Synthesis and electrical properties of novel oligomer semiconductors for organic field-effect transistors (OFETs): Asymmetrically end-capped acene-heteroacene conjugated oligomers

Article abstract of DOI:10.1016/j.dyepig.2014.07.008

Noble small molecules composed of an aromatic phenylene-trithiophene and naphthalene-trithiophene conjugated core were synthesized and examined for their property in p-type OFET devices. The phenylene and naphthalene moieties were combined with a trithiophene to introduce torsional structures into the conjugated core and improve the solubility compared with the solubility of the quaterthiophene core containing molecule. Although phenylene-trithiophene and naphthalene-trithiophene containing molecules showed lower mobilities (2.9 × 10^-3 and 1.04 × 10^-2 cm²/V, respectively) than quaterthiophene containing molecule (3.21 × 10 ^-2 cm²/V), they showed more film uniformity caused by improved solubility. The thermal stability, photochemical properties, and morphological characteristics were investigated using TGA, DSC, cyclic voltammetry, and XRD techniques.

Full text of DOI:10.1016/j.dyepig.2014.07.008

Dyes and Pigments 112 (2015) 220e226
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and electrical properties of novel oligomer semiconductors
for organic field-effect transistors (OFETs): Asymmetrically
end-capped acene-heteroacene conjugated oligomers
,
,
, ***
Jang Yeol Back a 1, Yebyeol Kim b 1, Tae Kyu An b, Moon Seong Kang c, Soon-Ki Kwon a
,
Chan Eon Park b *, Yun-Hi Kim c
,
, **
a School of Materials Science and Engineering and Research Institute for Green Energy Convergence Technology (REGET), Gyeongsang National University,
Jin-ju 660-701, Republic of Korea
b POSTECH Organic Electronics Laboratory, Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology,
Pohang 790-784, Republic of Korea
c Department of Chemistry and ERI, Gyeongsang National University, Jin-ju 660-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Noble small molecules composed of an aromatic phenylene-trithiophene and naphthalene-trithiophene
conjugated core were synthesized and examined for their property in p-type OFET devices. The phe-
nylene and naphthalene moieties were combined with a trithiophene to introduce torsional structures
into the conjugated core and improve the solubility compared with the solubility of the quaterthiophene
core containing molecule. Although phenylene-trithiophene and naphthalene-trithiophene containing
molecules showed lower mobilities (2.9 ꢀ 10ꢁ3 and 1.04 ꢀ 10ꢁ2 cm2/V, respectively) than quaterthio-
phene containing molecule (3.21 ꢀ 10ꢁ2 cm2/V), they showed more film uniformity caused by improved
solubility. The thermal stability, photochemical properties, and morphological characteristics were
investigated using TGA, DSC, cyclic voltammetry, and XRD techniques.
Received 25 April 2014
Received in revised form
7 July 2014
Accepted 10 July 2014
Available online 18 July 2014
Keywords:
Organic field-effect transistor
Small molecule semiconductor
Solubility
© 2014 Elsevier Ltd. All rights reserved.
Conjugated core modification
End-group
1. Introduction
developed morphology that includes short pep distances, favor-
able chain conformations, and submicron-sized crystal structures
without defects [7,11,13e15]; and ii) the molecular structure must
provide an appropriate highest occupied molecular orbital
(HOMO), lowest unoccupied molecular orbital (LUMO), and must
yield devices that provide stable operation and resist oxidation over
time [16,17].
The components of small molecules, including the end groups,
conjugated core, and substituted heteroatoms, may be modified
through systematic investigations in an attempt to optimize the
molecular structures [18]. Above all, modifications to the conju-
gated core provide a key method for controlling the electrical
properties of the organic semiconductor, such as the electron af-
finity, ionization potential, inter/intramolecular interactions, and
solubility in organic solvents. Previous work in our group has
investigated the quaterthiophene conjugated core with bulky end
groups, which displayed adequate field-effect mobility [19,20];
however, the quaterthiophene core did not convey sufficient solu-
bility to provide uniform film morphology during solution-
processing as the organic semiconductor.
Solution-deposited organic semiconductors have been widely
studied due to their suitability for large-area processing, their
mechanical flexibility, and their light weight [1e10]. Small mole-
cule semiconductors are particularly profitable for electronic ap-
plications because they can be reliably synthesized with accurate
molecular weight control without significant batch-to-batch vari-
ations [11,12]. Additionally, small molecule semiconductors can
potentially provide high-quality chain alignment.
Electronic applications require that small molecule semi-
conductors satisfy several criteria, in addition to providing good
electrical properties: i) they must form a thin film with a well-
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: skwon@gnu.ac.kr (S.-K. Kwon), cep@postech.ac.kr (C.E. Park),
ykim@gnu.ac.kr (Y.-H. Kim).
1
Jang Yeol Back and Yebyeol Kim contributed equally.
http://dx.doi.org/10.1016/j.dyepig.2014.07.008
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

J.Y. Back et al. / Dyes and Pigments 112 (2015) 220e226
221
This study sought to enhance the solubility of the small mole-
cule semiconductors by combining phenylene and naphthalene
aromatic rings into the tri-thiophene conjugated core, which offer a
rigid torsional structure due to topological constraints [21] to yield
product was extracted with dichloromethane and purified by col-
umn chromatography using hexane as the eluent. Yield: 2.3 g
(60%); IR (KBr, cmꢁ1); 3031e3064 (aromatic CH),1H NMR (300 MHz,
CDCl3, ppm):
d
¼ 7.62e7.59 (m, J ¼ 5.2 Hz, 2H), 7.40e7.38 (t,
a
homogeneous thin film. The phenylene-modified cyclo-
J ¼ 3.9 Hz, 2H), 7.31e7.28 (m, 1H), 7.24e7.23 (d, J ¼ 3 Hz, 1H),
7.10e7.09(d, J ¼ 3 Hz, 1H), 7.01e6.95 (dd, J ¼ 9 Hz, 2H).
hexylethynyltrithiophenephenylene (CHE3TP) and naphthalene-
modified cyclohexylethynyltrithiophenenaphthalene (CHE3TN)
were synthesized and characterized as semiconductors for use in
OFETs. The electron-rich triple bond in CHE3TP and CHE3TN
2.1.5. Synthesis of 5-bromo-5'-(naphthalen-2-yl)-2,2'-bithiophene
(4)
extended the
molecular rigidity [22].
p
-system of the conjugated core and improved the
A mixture of 5,5'-dibromo-2,2'-bithiophene (6 g, 18.51 mmol)
and naphthalene-2-ylboronic acid (2.22 g, 12.95 mmol) were added
CHE3TP and CHE3TN compounds were characterized by ther-
mogravimetric analysis (TGA), differential scanning calorimetry
(DSC), UVevis spectroscopy, and X-ray diffraction (XRD) tech-
niques. Density functional theory (DFT) calculations were per-
formed to investigate their electronic structures.
to toluene (80 mL), 2 M K2CO3 (15 mL), and tetrakis(-
triphenylphosphine) palladium(0) (0.17 g, 1.47 ꢀ 10ꢁ4 mol). After
stirring for 48 h at 85 ꢂC, 2 N HCl (40 mL) were added. The crude
product was extracted with dichloromethane and purified by col-
umn chromatography using hexane as the eluent. Yield: 2.5 g
(53%); IR (KBr, cmꢁ1); 3031e3064 (aromatic CH), 1H NMR
2. Experimental
(300 MHz, CDCl3, ppm):
7.76e7.75 (d, J ¼ 1.8 Hz, 1H), 7.52e7.48 (m, 2H), 7.37e7.35 (d,
J ¼ 3.9 Hz, 1H), 7.15e7.14 (d, J ¼ 3.6 Hz 1H), 7.02e6.98 (m, 2H).
d
¼ 8.04 (s, 1H), 7.88e7.86 (m, 3H),
2.1. Materials and synthesis
2.1.1. Materials
2.1.6. Synthesis of 5-cyclohexylethynyl-5'-phenyl-[2.2',5',2']
quaterthiophene (CHE3TP)
2-Bromothiophene, N-bromosuccinimide (NBS), n-butyllithium,
2-isopropoxy-3,3,4,4-tetramethyl-1,3,2-dioxaborolane,
bis(diphenylphosphinopropane)-dichloronickel,
1,3-
N,N-dime-
2-(5-(Cyclohexylethynyl)thiophen-2-yl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (1.77 g, 5.6 mmol) and 5-bromo-5'-phenyl-
2,2'-bithiophene (1.8 g, 5.6 mmol) were added to toluene (50 mL),
2 M K2CO3 (10 mL), THF (8 mL), and tetrakis(triphenylphosphine)
palladium(0) (0.05 g, 0.43 ꢀ 10ꢁ4 mol). The reaction mixture was
stirred for 48 h at 85 ꢂC, and 2 N HCl (400 mL) were added at room
temperature. The crude product was extracted with dichloro-
methane (3 ꢀ 300 mL) and purified by column chromatography
using hexane as the eluent. Yield: 1.1 g (46%); mp ¼ 191 ꢂC, IR (KBr,
cmꢁ1): 3050 (aromatic CeH), 2215 (alkyne), and 2847e2973
thylformamide (DMF), copper(I), sodium carbonate, 2,2-bipyridine,
1,5-cyclooctadiene, bis(cyclooctadiene)nickel(0), dichloro-((bis-
diphenylphosphino)ferrocenyl)palladium(II), cyclohexylacetylene,
and tetrakis(triphenylphosphine)palladium(0) were purchased
from Aldrich.
2.1.2. Synthesis of 2-(cyclohexylethynyl)thiophene (1)
CuI (1.07 g, 5.62 mmol) and Pd(dppf)Cl2 (0.75 g, 0.93 mmol)
were added to a mixture of 2-bromothiophene (10 g, 98.30 mmol)
and di(isopropylamine) (200 mL), followed by heating at 50 ꢂC.
After the slow addition of cyclohexylacetylene (15.29 g,
93.80 mmol) to the mixture, the reaction mixture was stirred for 4 h
at 80 ꢂC. The crude product was extracted with ethyl acetate
(3 ꢀ 200 mL) and the combined extracts were dried over anhydrous
magnesium sulfate and purified by column chromatography using
hexane as the eluent. Yield: 16.5 g (92%); IR (KBr, cmꢁ1): 2221
(aliphatic CeH) 1H NMR (300 MHz, CDCl3, ppm):
d
¼ 7.63e7.60 (t,
J ¼ 7.2 Hz, 2H), 7.43e7.38 (t, J ¼ 7.2 Hz, 2H), 7.30e7.24 (m, 2H),
7.17e7.15 (d, J ¼ 7.2 Hz, 1H), 7.11e7.07 (m, 2H), 7.03e7.00 (m, 2H),
2.66e2.60 (m, 1H), 1.92e1.75 (m, 5H), 1.53e1.27 (m, 5H).). HR-MS
Found 430.0891, requires 430.0884.
2.1.7. Synthesis of 5-cyclohexylethynyl-5'-naphthalene-2-yl-
[2,2',5'2']terthiophene (CHE3TN)
(alkyne), 1H NMR (CDCl3, 300 MHz, ppm):
d
¼ 7.15 (dd, J ¼ 5.2 Hz,
2-(5-(Cyclohexylethynyl)thiophen-2-yl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (1.7 g, 4.57 mmol) and 5-bromo-5'-(naph-
thalene-2-yl)-2,2'-bithiophene (1.54 g, 4.57 mmol) were added to
toluene (50 mL), 2 M K2CO3 (10 mL), THF (8 mL) and tetrakis(-
triphenylphosphine) palladium(0) (0.05 g, 0.43 ꢀ 10ꢁ4 mol). The
reaction mixture was stirred for 48 h at 85 ꢂC, and 2 N HCl
(400 mL) were added at room temperature. The crude product was
extracted with dichloromethane (3 ꢀ 300 mL) and purified by
column chromatography using hexane as the eluent. Yield: 1.05 g
1H), 7.10 (dd, J ¼ 3.7 Hz, 1H), 6.92 (dd, J ¼ 5.2 Hz, 1H), 2.42e2.41 (m,
1H), 1.90e1.86 (m, 5H), 1.46e1.23 (m, 5H).
2.1.3. Synthesis of 2-(5-(cyclohexylethynyl)thiophen-2-yl)-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (2) [23]
2.5 M n-BuLi (32.8 mL, 95.63 mmol) was added to a mixture of
2-(cyclohexylethynyl)thiophene (13 g, 68.31 mmol) and tetrahy-
drofuran (THF) (250 mL) at ꢁ78 ꢂC. After stirring for 1 h, 2-
isopropoxy-4,4,5,5,-tetramethyl-1,3,2-dioxaborolane
(17.8
g,
(48%); mp ¼ 220 ꢂC, 1H NMR (300 MHz, CDCl3, ppm):
d
¼ 8.05 (s,
81.97 mmol) were added. After stirring for 12 h, the crude product
was extracted with ethyl acetate (3 ꢀ 200 mL) and water (200 mL).
The pure product was obtained by column chromatography using
hexane as the eluent. Yield: 4 g (25%); IR (KBr, cmꢁ1): 2215 (alkyne),
2847e2973 (aliphatic CH3), 1H NMR (CDCl3, 300 MHz, ppm):
1H), 7.88e7.83 (t, J ¼ 6.3 Hz, 3H), 7.77e7.56 (d, J ¼ 3.6 Hz, 1H),
7.52e7.47 (m, 2H), 7.39e7.37 (d, J ¼ 3.9 Hz, 1H), 7.21e7.20 (d,
J ¼ 3.9 Hz, 1H), 7.14e7.08 (dd, J ¼ 3.9 Hz, 2H), 7.04e7.03 (d,
J ¼ 3.9 Hz, 2H), 2.65e2.62 (m, 1H), 1.92e1.75(m, 5H), 1.53e1.27(m,
5H). IR (KBr, cmꢁ1): 3063 (aromatic CeH), 2215 (alkyne),
2847e2973 (aliphatic CeH). HR-MS Found 480.1046, requires
480.1040.
d
¼ 7.46e7.45 (d, J ¼ 3 Hz, 1H), 7.16e7.15 (d, J ¼ 3 Hz, 1H), 2.65e2.59
(m, 1H), 1.95e1.73 (m, 5H), 1.60e1.41 (m, 5H), 1.34 (s, 12H).
2.1.4. Synthesis of 5-bromo-5'-phenyl-2,2'-bithiophene (3)
A mixture of 5,5'-dibromo-2,2'-bithiophene (5 g, 15.42 mmol)
and phenylboronic acid (5 g, 12.34 mmol) were added to a mixture
2.2. Measurements
The 1H NMR spectra were recorded using a Bruker AM-300
spectrometer. The FT-IR spectra were measured on a Bomem
Michelson series FT-IR spectrometer. The thermal analysis was
performed using a TA TGA 2100 thermogravimetric analyzer under
toluene (50 mL),
2
M
K2CO3 (10 mL), and tetrakis(-
triphenylphosphine) palladium(0) (0.17 g, 1.47 ꢀ 10ꢁ4 mol). After
stirring for 48 h at 85 ꢂC, 2 N HCl (40 mL) was added. The crude

222
J.Y. Back et al. / Dyes and Pigments 112 (2015) 220e226
a nitrogen atmosphere with a heating rate of 20 ꢂC/min. Differential
scanning calorimetry (DSC) measurements were conducted under
nitrogen using a TA instrument 2100 DSC. The samples were heated
with a rate of 20 ꢂC/min from 30 ꢂC to 250 ꢂC. UVevis absorption
spectra were measured using a Perkin Elmer LAMBDA-900 UV/VIS/
IR spectrophotometer. UVevis absorption studies were carried out
using PerkineElmer LAMBDA-900 UV/VIS/IR spectrophotometer.
Cyclic voltammetry (CV) was performed on an EG and G Parc model
273 Å potentiostat/galvanostat system with a three-electrode cell
in a solution of Bu4NCIO4 (0.1 M) in acetonitrile at a scan rate of
100 mV sꢁ1. CHE3TP and CHE3TN films were coated onto a square
Pt electrode (0.50 cm2) by dipping into xylene solutions containing
each of the molecules and then drying in air. A Pt wire was used as
the counter electrode, and an Ag/AgNO3 (0.1 M) electrode was used
as the reference electrode.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic scheme for preparing CHE3TP and CHE3TN is
outlined in Scheme 1. CHE3TP and CHE3TN were synthesized by a
Sonogashira and Suzuki coupling reaction. CHE3TP and CHE3TN
were characterized by H NMR, IR, and Mass spectroscopies.
CHE3TN was more soluble than CHE3TP in xylene, although both
small molecules showed better solubilities than cyclo-
hexylethynylquaterthiophene (CHE4T: the molecular structure is
shown in Fig. 4).
As shown in Fig. 1, the thermal properties of the synthesized
CHE3TP and CHE3TN were evaluated by TGA and DSC. The TGA
measurements were conducted to investigate the device longevity
[25,26]. CHE3TP and CHE3TN showed good thermal stability for use
in organic electronics (5% decomposition after heating at 365 ꢂC or
415 ꢂC, respectively). The DSC measurements revealed that CHE3TN
displayed a higher melting point (222 ꢂC) than CHE3TP (192 ꢂC),
indicating the presence of stronger intermolecular interactions in
the crystal structure of CHE3TN [27,28].
Microstructural characterization of the CHE3TP and CHE3TN
thin films was confirmed using XRD (beam energy: 8 keV). The
samples were prepared by spin-coating a xylene solution onto the
octyltrichlorosilane (OTS)-chemisorbed SiO2/Si wafer (film thick-
ness: 50 nm), and measurements were conducted in a vacuum
chamber.
2.3. Fabrication and characterization of OFETs
3.2. Photochemical properties
The electrical properties of the CHE3TP and CHE3TN films were
investigated using a top-contact bottom-gated OFET configuration,
with a 3000 Å thick SiO2 dielectric on a heavily N-type doped sil-
icon substrate as the gate electrode. The substrate was cleaned in
piranha solution, composed of concentrated H2SO4 (60 mL) and
H2O2 (40 mL), and modified by OTS to reduce the number of hy-
droxyl groups present on the SiO2 surface. The organic semi-
conductor thin films were formed by spin-coating at 2000 rpm
from a 0.6 wt% xylene solution to a nominal thickness of 50 nm, as
confirmed using an ellipsometer (J.A.WOLLAM Co., Inc.). After
depositing the semiconductor film, gold source/drain electrodes
were deposited (100 nm) with width-to-length ratios (W/
Fig. 2 shows the photochemical absorption spectra as measured
in the UVevis region in the solution and thin film states. CHE3TP
and CHE3TN revealed absorption maxima in a dilute chloroform
solution at 402 and 408 nm, respectively. The edges of the ab-
sorption peaks in the film state in both semiconductors were red-
shifted relative to the peaks obtained in the solution state.
The HOMO levels of CHE3TP and CHE3TN were calculated from
the oxidation onset measured using cyclic voltammetry (CV) and
were shown to be 5.43 eV and 5.55 eV for CHE3TP and CHE3TN,
respectively. The optical band gaps of CHE3TP and CHE3TN were
extracted from the band edge obtained in the thin film state. All
photochemical properties are summarized in Table 1. Computa-
tional simulations were carried out to better understand the elec-
tronic and geometric structures, as shown in Fig. 3. The HOMO and
LUMO levels of CHE3TN were slightly lower than those of CHE3TP,
and these results corresponded to the photochemical measure-
ments. In both molecules, the phenylene and naphthalene moi-
eties, which were combined with three thiophene conjugated
L) ¼ 1000
mm/100 mm. The OFET measurements were carried out in
a dark room and ambient environment using Keithley 2400 and
236 source/measure units. The OFET parameters, such as the field-
effect mobilities, threshold voltage, and on-off ratio, were extracted
in the saturation regime (ID: drain current ¼ ꢁ80 V) using the
square root of the drain currentegate voltage (I1D/2eVG), plotted
according to the following formula: ID ¼ mCdielðW=2LÞðVG ꢁ VthÞ2,
cores, contributed less to the p-orbital delocalization of the HOMO
where
m
is the field-effect carrier mobility, Cdiel is the capacitance
and LUMO levels (Fig. 3) than the thiophene moiety in the qua-
terthiophene core (Fig. 4), although the tilted structure enhanced
the solubility of the CHE3TP and CHE3TN molecules.
per unit area of the dielectric layer, VG is the gate voltage, and Vth is
the threshold voltage [24].
Scheme 1. Synthetic routes of the CHE3TP and CHE3TN.

J.Y. Back et al. / Dyes and Pigments 112 (2015) 220e226
223
Fig. 1. DSC and TGA thermograms of CHE3TP (a) and CHE3TN (b).
Fig. 2. UV-absorption spectra of CHE3TP (a) and CHE3TN (b).
3.3. Crystalline morphology
plane direction and the long-range film uniformity were indi-
cated by the strong intensity and sharpness of the (100) diffraction
peak, which correlated with a higher charge carrier mobility than
was observed in CHE3TP.
The charge carrier mobility in an organic transistor is related to
the crystallinity of the semiconductor. Out-of-plane XRD studies
were performed on the CHE3TP, CHE3TN, and CHE4T films to
assess the crystallinities of these films (Fig. 5). The diffraction peaks
of the small molecules corresponded to the (100) planes grown up
from the semiconductoredielectric interface. These planes indi-
cated an edge-on orientation. The differences in the interlayer
Table 1
Photochemical properties.
a
b
b
a
b
b
Eox
lmax_solution
lmax_film
EHOMO
Eg
ELUMO
(V)
[nm]
film[nm]
(eV)
(eV)
(eV)
spacings of CHE3TN (29.24 Å, 2
q
¼ 3.02ꢂ), CHE3TP (25.97 Å,
CHE3TP
CHE3TN
0.97
1.09
402
408
389*, 424, 461
345*, 364, 465
5.43
5.55
2.64
2.61
2.79
2.94
2
q
¼ 3.4ꢂ), and CHE4T (22.87 Å, 2
q
¼ 3.86ꢂ) arose from the mo-
* Most strong intensity peak position.
lecular lengths of naphthalene, phenylene, and thiophene,
respectively. The highly-ordered CHE3TN structure in the out-of-
a
Electrochemical parameter which calculated with CV.
Electrochemical parameter according to the onset of UV absorption.
b

224
J.Y. Back et al. / Dyes and Pigments 112 (2015) 220e226
Fig. 3. Molecular orbital surfaces of the HOMOs and LUMOs of the CHE3TP (left) and CHE3TN (right).
Fig. 4. DFT calculation of CHE4T.
3.4. OFET characteristics
The electrical properties of the OFETs prepared using CHE3TP or
CHE3TN as semiconductors were characterized, as shown in Fig. 6.
The output curves displayed good linear and saturation behavior.
The transfer characteristics are summarized in Table 2. The mobility
and threshold voltage are average values (7 devices) and other
OFET parameters are extracted from plots of Fig. 6 (c) and (d). The
higher mobility of CHE3TN compared to CHE3TP arose from the
higher solubility of CHE3TN, which formed a highly ordered
structure (Fig. 5) and relatively strong intermolecular interactions,
corroborated by the UVevis absorption spectra (Fig. 2) and the DSC
measurements (Fig. 1).
Although the phenylene-tri-thiophene and naphthalene-tri-
thiophene small molecules showed adequate mobilities for p-
type solution processed semiconductors, both mobilities were
lower than the mobility of quarter-thiophene (CHE4T, mobility:
3.21 ꢀ 10ꢁ2 cm2/V), reported previously [19]. These results were
interpreted as being derived from a trade-off between the phe-
nylene and naphthalene. The tilted phenylene and naphthalene
attached to the three-thiophene cores provided better solubility
and more homogeneous films compared with the quaterthiophene
core (CHE4T); however, the tilted phenylene and naphthalene
Fig. 5. Out-of-plain direction XRD of CHE4T, CHE3TP, and CHE3TN.

J.Y. Back et al. / Dyes and Pigments 112 (2015) 220e226
225
Fig. 6. Output characteristics of thin film transistors based on CHE3TP (a) and CHE3TN (b), Transfer characteristics of CHE3TP (c) and CHE3TN (d).
Table 2
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On/off ratio
Vth_avg(V)
SS
CHE3TP
CHE3TN
0.0029
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ꢁ2.88
ꢁ5.50
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disturbed the
reduced the charge carrier transfer (Fig. 6).
p-orbital delocalization (Figs. 3 and 4), and thus
4. Conclusions
In conclusion, phenylene and naphthalene moieties were com-
bined with a three-thiophene core to form CHE3TP and CHE3TN,
respectively. The moieties were tested as replacements for the
thiophene in quaterthiophene (CHE4T) in an effort to enhance the
solution processability of the small molecules. A greater degree of
torsional structure in the CHE3TP and CHE3TN molecules increased
the solubility and film uniformity, although this structure also
reduced the co-planarity in the conjugated core and mobility.
CHE3TN showed the best solubility and crystallinity among the
CHE3TP, CHE3TN, and CHE4T molecules tested.
Acknowledgments
This study was supported by a grant (2011-0031639) from the
Center for Advanced Soft Electronics under the Global Frontier
Research Program of the Ministry of Education, Science and Tech-
nology. This work was supported by the New & Renewable Energy
Technology Development Program of KETEP (20113020010070)
and by Nano-Material Technology Development Program of NRF by
MSIP (2012M3A7B4049647)
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