Synthesis and characterization of a novel ambipolar polymer semiconductor based on a fumaronitrile core as an electron-withdrawing group

Article abstract of DOI:10.1002/pola.26456

Two conjugated polymers containing stilbene and fumaronitrile moieties were synthesized to investigate their electronic properties by the existence of electron-withdrawing cyano groups on a vinylene backbone. The cyclic voltammetry investigation and time-dependent density functional theory calculations indicated that the cyano substituents lowered the lowest unoccupied molecular orbital (LUMO) energy level by about 0.65 and 0.63 eV, respectively. The lowering of the LUMO energy levels due to the electron-withdrawing properties of the cyano substituents could enhance electron injection capability. Furthermore, bithiophene-fumaronitrile (donor-acceptor) intermolecular interaction facilitates the self-assembly of the polymer chains. Organic field-effect transistors (OFETs) based on PBTSB without the electron-withdrawing group only exhibit hole transport, while OFETs based on PBTFN with cyano substituents exhibit ambipolar characteristics. The growth of PBTFN crystalline fibrils was observed with increasing annealing temperature, which enhanced hole and electron mobility. A complementary-like inverter using PBTFN with ambipolar properties exhibited good symmetry with an inverting voltage nearly half that of the power supply with a gain of 9 at V_DD = 100 V. 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013. Copyright

Full text of DOI:10.1002/pola.26456

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
Synthesis and Characterization of a Novel Ambipolar Polymer
Semiconductor Based on a Fumaronitrile Core as an
Electron-Withdrawing Group
Hyung-Gu Jeong,1 Dongyoon Khim,1 Eunhwan Jung,1 Jin-Mun Yun,1 Juhwan Kim,1
Jamin Ku,1 Yun Hee Jang,1 Dong-Yu Kim1,2
1School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST),
261 Cheomdan-gwagiro, Buk-Gu, Gwangju 500-712, Republic of Korea
2School of Nanobio Materials and Electronics, Heeger Center for Advanced Materials (HCAM), GIST,
Gwangju 500-712, Republic of Korea
Correspondence to: D.-Y. Kim (E-mail: kimdy@gist.ac.kr)
Received 26 June 2012; accepted 21 October 2012; published online 5 December 2012
DOI: 10.1002/pola.26456
ABSTRACT: Two conjugated polymers containing stilbene and
fumaronitrile moieties were synthesized to investigate their
electronic properties by the existence of electron-withdrawing
cyano groups on a vinylene backbone. The cyclic voltammetry
investigation and time-dependent density functional theory cal-
culations indicated that the cyano substituents lowered the
lowest unoccupied molecular orbital (LUMO) energy level by
about 0.65 and 0.63 eV, respectively. The lowering of the
LUMO energy levels due to the electron-withdrawing proper-
ties of the cyano substituents could enhance electron injection
capability. Furthermore, bithiophene-fumaronitrile (donor-
acceptor) intermolecular interaction facilitates the self-assem-
bly of the polymer chains. Organic field-effect transistors
(OFETs) based on PBTSB without the electron-withdrawing
group only exhibit hole transport, while OFETs based on
PBTFN with cyano substituents exhibit ambipolar characteris-
tics. The growth of PBTFN crystalline fibrils was observed with
increasing annealing temperature, which enhanced hole and
electron mobility. A complementary-like inverter using PBTFN
with ambipolar properties exhibited good symmetry with an
inverting voltage nearly half that of the power supply with
C
V
a gain of 9 at VDD ¼ 100 V.
2012 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2013, 51, 1029–1039
KEYWORDS: ambipolar transport; complementary-like inverter;
cyano group; organic field-effect transistor; self-assembly
INTRODUCTION Conjugated polymers are attractive semicon-
ductors for organic field-effect transistors (OFETs),1,2 organic
solar cells (OSCs),3,4 organic light-emitting diodes (OLEDs),5,6
and complementary inverter circuits7,8 because they provide
major advantages such as low-cost, large-area, and roll-to-
roll manufacturing of organic devices due to simple solution
processability.9,10 Among the various electronic devices,
OFETs have been the subject of intensive investigation, and
ambipolar transistors are of particular interest because holes
and electrons are accumulated simultaneously in both p- and
n- channels.11 For example, complementary circuits require
both p- and n- channel operation, but ambipolar semicon-
ductors exhibiting both hole and electron charge transport
can be deposited without the need for micropatterning of
individual p- and n- channels.12 In general, ambipolar tran-
sistors have been demonstrated using three methods that
include a single-layer semiconductor with an intrinsic ambi-
polar nature, a mixture of p- and n- type semiconductors,
and double-layer semiconductor film with hole and electron
transport layers.13,14 Jenekhe and coworkers have reported
many promising results from ambipolar transistors using the
three methods explained above.15–17 Among these three
methods, to the best of our knowledge the method using a
single-layer semiconductor is the best way to fabricate com-
plementary circuits because semiconductor blend and dou-
ble-layer semiconductor film have problems such as nano-
morphology formation in the blend film and intermixing of
the bilayers.17,18
Electron-withdrawing groups lower the highest occupied mo-
lecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) levels; in particular, the lowering of the
LUMO energy level is dominated by an electron acceptor.19
Therefore, the electron can be easily injected from the
source-drain electrode into the organic layer due to a low-
lying LUMO energy level that is suitable for electron injection
and additionally organic materials can have increased oxida-
tive stability due to a low HOMO energy level.20–22 Here, the
energy level of ambipolar semiconductors should be
C
V
2012 Wiley Periodicals, Inc.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 1029–1039
1029

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
considered with the work function of the common electrode
for ensuring efficient electron injection into the LUMO and
hole injection into the HOMO, particularly in the case of a
complementary circuit that has a common electrode to mini-
mize the complex micropatterning process. Recently, many
low band-gap polymers with alternating donor-acceptor
groups have exhibited good ambipolar characteristics.15,23–25
One of the properties that these polymers have in common
is that they have high-lying HOMO energy levels (5.1–5.5 eV)
and low-lying LUMO energy levels (3.6–3.9 eV). In addition,
the hole-electron balance in the semiconductor films is
important for application to complementary circuits. There-
fore, it should be noted here that an adequate electron-with-
drawing group should be introduced to achieve good balance
between hole and electron transport.
ison, a reference semiconductor polymer (poly[4,40-bis(do-
decyl)-2,20-bithiophene-co-stilbene] PBTSB), which does not
contain cyano substituents, was synthesized to characterize
and compare the effects of cyano substituents. We also dem-
onstrated OFETs and complementary-like inverters based on
new synthesized polymers (PBTSB and PBTFN) to investi-
gate electronic properties by alteration of the backbone
structure and introduction of electron withdrawing groups.
Furthermore, the molecular geometries and electronic struc-
tures of the two synthesized polymers (repeating units of
PBTSB and PBTFN) were investigated by density functional
theory (DFT) quantum mechanics calculations.
EXPERIMENTAL
Measurements
1H NMR spectra were measured using a JEOL ECX-400P at
400 MHz. Mass data were obtained using a gas chromatogra-
phy spectrometry (GCMS) (Shimadzu, GCMS-QP2010). Mn
and Mw/Mn of the polymers were measured at 35 ꢁC by gel
permeation chromatography, on a system equipped with a
guard column (Shodex LF-G), two linear columns (Shodex
LF-804), a Model 501 pump (Analytical Scientific Instru-
ments), and an RI-2000 refractive index detector (Schambeck
SFD GmbH) using chloroform at 1 mL/min as an eluent. Lin-
ear polystyrene standards (Shodex SM-105) were applied for
the calibration. Thermal properties of the polymers were
determined by DSC (TA Instruments, DSC-Q20)ꢁand TGA (TA
Instruments, TGA-Q50) at a heating rate of 10 C/min under
a nitrogen atmosphere. The UV–vis spectra of the polymers
were measured by using a UV–vis spectrometer (Perki-
nElmer, lambda 750). Cyclic voltammetry (CV) was con-
ducted using an Autolab PGSTAT 30 potentiostat/galvanostat
in an acetonitrile solution with 0.1 M tetrabutylammonium
perchlorate as the supporting electrolyte, ITO as the working
electrode, a Pt wire as the counter electrode, and Ag wire as
the reference electrode at a scan rate of 100 mV sꢀ1. Atomic
force microscopy (AFM) images were obtained using a Digi-
tal Instruments Multimode AFM controlled by a Nanoscope
IIIa scanning probe micro controller. The POM images were
all taken on an optical microscope (Olympus BX51) equipped
with temperature controlled stages (Linkam LTS 350) and
temperature controller (Linkam TMS 94). X-ray diffraction
(XRD) was measured using a Rigaku RINT 2000 diffractome-
ter using Cu Ka radiation.
Typically, electron-withdrawing groups such as cyano, nitro,
and fluoroalkyl are introduced to a p-type conjugated back-
bone to synthesize n-type organic semiconductors.26 Vinyl-
ene derivatives containing cyano substituents have been
studied extensively for organic electronics such as OLEDs,
OFETs, and OSCs.27–34 Particularly, fumaronitrile containing
two cyano groups on the vinylic double bond could increase
electron injection due to a large electron-withdrawing abil-
ity.20 Recently, Katz and coworkers have demonstrated
OFETs based on small molecules containing fumaronitrile
moieties and reported the n-type characteristics.31 To the
best of our knowledge, however, ambipolar characteristics of
a semiconductor using a fumaronitrile group have not been
reported. Therefore, we attempted to synthesize an ambipo-
lar charge-transport semiconducting polymer containing
both the thiophene (hole-transporting) moiety and the
fumaronitrile (electron-transporting) moiety to demonstrate
ambipolar transistors with common source/drain electrodes.
It is difficult to produce a balance between hole and electron
mobility when the HOMO energy level is aligned to the work
function of the common Au electrode. Therefore, first, the
stilbene moiety, containing benzene derivatives instead of
thiophene, was introduced, which lowered the HOMO energy
level by adjusting the band gap. In addition, the vinylene
unit in the stilbene moiety increased the degree of coplanar-
ity. For example, PETV12T containing a thienylenevinylene
moiety, recently reported by our group, exhibited a high
hole-mobility of 0.15 cm2 Vꢀ1 sꢀ1 due to the coplanarity of
the vinylene unit and the interdigitated structure of
PETV12T.35 Second, two cyano electron-withdrawing groups
were introduced to the vinylene unit (fumaronitrile) to lower
the LUMO energy level because an elevated LUMO energy
level makes electron transport difficult without a buffer layer
to improve electron injection. The electronic and structural
features (e.g., HOMO/LUMO energy levels, crystallinity, mor-
phology, and charge carrier mobilities) are affected by the
nature of the cyano substituents introduced on the conju-
gated backbone.
5,50-Bis(trimethylstannyl)-4,40-bis(dodecyl)-
2,20-bithiophene (1)
5,50-Dibromo-4,40-bis(dodecyl)-2,20-bithiophene was prepared
according to the procedures in the literature.36 In a two-
neck bottom flask, 5,50-dibromo-4,40-bis(dodecyl)-2,20-bithio-
phene (0.25 g, 0.38 mmol) was dissolved in dry hexane (15
mL) and then cooled to 0 ꢁC. A solution of n-BuLi in cyclo-
hexane (0.57 mL, 1.14 mmol) was added dropwise to a solu-
tion of 5,50-dibromo-4,40-bis(dodecyl)-2,20-bithiophene. The
reaction mixture was gradually warmed to room tempera-
ture and then the mixture was vigorously stirred at 60 ꢁC
In this regard, we first report a fumaronitrile containing
ambipolar semiconductor polymer (poly[4,40-bis(dodecyl)-
2,20-bithiophene-co-bis(phenyl)fumaronitrile] PBTFN) via the
synthetic strategies for tuning the energy levels. For compar-
ꢁ
for 1 h. The reaction mixture was cooled to ꢀ0 C and then
trimethyltin chloride (1.20 mL; 1.20 mmol) was added
1030
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 1029–1039

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
dropwise to the reaction mixture, which was then let stand
overnight at room temperature. The resultant mixture was
extracted with methylene chloride and washed with distilled
water and brine. The combined organic fractions were dried
over anhydrous MgSO4, and then filtered under reduced
pressure to give the product. The crude product was fol-
lowed by purification using column chromatography with
hexanes as an eluent, which provided the analytical pure
mmol) was dissolved in dry chlorobenzene (10 mL). The
reaction mixture was stirred at 120 ꢁC for 48 h under N2.
Bromobenzene (0.28 g, 1.75 mmol) was added, and stirred
for 2 h, followed by the addition of 2-(tributylstannyl) thio-
phene (0.65 g, 1.75 mmol), and the reaction mixture was
stirred for 2 h. After cooling to room temperature, the result-
ant mixture was precipitated into a mixture of methanol (90
mL) and concentrated hydrochloric acid (10 mL) with stir-
ring for 1 h at room temperature. The precipitate was fil-
tered and washed several times with methanol. The
unreacted monomers and low-molecular-weight fractions
were removed by Soxhlet extraction with methanol, and hex-
ane. The soluble fraction was collected by extraction with
1
product (0.28 g) in an 89% yield. H NMR (400 MHz; CDCl3;
Me4Si): d (ppm) 7.10 (s, 2H), 2.54 (t, J ¼ 7.8 Hz, 4H), 1.58
(m, 4H), 1.26 (m, 36H), 0.88 (m, 6H), 0.37 (m, 18H); 13C
NMR (100 MHz; CDCl3; ppm): d ¼ 151.48, 142.68, 130.78,
125.83, 32.86, 32.00, 31.92, 29.84, 29.79, 29.66, 29.64,
29.63, 29.61, 29.58, 22.71, 14.09, ꢀ7.89; (Note: 29.84–29.58
peaks in the 13C NMR spectrum overlap).
1
chloroform. Yield: 0.124 g (47%); H NMR (400 MHz; CDCl3;
Me4Si): d (ppm) 7.94–7.45 (m, br, 8H) 7.14 (br, 2H), 2.71 (br,
4H), 1.68 (br, 4H) 1.26 (br, 36H), 0.87 (br, 6H).
Bis(4-bromophenyl)fumaronitrile (2)
Poly[4,40-bis(dodecyl)-2,20-bithiophene-co-stilbene] (PBTSB)
The same procedure as described for PBTFN was followed.
Compound 1 (0.350 g, 0.422 mmol), Compound 3 (0.140 g,
0.414 mmol), P(o-tolyl)3 (0.005 g, 0.016 mmol), and Pd2dba3
(0.009 g, 0.010 mmol) was dissolved in dry chlorobenzene.
Yield: 0.085 g (31%); 1H NMR (400 MHz; CDCl3; Me4Si): d
(ppm) 7.59 (br, 4H) 7.48 (br, 4H), 7.08 (br, 2H), 7.00 (br,
2H) 2.67 (br, 4H), 1.65 (br, 4H) 1.26 (br, 36H), and 0.87 (br,
6H).
A two-neck flask was charged with 4-bromophenylacetoni-
trile (2.00 g, 10.2 mmol) and iodine (2.59 g, 10.2 mmol) in
dry ether (50 mL) and cooled to ꢀ78 ꢁC. A solution of so-
dium methoxide (1.38 g, 25.5 mmol) in methanol (10 mL)
was added into the reaction solution at ꢀ78 ꢁC under a
nitrogen atmosphere. The reaction mixture was gradually
warmed to 0 C and then the mixture was stirred at 0 C for
4 h. The reaction was quenched with a 3% aqueous solution
of hydrochloric acid. The solution was filtered to isolate the
solid, which was rinsed with methanol to remove the ionic
residues. Finally, the remaining white solid was dried under
vacuum and the product was recrystallized from methylene
chloride as a white solid (1.45 g) in a 72% yield. 1H NMR
(400 MHz; CDCl3; Me4Si): d (ppm) 7.71–7.65 (m, 8H); 13C
NMR (100 MHz; CDCl3; ppm): d ¼ 132.7, 130.5, 130.1,
126.8, 124.6, 116.1; MS (EI): m/z ¼ 387.90 [M]þ (Calcd:
387.85).
ꢁ
ꢁ
OFET Device Fabrication and Characterization
OFETs were fabricated in a TG/BC configuration on a glass
substrate. The fabricated FETs had a channel length of 20
lm and a channel width of 1.0 mm. The source/drain elec-
trodes were patterned using photolithography and then were
deposited by thermal evaporation of 3 nm of Ni and 12 nm
of Au. For FET fabrication, the substrates were ultrasonicated
in acetone and again in isopropanol. The substrates were
exposed to UV/O3 treatment in air for 20 min. After the thin
films of the organic semiconductors were spin coated (2000
rpm, 60 s) from a solution of PBTSB and PBTFN (10 mg/
mL, chlorobenzene), substrates were annealed at 100, 150,
200, and 250 ꢁC for 20 min, then thin films of CYTOP:CT-
sol.180 (2:1 blend, thickness 500 nm, Asahi Glass) were spin
coated at 2,000 rpm for 60 s. The devices were completed
by deposition of a 50 nm Al layer by thermal evaporation.
The current–voltage (I–V) characteristics of the OFET devices
were measured using a Keithley 4200-SCS under N2 inert
conditions.
Trans-4,40-dibromostilbene (3)
A two-neck flask was charged with zinc powder (6.31 g, 96.6
mmol) in anhydrous THF (100 mL), and TiCl4 (9.16 g, 48.3
mmol) was added dropwise to the solution at 0 ꢁC. Then, a
solution of 4-bromobenzaldehyde (3 g, 16.2 mmol) in anhy-
drous THF (50 mL) was added into the reaction solution at
0
ꢁC. The reaction mixture was vigorously stirred at 70 ꢁC
for 5 h. After 5 h, the reaction mixture was poured into a
separating funnel with aqueous NaHCO3 solution, and then
the aqueous layer was extracted with methylene chloride
while the combined organic layer was dried over anhydrous
MgSO4 and concentrated under reduced pressure to give the
crude product. Finally, the crude product was recrystallized
from methylene chloride as a white solid (2.35 g) in an 86%
yield. 1H NMR (400 MHz; CDCl3; Me4Si): d (ppm) 7.49 (d, J
¼ 8.7 Hz, 4H), 7.37 (d, J ¼ 8.5 Hz, 4H), 7.02 (s, 2H); 13C
NMR (100 MHz; CDCl3; ppm): d ¼ 135.9, 131.9, 128.1,
128.0, 121.6; MS (EI): m/z ¼ 338.04 [M]þ (Calcd: 337.65).
Complementary Inverter Fabrication
The contact electrodes (Au/Ni) of the inverters were pat-
terned by applying the same method as described above and
consisted of individual FET electrodes [Lp,n ¼ 20 mm, Wp
¼
1 mm, Wn ¼ 10 mm]. The thin film of PBTFN was prepared
ꢁ
by spin-coating and annealed at 200 C for 20 min. The rest
of the procedure is the same as described above.
Poly[4,40-bis(dodecyl)-2,20-bithiophene-co-bis(phenyl)
fumaronitrile] (PBTFN)
RESULTS AND DISCUSSION
In a 50 mL, flame-dried, two-neck flask, Compound 1 (0.300
g, 0.362 mmol), Compound 2 (0.140 g, 0.361 mmol), P(o-
tolyl)3 (0.003 g, 0.010 mmol), and Pd2dba3 (0.007 g, 0.008
General Synthesis
The synthetic procedures are illustrated in Scheme 1. The
bis(4-bromophenyl)fumaronitrile
was
prepared
from
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 1029–1039
1031

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
SCHEME 1 Synthesis of PBTSB and PBTFN. (i) C12H25MgBr, Ni(dppp)Cl2, diethyl ether, 50 ꢁC. (ii) NBS, AcOH, chloroform, rt. (iii)
AgNO3, KF, PdCl2(PhCN)2, DMSO, 70 ꢁC . (iv) n-BuLi, trimethyltin chloride, hexane, 0 ꢁC. (v) I2, Na2OCH3, MeOH, diethyl ether,
ꢀ78 ꢁC. (vi) Zn, TiCl4, THF, 0 ꢁC. (vii) Pd2(dba)3, P(o-tolyl)3, chlorobenzene, reflux.
4-bromophenylacetonitrile, iodine, and sodium methoxide in
dry diethyl ether. When two equivalents of sodium methox-
ide, a critical component of this reaction, were added in
reaction mixtures, the formation of a (Z)-isomer could be
minimized.37 The purity was checked after twice recrystalli-
zations by measuring the proton NMR spectra. It was con-
firmed that 4,40-dibromostilbene and bis(4-bromophenyl)fu-
maronitrile did not contain a (Z)-isomer. The polymerization
was performed by Stille cross-coupling, and low molecular-
weight fractions were removed by Soxhlet extraction with
methanol, and hexane. The soluble fraction was collected by
extraction with chloroform, and then the polymer powders
were obtained by reprecipitation in methanol. The Mn of
PBTSB and PBTFN were 6,500 and 9,500 Da, respectively,
and the polydispersity index (PDI) (Mw/Mn) of PBTSB and
PBTFN were 1.15 and 1.65, respectively. The Mn was deter-
mined by polystyrene standards in chloroform. The polymers
were highly soluble in common organic solvents such as
THF, chloroform, and chlorobenzene, and the spin-coated
films had good uniformity.
470 and 495 nm in PBTFN correspond to the electronic
state of an electron acceptor and a charge-transfer character
by the electron-withdrawing group. The band-gap of PBTFN
was decreased by 0.38 eV after introduction of the cyano
group compared to that of PBTSB, and this was attributed
to an intramolecular charge transfer (ICT) interaction
between the bis(dodecyl)bithiophene (electron donor, D) and
bis(phenyl)fumaronitrile (electron acceptor, A) moieties
along the polymer chain. In addition, PBTFN exhibited two
absorption bands, unlike the single characteristic absorption
band of PBTSB, and the spectral band around 350 nm was
assigned to the p!p* transition for PBTFN. The two charac-
teristic absorption bands of the PBTFN thin films were quite
Optical Properties of PBTSB and PBTFN
The absorption spectra of PBTSB and PBTFN obtained in so-
lution and film states are shown in Figure 1, and the charac-
terization of PBTSB and PBTFN are summarized in Table 1.
The absorption maxima of PBTFN in the film and solution
states were significantly red-shifted compared with those of
PBTSB. In fact, the chemical structure of PBTFN was similar
to that of PBTSB except that it contained cyano groups.
These results demonstrate that the cyano group acts as an
electron-withdrawing group, and the absorption bands at
FIGURE 1 UV–vis spectra of PBTSB and PBTFN in chloroben-
zene, film, and annealed film states.
1032
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 1029–1039

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
TABLE 1 A Summary of the Characterization of PBTSB and PBTFN
a
b
f
g
Mn
Td
Absorbance kmax
[solutionc, filmd (nm)]
Eg (optical)e
EHOMO
ELUMO
Polymer
(kDa)
PDIa
(ꢁC)
(eV)
(eV)
(eV)
PBTSB
PBTFN
a
6.5
9.5
1.15
1.65
395
396
400, 422
470, 495
2.48 (2.94)
2.10 (2.51)
5.38
5.65
2.90 (3.22)
3.55 (3.53)
Mn and PDI determined by polystyrene standards in chloroform.
Td (5% weight loss) determined by TGA under N2.
Measured in chlorobenzene.
b
c
d
e
f
Cast from chlorobenzene.
Energy bandgap estimated from the onset wavelength (or from the absorbance maximum kmax) of the optical absorption in film states.
HOMO ¼ [Eox
onset
ꢀ E1/2(ferrocene)] þ 4.8 eV.
g
The LUMO energy level estimated from optical band gap (or from the onset potential of reduction wave).
similar in shape to those in the solution state. A maximum
absorption peak at 357 nm derived from the p!p* transi-
tion for PBTFN thin film was red-shifted 11 nm compared
with the corresponding peak in the solution state. Another
maximum absorption peak corresponding to the ICT was
red-shifted 25 nm in the film state. In general, the molecular
chain had a much more planar conformation by the forma-
tion of a p-stacked structure in the solid state, resulting in
an increased electronic delocalization length, so that maxi-
mum absorption moved to longer wavelengths. However,
maximum absorption peaks corresponding to the p!p* tran-
sition and ICT were slightly red-shifted in going from the so-
lution to the solid state. This small red-shift could be attrib-
uted to the planar and rigid properties of vinylene moieties
in the solution state; in other words, there was no significant
conformational difference between the solution and the solid
state. Meanwhile, the broad absorption bands of PBTSB and
PBTFN in the solid state suggested p-p stacking in the poly-
mer film. The absorption spectra of thermally annealed
PBTFN and PBTSB film exhibited broad absorption com-
pared with that of non-annealed film. Otherwise, there were
no significant differences, regardless of the morphological
changes.
ficient CN unit provided an additional intermolecular interac-
tion with the electron-rich thiophene unit.26
Electrochemical Properties
CV was performed to understand the electrochemical char-
acteristics of the PBTSB and PBTFN using 0.1 M Bu4NClO4
in acetonitrile as an electrolyte at a scan rate of 100 mV sꢀ1
.
To investigate the electrochemical properties of the poly-
mers, thin film was prepared on indium tin oxide
a
glass as a working electrode, and silver and platinum wires
Thermal Properties
Thermal properties of PBTSB and PBTFN were investigated
using thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC). The degradation temperatures
(Td) at 5% weight loss of PBTSB and PBTFN were detected
at 395 and 396 ꢁC, respectively, which demonstrates that
these polymers possess good thermal stability [Fig. 2(a)].
From the DSC measurements [Fig. 2(b)], PBTSB exhibited no
transition peak including Tg and Tm during the heating
and cooling scans. However, PBTFN revealed an obvious exo-
ꢁ
thermic transition at 163 and 185 C during the cooling scan
ꢁ
and a weak endothermic transition at 190 and 210 C during
the second heating scan. The exothermic (at 163 and
185 ꢁC) and endothermic (at 190 and 210 ꢁC) transitions
correspond, respectively, to the recrystallization and crystal
melting peaks. The two endothermic transitions at 190 and
210 ꢁC on the second heating scan of PBTFN may corre-
spond to crystal-to-liquid crystal and liquid crystal-to-iso-
tropic phase transitions.35 The reason why only PBTFN
showed the obvious transition peaks is that the electron-de-
FIGURE 2 TGA (a) DSC (b) curves of PBTSB and PBTFN with a
heating and cooling rate of 10 ꢁC minꢀ1 under N2 atmosphere.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 1029–1039
1033

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 3 Cyclic voltammogram of PBTSB and PBTFN in 0.1 M
Bu4NClO4, acetonitrile solution at a scan rate of 100 mV sꢀ1
.
FIGURE 4 Energy level diagrams of PBTSB and PBTFN (TDDFT
calculations and CV).
were used as the reference and counter electrodes, respec-
tively. As shown in Figure 3, the onset oxidation potentials
for PBTSB and PBTFN versus Ag/Agþ were found to be
þ1.01 and þ1.28 V, respectively. The HOMO energy level
was calculated with reference to the half-wave potential
of ferrocene/ferrocenium (Fc/Fcþ), 4.8 eV below the vac-
corresponding HOMO energy levels of PBTSB and PBTFN
were ꢀ5.38 and ꢀ5.65 eV, and the LUMO energy levels
were ꢀ2.90 and ꢀ3.55 eV, respectively. The introduction of
a stilbene unit instead of bis(thienyl)ethylene lowered the
HOMO energy level and elevated the LUMO energy level
due to the large band gaps of the phenylene derivatives.39
The HOMO and LUMO energy levels of PBTFN were, 0.27
and 0.65 eV, respectively, lower than those of PBTSB due
to the introduction of an electron withdrawing cyano group
uum level, according to the following equation: HOMO ¼
onset
–[Eox
ꢀ E1/2(ferrocene)] ꢀ4.8 eV.38 The LUMO energy
level was calculated from the estimated optical band gap
(not the onset potential of reduction wave) because the Eg
(TDDFT) was calculated using optical absorption data. The
FIGURE 5 Optimized molecular structures of PBTSB and PBTFN (a, b) and their molecular orbital (MO) diagrams (c,d). Several tor-
sion angles are shown together in red bold faces. Color code: black (H), gray (C), green (N), yellow (S), red, and blue (positive and
negative phases of MO).
1034
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 1029–1039

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 6 I–V characteristics of an OFET device with PBTSB and PBTFN as the semiconductor: Transfer and output curves of
PBTSB (a); transfer (b) and output (c) curves of ambipolar PBTFN.
as we originally intended. In particular, the large differences
(0.65 eV) of the LUMO energy levels between PBTFN
and PBTSB were attributed to the large variation in LUMO
energy levels of PBTFN due to the strong electron-
withdrawing group (fumaronitrile) and to the effect of
lowering the HOMO-LUMO energy gap by the push-pull
structure. HOMO and LUMO energy levels of PBTFN were
significantly lower than those of the P3HT, which is well
known as a typical p-type material. Taking these facts into
consideration, PBTFN could reduce the electron injection
barrier.
Molecular Geometry and Electronic Structure
It is well known that the carrier mobility of organic semicon-
ductors depends significantly on molecular geometry (pla-
narity) and electronic structure. The influences of the two
cyano groups on the geometry and on the electronic struc-
ture were investigated with DFT calculations. The geometries
of repeating-unit models of PBTSB and PBTFN were opti-
mized in the gas phase at the level of B3LYP/6-311G(d,p)
using the Jaguar v6.5 software.40,41 As shown in Figure
5(a,b), both PBTSB and PBTFN show significant twists
(43.6ꢁ and 40.5ꢁ, respectively) between thiophenes and
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 1029–1039
1035

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
TABLE 2 Summary of OFETs Mobilities for PBTSB and PBTFN
adjacent benzene rings, most likely due to the steric hin-
drance between the dodecyl (methyl in our models) side
chains and the benzene rings. A prominent influence of the
two cyano groups introduced to the vinylene moiety is a
large twisting of the bis(phenyl)fumaronitrile backbone
(from 17.5ꢁ of rather planar PBTSB to 40.2ꢁ of nonplanar
PBTFN).
at Various Temperatures
a
a
Tannealing
lh
le
Polymers
PBTSB
(ꢁC)
(cm2 Vꢀ1 sꢀ1
8.5 ꢂ 10ꢀ4
5.5 ꢂ 10ꢀ4
8.7 ꢂ 10ꢀ4
8.9 ꢂ 10ꢀ4
1.2 ꢂ 10ꢀ5
1.0 ꢂ 10ꢀ4
1.4 ꢂ 10ꢀ4
5.4 ꢂ 10ꢀ5
)
(cm2 Vꢀ1 sꢀ1
)
100
150
200
250
100
150
200
250
–
–
–
–
Such change in the molecular geometry explains (or origi-
nates from) the difference in the electronic structure
between the two polymers. Figure 5(c,d) shows the frontier
molecular orbitals (HOMO and LUMO) of each polymer
model, which are superimposed on top of the optimized
structure. In the case of PBTSB [Fig. 5(c)], both HOMO and
LUMO are well distributed over the whole backbone, except
a slight localization of the LUMO around the stilbene moiety.
A rather extended p-conjugation and in turn a certain degree
of planarity is expected for PBTSB. However, after the intro-
duction of the cyano substituents, PBTFN [Fig. 5(d)] exhibits
rather localized HOMO (on the thiophene ‘‘donor’’ moieties)
and LUMO (on the fumaronitrile ‘‘acceptor’’ moieties). This
explains the prominent twisting of the backbone around the
fumaronitrile.
PBTFN
8.1 ꢂ 10ꢀ4
1.2 ꢂ 10ꢀ3
3.0 ꢂ 10ꢀ3
1.4 ꢂ 10ꢀ3
a
Mobilities were calculated using equation ID ¼ (lWC/2L)(Vg ꢀ Vt)2.
C, capacitance; L, length; W, width.
on PBTFN exhibited ambipolar charge transport characteris-
tics due to the introduction of an electron-withdrawing
cyano group; moreover, electron mobility was higher than
hole mobility. These ambipolar characteristics have never
been observed in organic semiconductors using the fumaro-
nitrile moiety.
The MO energy levels (EHOMO and ELUMO) were taken from
the DFT eigenvalue (EHOMO; ꢀ5.28 and ꢀ5.62 eV for PBTSB
and PBTFN, respectively) and from the lowest electronic
transition energy (band gap; Eg; 2.94 and 2.51 eV) calculated
with a TDDFT method implemented in the Gaussian 03 pro-
gram [ELUMO ¼ EHOMO (DFT) þ Eg (TDDFT); ꢀ2.39 and
ꢀ3.35 eV].42 As shown in Figure 4, the calculated values are
in reasonable agreement with the experimental values
obtained from the CV (EHOMO) and the absorbance maximum
wavelength (Eg). They again indicate that the two cyano elec-
tron-withdrawing groups led to the low-lying HOMO and
LUMO of PBTFN. The lowering is more significant for fumar-
onitrile-centered LUMO than for thiophene-centered HOMO,
leading to the reduced band gap of PBTFN.
To obtain high carrier mobilities, it is important to arrange
the polymer molecules in a specific molecular direction for
preferable charge transport in the OFET architecture. There-
fore, thermal annealing usually improves mobility compared
with nonannealed devices due to enhanced intermolecular
ordering and p-p stacking. Thermal annealing effects such as
enhancement of charge-carrier mobility usually take place
when the polymers of the semiconducting layer have an or-
dered phase after annealing.43 The FET characteristics of
PBTSB and PBTFN at different annealing temperature are
summarized in Table 2. It can be inferred that PBTSB was
insensitive to thermal treatment and did not form an
ordered phase because there was no relative variation of
mobility in the transistor based on PBTSB after annealing.
However, the hole and electron mobilities of the transistor
based on PBTFN increased with increasing annealing tem-
Characterization of the FET Devices and
Complementary Circuits
ꢁ
perature up to 200 C but decreased at an annealing temper-
The charge-carrier mobility of the new polymers was meas-
ured using field-effect transistors (FETs) with top-gate/
bottom-contact (TG/BC) geometry, which were set up as
described in the experimental section. Although these newly
synthesized polymers (PBTSB and PBTFN) have a very simi-
lar chemical structure, electron injection could be enhanced
by the introduction of a cyano group into the stilbene moi-
ety. The n-type characteristics of transistors based on fumar-
onitrile containing two cyano groups have not yet been
reported without surface treatments for efficient electron
injection. The transistors based on PBTSB typically exhibited
p-type characteristics, as shown in Figure 6. The low mobil-
ity of PBTSB might be due to the large dihedral angle of
phenyl rings, which increases conformational disorder.
Indeed, a phase transition peak was not detected by DSC
measurements, and evidence of crystallinity was not con-
firmed by XRD analysis. Considering these results, PBTSB
was amorphous as expected. However, the transistor based
ature of 250 ꢁC. We observed the morphological changes of
PBTSB and PBTFN films with temperature variation using
AFM to investigate the relationship between annealing tem-
perature and charge-carrier mobility. Figure 7 shows the
AFM phase images of annealed PBTFN film at different
annealing temperatures. Interestingly, the PBTSB film
showed a slightly aggregated morphology at higher tempera-
ture, but otherwise there were no distinguishable morpho-
logical changes. However, PBTFN film began to show a fibril-
lar structure after annealing at 150 ꢁC, and mature fibrils
ꢁ
were observed in PBTFN film annealed at 200 C. The fibril-
lar structure disappeared, and a rough surface was observed
due to aggregation in PBTFN film annealed at 250 ꢁC. This
clearly showed that the annealing temperature dependence
of the mobility is related to the morphology of PBTFN film.
We also observed polarized optical microscopy (POM)
images to confirm the presence of the crystalline phase
because of the observation of a fibrillar structure. Crystalline
1036
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 1029–1039

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 7 AFM topographic images of spin-coated PBTSB (a–d) and PBTFN (e–h) films on SiO2/Si substrate at different annealing
temperature for 20 min; (a, e): 100 ꢁC, (b, f): 150 ꢁC, (c, g): 200 ꢁC, (d, h): 250 ꢁC.
PBTSB was not observed as we intended, but PBTFN film
exhibited crystals after annealing above 150 ꢁC. There was
no significant difference between the POM images of films
annealed at 150 ꢁC and those annealed at 200 ꢁC. However,
similar to the AFM image of PBTFN, the POM image exhib-
iations in charge-carrier mobility in the OFETs based on
PBTSB and PBTFN.
In general, the nonplanar molecular structure prevented
close molecular packing, and the localized molecular orbital
hindered charge transport.44 The DFT calculation revealed
that the PBTFN repeat unit has a twisted molecular struc-
ture and localized molecular orbital compared to PBTSB.
However, OFETs based on PBTFN exhibited higher charge-
ꢁ
ited large aggregates of crystals after annealing at 250 C as
shown in Figure 8. The AFM and POM images for various
annealing temperature were entirely consistent with the var-
FIGURE 8 POM images of PBTFN at different annealing temperature for 20 min: (a) 200 ꢁC and (b) 250 ꢁC.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 1029–1039
1037

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
transistors. The relative humidity during the period of mea-
surement was maintained at 30–35%. The p-channels of
PBTSB and PBTFN showed relatively stable performance
under air-exposed conditions, while the n-channel of PBTFN
showed poor air-stability, as shown in Figure 10. The low-
lying HOMO energy level (EHOMO; ꢀ5.38 and ꢀ5.65 eV for
PBTSB and PBTFN, respectively) led to stable device per-
formance under ambient conditions compared with the
n-channel of PBTFN. In addition, the poor stability of the
n-channel OFETs was attributed to the presence of electron
traps in the air.
CONCLUSIONS
FIGURE 9 VTC of a complementary inverter consisting of an
ambipolar PBTFN FETs.
Two conjugated polymers containing stilbene and fumaroni-
trile moieties were synthesized using a Stille cross-coupling
reaction, and OFETs were fabricated using synthesized poly-
mers. We investigated the effects of electron-withdrawing
cyano groups on the electronic properties of the new conju-
gated polymers. PBTFN possessed low HOMO and LUMO
energy levels compared to those of PBTSB due to the elec-
tron-withdrawing effects of the cyano groups; in particular,
the lowering of the LUMO energy levels was noticeable.
PBTFN exhibited ambipolar transport behavior, and PBTSB
as a reference material only exhibited hole transport proper-
ties in OFETs because the lower LUMO energy levels of
PBTFN facilitated electron injection from the electrodes into
the semiconductor. Although, based on DFT calculation,
PBTFN possessed localized electron density along the back-
bone and a twisted molecular structure, which might inter-
rupt charge transport, however, OFETs based on PBTFN
exhibited higher mobility compared with those of PBTSB
due to enhanced intermolecular ordering. Ambipolar trans-
port in OFETs using the polymer containing a fumaronitrile
moiety was observed for the first time, and a complemen-
tary-like inverter using two identical ambipolar FETs with a
common gate and a common drain exhibited a good voltage
gain despite relatively low mobility.
carrier mobility after thermal annealing. The fibrillar struc-
ture of PBTFN may be driven by an electron-withdrawing
cyano group that led to the strong intermolecular interac-
tions.45,46 It is well known that charge transport can be
dominated by the hopping mechanism, and intermolecular
p-p interactions could reduce disordered regions and insulat-
ing grain boundaries.47 In the case of OFETs based on
PBTFN, electron transport was improved because of their
well-ordered molecular packing in fibrillar structure.48,49 We
demonstrated a complementary-like inverter using two iden-
tical ambipolar FETs with a common gate as input voltages
and a common drain as output voltages.13 Figure 9 shows
the voltage-transfer characteristics (VTC) of a CMOS-like
inverter based on PBTFN (Lp,n ¼ 20 lm, Wp ¼ 1 mm, Wn ¼
10 mm). PBTFN CMOS inverters showed a Z-shaped VTC
curve, which is generally exhibited by CMOS inverters, based
on ambipolarity due to the parasitic current of a single tran-
sistor.50 However, the VTC of the inverter exhibited a good
symmetry with an inverting voltage nearly half that of the
power supply with a gain of 9 at VDD ¼ 100 V. Furthermore,
we investigated the stability of PBTSB- and PBTFN-based
FIGURE 10 Transfer curves of PBTSB and PBTFN OFETs with different time of exposed to ambient air: p-type of PBTSB (a); p-type
(b) and n-type (c) of ambipolar PBTFN.
1038
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 1029–1039

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
23 S. Cho, J. Lee, M. Tong, J. H. Seo, C. Yang, Adv. Funct.
Mater. 2011, 21, 1910–1916.
ACKNOWLEDGMENTS
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea Government (MEST)
(No. 2011-0029858), (NRF-2009-C1AAA001-2009-0092950),
(R31-10026); the Basic Science Research Program through the
NRF funded by the MEST (R15-2008-006-02001-0); the Bioi-
maging Research Center at GIST; the Ministry of Knowledge
Economy (MKE) (Grant No. 2010-H-003-00030100-2010); and
the GSR_IC Project through a grant provided by GIST in 2011.
24 A. J. Kronemeijer, E. Gili, M. Shahid, J. Rivnay, A. Salleo,
M. Heeney, H. Sirringhaus, Adv. Mater. 2012, 24, 1558–1565.
25 J. C. Bijleveld, R. A. M. Verstrijden, M. M. Wienk, R. A. J.
Janssen, J. Mater. Chem. 2011, 21, 9224–9231.
26 J. H. Kim, J. W. Chung, Y. Jung, S.-J. Yoon, B.-K. An, H. S.
Huh, S. W. Lee, S. Y. Park, J. Mater. Chem. 2010, 20,
10103–10106.
27 X. Zhan, Y. Liu, X. Wu, S. Wang, D. Zhu, Macromolecules
2002, 35, 2529–2537.
28 H.-C. Yeh, S.-J. Yeh, C.-T. Chen, Chem. Commun. 2003,
2632–2633.
REFERENCES AND NOTES
1 H. Usta, G. Lu, A. Facchetti, T. J. Marks, J. Am. Chem. Soc.
2006, 128, 9034–9035.
29 Y. Lin, M. E. El-Khouly, Y. Chen, M. Supur, L. Gu, Y. Li, S.
Fukuzumi, Chem. Eur. J. 2009, 15, 10818–10824.
2 H. Nakanotani, M. Saito, H. Nakamura, C. Adachi, Appl. Phys.
Lett. 2009, 95, 033308.
30 L. Huoa, Z. Tan, X. Wang, M. Hana, Y. Li, Synth. Met. 2007,
157, 690–695.
3 W. Ma, C. Yang, X. Gong, K. Lee, A. J. Heeger, Adv. Funct.
Mater. 2005, 15, 1617–1622.
31 T. Lee, C. A. Landis, B. M. Dhar, B. J. Jung, J. Sun, A. Sarjeant,
H.-J. Lee, H. E. Katz, J. Am. Chem. Soc. 2009, 131, 1692–1705.
4 C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct.
Mater. 2001, 11, 15–26.
32 J. H. Kim, A. Watanabe, J. W. Chung, Y. Jung, B.-K. An, H.
Tada, S. Y. Park, J. Mater. Chem. 2010, 20, 1062–1064.
5 D. Vak, C. Chun, C. L. Lee, J.-J. Kim, D.-Y. Kim, J. Mater.
Chem. 2004, 14, 1342–1346.
33 H. Padhy, D. Sahu, D. Patra, M. K. Pola, J.-H. Huang, C.-W.
Chu, K.-H. Wei, H.-C. Lin, J. Polym. Sci. Part A: Polym. Chem.
2011, 49, 3417–3425.
6 R. Yang, R. Tian, Q. Hou, W. Yang, Y. Cao, Macromolecules
2003, 36, 7453–7460.
34 J. A. Mikroyannidis, M. M. Stylianakis, P. Balraju, P. Suresh,
G. D. Sharma, ACS Appl. Mater. Interfaces 2009, 1, 1711–1718.
7 Z. Chen, H. Lemke, S. Albert-Seifried, M. Caironi, M. M. Niel-
sen, M. Heeney, W. Zhang, I. McCulloch, H. Sirringhaus, Adv.
Mater. 2010, 22, 2371–2375.
35 B. Lim, K.-J. Baeg, H.-G. Jeong, J. Jo, H. Kim, J.-W. Park,
Y.-Y. Noh, D. Vak, J.-H. Park, J.-W. Park, D.-Y. Kim, Adv. Mater.
2009, 21, 2808–2814.
8 K.-J. Baeg, D. Khim, J.-H. Kim, M. Kang, I.-K. You, D.-Y. Kim,
Y.-Y. Noh, Org. Electron. 2011, 12, 634–640.
36 I. McCulloch, M. Heeney, C. Bailey, K. Genevicius, I. Mac-
donald, M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W.
Zhang, M. L. Chabinyc, R. J. Kline, M. D. McGehee, M. F.
Toney, Nat. Mater. 2006, 5, 328–333.
9 F. C. Krebs, J. Fyenbob, M. Jørgensen, J. Mater. Chem. 2010,
20, 8994–9001.
10 J. H. Cho, J. Lee, Y. Xia, B. Kim, Y. He, M. J. Renn, T. P.
Lodge, C. D. Frisbie, Nat. Mater. 2008, 7, 900–906.
37 H.-C. Yeh, W.-C.; Wu, Y.-S. Wen, D.-C. Dai, J.-K. Wang, C.-T.
Chen, J. Org. Chem. 2004, 69, 6455–6462.
11 H. Nakanotani, M. Saito, H. Nakamura, C. Adachi, Appl.
Phys. Lett. 2009, 95, 103307.
38 J. Pomrnerehne, H. Vestweber, W. Guss, R. F. Mahrt, H.
€
Bassler, M. Porsch, J. Daub, Adv. Mater. 1995, 7, 551–554.
€
12 T. D. Anthopoulos, S. Setayesh, E. Smits, M. Colle, E. Canta-
39 G. Klaerner, R. D. Miller, Macromolecules 1998, 31,
2007–2009.
tore, B. de Boer, P. W. M. Blom, D. M. de Leeuw, Adv. Mater.
2006, 18, 1900–1904.
40 Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT,
2004.
13 K.-J. Baeg, J. Kim, D. Khim, M. Caironi, D.-Y. Kim, I.-K. You,
J. R. Quinn, A. Facchetti, Y.-Y. Noh, ACS Appl. Mater. Interfa-
ces 2011, 3, 3205–3214.
41 Jaguar, version 6.5; Schrodinger, LLC: New York, 2005.
14 L. Zhang, D. Taguchi, T. Manaka, M. Iwamoto, Appl. Phys.
Lett. 2011, 99, 083301.
42 J. Ku, Y. Lansac, Y. H. Jang, J. Phys. Chem. C 2011, 115,
21508.
15 F. S. Kim, X. Guo, M. D. Watson, S. A. Jenekhe, Adv. Mater.
2010, 22, 478–482.
43 H.-G. Jeong, B. Lim, S.-I. Na, K.-J. Baeg, J. Kim, J.-M. Yun,
D.-Y. Kim, Macromol. Chem. Phys. 2011, 212, 2308–2318.
16 A. Babel, Y. Zhu, K.-F. Cheng, W.-C. Chen, S. A. Jenekhe,
Adv. Funct. Mater. 2007, 17, 2542–2549.
44 S. B. Darling, J. Phys. Chem. B 2008, 112, 8891–8895.
45 J. Liu, I. A. Mikhaylov, J. Zou, I. Osaka, A. E. Masunov, R. D.
McCullough, L. Zhai, Polymer 2011, 52, 2302–2309.
17 F. S. Kim, E. Ahmed, S. Subramaniyan, S. A. Jenekhe, ACS
Appl. Mater. Interfaces 2010, 2, 2974–2977.
46 Y. Li, F. Li, H. Zhang, Z. Xie, W. Xie, H. Xu, B. Li, F. Shen, L.
Ye, M. Hanif, D. Ma, Y. Ma, Chem. Commun. 2007, 231–233.
18 T. B. Singh, S. Gunes, N. Marjanoviæ, N. S. Sariciftci, R.
¨
Menon, J. Appl. Phys. 2005, 97, 114508.
47 J. Liu, Y. Sun, X. Gao, R. Xing, L. Zheng, S. Wu, Y. Geng, Y.
Han, Langmuir 2011, 27, 4212–4219.
19 M. Shahid, T. McCarthy-Ward, J. Labram, S. Rossbauer, E.
B. Domingo, S. E. Watkins, N. Stingelin, T. D. Anthopoulosb,
M. Heeney, Chem. Sci. 2012, 3, 181–185.
48 M. Surin, E. Hennebicq, C. Ego, D. Marsitzky, A. C. Grims-
ꢀ
dale, K. Mullen, J.-L. Bredas, R. Lazzaroni, P. Lecle`re, Chem.
¨
20 D. Hu, G. Cheng, P. Lu, H. Liu, F. Shen, F. Li, Y. Lv, W.
Dong, Y. Ma, Macromol. Rapid Commun. 2011, 32, 1467–1471.
Mater. 2004, 16, 994–1001.
49 Lee, H. Kong, S.-Y. Oh, H.-K. Shim, I.-N. Kang, J. Polym.
Sci. Part A: Polym. Chem. 2009, 47, 111–120.
21 B. Pal, W.-C. Yen, J.-S. Yang, W.-F. Su, Macromolecules
2007, 40, 8189–8194.
50 E. J. Meijer, D. M. De Leeuw, S. Setayesh, E. Van Veenen-
daal, B.-H. Huisman, P. W. M. Blom, J. C. Hummelen, U.
Scherf, T. M. Klapwijk, Nat. Mater. 2003, 2, 678–682.
22 Y.-J. Cheng, S.-H. Yang, C.-S. Hsu, Chem. Rev. 2009, 109,
5868–5923.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 1029–1039
1039