Organic semiconductors based on annelated β-oligothiophenes and its application for organic field-effect transistors

Article abstract of DOI:10.1039/c1jm14383b

Organic field-effect transistors based on two derivatives of annelated β-oligothiophenes are fabricated. High mobility of 2.2 cm² V^-1 s^-1 is obtained for 2,5-distyryl-dithieno[2,3-b: 3′,2′-d]thiophene (DEP-DTT) while 2,5-d

Full text of DOI:10.1039/c1jm14383b

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Organic semiconductors based on annelated b-oligothiophenes and its
application for organic field-effect transistors†
a
Jianwu Shi, Yabo Li, Ming Jia, Li Xu and Hua Wang
a
a
b
a
*
*
Received 5th September 2011, Accepted 30th September 2011
DOI: 10.1039/c1jm14383b
b-oligothiophenes derivatives make the molecules adopt
a compressed herringbone arrangement and provide another charge
transport pathway.5 In our previous work,6 we found that there exists
several kinds of intermolecular interactions, such as C–S, S–S, p–p
and S–p interactions, in the helicenes based on annelated b-oligo-
thiophenes.7 In the presence of n-BuLi, the ring opening occurs at the
position of the middle S atom in derivatives of dithieno[2,3-b:30,20-d]
thiophene (DTT),8a which implies the middle S atom is more elec-
tropositive than the other two S atoms. It is likely to generate the
intermolecular interaction between the electropositive S atom and the
other atoms on the neighboring DTT unit. In this paper, we
synthesized two annelated b-oligothiophenes: 2,5-diphenyl-dithieno
[2,3-b:30,20-d]thiophene (DP-DTT) and 2,5-distyryl-dithieno
[2,3-b:30,20-d]thiophene (DEP-DTT). A high mobility of 2.2 cm2
Vꢀ1 sꢀ1 was obtained for DEP-DTT, which was attributed to the
formation of S–p intermolecular interactions.
Organic field-effect transistors based on two derivatives of anne-
lated b-oligothiophenes are fabricated. High mobility of 2.2 cm2 Vꢀ1
sꢀ1 is obtained for 2,5-distyryl-dithieno[2,3-b:30,20-d]thiophene
(DEP-DTT) while 2,5-diphenyl-dithieno[2,3-b:30,20-d]thiophene
(DP-DTT) presents no field-effect characteristics. The existence of
S–p intermolecular interaction in DEP-DTT, which is introduced
by C]C double bonds, plays an important role in the molecular
arrangement both in single crystal and thin film structures, and the
charge transport of organic field-effect transistors.
Organic electronics have attracted particular attention due to their
potential advantages of flexibility, large-area process, and light-
weight.1 As an important kind of organic electronics, organic field-
effect transistors (OFETs) have earned significant progress in the past
few decades.2 Some OFETs based on small organic molecules exhibit
better performance than their inorganic counterparts (conventional
a-H: silicon-based transistors).3,4 Usually, these small organic mole-
cules with high field-effect mobilities were synthesized by enhancing
The molecular structures of DP-DTT and DEP-DTT are shown in
Fig. 1(a) and (b). DP-DTT was synthesized according to our previous
work,8 starting from DTT. The compound of DEP-DTT was
the degree of extended p-conjugation, which is expected to provide
more efficient p orbital overlap and lead to high field-effect mobility.
Normally, organic semiconductors are coalesced with weak inter-
molecular interactions, such as Van der Waals, p–p, and C–H
interactions. On the contrary, it is known that the inorganic semi-
conductors are combined with strong covalent bonds and possess
high mobility even in amorphous structure. For organic semi-
conductors, strengthening intermolecular interactions may be
a possible way to improve the mobility.1b However, detailed studies in
this domain are rarely investigated. It is still a significant challenge to
understand the relationship between the intermolecular interaction
and the device performance.
prepared
from
dithieno[2,3-b:30,20-d]thiophene-2,5-dicarbox-
aldehyde8 through Wittig reaction with benzyl chloride. All
compounds were purified three times by thermal gradient sublima-
tion prior to processing. All these materials have thermal stability
with a decomposition temperature of 307 ꢁC for DP-DTT and
330 ꢁC for DEP-DTT (supporting information†), respectively.
OFETs based on these two compounds were fabricated with top
contact geometry. It is surprising that a high field-effect mobility of
Recently, a high field-effect mobility of 2.0 cm2 Vꢀ1 sꢀ1 was
obtained in annelated b-oligothiophenes derivatives.5a It is very
interesting that the multiple S–S close contacts in annelated
aKey Lab for Special Functional Materials of Ministry of Education,
Henan University, Kaifeng, 475004, China. E-mail: jwshi@henu.edu.cn;
hwang@henu.edu.cn
bCollege of Chemistry and Chemical Engineering, Henan University,
Kaifeng, 475004, China
† Electronic supplementary information (ESI) available: Supplementary
information. CCDC reference numbers 836920 and 836921. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1jm14383b
Fig. 1 The molecular structures of DP-DTT (a) and DEP-DTT (b).
Output (c) and transfer (d) curves of OFETs based on DEP-DTT at
substrate temperature of 70 ꢁC.
17612 | J. Mater. Chem., 2011, 21, 17612–17614
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2.2 cm2 Vꢀ1 sꢀ1 is obtained in the devices with DEP-DTT as the active
layer, while the devices based on DP-DTT present no field-effect
characteristics. The output and transfer curves of OFETs based on
DEP-DTT with a substrate temperature (TS) of 70 ꢁC are shown in
Fig. 1(c) and (d). The output and transfer curves possess typical
p-type characteristics. Unfortunately, the devices present a high
threshold voltage (VT) and a low contact resistance in spite of the
HOMO value at 4.80 eV. Table 1 lists the characteristics of OFETs
with DEP-DTT as the active layer at different TS. There is no
dramatic change of device mobility after three months in the natural
environment.
In order to further understand the tremendous difference between
the properties of OFETs based on DP-DTT and DEP-DTT, the
topographical images of these compounds films were investigated by
atomic force microscope (AFM). Fig. 2 shows the topographical
images of DP-DTT (Fig. 2a–b) and DEP-DTT (Fig. 2c–e) films at
different TS. As shown in Fig. 2(a) and (b), the films of DP-DTT are
homogeneous and amorphous. For DEP-DTT, the topographical
images present a lamellar structure even when the TS is room
temperature (RT). For these three images of DEP-DTT films at
different TS, the average layer spacing is around 2.0 nm. On the other
hand, a series of progressional peaks (00l) are observed in the X-ray
diffraction patterns of DEP-DTT as shown in Fig. 2(f). This indicates
the films of DEP-DTT are highly crystalline. The d-spacing calcu-
lated according to Bragg’s equation is about 2.0 nm, which is
consistent with that obtained from AFM. But for DP-DTT, the
X-ray diffraction patterns indicate that the films of DP-DTT are
amorphous.
Fig. 2 Topographical images and X-ray diffraction patterns at different
substrate temperature: (a) RT of DP-DTT; (b) 70 ꢁC of DP-DTT; (c) RT
of DEP-DTT; (d) 70 ꢁC of DEP-DTT; (e) 100 ꢁC of DEP-DTT; (f) X-ray
diffraction patterns. The areas of the topographical images are all 4 mm ꢂ
4 mm.
As we know, the difference between the molecular structures of
DP-DTT and DEP-DTT is the C]C double bond. So it becomes
necessary to further investigate the effect of introducing C]C
double bonds into the molecular structures. Fig. 3 shows the single
crystal structures and the intermolecular interactions of DP-DTT
and DEP-DTT. For DP-DTT, the molecular long axis is the c axis
ꢀ
with a length of 12.7 A. The terminal phenyl groups contort from
the plane of the DTT unit, as shown in Fig. 3(a). The twisted angles
are about 14.2ꢁ and 18.8ꢁ, respectively. Due to these large twisted
angles, it may be implied that the extended p orbit is partially
formed between the phenyl groups and the DTT unit. As shown in
Fig. 3(b), the distance between the middle S atoms of two neigh-
ꢀ
boring DTT units is 3.560 A, which is smaller than the sum of the S
ꢀ
atoms’ Van der Waals radius (3.70 A). This short contact is nor-
mally considered as an intermolecular interaction. In addition, the
DP-DTT molecules adopt a herringbone arrangement with a p–p
ꢁ
ꢀ
spacing of 3.578 A. The herringbone angle is about 79.3 . However,
the S–S intermolecular interaction is not observed among the
molecules at the neighboring rows marked with the dot-dashed line
in Fig. 3(c). In DEP-DTT, the terminal phenyl groups almost lie on
Table 1 The characteristics of OFETs based on DEP-DTT at different
substrate temperature (TS)
a
Compound
DEP-DTT
TS (ꢁC)
m (cm2 Vꢀ1 sꢀ1
)
VT (V)a
Ion/Ioff
Fig. 3 Single crystal structures and intermolecular interactions in the
single crystals of DP-DTT (left column) and DEP-DTT (right column):
(a) and (d) side-view; (b) and (e) intermolecular interaction; (c) and (f)
crystal packing. The dashed line presents the close contact between
atoms. The dot-dashed line indicates the molecular situation in one row.
RT
70
100
1.2–2.0
1.5–2.2
0.6–1.5
ꢀ65 to ꢀ68
ꢀ58 to ꢀ68
ꢀ64 to ꢀ69
105
105
105
a
All data were extracted from more than ten devices.
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the plane of the DTT unit with smaller dihedral angles of 2.9ꢁ and
4.3ꢁ. As a consequence, the DEP-DTT molecule presents a bowed
shape, as shown in Fig. 3(d). Furthermore, it is unexpected that the
S–S intermolecular interaction disappears with the emergence of
multiple S–C intermolecular interactions in DEP-DTT, as shown in
Fig. 3(e). The distances between C atoms and the middle S atom of
no field-effect characteristics. The introduction of a C]C
double bond causes the formation of S–p intermolecular inter-
action in DEP-DTT, which plays an important role in the
molecular arrangement in single crystal and film structures, and
has a significant contribution to charge transport in OFETs. It
will help us to design and synthesize new organic semicon-
ductors with high field-effect mobility by utilizing the various inter-
molecular interactions.
ꢀ
the neighboring DTT unit are 3.422 A, 3.378 A, 3.481 A, and 3.417
ꢀ
ꢀ
ꢀ
A, respectively. As discussed above, the middle S atom of the DTT
unit is more electropositive, which causes the formation of these
multiple S–C intermolecular interaction. Due to the coplanar
structure of the styryl groups and the DTT unit, the extended p
orbit is formed and these multiple S–C intermolecular interactions
convert into S–p intermolecular interactions. At the same time,
DEP-DTT molecules also adopt a herringbone arrangement with
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (50803015, 20672028, and 20972041), Program
for Innovation Scientists and Technicians Troop Construction
Projects of Henan Province (104100510011).
ꢁ
ꢀ
a p–p spacing of 2.575 A. The herringbone angle is about 52.0 ,
which is smaller than that of sexithiophene (66ꢁ) and nearly equal to
that of pentacene (53ꢁ).9 Compared with that of DP-DTT, it can be
concluded that DEP-DTT molecules adopt a condensed packing. It
is evident that the introduction of a C]C double bond into DEP-
DTT causes the formation of S–p intermolecular interaction and
results in a condensed molecular packing. Furthermore, the
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is consistent with the layer spacing (2.0 nm) obtained from AFM
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Fig. 4 The effect of the S–p intermolecular interaction on charge
transport. The line with an arrow presents the charge transport under
field-effect. The dashed line with an arrow illustrates the charge transport
assisted by the S–p intermolecular interaction.
17614 | J. Mater. Chem., 2011, 21, 17612–17614
This journal is ª The Royal Society of Chemistry 2011