Organic thin-film transistor properties and the structural relationships between various aromatic end-capped triisopropylsilylethynyl anthracene derivatives

Article abstract of DOI:10.1016/j.orgel.2010.01.024

We describe the preparation and characterization of single crystals and thin films composed of three new small molecules based on soluble triisopropylsilylethynyl (TIPS)-substituted anthracene (TIPSAnt) for use in organic thin-film transistors (TFTs); the three molecules examined were TIPSAnt derivatives containing thiophene (TIPSAntT), benzothiophene (TIPSAntBeT), or phenyl thiophene (TIPSAntPT) end cappers. We elucidated the relationship between the molecular structures and TFT performances by measuring the thermal, optical, electrochemical, crystalline, and morphological properties of these molecules. TIPSAntPT was able to achieve a bricklayer type of molecular stacking structure, facilitated by the bulky phenyl thiophene substituents. This is the first report of a bricklayer stacking structure of triisopropylsilylethynyl anthracene derivatives. Among the three molecules, TIPSAntPT showed the highest TFT mobility, 0.063 cm²/Vs for the film state and 0.12 cm²/Vs for single crystals. By contrast, in the case of in TIPSAntBeT, the introduction of the rigid substituent group led to poor TFT device performance. TFT device performances could be explained in terms of thin-film morphology, investigated by atomic force microscopy (AFM) and X-ray diffraction (XRD) analysis.

Full text of DOI:10.1016/j.orgel.2010.01.024

Organic Electronics 11 (2010) 820–830
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Organic thin-film transistor properties and the structural relationships
between various aromatic end-capped triisopropylsilylethynyl
anthracene derivatives
Jong-Hwa Park a, Dong Hoon Lee b, Hoyoul Kong a, Moo-Jin Park c, In Hwan Jung a,
a,
Chan Eon Park b, , Hong-Ku Shim
*
*
a Department of Chemistry, Korea Advanced Institute of Science and Technology, Guseong-Dong, Yuseong-Gu, Daejeon 305-701, Republic of Korea
b Department of Chemical Engineering, Pohang University of Science and Technology, Hyoja-Dong, Nam-Gu, Pohang, Gyungbuk 790-784, Republic of Korea
c Polymer Science and Engineering Program, Korea Advanced Institute of Science and Technology, Guseong-Dong, Yuseong-Gu, Daejeon 305-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We describe the preparation and characterization of single crystals and thin films
composed of three new small molecules based on soluble triisopropylsilylethynyl (TIPS)-
substituted anthracene (TIPSAnt) for use in organic thin-film transistors (TFTs); the three
molecules examined were TIPSAnt derivatives containing thiophene (TIPSAntT), benzothio-
phene (TIPSAntBeT), or phenyl thiophene (TIPSAntPT) end cappers. We elucidated the rela-
tionship between the molecular structures and TFT performances by measuring the
thermal, optical, electrochemical, crystalline, and morphological properties of these mole-
cules. TIPSAntPT was able to achieve a ‘‘bricklayer” type of molecular stacking structure,
facilitated by the bulky phenyl thiophene substituents. This is the first report of a ‘‘brick-
layer” stacking structure of triisopropylsilylethynyl anthracene derivatives. Among the
three molecules, TIPSAntPT showed the highest TFT mobility, 0.063 cm2/Vs for the film
state and 0.12 cm2/Vs for single crystals. By contrast, in the case of in TIPSAntBeT, the intro-
duction of the rigid substituent group led to poor TFT device performance. TFT device per-
formances could be explained in terms of thin-film morphology, investigated by atomic
force microscopy (AFM) and X-ray diffraction (XRD) analysis.
Received 23 November 2009
Received in revised form 15 January 2010
Accepted 28 January 2010
Available online 4 February 2010
Keywords:
Organic thin-film transistor
Anthracene
Charge-carrier mobility
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
Apart from polymer-based OTFTs, a variety of small
molecule OTFTs have been reported to be solution-
Many classes of
p
-conjugated organic small molecules
processable. Among these, triisopropylsilylethynyl (TIPS)-
substituted pentacene derivatives are considered to be
most promising candidates [7,8]. Anthony et al. developed
the TIPS-pentacene system by introducing bulky TIPS
groups into highly crystalline, insoluble, and unstable
pentacene at the peri position, and, surprisingly, the bulky
groups improved not only the solubility and stability, but
also the crystallinity of the molecules [9]. The single crys-
tals packed in either a ‘‘bricklayer” structure (each mole-
and polymers have been developed into semiconductors
for organic thin-film transistors (OTFTs) over the past
several decades [1–3]. Recently, interest in solution-
processable materials that enable inexpensive device fabri-
cation has increased, and remarkable achievements have
been reported [4]. By using simple solution techniques,
such as spin-coating, drop-casting, or inkjet-printing,
OTFTs can be fabricated at prices that are comparable to
conventional inorganic transistors [5,6].
cule engaged in
or a ‘‘slipped-stacked” structure (each molecule engaged
in -overlap with only one adjacent molecule), depending
p-overlap with two adjacent molecules)
p
* Corresponding authors.
E-mail addresses: cep@postech.ac.kr (C.E. Park), hkshim@kaist.ac.kr
on the relative bulkiness of the substituents of the silyle-
thynyl group with respect to the length of the conjugated
(H.-K. Shim).
1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2010.01.024

J.-H. Park et al. / Organic Electronics 11 (2010) 820–830
821
core [9]. It was found that the ‘‘bricklayer” structure
showed much better TFT performance than the ‘‘slipped-
stacked” structure, and that the bulkiness of the substitu-
ents (relative to the molecular size) which produced the
bricklayer structure was required to fall within a narrow
range; If the size of the substituents was too large or too
small, a ‘‘slipped-stacked” or ‘‘herringbone” structure oc-
curred, which showed poorer TFT performance. Based on
this strategy, many small molecules yielding promising
TFT performances have been developed [10–12].
In 2007, our group reported the development of prom-
ising TIPS anthracene derivatives (TIPSAnts) using the
anthracene moiety in place of pentacene [13]. Anthracene
is much more stable with respect to Diels–Alder reactions,
and this stability prevents butterfly dimer decomposition,
improves processability, and yields high crystallinity [14–
16]. Small molecules containing TIPSAnt showed promising
optical, electrochemical, and crystalline properties that re-
sulted in moderate charge-carrier mobilities. However, to
achieve higher TFT performances, further improvements
were required. Because TIPSAnt contains dibromo moieties,
it can serve as a soluble oligomer core for aromatic cou-
pling reactions. The synthetic functionalities of TIPSAnt
led to the development of a variety of new TIPSAnt-based
small molecules.
internal reference. For the NMR measurements, chloro-
form-d (CDCl3) was used as the solvent for the materials
except TIPSAntBeT (1,1,2,2,-tetrachloroethane-d2). Elemen-
tal analysis was performed using EA-1110-FISONS elemen-
tal analyzer. UV–vis spectra were measured by using a
Jasco V-530 UV/vis Spectrometer. Thermogravimetric anal-
ysis (TGA) measurement was performed under nitrogen
atmosphere at a heating rate of 10 °C/min using TA Q500
instrument, and the differential scanning calorimetric
(DSC) measurement was made using TA Q100 instrument
operated under nitrogen atmosphere. Cyclic voltammetry
(CV) measurement was performed on an AUTOLAB/
PGSTAT12 at room temperature with a three-electrode cell
in a solution of 0.1 M tetrabutylammonium tetrafluorobo-
rate (TBABF4) in anhydrous acetonitrile at room tempera-
ture under nitrogen gas with a scan rate of 50 mV/s. The
working electrode was the oligomer coated platinum, the
counter electrode was a platinum wire, and the reference
electrode was Ag/AgNO3 (0.10 M) which was separated
by a diaphragm. The measurements were calibrated using
ferrocene as a standard. The single crystal for X-ray analy-
sis was typically grown from hexane and chlorobenzene.
Analyses were performed on a Bruker SMART Apex II X-
ray Diffractometer. Data have been submitted as .cif files.
AFM (Multimode IIIa, Digital Instruments) operating in
tapping-mode was used to image surface morphology of
our semiconductor. Synchrotron X-ray diffraction analyses
of PAT4 films were performed at the 10C1 beam line
(wavelength ꢀ1.54 Å) of the Pohang Accelerator Labora-
tory (PAL). Top-contact OTFTs were fabricated on a com-
mon gate of highly n-doped silicon with a 300 nm thick
thermally grown SiO2 dielectric layer. Substrates were
modified with octyltrichlorosilane from a toluene solution
for 2 h at room temperature. Films of organic semiconduc-
tor were spin-coated at 2000 rpm from 0.7 wt.% chloro-
form or chlorobenzene solution, with nominal thickness
45 nm. Gold source and drain electrodes were evaporated
on the top of semiconductor (100 nm). For all measure-
Recently, we reported single crystal TFT performances
of TIPSAnt derivatives by incorporating naphthalene and
bithiophene moieties as counterparts of TIPSAnt [17]. De-
vice performance was increased by increasing the molecu-
lar surface area available for
p-stacking overlap. However,
TIPSAntNa and TIPSAntBT single crystals showed only a
‘‘slipped-stacked” structure.
In this paper, we report three new small molecules
based on TIPSAnt, 2,6-bis(thiophene-20-yl)-9,10-bis(triiso-
propylsilylethynyl)anthracene (TIPSAntT), 2,6-bis(benzo[b]
thiophene-20-yl)-9,10-bis(triisopropylsilylethynyl)anthra-
cene (TIPSAntBeT), and 2,6-bis(50-phenyl-thiophene-20-yl)-
9,10-bis(triisopropylsilylethynyl)anthracene (TIPSAntPT).
We systematically designed these new small molecules
to elucidate the relationship between thin-film structure
and TFT device performance for aromatic end-capped TIP-
SAnt derivatives. Thiophene, benzothiophene, and phenyl
thiophene were used as end cappers for the TIPSAnt series.
Two structural factors were considered: (1) the increase in
the rigidity caused by fusing the aromatic rings, and (2) the
ments, we used channel lengths (L) of 150
lm and channel
widths (W) of 1500 m. Electrical characteristics of the
l
TFTs were measured in air using both Keithley 236
source/measure units. The carrier mobility was calculated
in the saturation regime from the slope of a plot of the
square root of the drain current versus gate voltage (VG)
by fitting the data to the following equation: IDS = (WCi/
increase in the effective
p-stacking area of the ‘‘bricklayer”
2L)
l
(VG ꢁ VT)2, where IDS is the drain current,
l is the car-
structure. We performed a comparative study to investi-
gate the relationship between the molecular structures
and TFT performances by measuring the thermal, optical,
electrochemical, crystalline, and morphological properties
of these molecules. This study demonstrated, for the first
time, that the preparation of a ‘‘bricklayer” stacking struc-
ture using a TIPSAnt derivative yields high TFT performance.
rier mobility and VT is the threshold voltage. For single
crystal TFTs, saturated chlorobenzene solution of the small
molecules was slowly evaporated at room temperature on
OTS-modified SiO2 substrates. After 1 or 2 days, well-de-
fined rod-type single crystals were obtained and gold elec-
trode was patterned onto the single crystals by thermal
evaporation with thickness of 100 nm. Width and length
of the channels were calculated from optical microscopy
(OM) images. The thickness of single crystals was mea-
sured using an alpha stepper (Alpah Step 500, Tencor).
Chemical reagents were purchased from Aldrich Chem-
ical Co. and were used without further purification. Sol-
vents with analytical-grade were used during the whole
experiments, and all chemicals were used without further
purification. 2,6-Dibromo-9,10-bis-(triisopropylsilylethy-
2. Experimental
2.1. Instruments and reagents
1H and 13C NMR spectra were recorded on a Bruker
AVANCE 400 spectrometer with tetramethylsilane as an

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J.-H. Park et al. / Organic Electronics 11 (2010) 820–830
nyl)anthracene (TIPSAnt) [13], 2-pinacolborylthiophene
(1a) [18], and 2-(40,40,50,50-tetramethyl-10,30,20-dioxaboro-
lane)benzo[b]thiophene (1b) [19], 5-phenyl-1-tributyl
stannyl thiophene (1c) [20] were synthesized according
to the reported procedures.
124.84, 124.77, 123.89, 122.46, 121.02, 119.35, 106.50,
103.41, 19.10, 12.01. Anal. calcd for C52H58S2Si2: C,
77.75%; H, 7.28%; S, 7.98%. Found: C, 77.24%; H, 7.02%; S,
7.68%.
2.2.3. Synthesis of 2,6-bis(50-phenyl-thiophene-20-yl)-9,10-
bis(triisopropylsilylethynyl)anthracene (TIPSAntPT)
2.2. Synthesis
To a 150 mL two neck round-bottom flask equipped
with a stir bar under N2 was added TIPSAnt (0.60 g, 0.86
mmol) and 5-phenyl-1-tributylstannyl thiophene (1c)
(1.16 g, 2.58 mmol). Tris(dibenzylideneacetone)dipalla-
dium (0.016 g, 0.017 mmol) and tri-o-tolyphosphine
(0.021 g, 0.069 mol) was transferred into the mixture in
dry box. Subsequently anhydrous toluene (20 mL) was
added and purged under nitrogen for 30 min. The reaction
mixture was stirred at 120 °C for 3 days and cooled to
room temperature. Then, solvent was evaporated with ro-
tary evaporator and dried at vacuum. The crude product
was column chromatographed with hexane and MC mix-
ture as the eluent, and then crystallized from hexane and
chlorobenzene to yield TIPSAntPT (0.56 g, 75.7%) as bright
orange crystal. 1H NMR (CDCl3, ppm) d = 8.86 (d, 2H),
8.59 (d, 2H), 7.91 (dd, 2H), 7.66 (dd, 4H), 7.49 (d, 2H),
7.41 (t, 4H), 7.35 (d, 2H), 7.30 (t, 2H), 1.32 (m, 42H). 13C
NMR (CDCl3, ppm) d = 144.39, 143.44, 134.24, 132.60,
132.35, 132.22, 128.99, 127.90, 127.67, 125.58, 125.28,
124.94, 124.23, 122.79, 118.51, 105.29, 103.13, 19.04,
11.61. Anal. calcd for C56H62S2Si2: C, 78.63%; H, 7.31%; S,
7.50%. Found: C, 78.52%; H, 7.28%; S, 7.52%.
2.2.1. Synthesis of 2,6-Bis(thiophene-20-yl)-9,10-
bis(triisopropylsilylethynyl)anthracene (TIPSAntT)
Into 150 mL two-neck flask were added TIPSAnt (1 g,
1.43 mmol) and 2-pinacolborylthiophene (1a) (0.66 g,
3.16 mmol) in 15 mL of anhydrous toluene. Catalytic
amount of tetrakis(triphenyl phosphine)palladium(0) was
transferred into the mixture in dry box. Subsequently,
(2 M Na2CO3) sodium carbonate (0.76 g, 7.21 mmol) deaer-
ated for 30 min and the phase transfer catalyst, Ali-
quatÒ336 (0.29 g, 0.72 mmol) in toluene purged under
nitrogen for 1 h was transferred via cannula. The reaction
mixture was stirred at 90 °C for 3 days and cooled to room
temperature. Then, the reaction mixture was poured into
saturated sodium bicarbonate solution and extracted with
EA. The organic layer was washed with brine, and then
dried over anhydrous MgSO4. The crude product was col-
umn chromatographed with hexane and MC mixture as
the eluent, and then crystallized from hexane and MC to
yield TIPSAntT (0.72 g, 71.0%) as bright orange crystal. 1H
NMR (CDCl3, ppm) d = 8.84 (d, 2H), 8.60 (d, 2H), 7.87 (dd,
2H), 7.53 (d, 2H), 7.37 (d, 2H), 7.14 (dd, 2H), 1.31 (m,
42H). 13C NMR (CDCl3, ppm) d = 144.31, 132.57, 132.49,
132.19, 128.25, 127.92, 125.79, 125.72, 123.99, 122.97,
118.53, 105.28, 103.10, 19.00, 11.55. Anal. calcd for
C44H54S2Si2: C, 75.15%; H, 7.74%; S, 9.12%. Found: C,
74.73%; H, 7.58%; S, 8.99%.
3. Results and discussion
3.1. Synthesis of the small molecules
2.2.2. Synthesis of 2,6-bis(benzo[b]thiophene-20-yl)-9,10-
bis(triisopropylsilylethynyl)anthracene (TIPSAntBeT)
The soluble oligomer core, TIPSAnt, was synthesized
according to procedures developed previously [13]. The
bromo groups of TIPSAnt enabled coupling of the molecules
to a variety of borolanylaryl or stannylaryl molecules, for
example, acene, thiophene, or fluorene derivatives. In the
present work, we chose thiophene, benzothiophene, and
phenyl thiophene for TIPSAntT, TIPSAntBeT, and TIPSAntPT,
respectively, as counterparts of TIPSAnt. Aromatic end cap-
pers (1a–1c) were easily synthesized via well-known lithi-
ation methods. TIPSAntT and TIPSAntBeT were synthesized
via a Suzuki coupling reaction, and TIPSAntPT was synthe-
sized via a Stille coupling reaction with high yields, as
shown in Scheme 1. To achieve high purity, we purified
the molecules by column chromatography, followed by
several consecutive recrystallizations. The products were
bright orange highly crystalline solids. The relative solubil-
ities of the resulting products were as follows: TIP-
SAntT > TIPSAntPT > TIPSAntBeT. TIPSAntT showed good
solubility in common organic solvents, such as THF, tolu-
ene, and chloroform, whereas TIPSAntBeT was soluble only
in hot chlorobenzene. TIPS groups improved the solubility
Into 150 mL two-neck flask were added TIPSAnt (1 g,
1.43 mmol) and 2-(40,40,50,50-tetramethyl-10,30,20-dioxa-
borolane)benzo[b] thiophene (1b) (0.86 g, 3.31 mmol) in
20 mL of anhydrous toluene. Catalytic amount of tetra-
kis(triphenyl phosphine)palladium(0) was transferred into
the mixture in dry box. Subsequently, (2 M Na2CO3) so-
dium carbonate (0.76 g, 7.21 mmol) deaerated for 30 min
and the phase transfer catalyst, AliquatÒ336 (0.29 g,
0.72 mmol) in toluene purged under nitrogen for 1 h was
transferred via cannula. The reaction mixture was stirred
at 90 °C for 3 days and cooled to room temperature. Then,
the reaction mixture was poured into saturated sodium
bicarbonate solution and extracted with MC. The organic
layer was washed with brine, and then dried over anhy-
drous MgSO4. The crude product was column chromato-
graphed with hexane and MC mixture as the eluent, and
then crystallized from chlorobenzene to yield TIPSAntBeT
(0.63 g, 55.1%) as bright orange crystal. 1H NMR (1,1,2,2-
tetrachloroethane-d2 at 100 °C, ppm) d = 9.06 (s, 2H), 8.74
(d, 2H), 8.05 (d, 2H), 7.93 (d, 2H), 7.86 (d, 2H), 7.83 (s,
2H), 7.41 (m, 4H), 1.44 (m, 42H). 13C NMR (1,1,2,2-tetra-
chloroethane-d2 at 100 °C, ppm) d = 144.34, 141.05,
140.35, 133.07, 132.98, 132.83, 128.25, 126.00, 124.85,
of the
p-conjugated rings, but as the molecular size and
rigidity increased, the solubility of the molecules de-
creased. The compounds were characterized by 1H NMR,
13C NMR, and elemental analysis.

J.-H. Park et al. / Organic Electronics 11 (2010) 820–830
823
Si
Si
O
S
S
B
S
O
Pd(PPh3)4, Aliquat336, Na2CO3
Toluene/H2O, reflux
1a
S
Si
S
S
O
Si
Si
B
Br
O
TIPSAntT
TIPSAntBeT
1b
Br
Si
Si
Si
Sn(C4H9)3
Pd2(dba)3, P(o-Tol)3
Toluene, reflux
S
S
S
TIPSAnt
1c
TIPSAntPT
Scheme 1. Syntheses of the small molecules.
3.2. Thermal properties
more extensive conjugation. The lowest energy transitions
in the film states were more red-shifted (18–37 nm) than
The thermal stabilities of the small molecules were
characterized by thermogravimetric analysis (TGA) under
a nitrogen atmosphere. TIPSAntT exhibited good thermal
stability, with a decomposition temperature, based on 5%
weight loss (Td5), of 355 °C, whereas TIPSAntBeT and TIPS-
AntPT exhibited excellent thermal stabilities, showing a
Td5 of 416° and 420 °C, respectively. The larger molecular
weights of TIPSAntBeT and TIPSAntPT may have contributed
to their improved thermal stabilities. The thermal transi-
tions of the small molecules were studied by differential
scanning calorimetry (DSC) under a nitrogen atmosphere.
TIPSAntT exhibited liquid crystalline properties, with the
first endothermal peak at 173 °C and the second endother-
mal peak at 290 °C, and corresponding exothermal peaks at
140 °C and 220 °C. TIPSAntBeT and TIPSAntPT exhibited
similar transition behavior, with major melting endo-
therms at 209 and 304 °C, and corresponding crystalliza-
tion exotherms upon cooling at 206 and 262 °C. All peaks
were significant and reversible, without the presence of
additional thermal transitions. From these thermal re-
sponses, we concluded that the small molecules were
highly crystalline and should form well-ordered thin films
when deposited onto a substrate. DSC results are shown in
Fig. 1.
those in solution states for all small molecules (see
D2nd
Max in Table 1). The intensities of the lowest energy tran-
sitions were also larger in the film states compared to the
intensities in the solution states. The red-shifts and in-
creased intensities demonstrated extended
p-conjugation
and substantially more intermolecular ordering within
the 2D stacking of the small molecules in the film state.
The most striking red-shift, observed for TIPSAntPT
(37 nm), indicated that this molecule has the strongest
intermolecular interactions. The existence of strong inter-
actions between the p-conjugated systems of neighboring
molecules in the solid state is desirable for good TFT device
performance.
3.4. Electrochemical properties
To investigate the electrochemical stability and the
charge transport properties of the small molecules, we car-
ried out cyclic voltammetry (CV) measurements on thin
films of the small molecules, summarized in Table 2. The
small molecules showed reversible waves for both p-dop-
ing and n-doping processes. The oxidation potentials Eox
(p-doping) for TIPSAntT, TIPSAntBeT, and TIPSAntPT were
0.99, 1.13, and 0.96 V, respectively. The relatively high oxi-
dation potentials for TIPSAnts, compared to TIPS pentacene
(380 mV), [21] resulted in better oxidation stability of the
materials. The highest occupied molecular orbital (HOMO)
levels of TIPSAntT, TIPSAntBeT, and TIPSAntPT were esti-
mated to be ꢁ5.39, ꢁ5.53, and ꢁ5.36 eV, respectively,
using the previously reported empirical equation [22].
The observation that TIPSAntBeT had the lowest HOMO
was attributed to reduced delocalization along the back-
bone through the introduction of a fused aromatic unit,
benzothiophene. The delocalization of electrons from the
benzothiophene unit onto the backbone was less favorable
than from a single thiophene and benzene ring, due to the
larger resonance stabilization energy of the fused ring [23].
It is worth noting that among the three materials, the
3.3. Optical properties
The UV–vis absorption spectra of the small molecules
were recorded both in solution (chlorobenzene) and in film
states, as shown in Fig. 2. The UV–vis data for the small
molecules is summarized in Table 1. All small molecules
showed good absorption properties, with vibronic struc-
tures in both solution and film states. In both the solution
and film states, the absorption spectra of the small mole-
cules showed relative red-shifts as follows: TIPSAntT < TIP-
SAntBeT < TIPSAntPT. The largest red-shift in the absorption
spectrum of TIPSAntPT suggested the presence of increas-
ing electron delocalization along the main chain due to

824
J.-H. Park et al. / Organic Electronics 11 (2010) 820–830
Fig. 1. DSC scans of the small molecules.
Table 1
UV–vis absorption properties of the small molecules.
kmax
UV_sola
UV_filmb
D
1st Maxc
D
2nd Maxd
TIPSAntT
TIPSAntBeT
TIPSAntPT
342, 475
357, 476
373, 494
353, 496
373, 494
366, 531
11
16
ꢁ7
21
18
37
a
b
c
In chlorobenzene solution.
Spin-coated thin film from chlorobenzene solution.
The 1st maximum peak difference between solution and film states
around 340–370 nm.
d
The 2nd maximum peak difference between solution and film states
around 470–530 nm.
The low band gap of TIPSAntPT was attributed to the ex-
tended conjugation system of the material, which bene-
fited from the long conjugation length of the main chain,
as described in Section 3.3.
3.5. X-ray crystallography
Crystallographic analysis allows for the rapid screening
of potential OTFT materials. Single crystals of small mole-
cules, easily grown from hexane and chlorobenzene, were
analyzed by X-ray crystallography.
Fig. 3 shows the p-stacking structures of the small mol-
ecules in single crystals. All of the molecules showed face-
to face molecular interactions similar to our previous re-
sults [13]. The interplanar distances in the three molecules
were almost identical: 3.54, 3.55, and 3.54 Å for TIPSAntT,
TIPSAntBeT, and TIPSAntPT, respectively. Single crystals of
TIPSAntT and TIPSAntBeT showed ‘‘slipped-stacked” struc-
tures similar to that previously reported for TIPSAnts
[13,17], as shown in Fig. 3b and d. However, single crystals
of TIPSAntPT showed a ‘‘bricklayer” structure, as shown in
Fig. 3f. How the TIPSAnt columns interdigitate determines
whether a ‘‘slipped-stacked” or ‘‘bricklayer” structure is
formed. When TIPSAntT of TIPSAntBeT columns packed to-
gether, each conjugated core in one column adjoined two
TIPS groups in an adjacent column to form a ‘‘slipped-
stacked” structure, as shown in Fig. 3a and c. For TIPSAntPT,
by contrast, the bulky phenyl groups prevented the conju-
gated cores of the TIPSAntPT molecules in one column from
adjoining the two TIPS groups in an adjacent column. As a
result, the conjugated cores of TIPSAntPT molecules in one
column adjoined the conjugated cores of molecules in
Fig. 2. UV–vis absorption of the small molecules. (a) In a chlorobenzene
solution (b) in the spin-coated film state.
HOMO level of TIPSAntPT provided the best match to the
work function of gold metal [24]. As a result, hole injection
from the gold source electrode in p-type thin-film transis-
tors composed of TIPSAntPT was expected to be the most
efficient among the three materials. In contrast, the lowest
lying HOMO level of TIPSAntBeT would be expected to in-
crease the hole injection barrier between the gold elec-
trodes and the TIPSAntBeT film. The relative band gaps of
TIPSAnts, calculated from CV measurements and the onset
of absorption in the film states, decreased in the order TIP-
SAntT > TIPSAntBeT > TIPSAntPT, as summarized in Table 2.

J.-H. Park et al. / Organic Electronics 11 (2010) 820–830
825
Table 2
Electrochemical properties of the small molecules.
c
Eox (mV)a
Ered (mV)a
HOMO (eV)b
LUMO (eV)b
Eg (eV)
Eg_opt (eV)
TIPSAntT
TIPSAntBeT
TIPSAntPT
990
1130
960
ꢁ1680
ꢁ1310
ꢁ1440
ꢁ5.39
ꢁ5.53
ꢁ5.36
ꢁ2.72
ꢁ3.09
ꢁ2.97
2.67
2.44
2.39
2.38
2.36
2.18
a
b
c
Eox, and Ered stand for the oxidation potential and the reduction potential, respectively.
HOMO = ꢁ(Eox + 4.4) eV, LUMO = ꢁ(Ered + 4.4) eV.
Optical bandgap derived from UV–vis onset.
Fig. 3. Single crystal structures of (a and b) TIPSAntT and (c and d) TIPSAntBeT, which show ‘‘slipped-stacked” structures, and (e and f) TIPSAntPT, which
shows a ‘‘bricklayer” structure. (a, c and e): view down crystallographic a axis, (b, d, and f): view down crystallographic c axis. Schematic figures of columns
of TIPSAnts stacks: (g) for TIPSAntT and TIPSAntBeT, and (h) for TIPSAntPT. Yellow dotted squares indicate columns of TIPSAnts stack and red dotted circles
indicate adjoining areas between adjacent columns. (For interpretation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
adjacent columns, resulting in a ‘‘bricklayer” structure, as
shown in Fig. 3e. TIPSAntPT is the first TIPSAnt derivative
to yield a ‘‘bricklayer” stacking structure. In Fig. 3g and h,
the TIPSAnt columns and the adjoining areas between adja-
cent columns are indicated by yellow and red dotted lines,
respectively.
creased. As a result, TIPSAntBeT had a larger
than TIPSAntT. Only TIPSAntPT showed an additional area of
-overlap with adjacent molecules due to the ‘‘bricklayer”
stacking structure. This additional area of -overlap signif-
p-overlap area
p
p
icantly improved the charge-carrier mobility.
Fig. 4 shows the area of
and above adjacent molecules of TIPSAnt derivatives. (The
area of -overlap with below adjacent molecules was
omitted for clarity.) When bulkier substituents were incor-
porated into the conjugated core, the area of -overlap in-
p
-overlap between a molecule
3.6. TFT performances
p
Top-contact OTFTs were fabricated on a common gate
of highly n-doped silicon with a 300 nm thick thermally
grown SiO2 dielectric layer. Substrates were modified with
p

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J.-H. Park et al. / Organic Electronics 11 (2010) 820–830
Fig. 4. Overlap of the molecules (dark-shaded molecules) along with their closest neighbors in the plane above the molecule (light-shaded molecules).
octyltrichlorosilane (OTS) by incubation in a toluene solu-
tion of OTS for 2 h at room temperature. Films of the organ-
ic semiconductors were spin-coated at 2000 rpm from a
0.5 wt.% chloroform solution of TIPSAntT and TIPSAntPT,
and chlorobenzene for TIPSAntBeT, due to solubility limita-
tions. All of the TFT results were obtained in ambient air
condition and the devices did not show any unstable prob-
lems. As shown in Fig. 5, the TFT devices of all molecules
showed typical p-channel responses and good saturation
behavior. The TFT performances of TIPSAnts are summa-
rized in Table 3. TIPSAntPT showed the most improved
TFT performance among the three molecules, with an aver-
age mobility of 0.063 cm2/Vs, on/off ratio of 4 ꢂ 105, and
threshold voltage (VT) of ꢁ4.0 V. TIPSAntPT showed an or-
der of magnitude larger charge-carrier mobility than TIP-
SAntT, most likely because of the ‘‘bricklayer” structure of
the film. TIPSAntBeT, however, showed a lower mobility
lar, single crystals of functionalized pentacene reportedly
yielded high mobilities because they were largely free from
grain boundaries and molecular disorder, and, thus, over-
lap of the p-orbitals was enhanced [27]. A saturated chlo-
robenzene solution of the small molecules was slowly
evaporated at room temperature onto OTS-modified SiO2
substrates. After 1 or 2 days, well-defined rod-type single
crystals were obtained, and gold electrodes were patterned
onto the single crystals by thermal evaporation. The width
and length of the channels were calculated from optical
microscopy (OM) images. OM images of single crystal TFTs
and a schematic device structure are shown in Fig. 6 The
mobility trend for single crystal devices was the same as
that of the film devices (TIPSAntBeT < TIPSAntT < TIPSAntPT).
As expected, the highest TFT performances, showing a
mobility of 0.12 cm2/Vs, were achieved from one-dimen-
sional (1D) single crystalline TIPSAntPT microribbons. The
than TIPSAntT. Although a larger area of
p-overlap was ob-
improved performance arose from the highly efficient p-
served, as shown in Fig. 4, TIPSAntBeT exhibited decreased
mobility on account of its poorer solubility and higher
crystallinity arising from the rigid benzothiophene. In
addition, high crystallinity increased the number of hole-
trapping sites at the interface between the OTS monolayer
and the TIPSAntBeT film. If there were too many hole-trap-
ping sites, a very large negative gate bias would be re-
quired to swing the polymer into hole accumulation
mode, by inducing a high threshold voltage, VT. The low
HOMO level of TIPSAntBeT also produced a hole injection
barrier. As a result, VT of TIPSAntBeT (ꢁ28.7 V) was much
larger than VT for the other small molecules. The trend in
TFT performance among the three small molecules corre-
sponded with the morphological properties, which are dis-
cussed in the next section.
orbital overlap of the ‘‘bricklayer” structure of the TIPS-
AntPT single crystals. TIPSAntT also grew as 1D single crys-
talline microribbons, resulting in a high mobility, but the
less efficient
p-orbital overlap of the ‘‘slipped-stacked”
arrangement of TIPSAntT single crystals resulted in a lower
mobility than was observed for TIPSAntPT single crystals. In
contrast, TIPSAntBeT grew as three-dimensional (3D) poly-
hedron single crystals with a larger thickness than TIPS-
AntPT. (The thickness of single crystals was 80 and
200 nm for TIPSAntPT and TIPSAntBeT, respectively, mea-
sured with an Alpha Step 400, Tencor.) In single crystal
TFTs, 1D or 2D microrods showed much higher TFT perfor-
mance than 3D polyhedra because 3D polyhedra have dif-
ficulties contacting the gold electrodes [28,29]. As a result,
the mobility of TIPSAntBeT single crystals was poor. TFT re-
sults of single crystals of the TIPSAnt derivatives are sum-
marized in Table 3. We are currently working on
improving device performances including operational sta-
bility and we will publish the results elsewhere.
Single crystal TFTs based on the small molecules were
fabricated because single organic crystals allow the facile
investigation of the intrinsic material parameters that
determine charge transport efficiency [25,26]. In particu-

J.-H. Park et al. / Organic Electronics 11 (2010) 820–830
827
Fig. 5. Transfer characteristics and output characteristics of TIPSAntT (a and b), TIPSAntBeT (c and d), and TIPSAntPT (e and f).
3.7. Morphological properties
on TIPSAntPT. In contrast, TIPSAntT and TIPSAntBeT under-
went 3D film growth, as shown in Fig. 7a and b, due to
We performed atomic force microscopy (AFM) mea-
surements to further investigate the relationship between
molecular alignment in films and device performance as
AFM provides the thin-film morphologies in micrometer
scales to hundreds of nanometer scales. As shown in
Fig. 7c, TIPSAntPT showed film morphologies dominated
by 2D growth after spin-casting, due to the strong elec-
the limited p-overlap in the ‘‘slipped-stacked” structures.
TIPSAntBeT, especially, showed poor substrate coverage,
with the needle-like crystallites produced due to poor sol-
ubility. The high crystallinity also led to a very low mobil-
ity and high VT. The rotational invariance of the fused
benzothiophene in the backbone facilitated the adoption
of a low energy backbone conformation, promoting the for-
mation of highly ordered crystalline domains.
tronic interactions in the ‘‘bricklayer” p-stacking structure.
Each crystalline domain connected smoothly in the as-cast
film, yielding devices with high TFT hole mobilities based
Out-of-plane and in-plane mode X-ray diffraction
(XRD) spectra of films of the molecules were measured,

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J.-H. Park et al. / Organic Electronics 11 (2010) 820–830
Table 3
TFT performances of the small molecules.
Film
Single crystal
Mobility (cm2/Vs)
Ion/Ioff
VT (V)
Mobility (cm2/Vs)
Ion/Ioff
VT (V)
TIPSAntT
TIPSAntBeT
TIPSAntPT
0.0039
0.0013
0.063
6 ꢂ 104
7 ꢂ 103
4 ꢂ 105
ꢁ11.2
ꢁ28.7
ꢁ4.0
0.059
0.0067
0.12
5 ꢂ 103
3 ꢂ 10
2 ꢂ 102
ꢁ24.1
ꢁ8.5
ꢁ6.1
Fig. 6. Optical microscopy images of single crystal TFTs of (a) TIPSAntT, (b) TIPSAntBeT, and (c) TIPSAntPT. (d) Schematic device structure of single crystal
TFTs.
Fig. 7. AFM images (5l ꢂ 5l).
as shown in Fig. 8. The TIPSAntT film showed the lowest
degree of crystallinity, exhibiting no specific ordering in
the peaks of the out-of-plane mode XRD. Although a
(0 1 0) peak was observed in the in-plane mode XRD of
the TIPSAntT film, the interlayer spacing d was 4.28 Å,
which is the largest spacing measured among the three
properties, with (1 0 0), (2 0 0), and (3 0 0) reflections in
the out-of-plane mode XRD indicating well-organized
intraplanar lamella-like structures oriented normal to the
substrate. The molecular length of the TIPSAnt derivatives
along the TIPS axis was 16.7 Å, as determined by X-ray
crystallography. The d spacing of the (1 0 0) peak was
14.9 Å, which implied that TIPSAntBeT molecules were
aligned perpendicular to the substrate, with respect to
molecules, indicating less effective intermolecular
p-stack-
ing. In contrast, TIPSAntBeT films showed highly crystalline

J.-H. Park et al. / Organic Electronics 11 (2010) 820–830
829
Fig. 8. XRD patterns (a) TIPSAntT_out of plane, (b) TIPSAntT_in-plane, (c) TIPSAntBeT_out of plane, (d) TIPSAntBeT_in-plane, (e) TIPSAntPT_out of plane, and (f)
TIPSAntPT_in-plane.
the TIPS axis, with a tilt angle of 27°. Although the (0 1 0)
peak in the in-plane mode XRD of the TIPSAntBeT films
showed a closer interlayer d spacing of 4.15 Å, the high
crystallinity of the TIPSAntBeT films resulted in poorer de-
vice performances than TIPSAntT. The high crystallinity of
dicularly to the substrate along the TIPS axis than was TIP-
SAntBeT, with a smaller tilt angle of 15°. When the
molecules were aligned perpendicular to the substrate,
the
p-overlap between adjacent molecules was maxi-
mized, and carrier transport could occur more effectively.
In-plane XRD of TIPSAntPT showed a (0 1 0) peak that indi-
cated the interlayer d spacing of 3.49 Å. This value was
very close to the interplanar distances (3.54 Å) determined
by X-ray crystallography measurements, indicating highly
the TIPSAntBeT probably arose from a
p-overlap area that
was larger than that of TIPSAntT, as expected from X-ray
crystallography. TIPSAntPT films showed the most promis-
ing XRD results in both out-of-plane and in-plane mode
XRD experiments. The d spacing of the (1 0 0) peak was
16.1 Å, implying that TIPSAntPT was aligned more perpen-
effective
p-orbital overlap due to the ‘‘bricklayer”
structure.

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J.-H. Park et al. / Organic Electronics 11 (2010) 820–830
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We synthesized three new TIPSAnt derivatives and car-
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structures and TFT performances. Only TIPSAntPT showed
a ‘‘bricklayer” stacking structure that resulted in a high
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agreed well with the AFM and XRD findings. Although
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Acknowledgements
This research was supported by a grant (F0004011-
2008-31) from the Information Display R&D Center, one
of the 21st century Frontier R&D Programs funded by the
Ministry of Commerce, Industry, and Energy of the Korean
Government. We gratefully acknowledge Dr. J.-H. Kim and
Dr. S.K. Lee for assistance with XRC measurements.
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