n-type organic field-effect transistors with very high electron mobility based on thiazole oligomers with trifluoromethylphenyl groups

Article abstract of DOI:10.1021/ja055686f

Novel thiazole oligomers and thiazole/thiophene co-oligomers with trifluoromethylphenyl groups were developed as n-type semiconductors for OFETs. They showed excellent n-type performances with high electron mobilities. A 5,5′-bithiazole with trifluorometh

Full text of DOI:10.1021/ja055686f

Published on Web 10/08/2005
n-Type Organic Field-Effect Transistors with Very High Electron Mobility
Based on Thiazole Oligomers with Trifluoromethylphenyl Groups
Shinji Ando,† Ryo Murakami,† Jun-ichi Nishida,† Hirokazu Tada,‡ Youji Inoue,§ Shizuo Tokito,§ and
Yoshiro Yamashita*,†
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering,
Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan, Institute for Molecular Science,
Myodaiji, Okazaki 444-8585, Japan, and NHK Science and Technical Research Laboratories,
Kinuta, Setagaya-ku, Tokyo 157-8510, Japan
Received August 25, 2005; E-mail: yoshiro@echem.titech.ac.jp
n-Type organic field-effect transistors (OFETs) are envisioned
as key components of organic p-n junctions, bipolar transistors,
and complementary integrated circuits leading to flexible, large-
area, and low-cost electronic applications.1 Compared to the p-type
semiconductors, such as pentacene, n-type semiconductors are still
not fully developed, and the performances are not satisfactory. Thus,
pentacene afforded a thin film FET with the highest hole mobility
of 3.0 cm2/Vs,2 whereas the highest electron mobility was reported
to be 0.62 cm2/Vs using a perylene diimide derivative.3 To obtain
the high electron mobility, the organic semiconductor layer should
be highly ordered with strong intermolecular interactions and should
also have a proper LUMO energy level near the work functions of
source/drain electrodes.4 Recently, high performance n-type organic
semiconductors have been obtained by introducing fluoro or
fluoroalkyl substituents into pentacene and oligothiophene deriva-
tives, which are known as hole-transporting systems.5 However,
Figure 1. X-ray structure of 1 and 2. (a) Stacking structure of 1 along the
c-axis. (b) Stacking structure of 2.
they still have disadvantages of insufficient intermolecular π-π
(-1.81 V), 1 (-1.63 V), 2 (-1.46 V), 3 (-1.35 V), 4 (-1.62 V),
5 (-1.44 V). The reduction peak of 6 was not observed. As
expected, the electron affinity increases with an increase in the
number of thiazole rings. This is ascribed to the electron-
withdrawing property of the thiazole ring. The electron affinity of
2,2′-bithiazole 2 is higher than that of 5,5′-bithiazole 1. The
reduction potential of 5 with 2,2′-bithiazole is also more positively
shifted than that of 4. This can be explained by the fact that the
LUMO of 2 has large coefficients on the electron-negative nitrogen
atoms, whereas the LUMO of 1 has almost no coefficient on the
nitrogens (see Supporting Information). The HOMO-LUMO gaps
obtained from the absorption onsets are 2.77 eV for bis(4-
overlap6 and inefficient electron injection.4 These disadvantages
were expected to be overcome by using electron-accepting hetero-
cycles as core π-electron systems and a trifluoromethylphenyl group
as an end substituent. Actually, we have recently succeeded in
fabricating an n-type OFET with a high electron mobility of 0.30
cm2/Vs based on a thiazolothiazole derivative with trifluorometh-
ylphenyl groups.7 In this context, we have now turned our attention
to thiazole oligomers8 and designed regioselectively linked thiazole
oligomers 1-3 and thiazole/thiophene co-oligomers 4 and 5 with
4-trifluoromethylphenyl groups for high performance n-type organic
semiconductors. We report herein their synthesis, characterization,
and structures along with those of thiophene oligomer 6. OFETs
trifluoromethylphenyl)bithiophene, 2.90 eV for 1, 2.83 eV for 2,
based on them have been constructed, and their very high electron
2.57 eV for 3, 2.44 eV for 4, 2.42 eV for 5, and 2.45 eV for 6.
The single crystals of 1 and 2 were obtained by slow sublimation.
To investigate the molecular structures and intermolecular interac-
tions in the solid state, their X-ray structure analyses were carried
out. Their crystal structures are shown in Figure 1. Both of the
molecules have a torsion angle of 0° between the thiazole rings.
Interestingly, the molecule of 1 has a completely planar geometry,
as shown in Figure 1a, where the torsion angle between the thiazole
and 4-trifluoromethylphenyl rings is also 0°. Moreover, the molecule
forms a unique two-dimensional columnar structure, where one
molecule bridges two others with an intermolecular short separation
of 3.37 Å between the stacked molecules. The molecule 2 also
affords a columnar structure, as shown in Figure 1b. However, two
kinds of crystallographically independent molecules exist in the
unit cell, and they have torsion angles of 1.6 and 10.4° between
mobilities are presented here.
5,5′-Bithiazole 1 was prepared by the homo-cross-coupling
reaction of 5-bromo-2-(4-trifluoromethylphenyl)thiazole with Fe-
(C5H7O2)3 in THF in 30% yield. 2,2′-Bithiazole 2 was obtained by
the Suzuki coupling reaction of 5-bromo-2-(5-bromothiazol-2-yl)-
thiazole and 4-trifluoromethylphenylboronic acid with Pd(PPh3)4
in refluxing toluene in 14% yield. Other derivatives 3-6 were
prepared using the Stille coupling reactions of the corresponding
dibromo precursors and stannyl reagents with Pd(PPh3)4 in refluxing
toluene in 6-18% yields. All of compounds 1-6 were purified by
sublimation, and the structures were determined by the spectral data
along with elemental analysis.
The reduction potentials of 1-6 measured by differential pulse
voltammetry are as follows: bis(4-trifluoromethylphenyl)bithiophene7
the thiazole and 4-trifluoromethylphenyl rings owing to the steric
interactions between the hydrogen atoms. This result suggests that
the 2-(4-trifluoromethylphenyl)thiazole unit is more effective for
† Tokyo Institute of Technology.
‡ Institute for Molecular Science.
§ NHK Science and Technical Research Laboratories.
9
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J. AM. CHEM. SOC. 2005, 127, 14996-14997
10.1021/ja055686f CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
device of 1 fabricated on the OTS-treated substrate. The highest
mobility of 1.83 cm2/Vs is 3 times as high as the highest value
reported so far.3
The four kinds of films of 1 deposited under various conditions
were investigated by X-ray diffraction in reflection mode (XRD)
and AFM study. Sharp reflections up to the second order were
observed in all films, indicating formation of lamellar ordering and
crystallinity on the substrate. The d-spacing obtained from the first
reflection peak (200) is ca. 9.68 Å. Since the molecular length of
1 obtained from the single-crystal X-ray analysis is 19.1 Å, the
molecule of 1 is considered to be perpendicular to the substrate.
On the other hand, the AFM study reveals that the growth of grains
depends significantly on the substrate temperature and surface
conditions. Although the grain size deposited on SiO2 at 25 and 50
°C was almost similar and small, the larger boundary gap between
the grains was observed in the film deposited at 50 °C. The lower
mobility observed at 50 °C can be attributed to the large boundary
distance, which is unfavorable for the hopping of carriers at the
grain boundaries. In contrast, the grain size deposited on HMDS
or OTS-treated substrates dramatically increased in the order HMDS
> OTS, where smooth layer-by-layer structures were also observed.
Therefore, the highest electron mobility of 1.83 cm2/Vs observed
at the OTS-treated film could be attributed to the film formation
with a smooth surface and large size of grains.
Figure 2. (a) Drain current (Id) versus drain voltage (Vd) characteristics as
a function of gate voltage (Vg) for 1 OFET on OTS-modified SiO2. (b) Id
1/2
and Id versus Vg plots for the same devices. The field-effect mobility
calculated in the saturation regime is 1.83 cm2/Vs.
Table 1. Field-Effect Transistor Characteristics Deposited with
Various Conditions
T
mobility
(cm2/Vs)
on/off
ratio
threshold
(V)
compound
surface
(
°
C)
1
SiO2
SiO2
HMDS
OTS
SiO2
SiO2
SiO2
SiO2
SiO2
25
50
25
25
25
25
25
25
25
0.21
0.045
0.52
5 × 105
2 × 104
5 × 105
1 × 104
2 × 104
1 × 104
1 × 104
8 × 103
67
55
59
78
1.83
2
3
4
5
6
not observed
0.0028
0.085
0.018
0.025
63
63
61
76
In summary, we have developed new thiazole oligomers and
thiazole/thiophene co-oligomers with trifluoromethylphenyl groups
having strong intermolecular interactions and electron-accepting
properties. Some FET devices based on them showed excellent
n-type performances with high electron mobilities. A 5,5′-bithiazole
with trifluoromethylphenyl groups has a two-dimensional columnar
structure leading to a high performance n-type FET. The electron
mobility was enhanced to 1.83 cm2/Vs on the OTS-treated substrate.
forming a closely packed columnar structure favorable for efficient
intermolecular π-π interactions.
Top contact OFETs were fabricated by vapor-deposition onto
SiO2 (200 nm), followed by Au deposition through shadow masks
with W/L of 1.0 mm/100 or 50 µm. Although 2 did not show FET
characteristics, 1 and 3-6 exhibited good n-type performances,
which were not observed in air. The FET characteristics are
summarized in Table 1. The electron mobility of 1 was found to
be 0.21 cm2/Vs and was 3 times as high as that of the trifluoro-
methylphenylbithiophene derivative under the same conditions.7
This is attributed to the π-stacking structure leading to stronger
intermolecular interactions.9 The two-dimensional structure of 1,
as shown in Figure 1, may play an important role in the high
mobility. The thiazole derivative 4 also showed good n-type
performance. The mobility of 0.085 cm2/Vs is about 3 times as
high as that of the quaterthiophene derivative 6, indicating that the
2-(4-trifluoromethylphenyl)thiazole unit is more effective for
electron transport than the 2-(4-trifluoromethylphenyl)thiophene
unit. All the threshold voltages of these oligomers were relatively
high, probably due to their low electron affinities. However, the
threshold voltages of thiazole derivatives 1 and 3-5 were ca. 10
V lower than those of the corresponding thiophene derivatives. This
is attributed to the electron-withdrawing property of the thiazole
ring as revealed by the DPV measurements. On the other hand, 2
did not show FET behavior, and the mobilities of 2,2′-bithiazole
derivatives 3 and 5 were lower than those of the corresponding
bithiophene derivatives 4 and 6, indicating that the 2,2′-bithiazole
unit is not suitable for affording high performance FETs. Further-
more, to improve the FET performance of 1, the devices were
fabricated by increasing the substrate temperature or treating SiO2
substrate with hexamethyldisilazan (HMDS) or octadecyltrichlo-
rosilane (OTS). Although the mobility decreased with an increase
of temperature, the treatment of the substrate brought about drastic
enhancement of the electron mobility. Figure 2 shows the drain
current (Id) versus drain voltage (Vd) characteristics for the FET
Acknowledgment. This work was supported by The 21st
Century COE program, a Grant-in-Aid for Scientific Research on
Priority Areas (No. 15073212), and Nanotechnology Support Project
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
Supporting Information Available: Experimental details, absorp-
tion and emission spectra, XRD, AFM, Id versus Vd characteristics for
compounds 1-6, and X-ray crystallographic data for 1 and 2 in CIF
format and LUMO of 1 and 2 calculated by the PM5 method. This
material is available free of charge via the Internet at http://pubs.acs.org.
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