High-performance organic semiconductors for thin-film transistors based on 2,7-divinyl[1]benzothieno[3,2-b]benzothiophene

Article abstract of DOI:10.1039/b808438f

We have synthesized two novel organic semiconductors, which have a symmetrically substituted 2,7-divinyl[1]benzothieno[3,2-b]benzothiophene backbone. They show good electrical performances on a SiO₂/Si substrate, with high field-effect mobiliti

Full text of DOI:10.1039/b808438f

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www.rsc.org/materials | Journal of Materials Chemistry
High-performance organic semiconductors for thin-film transistors based
on 2,7-divinyl[1]benzothieno[3,2-b]benzothiophene†
Myoung-Chul Um,a Jeonghun Kwak,b Jung-Pyo Hong,a Jihoon Kang,a Do Yeung Yoon,a Seong Hoon Lee,a
b
Changhee Lee and Jong-In Hong
a
*
*
Received 19th May 2008, Accepted 10th July 2008
First published as an Advance Article on the web 27th August 2008
DOI: 10.1039/b808438f
We have synthesized two novel organic semiconductors, which have a symmetrically substituted
2,7-divinyl[1]benzothieno[3,2-b]benzothiophene backbone. They show good electrical performances on
a SiO2/Si substrate, with high field-effect mobilities of up to 0.4 cm2 Vꢀ1 sꢀ1, and can easily be
synthesized in large quantities. In addition, there is no significant change in the mobility of DPV-BTBT
even after the device is exposed to air for at least 60 days (further monitoring is in progress), showing
that it is a promising air-stable p-channel organic semiconductor that can be applied to all-organic
flexible electronic devices.
coplanar molecules with an extended p-conjugated length, which
Introduction
should help maximize the organization of the molecules in thin
films. Devices derived from vinylene-substituted compounds
such as 2,6-bis[2-(phenyl)vinyl]anthracene (DPVAnt) not only
show very high field-effect mobilities (up to 1.3 cm2 Vꢀ1 sꢀ1) and
on/off ratios (up to 107) but are also found to be exceptionally
long-lived and stable even when continuously operated under
atmospheric conditions.12 In addition, OTFT devices derived
from 2,6-bis[2-(4-pentylphenyl)vinyl]anthracene (DPPVAnt)
also showed high mobilities (0.1–1.28 cm2 Vꢀ1 sꢀ1) and on/off
ratios (106–107).13 However, there are still very few organic
semiconductors endowed with both high durability and
high OTFT performance. Hence, there is a demand for new
semiconductor materials that have both high mobilities and high
durabilities.
Conjugated organic materials are of tremendous interest
because of their potential use as organic semiconductors for
various optoelectronic devices, some of which include organic
thin-film transistors (OTFTs),1–3 organic light-emitting diodes
(OLEDs),4 photovoltaic cells,5 sensors,6 and radio-frequency
identification (RF-ID) tags.7 The design and synthesis of new
conjugated organic semiconductors have attracted significant
research interest in the past decade. In particular, p-extended
heteroarenes containing chalcogenophenes (i.e., thiophene and/
or selenophene) in fused aromatic ring systems have been
actively investigated because of their structural resemblance to
oligoacenes.8 Currently, the mobilities of the charge carriers in
organic semiconductors have outperformed the mobility of
amorphous silicon (0.5 cm2 Vꢀ1 sꢀ1), i.e., 2,7-diphenyl[1]benzo-
With the above consideration in mind, we focused our
attention on p-conjugated heteroarene cores with vinyl groups
(DCV-BTBT and DPV-BTBT, Fig. 1) as new promising organic
semiconductors.
thieno[3,2-b]benzothiophene (DPh-BTBT): ꢁ2.0 cm2 Vꢀ1 sꢀ1
;
2,7-diphenyl[1]benzoselenopheno[3,2-b]benzobenzoselenophene
(DPh-BSBS):8a ꢁ0.3 cm2 Vꢀ1 sꢀ1; dinaphtho[2,3-b:20,30-f]chalco-
genopheno[3,2-b]chalcogenophene (DNTT and DNSS):
ꢁ2.9 and ꢁ1.0 cm2 Vꢀ1 sꢀ1.9 These compounds were recently
reported to be high-performance OTFT materials with reason-
able stabilities when operated under ambient conditions. In
addition, very recently, several research groups have developed
semiconductors with high stabilities and conductivities by
employing conjugated vinylene-based oligomers.10 In fact, the
presence of a double bond of a defined configuration has been
found to result in a reduction in the overall aromatic character of
the planar structures, and hence an increase in the p-electron
localization.11 The introduction of a vinylene unit into p-conju-
gated oligomeric structures is a well-known method for forming
Experimental
General
The 1H and 13C NMR spectra were recorded in CDCl3 and
DMSO-d6 using an Advance 300 MHz Bruker spectrometer.
The 1H NMR and 13C NMR chemical shifts in CDCl3 were
reported relative to those in CHCl3 (7.27 ppm and 77.23 ppm,
respectively).
aDepartment of Chemistry, College of Natural Sciences, Seoul National
University, Seoul, 151-747, Korea. E-mail: jihong@snu.ac.kr; Fax:
+82-2-889-1568; Tel: +82-2-880-6682
bSchool of Electrical Engineering and Computer Science, Seoul National
University Kwanak, P.O. Box 34, Seoul, 151-600, Korea. E-mail:
chlee7@snu.ac.kr; Fax: +82-2-872-6451; Tel: +82-2-880-955
† Electronic supplementary information (ESI) available: CV, UV, PL,
AFM, XRD and more detailed device data. See DOI: 10.1039/b808438f
Fig. 1 Structures of DC(P)V-BTBT.
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Physical measurements
8.1 Hz), 7.48 (d, 2H, J ¼ 8.2 Hz), 5.38 (t, 2H, J ¼ 5.6 Hz), 4.69
(d, 4H, J ¼ 5.3 Hz).
The TGA analyses were performed on a TGA Q50 TA instru-
ment at 10 ꢂC minꢀ1 under a nitrogen atmosphere. The DSC
analyses were performed on a DSC2910 TA instrument at 10 ꢂC
minꢀ1 under nitrogen flow. The UV-vis absorption spectra were
recorded on a Beckman Coulter DU 800 spectrophotometer
using quartz cells with path lengths of 2.5 cm. For solid-state
measurements, the oligomers were thermally evaporated in
2,7-Dibromomethyl[1]benzothieno[3,2-b]benzothiophene. Phos-
phorus tribromide (3.24 g, 11.9 mmol) was added dropwise
to a suspension of 2,7-dihydroxymethyl[1]benzothieno[3,2-b]
benzothiophene (0.9 g, 2.99 mmol) in DMF (20 mL) at 0 ꢂC.
Upon the formation of a yellow precipitate, the mixture was
warmed to room temperature and stirred for 4 h. The solids were
collected by filtration and were washed with water and hexane to
afford 2,7-dibromomethyl[1]benzothieno[3,2-b]benzothiophene
as a yellow solid (1.1 g, 78%). The product was further purified by
˚
a vacuum chamber on quartz plates to form 300 A-thick films at
a deposition rate of 0.5 A sꢀ1. The XRD analyses were carried out
˚
at room temperature with a Mac Science (M18XHF-22)
diffraction meter using CuKa radiation as the X-ray source
at 50 kV and 100 mA. The data were collected in the conven-
tional q–2q configuration (2.5–30ꢂ) from thin films thermally
evaporated on SiO2/Si substrates in a vacuum chamber to form
1
recrystallization from DMF. H NMR (300 MHz, DMSO-d6):
d 8.24 (s, 2H), 8.08 (d, 2H, J ¼ 8.2 Hz), 7.63 (d, 2H, J ¼ 8,1 Hz),
4.91 (s, 4H).
300 A-thick films at a rate of 0.5 A sꢀ1. The AFM images of the
vacuum-deposited thin films were recorded using a PSIA XE-100
Advanced Scanning Microscope. The voltammetric apparatus
used was a CH Instruments model 700C electrochemical work-
station. The cyclic voltammograms (CVs) were obtained at room
temperature in a three-electrode cell equipped with a working
electrode (Au), a reference electrode (Ag/AgCl), and a counter
electrode (Pt) in dichlorobenzene containing tetrabutylammo-
nium hexafluorophosphate (Bu4N+PF6ꢀ, 0.1 M) as the support-
ing electrolyte at a scan rate of 100 mV sꢀ1. All the potentials
were calibrated with the standard ferrocene/ferrocenium redox
couple (E ¼ + 0.41 V measured).
˚
˚
2,7-Bis(diethylphosphorylmethyl)[1]benzothieno[3,2-b]benzo-
thiophene. 2,7-Dibromomethyl[1]benzothieno[3,2-b]benzothio-
phene (1.1 g, 2.58 mmol) was added to triethylphosphite (30 mL),
and the resulting solution was refluxed for 12 h. The solvent was
removed in vacuo, and the residue was purified by column
chromatography on silica gel using ethyl acetate–dichloro-
methane (2 : 1) as the eluent to obtain the product in 90% yield.
1H NMR (300 MHz, CDCl3): d 7.87 (s, 2H), 7.84 (d, 2H, J ¼
8.2 Hz), 7.42 (d, 2H, J ¼ 8.1 Hz), 4.05 (m, 8H), 3.36 (d, 4H, J ¼
21.5 Hz), 1.27 (t, 12H, J ¼ 7.0 Hz). 13C NMR (75 MHz, CDCl3):
(142.62, 142.58), 133.18, 131.90, (128.79, 128.67), (126.92,
126.84), (124.98, 124.88), 121.40, (62.33, 62.24), (34.81, 32.97),
(16.45, 16.37).
Fabrication of TFT devices
2,7-Bis(2-cyclohexylvinyl)[1]benzothieno[3,2-b]benzothiophene
(DCV-BTBT). LDA (1.5 M in cyclohexane, 4.0 mL, 6.0 mmol)
was added dropwise to a stirred solution of 2,7-bis(diethyl-
phosphorylmethyl)[1]benzothieno[3,2-b]benzothiophene (1.3 g,
2.41 mmol) in anhydrous THF (50 mL) at ꢀ78 ꢂC under
a nitrogen atmosphere. The mixture was stirred for 1 h, and then
cyclohexanecarbaldehyde (0.67 g, 6.02 mmol) in THF (10 mL)
was added dropwise over a period of 10 min. After the mixture
was stirred for 2 h at ꢀ78 ꢂC and for 12 h at room temperature,
5 mL of water was added, and the solvent was evaporated off.
The residue was washed with water and MeOH. The desired
product was separated by sublimation (0.77 g, 70%). High-
resolution mass spectrometry (HRMS): Calcd for C30H32S2:
456.1945. Found: 456.1951. Anal. Calcd for CHS: C, 78.90; H,
7.06; S, 14.04. Found: C, 78.48; H, 7.14; S, 14.36%.
The field-effect measurements were carried out using top-contact
FETs. TFT devices with a channel length (L) of 50 mm and
a channel width (W) of 1000 mm were fabricated on thermally
oxidized highly n-doped silicon substrates. The SiO2 gate
dielectric was 300 nm in thickness. The organic semiconductor
(300 A) was evaporated (0.1 A sꢀ1 at 1 ꢃ 10ꢀ6 Torr) onto a non-
pretreated or octadecyltrichlorosilane (OTS)-pretreated oxide
surface. Gold source/drain electrodes were evaporated on top of
the films through a shadow mask. All the measurements were
performed at room temperature using a 4155C Agilent semi-
conductor parameter analyzer, and the mobilities (m) were
calculated in the saturation regime by using the relationship
msat ¼ (2IDSL)/(WC(Vg ꢀ Vth)2), where IDS is the source–drain
saturation current; C (1.18 ꢃ 10ꢀ8 F), the oxide capacitance; Vg,
the gate voltage; Vth, the threshold voltage.
˚
˚
2,7-Distyryl[1]benzothieno[3,2-b]benzothiophene (DPV-BTBT).
LDA (1.5 M in cylohexane, 4.0 mL, 6.0 mmol) was added
dropwise to a stirred solution of 2,7-bis(diethylphosphor-
ylmethyl)[1]benzothieno[3,2-b]benzothiopheꢂne (1.3 g, 2.41
mmol) in anhydrous THF (50 mL) at ꢀ78 C under a nitrogen
atmosphere. The mixture was stirred for 1 h, and then benz-
aldehyde (0.67 g, 6.0 mmol) in THF (20 mL) was added
dropwise over a period of 10 min. After the mixture was stirred
Synthesis
2,7-Dihydroxymethyl[1]benzothieno[3,2-b]benzothiophene. To
solution of [1]benzothieno[3,2-b]benzothiophene-2,7-dicar-
a
boxylate (1.50 g, 3.9 mmol) in THF (40 mL) was added LiAlH4
(0.74 g, 19.5 mmol). The reaction mixture was stirred overnight.
The insoluble material was removed by filtration and washed
with hot DMSO. The filtrate and washings were collected, and
the product was precipitated by adding 50 mL of 1 N HCl. The
product was collected by filtration to afford 1.86 g (75%) of
pure 2,7-dihydroxymethyl[1]benzothieno[3,2-b]benzothiophene.
1H NMR (300 MHz, DMSO-d6): d 8.05 (s, 2H), 7.98 (d, 2H, J ¼
ꢂ
for 2 h at ꢀ78 C and for 12 h at room temperature, 5 mL of
water was added, and the solvent was evaporated off. The
residue was washed with water and MeOH. The desired
product was separated by sublimation (0.68 g, 64%). HRMS:
Calcd for C30H20S2: 444.1006. Found: 444.1008. Anal. Calcd
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For CHS: C, 81.04; H, 4.53; S, 14.42. Found: C, 81.04; H, 4.55;
S, 14.40%.
the reaction of the dibromide with triethylphosphite. The orga-
nization of oligomers in thin films can be maximized by the
introduction of vinylene units. The final step of the conversion
was accomplished by Horner–Emmons coupling reactions
between the phosphonate and the aldehyde derivatives, as shown
in Scheme 1. After purification by sublimation, DCV-DTBT and
DPV-BTBT were identified by HRMS and elemental analyses.
The thermal stabilities of DCV-DTBT and DPV-BTBT were
investigated by thermal gravimetric analysis (TGA). The TGA
analysis showed that DPV-BTBT is thermally more stable than
DCV-BTBT, probably due to the difference between the thermal
stabilities of the phenyl and cyclohexane groups. The thermal
decomposition temperatures were observed to be 347 ꢂC for
DCV-DTBT and 400 ꢂC for DPV-BTBT while, by contrast,
Results and discussion
Although there are several possible synthetic approaches to the
target compounds, we selected the Horner–Emmons coupling
reaction between phosphonates and aldehyde derivatives for
introducing vinyl groups, as shown in Scheme 1. The Horner–
Emmons coupling reaction is well known for yielding products
with all-trans configurations.14 The synthetic route for the
preparation of DCV-BTBT and DPV-BTBT is presented in
Scheme 1. The synthesis of the [1]benzothieno[3,2-b]benzothio-
phene-2,7-dicarboxylate was carried out from 2,20-diamino-(E)-
stilbene-4,40-dicarboxylate, which was prepared as already
reported.15a The entire procedure for the conversion to DCV-
BTBT and DPV-BTBT consists of ten steps. First, commercially
available 4-(chloromethyl)benzoic acid was converted into its
ethyl ester and nitrated to yield ethyl 4-(chloromethyl)-3-nitro-
benzoate. Two of the ester molecules were then condensed to
afford diethyl 2,20-dinitro-(E)-stilbene-4,40-dicarboxylate by
treatment with sodium ethoxide. The nitro groups were reduced
using iron powder in ethanol in the presence of hydrochloric acid
to give diethyl 2,20-diamino-(E)-stilbene-4,40-dicarboxylate. The
amino groups so formed were converted into xanthate groups via
a bisdiazonium salt. Treatment of the stilbene bisxanthate with
bromine in acetic acid yielded the fused thiophene ring, dieth-
yl[1]benzothieno[3,2-b]benzothiophene-2,6-dicarboxylate, which
was then reduced with LiAlH4 in THF to afford 2,7-dihydrox-
ymethyl[1]benzothieno[3,2-b]benzothiophene in good yield.
Treatment of the resulting diol with phosphorus tribromide in
DMF at room temperature afforded 2,7-dibromome-
thyl[1]benzothieno[3,2-b]benzothiophene. One of the precursors
of the Horner–Emmons olefination, 2,7-diethylphosphor-
ylmethyl[1]benzothieno[3,2-b]benzothiophene, was prepared by
ꢂ
pentacene began to decompose at 260 C (due to sublimation),
indicating that DC(P)V-DTBT compounds have high thermal
stabilities (Fig. 2).
The DSC results reveal the melting features of the two mate-
rials. In the case of DCV-BTBT, an endothermic transition and
an exothermic transition were observed at temperatures at 301
Fig. 2 Thermal gravimetric analysis (TGA) of DC(P)V-BTBT.
Scheme 1 Synthesis of DC(P)V-BTBT.
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Fig. 3 UV-vis absorption spectra of DPV-BTBT.
Fig. 4 XRD patterns of DPV-BTBT thin films vacuum-deposited on
OTS-treated SiO2/Si at Tsub ¼ 25, 50, and 80 ꢂC.
ꢂC (56.00 J gꢀ1) and 255 ꢂC (41.29 J gꢀ1), respectively, during the
heating and cooling cycles. On the other hand, DPV-BTBT was
with second- and third-order diffraction peaks at 2q ¼ 8.56ꢂ and
12.36ꢂ, respectively. The strong intensity of the X-ray diffraction
peaks indicated the formation of lamellar ordering and crystal-
linity on the substrate. The d-spacing of DPV-BTBT obtained
observed to have two endothermic peaks at 325 ꢂC (35.87 J gꢀ1
)
and 352 ꢂC (58.63 J gꢀ1). In the cooling trace, only one
exothermic peak at 330 ꢂC (47.84 J gꢀ1) was observed.
˚
from the first reflection peak was 20.15 A, which was comparable
The UV-vis spectrum of a dilute solution of DPV-BTBT in
xylene showed absorption peaks at 399, 379, and 359 nm (Fig. 3).
It is generally recognized that an increase in the planarity of
conjugated systems leads to a decrease in the HOMO–LUMO
gap and a corresponding red shift of the absorption spectra. The
long-wavelength absorptions in the UV-vis spectrum of the
DPV-BTBT solution showed greater red shifts (38 nm) than
those in the UV-vis spectrum of the DCV-BTBT solution. The
DPV-BTBT films showed a greater blue shift of their main
absorption peaks as compared to those of their diluted xylene
solutions, which suggests H-aggregate formation by analogy
with previous studies.15b,c Interestingly, in the PL spectra, the
differences in the emission maxima between the solution and
film states of DCV-BTBT and DPV-BTBT were 47 and 60 nm,
respectively, indicating the presence of extremely strong inter-
molecular interactions in the film states.16
to the molecular length obtained from the MM2 calculation
˚
(22.42 A). These spacings were consistent with the mono-
molecular layer thickness obtained by atomic force microscopy
(AFM), as shown in Fig. 7 (later) and Fig. S3 (ESI†), which
indicated a near-perpendicular alignment of the molecules with
respect to the substrate surface. In contrast, the XRD results of
the DCV-BTBT films exhibited very weak reflection peaks. This
may be attributed to the unique molecular stacking structure of
DCV-BTBT, where a molecular layer structure is not well
formed along the molecular long axis. It is assumed that the weak
reflection peaks of DCV-BTBT have a negative effect on the
mobility of the OFETs. This assumption is also in agreement
with the performance of the devices. When used as a channel
semiconductor in OTFTs, DCV-BTBT provided lower FET
mobility than DPV-BTBT. XRD measurements at higher
substrate temperatures of DCV-BTBT thin films vacuum-
deposited on OTS-treated SiO2/Si did not show increased
reflection peaks.
The UV-vis spectra of DCV-BTBT and DPV-BTBT showed
long-wavelength absorption edges at 379 and 419 nm, respec-
tively, which corresponded to HOMO–LUMO energy gaps of
3.25 and 2.97 eV, respectively; these values were significantly
higher than the energy gap of pentacene (2.2 eV).17 The HOMO–
LUMO energy gap decreases with an increase in the p-conju-
gation length.
Thin films of the two conjugated oligomers were formed by
vacuum evaporation onto either untreated or OTS-coated Si/
SiO2 substrates at various temperatures (Tsub ¼ 25 ꢂC, 50 ꢂC and
ꢂ
80 C). All the OTFTs showed typical p-channel TFT charac-
Further insight into the electronic properties of these
compounds was provided by cyclic voltammetry (CV). The
CV measurements of DCV-BTBT and DPV-BTBT in 0.1 M
Bu4N+PF6ꢀ/dichlorobenzene solution showed an irreversible
oxidation peak. The onset oxidation potentials were 0.69 eV and
0.76 eV vs. ferrocene (FOC). With respect to the energy level of
the FOC/ferrocenium reference (ꢀ4.8 eV), the HOMO energy
levels of DCV-BTBT and DPV-BTBT were ꢀ5.49 and ꢀ5.56 eV,
respectively, which were lower than that of pentacene, thus
indicating their high oxidative stability (ESI† Fig. S5).18 The
morphological characteristics were investigated by X-ray
diffraction (XRD) (Fig. 4 and ESI† Fig. S6).
teristics. The OTFTs of DCV-BTBT and DPV-BTBT were
fabricated with Au electrodes using top-contact geometry. Gold
source and drain contacts (50 nm) were deposited onto the
organic layer through a shadow mask. The channel length (L)
and width (W) were 50 mm and 1000 mm, respectively.
Fig. 5 shows the plot of the drain current (IDS) versus
drain-source voltage (VDS) and the transfer characteristics of
DPꢂV-BTBT TFTs grown at a substrate temperature (Tsub) of
80 C. From the electrical transfer characteristics, we estimated
several parameters such as the carrier mobility, on/off current
ratio, threshold voltage, and subthreshold swing for each device.
These are summarized in Table 1. We have demonstrated that
a high carrier mobility can be obtained with DPV-BTBT even
after the devices had been exposed to air for 2 months (Fig. 5).
The thin-film XRD pattern of DPV-BTBT displayed
a primary diffraction peak at 2q ¼ 4.38ꢂ (d-spacing: 20.15 A),
˚
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Fig. 5 Source–drain current (IDS) versus source–drain voltage (VDS) at
various gate voltages (VG) for top-contact field-effect transistor using
DPV-BTBT deposited at Tsub ¼ 80 ꢂC on OTS-treated SiO2. The transfer
characteristics in the saturation regime at a constant source–drain voltage
(VDS ¼ ꢀ100 V) are also included.
Fig. 6 a) The transfer characteristics in the saturation regime at
a constant source–drain voltage (VDS ¼ ꢀ100 V) with different times. b)
OTFT hole mobilities of DPV-BTBT were collected under ambient
conditions at different times and substrate temperatures.
oligomer is expected to increase its solubility, and thus enhance
the solution processing.19
Table 1 Field-effect mobility (mTFT), on/off current ratio (Ion/Ioff),
threshold voltage (Vth), and subthreshold swing (S) of DPV-BTBT
vacuum-deposited on differently treated SiO2 surfaces and at different
substrate temperatures (Tsub) after 2 months
In our studies, however, we found that DCV-BTBT is not
sufficiently soluble in any organic solvent. Moreover, optical
microscopic observations revealed that films of cyclohexyl-
substituted vinyl-BTBT did not have a continuous morphology
SiO2 surface Tsub mTFT/cm2 Vꢀ1 sꢀ1 Ion/Ioff Vth/V S/V decadeꢀ1
Bare
25
50
80
0.003
0.024
0.021
0.015
0.244
0.437
105
106
107
105
106
107
ꢀ5.5 2.7
ꢀ8.8 2.0
ꢀ7.0 1.8
ꢀ3.5 1.7
ꢀ5.7 1.2
ꢀ4.4 0.9
OTS-treated 25
50
80
The DPV-BTBT devices fabricated under various conditions
showed mFET in the range of 0.003–0.437 cm2 Vꢀ1 sꢀ1 and on/off
ratios of >105–107 under ambient conditions (Table 1). In
particular, excellent FET characteristics with mFET higher than
0.437 cm2 Vꢀ1 sꢀ1 (measured in the saturation regime) and on/off
ratios of >105 were observed in DPV-BTBT devices fabricated on
the OTS-treated substrate at Tsub ¼ 80 ꢂC. As evident from
Fig. 6, the mobility of DPV-BTBT showed negligible changes
even after 60 days of operation in air (further monitoring is in
progress). This observed trend clearly suggests that 2,7-bis(vinyl)
BTBT with a lower HOMO level tends to exhibit better air
stability.
We found that the TFT performance depends critically on the
side-end group of the active materials. Typically, the addition of
bulky substituents such as cyclohexyl groups to the ends of the
Fig. 7 AFM topography image of 30 nm thin films of DPV-BTBT
deposited on OTS treated SiO2 at various substrate temperatures: a)
25 ꢂC; b) 50 ꢂC; c) 80 ꢂC; and d) 100 ꢂC (2 ꢃ 2 mm area).
4702 | J. Mater. Chem., 2008, 18, 4698–4703
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3 C. D. Dimitrakopoulos and R. L. Malenfant, Adv. Mater., 2002, 14,
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(ESI† Fig. S4). The mobility of DPV-BTBT was 20 times higher
than that of DCV-BTBT. When used as a channel semiconductor
in OTFTs, DCV-BTBT exhibited lower FET mobility than
DPV-BTBT. This is not surprising since the charge transport in
organic semiconductors is dominated by the crystal structure,
and thus the less-ordered DCV-BTBT would not be expected to
exhibit a high mobility (0.024 cm2 Vꢀ1 sꢀ1, ESI† Fig. S1).20
Fig. 7 shows the AFM images of 30- nm-thick films of
DPV-BTBT deposited on OTS-treated SiO2/Si at 25, 50, 80 and
100 ꢂC. At 80 ꢂC, the molecules become more ordered, and
a network of interconnected grains can be observed in the
DPV-BTBT sample. The AFM step heights for the lamellar
structure of the DPV-BTBT grains (as obtained from the films
deposited at 80 ꢂC) correspond well to the d-spacing obtained
from XRD and the calculated molecular length (ESI† Fig. S3).
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Conclusions
In summary, a series of substituted vinyl-BTBT molecules were
synthesized by a route involving the Horner–Emmons coupling
reaction. The oligomers show high thermal stability. DPV-BTBT
exhibits excellent field-effect performances, with a mobility as
high as 0.46 and an on/off ratio of up to 1.2 ꢃ 107. It is notable
that the mobility of DPV-BTBT does not significantly change
even after the device has been exposed to air for at least 60 days
(further monitoring is in progress), showing that it is a promising
air-stable p-channel organic semiconductor for application in
all-organic flexible electronic devices.
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We are currently investigating the OTFT devices based on
DCV-BTBT and DPV-BTBT for improving their performances.
In addition, we are attempting the introduction of other alkyl
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Acknowledgements
This work was supported by grants (F0004030-2007-23,
F0004071-2007-23) from the Information Display R&D Center,
one of the 21st Century Frontier R&D Program funded by the
Ministry of Commerce, Industry, and Energy of the Korean
Government, and Seoul R&BD.
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This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 4698–4703 | 4703