Highly soluble [1]benzothieno[3,2-b]benzothiophene (BTBT) derivatives for high-performance, solution-processed organic field-effect transistors

Article abstract of DOI:10.1021/ja074841i

2,7-Dialkyl[1]benzothieno[3,2-b]benzothiophenes were tested as solution-processible molecular semiconductors. Thin films of the organic semiconductors deposited on Si/SiO₂ substrates by spin coating have well-ordered structures as confirmed by XRD analysis. Evaluations of the devices under ambient conditions showed typical p-channel FET responses with the field-effect mobility higher than 1.0 cm² V^-1 s^-1 and I_on/I_off of ～10⁷. Copyright

Full text of DOI:10.1021/ja074841i

Published on Web 11/29/2007
Highly Soluble [1]Benzothieno[3,2-b]benzothiophene (BTBT) Derivatives for
High-Performance, Solution-Processed Organic Field-Effect Transistors
Hideaki Ebata, Takafumi Izawa, Eigo Miyazaki, Kazuo Takimiya,* Masaaki Ikeda,§
†
†
†
,†,‡
§
§
Hirokazu Kuwabara, and Tatsuto Yui
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima UniVersity, Higashi-Hiroshima
39-8527, Japan, Institute for AdVanced Materials Research, Hiroshima UniVersity, Higashi-Hiroshima 739-8530,
Japan, and Functional Chemicals R&D Laboratories Nippon Kayaku Co., Ltd., 3-31-12 Shimo,
Kita-ku, Tokyo 115-8588, Japan
7
Received July 2, 2007; E-mail: ktakimi@hiroshima-u.ac.jp
π-Conjugated polymers with aromatic and/or heteroaromatic
backbones, represented by poly(3-hexylthiophene) (P3HT), have
been widely investigated as soluble organic semiconductors for
1
solution-processed organic field-effect transistors (OFETs). Re-
2
-1
cently, the very high field-effect mobility (µFET) of 0.7 cm V
-
1
s
was reported for solution-processed OFETs possessing the
thieno[3,2-b]thiophene-thiophene copolymer-based active semi-
2
conducting channel. For further improvement of device perfor-
mance, however, polymer materials may have disadvantages owing
to the statistical distribution of molecular size and structural defects
caused by mislinkage of monomers, which may act as carrier traps
in the semiconducting channel.
In this regard, molecular materials are advantageous in terms of
their well-defined structure, ease of purification, and controllable
3
properties. In fact, a recent impressive example of molecular-based
2
-1
Figure 1. Structure of 2,7-dialkyl[1]benzothieno[3,2-b][1]benzothiophenes
(inset) and XRD pattern of spin-coated thin film of C12-BTBT. Calculated
d-spacing is 37.5 Å.
solution-processed OFETs showing µFET higher than 1.0 cm V
-1
s
was reported using soluble pentacene derivatives, in which bulky
2
-(trialkylsilyl)ethynyl substituents at the peri position (the molec-
4
ular short-axis direction) act as solubilizing groups. Owing to the
presence of substituents that hinder intermolecular CH-π interaction
in the lateral direction, these pentacene derivatives take the two-
dimensional (2D) π-stack structure, not the herringbone-type
packing, a typical solid-state structure for molecular organic
semiconductors such as pentacene.
Table 1. FET Characteristics of Cn-BTBT Devices Fabricated on
Si/SiO2 Substrates
solubilitya
(g L-1)
d-spacing
(Å)
µ
b
V
th
b
FET
n
(cm2 V- s-1)
1
I
on/Ioffb
(V)
108
5
>60
70
70
80
90
24
13
8.6
5.0
2.3
22.8
24.5
27.1
29.0
31.6
33.3
35.9
37.5
40.2
41.8
0.16-0.43
0.36-0.45
0.52-0.84
0.46-1.80
0.23-0.61
0.28-0.86
0.73-1.76
0.44-1.71
1.20-2.75
0.19-0.72
-21
-25
-29
-17
-21
-23
-20
-20
-27
-18
8
6
7
8
9
1
10
10
10
10
7
Recently, we developed several high-performance, air-stable,
7
5
molecular semiconductors for vacuum-processed OFETs. In the
8
course of our studies, we found that [1]benzothieno[3,2-b][1]-
benzothiophene (BTBT) is a promising core structure for air-stable
organic semiconductors. As solution-processible organic semicon-
8
7
8
7
8
0
10
10
10
10
10
11
1
1
1
2
3
4
ductors based on the BTBT core structure, we focused on 2,7-dialkyl
6
derivatives (C
n
-BTBT, Figure 1 inset), in which two solubilizing
long alkyl groups are introduced in the molecular long-axis direction
of the core to facilitate lateral intermolecular interaction. We here
a
Concentration of saturated solution in chloroform at room temperature.
Typical device characteristics obtained from more than 10 devices.
7
b
n
report the syntheses and characterization of a series of C -BTBTs
and their application to an active semiconducting channel in
solution-processed OFETs.
freely soluble, while those with alkyl groups longer than the C12
group showed reduced solubilities.
H
25
Homogeneous thin films were deposited on a Si/SiO
2
substrate
n
A series of C -BTBTs were synthesized via two reaction steps,
by spin coating using a 0.4 wt % solution of C -BTBT in chloroform
n
namely, Friedel-Crafts acylation and Wolff-Kishner reduction,
6
at 3000-4000 rpm for 30 s. The spin-coated thin films have well-
ordered structure as indicated by X-ray diffraction (XRD) measure-
ments: as a representative, an XRD pattern of C12-BTBT thin film
is shown in Figure 1, where a series of peaks assignable to (00l)
reflections were clearly observed, and the interlayer distance (d-
spacing) calculated from these reflections is 37.5 Å. d-Spacings of
the other derivatives depended on the length of the alkyl groups:
with longer alkyl groups, larger d-spacings were obtained (Table
1), indicating that all the derivatives take similar molecular packing
structures.
using parent BTBT as starting material, or via the palladium-
catalyzed Sonogashira coupling of 2,7-diiodo-BTBT with 1-alkynes,
followed by catalytic hydrogenation (see Supporting Information).
n
Thus synthesized C -BTBTs are quite soluble in common organic
solvents; their solubilities in chloroform at room temperature are
5 9
listed in Table 1. Derivatives with C H11-C H19 groups are almost
†
Graduate School of Engineering, Hiroshima University.
Institute for Advanced Materials Research, Hiroshima University.
Nippon Kayaku Co., Ltd.
‡
§
15732
9
J. AM. CHEM. SOC. 2007, 129, 15732-15733
10.1021/ja074841i CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
core layers (Figure 4a). In the BTBT core layer, the molecules take
herringbone packing (Figure 4b) to facilitate 2D carrier transport
property. In addition, a network of intermolecular interactions
through short S-S contacts (3.54 and 3.63 Å) exists. These
n
structural aspects can be related to the high mobility of C -BTBT-
based FET devices, since the existence of 2D semiconducting layers
with strong intermolecular overlap is considered to be one of the
8
prerequisites to realizing high-performance OFET devices.
In summary, we have successfully developed a series of highly
soluble molecular semiconductors, 2,7-dialkyl[1]benzothieno[3,2-
b]benzothiophenes, and tested their utility as active layers for
solution-processed OFETs. The OFET devices showed typical
p-channel FET responses with field-effect mobility higher than 1.0
Figure 2. FET characteristics of C12-BTBT-based OFET: output charac-
teristics (left) and transfer characteristics at Vd ) -60 V (right).
2
-1 -1
7
cm V
s
and Ion/Ioff of ∼10 . These results indicate that small
molecules possessing an extended aromatic core and solubilizing
long aliphatic chains are promising candidates for solution-
processible organic semiconductors.
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, and an Industrial
Technology Research Grant Program in 2006 from the New Energy
and Industrial Technology Development Organization (NEDO),
Japan. We also thank Rigaku Corp. for the measurement of in-
plane XRD.
Figure 3. In-plane XRD (2θ ø/φ scan) of a spin-coated thin-film of C12-
BTBT on Si/SiO2 substrate (incident angle, 0.19°). All the peaks are
assignable with the structural data from the single-crystal X-ray analysis
of C12-BTBT.
Supporting Information Available: Experimental details for the
syntheses, characterization and device fabrication of C
n
-BTBTs,
Crystallographic information file (CIF) and in-plane XRD for C12
-
BTBT. This material is available free of charge via the Internet at http://
pubs.acs.org.
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