Novel semiconductors based on functionalized benzo[ d, d′]thieno[3,2- b;4,5- b′]dithiophenes and the effects of thin film growth conditions on organic field effect transistor performance

Article abstract of DOI:10.1021/cm101435s

A series of benzo[d,d]thieno[3,2-b;4,5-b]dithiophene (BTDT) derivatives, end-functionalized with phenyl (P) and benzothiophenyl (BT), were synthesized and characterized. A facile, one-pot synthesis of BTDT was developed which enables the efficient realiza

Full text of DOI:10.1021/cm101435s

Chem. Mater. 2010, 22, 5031–5041 5031
DOI:10.1021/cm101435s
Novel Semiconductors Based on Functionalized
Benzo[d,d0]thieno[3,2-b;4,5-b0]dithiophenes and the Effects of
Thin Film Growth Conditions on Organic Field Effect
Transistor Performance
Jangdae Youn,† Ming-Chou Chen,*,‡ You-jhih Liang,‡ Hui Huang,† Rocio Ponce Ortiz,†,^
Choongik Kim,† Charlotte Stern,† Tarng-Shiang Hu,§ Liang-Hsiang Chen,§ Jing-Yi Yan,§
Antonio Facchetti,*,† and Tobin J. Marks*,†
†Department of Chemistry and the Materials Research Center, Northwestern University,
2145 Sheridan Road, Evanston, Illinois 60208-3113, ‡Department of Chemistry, National Central University,
Chung-Li, Taiwan 32054, ROC, §Industrial Technology Research Institute, Taiwan, ROC, and
^Department of Physical Chemistry, University of Malaga, Malaga 29071, Spain
Received May 21, 2010. Revised Manuscript Received June 24, 2010
A series of benzo[d,d]thieno[3,2-b;4,5-b]dithiophene (BTDT) derivatives, end-functionalized with
phenyl (P) and benzothiophenyl (BT), were synthesized and characterized. A facile, one-pot synthesis
of BTDT was developed which enables the efficient realization of a new BTDT-based semiconductor
series for organic thin-film transistors (OTFTs). The crystal structure of P-BTDT was determined via
single-crystal X-ray diffraction. Various combinations of dielectric surface treatment methods,
substrate temperature, and deposition flux rate sequences have significant effects on device
performance. Films deposited on octadecyltrichlorosilane (OTS)-treated SiO2 substrates under
properly adjusted substrate temperature and deposition flux rate achieve an efficaceous compromise
between high film crystallinity and good film grain interconnectivity, resulting in good OTFT
performance, with mobility greater than 0.70 cm2 V-1 s-1 and Ion/Ioff greater than 108.
1. Introduction
components, and e-paper.1-16 For recent reviews of this
subject see refs 2 and 17-19. Among these materials,
pentacene has attracted the greatest interest owing to its
high performance, with typical field-effect mobilities of
Over the past decade, organic semiconducting materials
have been extensively developed for the fabrication of
organic thin-film transistors (OTFTs), which have many
potential applications in low-cost/printable electronics, such
as flexible displays, Radio-frequency identification (RFID)
0.5-1.0 cm2 V-1 s-1 20,21
Unfortunately, pentacene and
.
several of its 6,13-functionalized derivatives suffer from
rapid degradation in ambient conditions due to photoin-
duced oxidative decomposition and the formation of dimeric
Diels-Alder adducts at the electron-rich central ring.22-27
Therefore, sizable efforts have been devoted to improving
pentacene stability by developing new heteroacenes with
*To whom correspondence should be addressed. E-mail: mcchen@
ncu.edu.tw (M.-C.C.), a-facchetti@northwestern.edu (A.F.), t-marks@
northwestern.edu (T.J.M.).
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5032 Chem. Mater., Vol. 22, No. 17, 2010
Youn et al.
is also critically important for optimizing OTFT perfor-
mance. The film morphology of organic semiconductors
depends on several important and poorly understood
substrate/film growth parameters such as the gate di-
electric surface characteristics, substrate temperature
during film growth, and film deposition flux rate. The
dielectric surface functionalization can change interfacial
interactions between the gate dielectric and the semicon-
ductor layer. Since charge transport takes place primarily
within the first few semiconductor film monolayers at the
dielectric-semiconductor interface,37 device performance
can be enhanced significantly by appropriate surface mod-
ification of the gate dielectric. One of the most effective
surface treatment methods for SiO2 gate dielectrics is via
alkylsilane self-assembledmonolayer (SAM) formation on
the SiO2 surface.38 Indeed, acidic functional groups on the
bare SiO2 surface can act as charge traps, which can
significantly decrease mobile charge carrier density and
the OTFT performance. Organosilane SAM functionali-
zation of the SiO2 gate dielectric reduces the surface
SiOH density, as well as decreases the surface energy of
the SiO2, which generally enhances the semiconductor film
crystallinity.39 At the initial stage of film growth, the
adhesion between the π-cores of the adsorbate molecules
and the substrate is much stronger than the adsorbate-
adsorbate molecular adhesion strength.40 As a result, the
molecules at the interfacial region generally lie flat on the
surface. SAM-functionalized surfaces with low surface
energy reduce the adhesion strength between the adsorbate
and the substrate, resulting in vertically aligned molecular
orientations on the surface. For efficient transport of
charge carriers, the π-π stacking direction of the semicon-
ductor molecules should ideally be in the same direction as
the current flow. Therefore, silane SAM functionalization
is instrumental in achieveing well-aligned π-π conjugated
films at the semiconductor-dielectric interface.
Figure 1. Examples of fused oligothiophene semiconductors used in
OTFTs.
high charge carrier mobility. Fused thiophene-based mate-
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due to their extensive conjugation, strong intermolecular
S
S interactions, large band gaps, and high ambient
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Article
Chem. Mater., Vol. 22, No. 17, 2010 5033
To the best of our knowledge, this is among the best device
performance reported to date for four-ring fused thio-
phene organic semiconductors.
Figure 2. Chemical structures of the benzo[d,d0]thieno[3,2-b;4,5-b0] dithio-
phene (BTDT) semiconductors developed in this study.
2. Experimental Section
2.1. Materials and Methods. All chemicals and solvents were
of reagent grade and were obtained from Aldrich, Arco, or TCI
Chemical Co. Reaction solvents (toluene, benzene, ether, and
THF) were distilled under nitrogen from sodium/benzophenone
ketyl, and halogenated solvents were distilled from CaH2.
1H and 13C NMR spectra were recorded on a Bruker 500 or a
at the edges of the islands, facilitating grain site growth. For
this reason, films deposited at higher substrate temperatures
typically exhibit large grain sizes, resulting in superior device
performance if cracks do not form.47,48
Finally, the molecular deposition flux rate also appears
to have a significant effect on film morphology. Grain
growth kinetics can increase the size and depth of each
grain boundary region. When films are grown slowly,
molecules do not migrate to the lower layer but remain on
top of islands, resulting in a 3D growth mode due to the
potential energy barriers at the step edges. At slow
deposition flux rates with low kinetic energy, the mole-
cules cannot overcome the barrier at the step edges.
Therefore, the semiconductor molecules do not comple-
tely fill the valleys between grains, resulting in large grain
boundaries and voids. On the other hand, larger deposi-
tion fluxes can overcome the potential energy barriers for
molecular diffusion at each grain boundary, and the
molecules can reach the bottoms of the grain valleys.
The outcome is a large density of nucleation sites with
much smaller grain sizes. As a result, the interconnecti-
vity between grains in the film is improved since large,
deep grain boundaries are prevented. Therefore, several
research groups have recently introduced a two-stage
deposition technique to simultaneously maximize the film
crystallinity and interconnectivity.29,49 In this two-stage
process, semiconductor films are initially grown at a very
slow rate and, at a later stage, at a higher deposition rate.
This combination of high crystallinity and good inter-
connectivity sometimes results in excellent charge trans-
porting films.
In this contribution, we describe the facile synthesis of a
new class four fused-ring heteroacene derivatives, based
on the benzo[d,d0]thieno[3,2-b;4,5-b0]dithiophene (BTDT)
core (Figure 2). In addition, molecular structure-property
relationships are elucidated by introducing various func-
tional groups on the BTDT core. Furthermore, film
growth parameters relating to the SiO2 gate dielectric
surface treatment, the semiconductor film substrate tem-
perature, and the molecular deposition flux rate are in-
vestigated to assess their effects on film microstructure and
morphology. Our results reveal that TFTs based on phenyl
benzo[d,d]thieno[3,2-b;4,5-b]dithiophene (P-BTDT) films
deposited on octadecyltrichlorosilane (OTS)-treated sub-
strates at 40 ꢀC exhibit excellent p-channel mobilities up to
0.70 cm2 V-1 s-1 and with good environmental stability.
1
300 instrument. Chemical shifts for H and 13C NMR spectra
were referenced to solvent signals. Differential scanning calo-
rimetry (DSC) was carried out on a Mettler DSC 822 instru-
ment, calibrated with a pure indium sample at a scan rate of
10 K/min. Thermogravimetric analysis (TGA) was performed on
a Perkin-Elmer TGA-7 thermal analysis system using dry nitro-
gen as the carrier gas at a flow rate of 40 mL/min. The UV-Vis
absorption and fluorescence spectra were obtained using JAS-
CO V-530 and Hitachi F-4500 spectrometers, respectively, and
all spectra were measured in the indicated solvents at room
temperature. IR spectra were obtained using a JASCO FT/IR-
4100 spectrometer. Differential pulse voltammetry experiments
were performed with a CH Instruments model CHI621C electro-
chemical analyzer. All measurements were carried out at the
temperature indicated with a conventional three-electrode con-
figuration consisting of a platinum disk working electrode, an
auxiliary platinum wire electrode, and a nonaqueous Ag refer-
ence electrode. The supporting electrolyte was 0.1 M tetrabuty-
lammonium hexafluorophosphate (TBAPF6) in a specified dry
solvent. All potentials reported are referenced to a Fcþ/Fc
internal standard (at þ0.6 V). Elemental analyses were per-
formed on a Heraeus CHN-O-Rapid elemental analyzer. Mass
spectrometric data were obtained with a JMS-700 HRMS
instrument. Prime grade silicon wafers (pþ-Si) with ∼300 nm
((5%) thermally grown oxide (from Montco Silicon) were used
as device substrates.
2.2. One-Pot Synthesis of Benzo[d,d0]thieno[3,2-b;4,5-b0]-
dithiophene (BTDT). Under nitrogen and anhydrous conditions
at -78 ꢀC, 2.5 M n-BuLi (21 mL in hexanes, 0.053 mol) was
slowly added to an ether solution (30 mL) of 3-bromothiophene
(5.1 mL, 0.053 mol), and the mixture was stirred for 40 min. This
solution was then warmed to 0 ꢀC under vacuum to remove
C4H9Br, and then, ether (30 mL) was added. Note: without
removing the n-C4H9Br, 3-butyl-S-thiophene is generated as a
major side product after sulfur addition in the next step. At -78
ꢀC, sulfur (1.68 g, 0.053 mol) was added to this ether solution,
which was then stirred for 30 min, warmed to 0 ꢀC, and stirred
for another 30 min. Next, p-toluenesulfonyl chloride (TsCl,
10.62 g, 0.053 mol) was added to this solution at 0 ꢀC; stirring
was continued for 10 min, and then, the mixture was warmed up
to 45 ꢀC for 4 h. The reagent 3-Li-benzo[b]thiophene was
prepared as in the above procedure from 2.5 M n-BuLi
(19.1 mL in hexanes, 0.048 mol) and 3-bromobenzo[b]thiophene
(10.2 mL, 0.048 mol, see preparation below). Again, this solu-
tion was warmed to 0 ꢀC under vacuum to remove n-C4H9Br,
and 40 mL of ether was added. At -78 ꢀC, the second portion of
3-Li-benzo[b]thiophene ether solution was added to the first
reaction mixture, stirred for 1 h, then warmed to room tem-
perature, and stirred overnight. Next at 0 ꢀC, 2.5 M n-BuLi
(42.2 mL in hexanes, 0.105 mol) was slowly added to this mix-
ture, which was stirred for 30 min, and then refluxed for 1 h.
Next, CuCl2 (15.4 g, 0.115 mol) was added to this mixture at 0 ꢀC,
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Adv. Funct. Mater. 2006, 16, 1859.

5034 Chem. Mater., Vol. 22, No. 17, 2010
Youn et al.
stirred for 1 h, warmed to room temperature, and stirred over-
night. The resulting solids were collected by filtration, washed
with water, extracted with benzene, and then chromatographed
(silica gel; n-hexane as the eluent). The BTDT product was
recrystallized from hexanes as a light-yellow powder to give a
total of 3.85 g in a yield of 32%. 1H NMR (CDCl3; 300 MHz): δ
7.86 (dd, J = 7.8, 0.6 Hz, 1H), 7.82 (dd, J = 7.8, 0.6 Hz, 1H), 7.42
THF solution of benzo[b]thiophene (114.3 mg, 0.85 mmol),
and the mixture was stirred for 1 h. Next, tri-n-butyltinchloride
(0.25 mL, 0.89 mmol) was added, and the mixture was stirred for
30 min at this temperature, then warmed to room temperature,
and stirred overnight. After simple filtration, THF was removed
under vacuum and 30 mL of toluene was added. The toluene
solution was then transferred to a 2-bromobenzo[d,d0]thieno[3,2-
b;4,5-b0]dithiophene (277.2 mg, 0.85 mmol) and tetrakis(triphenyl-
phosphine)palladium (39.3 mg, 0.034 mmol) in toluene (20 mL)
solution, and the mixture was refluxed for 2 days. The desired solid
product was collected by filtration, washed with hexanes and ether,
and then purified by gradient sublimation at pressures of ∼10-5
Torr, giving a bright yellow solid, 114 mg; yield, 35%. 1H NMR
(CD2Cl2; 500 MHz): δ 7.89 (d, J = 7.5 Hz, 1H), 7.85 (d, J = 7.5
Hz, 1H), 7.7.83 (d, J = 7.5 Hz, 1H), 7.77 (d, J = 7.5 Hz, 1H), 7.59
(s, 1H), 7.51 (s, 1H), 7.46 (t, J = 7.5 Hz, 1H), 7.39 (t, J = 7.5 Hz,
1H), 7.37 (t, J = 7.5 Hz, 1H), 7.33 (t, J = 7.5 Hz, 1H). This
material was insufficiently soluble to obtain a useful 13C NMR
spectrum. Anal. Calcd for C20H10S4: C, 63.46; H, 2.66. Found: C,
63.41; H, 2.72. HRMS (EI) m/z: calcd 377.9674 (Mþ); found,
377.9665.
(d, J = 5.1 Hz, 1H), 7.38 (m, 2H), 7.34 (d, J = 5.1 Hz, 1H). 13
C
NMR (CDCl3; 300 MHz): δ 141.58, 141.49, 136.49, 133.51,
131.53, 129.51, 126.93, 124.88, 124.44, 123.90, 120.78, 120.61.
MS (FAB) m/z: calcd for C12H6S3, 246.3 (Mþ); found, 246.3.
2.3. Synthesis of 3-Bromobenzo[b]thiophene.50 At 0 ꢀC, NBS
(33.8 g, 0.19 mol) was added to a 400 mL THF solution of
benzo[b]thiophene51 (17.17 g, 0.13 mol), and the mixture was
stirred for 0.5 h, then warmed to room temperature, and stirred
for 36 h. Aqueous Na2S2O3 solution was added next, and the
desired product was extracted with ether and chromatographed
(silica gel; hexanes as the eluent). The product was then further
purified by distillation, giving a bright yellow oil, 25.9 g; yield,
1
95%. H NMR (CDCl3): δ 7.85 (m, 2H), 7.45 (m, 3H). 13C
NMR (CDCl3; 300 MHz): δ 138.26, 137.18, 125, 124.74, 123.28,
122.75, 122.42, 107.51.
2.6. X-ray Crystal Structure Determination of P-BTDT. Yellow
crystals suitable for X-ray diffraction were crystallized from a hot
trimethylbenzene solution of P-BTDT. The chosen crystals were
mounted on a glass fiber. Data collection for P-BTDT was carried
out on a Bruker Smart Apex2 CCD diffractometer with Cu KR
2.4. Synthesis of 2-Phenylbenzo[d,d0]thieno[3,2-b;4,5-b0]dithio-
phene (P-BTDT). Under nitrogen and anhydrous conditions at
0 ꢀC, 2.5 M n-BuLi (0.49 mL in hexanes, 1.22 mmol) was slowly
added to a 10 mL THF solution of BTDT (302 mg, 1.22 mmol),
and the mixture was stirred for 40 min. Next, tri-n-butyltin
chloride (0.38 mL, 1.35 mmol) was added, and the mixture was
stirred for 30 min at this temperature, then warmed to room
temperature, and stirred overnight. After simple filtration, THF
was removed under vacuum, and 20 mL of toluene was loaded.
The toluene solution was then transferred into a bromobenzene
(0.14 mL, 1.35 mmol) and tetrakis (triphenylphosphine)palla-
dium (57 mg, 0.049 mmol) toluene (10 mL) solution and was
refluxed for 2 days. The desired solid product was collected by
filtration, washed with hexanes and ether, and then purified by
gradient sublimation at pressures of ∼10-5 Torr, giving a bright
yellow solid, 182 mg; yield, 46%. 1H NMR (CDCl3; 500 MHz): δ
7.88 (d, J = 8 Hz, 1H), 7.83 (d, J = 8 Hz, 1H), 7.67 (d, J = 7.5Hz,
2H), 7.58 (s, 1H), 7.44 (m, 3H), 7.35 (t, 2H). 13C NMR (CDCl3;
500 MHz): δ 146.42, 142.37, 141.86, 136.20, 134.66, 133.72,
130.93, 130.02, 129.13, 128.11, 125.97, 125.07, 124.58, 124.03,
120.72, 116.74. Anal. Calcd for C18H10S3: C, 67.04; H, 3.13.
Found: C, 67.15; H, 3.02. HRMS (EI) m/z: calcd 321.9937 (Mþ);
found, 321.9945.
˚
radiation (λ = 1.54178 A) at 100(2) K. After data collection, the
frames were integrated and absorption corrections were applied.
The initial crystal structure was solved by direct methods, the
structure solution was expanded through successive least-squares
cycles, and the final solution was determined. All of the nonhydro-
gen atoms were refined anisotropically. Hydrogen atoms attached
to carbon atoms were fixed at calculated positions and refined
using a riding mode. Crystal data, data collection, and refinement
parameters are summarized in Table S1 (Supporting Information).
2.7. OTFT Fabrication. Thin film transistors were fabricated
in a bottom gate-top contact configuration. Highly doped p-type
(100) silicon wafers (<0.004Ω cm) were used as gate electrodes as
well as substrates, and 300 nm SiO2 thermally grown on Si was
used as the gate insulator. The unit area capacitance is 10nF. The
substrate surface was treated with octadecatrichlorosilane (OTS)
or hexamethyldisilazane (HMDS), both purchased from Sigma-
Aldrich ChemicalCo. A fewdropsofHMDSwereloadedinsidea
self-assembly chamber under an N2 blanket. The SiO2/Si sub-
strates were exposed to this atmosphere for at least 7.0 days to
give a hydrophobic surface. After HMDS deposition, the advan-
cingaqueous contactangleis95ꢀ. OTS was depositedunderanN2
blanket inside a self-assembly chamber. The SiO2/Si substrates
were exposed to an OTS toluene solution for 10 h to give a
hydrophobic surface. After OTS deposition, the advancing aqu-
eous contact angle of a water drop is 105ꢀ. Semiconductor thin
films (50 nm) were next vapor-deposited onto the Si/SiO2 sub-
strates held at predetermined temperatures of 25 and 50 ꢀC for
P-BTDT and 25, 50, and 80 ꢀC for BT-BTDT with a deposition
2.5. Synthesis of 2-Benzothienylbenzo[d,d0]thieno[3,2-b;4,5-b0]-
dithiophene (BT-BTDT). At 0 ꢀC, NBS (595.5 g, 3.31 mol) was
added to a 100 mL THF solution of BTDT (816.3 mg, 3.31 mol),
and the mixture was stirred for 0.5 h, then warmed to room
temperature, and stirred overnight. Aqueous Na2S2O3 solution
was added next, and the desired 2-bromobenzo[d,d0]thieno[3,2-b;
4,5-b0]dithiophene was extracted with CH2Cl2, chromatographed
(silica gel; hexanes as the eluent), and recrystallized to give a
bright yellow solid, 932.2 mg; yield, 89%.1H NMR (CDCl3; 300
MHz): δ 7.87 (d, J = 7.8 Hz, 1H), 7.82 (d, J = 7.8 Hz, 1H), 7.44
-6
˚
rate of 0.1 A/s at 6 ꢀ 10 Torr, employing a high-vacuum
(t, J = 7.5 Hz, 1H), 7.37 (d, J = 7.5 Hz, 1H), 7.36 (s, 1H). 13
C
deposition chamber (Denton Vacuum, Inc., USA). Gold source
and drain electrodes (50 nm) were vapor-deposited at 2 ꢀ 10-6
Torr through a shadow mask in the vacuum deposition chamber.
Devices were fabricated with typical channel lengths of 50 and
100 μm and a channel width of 2000 μm.
NMR (CDCl3; 300 MHz): δ 141.42, 139.90, 135.72, 133.12,
131.78, 129.25, 125.06, 124.73, 123.95, 123.41, 120.72, 113.09.
Under nitrogen and anhydrous conditions at 0 ꢀC, 2.5 M n-BuLi
(0.34 mL in hexanes, 0.85 mmol) was slowly added to a 10 mL
2.8. OTFT Characterization. I-V plots of device perfor-
mance were measured under vacuum, and transfer and output
plots were recorded for each device. The current-voltage (I-V)
characteristics of the devices were measured using a Keithley
(50) Marc, L.; Jeremie, F. D. C.; Lionel, J.; Emilie, D. Tetrahedron 2004,
60, 3221.
(51) Kashiki, T.; Shinamura, S.; Kohara, M.; Miyazaki, E.; Takimiya,
K.; Ikeda, M.; Kuwabara, H. Org. Lett. 2009, 11, 2473.

Article
Chem. Mater., Vol. 22, No. 17, 2010 5035
n-BuLi and then alkylstannylation affords the corres-
ponding 2-BTDT-SnR3 derivatives generated in situ,
which are then coupled with the corresponding aryl
bromide to give P-BTDT in ∼46% yield, using a Stille
coupling protocol. Since 2-benzothiophenyl tributyltin
(2-BT-SnR3) is easier to access than 2-benzothiophenyl
bromide (2-BT-Br), BT-BTDT was prepared by coupling
2-BT-SnR3 with 2-BTDT-Br in 35% yield (rather than
2-BT-Br þ 2-BTDT-SnR3).
Figure 3. One-pot synthesis of the BTDT semiconductor core.
6430 subfemtoamperometer and a Keithly 2400 source meter,
operated by a local Labview program and GPIB communica-
tion. Key device parameters, such as charge carrier mobility (μ)
and on-to-off current ratio (Ion/Ioff), were extracted from the
3.2. BTDT Derivative Thermal and Optical Properties.
Differential scanning calorimetry (DSC) data for BT-
BTDT exhibit sharp endotherms above 258 ꢀC, and
thermogravimetric analysis plots reveal weight loss
(∼5%) only on heating above 255 ꢀC (Table 1). The high
melting temperatures of these new materials indicate
excellent thermal stability. Lower melting points and
lower weight loss temperatures are observed for BTDT
derivatives with smaller molecular weights. The optical
absorption spectra of compounds P-BTDT and BT-
BTDT in C6H5Cl solution (Figure 5) are significantly
red shifted (λmax > 353 nm) compared to that of the
BTDT core (λmax ∼ 315 nm). Compared to phenyl-
substituted P-BTDT, greater π-electron delocalization is
observed for the benzothiophenyl derivative (BT-BTDT)
giving the latter the lowest energy gap among the com-
pounds in this series (Table 1). This result indicates
substantial delocalization extending from the BTDT core
to the substituents. Such delocalization is evident in
Figure 6, where the computed topologies of the frontier
molecular orbitals are depicted. As can be appreciated,
both the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO)
are fully delocalized over the entire π-conjugated skele-
tons for all the systems under study here.
To better understand the optical spectra, time-depen-
dent DFT (TD-DFT) calculations were performed and
the 12 lowest energy levels were estimated. These calcula-
tions are in very good agreement with the experimental
data and predict that the electronic transitions having the
highest oscillator strengths appear at 313, 354, and 389
for BTDT, P-BTDT, and BT-BTDT, respectively. These
transitions can be assigned principally to monoelectronic
HOMO to LUMO excitation. According to the topo-
logies of these orbitals, the electronic transitions do not
involve significant intramolecular charge transfer, which
is also supported by the vibronic structure found in the
optical spectra.
source-drain current (ISD) versus source-gate voltage (VSG
)
characteristics employing standard procedures. Mobilities were
obtained from the formula defined by the saturation regime in
transfer plots, μ = 2ISDL/[CiW(VSG - VT)2 ], where ISD is the
source-drain current, VSG is source-gate voltage, and VT is the
threshold voltage. Threshold voltage was obtained from x
1/2
intercept of VSG vs ISD
plots. Atomic force microscopy
(AFM) measurements were performed using a JEOL 5200
Scanning Probe Microscope (JEOL Ltd. Japan) in the tapping
mode.
2.9. DFT Calculations. Density functional theory (DFT)
calculations on the present semiconductors were performed
using the B3LYP funtional52 and the 6-31G** basis set53 as
implemented in Gaussian 03 program. Vertical electronic
excitation energies were computed using the time-dependent
DFT (TDDFT) approach.54,55 The 12 lowest energy electronic
excitations were computed for all the molecules. TDDFT
calculations were carried out using the B3LYP functional and
the 6-31G** basis set on the previously optimized molecular
geometries obtained at the same level of calculation.
3. Results and Discussion
3.1. Synthesis. The key building block of this semicon-
ductor family is the new BTDT core (Figure 2), and therefore,
a convenient, efficient, and inexpensive one-pot BTDT
synthesis was developed. The synthetic route starts from
inexpensive, commercially available materials, which do not
require the expensive bis(phenyl-sulfonyl)sulfide, used in
the published DTT synthesis.56-58 As shown in Figure 3,
3-bromothiophene is first lithiated with n-BuLi, followed by
S8 and then by TsCl addition. Next, the mixture is treated
with 3-lithiumthiophenylbenzene. Without product isola-
tion, the crude mixture is subsequently dilithiated with n-
BuLi and ring closure is achieved with CuCl2 to afford
BDTT in >32% yield. This simple route enables the
synthesis of new asymmetric fused rings which are not
accessible using previous fused-ring synthetic routes.59
The syntheses of functionalized BTDTs were achieved
as shown in Figure 4. Deprotonation of BTDT with
The HOMO-LUMO energy gaps calculated from
the onset of the experimental optical absorption (2.99-
3.44 eV) increase in the order of BT-BTDT < P-BTDT <
BTDT. The same trend is found in the computational
results, even if the theoretical HOMO-LUMO gaps are
estimated to be somewhat larger (3.13-4.24 eV). These
BTDT energy gaps are obviously larger than that of
pentacene (1.77 eV). The larger optical bandgaps imply
that these compounds are not easily oxidized and may
have better stability in air. The photooxidative stability of
the BTDT derivatives was also investigated by monitor-
ing the absorbance decay at λmax of P-BTDT and
(52) Becke, A. D. J. Chem. Phys. 1993, 98, 1372.
(53) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon,
M. S.; Defrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654.
(54) Gross, E. K. U.; Ullrich, C. A.; Gossmann, U. J. In Density
Functional Theory; Gross, E. K. U., Dreizler, R. M., Eds.; NATO
Advanced Study Institute Series B: Physics; Plenum: New York, 1995;
Vol. 337, pp 149-171.
(55) Chong, D. P. Recent Advances in Density Functional Methods;
World Scientific: Singapore, 1995; p 115.
(56) Jong, F. D.; Janssen, M. J. J. Org. Chem. 1971, 36, 1645.
(57) Allared, F.; Hellberg, J.; Remonen, T. Tetrahedron Lett. 2002, 43,
1553.
(58) Frey, J.; Bond, A. D.; Holmes, A. B. Chem. Commun. 2002, 20,
2424.
(59) See Experimental Section for details.

5036 Chem. Mater., Vol. 22, No. 17, 2010
Youn et al.
Figure 4. Synthetic routes to BTDT semiconductors.
Table 1. Thermal, Optical Absorption/Emission, and Electrochemical Data for BTDT
TGA
(ꢀC, 5%)
UV-visa
reduction
potential (V)b
oxidation
potential (V)b
Vgap (eV)
(optical)a
compound
DSC Tm (ꢀC)
λ
max (nm)
(DPV)b
BTDT
P-BTDT
BT-BTDT
175
258
341
175
255
309
315
353
378
-1.89
-1.93
-1.86
1.5
1.33
1.3
3.44
3.17
2.99
3.39
3.26
3.16
a in C6H5Cl. b by DPV in C6H4C12 at 25 ꢀC.
Figure 5. Optical spectra of BTDT derivatives in C6H5Cl solution.
Figure 7. DPV-derived HOMO and LUMO energy levels of BTDT
semiconductors.
conjugative effects of the aryl substituents. The electroche-
mically derived HOMO-LUMO energy gaps obtained
from the DPV data are ranked in the order of BT-BTDT
(3.16 eV) < P-BTDT (3.26 eV) < BTDT (3.39 eV; Figure 7;
assuming ferrocene/ferrocenium oxidation at 4.8 eV), which
is consistent with the values obtained from optical spectros-
copy: BT-BTDT (2.99 eV) < P-BTDT (3.17 eV) < BTDT
(3.44 eV; Table 1). The electronically derived HOMO
energy of the BTDTs is significantly lower than that of
pentacene (-5.14 eV). The combination of the relatively low
HOMO energies and large band gaps of BTDT series
suggests that these are some of the most environmentally
stable oligoacene semiconductors.
3.4. Semiconductor Film Growth. The TFT properties of
P-BTDT and BT-BTDT were investigated on doped Si
(gate)/SiO2 (gate insulator) with several different gate di-
electric surface treatments. Hexamethyldisilazane (HMDS)-
modified substrates were prepared by exposing the Si/SiO2
substrates to HMDS vapor for 7 days in a nitrogen atmo-
sphere to yield a trimethylsilyl-coated surface. Octadecyltri-
chlorosilane (OTS)-modified substrates were fabricated by
immersion of the Si/SiO2 substrates in 3.0 mM dry toluene
Figure 6. B3LYP/6-31G** electronic density contours for the frontier
molecular orbitals of the molecules under study.
BT-BTDT in aerated C6H5Cl solutions exposed to the
white light of a fluorescent lamp at room temperature.
Under these conditions, no decomposition is observed after
4 to 5 days for any of these compounds, demonstrating the
good stability of these materials.
3.3. Electrochemical Characterization. Differential pulse
voltammograms (DPVs) of the present BTDT derivatives
were recorded in dichlorobenzene at 25 ꢀC, and the resulting
reduction and oxidation potentials are summarized in
Table 1. The DPV of P-BTDT exhibits an oxidation peak
at þ1.33 V and a reduction peak at -1.93 V (using ferro-
cene/ferrocenium as the internal standard at þ0.6 V). As
expected, the oxidation potentials of BT-BTDT (Eox
=
þ1.30 V, Ered = -1.86) are shifted to more negative values
than those for P-BTDT, while the reduction potentials shift
to less negative values, which can be attributed to the

Article
Chem. Mater., Vol. 22, No. 17, 2010 5037
Figure 8. Crystal structure of P-BTDT. (A) Side view of the molecular stacking where the BTDT cores adopt an edge-to-face packing with a herringbone
˚
angle of 49.2ꢀ. (B) Front view and S-S/S-C/C-C/H-C distances in A. (C, D, E) The side view, the front view, and the top view in space-filling models,
respectively.
solutions of the silane reagent under nitrogen for 10 h. All
substrates were characterized by advancing aqueous contact
angle measurements, which indicate increasing hydrophobi-
city in the order of SiO2 (<5ꢀ), HMDS (97ꢀ), and OTS
(104ꢀ). Additionally, the surface roughness was evaluated by
tapping mode AFM, revealing a root-mean square (rms)
roughness of 0.15 nm for SiO2, 0.20 nm for HMDS, and
0.47 nm for OTS.
aligned with the phenyl units packed on the same side
(Figure 8B).60 The shortest intermolecular S C dis-
tance between the edge-to-face P-BTDT molecules is
3 3 3
˚
approximately 3.377 A, with 3.665 A for the shortest
˚
intermolecular S S distance (Figure 8B). The molecu-
lar sheets are stacked along the b-axis direction and form
3 3 3
˚
a columnar array with an interplanar separation of 3.9 A.
Between the stacks, this interaction is reinforced by the
close interplanar S S contacts. In contrast, the theore-
All semiconductor films (∼50 nm thick) were vapor-
deposited while maintaining the substrates at the tem-
peratures (Td) of 25, 50, 80, and 110 ꢀC and with a
3 3 3
tical calculations predict this molecule to be somewhat
more twisted, with a dihedral angle between the phenyl
ring and the BTDT moiety of ∼25ꢀ. This disagreement
may be explained by the fact that the calculations simu-
late an isolated molecule in vacuum while strong inter-
molecular interactions are clearly present in the crystal
structure, which are not considered in the calculations.
With the crystal structure data in hand, it is straightfor-
ward to simulate the XRD powder pattern and, therefore,
assign the reflections observed in the film XRD measure-
ments (Figure 9).61 The reflections from the simulated
(random crystallite orientation) powder patterns that can
be readily identified in the θ-2θ scans and the 2θ values of
the experimentally observed reflections are summarized
in Table 2. Figure 9 shows a typical graphical comparison
of the experimental and simulated data, with a 2θ scan for
a P-BTDT film grown at Td = 40 ꢀC on bare SiO2
substrate and a P-BTDT powder pattern generated from
the single-crystal data. The (0 0 l) reflection family is
particularly pronounced, from (0 0 2) to (0 0 10). The
(0 0 2) reflection in the film XRD is observed at 2θ = 5.7ꢀ,
˚
deposition flux rate of 0.1 A/s. For P-BTDT, an addi-
tional Td = 40 ꢀC with two-stage deposition flux rates of
˚
˚
0.1 A/s and 0.4 A/s was also investigated. All films were
characterized by θ-2θ X-ray diffraction (XRD) and by
tapping mode AFM.
3.5. Single Crystal versus Polycrystalline Film XRD
Analysis. Before discussing the XRD results for the BTDT
films, it is useful to begin by correlating film X-ray θ-2θ
scan data for the compound of known crystal structure.
This initial study allows a much more thorough analysis of
the molecular ordering in these compounds. The diffrac-
tion-derived single-crystal structure of P-BTDT is shown
in Figure 8, and crystal structure determination data are
summarized in the Supporting Information (Table S1).
Similar to other fused thiophenes,22,28,29,31 the unit cell
of P-BTDT exhibits the commonly observed herringbone
packing motif (Figure 8A). The fused phenylene moiety is
nearly coplanar with the three fused thiophenes, with a
dihedral angle of 2.6ꢀ, while the dihedral angle between
the phenyl moiety and the BTDT plane is 8.2ꢀ. P-BTDT
molecules adopt an edge-to-face (herringbone) packing
motif with an intermolecular packing angle of approxi-
mately 49.2ꢀ. The BTDT cores in P-BTDT are well
˚
corresponding to a d spacing of 15 A (Table 2), approxi-
mately one-half the length of the unit cell c axis. This set of
(0 0 l) reflections is predominant, even for P-BTDT films
grown at room temperature (Figure S1A, Supporting
Information). This indicates that the films are highly
textured and that the P-BTDT molecules in the films
(60) Although the present molecule has an unusual unsymmetrical
structure, no phenyl to fused phenylene (C6H5 to C6H4) π-π
stacking is evident in the crystal packing. Thus, apparently, the
low symmetry of this molecule does not affect the crystallization
arrangement.
(61) Facchetti, A.; Mushrush, M.; Yoon, M. H.; Hutchison, G. R.;
Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 13859.

5038 Chem. Mater., Vol. 22, No. 17, 2010
Youn et al.
Figure 10. θ-2θ XRD scan of a BT-BTDT film grown at 80 ꢀC on a bare
Si/SiO2 substrates.
Figure 9. Comparison of the θ-2θ XRD scan of a P-BTDT film grown at
40 ꢀC on bare Si/SiO2 (A) with the simulated powder pattern (B).
P-BTDT crystal structure. BT-BTDT thin films depo-
sited at 80 ꢀC on bare Si/SiO2 exhibit only the family of
(0 0 l) reflections (Figure.10). These data indicate good
crystallinity of the film with long-range order. The (0 0 2)
reflection in the film XRD is observed at 2θ = 5.0ꢀ,
Table 2. Thin Film X-Ray Diffraction Data for Semiconductors
substrate
2θ
d spacing (A)
(XRD)
compound temperature (ꢀC) (deg)
(DFT cal.)
P-BTDT
BT-BTDT
25, 50
25, 50
5.69
5.00
15.5
17.7
15.1
17.0
˚
corresponding to a d spacing of 17.7 A (Table 2). This
result is coincident with a computed molecular length of
˚
17.0 A from the DFT calculations. The fact that the
are predominantly aligned with their long molecular axes
along the substrate normal; that is that film growth is
favored in the c direction. It is also apparent that higher
deposition/annealing temperatures make this c-direc-
tional alignment even more favorable, as indicated by
sharpening and increased intensity of the (0 0 l) reflections
in the θ-2θ scans of the corresponding films.
One question that arises is the absence in the θ-2θ scans
of reflection belonging to (0 k 0). The lowest observable
reflections (0 2 0) would be observed at 2θ = 23.42ꢀ
according to the simulated powder diffraction pattern.
calculated molecular length is so similar to the d spacing
from the thin film XRD data suggests that the orientation of
molecules is almost perpendicular to the dielectric surface,
providing an ideal π-π stacking arrangement for charge
transport. Furthermore, as Td is increased, the reflection
intensities become more pronounced (Figure S1B, Support-
ing Information). The effect of the surface treatment is again
relatively small when compared to microstructural variation
with Tds (Figure S2B, Supporting Information).
3.6. Organic Thin Film Transistor Fabrication and
Characterization. Thin film transistors were fabricated
in bottom gate-top contact configurations. Highly doped
p-type (100) silicon wafers were used as gate electrodes as
well as substrates, and 300 nm SiO2 thermally grown on
the Si was used as the gate insulator. Organic semicon-
ductor thin films (50 nm) were vapor-deposited onto the
Si/SiO2, HMDS-treated, and OTS-treated substrates as
described previously. Next, 50 nm gold source and drain
electrodes were vapor-deposited at 2 ꢀ 10-6 Torr through
a shadow mask in a high vacuum deposition chamber.
Devices were fabricated with typical channel lengths of
25, 50, and 100 μm, with a channel width of 2000 μm.
Current-voltage (I-V) transfer and output plots were
measured for each device under vacuum and in air. To
illustrate the precision of each measurement, the reported
data are an average of at least three devices tested at
different regions of the semiconductor layer. Key device
performance parameters, such as field-effect carrier mo-
bility (μ), threshold voltage (VT), and on-to-off current
ratio (Ion/Ioff), were extracted using standard proce-
dures.62 The results are summarized in Table 3.
˚
This corresponds to d spacing of 3.80 A, approximately
one-half of the unit cell b axis. Although the (0 2 0)
reflection is predicted in the simulated powder pattern
to be intense, the absence of this peak in the film XRD of
P-BTDT is likely due to the predominant ordering in the
c direction.
Over the entire Td range (25-50 ꢀC), the P-BTDT films
deposited on bare SiO2 substrates exhibit reflections at
the same values of 2θ (Figure S1A, Supporting Infor-
mation). As the deposition temperature is increased, the
peaks become sharper, and their intensities increase with
increasing Td (Figure S1A, Supporting Information). The
consistency in reflection positions indicates the presence
of a single polymorph and growth orientation across the
Td range.3 The increase in relative intensity of the higher
order peaks on going from room temperature to higher
growth temperatures indicates enhancements in the long-
range order (Figure S1A, Supporting Information). The
effect of surface treatment is relatively minor compared to
the microstructural variation arising from different Tds
(Figure S2A, Supporting Information).
Although the single crystal structure of BT-BTDT was
not determined, it is reasonable to assume that they have
thin film microstructures similar to that of P-BTDT.
Therefore, their peak orientations were assigned based
on the simulated powder pattern extracted from the
Since device parameters measured in air are statistically
identical to those measured in vacuum, only the ambient
(62) Sze, S. M. Physics of Semiconductor Devices, 2 ed.; John Wiley &
Sons: Hoboken, NJ, 1981.

Article
Chem. Mater., Vol. 22, No. 17, 2010 5039
Table 3. Device Performance of BTDT Series
tion, S3.)64,65 In the present case, the reorganization
energies are quite similar for all the semiconductors,
0.289 and 0.267 for P-BTDT and BT-BTDT, respectively.
These values are larger than that reported for pentacene66
(0.097 eV) or for other polycyclic benzene-thiophene
structures67 (0.118-0.204 eV) but are slightly smaller
than that of pentathienoacene68 (0.306 eV). From the
theoretical results, it is apparent that neither trends in
reorganization energies nor those in HOMO energies
correlate with electrical performance. This argues
that film morphological and microstructural charac-
teristics rather than isolated molecule properties
dominate TFT performance trends within this series
of semiconductors.
3.7. Semiconductor Film Morphology and TFT Perfor-
mance. The surface morphology of organic semiconduc-
tor thin films is often used to evaluate grain size and
crystalline microstructure. The relevance of these data to
TFT performance assumes that the surface microstruc-
ture is also characteristic of the thin film interface with the
substrate/gate dielectric which is the active region for
charge transport.69 The highest carrier mobilities are
obtained for films having the appropriate balance of large
grain size and space-filling grain connectivity. The thin
film morphology of BTDT films grown on different
substrates was characterized by tapping-mode AFM,
and the results highlight the effects of dielectric surface
treatment, substrate temperature, and deposition flux
rate. First, for all present semiconductors, the grain size
increases when Td is increased. Second, although a hydro-
phobic surface usually results in large grains,39 the trend is
not obvious in the present BTDT family. Third, higher
deposition flux rates produce smaller grains; however, the
crystalline domains are well interconnected.
In the case of P-BTDT, films deposited on bare SiO2
surfaces have a quite different film morphology com-
pared with films deposited on alkyl silane SAM-treated
surfaces (Figure 12A). Although the grain sizes are
comparable to that on hydrophobic surfaces, the films
deposited on bare SiO2 exhibit larger voids between
grains. The sizes of these void structures increase with
the grain size when the films are deposited at higher
substrate temperatures. For films deposited at 50 ꢀC,
the grains are disconnected, so that devices fabricated
on these films do not exhibit measurable response
(Table 3). If the poor film interconnectivity is the source
of this low response, then device performance might be
significantly improved by filling these voids with semi-
conductor. To test this hypothesis, the two-stage deposi-
tion method was introduced in this study. Thus, P-BTDT
air
substrate
semi-
temperature
surface
mobility
threshold
conductor
T
d (ꢀC)
treatment (cm2 V-1 s-1
)
voltage (V) Ion/Ioff
P-BTDT
25
bare
0.04
0.04
0.07
0.02
0.05
0.70
NAa
0.07
0.11
0.01
0.02
0.01
0.01
0.03
0.02
0.008
0.02
0.02
-20
-23
-29
-20
-24
-41
2.8 ꢀ 107
1.4 ꢀ 107
3.3 ꢀ 107
6.3 ꢀ 106
2.6 ꢀ 107
1.2 ꢀ 108
HMDS
OTS
40
50
25
50
80
bare
HMDS
OTS
bare
HMDS
OTS
-22
-22
-35
-36
-33
-32
-35
-42
-26
-31
-19
1.2 ꢀ 106
7.6 ꢀ 106
3.1 ꢀ 105
2.2 ꢀ 105
1.2 ꢀ 106
5.3 ꢀ 104
4.6 ꢀ 105
1.1 ꢀ 105
2.0 ꢀ 106
1.8 ꢀ 106
6.6 ꢀ 106
BT-BTDT
bare
HMDS
OTS
bare
HMDS
OTS
bare
HMDS
OTS
a NA = not active.
results are presented in the table. Interesting insights can
be drawn from these data. First, hydrophobic dielectric
surface treatment, especially OTS treatment, yields
the best p-type device performance. Second, TFT para-
meters including mobility (μ) and on-to-off current ratio
(Ion/Ioff) are enhanced as the deposition temperature
increases. Third, within the BTDT series, P-BTDT ex-
hibits the best p-type TFT performance metrics with
mobility = 0.70 cm2 V-1 s-1, threshold voltage (Vth) =
-41 V, and on-to-off current ratio = 1.2 ꢀ 108 in air
(Figure 11). This superior device performance is obtained
by optimization of three different semiconductor film
growth conditions: the gate dielectric surface treatment,
Td, and the film growth flux rate. Films deposited on
OTS-functionalized substrates all afford the best device
performance at the majority of the Tds investigated. For
P-BTDT, the mobility increases from 0.11 cm2 V-1 s-1
for semiconductor films deposited at Td = 25 ꢀC to
0.70 cm2 V-1 s-1 for films deposited at Td = 40 ꢀC.63
In order to better understand the electrical perfor-
mance within this semiconductor series, intramolecular
reorganization energies were estimated theoretically,
using the DFT approach. The intramolecular reorganiza-
tion energy (λ) describes the structural reorganization
energetics required to accommodate delivered charges in
transport. It consists of two terms associated with the
geometric relaxation energies upon going from the neu-
tral to the charged species and vice versa and is calculated
directly from the adiabatic potential energy surfaces, as
described elsewhere. (See details in Supporting Informa-
˚
films were first deposited at 0.1 A/s (10 nm) and next at
(66) Coropceanu, V.; Malagoli, M.; da Silva, D. A.; Gruhn, N. E.; Bill,
T. G.; Bredas, J. L. Phys. Rev. Lett. 2002, 89, 275503.
(67) Coropceanu, V.; Kwon, O.; Wex, B.; Kaafarani, B. R.; Gruhn,
N. E.; Durivage, J. C.; Neckers, D. C.; Bredas, J. L. Chem.;Eur. J.
2006, 12, 2073.
(68) Kim, E. G.; Coropceanu, V.; Gruhn, N. E.; Sanchez-Carrera, R. S.;
Snoeberger, R.; Matzger, A. J.; Bredas, J. L. J. Am. Chem. Soc.
2007, 129, 13072.
(63) Standard deviations between five data were <20%.
(64) Malagoli, M.; Bredas, J. L. Chem. Phys. Lett. 2000, 327, 13.
(65) (a) Delgado, M. C. R.; Pigg, K. R.; Filho, D.; Gruhn, N. E.;
Sakamoto, Y.; Suzuki, T.; Osuna, R. M.; Casado, J.; Hernandez, V.;
Navarrete, J. T. L.; Martinelli, N. G.; Cornil, J.; Sanchez-Carrera,
R. S.; Coropceanu, V.; Bredas, J. L. J. Am. Chem. Soc. 2009, 131,
1502. (b) Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem.
Soc., 2005, 2339, 16866.
(69) Jones, B. A.; Facchetti, A.; Wasielewski, M. R.; Marks, T. J. Adv.
Funct. Mater. 2008, 18, 1329.

5040 Chem. Mater., Vol. 22, No. 17, 2010
Youn et al.
Figure 11. Transfer and output plots of an OFET device fabricated from P-BTDT films grown on an OTS-coated substrate: (A) transfer plot; (B) output
˚
plot. Substrate temperature = 40 ꢀC, deposition flux rate = 0.1 A/s, channel length = 25 μm, and channel width = 2000 μm. The device performance is as
follows: μ = 0.70 cm2 V-1 s-1, Vth = -41 V, and Ion/Ioff = 1.2 ꢀ 108.
Figure 12. AFM images of P-BTDT films grown on different substrates and at different temperatures: (A) bare SiO2, (B) SiO2 with HMDS treatment, and
(C) SiO2 with OTS treatment (tapping mode, topography, 9 μm ꢀ 9 μm).
˚
0.4 A/s (40 nm) on bare SiO2. As judged by AFM images,
this approach significantly minimizes the grain boundary
region in the semiconductor films (Figure 13).
Films deposited on OTS at Td = 40 ꢀC exhibit a good
combination of large grain and film interconnectivity,
resulting in TFTs exhibiting the highest performance
metrics (Table 3).
Due to the enhanced film interconnectivity (Table 4), the
TFT mobility increases from μ = 0.04 cm2 V-1 s-1 to μ =
0.11 cm2 V-1 s-1 for Td = 25 ꢀC and from inactive to μ =
0.02 cm2 V-1 s-1 for Td = 50 ꢀC. Films on HMDS- and
OTS-treated substrates have quite similar film morpholo-
gies (Figure 12B,C). P-BTDT is a representative example
of the pronounced growth temperature sensitivity of film
microstructure. Within a small deposition temperature
range (25-50 ꢀC), the film morphology changes signifi-
cantly. As the substrate temperature increases, so does
the grain size, and for Td = 50 ꢀC, voids begin to appear in
the film. The device performance degrades as a result.
BT-BTDT films deposited at 25 ꢀC (Figure S3, Sup-
porting Information) exhibit similar surface morpho-
logies when grown on HMDS-treated and bare Si/SiO2
substrates. Very large crystalline domains (>3 μm) are
observed in the films. However, the grain size of the BT-
BTDT films on OTS-treated substrates is far smaller at all
deposition temperatures. Considering that BT-BTDT
TFT performance on all substrates is comparable, it is
assumed that the smaller grain size observed on OTS is
compensated by the well-connected film morphology. In
addition, for all substrates, the mobility falls when Td is

Article
Chem. Mater., Vol. 22, No. 17, 2010 5041
an ambient laboratory, P-BTDT-derived devices still exhibit
μ = 0.3 cm2 V-1 s-1 and Ion/Ioff = 107.
4. Conclusions
A family of new benzo[d,d0]thieno[3,2-b;4,5-b0]dithiophene-
based semiconductors was synthesized and characterized.
TFT devices fabricated from these molecules exhibit good
device performance with a good air stability. Various combi-
nations of surface treatment methods, substrate temperature,
and deposition flux rate sequences have significant effects on
device performance. Films deposited on OTS-treated SiO2
substrates under properly adjusted substrate temperature and
deposition flux rate, achieve an efficacious compromise be-
tween high film crystallinity and good film grain interconnec-
tivity, resulting in high OFET performance, with mobility
greater than 0.70 cm2 V-1 s-1 and Ion/Ioff greater than 108.
Further investigations on modifications of these molecules to
better understand the structure/property relationships are
currently underway.
Figure 13. Comparison of AFM images of P-BTDT films grown on bare
˚
SiO2: (A) slow deposition (50 nm film deposited at a growth rate = 0.1 A/
˚
s) and (B) two-stagedepositionprocess (10nmat0.1 A/s, 40 nmat 0.4 A/s;
˚
Acknowledgment. We thank AFOSR (FA9550-08-01-
0331) and the NSF-MRSEC program through the North-
western Materials Research Center (DMR-0520513) for
support of this research at Northwestern University. Finan-
cial assistance for this research was also provided by the
National Science Council, Taiwan, Republic of China
(Grant Numbers NSC98-2628-M-008-003, NSC97-2113-
M-008-003, and NSC98-2627-E-006-003) and partially
provided by Industrial Technology Research Institute of
Taiwan under contract number ITRI 99-B-069351AA5130.
R.P.O. acknowledges funding from the European Community’s
Seventh Framework Programme under Grant Agreement
234808.
tapping mode, topography, 9 μm ꢀ 9 μm).
Table 4. TFT Performance of P-BTDT Grown via Two-Stage Deposition
air
substrate
temperature
surface
mobility
threshold
compound
P-BTDT
T
d (ꢀC)
treatment (cm2 V-1 s-1
)
voltage (V) Ion/Ioff
25
bare
0.11
0.05
0.13
0.02
0.07
0.12
-29
-22
-30
-21
-31
-31
2.9 ꢀ 107
2.0 ꢀ 106
1.8 ꢀ 107
3.5 ꢀ 106
5.7 ꢀ 106
2.9 ꢀ 106
HMDS
OTS
50
bare
HMDS
OTS
increased from 50 to 80 ꢀC. The size and depth of grain
boundary regions in these highly crystalline films must
have diminished the film interconnectivity.
3.8. Shelf Life Test. To monitor the stability of the
present TFTs, shelf life tests for about 2 months were carried
out. In agreement with the optical and thermal data,
P-BTDT films are stable in air. After 2 months of shelf life in
Supporting Information Available: Table of crystal structure
data for P-BTDT; θ-2θ X-ray diffraction scans of P-BTDT and
BT-BTDT films deposited at the indicated Td values and on the
indicated substrates; reorganization energies; AFM images of
BT-BTDT films grown on various substrates (PDF). This
material is available free of charge via the Internet at http://
pubs.acs.org.