Organic field-effect transistors based on solution-processible dibenzotetrathiafulvalene derivatives

Article abstract of DOI:10.1246/cl.2009.200

Organic field-effect transistors based on alkyl-substituted dibenzotetrathiafulvalenes (DBTTF) are fabricated by solution process. The molecules with butyl or longer alkyl groups are standing perpendicular to the substrates in the thin films, and the tran

Full text of DOI:10.1246/cl.2009.200

200
Chemistry Letters Vol.38, No.3 (2009)
Organic Field-effect Transistors Based on Solution-processible
Dibenzotetrathiafulvalene Derivatives
Takamasa Yoshino,1 Koji Shibata,2 Hiroshi Wada,1 Yoshimasa Bando,2 Ken Ishikawa,2 Hideo Takezoe,2 and Takehiko Moriꢀ1
1Department of Chemistry and Materials Science, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552
2Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552
(Received November 28, 2008; CL-081122; E-mail: mori.t.ae@m.titech.ac.jp)
Organic field-effect transistors based on alkyl-substituted
dibenzotetrathiafulvalenes (DBTTF) are fabricated by solution
process. The molecules with butyl or longer alkyl groups are
standing perpendicular to the substrates in the thin films, and
the transistors exhibit comparable performance to the vacuum-
deposited DBTTF devices.
Scheme 1. Synthesis of DCn-DBTTF. i: 1) chlorale hydrate, hy-
droxylamine hydrochloride, HCl, Na2SO4, 2) H2SO4, ii: NaOH,
Solution-processible organic semiconductors have been at-
H2O2, iii: 3-methylbutylnitrite, 3-methylbutanol, CS2, iv: HBF4,
tracting considerable interest due to their potential applications
to organic field-effect transistors (OFETs). Although solution-
processed OFETs have generally offered lower performance
v: Et3N.
c
c
o
than the vacuum-deposited counterparts, recent investigations
have attained high mobilities exceeding 1.0 cm2 Vꢁ1 sꢁ1 by
using functionalized pentacenes and benzothiophene deriva-
tives.1,2 Considerably high mobilities reaching 0.1–0.6 cm2
a
q
b
p
c
c
o
a
p
q
V
ꢁ1 sꢁ1 have been realized in poly(3-hexylthiophene) (P3HT)
b
derivatives as well.3 Tetrathiafulvalene (TTF) derivatives also
afford high-performance OFETs exceeding 1.0 cm2 Vꢁ1 sꢁ1 in
solution-processed crystals4 and in the range of 0.1–0.4 cm2
Figure 1. Crystal structure of DC1-DBTTF.
V
ꢁ1 sꢁ1 by vacuum-deposited thin films.5 However, solution-
processed thin-film OFETs of TTF derivatives still usually
forms with respect to the alkyl groups, whereas only the cis form
is found. The molecular arrangement is a kind of modified her-
ringbone packing, where the molecules are shifted along the
molecular long axis from the basic herringbone pattern.11
Compounds with n ꢂ 2 can be vacuum deposited on
hexamethylenedisilazane (HMDS)-treated Si/SiO2 substrates,
whereas the n ꢃ 2 compounds are spin coated from the chloro-
form solutions. The X-ray diffraction (XRD) measurements of
the thin films show sharp peaks (Figure 2a), and the evaluated
d spacings are listed in Table 1. When n ꢂ 3, the observed d
spacings are considerably shorter than the expected molecular
lengths l, suggesting tilted or modified layer structures as found
in the DC1-DBTTF crystal. For n ꢃ 4, however, the observed d
spacings are nearly equal to the molecular lengths, indicating
that the molecules are standing perpendicular to the substrates.
remain in the order of 10ꢁ2 cm2 Vꢁ1 sꢁ1
.
Among them,
6
dibenzo-TTF (DBTTF) is a representative material,5b,5d but the
solubility is very low. Here we report alkyl-substituted DBTTFs
(DCn-DBTTF, Chart 1) with improved solubility. The OFETs
fabricated by spin coating attain 0.11 cm2 Vꢁ1 sꢁ1, that is
almost comparable to 0.19 cm2 Vꢁ1 sꢁ1 of the vacuum-deposited
DBTTF.5d
A series of DCn-DBTTFs were synthesized similarly to
DBTTF from the corresponding anthranilic acid (3 in
Scheme 1)7–9 and purified by recrystallization and/or sublima-
tion. The oxidation potentials, 0.55 and 0.97 V for DC8-
DBTTF,9 were slightly smaller than 0.59 and 1.01 V of DBTTF,
reflecting the enhanced donor ability owing to the electron-
donating alkyl substitution.
The molecular and crystal structures of DC1-DBTTF
were investigated by X-ray single-crystal structure analysis
(Figure 1).10 The preparation gives potentially cis and trans
S
S
S
S
R = C2H5
C3H7
DC2-DBTTF
DC3-DBTTF
DC4-DBTTF
DBTTF
C4H9
R
S
S
C6H13 DC6-DBTTF
C8H17 DC8-DBTTF
C12H25 DC12-DBTTF
S
S
R
Figure 2. (a) X-ray diffraction diagram and (b) AFM image of
DC8-DBTTF thin film, spin coated on an HMDS-treated SiO2
substrate, after 80 ꢄC annealing.
DCn-DBTTF
Chart 1.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.3 (2009)
201
Table 1. FET characteristics of DCn-DBTTF transistors, with
d spacings and the expected molecular lengths (l)
This work was partly supported by Grant-in-Aid for Scien-
tific Research (B) (No. 18350095) and on Priority Areas of
Molecular Conductors (No. 15073211) from MEXT.
Mobility
/cm2 Vꢁ1 sꢁ1
Compoundsa
On/off ratio
˚
d/A
˚
l/A
References and Notes
DBTTF
DC1-DBTTF
DC2-DBTTF
v
v
v
s
s
s
s
s
s
s
0.17
6:4 ꢅ 10ꢁ5
0.079
0.014
0.013
0.054
0.054
0.081
0.11
0.0015
9:2 ꢅ 104 13.5
13.5
13.7
1
2
3
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64 12.9
7:4 ꢅ 102 14.2
1:2 ꢅ 102 14.6
17.2
DC3-DBTTF
DC4-DBTTF
DC6-DBTTF
DC8-DBTTF
Tsub 80 ꢄC
6:2 ꢅ 103 13.5, 17.2 19.6
1:4 ꢅ 102 22.3
7:4 ꢅ 102 26.7
1:0 ꢅ 103 31.0
3:4 ꢅ 104 31.4
22.4
27.2
32.2
DC12-DBTTF
48
—
as: spin coat, v: vacuum deposition.
4
5
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Figure 3. (a) Output and (b) transfer characteristics of a DC8-
DBTTF transistor after annealing.
The n ¼ 3 compound is the boundary, where two independent
peaks appear in the thin-film XRD (Table 1). Sharp XRD peaks
are obvious in the large n compounds, and at the same time the
domain structure, observed by AFM, becomes large (Figure 2b).
Significant improvement of the film morphology is observed by
increasing the alkyl chain length. Accordingly, the alkyl chains
with n ꢃ 4 improve not only the solubility but also molecular
packing. Solubility of the n ¼ 8 compound is as large as
20 mg in 1 mL of chloroform, but the n ¼ 12 compound exhibits
reduced solubility (2 mg/1 mL) and relatively poor thin-film
quality.
Gold source and drain electrodes were vapor-deposited on
the top of the thin films through a shadow mask, and the OFET
characteristics were measured in air (Table 1). All compounds
exhibit p-channel properties, and the field-effect mobility in-
creases with increasing n, but the n ¼ 12 compound shows
reduced performance on account of the reduced solubility. The
best results are obtained for n ¼ 8. When the device is annealed
at 80 ꢄC for 1 h, the film quality is further improved (Figure 2),
and the mobility increases to 0.11 cm2 Vꢁ1 sꢁ1 (Figure 3), which
is comparable to 0.19 cm2 Vꢁ1 sꢁ1 of the vacuum-deposited
DBTTF.5d As-grown films of these compounds tend to give rel-
atively large threshold voltages, but it is improved by annealing.
In conclusion, alkyl-substituted DBTTF shows improved
solubility and excellent quality of the spin-coated thin films,
and the solution-processed OFETs realize comparable perform-
ance to the vacuum-deposited DBTTF OFETs.
6
7
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8
9
Supporting Information is available electrically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
10 Crystal data of DC1-DBTTF: C16H12S4, monoclinic P21,
ꢄ
˚
a ¼ 16:467ð6Þ, b ¼ 7:533ð4Þ, c ¼ 6:080ð2Þ A, ꢀ ¼ 98:81ð3Þ ,
3
˚
V ¼ 745:3ð5Þ A , Z ¼ 2, and R1 ¼ 0:045. Crystallographic data
reported in this manuscript have been deposited with Cambridge
Crystallographic Data Centre as supplementary publication
no. CCDC-714681.
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Published on the web (Advance View) January 31, 2009; doi:10.1246/cl.2009.200