H-aggregation strategy in the design of molecular semiconductors for highly reliable organic thin film transistors

Article abstract of DOI:10.1002/adfm.201002367

Four new quaterthiophene derivatives with end-groups composed of dicyclohexyl ethyl (DCE4T), dicyclohexyl butyl (DCB4T), cyclohexyl ethyl (CE4T), and cyclohexyl butyl (CB4T) were designed. All materials showed high solubility in common organic solvents. UV-vis absorption measurements showed that the quaterthiophene derivatives with asymmetrically substituted cyclohexyl end-groups (CE4T and CB4T) preferred H-type aggregation whereas those with symmetrically substituted cyclohexyl end-groups (DCE4T and DCB4T) preferred J-type aggregation. The molecular structure-dependent packing (H or J) of the new quaterthiophene derivatives was analyzed by grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. The field-effect mobilities of devices that incorporated the asymmetrical molecules, CE4T and CB4T, were quite high, above 10 ^{- 2} cm ² V ^{- 1} s ^{- 1} , due to H-aggregation, whereas the field-effect mobilities of devices that incorporated symmetrical molecules, DCE4T and DCB4T, were poor, below 10 ^{- 4} cm ² V ^{- 1} s ^{- 1} , due to J-aggregation. More importantly, H-aggregation within the thin film provided stable crystalline morphologies in the spin-coated films, and, thus, thin film transistors (TFTs) using cyclohexylated quaterthiophenes yielded highly reproducible transistor performances. The distributions of measured field-effect mobilities in transistors based on cyclohexylated quaterthiophenes with H-aggregation were remarkably narrow.

Full text of DOI:10.1002/adfm.201002367

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H-Aggregation Strategy in the Design of Molecular
Semiconductors for Highly Reliable Organic Thin Film
Transistors
Seul-ong Kim, Tae Kyu An, Jun Chen, Il Kang, So Hee Kang, Dae Sung Chung,
Chan Eon Park,* Yun-Hi Kim,* and Soon-Ki Kwon*
semiconductor applications to replace
their higher-cost inorganic counterparts.
The most attractive aspect of these organic
materials is that their structures can be
diversified and their physical or chem-
ical properties tuned through strategic
molecular design.[1–9]
Four new quaterthiophene derivatives with end-groups composed of dicy-
clohexyl ethyl (DCE4T), dicyclohexyl butyl (DCB4T), cyclohexyl ethyl (CE4T),
and cyclohexyl butyl (CB4T) were designed. All materials showed high solubility
in common organic solvents. UV–vis absorption measurements showed that
the quaterthiophene derivatives with asymmetrically substituted cyclohexyl
end-groups (CE4T and CB4T) preferred H-type aggregation whereas those with
symmetrically substituted cyclohexyl end-groups (DCE4T and DCB4T) preferred
J-type aggregation. The molecular structure-dependent packing (H or J) of the
new quaterthiophene derivatives was analyzed by grazing-incidence wide-angle
X-ray scattering (GIWAXS) measurements. The field-effect mobilities of devices
that incorporated the asymmetrical molecules, CE4T and CB4T, were quite high,
above 10−2 cm2 V−1 s−1 , due to H-aggregation, whereas the field-effect mobili-
ties of devices that incorporated symmetrical molecules, DCE4T and DCB4T,
were poor, below 10−4 cm2 V−1 s−1 , due to J-aggregation. More importantly,
H-aggregation within the thin film provided stable crystalline morphologies in
the spin-coated films, and, thus, thin film transistors (TFTs) using cyclohexy-
lated quaterthiophenes yielded highly reproducible transistor performances. The
distributions of measured field-effect mobilities in transistors based on cyclohex-
ylated quaterthiophenes with H-aggregation were remarkably narrow.
Among the variety of organic semi-
conductors developed, thiophene-based
materials have emerged as an important
class because of their high chemical and
electrochemical stability, the accessi-
bility of their preparation for thiophene
synthesis, and the availability of well-
developed/regioselective ring–ring cou-
pling methodologies.[10,11] Furthermore,
the properties of oligo/polythiophene
cores can be efficiently tuned by intro-
ducing appropriate substitutions. To date,
the field-effect transistor (FET) mobili-
ties in devices composed of oligomeric
thiophene semiconductors are generally
higher than those obtained from conju-
gated polymers. Oligomeric thiophene
semiconductor polymers engage in long-
range packing, and efficient charge transport is directly related
to the long-range packing of molecules in semiconductor
films. Many synthetic methods have been developed to func-
tionalize the α- and ω-ends of conjugated thiophene ring sys-
tems to increase the solubility and stability toward oxidation or
to influence the solid-state ordering of oligothiophenes without
affecting the planarity of the conjugated backbone.[12,13]
The present study was devised based on the following three
considerations: i) previously reported organic semiconduc-
tors have generally shown J-aggregation[14] with head-to-tail
molecular stacking;[15] ii) large area π-stacking between adjacent
molecules can be realized by H-aggregation, which occurs when
molecules stack side by side;[16] and iii) H-aggregation induces
stable morphologies in thin films and reproducible transistor
performances. We describe a novel synthetic strategy to induce
H-aggregation between adjacent molecules in the thin film
state. We designed four types of quaterthiophene derivative with
end-groups composed of cyclohexyl ethyl (CE4T), cyclohexyl
butyl (CB4T), dicyclohexyl ethyl (DCE4T), and dicyclohexyl
butyl (DCB4T). UV–vis absorption and grazing-incidence wide-
angle X-ray scattering (GIWAXS) analyses indicated that the
asymmetric derivatives, CE4T and CB4T, tended to undergo
1. Introduction
Over the past decade, several conjugated organic oligomers
and polymers have been developed for potential use in low-cost
S.-o. Kim, J. Chen, I. Kang, Prof. S.-K. Kwon
School of Materials Science and Engineering
Engineering Research Institute
Gyeongsang National University
Jinju 660–701, Korea
E-mail: skwon@gnu.ac.kr
S. H. Kang, Prof. Y.-H. Kim
Department of Chemistry
Gyeongsang National University JinJu
600–701, Korea
E-mail: ykim@gnu.ac.kr
T. K. An, Dr. D. S. Chung, Prof. C. E. Park
Polymer Research Institute
Department of Chemical Engineering
Pohang University of Science Technology
Pohang 790–784, South Korea
E-mail: cep@postech.ac.kr
DOI: 10.1002/adfm.201002367
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SOCl2
Mg / ether
OH
n-BuLi
Sn(Bu)3Cl
Cl
+
Pyridine
Br
S
Ni(dppp)Cl2
S
S
S
Br
S
Pd(PPh3)4 / Toluene
Pd(PPh3)4 / Toluene
Br
Sn(Bu)3
S
S
+
S
S
Compound 1
DCE4T
S
S
S
S
S
Br
Compound 1
+
S
S
CE4T
Mg / ether
n-BuLi
5 mol% copper(I) bromide
THF
Br
+
Br
Br
+
Br
Br
Sn(Bu)3Cl
S
Ni(dppp)Cl2
S
S
Br
Br
S
S
S
Sn(Bu)3
S
S
S
Compound 2
DCB4T
S
Pd(PPh3
)
/ Toluene
S
S
S
4
Compound 2
S
+
Br
S
S
CB4T
Scheme 1. Synthetic scheme for the alkyl cyclohexyl-substituted quaterthiophenes.
H-aggregation whereas the symmetric derivatives, DCE4T and
DCB4T, tended to undergo J-aggregation.[14]
heating–cooling–heating cycle under a nitrogen atmosphere.
Good thermal stability is important because it conveys device
longevity.[17] TGA revealed good thermal stability of the cyclohex-
ylated alkyl substituted quaterthiophene. A 5% weight loss was
observed at 305 °C for CE4T, 372 °C for DCE4T, 325 °C for
CB4T, and 389.7 °C for DCB4T (Figure S1). The symmetrically
substituted DCE4T and DCB4T (Figure S1) materials showed
enhanced thermal stability. Interestingly, two exothermic peaks
appeared in the DSC curve in a narrow temperature range for
the case of the asymmetrically substituted molecules, CE4T and
CB4T, compared with single peaks for the case of the symmetri-
cally substituted molecules, DCE4T and DCB4T (Figure S2 and
Table 1) .
We fabricated a thin-film transistor using the synthesized
materials as active layers. The charge carrier mobility in
asymmetric molecules with H-aggregation was much higher
than in symmetric molecules with J-aggregation. The correla-
tion between structure and mobility is discussed to describe
the aggregation type-dependent charge carrier mobility in
detail. In addition, thin films formed by asymmetric mol-
ecules yielded a morphology that was more uniform than
that of thin films formed by symmetric molecules. Therefore,
transistors based on asymmetric molecules showed excellent
reproducibility.
2. Results and Discussion
2.3. UV–Vis Absorption Spectroscopy
2.1. Synthesis
Intermolecular interactions between adjacent molecules in the
solid state were characterized by UV–vis spectroscopy. We dis-
covered that the absorption peaks of the asymmetric molecules
(cyclohexylated quaterthiophenes) in thin films were blue-shifted
relative to those in solution (Figure 1a,c and Table 2). This
result could be explained by H-aggregation of the molecules,
which was correlated with the specific molecular packing struc-
ture.[18] In other words, the cyclohexylated molecules, CE4T and
We carried out the synthesis of the cyclohexylated alkyl sub-
stituted quaterthiophenes by Grignard reaction and Stille
coupling. The synthetic scheme is outlined in Scheme 1. The
obtained quaterthiophenes were purified by recrystalliza-
tion using MeOH–CH2Cl2 or MeOH–CHCl3 and subjected to
Soxhlet extraction using toluene. The quaterthiophenes were
1
characterized by H NMR, mass spectrometry, and elemental
analysis. All quaterthiophenes were readily soluble in chloro-
form and other nonpolar solvents, such as toluene and benzene,
whereas the previously reported dicyclohexylated quaterthi-
ophene showed limited solubility.[1]
T a b le 1 . DSC and TGA data for CE4T, DCE4T, CB4T, and DCB4T.
CE4T
305 °C
DCE4T
372 °C
212 °C
204 °C
212 °C
CB4T
325 °C
DCB4T
389.7 °C
174 °C
Td (5% weight loss)
1st heating
146 °C
147,157 °C
155 °C
2.2. Thermal Analysis
Cooling
138,149 °C
146 °C
164,173 °C
175 °C
The thermal properties of the synthesized quaterthiophenes
were evaluated by TGA and DSC by applying a repeated
2nd heating
147,157 °C
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(a)
(b)
Solution
Film
Solution
Film
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
300
350
400
450
500
550
600
300
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
(d)
(c)
Solution
Film
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Solution
Film
250 300 350 400 450 500 550 600
300
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Figure 1. UV–vis absorption spectra of a) CE4T, b) DCE4T, c) CB4T, and d) DCB4T in toluene solution.
CB4T, tended to stack in parallel.[19] In contrast, the absorption
peaks of the symmetric molecules (dicyclohexylated quaterthi-
ophenes) in thin films were red-shifted relative to those in solu-
tion (Figure 1b,d and Table 2). These observations have been
noted in many conventional molecular semiconductors and are
explained well by J-aggregation.[14] J-aggregation is observed
when molecules stack by a head-to-tail arrangement.[15]
A molecule can be approximated as a point dipole, and the
potential energy of interaction between two molecules is a
function of their relative dipole configuration or orientation.
To calculate the potential energy of interaction between two
dipoles, one dipole moment is defined by μ1 and the other
dipole is defined by μ2, separated by a vector r. The potential
energy of μ2 is calculated using classical electromagnetic theory
in presence of the electric field generated by the dipole moment
μ1. In the thin film systems, two assumptions are appro-
priate. Firstly, we can apply a Taylor expansion to the Coulomb
potential because the intermolecular distance (between adjacent
molecular dipoles) is very short. Secondly, the sum of charges is
0, that is, the charge distribution is neutral. Finally, the poten-
tial energy of interaction between two dipoles can be described
as follows:
:1 ·
:
(:1 · r)(:2 · r)
4Bg0r5
:1:2(1 − cos2 2)
4Bg0r3
2
(1)
V =
− 3
=
,
4Bg0r3
because :1 · r = :1r cos2: 2 · r = :2r cos2, as shown in
Figure 2a.[20]
Next, we can estimate the molecular configuration using the
equation for the potential energy of interaction between two
dipoles. From Equation 1, we can see that the energy of the system
increases when the value of θ is close to 90° (H-aggregation)
and decreases when it is close to 0° (J-aggregation). In other
words, the excitonic state of the molecular aggregate splits
into two energy levels, one that is higher and another that is
lower than the energy of the monomer, as shown in Figure 2b,
depending on the interaction between the transition dipoles.[21]
Because the blue shift in the UV absorption could be inter-
preted as an increase in energy and the red shift in the
absorption as a decrease in the energy of the excitonic energy
states, the cyclohexylated quaterthiophenes were thought to
T a b le 2 . Optical properties of CE4T, DCE4T, CB4T, and DCB4T.
CE4T
398
DCE4T
404
CB4T
401
DCB4T
404
UV–vis [nm]
Solution
Film
339
385, 458
269
365, 462
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(a)
q2
-q2
(a)
r
θ
Perpendicular
direction on the
substrate
-q1
q1
C
(b)
Parallel
Head-to-tail
(b)
Perpendicular
direction on the
substrate
H-aggregate
blue shift
Monomer
J-aggregate
red shift
C
Figure 2. a) Arrangement of the molecular dipoles separated by a dis-
tance r. b) Scheme describing the excitonic energy state splitting, into two
levels, in H-aggregated and J-aggregated materials.
Figure 4. Estimated packing structures for a) asymmetric molecules, and
b) symmetric molecules.
form H-aggregation-type molecular packing structures and
dicyclohexylated quaterthiophenes were thought to form J-
aggregation-type molecular packing structures, respectively.[18,19]
Previously described high-performance organic semiconduc-
tors like TIPS-pentacene and its derivatives were usually charac-
terized by a packing type that resembled J-aggregation and not
H-aggregation. However, the area of pi-overlap between adja-
cent molecules, which is believed to be central to enhancing
the charge carrier mobility of molecular semiconductors,[16] is
higher in H-aggregated materials such that they yield better
charge carrier mobilities than J-aggregated materials. Although
H-aggregation is difficult to achieve, as mentioned above, it can
be induced through strategic molecular design. Dividing mole-
cular semiconductors into a central group (in our cases, quater-
thiophene) and side groups (cyclohexylated alkyl chain) sug-
gests that large asymmetrically substituted side groups would
favor packing of adjacent molecules in parallel so that the bulky
side groups avoid proximity and minimize steric hindrance. On
the other hand, dicyclohexylated quaterthiophenes stack such
that molecules are tilted on the substrate (J-aggregation) to
avoid bulky side groups.
the hypothesis described above. These results indicated that
cyclohexylated quaterthiophenes were vertically oriented on the
substrate and grew along the out-of-plane direction, as shown
in Figure 4a.[22] Diffraction patterns of dicyclohexylated quater-
thiophenes could not be observed along the out-of-plane and
in-plane directions. However, diagonal diffraction patterns were
observed. These diffraction results suggest that cyclohexylated
quaterthiophenes are vertically aligned in a face-to-face packing
structure, and dicyclohexylated quaterthiophenes are diagonally
packed on the substrate, as shown in Figure 4b.[23,24]
It should be noted that cyclohexylated quaterthiophenes
can engage in two types of crystal packing for a given mole-
cular dipole, as shown in Figure 4a (compare the left and right
models). The out-of-plane scattering profiles of cyclohexylated
quaterthiophenes, which were extracted along the αf-direction
at 2θf = 0° from the scattering patterns in Figure 3, showed two
types of n-numbered diffraction peaks, as shown in Figure S3.
The presence of two exothermic peaks for the cyclohexylated
quaterthiophenes in DSC measurements (Figures S2a and c)
also supports this model, because domains with different
packing structures had different durabilities under thermal
exposure.
The molecular packing of the designed semiconductors
was examined by GIWAXS, as shown in Figure 3, to test
2.4. FET Behavior
To test the potential of the quaterthiophene derivatives as
organic semiconductors, field-effect transistors (FETs) of these
derivatives were fabricated using a soluble process. As shown
in Figure 5, FET devices composed of CE4T and CB4T showed
typical p-channel responses, and the output curves showed
excellent saturation behavior. The field-effect mobilities of the
devices based on CE4T and CB4T were 1.14 × 10−–2 cm2 V−1 s−1
(Ion/Ioff = 4.76 × 105) and 1.55 × 10−2 cm2 V−1 s−1 (Ion/off = 2.06 ×
106) in the saturation regime, whereas the field-effect mobilities
of devices based on DCE4T and DCB4T were 3.10 × 10−4 cm2
V−1 s−1 ) (Ion/Ioff = 2.05 × 102) and 2.35 × 10−5 cm2 V−1 s−1
(Ion/off = 8.04 × 10).
Figure 3. GIWAXS scattering results for a) CE4T, b) DCE4T, c) CB4T, and
d) DCB4T.
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to that previously reported.[25] The difference of
tendency of charge carrier mobility may come
from substituent alkyl groups. It is well known
that lipophilic alkyl chain at the α-position of
oligothiophenes can induce stronger intermo-
lecular interaction between adjacent molecules
by Van der Waals interactions.[26] On the other
hand, in our study, the bulky side groups
generate steric hindrance and the bulky side
groups push each molecule away. Therefore,
cyclohexylated quaterthiophenes are vertically
aligned in a face-to-face packing structure, and
dicyclohexylated quaterthiophenes are diago-
nally packed on the substrate.
(a)
(b)
4.0x10-7
0.0020
-60V
-40V
-20V
0V
10-6
10-7
10-8
10-9
0.0016
0.0012
0.0008
0.0004
0.0000
2.0x10-7
0.0
20
0
-20 -40 -60 -80 -100
0
-10 -20 -30 -40 -50 -60
Drain Voltage (V)
H-aggregation has another advantage over
thin film transistors (TFTs) in that the repro-
ducibility of device properties with this type of
molecular stacking is superior. The high crys-
tallinity of cyclohexylated quaterthiophenes
with H-type packing produced remarkably
uniform and reproducible TFT performances.
We compared the reproducibility of charge
carrier mobility in cyclohexylated and
dicyclohexylated quaterthiophenes, poly[9,9-
dioctylfluorenyl-2,7-diyl]-co-(bithiophene)
(F8T2) and 6,13-bis(triisopropyl-silylethynyl)
pentacene (TIPS-pentacene)-based TFTs. We
measured the performances of dozens of
transistors. TFTs based on dicyclohexylated
quaterthiophenes yielded poor TFT perform-
ance, such that it was difficult to compare the
performance distribution with the distribu-
tions of other materials. Although it is well-
known that drop-cast slow solvent-drying
Drain Voltage (V)
(c)
(d)
1.4x10-6
10-5
10-6
10-7
10-8
10-9
10-10
0.0028
0.0024
0.0020
0.0016
0.0012
0.0008
0.0004
0.0000
-60V
-40V
-20V
0V
1.2x10-6
1.0x10-6
8.0x10-7
6.0x10-7
4.0x10-7
2.0x10-7
0.0
20
0
-20 -40 -60 -80-100
0
-10 -20 -30 -40 -50 -60
Drain Voltage (V)
Drain Voltage (V)
Figure 5. Transfer and output characteristics of OFETs based on CE4T (a and b) and CB4T
(c and d).
methods for TIPS-pentacene films produce high mobilities that
exceed 1 cm2 V−1 s−1 ,[27] the poor performance reproducibility in
such films forced us to use spin-coating methods that yielded
relatively lower mobilities, simply to compare the reproduc-
ibility of TFT performance.
A structural analysis, based on the UV–vis absorption and
GIWAXS results, revealed specific features that explained the
pronounced differences in field-effect mobilities between the
cyclohexylated and dicyclohexylated quaterthiophene derivatives.
Clear correlations were observed, as expected: H-aggregation
(CB4T and CE4T) produced higher charge carrier mobilities, and
J-aggregation (DCB4T and DCE4T) produced lower mobilities. As
the number of molecules stacked in parallel increased to produce
a large area of π-overlap, hole mobility among adjacent molecules
improved, thereby producing higher field-effect mobilities in the
devices. In addition, the packing patterns on substrates, shown
in Figure 4, revealed that the orientations of cyclohexylated or
dicyclohexylated quaterthiophene molecules in the lattices were
different. In other words, lattices composed of dicyclohexylated
quaterthiophenes were tilted, and, thus, the thin films were
expected to contain some defects because the orientations in
the tilted lattices were not uniform. Dicyclohexylated quaterthi-
ophenes were found to stack by a head-to-tail arrangement. There-
fore, the connectivity in the dicyclohexylated quaterthiophene
film was poorer than the connectivity in the cyclohexylated
quaterthiophenes, resulting in different field-effect mobilities in
films composed of each type of substituent molecules.
As shown in Figure 6a and b, the distribution of mobilities in
TIPS-pentacene TFTs fabricated by spin-coating was very broad.
The distributions of mobilities in CE4T and CB4T TFTs were
remarkably narrower than those of TIPS-pentacene and F8T2
TFTs. Devices based on cyclohexylated quaterthiophenes yielded
the most reproducible data. The standard deviations of CE4T,
CB4T, and F8T2 TFT mobilities were 7.67 × 10−4 , 1.02 × 10−3 and
2.54 × 10−3 . Furthermore, we fabricated a 4 inch diameter sample
via one-step spin-coating of CE4T to highlight the superior per-
formance reproducibility of films based on this molecule. The
results are shown in Figure 7. Over almost the whole area, a con-
stant FET performance (equal to the average value) was obtained.
Therefore, devices based on cyclohexylated quaterthiophenes
yielded good performance reproducibility by improving the molec-
ular ordering within the thin films as a result of H-aggregation.
3. Conclusions
The tendency that the charge carrier mobility of FET devices
composed of cyclohexylated quaterthiophenes is higher than that
of devices based on dicyclohexylated quaterthiophenes is opposite
We designed four types of quaterthiophene derivatives with
end-groups composed of DCE4T, DCB4T, CE4T, and CB4T.
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molecular semiconductors. This observation was made on
the basis of an analysis of the absorption and diffraction
measurements. The large π-overlap area in H-type aggrega-
tion materials produced FET mobilities in the asymmetrical
molecules that were clearly higher than those measured in
symmetrical molecules. In addition, good ordered packing
caused by H-aggregation produced TFTs with stable mor-
phologies and highly reproducible performances. These find-
ings revealed that, through strategic molecular structural
design, H-aggregation can be introduced to molecular semi-
conductors to achieve higher performance and better repro-
ducibility in organic electronics.
(a)
TIPS-pentacene
0.4
0.3
0.2
0.1
0.0
0.000
0.002
0.004
Field-effect mobility
(b)
0.8
CE4T
CB4T
F8T2
4. Experimental Section
0.6
0.4
0.2
0.0
Materials: All reagents were purchased from Aldrich or Alfa, and
chemical agents were purchased from Aldrich, Acros, and Lancaster.
Tetrahydrofuran (THF) and diethyl ether were distilled over sodium in
the presence of benzophenone to remove water.[28] Toluene was distilled
over calcium hydride prior to use to ensure that it was anhydrous.
Chloroform, dichloromethane, and methanol were used without
further purification. The reagents 5,5’-dibromo-[2,2’]bithiophene,
5-bromo-[2,2’;5’,2’’]terthiophene, and (4-bromo-butyl)-cyclohexane were
synthesized according to known procedures.[28–31]
CE4T
CB4T
F8T2
0.000 0.004 0.008 0.012 0.016
Field-effect mobility
Measurements: The 1H-NMR spectra were recorded using a Bruker
AM-200 spectrometer. FT-IR spectra were measured using a Bomen
Michelson series FT-IR spectrometer. The melting points were determined
using an Electrothermal Mode 1307 digital analyzer. Thermogravimetric
analysis (TGA) was performed under nitrogen using a TA Instruments
2050 thermogravimetric analyzer. The sample was heated at a 10 °C min−1
heating rate from 30 °C to 700 °C. Differential scanning calorimetry (DSC)
was conducted under nitrogen using a TA Instruments 2100 differential
scanning calorimeter. The samples were heated at 10 °C min−1 from 30 °C
to 300 °C. UV-vis absorption spectra were measured using a Perkin-Elmer
LAMBDA-900 UV/vis/IR spectrophotometer.
Synthesis of (2-Chloro-ethyl)-cyclohexane: 2-Cyclohexyl-ethanol (20.0 g,
0.16 mol) was added slowly to a mixture of purified thionyl chloride (46.4 g,
0.39 mol) and pyridine (24.7 g, 0.31 mol) with cooling at 0 °C over an
ice bath. After one hour, the solution was heated to 115 °C and stirred
overnight. After the mixture was cooled to room
Figure 6. The distribution of field-effect mobilities of OFETs based on
a) TIPS-pentacene, b) CE4T, CB4T, and F8T2.
These four materials were suitable for soluble processing,
suggesting that all materials had good solubility and good
wettability in nonpolar solvents. Interestingly, we found that
quaterthiophene derivatives with asymmetrically substituted
cyclohexyl end-groups, CE4T and CB4T, produced H-type
aggregation that differed from the aggregation observed
for materials with symmetrically substituted cyclohexyl
end-groups, DCE4T and DCB4T, or other conventional
temperature, it was quenched with ice water and
extracted with CH2Cl2. The combined organic layer
was dried over anhydrous magnesium sulfate.
The solvent was removed by rotary evaporation,
and the resulting residue was purified by vacuum
fractional distillation to give the pure product
as a colorless liquid. Yield: 12.4 g (54.4%). BP:
50–52 °C/3 mmHg. 1H NMR (300 MHz, CDCl3), δH
3.5 (t, 2H), 1.7 (m,7H), 1.65 (m, 1H), 1.2 (m, 3H),
0.9 (m, 2H).
Synthesis
of
2-(2-Cyclohexyl-ethyl)-thiophene:
Magnesium (2.18 g, 89.6 mmol) was added to a
three-neck flask, and water and air were removed.
Subsequently, (2-chloro-ethyl)-cyclohexane (12.0 g,
81.5 mmol) and diethyl ether (250 mL) were
added dropwise to the flask. The reactant mixture
was refluxed for
reagent solution was then added dropwise
to solution of 2-bromothiophene (13.29 g,
1 h. The resulting Grignard
a
81.5 mmol) and Ni(dppp)Cl2 (where dppp is
diphenylphosphinopropane) (0.44 g, 0.8 mmol)
with cooling over an ice bath. The mixture was then
Figure 7. Four inch diameter sample using CE4T. Each sample size was 1 cm × 1 cm, and the stirred overnight at room temperature and quenched
number of samples was 52. The gradation of colors shows the average field-effect mobility in with 1 N HCl. The organic layer was separated
each sample, and 4 transistors were measured in each sample.
and extracted with ether. The combined ether
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solution was washed with H2O three times and dried over anhydrous
MgSO4. After removal of the solvent, the resulting liquid was subjected
to purification by flash chromatography using silica gel with hexane
as the eluent. Yield: 6.65 g (42.1%). 1H NMR (300 MHz, CDCl3), δH
7.14 (m, 1H), 6.96 (m, 1H), 6.81 (m, 1H), 2.88 (t, 2H), 1.58–1.82 (m, 7H),
1.25 (m, 4H), 0.97 (t, 2H).
Synthesis of Tributyl-[5-(2-cyclohexyl-ethyl)-thiophen-2-yl]-stannane: BuLi
(2.5 M in hexane) (14.45 mL, 35.9 mmol) was added slowly to a solution
of 2-(2-cyclohexyl-ethyl)-thiophene (6.65 g, 34.2 mmol) in THF (70 mL)
at –78 °C. After lithiation, tributyltin chloride (11.76 g, 36.1 mmol) was
added to the mixture at –78 °C. The reaction was allowed to warm to
room temperature and stirred overnight. After normal workup, the
organic layer was separated and extracted with hexane. The combined
hexane solution was washed with saturated NaHCO3 three times and
dried over anhydrous MgSO4. After removal of the solvent, the resulting
crude product was used without further purification. Yield: 11.58 g (80%).
1H NMR (300 MHz, CDCl3), δH 7.01 (d, 1H), 6.91 (d, 1H), 2.89 (t, 2H),
1.56–1.69 (m, 7H), 1.32–1.39 (m, 18H), 1.21 (m, 4H), 1.07–1.12 (m, 9H),
0.93 (t, 2H).
Synthesis of 5,5’’’-Dicyclohexyl-ethyl-[2,2’;5’,2’’;5’’,2’’’’-quaterthiophene
(DCE4T): Tributyl-[5-(2-cyclohexyl-ethyl)-thiophen-2-yl]-stannane (2.74 g,
5.67 mmol) and 5,5’-dibromo-[2,2’]-bithiophene (0.80 g, 2.46 mmol)
were added to toluene (30 mL) in a one-neck flask equipped with a
condenser. After three freeze-pump-thaw cycles, Pd(PPh3)4 (0.17 g,
6 mol%) was added to the mixture under a nitrogen atmosphere, and the
solution was heated to 100 °C for 24 h. The mixture was cooled to room
temperature, filtered, and washed with hexane and THF to the remove
starting material and any monocoupled product. The resulting solid
was subjected to purification by chromatography using silica gel with
toluene as the eluent to afford nearly pure solid product. This solid was
further purified by recrystallization from MeOH–CHCl3 to give yellow-
orange crystals. Yield: 0.83 g (61.2%). 1H NMR (300 MHz, CDCl3), δ H
7.05 (d, 1H), 7.00 (t, 2H), 6.69 (t, 1H), 2.83 (t, 2H), 1.56–1.80 (m, 7H),
1.23 (m, 4H), 0.95 (m, 2H). MS (EI) m/z: 550 (M+).
Synthesis of 5-(2-Cyclohexyl-ethyl)-[2,2’;5’,2’’;5’’,2’’’’-quaterthiophene
(CE4T): Tributyl-[5-(2-cyclohexyl-ethyl)-thiophen-2-yl]-stannane (3.0 g,
6.20 mmol) and 5-bromo-[2,2’;5’,2’’]terthiophene (2.03 g, 6.20 mmol)
were added to toluene (40 mL) in a one-neck flask equipped with a
condenser. After three freeze-pump-thaw cycles, Pd(PPh3)4 (0.43 g,
6 mol%) was added to the mixture under a nitrogen atmosphere, and
the solution was heated to 90 °C for 24 h. The mixture was cooled to
room temperature, quenched with water, and extracted with CHCl3.
The combined organic layer was dried over anhydrous magnesium
sulfate. After removing the solvent, the resulting solid was subjected
to purification by chromatography using silica gel with hexane as the
eluent. Finally, the solid was recrystallized from MeOH–CH2Cl2 to give
the pure target compound as yellow crystals. Yield: 1.88 g (69.0%). 1H
NMR (300 Hz, CDCl3), δH 7.23 (d, 1H), 7.19 (d, 1H), 7.05 (m, 6H),
6.70 (d, 1H), 2.81 (t, 2H), 1.62–1.79 (m, 7H), 1.25 (m, 4H), 0.89 (m, 2H).
MS (EI) m/z: 440 (M+)
of 2-(4-cyclohexyl-butyl)-thiophene (2.77 g, 14.2 mmol) in THF (30 mL)
at –78 °C. After lithiation, tributyltin chloride (4.88 g, 14.98 mmol) was
added to the mixture at –78 °C. The reaction was allowed to warm to
room temperature and stirred overnight. After normal workup, the
organic layer was separated and extracted with hexane. The combined
hexane solution was washed with saturated NaHCO3 three times and
dried over anhydrous MgSO4. After removal of the solvent, the crude
product was used without further purification. Yield: 5.83 g (80%). 1H
NMR (300 MHz, CDCl3), δH 7.01 (d, 1H), 6.93 (d, 1H), 2.85 (t, 2H), 1.69
(m, 7H), 1.37 (m, 20H), 1.23 (m, 6H), 1.09 (m, 9H), 0.89 (d, 2H).
Synthesis of 5,5’’’-Dicyclohexyl-butyl-[2,2’;5’,2’’;5’’,2’’’]quaterthiophene
(DCB4T): Tributyl-[5-(4-cyclohexyl-butyl)-thiophen-2-yl]-stannane (2.96 g,
5.8 mmol) and 5,5’-dibromo-[2,2’]bithiophene (0.75 g, 2.3 mmol) were
added to toluene (20 mL) in a one-neck flask equipped with a condenser.
After three freeze-pump-thaw cycles, Pd(PPh3)4 (0.16 g, 6 mol%) was
added to the mixture under a nitrogen atmosphere, and the solution
was heated to 100 °C for 24 h. Subsequently, the mixture was cooled
to room temperature, quenched with water, and extracted with CHCl3.
The combined organic layer was dried over anhydrous magnesium
sulfate. After removing the solvent, the resulting solid was subjected to
purification by chromatography using silica gel with hexane as the eluent
to afford a nearly pure solid product. This solid was further purified by
recrystallization from MeOH–CHCl3 to give yellow crystals. Yield: 0.84 g
(59.8%). 1H NMR (300 MHz, CDCl3), δH 7.05 (d, 1H), 7.00 (t, 2H),
6.69 (d, 1H), 2.81 (t, 2H), 1.68 (m, 7H), 1.38 (m, 2H), 1.22 (m, 6H),
0.89 (d, 2H). MS (EI) m/z: 607 (M+)
Synthesis of 5-(2-Cyclohexyl-butyl)-[2,2’;5’,2’’;5’’,2’’’]-quaterthiophene
(CB4T): Tributyl-[5-(2-cyclohexyl-ethyl)-thiophen-2-yl]-stannane (2.36 g,
4.6 mmol) and 5-bromo-[2,2’;5’,2’’]terthiophene (1.51 g, 4.6 mmol) were
addedtotoluene(30mL)inaone-neckflaskequippedwithacondenser. After
three freeze–pump–thaw cycles, Pd(PPh3)4 (0.32 g, 6 mol%) was added to
the mixture under a nitrogen atmosphere, and the solution was heated to
90 °C for 24 h. After the mixture was cooled to room temperature, it was
quenched with water and extracted with CHCl3. The combined organic layer
was dried over anhydrous magnesium sulfate. After removing the solvent,
the resulting solid was subjected to purification by chromatography using
silica gel with hexane as the eluent. Finally, this solid was recrystallized
from MeOH–CH2Cl2 to give the pure target compound as yellow crystals.
Yield:1.54g(71.2%).1HNMR(300MHz,CDCl3),δH7.23(t,1H),7.18(d,1H),
7.06 (m, 6H), 6.69 (d, 1H), 2.81 (t, 2H), 1.68 (m, 7H), 1.38 (m, 2H),
1.22 (m, 6H), 0.89 (d, 2H). MS (EI) m/z: 468 (M+).
Fabrication and Characterization of the OFET Devices: Top-contact OFETs
were fabricated on a common gate of highly n-doped silicon with a 300 nm
thick thermally grown SiO2 dielectric layer. The octadecyltrichlorosilane
monolayer was treated in toluene solution for 2 h. Solutions of the organic
semiconductors were spin-coated at 2000 rpm from 0.7 wt.% chloroform
solutions to form thin films with a nominal thickness of 45 nm, confirmed
using a surface profiler (Alpha Step 500, Tencor). Gold source and drain
electrodes were evaporated on top of the semiconductor layers (100 nm).[32]
For all measurements, we used channel lengths (L) of 100 μm and channel
widths (W) of 2000 μm. The electrical characteristics of the FETs were
measured in air using both Keithley 2400 and 236 source/measure units.
Field-effect mobilities were extracted in the saturation regime from the slope
of the source–drain current. X-ray diffraction (XRD) studies were performed
using the 4C1 and 4C2 beamlines at the Pohang Accelerator Laboratory
(PAL). The measurements were carried out with a sample-to-detector
distance of 136 mm. Data were typically collected for ten seconds using an
X-ray radiation source of λ = 0.138 nm with a 2D charge-coupled detector
(CCD) (Roper Scientific, Trenton, NJ, USA). The samples were mounted
onto a home-built z-axis goniometer equipped with a vacuum chamber. The
incidence angle ai of the X-ray beam was set to 0.14°, which is intermediate
between the critical angles of the films and the substrate (ac,f and ac,s).
Synthesis of 2-(4-cyclohexyl-butyl)-thiophene: Magnesium (2.16 g,
89.0 mmol) was added to a 3-neck flask, and water and air were removed.
Subsequently, (4-bromo-butyl)-cyclohexane (15 g, 68.4 mmol) and
diethyl ether (200 mL) were added dropwise to the flask. The reactant
mixture was refluxed for 2 h. The resulting Grignard reagent solution
was then added dropwise to the solution of 2-bromothiophene (11.16 g,
68.4 mmol) and Ni(dppp)Cl2 (where dppp is diphenylphosphinopropane)
(2.22 g, 6 mol%) with cooling over an ice bath. The mixture was then
stirred overnight at room temperature and quenched with 1 N HCl. The
organic layer was separated and extracted with ether. The combined ether
solution was washed with H2O three times and dried over anhydrous
MgSO4. After removing the solvent, the resulting liquid was subjected to
purification by flash chromatography using silica gel with hexane as the
eluent. Yield: 6.19 g (40.7%). 1H NMR (300 MHz, CDCl3), δH 7.16 (m, 1H),
6.95 (m, 1H), 6.83 (m, 1H), 2.85 (t, 2H), 1.72 (m, 7H), 1.39 (m, 2H),
1.25 (m, 6H), 0.91 (d, 2H).
Supporting Information
Synthesis of Tributyl-[5-(4-cyclohexyl-butyl)-thiophen-2-yl]-stannane: BuLi
(2.5 M in hexane) (6 mL, 14.91 mmol) was added slowly to a solution
Supporting Information is available from the Wiley Online Library or
from the author.
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[10] C. Nogues, P. Lang, B. Desbat, T. Buffeteau, L. Leiserowitz, Lang-
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Acknowledgements
S.-O.K. and T.K.A. contributed equally to this work. This work was
supported by a grant (F0004011–2010-33) from the Information Display
R&D Center, one of the 21st Century Frontier R&D Program funded by the
Ministry of Knowledge Economy and Basic Science Research Program;
by a grant (2010–000826) from the National Research Foundation
(NRF) funded by the Ministry of Education, Science and Technology
and the Ministry of Knowledge Economy (MKE) and Korea Institute for
Advancement in Technology (KIAT) through the Workforce Development
Program in Strategic Technology.
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Adv. Funct. Mater. 2011, 21, 1616–1623
2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1623