm²/Vs, an on/off ratio of 0.57×10³ and a threshold voltage of 1.6V have been obtained with OTFTs using a P3EHT/TIPS-Pentacene blend.

Full text of DOI:10.1080/15421401003613675

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An Amorphous Polythiophene as a Binder
Material for Organic Thin-Film Transistor
Channel Applications
Jaemin Lee a , Jiyoung Kwon a , Jongsun Lim a & Changjin Lee a
a Advanced Materials Division , Korea Research Institute of Chemical
Technology , Yusong-gu, Daejo, Korea
Published online: 19 Apr 2010.
To cite this article: Jaemin Lee , Jiyoung Kwon , Jongsun Lim & Changjin Lee (2010) An Amorphous
Polythiophene as a Binder Material for Organic Thin-Film Transistor Channel Applications, Molecular
Crystals and Liquid Crystals, 519:1, 179-186, DOI: 10.1080/15421401003613675
To link to this article: http://dx.doi.org/10.1080/15421401003613675
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Mol. Cryst. Liq. Cryst., Vol. 519: pp. 179–186, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421401003613675
An Amorphous Polythiophene as a Binder
Material for Organic Thin-Film Transistor
Channel Applications
JAEMIN LEE, JIYOUNG KWON,
JONGSUN LIM, AND CHANGJIN LEE
Advanced Materials Division, Korea Research Institute of Chemical
Technology, Yusong-gu, Daejo, Korea
Blending of crystalline low molecular-weight organic semiconductors, e.g., 6,13-bis
(triisopropylsilylethynyl)pentacene (TIPS-Pentacene), and binder polymers has
recently been known to be an effective way to achieve high performance organic
thin-film transistors (OTFTs). However, only limited number of examples has been
reported as binder materials for OTFT applications, e.g., poly(triarylamine),
poly(a-methylstyrene), and etc. In this work, an amorphous polythiophene deriva-
tive, poly(3-(2-ethylhexyl)thiophene) (P3EHT), was prepared by introducing
2-ethylhexyl group as a side-chain to the polythiophene. The P3EHT was synthe-
sized by Grignard metathesis (GRIM) polymerization to give a molecular weight
(Mn) of 4,100 (PDI ¼ 3.4), and it was readily soluble in common organic solvents,
such as chloroform, tetrahydrofuran, toluene, and etc. An amorphous nature of
the P3EHT was confirmed from differential scanning calorimetry (DSC) and
ultraviolet-visible spectroscopy results. A mobility of 2.5 ꢀ 10ꢁ4 cm2=Vs, an on=
off ratio of 0.57 ꢀ 103 and a threshold voltage of 1.6 V have been obtained with
OTFTs using a P3EHT=TIPS-Pentacene blend.
Keywords Binder; GRIM polymerization; organic semiconductor; OTFT;
thiophene
Introduction
Solution-processable transistors open a new possibility to future electronics, because
the manufacturing cost can be lowered and large-area fabrication becomes possible
with solution processing or printing. Among the various kinds of technologies
related with solution-processable transistors, organic thin-film transistors (OTFTs)
have attracted much research interest during the past decades [1]. At the initial stage
of soluble OTFT research, thiophene-based p-conjugated polymers have been widely
used as p-type semiconductors due to their high charge carrier mobility [2,3].
Recently, another approach utilizing crystalline low molecular-weight organic
semiconductors, such as TIPS-Pentacene and its derivatives, has also proved its
possibility [4]. Moreover, improved performance and uniformity of the device has
Address correspondence to Changjin Lee, Advanced Materials Division, Korea Research
Institute of Chemical Technology, P.O. Box 107, Sinseongno 19, Yuseong-gu, Daejeon
305-600, Korea. Tel.: 82-42-860-7208; Fax: 82-42-860-7200; E-mail: cjlee@krict.re.kr
179

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J. Lee et al.
been reported by blending such small molecules with binder polymers [5]. However,
only limited number of examples has been reported as binder materials for OTFT
applications, e.g., poly(triarylamine) (PTAA) [5,6], poly(a-methylstyrene) (PAMS)
[5–7], polystyrene (PS) [8], poly(4-vinylbiphenyl) (PVBP) [6], and etc.
In this work, we prepared and applied an amorphous polythiophene as a binder
polymer for a crystalline low molecular-weight organic semiconductor, e.g., TIPS-
Pentacene. The good crystallinity of the well-known P3HT comes from its planar
molecular structure along the main-chain and the side-chain. We therefore intro-
duced 2-ethylhexyl group to the 3-position of thiophene resulting in bulky and
non-planar structure, and synthesized poly(3-(2-ethylhexyl)thiophene) (P3EHT)
which is different in its side-chain compared with the most known polymer semicon-
ductor, P3HT. The physical, optical and electrochemical properties of the P3EHT
were revealed using differential scanning calorimetry (DSC), UV-Vis absorption
spectroscopy and cyclic voltammetry. OTFT device characteristics using a blend
of P3EHT and TIPS-Pentacene were also investigated.
Experiments
Instrumentation and Characterization
The NMR spectra were recorded at room temperature using a Bruker 300 NMR spec-
trometer. The molecular weight and the molecular weight distribution (relative to
polystyrene standards) of the polymers were obtained by gel permeation chromato-
graphy (GPC) using Shodex GPC columns with a chloroform eluent. DSC was
conducted using METTLER TOLEDO DSC1. Absorption spectra were measured
using a SHIMADZU UV-2550 UV-Visible Spectrophotometer. Cyclic voltammetric
measurements were performed on a BAS 100 voltammetric system with a three-
electrode cell in a solution of 0.10 M tetrabutylammonium hexafluorophosphate in
acetonitrile at a scanning rate of 50 mVsꢁ1. An Ag=Agþ electrode (0.01 M AgNO3
in acetonitrile) was used as a reference electrode, and platinum wires were used as a
counter electrode and a working electrode. Current-voltage characteristics of the
OTFT device were measured using Agilent 4155C semiconductor parameter analyzer.
Polymerization
To a solution of 2,5-dibromo-3-(2-ethylhexyl)thiophene 1 (3 g, 8.47 mmol) in anhydrous
tetrahydrofuran was added methylmagnesium bromide solution (1.0 M in THF,
7.62 mL, 7.62mmol) dropwisely while refluxing. Ni(dppp)Cl2 (0.046 g, 0.08mmol) was
added in one portion after 1 hour. The solution was further refluxed for two hours,
and then the reaction mixture was dropped into 900mL of methanol to precipitate the
polymer. Then the precipitated dark brown solid was filtered and subjected to sequential
Soxhlet extraction with methanol to remove low molecular weight fraction of the
material. Theyield ofP3EHTbythisprocedure was19.9%.1H-NMR(300 MHz, CDCl3)
d (ppm): 6.94 (s, 2H), 2.73 (br d, 2H), 1.69 (br s, 1H), 1.29 (br m, 8H), 0.88 (br t, 6H).
The same procedure was applied for P3HT synthesis.
OTFT Device Fabrication
Predefined substrates with gold source=drain electrodes were carefully cleaned by
methanol, isopropyl alcohol and deionized water, and then further cleaned by

An Amorphous Polythiophene Binder for OTFT
181
UV=ozone treatment. The organic semiconductor solution (1:1 blend of P3EHT:
TIPS-Pentacene in toluene) was spin-coated at 500 rpm for 30 seconds, and the film
was dried at 100ꢂC for 30 minutes. A diluted solution of CTL-809 M (from Asahi
Glass Co., LTD) was spin-coated onto the organic semiconductor layer at
900 rpm for 30 seconds to give a gate dielectric layer of CYTOP. The CYTOP gate
dielectric layer was dried in a vacuum oven at 90ꢂC for 1 hour. A gate electrode was
vacuum-deposited from gold to finalize the device fabrication.
Results and Discussion
For monomer synthesis, 2-ethylhexyl side-chain was introduced to the 3-position
of thiophene by Kumada coupling reaction between 3-bromothiophene and
2-ethylhexylmagnesium bromide. Following bromination with N-bromosuccinimide
(NBS), 2,5-dibromo-3-(2-ethylhexyl)thiophene was obtained. The polymerization was
carried out according to a GRIM polymerization method to yield P3EHT (Scheme 1)
[9]. P3HT was also synthesized by the same procedure for comparison. The
number-average molecular weight (Mn) of P3EHT and P3HT were 4,100 (PDI¼ 3.4)
and 4,600 (PDI ¼ 2.3), respectively. Both of the polymers were readily soluble in
common organic solvents, such as chloroform, tetrahydrofuran, toluene, and etc.
Figure 1 shows the DSC thermograms of the two polymers, P3EHT and P3HT.
DSC measurements were performed to examine the crystallinity of the polymers.
Due to its planar structure, P3HT is known to be crystalline, and DSC thermogram
also showed reversible melting (endothermal) and crystallization (exothermal) peaks
during heating and cooling scans. In contrast, no peak was observed for P3EHT,
thus implying the amorphous characteristic of P3EHT. The UV-Vis absorption spec-
troscopy also supports this. Figure 2 shows the absorption spectra of each of the two
polymers in solution state and in film state. P3EHT showed an absorption peak with
k
max at 433 nm in solution, and kmax at 443 nm in film (4kmax ¼ 10 nm). In the case of
P3HT, much larger shift of kmax was observed from solution to film (4kmax
¼
63 nm). P3HT showed an absorption peak with kmax at 443 nm in solution, and kmax
506 nm in film. The degree of red-shift of absorption maximum from solution to film
is known to be a criterion for estimating polymers’ chain ordering and alignment
[10]. The relatively small shift of kmax for P3EHT means that P3EHT has little tend-
ency to ordering and crystallization, while P3HT has a strong tendency to ordering
and crystallization. From these results including DSC and UV-Vis absorption spec-
troscopy, it is apparent that P3EHT which has branched alkyl side-chains is a kind
of amorphous polymers.
In addition to the characterization of the amorphous property of P3EHT, we
also carried out absorption measurements of the 1:1 ratio of polymer=TIPS-Pentacene
Scheme 1. Synthesis of P3EHT.

182
J. Lee et al.
Figure 1. DSC thermograms of P3EHT (a) and P3HT (b).
blend films (Figure 2c). Because the polymers will be utilized in the form of blend
film with TIPS-Pentacene for OTFT applications, investigation of those blend films
is important. The absorption spectrum of the TIPS-Pentacene film without any
binder is also shown in Figure 2c. All of the three spectra were normalized with
Figure 2. UV-Vis absorption spectra of P3EHT (a), P3HT (b), and polymer=TIPS-Pentacene
blend film (c).

An Amorphous Polythiophene Binder for OTFT
183
respect to the highest peak around 343 nm, which originates from the TIPS-
Pentacene. Other peaks at 450, 588, 650, and 705 nm also originate from
TIPS-Pentacene not P3EHT nor P3HT. The spectrum of the P3EHT-based blend
film coincides well with that of the TIPS-Pentacene film in the long wavelength
region (>550 nm), and this implies that the thin-film structure of the TIPS-Pentacene
is maintained inside the blend film based on amorphous P3EHT. However, the dif-
ference is clearly observed for the P3HT-based blend film. The broad absorption
around 500 nm mainly comes from P3HT film, but what is noteworthy is that the
peak intensity at 705 nm is relatively low compared with that of the TIPS-Pentacene
or P3EHT-based blend film. We therefore infer that the thin-film structure of the
TIPS-Pentacene has changed by blending with crystalline P3HT.
The electronic properties of the binder polymer as well as the amorphous char-
acteristics are also very important for OTFT applications, because charges pass
through the polymers during operation. Figure 3a shows the cyclic voltammogram
of P3EHT and P3HT during oxidative scan. The half-wave oxidation potential
(Eox,1=2) of P3EHT was 551 mV, while P3HT was 196 mV vs. Ag=Agþ. Although
not displayed in Figure 3a, the first half-wave oxidation potential (Eox,1=2) of
ferrocene was measured to be 87 mV in our experiment. So the HOMO (highest
occupied molecular orbital) energy level of P3EHT was estimated to be ꢁ5.26 eV
from its Eox,1=2 by regarding Eox,1=2 of ferrocene as ꢁ4.80 eV [11]. The HOMO
level of P3HT was also estimated to be ꢁ4.91 eV in the same manner. The optical
band-gap was estimated from the onset position of the absorption spectrum, and
P3EHT and P3HT showed 2.28 and 1.92 eV, respectively. The energy level diagram
of the two polymers is depicted in Figure 3b, and it was found that the LUMO
(Lowest Unoccupied Molecular Orbital) energy levels of the two polymers are
almost the same. This means that the increase of band-gap from P3HT to P3EHT
was mainly affected by the HOMO energy levels rather than the LUMO energy
levels. The relatively low HOMO energy level of P3EHT originates from reduced
p-conjugation of polythiophene main-chain due to the branched alkyl side-chain.
But the value is within a range of other hole-transporting materials [12], so P3EHT
can also be used as a hole-transporting polymer for p-type OTFT applications.
Figure 3. Cyclic voltammogram (a) and energy level diagram (b) of the polymers.

184
J. Lee et al.
To characterize the TFT performance of the polymer=TIPS-Pentacene blend,
top-gate OTFT test device was fabricated. A 1:1 ratio of blend between the polymer
and TIPS-Pentacene was used as a semiconducting channel, and CYTOP was used as
a gate dielectric. The channel width and length was 3000 mm and 50 mm, respectively.
But OTFT devices using P3HT-based blend did not operated at all, and only devices
using P3EHT-based blend operated well in a p-type mode. Figure 4 shows
the representative OTFT device characteristics of P3EHT=TIPS-Pentacene blend.
Saturation behavior at high drain voltage was clearly observed in the output curve
(Figure 4a). However, drain current offset is observed at low drain voltage region.
This phenomenon is known to be related to gate leakage, and it can be overcome
by optimization of device structure, such as patterning of the semiconductor film
[13]. For the transfer characteristics measurement, gate voltage (VG) was swept
from þ10 V to ꢁ60 V at constant drain voltage (VD) of ꢁ60 V (Figure 4b). In this
saturation regime, the extracted mobility of the P3EHT=TIPS-Pentacene blend
device was 2.5 ꢀ 10ꢁ4 cm2=Vs with on=off ratio of 0.57 ꢀ 103 and threshold voltage
(VT) of 1.6 V.
Figure 4. Output curve (a) and transfer curve (b) of P3EHT=TIPS-Pentacene OTFT device.

An Amorphous Polythiophene Binder for OTFT
185
Although our OTFT device did not show the higher performance than the
previously reported works [5–8], to our knowledge, this is the first example of a poly-
thiophene derivative used as a binder polymer for TIPS-Pentacene. As a preliminary
data, this result indicates that an amorphous polythiophene, P3EHT, can be success-
fully applied as a binder polymer for TIPS-Pentacene-based OTFT applications.
From the viewpoint of materials, we believe that control of the crystallinity or
amorphous characteristics of the conjugated polymer is a key to achieve high per-
formance OTFTs. Further device optimization such as varying composition ratio,
solvents, annealing conditions, device geometry, and etc. would be able to improve
the OTFT characteristics.
Conclusion
In this paper, an amorphous polythiophene containing branched alkyl side-chain
was prepared and applied as a binder polymer for TIPS-Pentacene-based OTFT
device. Poly(3-(2-ethylhexyl)thiophene) (P3EHT) was successfully synthesized via
GRIM polymerization, and its amorphous characteristic was analyzed by DSC
and UV-Vis spectroscopy. Due to the branched alkyl side-chain, P3EHT showed
the larger band gap and the lower HOMO energy level compared to P3HT. From
the OTFT results using 1:1 blend of P3EHT=TIPS-Pentacene, the extracted mobility
of the device was 2.5 Â 10À4 cm2=Vs with an on=off ratio of 0.57 Â 103 and VT of
1.6 V. Considering this preliminary results, P3EHT is a promising binder polymer for
p-type OTFT applications based on crystalline low molecular-weight organic semi-
conductors. Further characterization of the thin-film structure and optimization of
the device fabrication would be expected to improve the device performance of
P3EHT=TIPS-Pentacene blend.
Acknowledgment
This work was supported by a grant from the Fundamental R&D Program for Core
Technology of Materials (M200701004) and the Joint Research Program (B551179-
09-07-00) funded by the Ministry of Knowledge Economy, Republic of Korea.
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