Functionalized anthradithiophenes for organic field-effect transistors

Article abstract of DOI:10.1039/b715746k

Two new semiconductors for organic thin-film transistors (OTFTs), diperfluorophenyl anthradithiophene (DFPADT) and dimethyl anthradithiophene (DMADT), have been synthesized and characterized. The first material exhibits ambipolar transport in OTFT devices

Full text of DOI:10.1039/b715746k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Functionalized anthradithiophenes for organic field-effect transistors†
Ming-Chou Chen, Choongik Kim,b Sheng-Yu Chen,a Yen-Ju Chiang,a Ming-Che Chung,a
a
*
b
Antonio Facchetti and Tobin J. Marks
b
*
*
Received 11th October 2007, Accepted 3rd January 2008
First published as an Advance Article on the web 25th January 2008
DOI: 10.1039/b715746k
Two new semiconductors for organic thin-film transistors (OTFTs), diperfluorophenyl
anthradithiophene (DFPADT) and dimethyl anthradithiophene (DMADT), have been synthesized and
characterized. The first material exhibits ambipolar transport in OTFT devices with field-effect
mobilities (m) of 6 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1 and 0.05 cm2 Vꢁ1 sꢁ1 for electrons and holes, respectively.
Therefore, diperfluorophenyl substitution was found to be effective to induce n-type transport.
Dimethyl-substituted anthradithiophene (DMADT) was also synthesized for comparison and exhibited
exclusively hole transport with carrier mobility of ꢂ0.1 cm2 Vꢁ1 sꢁ1. Within this semiconductor family,
OTFT carrier mobility values are strongly dependent on the semiconductor film growth conditions,
substrate deposition temperatures, and gate dielectric surface treatment.
the core substituents.10 Following the same design strategy, the
Introduction
potential use of the ADT core as a potential n-channel semicon-
ductor candidate is proposed for the first time. The synthesis,
characterization, and OTFT performance of a new diperfluoro-
phenyl-substituted ADT compound (DFPADT) is presented
herein. Dimethyl anthradithiophene (DMADT) and anthradi-
thiophene (ADT) have been also synthesized and characterized
for comparison.
Organic thin-film transistors (OTFTs)1,2 have attracted intensive
attention for potential applications in flexible displays, inexpen-
sive electronic papers, radio-frequency identification (RFID)
components, and smart textiles, as well as smart memory/sensor
elements in the automotive and transportation industries.
Among previously developed OTFT semiconductors, most of
them are hole-transporting (p-type) materials, such as pentacene
derivatives,3 oligothiophenes,4 fused-thiophenes,5 and anthradi-
thiophenes (ADTs),6 while n-type semiconductors are still
rare.7 Since n-type semiconductors are crucial for the fabrication
of organic p–n junctions, bipolar transistors, and complemen-
tary circuits,8 we are particularly interested in developing new
candidates suitable for electron transport.
Experimental
Materials and methods
All chemicals and solvents were of reagent grade obtained from
Aldrich, Arco, or TCI Chemical Co. Solvents (toluene, benzene,
ether, THF, and hexane) were distilled under nitrogen from
Na/benzophenone ketyl, and halogenated solvents were distilled
from CaH2. Compound 3 and 2,3-bis(1,3-dioxolan-2-yl)thio-
phene (6) were prepared according to literature procedures.6a
1H and 13C NMR spectra were recorded on Bruker 500 or Bruker
Anthradithiophene-based materials have demonstrated great
potential for OTFT fabrication,6 and a solution processable
p-channel ADT derivative with a hole mobility of 1.0 cm2 Vꢁ1 sꢁ1
has been reported.6c However, n-channel ADT semiconductors
are unknown. Recently, fluoro-,8a perfluoroalkyl-,4c,9 per-
fluoroaryl-,10 carbonyl-,8c,10a cyano-11 and trifluoromethylphe-
nyl-2b,4a,5g,7c modified oligothiophenes have been synthesized
and proven to be excellent n-channel semiconductors. From
single crystal X-ray diffraction (XRD) analysis, the perfluoro-
phenyl-substituted oligothiophenes exhibit unique packing char-
acteristics characterized by strong cofacial intermolecular
packing. Enhanced electron affinity (low LUMO energy) is
enabled by the strong electron-withdrawing characteristics of
1
DRX-200 instruments. Chemical shifts for H and 13C NMR
spectra were referenced to solvent peaks. 19F NMR spectra
were referenced to external CFCl3. DSC thermographs were
carried out on a Mettler DSC 822 and calibrated with a pure
indiumsample witha scanrateof 10.0ꢃC minꢁ1. Thermogravimet-
ric analysis (TGA) was performed with a Perkin Elmer TGA-7
thermal analysis system using dry nitrogen as a carrier gas at
a flow rate of 100 mL sꢁ1. The UV-Vis absorption and fluores-
cence spectra were obtained using a JASCO V-530 and a Hitachi
F-4500 spectrometer, and all spectra were measured in the speci-
fied solvent at room temperature. The IR spectra were obtained
using a JASCO FT/IR-4100 spectrometer. Differential pulse
voltammetry experiments were performed with a CH Instruments
model CHI621C Electrochemical Analyzer. All measurements
aDepartment of Chemistry, National Central University, Chung-Li,
Taiwan, 32054, ROC. E-mail: mcchen@ncu.edu.tw; Tel: +886-3-4273253
bDepartment of Chemistry and the Materials Research Center,
Northwestern University, 2145 Sheridan Rd, Evanston, Illinois, 60208-
3113,
USA.
E-mail:
a-facchetti@northwestern.edu;
northwestern.edu; Tel: +1-847-491-3295; Tel: +1-847-491-5658
t-marks@
ꢃ
were carried out at 100 C with a conventional three-electrode
† Electronic supplementary information (ESI) available: Output
characteristics of 50 nm thick DFPADT FET as an n-channel
semiconductor; XRD patterns of 50 nm thick DFPADT films on
HMDS-treated substrates at various temperatures. See DOI:
10.1039/b715746k
configuration consisting of a platinum disk working electrode,
an auxiliary platinum wire electrode, and a nonaqueous
Ag reference electrode, and the supporting electrolyte was 0.1
M tetrabutylammonium hexafluorophosphate (TBAPF6) in
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dichlorobenzene. All potentials reported are referenced to an Fc+/
Fc internal standard. Elemental analyses were performed on
a Heraeus CHN-O-Rapid elemental analyzer. Mass spectra
were obtained on a JMS-700 HRMS instrument. Prime grade
silicon wafers (p+-Si) with ꢂ300 nm (ꢄ5%) thermally grown
oxide (from Montco Silicon) were used as device substrates.
was slowly added into a THF solution (60 mL) of 2,3-bis(1,3-
dioxolan-2-yl)thiophene (6; 4.56 g, 0.02 mol)6a at ꢁ78 ꢃC and
the mixture was stirred for 0.5 h at this temperature. Tri-n-butyl-
stannyl chloride (7.84 g, 0.026 mol) was then slowly added and
the mixture was warmed to room temperature and stirred for 8
h. NH4Cl saturated aqueous solution was added and 7 was
extracted with ether, 10.1 g; yield, 97%. The product was used in
1
the next step without further purification. H NMR (CDCl3):
Film deposition and characterization
d 0.90 (t, 9 H, CH3), 1.28 (m, 6 H, CH2), 1.48 (t, 6 H, CH2),
4.02 (m, 4 H, –OCH2CH2O–), 4.13 (m, 4 H, –OCH2CH2O–),
6.06 (s, 1 H, CH), 6.35 (s, 1 H, CH), 7.14 (s, 1 H, CH). Under
nitrogen, a toluene solution (60 mL) of 7 (5.18 g, 0.01 mol),
bromopentafluorobenzene (1.87 mL, 0.015 mol), and tetrakis
(triphenylphosphine)palladium (0.693 g, 0.6 mmol in 10 mL of
toluene) was refluxed for 4 days. NH4Cl saturated aqueous solu-
tion was added and the product was extracted with hexanes and
was recrystallized to give 5-perfluorophenyl-2,3-bis(1,3-dioxo-
lan-2-yl)thiophene, 1.95 g; yield, 50%. This product was used
in the next step without further purification. 1H NMR
(CDCl3): d 4.04 (m, 4 H, –OCH2CH2O–), 4.12 (m, 4 H, –OCH2-
CH2O-), 6.06 (s, 1 H, CH), 6.40 (s, 1 H, CH), 7.50 (s, 1 H, CH).
19F NMR (CDCl3): d ꢁ143.75 (d, 2F), ꢁ159.30 (t, 1F), ꢁ165.85
(t, 2F). The above dioxolanylthiophene (1.95 g) THF (10 mL)
solution was added to 3 M HCl (30 mL) and stirred for 30 min
at room temperature. 4a was extracted with ether and purified by
recrystallization in ether–hexanes to give yellow-brown crystals,
1.38 g; yield, 90%. 1H NMR (CDCl3): d 7.99 (s, 1 H, CH), 10.38
(s, 1 H, CHO), 10.55 (s, 1 H, CHO). Anal. calcd for C12H3F5O2S:
C, 47.07; H, 0.99. Found: 46.92; H, 1.05%. Under nitrogen, a
ethanol solution (60 mL) of 4a (0.612 g, 2.0 mmol) and 1,4-cyclo-
hexanedione (0.108 g, 1.0 mmol) was slowly added to 2.0 mL
15% KOH (in ethanol) at room temperature and the mixture
was stirred overnight. The crude residue was centrifuged and
washed with water (2 ꢀ 30 mL), methanol (2 ꢀ 30 mL), ether
(2 ꢀ 30 mL) to give compound 5a (0.63 g, 97%). (The product
was used in the next step without further purification. No
attempt was made to separate the syn and the anti isomers.
DFPADT (1) was first prepared by reduction of the ADT
quinone 5a with LiAlH4. LiAlH4 (0.152 g, 4.0 mmol) was added
to an ice-cooled suspension of 5a (0.63 g, 1.0 mmol) in dry THF
(100 mL) under nitrogen atmosphere. The deep blue suspension
was refluxed for 1 h. The mixture was cooled to room tempera-
ture and HCl (6 M, 40 mL) was added under cooling with ice.
The mixture was then refluxed for another 3 h. The pink-red
residue was centrifuged, washed with water (2 ꢀ 30 mL),
dichloromethane (2 ꢀ 30 mL), MeOH (2 ꢀ 30 mL) and ether
(2 ꢀ 30 mL). After drying, the crude product was again treated
with LiAlH4 and HCl and the same purified procedure was
repeated to afford 1. The product was purified by vacuum gradient
All p+-Si/SiO2 substrates were cleaned by sonication in absolute
ethanol (200 proof) for 3 min and then by oxygen plasma treat-
ment for 5 min (20 W). For the SiO2 coating layer, –SiMe3
groups were introduced using hexamethyldisilazane (HMDS),
deposited by placing the SiO2 substrates in an N2-filled chamber
saturated with HMDS vapor for 48 h. PS (5.0 mg mLꢁ1 in anhy-
drous toluene) was spin-coated onto substrates at 5000 rpm in the
air (relative humidity ꢂ30%) and cured in a vacuum oven at 80
ꢃC overnight. Film thicknesses were measured by profilometry
(Tencor, P10). Atomic force microscopic (AFM) images were
obtained using a JEOL-5200 Scanning Probe Microscope with
silicon cantilevers in the tapping mode. All of the semiconducting
materials were vacuum deposited at 5 ꢀ 10ꢁ6 Torr (ꢂ500 A thick-
˚
ness, 0.2 A sꢁ1 growth rate) at preset substrate temperatures. Thin
˚
films of organic semiconductors were analyzed by standard wide
angle q–2q X-ray diffraction (WAXRD) using monochromated
Cu-Ka radiation. For FET device fabrication, top-contact
electrodes (ꢂ70 nm) were deposited by evaporating gold (<1 ꢀ
10ꢁ6 Torr) through a shadow mask with the channel length (L)
and width (W) defined as 100 mm and 5000 mm, respectively.
OTFT and capacitance measurements
The capacitance of the bilayer dielectrics was measured on
metal–insulator–semiconductor (MIS) structures using a Signa-
tone probe station equipped with a digital capacitance meter
(Model 3000, GLK Instruments). All OTFT measurements
were carried out under vacuum (<1 ꢀ 10ꢁ5 Torr) using a Keithly
6430 subfemtoammeter and a Keithly 2400 source meter, oper-
ated by a local Labview program and GPIB communication.
Mobilities (m) were calculated in the saturation regime using
the relationship:12 msat ¼ (2IDSL)/[WCi(VG ꢁ VT)2], where IDS
is the source–drain saturation current; Ci is the gate dielectric
capacitance (per area), VG is the gate voltage, and VT is the
threshold voltage. The latter can be estimated as the x intercept
1/2
of the linear section of the plot of VG vs. (IDS) .
Material synthesis
Synthesis of diperfluorophenylanthradithiophene (DFPADT; 1).
Under nitrogen, 2.5 M n-BuLi (9.6 mL in hexanes, 0.024 mol)
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sublimation at pressures of <10ꢁ4 Torr, giving a purple-red solid,
80 mg; yield, 12%. Because of the low yield from the above
procedure, DFPADT (1) was prepared according to an alterna-
tive route.6a A mixture of aluminium wire (0.208 g, 7.7 mmol)
and mercuric chloride (5.2 mg, 0.02 mmol) in 30 mL of dry cyclo-
hexanol and carbon tetrachloride (0.2 mL) was reacted overnight
under mild reflux. 5a (0.50 g, 0.77 mmol) was then added to the
solution and the resulting mixture was refluxed under nitrogen
for 48 h. The reaction mixture was cooled to room temperature
and the solid collected by centrifugation then washed with warm
cyclohexanol followed by glacial acetic acid, 10% HCl, ethanol,
CHCl3, THF, and ether several times. The product was purified
by vacuum gradient sublimation at a pressure <10ꢁꢃ4 Torr to give
a purple-red solid, 0.16 g; yield, 34%. Mp > 400 C (decomp.);
Anal. calcd for C30H8F10S2: C, 57.88; H, 1.30. Found: C,
57.79; H, 1.42%.
by centrifugation and washed with water (2 ꢀ 30 mL), methanol
(2 ꢀ 30 mL), ether (2 ꢀ 30 mL) to give compound 5b (0.86 g,
1
97%). H NMR (CDCl3): d 2.68 (s, 6 H, CH3), 7.26 (s, 2 H),
8.62 (s, 2 H), 8.76 (s, 2 H). The product was used in the next
step without further purification. No attempt was made to sepa-
rate the syn and the anti isomers.
DMADT (2) was prepared by reduction of the ADT quinone
5b with LiAlH4. LiAlH4 (0.152 g, 4.0 mmol) was added to an
ice-cooled suspension of 5b (0.348 g, 1.0 mmol) in dry THF (100
mL) under nitrogen atmosphere. Through a similar procedure as
mentioned above, 2 was obtained as a bright orange solid (0.19
g) in 60% yield. Mp > 400 ꢃC (decomp.); Anal. calcd for
C20H14S2: C, 75.43; H, 4.43. Found: C, 75.22; H, 4.50%. MS
(EI) m/z calcd for C20H14S2: 318 (M+).
Results and discussion
Synthesis
Synthesis of dimethylanthradithiophene (DMADT; 2). Under
nitrogen, 2.5 M n-BuLi (6 mL in hexanes, 0.015 mol) was slowly
added into a THF solution (75 mL) of 2,3ꢃ-bis(1,3-dioxolan-2-
yl)thiophene (6; 2.28 g, 0.01 mol)6a at ꢁ78 C and the mixture
was stirred for 0.5 h at this temperature. Iodomethane (2.84 g,
0.02 mol) was then slowly added and the mixture was warmed
to room temperature and stirred for 8 h. The product was
extracted with ether (1H NMR (CDCl3): d 2.41 (s, 3 H, CH3),
4.02 (m, 4 H, –OCH2CH2O–), 4.10 (m, 4 H, –OCH2CH2O–),
5.95 (s, 1 H, CH), 6.27 (s, 1 H, CH), 6.75 (s, 1 H, CH)), concen-
trated to 30 mL, 3 M HCl (20 mL) was added, and the mixture
was stirred for 30 min at room temperature. Compound 4b was
extracted with ether and the crude product was then purified by
recrystallization in ether–hexanes to give yellow crystals, 1.39 g;
yield, 90%. 1H NMR (CDCl3): d 2.57 (s, 3 H, CH3), 7.29 (s, 1 H,
CH), 10.29 (s, 1 H, CHO), 10.36 (s, 1 H, CHO). Anal. calcd for
C7H6O2S: C, 54.53; H, 3.92. Found: 54.25; H, 4.11%. Under
nitrogen, a ethanol solution (60 mL) of 4b (0.788 g, 5.0 mmol)
and 1,4-cyclohexanedione (0.275 g, 2.5 mmol) was slowly added
to 2.5 mL 15% KOH (in ethanol) at room temperature and the
mixture was stirred for 1 h. The crude solid product was collect
The synthesis of functionalized anthradithiophenes 1 and 2 was
achieved according to Scheme 1 following known ADT core
synthetic approaches.6 Starting from 5-perfluorophenyl-2,3-
thiophenedicarboxaldehyde (4a), DFPADT (1) was obtained in
ꢂ30% yield, while DMADT (2) was obtained in ꢂ60% yield
from 4b. 4a was prepared in ꢂ45% yield by Stille coupling reac-
tion of 5-(tributylstannyl)-2,3-bis(1,3-dioxolan-2-yl)-thiophene
(7) with bromopentafluorobenzene. The dimethyl derivative 4b
was obtained in ꢂ90% yield from the methylation of 2,3-bis
(1,3-dioxolan-2-yl)thiophene 6. ADT (3) was synthesized accord-
ing to a known procedure.6 For both ADT derivatives 1 and 2,
differential scanning calorimetry (DSC)ꢃmeasurements did not
show any melting endotherm up to 400 C. Thermogravimetric
analꢃysis plots demonstrate weight loss only after heating above
400 C.
UV-Vis spectroscopy
As shown in Fig. 1A, the UV-Vis absorption spectrum of
DFPADT (1) in CHCl3 is significantly red shifted (lmax ¼ 515
Scheme 1 Synthesis of compounds 1–3.
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Fig. 1 (A) UV-Vis spectra of ADTs 1–3 in CHCl3. (B) Reflection UV-
Vis spectra of ADTs (1–3) powder.
nm) compared to ADT (3) (lmax ¼ 488 nm) and DMADT (2)
(lmax ¼ 479 nm). This result is in line with the substituent group
directing effects.10 Furthermore, DFPADT spectrum exhibits an
additional absorption located at lmax ¼ 595 nm, indicating
considerable p-electron delocalization extending from the ADT
core to the two electron withdrawing perfluorophenyl substitu-
ents. The HOMO–LUMO energy gaps calculated from the onset
of the absorption are 2.50 eV for DMADT, 2.46 eV for ADT, and
1.95 eV for DFPADT. The absorption red shifts going from 3
and 2 to 1 were also observed in reflection UV-Vis absorption
spectra of 1–3 powders (Fig. 1B). This result suggests large
core conjugation for 1 in the solid state and/or possible forma-
tion of J-aggregates.2b,n
Fig. 2 DPV of compounds 1–3 in dichlorobenzene. (A) Oxidation and
(B) reduction scans.
Fig. 3 The electrochemically derived HOMO and LUMO energies of
compounds 1–3.
The photooxidative stability of the ADT derivatives was inves-
tigated by monitoring the absorbance decay at lmax of 1–3 in
aerated CHCl3 solutions exposed to white light (fluorescent
lamp) at room temperature. Under these conditions for 24 h,
DFPADT exhibits the longest life time (ꢂ37% decay), demon-
strating the substituent effects on molecular stability, while
DMADT shows the shortest (ꢂ75% decay). ADT exhibits ꢂ45%
decay under the same conditions. Clearly, perfluorophenyl
substitution stabilizes the ADT core as previously observed for
thiophene- and pentacene-based derivatives.13
Ered ¼ ꢁ1.67) and DMADT (Eox ¼ +0.86 V, Ered ¼ ꢁ1.83) are
shifted to more positive values than those observed for DFPADT
(Fig. 2), which can be attributed to the electron-withdrawing
effect of the perfluorophenyl substituents. The derived molecular
orbital energy orderings (Fig. 3) demonstrate that fluoroaryl
substitution strongly lowers both HOMO and LUMO
energies.8c,10b The electrochemically derived HOMO–LUMO
energy gaps are 1.98 eV for DFPADT, 2.61 eV for ADT, and
2.69 eV for DMADT. The calculated energy levels from DPV plots
are consistent with those obtained from UV-Vis spectroscopy.
Differential pulse voltammetry
Differential pulse voltammꢃograms (DPV) of 1–3 were recorded
in dichlorobenzene at 100 C and the DPV plots are shown in
Fig. 2.14 The differential pulse voltammogram of DFPADT
exhibits an oxidation peak at +1.04 V and a reduction peak at
ꢁ0.94 V (ferrocene/ferrocenium as internal standard). Both the
oxidation and reduction potentials of ADT (Eox ¼ +0.94 V,
Field-effect transistor performance
Top-contact OTFTs were fabricated by vapor-depositing 50 nm
thick 1–3 films on bare, hexamethyldisilazane (HMDS)-treated,
or polystyrene (PS)-coated p+-Si/SiO2 substrates, followed by
Au deposition through a shadow mask to define the source
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and drain electrodes. The substrate temperature (TD) during
semiconductor film deposition was controlled with a thermo-
couple. OTFT characterization was performed under vacuum
and the device performance data are summarized in Table 1.15
For data analysis it is clear that ADT derivative-based OTFT
performance strongly depends on both dielectric surface treat-
ment and TD value. As a general trend, for all compounds,
p-channel performance increases at higher TDs, similarly to the
previous ADT derivatives.6 As TD increases, the hole mobilities
and Ion : Ioff increase, while the threshold voltage shifts to less
negative values. Overall, DMADT-based OTFTs perform better
on HMDS-treated substrates,16 whereas those based on DFPADT
exhibit the best performance on PS-coated p+-Si/SiO2 substrates.
DFPADT-based TFTs exhibit electron mobilities ꢂ100ꢀ lower
than the hole mobilities. This result is typical for most fluor-
inated semiconductors17 where electron transport is much more
sensitive than hole transport.
Fig. 4 Transfer plots of OTFT devices fabricated with DFPADT. A.
p-type character; film deposited at TD ¼ 150 ꢃC. B. n-type character;
film deposited at TD ¼ 25 ꢃC.
report of an ambipolar ADT derivative. Clearly, ADT end-
substitution with strongly electron withdrawing perfluorophenyl
groups does promote electron transport as previously demon-
strated in other studies,10 but it does not prevent efficient hole
transport.
For DFPADT-based TFTs, the highest hole mobility (mh) of
0.05ꢃcm2 Vꢁ1 sꢁ1 is observed on PS-coated substrates (TD
¼
Fluoroarene-free DMADT (2) only exhibits hole transport
with the highest mobilities (mh) of ꢂ0.1 cm2 Vꢁ1 sꢁ1 on HMDS-
treated substrates (TD ¼ 85 ꢃC), a threshold voltage (VT) of
ꢁ8 V, and Ion : Ioff of ꢂ103 (Fig. 5), which is comparable to
the TFT performance of previously reported dihexyl-substituted
ADT (ꢂ0.15 cm2 Vꢁ1 sꢁ1 at 85 ꢃC).6 ADT (3) is also a p-type
semiconductor and exhibits maximum hole mobilities (mh) of
150 C) with a threshold voltage (VT) of ꢁ25 V and Ion : Ioff of
ꢂ106 (Fig. 4a), while the highest electron mobility (me) of ꢂ6 ꢀ
10ꢁ4 cm2 Vꢁ1 sꢁ1 is observed on the same substrates (TD ¼ 25
ꢃC) with a threshold voltage (VT) of +55 V and Ion : Ioff of
ꢂ102 (Fig. 4b). A distinctive feature of ambipolar transport
can be observed in the output characteristic of DFPADT
(Fig. S1 in ESI†) where for VG ꢅ VDS the IDS increases consid-
0.01 cm2 Vꢁ1 sꢁ1, a threshold voltage (VT) of ꢁ21 V, and Ion
:
18
erably with increasing VDS
.
To our knowledge this is the first
Ioff of ꢂ105 on HMDS-treated substrates (TD ¼ 25 C), which
ꢃ
is similar to the literature values.19
Table 1 Charge carrier mobilities (m/cm2 Vꢁ1 sꢁ1)a, current on/off ratios
(Ion : Ioff), and threshold voltages (VT/V) for OFETs fabricated with
semiconductors 1 to 3
The present ADT-based compounds also possess great oxida-
tive stability, as previously demonstrated for other end-
substituted anthradithiophenes.6 Note that pentacene suffers
from oxidative instability,20 making the ADT family attractive
for future application. OTFTs fabricated with 1 and 2 exhibited
excellent stability when the devices were kept in the dark under
ambient conditions for several months. Negligible degradation
of p-channel mobility, threshold voltage, and Ion/Ioff over six
months were observed for DFPADT-based TFTs, demonstrat-
ing the advantages of using this stable ADT core for next-
generation solution processable semiconductors.21 One
drawback of n-type TFTs based on DFPADT is the very large
threshold voltages. This may be related to the quite negative
reduction potential (much more negative than typical n-type
semiconductors) of DFPADT and/or the high injection barrier
between the LUMO of DFPADT (3.26 eV) and the gold elec-
trode work function (5.1 eV).
Compound
Substrate TD/ꢃCb
m
Ion : Ioff VT
DFPADT (1) (p-type) Bare
HMDS
PS
25
85
5.0 ꢀ 10ꢁ6 103
7.0 ꢀ 10ꢁ6 103
7.3 ꢀ 10ꢁ6 103
6.8 ꢀ 10ꢁ6 103
1.7 ꢀ 10ꢁ5 103
1.3 ꢀ 10ꢁ5 103
1.6 ꢀ 10ꢁ4 102
5.0 ꢀ 10ꢁ4 103
3.8 ꢀ 10ꢁ4 104
3.0 ꢀ 10ꢁ3 105
4.0 ꢀ 10ꢁ2 106
4.8 ꢀ 10ꢁ2 106
2.6 ꢀ 10ꢁ6 101
6.1 ꢀ 10ꢁ4 102
6.0 ꢀ 10ꢁ7 101
2.0 ꢀ 10ꢁ5 103
1.9 ꢀ 10ꢁ4 101
2.2 ꢀ 10ꢁ5 103
2.0 ꢀ 10ꢁ4 101
3.0 ꢀ 10ꢁ4 103
ꢁ35
ꢁ34
ꢁ29
ꢁ31
ꢁ34
ꢁ35
ꢁ21
ꢁ29
ꢁ39
ꢁ31
ꢁ27
ꢁ25
+142
+55
+145
+82
+109
+130
+106
+58
ꢁ2
Bare
HMDS
PS
Bare
HMDS
PS
Bare
HMDS
120
150
PS
DFPADT (1) (n-type) HMDS
25
85
PS
HMDS
PS
HMDS
PS
HMDS
120
150
25
PS
DMADT (2) (p-type) Bare
0.048
0.062
0.070
0.070
0.10
103
104
104
102
103
105
105
105
105
HMDS
PS
Bare
HMDS
PS
ꢁ19
ꢁ33
ꢁ3
85
25
ꢁ8
0.011
0.008
0.010
0.012
ꢁ10
ꢁ20
ꢁ21
ꢁ33
ADT (3) (p-type)
Bare
HMDS
PS
a
Calculated in the saturation regime. Typical standard deviations are
<10%. Substrate deposition temperature.
b
Fig. 5 Transfer plot of OTFT device fabricated with DMADT, film
deposited at TD ¼ 85 ꢃC.
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Fig. 7 AFM images (5 mm ꢀ 5 mm) of 50 nm thick films of DFPADT
grown on HMDS-treated substrates at A TD ¼ 25 ꢃC, B TD ¼ 150 ꢃC.
Fig. 6 XRD patterns of 50 nm thick films of the indicated semiconduc-
tors grown on HMDS-treated substrates at TD ¼ 85 ꢃC.
DFPADT grown on PS-coated substrates at C TD ¼ 25 ꢃC, D TD
¼
150 ꢃC.
To investigate ADT-based semiconductor film microstructure
and correlate it to the corresponding TFT performance, wide-
angle X-ray diffraction (WAXRD) experiments were performed
for the DFPADT (1) and DMADT (2) films. Fig. 6 shows XRD
ꢃ
patterns of 1 and 2 films deposited at TD ¼ 85 C on HMDS-
treated substrates, demonstrating that these films are highly
crystalline. For both films only one set of strong reflections is
observed in the q–2q scan, with the first reflection located at
2q ¼ 3.75ꢃ for 1 and 2q ¼ 5.25ꢃ for 2. These angles correspond to
˚
˚
a film d-spacing of 24.6 A for 1 and 16.8 A for 2 and demonstrate
an end-on-substrate molecular orientation.22 Since the TFT
performance of 1 is quite sensitive to the substrate temperature,
the XRD patterns were also examined at various TDs (ESI†
Fig. S2). As the TD increases, the first Bragg reflection intensity
increases and shifts to smaller angle corresponding to a slightly
larger d-spacing. This result may partially account for the
improved TFT performance. However, film morphology varia-
tions with TD may also play an important role. Indeed, AFM
images of the ADT films demonstrate that film growth depends
significantly on both TD and the dielectric surface treatment.9 In
all cases, semiconductor films deposited at 25 ꢃC consist of small
grains (Fig. 7A, C and 8A), likely resulting from slow molecular
diffusion on the cold dielectric surface of the film nucleation
sites.23 For the films of DFPADT, as TD is increased from 25
ꢃC to 150 ꢃC, the film grains become larger and longer
(Fig. 7B, D and 8B) on either surface treatment. This result
corroborates the improved electron mobilities of DFPADT-
based TFTs when the films are deposited at higher substrate
temperatures (see Table 1). In the case of DMADT, the grain
size of the films grown on HMDS-treated substrates at 85 ꢃC
(Fig. 8B) is extremely large (ca. 1 mm) in agreement with the
higher device performance.
Fig. 8 AFM images (5 mm ꢀ 5 mm) of 50 nm thick films of DMADT
grown on HMDS-treated Si/SiO2 substrates. A at TD ¼ 25 ꢃC, B at TD
¼ 85 ꢃC.
Conclusions
In summary, the first diperfluorophenyl-substituted anthradi-
thiophene (DFPADT) has been synthesized and exhibited TFT
ambipolar transport. The perfluorophenyl group was found to
be effective in inducing n-channel transport while preserving
p-channel transport. The devices based on DFPADT exhibit
field-effect mobilities (m) of 6 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1 for electrons
and 0.05 cm2 Vꢁ1 sꢁ1 for holes. Dimethyl anthradithiophene
(DMADT) was also synthesized for comparison and exhibited
only p-channel transport with hole mobilities approaching 0.1
cm2 Vꢁ1 sꢁ1. The carrier mobilities of these derivatives depend
on the substrate deposition temperature and dielectric surface
treatment. P-Type operation of these ADT-based OTFTs
exhibited excellent stability over months (nearly one year),
demonstrating the advantages of using this core for the realization
of solution processable organic semiconductors.
1034 | J. Mater. Chem., 2008, 18, 1029–1036
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H. Klauk, U. Zschieschang, G. Schmid, S. Ponomarenko,
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We thank the NSF-MRSEC program through the Northwestern
Materials Research Center (DMR-0520513) and Polyera Corpo-
ration for financial support. Financial assistance for this research
was partially provided by the National Science Council, Repub-
lic of China (Grant Number NSC 95-2113-M-008-008-MY2).
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using 0.1 mol dmꢁ3 Bu4NPF6 as the supporting electrolyte.
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15 The FET measurements were carried out in vacuum. No change in
TFT carrier mobility was observed for p-channel operation of 1–3-
based devices. However electron transport for DFPADT was not
observed when the devices were tested in air.
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16 In general, simple SiO2 hydrocarbon functionalization using
octadecyltrichlorosilane (OTS) or hexamethyldisilazane (HMDS)
enhances the mobilities and lowers the off-currents of most OTFTs.
We reported previously that optimized DHCO-4T-based TFTs on
HMDS exhibit both p- and n-type transport with carrier mobilities
of 0.22 and 0.002 cm2 Vꢁ1 sꢁ1, respectively. See (a) J. A. Letizia,
A. Facchetti, C. L. Stern, M. A. Ratner and T. J. Marks, J. Am.
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19 The mobility number (0.01 cm2 Vꢁ1 sꢁ1) slightly lower than the
published data (0.02 cm2 Vꢁ1 sꢁ1) in ref. 6 might be due to the
unoptimized deposition conditions in this study.
20 M. Yamada, I. Ikemote and H. Kuroda, Bull. Chem. Soc. Jpn., 1988,
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21 However, under similar conditions the electron mobilities for
the same TFTs decreased by 30–50%. For example, the
mobility had decreased from 1.9 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1 to 5.8 ꢀ
10ꢁ5 cm2 Vꢁ1 sꢁ1 (TD ¼ 120 ꢃC) and from 2.0 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1
to 1.3 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1 (TD ¼ 150 ꢃC) on HMDS-treated
substrates.
˚
22 The d-spacing of ADT (3) films obtained from XRD is 14.15 A
demonstrating an almost perpendicular orientation of the molecules
on the substrate as reported in ref. 6.
23 C. Kim, A. Facchetti and T. J. Marks, Science, 2007, 318, 76.
1036 | J. Mater. Chem., 2008, 18, 1029–1036
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