Dibenzo[b,d]thiophene based oligomers with carbon-carbon unsaturated bonds for high performance field-effect transistors

Article abstract of DOI:10.1016/j.orgel.2009.12.011

Dibenzo[b,d]thiophene (DBT) based oligomers with carbon-carbon double and triple bonds were synthesized. Their thermal stability and energy levels were studied by thermal analyses, UV-vis absorption spectra and electrochemistry. Single crystals of 3,7-bis

Full text of DOI:10.1016/j.orgel.2009.12.011

Organic Electronics 11 (2010) 544–551
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Dibenzo[b,d]thiophene based oligomers with carbon–carbon
unsaturated bonds for high performance field-effect transistors
Chengliang Wang a,c, Zhongming Wei a,c, Qing Meng a,c, Huaping Zhao a, Wei Xu a,
a,
Hongxiang Li b, , Wenping Hu
*
*
a Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
b Laboratory of Material Science, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, PR China
c Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 October 2009
Received in revised form 18 December 2009
Accepted 18 December 2009
Available online 24 December 2009
Dibenzo[b,d]thiophene (DBT) based oligomers with carbon–carbon double and triple
bonds were synthesized. Their thermal stability and energy levels were studied by thermal
analyses, UV–vis absorption spectra and electrochemistry. Single crystals of 3,7-bis(phen-
ylethynyl)dibenzo[b,d]thiophene (BEDBT) revealed the introduction of unsaturated bonds
eliminated the steric repulsion between adjacent aromatic rings and BEDBT displayed pla-
nar structure in crystals. Thin film transistors of these compounds displayed typical p-type
behaviour. The best performance was obtained from 3,7-distyryldibenzo[b,d]thiophene
(DSDBT) on OTS modified substrates with mobility as high as 0.15 cm2/Vs and on/off cur-
rent ratio up to 108, one of the highest performance for DBT based oligomers.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Thin film transistors
Organic semiconductors
Oligomer
1. Introduction
would lead to large charge transfer integral and low reor-
ganization energy. However, these fused ring compounds
Organic field-effect transistors (OFETs) have attracted
great interest because of their merits (low cost, could be
fabricated on large scale and on flexible substrates) and po-
tential applications in flexible electronic devices (roll-up
displays, smart card, RFID and so on) [1–4]. The typical
structure of OFETs includes source, drain and gate elec-
trodes, an organic semiconductor layer, and a dielectrical
layer. Among them, organic semiconductor layer is the
key point to determine the performance of OFETs. Till
now, much progress has been made to the design and
synthesis of novel organic semiconductors for OFETs. Some
organic semiconductors such as pentacene and heteroac-
enes with mobility larger than 0.5 cm2/Vs have been
reported [5–11]. The reason for the high performance of
these fused ring organic semiconductors could be attrib-
uted to their large planar conjugated structures which
usually suffered tedious synthesis steps, complex purifica-
tions, low stabilities and solubilities. Studies have showed
that oligomers would have good solubility and are easy to
be modified to fit different applications, but oligomers usu-
ally exhibit low device performance because of their non-
planar structures caused by the steric repulsion between
adjacent aromatic rings [12–17]. We and some researchers
have reported, by introduction of carbon–carbon double
[18–24] and triple bonds [25–32], planar conjugated oligo-
mers which combined the merits of fused ring compounds
and single bonds linked oligomers can be obtained. For
instance, phenylvinylene oligomer [19] and its alkyl func-
tionalized derivates [20] showed a maximum mobility of
1.3 cm2/Vs; similar thiophenylvinylene oligomers [21]
exhibited mobility up to 0.4 cm2/Vs, seven times of its sin-
gle bonds linked components [13,14]. Similar results have
also been observed from the organic semiconductors with
carbon–carbon triple bonds. For example, single crystals of
9,10-bisphenylethynyleneanthracene showed mobility as
high as 0.73 cm2/Vs [26]; the performances of transistors
* Corresponding authors.
E-mail addresses: lhx@mail.sioc.ac.cn (H. Li), huwp@iccas.ac.cn
(W. Hu).
1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2009.12.011

C. Wang et al. / Organic Electronics 11 (2010) 544–551
545
Scheme 1. The chemical structures of DBT and some of its single bonds linked oligomers.
of thiophene-phenyl cooligomers with carbon–carbon tri-
ple bonds was much better than those of their single bond
components [25].
Dibenzo[b,d]thiophene (DBT, chemical structure see
Scheme 1) is a commercial available chalcogenophene
coumpound [33,34]. It has high ionization potential com-
pared with anthracene, fluorene and carbazole which has
conjugated planar structure and is commonly used as
blocking unit in semiconductors. Additionally, the S atoms
(relative to S atoms). Fig. 1B showed the intermolecular
interactions among neighbor molecules along a axis. SꢀꢀꢀH,
Sꢀꢀꢀ
p
and C–Hꢀꢀꢀ
p intermolecular interactions were found
in Head-to-Head pairs, and only C–Hꢀꢀꢀ
p intermolecular
interactions existed in Tail-to-Tail pairs.
The thermal stabilities of BEDBT and DSDBT were char-
acterized by thermogravimetric analysis (TGA). From their
TGA curves (see Supplementary data), we could see BEDBT
and DSDBT decomposed onset of 300 °C and 350 °C respec-
tively, suggesting their good thermal stabilities. Fig. 2A
showed the UV–vis absorption spectra of BEDBT and
DSDBT in CH2Cl2 solution at the concentration of 10ꢁ5 M.
The two compounds showed similar absorption peaks,
but the maximum absorption of DSDBT was about 30 nm
in DBT could introduce SꢀꢀꢀS and Sꢀꢀꢀ
p intermolecular inter-
actions which might facilitate charge transport. All of these
merits suggested that DBT should be an ideal conjugated
unit for organic semiconductors. In our previous work
[34], we have observed single bonds linked DBT oligomer
3,7-bis(50-hexylthiophene-20-yl)dibenzothiophene
(3,7-
red shift than that of BEDBT, indicating a larger p-conjuga-
DHTDBT) displayed good transistor performance. In order
to further investigate the application of DBT in organic
semiconductors and the effect of carbon–carbon double
and triple bonds to the performance of materials, herein
two new 3,7-disubstituted DBT derivates, 3,7-bis(phenyl-
ethynyl)dibenzo[b,d]thiophene (BEDBT) and 3,7-dist-
yryldibenzo[b,d]thiophene (DSDBT) were synthesized.
Their physicochemical properties and charge transport
characteristics were studied.
tion length of DSDBT. This phenomenon has also been
observed by others [25,37]. The optical energy gaps
estimated from UV–vis spectra were 3.35 and 3.1 eV for
BEDBT and DSDBT respectively. The UV–vis absorption of
vacuum deposited thin films of BEDBT and DSDBT were
also tested to show the molecular arrangement in solids.
They all showed very broad absorptions and the initial
absorptions were red shifted compared with those of the
solutions, indicating the ordered arrangements in solid
states. The cyclic voltammetry of BEDBT and DSDBT in
CH2Cl2 solutions at the concentration of 10ꢁ3 M were
illustrated in Fig. 2B and C (using ferrocene (Cp2Fe) as
internal standard substance). The HOMO energy levels
calculated from electrochemistry were ꢁ5.87 eV for BEDBT
and ꢁ5.57 eV for DSDBT (the HOMO energy level of
ferrocene was about ꢁ4.8 eV). The HOMO energy level of
BEDBT was even lower than 2,8-disubstituted DBT deriva-
2. Results and discussion
The synthesis routes of BEDBT and DSDBT were
outlined in Scheme 2. BEDBT were synthesized through
modified Pd catalyzed Sonogashira coupling reaction
[35]. DSDBT were synthesized through Heck reaction using
Pd(OAc)2 as catalyst [36]. BEDBT was easy to form crystals
and was purified through recrystallization from toluene.
DSDBT was obtained through column chromatography
and further purified by vacuum sublimation. Both com-
pounds were soluble in normal organic solvents such as
CH2Cl2, THF, toluene and chlorobenzene, and totally char-
acterized by 1H NMR, MS and elementary analysis.
Single crystals of BEDBT suitable for X-ray diffraction
(XRD) measurements were grown from toluene solution.
As we expected, the single crystals of BEDBT revealed that
it had nearly planar structure and belonged to a crystal sys-
tem of monoclinic space group P21 with unit-cell parame-
ters of a = 11.666(2), b = 7.6007(15), c = 22.162(4) Å and
b = 94.89(3)°. As showed in Fig. 1A, BEDBT adopted herring-
bone geometry in the crystals, and the herringbone angle
was about 51.7°. Interestingly, adjacent BEDBT molecules
along a axis formed Head-to-Head and Tail-to-Tail pairs
tives
2,8-di(5’-hexyl-thiophen-2’-yl)-dibenzothiophene
(2,8-DHTDBT, chemical structure see Scheme 1) reported
in our previous work [33]. The lower HOMO energy level
of BEDBT should be attributed to its short conjugated
length (which was determined by UV–vis spectrum), the
high ionic potential of DBT and the stronger electron-with-
drawing effect of ethynylene group than that of the vinyl
group. And this low HOMO energy level would cause carri-
ers accumulation difficulty and charge injection barrier,
which usually leads to low transistor performance. The en-
ergy gaps and HOMO/LUMO energy levels of BEDBT and
DSDBT were summarized in Fig. 2D. The low HOMO energy
levels suggested the good environmental stability of BED-
BT and DSDBT.
The charge transport properties of BEDBT and DSDBT
were examined to study their applications in OFETs. The

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C. Wang et al. / Organic Electronics 11 (2010) 544–551
Scheme 2. Synthesis routes of DSDBT and BEDBT.
Fig. 1. (A) Packing diagram of BEDBT, (B) intermolecular interactions between Head-to-Head and Tail-to-Tail pairs along a axis in BEDBT crystals.

C. Wang et al. / Organic Electronics 11 (2010) 544–551
547
Fig. 3. (A) An example transfer with hysteresis loop (VDS = ꢁ100 V) and
(B) output curves of devices based on DSDBT films deposited on OTS
modified SiO2/Si substrates at 50 °C.
length, respectively, VG and VT are the gate and threshold
voltage, respectively. All the devices showed typical p-type
channel FET performance under ambient conditions. Fig. 3
showed typical transfer and output curves of an example
device obtained at 50 °C on octadecyltrichlorosilane (OTS)
modified SiO2/Si substrates using DSDBT as a semiconduc-
tor layer. The FET performance of DSDBT and BEDBT on
different deposition temperature were summarized in
Table 1. From Table 1, we could see the devices of DSDBT
displayed mobility larger than 0.03 cm2/Vs. And when
the temperature of substrate increased (from 25 °C to
50 °C), the mobility of devices increased; further increasing
the substrate temperature, the performance of devices
decreased. The highest mobility of DSDBT could reach up
to 0.15 cm2/Vs, one of the highest mobility reported for
DBT based oligomers. While scanning VG from
0 to
ꢁ120 V and then retraced scanning from ꢁ120 to 0 V at
VDS = ꢁ100 V, hysteresis behaviour was observed
(Fig. 3A). This might be attributed to the instability of the
oxidized species which was proved by the irreversible
oxidations from the electrochemistry results. The perfor-
mance of BEDBT devices was much lower compared with
DSDBT, which was probably due to its lower HOMO energy
level. The highest performance of BEDBT devices was
obtained at Tsub = 25 °C.
In order to investigate the mobility-substrate tempera-
ture dependence, the morphologies and the structures of
DSDBT and BEDBT films deposited at different substrate
temperature were investigated by atomic force microscopy
(Fig. 4 for DSDBT and Fig. 5 for BEDBT) and X-ray diffrac-
tion (Fig. 6) respectively. From the AFM images, we could
see with the increase of substrate temperature, the grain
size of DSDBT film increased. The step height of crystalline
terrace layers at Tsub = 50 °C was about 2.2 nm (Fig. 4E, left)
Fig. 2. (A) UV–vis absorption spectra, (B and C) cyclic voltammograms
and (D) orbital levels of BEDBT and DSDBT.
transistors were fabricated in top-contact mode using gold
as the source and drain electrodes. The field-effect mobili-
ties (
l) were calculated in the saturation regime according
to the following equation: ID
ID is the drain current, Ci is the capacitance per unit area of
the gate dielectric, W and L are the channel width and
=
l
Ci(W/2L)(VG ꢁ VT)2, where

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C. Wang et al. / Organic Electronics 11 (2010) 544–551
Table 1
which was nearly equal to the simulated molecular length
(about 2.24 nm) by minimized energy optimization
(Fig. 4E, right), indicating that the DSDBT molecules grown
layer-by-layer and took nearly perpendicular orientation
on the substrate in the films. Further increasing the sub-
strate temperature to 100 °C, larger grain was obtained.
However, the continuity of the films became worse (see
Supplementary data), and this should take the responsibil-
ity for the lower device performance at Tsub = 100 °C. From
FET characteristics of DSDBT and BEDBT prepared by vacuum deposition on
OTS modified SiO2/Si substrates at different temperatures.
Entry
Tsub (°C)
lmax (cm2/Vs)
On/off ratio
DSDBT
25
50
100
25
0.039
0.15
4 ꢂ 106
2 ꢂ 108
3 ꢂ 107
7 ꢂ 105
9 ꢂ 104
0.06
BEDBT
1.3 ꢂ 10ꢁ4
50
8.0 ꢂ 10ꢁ5
Fig. 4. (A–C) AFM images (5 ꢂ 5
100 °C. (D) AFM images (2 ꢂ 2 m) of DSDBT thin films on OTS modified SiO2/Si substrate at the substrate temperature of 50 °C and (E) the step heights of
crystalline terrace layers as measured by a section analysis along the line marked in (D) (left) and optimized model molecular structure (right).
lm) of thin films of DSDBT on OTS modified SiO2/Si substrates at the substrate temperature of (A) 25 °C, (B) 50 °C, (C)
l

C. Wang et al. / Organic Electronics 11 (2010) 544–551
549
Fig. 5. (A and B) AFM images (5 ꢂ 5
images (1 ꢂ 1 m) of BEDBT thin films on OTS modified SiO2/Si substrates at the substrate temperature of 25 °C and (D) the step heights of crystalline
terrace layers as measured by a section analysis along the line marked in (C) (top) and molecular structure in crystal (bottom).
lm) of BEDBT thin films on OTS modified SiO2/Si substrates at the substrate temperature of (A) 25 °C, (B) 50 °C. (C) AFM
l
the XRD patterns of DSDBT films (Fig. 6B), the d-spacing
determined from their dominant peaks were 1.97 nm,
closed to the simulated molecular length (2.24 nm), which
also proved that the molecules were grown almost
vertically to the substrates when deposited. BEDBT films
exhibited similar results as DSDBT. When the substrate
temperature increased to 50 °C, larger grains were
obtained, but larger grain boundaries were also formed at
the same time. The step height of layers of thin films deter-
mined from AFM results was about 2.3 nm (very close to
the molecular length, Fig. 5D), suggesting BEDBT was
grown layer-by-layer and nearly vertically on the
substrate.
3. Conclusions
In summary, dibenzothiophene oligomers with carbon–
carbon double and triple bonds were synthesized. TGA,
UV–vis spectra and electrochemistry results indicated that
both compounds had high stability and high oxidation
resistance. Single crystal structure of BEDBT revealed that
the introduction of carbon–carbon unsaturated bonds
eliminated the steric repulsions between adjacent aro-
matic rings. Thin film transistors of these compounds
exhibited typical p-channel performance. DSDBT displayed
high FET performance with mobility up to 0.15 cm2/Vs, and
Fig. 6. XRD patterns of thin films of (A) DSDBT and (B) BEDBT on bare and
OTS modified SiO2/Si substrate at 25 °C.

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C. Wang et al. / Organic Electronics 11 (2010) 544–551
on/off ratio as high as 108. The performance of BEDBT was
much lower compared with DSDBT, which could be attrib-
uted to its smaller p-conjugated system and lower HOMO
energy level. Further optimization of the devices, field-ef-
fect transistors based on solution-processed films and sin-
gle crystals of these materials are underway.
phenylacetylene (1.86 g, 18 mmol) was added. The reac-
tion solution was stirred at 80 °C overnight. THF was re-
moved, and the aqueous layer was extracted by CH2Cl2.
The combined organic layer was evaporated under reduced
pressure. The residual solid was purified by recrystalliza-
tion from toluene to give BEDBT as white crystals (yield:
1.15 g, 60%). mp: 234 °C. MS (EI) m/z: 384 [M+]. 1H NMR
(400 MHz, CDCl3): 8.10 (d, 2H), 8.03 (s, 2H), 7.63 (d, 2H),
7.57 (m, 4H), 7.37 (m, 6H). Anal. calcd for C28H16S: C,
87.47; H, 4.19; found: C, 87.25; H, 4.29.
4. Experimental
4.1. General
4.3. Device fabrication and electrical characterization
All reactants and solvents were purchased from com-
mercial suppliers and used as received unless ultra noted.
The reactant 3,7-dibromodibenzo[b,d]thiophene was syn-
thesized using N-bromosuccinimide (NBS) as bromination
reagent according to literature [34,38–39]. MS (EI) was re-
corded on a Shimadzu GCMS-QP2010 Plus mass spectrom-
eter. 1H NMR spectra were obtained on a Varian 400 MHz
spectrometer in CDCl3 with tetramethylsilane as an inter-
nal reference. Elemental analyses were performed on a
Carlo Erba model 1160 elemental analyzer. UV–vis spectra
were recorded on a Hitachi U-3010 spectrometer. Cyclic
voltammograms were obtained on a CHI660C analyzer in
a conventional three-electrode cell using a glassy carbon
working electrode, a platinum wire counter electrode, an
Ag/AgCl reference electrode and Bu4NPF6 as supporting
electrolyte. X-ray diffraction (XRD) measurements for films
The SiO2/Si substrates used were heavily doped n-type
Si wafer with 500 nm-thick SiO2 layer and capacitance of
7.5 nF/cm2. After rinsed with conc. H2SO4/H2O2, water,
and iso-propanol, they were surface modified with OTS.
Before sublimation, the OTS modified substrates were
rinsed with hexane, chloroform and iso-propanol succes-
sively. The organic semiconductors were deposited at a
rate increasing gradually under
a pressure of about
10ꢁ4 Pa to a final thickness of 50 nm determined by a
quartz crystal monitor. The source and drain electrodes
were deposited with gold by using shadow masks with
W/L of ca. 48.2. Device characteristics were obtained with
a Keithley 4200 SCS and Micromanipulator 6150 probe sta-
tion in a clean and shielded box at room temperature in air.
were carried out on a D/max2500 instrument (Cu Ka radi-
ation, k = 1.54056 Å). XRD measurements for single crys-
Acknowledgements
tals were carried out in the reflection mode using a
Rigaku Saturn CCD area detector system (Mo Ka radiation,
k = 0.71073 Å). AFM images were obtained in air using a
Digital Instruments Nanoscope III in tapping mode.
This work was supported by National Natural Science
Foundation of China (60771031, 60736004, 20571079,
20721061 and 50725311), Ministry of Science and Tech-
nology of China (2006CB806200, 2006CB932100) and Chi-
nese Academy of Sciences.
4.2. Materials synthesis
Appendix A. Supplementary data
4.2.1. Synthesis of 3,7-distyryldibenzo[b,d]thiophene
(DSDBT)
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.orgel.2009.
12.011.
A 100 mL flask was charged with 3,7-dibromodibenzo-
[b,d]thiophene (1.71 g, 5 mmol), PPh3 (0.13 g, 0.5 mmol),
Pd(OAc)2 (0.112 g, 0.5 mmol) and dry Et3N (20 mL). After
the mixture was degassed three times and backfilled with
N2, styrene (5.2 g, 50 mmol) was added. The reaction solu-
tion was stirred at 100 °C for 3 days. The mixture was
evaporated under reduced pressure. The residual solid
was purified by chromatography on silica gel (petroleum
ether:dichloromethane = 1:1) to provide DSDBT as light
yellow powder (yield: 1.2 g, 62%). mp: >300 °C. MS (EI)
m/z: 388 [M+]. 1H NMR (400 MHz, CDCl3): 8.01 (d, 2H),
7.97 (s, 2H), 7.64 (d, 2H), 7.56 (d, 4H), 7.39 (t, 4H), 7.29
(t, 2H), 7.24 (d, 4H). Anal. calcd for C28H20S: C, 86.56; H,
5.19; found: C, 86.31; H, 5.28.
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