Synthesis and photovoltaic properties of quinoxaline-based alternating copolymers for high-efficiency bulk-heterojunction polymer solar cells

Article abstract of DOI:10.1021/ma102943g

A series of quinoxaline-based copolymers, namely, poly[N-9″- heptadecanyl-2,7-carbazole-alt-5,5-(5′,8′-di-2-thienylquinoxaline)] (P1), poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(5′,8′-di- 2-thienyl-2,3-bis(4-octyloxyl)phenyl)quinoxaline] (P2), and poly

Full text of DOI:10.1021/ma102943g

ARTICLE
pubs.acs.org/Macromolecules
Synthesis and Photovoltaic Properties of Quinoxaline-Based
Alternating Copolymers for High-Efficiency Bulk-Heterojunction
Polymer Solar Cells
Sang Kyu Lee,† Woo-Hyung Lee,‡ Jung Min Cho,† Song Ju Park,† Jin-Uk Park,† Won Suk Shin,†
Jong-Cheol Lee,† In-Nam Kang,*,‡ and Sang-Jin Moon*,†
†Energy Materials Research Center, Korea Research Institute of Chemical Technology (KRICT), 100 Jang-dong,
Yuseong-gu, Daejeon 305-600, Korea
‡Department of Chemistry, The Catholic University of Korea, Bucheon, Gyeonggi-do 420-743, Korea
S Supporting Information
b
ABSTRACT: A series of quinoxaline-based copolymers, namely,
poly[N-900-heptadecanyl-2,7-carbazole-alt-5,5-(50,80-di-2-thienyl-
quinoxaline)] (P1), poly[N-900-heptadecanyl-2,7-carbazole-alt-
5,5-(50,80-di-2-thienyl-2,3-bis(4-octyloxyl)phenyl)quinoxaline]
(P2), and poly[N-900-heptadecanyl-2,7-carbazole-alt-5,5-(50,80-
di-2-thienyl-2,3-bis(4-(3,7-dimethyloctyloxy)phenyl)quinoxaline]
(P3), were synthesized and characterized for use in polymer solar
cells (PSCs). We describe the effect of modifying the alkyl group of
the side chain of the quinoxaline derivatives on the electronic
and optoelectronic properties of the polymers. The field-effect
hole mobility as well as the electronic energy levels and processability of the materials for PSC applications were investigated. Among
the studied quinoxaline-based copolymers, P2 showed the best photovoltaic performance with an open-circuit voltage (VOC) of
0.82 V, a short-circuit current density (JSC) of 9.96 mA/cm2, a fill factor (FF) of 0.49, and a power-conversion efficiency of 4.0%
when a P2/PC71BM blend film was used as the active layer under AM 1.5 G irradiation (100 mW/cm2).
’ INTRODUCTION
chains, to increase the conjugation length.16,17 However, pendant
chains can give rise to affect steric interactions and the coplanar-
ity of the polymer backbone, resulting in a change of the optical
and electrochemical properties of the donorꢀacceptor polymers.
Among the various conjugated polymers, polyfluorene and
polycarbazole derivatives have deep HOMO levels that increase
the open-circuit voltage (VOC) of the PSCs.18,19 Recently,
Leclerc et al. reported a systematic study of PSCs using various
poly(2,7-carbazole) derivatives. In that work, the researchers
varied the electron-withdrawing unit of the polymers.20,21
Poly[N-900-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-
20,10,30-benzothiadiazole) (PCDTBT) and PC61BM composites
provide a promising efficiency of 3.6%. Further optimization of
the device for PCDTBT led to a PCE of 6.1%.6 However, all
these polymers, except for PCDTBT, show hole-mobility
values between 10ꢀ5 and 10ꢀ4 cm2/(V s) in organic field-effect
transistors (OFETs), resulting in a low power-conversion
efficiency of 1ꢀ2%.
Recently, polymer solar cells (PSCs) have attracted consider-
able attention because of their unique advantages, which include
low cost, lightweight, solution processability, and flexibility.
Thus, enormous effort has been focused on improving the power
conversion efficiency (PCE) of PSCs for future commercial
applications.1ꢀ8 To date, highly efficient PSCs based on poly-
(3-hexylthiophene) (P3HT) as the electron-donor material and
[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the
acceptor have been reported with a power conversion efficiency
of 4ꢀ5%.9,10 One of the main factors limiting the performance of
PSCs is the mismatch of their absorption to the terrestrial solar
spectrum. Recently, several studies on low-band gap conjugated
polymers have been pursued in order to match the absorption
spectrum of the active layer to the solar spectrum.11ꢀ13 The most
successful approach to achieving low-band gap polymers is a
copolymerized donorꢀacceptor structure. Copolymerization of
the donor, with a higher HOMO (highest occupied molecular
orbital) energy level, and the acceptor, with a lower LUMO
(lowest unoccupied molecular orbital) energy level, results in a
lower band gap polymer due to an intramolecular charge transfer
(ICT) from the donor to the acceptor.14,15 Another approach to
designing low-band gap conjugated polymers is the introduction
of electron-releasing effects, such as long flexible alkyl and alkoxy
In this study, poly[N-900-heptadecanyl-2,7-carbazole-alt-5,5-
(50,80-di-2-thienylquinoxaline)] (P1), poly[N-900-heptadecanyl-2,
Received: December 26, 2010
Revised:
May 24, 2011
Published: July 07, 2011
r
2011 American Chemical Society
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7-carbazole-alt-5,5-(50,80-di-2-thienyl-2,3-bis(4-octyloxyl)phenyl)-
quinoxaline] (P2), and poly[N-900-heptadecanyl-2,7-carbazole-alt-5,5-
(50,80-di-2-thienyl-2,3-bis(4-(3,7-dimethyloctyloxy)phenyl)quinoxaline]
(P3), which have the same polymer backbone but different side
chains, were studied in order to achieve a higher efficiency value
and investigate the side-chain effect. Actually, the synthesis and
solar cell devices of the polymer called P1 has actually been
published earlier,21 and P1 was synthesized for the purpose of
direct comparison. Although the backbone of the copolymers
with similar structure have been described in the literature except
differences in the solubilizing side chains,22ꢀ25 the effect of the
alkoxy side chains on the electronic, optoelectronic, and photo-
voltaic properties has not been reported for PSCs based on
polymers with quinoxaline units. Compared to P1, the alkoxy
side chains on the quinoxaline units, which have been used earlier
in other polymers for solar cells,26 can enhance the solubility and
the charge transfer to electron acceptors such as PCBM. In
general, bulky side chains have a negative effect on the carrier
mobility, since interchain hopping of carriers requires a favorable
overlapping of the electron wave function of adjacent conjugated
units on the polymer main chains.27 Therefore, from the view-
point of πꢀπ* stacking, the quinoxaline- (Qx-) based copoly-
mers with different side chains were designed and synthesized.
We systematically investigated the synthesis, thermal stability,
optical and electrochemical properties, field-effect carrier mobi-
lities, and photovoltaic characteristics of the resulting polymers.
Our results provide valuable information on the relationship be-
tween polymer design at the molecular level and its electrical
performance in field-effect transistors (FETs) and polymer solar
cells.
differential scanning calorimetry (DSC) at a heating rate of
10 °C/min. All the polymers exhibited a good thermal stability,
showing less than 5% weight loss at temperatures up to 400 °C.
Compared to P1, P2 and P3 have a lower glass transition
temperature (Tg), which is attributed to the alkoxy side chains
on the quinoxaline units. The physical properties of the polymers
are summarized in Table 1.
Optical Properties. The UVꢀvis absorption spectra of the
polymers in chloroform and in the spin-cast film are shown in
Figures 1 and 2, respectively, and the corresponding absorption
properties are summarized in Table 2. Both in solution and in the
films, the polymers exhibited two distinct absorption bands: one
at about 375ꢀ400 nm and the other one in the range: 495ꢀ545 nm.
The nature of these bands, observed for the low-band gap polymer
with pushꢀpull units, still remains to be elucidated. Jesperson
et al. attributed the highest energy band to a πꢀπ* transition
whereas the lowest energy band was assigned to an intramolecular
charge transfer (ICT) between the donor and the acceptor.28,29
To obtain further information about the ICT electronic structure
of the polymers, we carried out density functional theory (DFT)
calculations for the model compound using the DMol 3 software.
Figure S1 (Supporting Information) shows the calculated mo-
lecular orbitals of the model compounds. As shown in Figure S1,
the HOMO was delocalized over the polymer backbone, whereas
the LUMO was highly localized on the Qx unit (Supporting
Information).
The red shift observed for P2 and P3, compared to P1, is due to
the introduction of the benzene groups on the quinoxaline unit
and not electron-donating effect of the alkoxy chains on the para
position of the benzene rings. The dihedral angle between the
benzene rings and the quinoxlaine rings are significant, as can be
seen from the calculated structures (Figure S1, Supporting
Information). However, this result currently lacks a theoretical
explanation. The absorption maxima of the polymer films are
red-shifted by 25ꢀ33 nm compared to those of the polymer
solutions. This red-shift indicates higher coplanarity of the
polymer and/or enhanced intermolecular electronic interactions
in the solid state. The optical band gap obtained from the
extrapolation of the absorption edges of the film are in the order
of P3(1.91 eV) < P2(1.97 eV) < P1(2.01 eV). The smaller band
gap should help improve the absorption efficiency in the solar
spectrum.
Electrochemical Properties. To determine the energy levels
of the HOMO and the LUMO, we investigated the electroche-
mical properties of the polymers by using cyclic voltammetry
(CV). A platinum (Pt) electrode, modified with a polymer film
by means of dip-coating, was used as the working electrode while
a Pt wire was used as the counter electrode and Ag/AgNO3 (0.10
M) served as the reference electrode. The CV measurements
were carried out in tetrabutylammonium tetrafluoroborate
(TBABF4, 0.1 M)/acetonitrile electrolyte at room temperature
under nitrogen atmosphere at a scan rate of 50 mV/s.30 The
HOMO level of the polymers can be deduced from the oxidation
onsets assuming that the energy level of ferrocene (Fc) is 4.8 eV
below the vacuum level.31 In the anodic scan, the onset of
oxidation for P1, P2, and P3 occurred at 0.54, 0.52, and 0.50 V,
which corresponds to ionization-potential values of ꢀ5.34,
ꢀ5.32, and ꢀ5.30 eV, respectively (Figure 3). The electron-
donating alkoxy groups in P2 and P3 slightly raise the HOMO
level and consequently reduce the band gap of the copolymer.
Unfortunately, it was difficult to obtain the LUMO energies
for the copolymers using this technique, so these values were
’ RESULTS AND DISCUSSION
Synthesis and Characterization of the Polymers. The
synthetic routes for preparing the Qx-based polymers are shown
in Scheme 1. Three types of Qx-based copolymers, namely,
poly[N-900-heptadecanyl-2,7-carbazole-alt-5,5-(50,80-di-2-thieny-
lquinoxaline)] (P1), poly[N-900-heptadecanyl-2,7-carbazole-alt-
5,5-(50,80-di-2-thienyl-2,3-bis(4-octyloxyl)phenyl)quinoxaline]
(P2), and poly[N-900-heptadecanyl-2,7-carbazole-alt-5,5-(50,80-
di-2-thienyl-2,3-bis(4-(3,7-dimethyloctyloxy)phenyl)quinoxaline]
(P3), were synthesized by polycondensation of 2,7-bis(40,40,50,50-
tetramethyl-10,30,20-dioxaborolane-20-yl)-N-900-heptadecanylcar-
bazole and the corresponding dibrominated quinoxaline deriva-
tives through the palladium-catalyzed Suzuki reaction. The crude
polymers were extracted with chloroform, recollected by precipi-
tating them in methanol, and extracted again with methanol and
acetone successively using a Soxhlet apparatus to remove bypro-
ducts and oligomers. The chemical structure of the polymers was
verified with 1H NMR spectroscopy and elemental analysis. P2 and
P3 were readily soluble in common organic solvents, such as
toluene, chloroform, and tetrahydrofuran (THF), whereas P1
dissolved only in warm chloroform or warm chlorinated benzene.
The molecular weights were determined by using gel permeation
chromatography (GPC) against polystyrene standards in a chloro-
form eluent and were found to be in the range of 6ꢀ37 kDa with a
polydispersity index of 2.0ꢀ2.9 (Table 1). With additional
solubilizing chains on the Qx unit, P2 and P3 displayed a higher
molecular weight, which is ascribable to the increased solubility of
these polymers.
The thermal properties of the copolymers were evaluated in a
nitrogen atmosphere by thermogravimetric analysis (TGA) and
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Scheme 1. Synthetic Scheme for the Monomer and Copolymers
estimated from the optical band gaps (taken as the absorption
onsets of the UVꢀvis spectra for the polymer films) and the
HOMO energies.32 The energies of the LUMO levels of P1, P2,
and P3 are listed in Table 2.
Field-Effect Transistor Characteristics. The effects of the
side-chain substitutions on the electrical transport properties of
the resulting materials were examined by measuring the field-
effect properties of each of the three copolymers. The field-effect
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Table 1. Physical Properties of the Polymers
polymer
Mn [kg/mol]a
PDI
yield (%)
Tg (°C)b
Td (°C)c
P1
P2
P3
6
37
24
2.0
2.8
2.9
68
60
65
125
112
109
420
400
410
a The molecular weights were determined by using gel permeation
chromatography (GPC) against polystyrene standards in chloroform
eluent. b Glass transition temperature determined by differential scan-
ning calorimetry (DSC) curve. c Temperature resulting in 5% weight loss
based on initial weight.
carrier mobilities of the polymers were investigated by fabricating
thin-film transistors (TFTs) with a bottom-contact geometry
using Au electrodes. The detailed device-fabrication process is
described in the Experimental Section. To reduce the photo-
current loss and obtain high-performance PSC devices, a high
charge-carrier mobility (higher than or close to 10ꢀ3 cm2 Vꢀ1
Figure 1. UVꢀvis absorption of the copolymers in chloroform
solutions.
s
ꢀ1) is necessary.33,34 Figure S2 (Supporting Information) shows
plots of drain-source current (Ids) as a function of drain-source
voltage (Vds) at different gate voltages, as well as typical output
and transfer curves for the polymers (Supporting Information).
TFTs of the polymers were found to exhibit typical p-channel
TFT characteristics with good drain-current modulation and
well-defined linear and saturation regions. The TFT mobilities were
calculated in the saturation region using the following equation:4
2
Ids ¼ ðWCi=2LÞμðVG ꢀ VTÞ
where Ids is the drain-source current in the saturated region, Wand L
are the channel width (120 μm) and length (6 μm), respectively, μ
is the field-effect mobility, Ci is the capacitance per unit area of the
insulation layer (SiO2, 300 nm), and VG and VT are the gate and
threshold voltages, respectively.
Figure 2. UVꢀvis absorption of thin film of the copolymers.
The mobilities and on/off ratios of TFTs fabricated using the
resulting polymers are listed in Table 3. The field-effect mobilities
for P1, P2, and P3 were determined to be 4.2 ꢁ 10ꢀ6, 1.4 ꢁ 10ꢀ3
,
voltage (VOC) of 0.82 V, and a fill factor (FF) of 0.49 under AM
1.5 G irradiation (100 mW/cm2).
and 2.1 ꢁ 10ꢀ4 cm2 Vꢀ1
s
ꢀ1, respectively. The higher mobility
The VOC value is closely related to the energy difference
between the HOMO of the polymer and the LUMO of the
electron acceptor, PC71BM.35 As expected from the HOMO
energy levels of the polymers, those polymers with the same
backbone units have a similar VOC value because of their almost
identical electronic structures. However, the trend of VOC for P1
does not seem to fit the electrochemical potentials, which can be
explained in terms of the morphology of the blended film. A low
solubility has an impact on the mechanical properties of the film,
the interface resistance, and the nanoscale morphology, resulting
in a low open-circuit voltage and a low fill factor.21 Figure 5 shows
atomic force microscopy (AFM) images (1 μm ꢁ 1 μm scan
area) of the blend film (polymer: PC71BM = 1:3) for a better
understanding of the device characteristics. P2 shows homoge-
neously well-distributed small-sized domains whereas P1 exhibits
larger polymerꢀPC71BM domains and phase separations. These
results indicate that P2 may form a well-distributed bicontinuous
interpenetrating network of polymerꢀPC71BM domains, show-
ing good agreement with the improved currentꢀvoltage char-
acteristics observed for this system in comparison to the other
polymers. The measured short-circuit current (JSC) for P1, P2,
and P3 was 2.70, 9.96, and 6.24 mA/cm2, respectively. These
results are also in good agreement with the OFET measurements.
The hole mobility determined in pure P2 is about 1 order of
measured for P2 can be understood in terms of the better solubility
and πꢀπ* stacking.27 The hole mobility determined for P2 is 1
order of magnitude higher than that for P3. We guess that the high
mobility of P2 is due to the improved packing of P2 compared to
that of polymers containing dimethyloctyloxy chains. The hole
mobility of P2 was within the desired range for PSC materials (i.e.,
higher than or close to 10ꢀ3 cm2 Vꢀ1 sꢀ1), allowing for an efficient
charge extraction and a good fill factor (FF).
Photovoltaic Characteristics. Bulk-heterojunction polymer
solar cells were fabricated with the structure of ITO/PEDOT:
PSS/polymer:PC71BM/LiF/Al. The detailed device-fabrication
process is described in the Experimental Section. Optimal perfor-
mance of the polymers was obtained from a 14 mg/mL dichloro-
benzene solution, at a 400 rpm spin-coating rate and a polymer/
PC71BM ratio of 1:3 (w/w). The thicknesses of the layers were
PEDOT:PSS (60 nm), active layer (80 nm), LiF (0.7 nm), and Al
(100 nm). According to the optoelectronic and TFT properties
described earlier, P2 is a suitable material for PSC applications.
Figure 4 shows the currentꢀvoltage characteristics of a PSC
device based on blends of the polymer and fullerene. The output
characteristics of the resulting polymer-based devices are sum-
marized in Table 3. The best PSC performance was observed for
P2/PC71BM devices, which reached a PCE of 4.0%, with a short-
circuit current density (JSC) of 9.96 mA/cm2, an open-circuit
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Table 2. Optical and Electrochemical Properties of the Polymers
polymer
λmaxabs,sol (nm)a
λmaxabs,film (nm)a
HOMO (eV)b
LUMO (eV)c
Egopt (eV)d
P1
P2
P3
379, 496
389, 510
391, 521
390, 529
395, 535
396, 543
ꢀ5.34
ꢀ5.32
ꢀ5.30
ꢀ3.33
ꢀ3.35
ꢀ3.39
2.01
1.97
1.91
a The UVꢀvis absorption spectra of the polymers were measured in chloroform solution and thin film. b HOMO levels of the polymer were determined
from onset voltage of the first oxidation potential with reference to ferrocene at ꢀ4.8 eV. c LUMO levels of the polymer were estimated from the optical
band gaps and the HOMO energies. d Optical bandgap calculated from the UVꢀvis absorption onset in film.
Figure 3. Cyclic voltammograms of the copolymers.
Figure 5. AFM images of films spin coated from P1/PC71BM (a, d),
Table 3. FET and PSC Performances of the Polymers
P2/PC71BM (b, e), and P3/PC71BM(c, f). (aꢀc) AFM topography of
each film. (dꢀf) AFM phase images of each film.
VOC
JSC
PCE
a
a
polymer μ (cm2 Vꢀ1 sꢀ1
)
Ion/Ioff
(V)b (mA/cm2)b FFb (%)b
P1
P2
P3
4.2 ꢁ 10ꢀ6
1.4 ꢁ 10ꢀ3
2.1 ꢁ 10ꢀ4
102
103
0.61
0.82
2.70
9.96
6.24
0.29 0.48
0.49 4.0
0.46 2.3
6 ꢁ 103 0.81
a The field-effect carrier mobilities of the polymers were investigated by
fabricating thin film transistors (TFTs) with a bottom contact geometry
using Au electrodes. b The device was fabricated with a layered structure
of ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al.
Figure 6. External quantum efficiency of P1:PC71BM (black), P2:
PC71BM (red), and P3:PC71BM (green).
Figure 6 shows the external quantum efficiency (EQE) spectra
of PSC devices fabricated with P2/PC71BM and P3/PC71BM
under monochromatic light illumination. In both cases, the EQE
spectra agree well with the optical-absorption curves, resulting in
a close correlation with the photocurrents. Convolution of the
spectral response with the photon flux of the AM 1.5 G spectrum
provided an estimate of the value of JSC under irradiation. The
calculated JSC value for the P2-based devices was 8.46 mA/cm2.
Because of the discrepancy between the EQE results and the
photon flux under AM 1.5 illumination, an approximate mis-
match of 15% was present between the convolution and the solar-
simulator data.36
Figure 4. JꢀV characteristics of photovoltaic devices fabricated with
P1:PC71BM (black), P2:PC71BM (red), and P3:PC71BM (green) under
AM 1.5 G irradiation (100 mW/cm2).
magnitude higher than that obtained with P3, which could explain
the higher value of the short-circuit current measured for P2.
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Although the values of VOC and JSC were high, those of the fill
factors were low. We believe that the value of FF could be
improved by using different solvents,22 adding various additives,37
and/or using a buffer layer.38 With an enhancement of the FF,
the performance of the polymer solar cells could be greatly im-
proved.
the polymer (1 wt %) in dichlorobenzene. LiF and aluminum contacts
were formed by vacuum deposition at pressures below 3 ꢁ 10ꢀ6 Torr,
providing an active area of 0.09 cm2. Solar cell efficiencies were
characterized under simulated 100 mW/cm2 AM 1.5 G irradiation from
a Xe arc lamp with an AM 1.5 global filter. Simulator irradiance was
characterized using a calibrated spectrometer, and the illumination
intensity was set using an NREL-certified silicon diode with an
integrated KG1 optical filter: short-circuit currents were found to be
within 15% of the values calculated using the integrated external
quantum efficiency (EQE) spectra and the solar spectrum. The EQE
was measured by underfilling the device area using a reflective micro-
scope objective to focus the light output from a 100 W halogen
lamp outfitted with a monochromator and optical chopper; the photo-
current was measured using a lock-in amplifier, and the absolute photon
flux was determined using a calibrated silicon photodiode. All device
fabrication procedures and measurements were carried out in air at room
temperature.
’ CONCLUSIONS
In summary, we have synthesized a new series of conjugated
polymers consisting of alternating carbazole and quinoxaline
units by means of Suzuki coupling polymerization. Considering
the polymer solubility, the ease of polymer characterization and
photovoltaic-device fabrication, and the hole mobility, we were
able to design conjugated polymers with the same polymer
backbone but different side chains. The processability, hole
mobility, and, film morphology of the copolymers were influ-
enced by their size/shape. Interestingly, the new semiconducting
copolymer P2 exhibited a field-effect carrier mobility of up to 1.4 ꢁ
10ꢀ3 cm2/(V s). Bulk heterojunction solar cells fabricated from
blends of P2 with PC71BM exhibited a power-conversion effi-
ciency of 4.0% without any special treatments. Considering the
field-effect carrier mobility and photovoltaic properties of P2, this
polymer exhibits great potential as a candidate for new-genera-
tion solar-cell materials. Further modifications of the polymer
structure and/or the device structure (for example by introdu-
cing an optical space such as titanium oxide) are currently under-
way to achieve even better performance.
’ MATERIALS
2,1,3-Benzothiadiazole, bromine, sodium borohydride, diethyl
oxalate, 4-bromophenol, 1-bromooctane, 1-bromo-3,7-dimethyl-
octane, 2-(tribuylstannyl)thiophene, dimethylformamide (DMF),
tetraethylammonium hydroxide solution, tetrakis(triphenylphos-
phine)palladium, and toluene (99.8%, anhydrous) were purchased
from Aldrich. Palladium(II) acetate was purchased from Strem
Chemicals Co. All chemicals were used without further purifica-
tion. The monomer 4,7-dibromo-2,1,3-benzothiadiazole (1),39
3,6-dibromo-1,2-phenylenediamine (2),40 1,4-dimethylpipera-
zine-2,3-dione (3),41 1-bromo-4-octyloxybenzene (4a),42 1-(3,7-
dimethyloctyloxy)-4-bromobenzene(4b),43 4,40-bis(2-octyloxy)benzil
(5a),41 5,8-dibromo-2,3-bis(4-octyloxyphenyl)quinoxaline (6a),44
5,8-bis(50-bromodithien-2-yl)quinoxaline,21 2,3-bis(4-octyloxy-
phenyl)-5,8-dithien-2-yl-quinoxaline (7a),44 2,3-bis(4-octyloxy-
phenyl)-5,8-bis(50-bromodithien-2-yl)quinoxaline (8a),44 and
2,7-bis(40,40,50,50-tetramethyl-10,30,20-dioxaborolan-20-yl)-N-900-
heptadecanylcarbazole21 were prepared with previously described
methods.
’ EXPERIMENTAL SECTION
General Data. The synthesized compounds were characterized
with 1H NMR spectra obtained using a Bruker DPX-300 NMR spectro-
meter. UVꢀvisible analysis was performed with a Lambda 20 (Perkin-
Elmer) diode array spectrophotometer. The number- and weight-
average molecular weights of the polymers were determined by gel
permeation chromatography (GPC; Viscotek) equipped with TDA 302
detector and PL-gel (Varian) column, using chloroform as the eluent
and polystyrene as the standard. Thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC) were performed under a
nitrogen atmosphere at a heating rate of 10 °C/min with a Dupont 9900
analyzer.
Fabrication of the Organic Thin Film Transistors (OTFTs).
OTFT devices were fabricated in a bottom-contact geometry (channel
length = 6 μm, width = 120 μm). The source and drain contacts
consisted of gold (100 nm), and the dielectric was silicon oxide (SiO2)
with a thickness of 300 nm. The SiO2 surface was cleaned, dried, and
pretreated with a solution of 1.0 mM octyltrichlorosilane (OTS-8) in
toluene at room temperature for 2 h under nitrogen to produce apolar
and smooth surfaces onto which the polymers could be spin-coated. The
polymers were dissolved to a concentration of 0.5 wt % in chloroben-
zene. Films of the organic semiconductors were spin-coated at 1500 rpm
for 50 s to a thickness of 50 nm, followed by an annealing process. All
device fabrication procedures and measurements were carried out in air
at room temperature.
1,2-Bis(4-(3,7-dimethyloctyloxy)phenyl)ethane-1,2-dione
(5b). Into a solution of compound 4b (13.6 g, 43.3 mmol) in
THF (100 mL) at ꢀ50 °C was added, by syringe, 30.0 mL (47.8
mmol) of n-BuLi (1.6 M in hexane). The mixture was stirred at
ꢀ50 °C for 1 h. Compound 3 (3.00 g, 21.1 mmol) was added to
the solution, and the resulting mixture was stirred at ꢀ50 °C for
1 h and then warmed to room temperature and stirred for a further
12 h. The mixture was poured into water, extracted with ether,
and dried with anhydrous MgSO4. The solvent was removed via
rotary evaporation, and the residue was precipitated in methanol.
1
The product yield was 48% (5.29 g) H NMR (300 MHz,
CDCl3): δ (ppm) 7.91 (d, 4H), 6.93 (d, 4H), 4.05(t, 4H), 1.85
(m, 2H), 1.74ꢀ1.12 (m, 18H), 0.92 (d, 6H), 0.87 (d, 12H). 13C
NMR (75 MHz, CDCl3): δ (ppm) 193.77, 164.68, 132.56,
126.29, 114.92, 67.09, 39.41, 37.42, 36.11, 29.98, 28.17, 24.84,
22.90, 22.80, 19.80. Anal. Calcd for C34H50O4: C, 78.12; H, 9.64.
Found: C, 78.12; H, 9.68.
5,8-Dibromo-2,3-bis(4-(3,7-dimethyloctyloxy)phenyl)qui-
noxaline (6b). Compound 2 (2.50 g, 9.34 mmol) and 5b (4.48 g,
8.57 mmol) were dissolved in 60 mL of MeOH, then 5,5 mL of
acetic acid was added, the mixture was heated under reflux for 4 h.
After evaporation of the solvent, the mixture was poured into
water (100 mL) and extracted with CHCl3. After the reaction
had finished, the reaction mixture was extracted three times with
chloroform and brine. The organic layer was separated and dried
Fabrication of the Polymer Solar Cells (PSCs). In this study,
the devices were fabricated with the structure ITO/PEDOT:PSS/
polymer:PC71BM/LiF/Al. The procedure for cleaning the ITO surface
included sonication and rinsing in deionized water, methanol, and
acetone. The hole-transporting PEDOT:PSS layer was spin-coated onto
each ITO anode from a solution purchased from H. C. Starck. Each
polymer:PC71BM solution was then spin-coated onto the PEDOT:PSS
layer. The polymer solution for spin-coating was prepared by dissolving
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with anhydrous magnesium sulfate, and then the solvent was
removed by using a rotary evaporator. The crude product was
precipitated in methanol. The product yield was 95% (6.47 g) 1H
NMR (300 MHz, CDCl3): δ (ppm) 7.83 (s, 2H), 7.64 (d, 4H),
6.86 (d, 4H), 4.02 (t, 4H), 1.83 (m, 2H), 1.74ꢀ1.12 (m, 18H),
0.94 (d, 6H), 0.87 (d, 12H). 13C NMR (75 MHz, CDCl3): δ
(ppm) 160.67, 153.78, 139.22, 132.69, 131.87, 130.54, 123.64,
114.58, 66. 64, 39.77, 37.49, 36.33, 30.06, 28.17, 24.87, 22.92,
22.82, 19.87. Anal. Calcd for C40H54Br2N2O2: C, 63.66; H, 7.21;
N, 3.71. Found: C, 63.61; H, 7.17; N, 3.68.
2,3-Bis(4-(3,7-dimetyloctyloxy)phenyl)-5,8-dithien-2-ly-qui-
noxaline (7b). In a 250 mL three neck flask, compound 6b
(5.40 g, 7.18 mmol) and tetrakis (triphenylphosphine)palla-
dium(0) (0.194 g, 0.168 mmol) were mixed, and the flask was
evacuated and filled with nitrogen. After addition of tolene
(50 mL) the mixture was heated to 110 °C. Then, 2-(tri-n-
butylstannyl)thiophene (5.90 g, 15.7 mmol) was added via a
syringe and the reaction mixture was stirred for 24 h. The mixture
was poured into water (100 mL) and extracted with CHCl3. The
extract was then successively washed with water and brine. After
drying over anhydrous MgSO4, the solvent was evaporated and
the residue was precipitated with MeOH several times to give
4.50 g (83%) of red-yellow solid. 1H NMR (300 MHz, CDCl3):
δ (ppm) 8.09 (s, 2H), 7.85 (d, 2H), 7.72 (d, 4H), 7.50 (d, 2H),
7.18 (t, 2H), 6.90 (d, 4H), 4.05 (t, 4H), 1.83 (m, 2H), 1.75ꢀ1.11
(m, 18H), 0.94 (d, 6H), 0.87 (d, 12H). 13C NMR (75 MHz,
CDCl3): δ (ppm) 159.99, 151.32, 138.95, 136.98, 131.88,
131.04, 130.22, 128.74, 127.16, 126.55, 126.22, 114.25, 66.42,
39.27, 37.33, 36.22, 29.91, 28.01, 24.70, 22.75, 22.65, 19.72. Anal.
Calcd for C48H58N2O2S2: C, 75.95; H, 7.70; N, 3.69; S, 8.45.
Found: C, 75.91; H, 7.68; N, 3.62; S, 8.36.
added slowly to a vigorously stirred mixture consisting of 230 mL
methanol and 13 mL 1 N aqueous HCl. The polymer fibers were
collected by filtration and reprecipitation from methanol. The
polymer was purified by washing for 2 days in a Soxhlet apparatus
with acetone to remove oligomers and catalyst residues, and
column chromatographed with a chloroform/toluene solution of
the polymer. The reprecipitation procedure in toluene/methanol
was then repeated several times. The final product was obtained
1
after drying in vacuo at 40 °C (60%). H NMR (300 MHz,
CDCl3): δ (ppm) 8.29ꢀ6.70 (m, 20H); 4.75 (br, 1H); 4.03 (br,
4H); 2.61ꢀ0.61 (m, 64H). Anal. Calcd: C, 79.39; H, 8.44; N,
3.70; S, 5.65. Found: C, 79.26; H, 8.35; N, 3.62; S, 5.53.
Poly[N-900-heptadecanyl-2,7-carbazole-alt-5,5-(50,80-di-2-
thienylquinoxaline)] (P1). was synthesized with the procedure
described for P2. The copolymerization of the monomers 2,7-
bis(40,40,50,50-tetramethyl-10,30,20-dioxaborolan-20-yl)-N-900-hep-
tadecanylcarbazole (0.263 g, 0.400 mmol), and 5,8-bis(50-bro-
modithien-2-yl)quinoxaline (0.181 g, 0.400 mmol) gave P1
(68%). 1H NMR (300 MHz, CDCl3) δ (ppm): 8.75 (br, 2H);
8.19- 7.38 (m, 12H); 4.73 (br, 1H); 2.61- 0.61 (m, 34H). Anal.
Calcd: C, 77.75; H, 7.63; N, 5.79; S, 8.83. Found: C, 77.69; H,
7.54; N, 5.64; S, 8.75.
Poly[N-900-heptadecanyl-2,7-carbazole-alt-5,5-(50,80-di-2-
thienyl-2,3-bis(4-(3,7-dimethyloctyloxy)phenyl)quinoxaline]
(P3). This was synthesized with the procedure described for P2.
The copolymerization of the monomers 2,7-bis(40,40,50,50-tetra-
methyl-10,30,20-dioxaborolan-20-yl)-N-900-heptadecanylcarbazole
(0.263 g, 0.400 mmol), and 2,3-bis(3,7-dimetyloctyloxyphenyl)-
5,8-bis(50-bromodithien-2-yl)quinoxaline (0.367 g, 0.400 mmol)
gave P3 (65%). P3 was extracted again with acetone and hexane
successively using a Soxhlet apparatus to remove oligomers and
inpurity. 1H NMR (300 MHz, CDCl3): δ (ppm): 8.30ꢀ6.70 (m,
20H); 4.73 (br, 1H); 4.07 (br, 4H); 2.62ꢀ0.60 (m, 72H). Anal.
Calcd: C, 79.68; H, 8.72; N, 3.53; S, 5.39. Found: C, 79.55; H,
8.60; N, 3.46; S, 5.33.
2,3-Bis(4-(3,7-dimetyloctyloxy)phenyl)-5,8-bis(50-bromo-
dithien-2-ly)quinoxaline (8b). A solution of compound 7b
(3.80 g, 4.98 mmol) in CHCl3 was protected from light and
cooled to 0 °C. Then N-bromosuccinimide (1.79 g, 9.96 mmol)
was added in small portions over 20 min and then the mixture
was stirred for 3 h. The mixture was poured into water (100 mL)
and extracted with CHCl3. The organic layer was dried over
anhydrous MgSO4 and the solvent removed by rotary evapora-
tion. The crude product was purified by crystallization with
MeOH to yield compound 8b as a red-yellow solid (4.26 g, 93%).
1H NMR (300 MHz, CDCl3): δ (ppm) 7.94 (s, 2H), 7.66 (d,
4H), 7.49 (d, 2H), 7.08 (d, 2H), 6.91 (d, 4H), 4.05 (t, 4H), 1.85
’ ASSOCIATED CONTENT
S
Supporting Information. Density functional theory
b
(DFT) calculations for the model compound using the DMol
3 software, figures showing the structure of model compounds,
output characteristics of OTFT devices, JꢀV characteristics of
OPV composed of P2:PC71BM with and without postannealing
treatment, and photovoltaic performance. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
(m, 2H), 1.77ꢀ1.15 (m, 18H), 0.97 (d, 6H), 0.88 (d, 12H). 13
C
NMR (75 MHz, CDCl3): δ (ppm) 160.34, 151.99, 139.96,
136.58, 132.13, 130.85, 130.52, 129.25, 125.54, 125.41, 117.16,
114.48, 66.65, 39.48, 37.55, 36.45, 30.11, 28.21, 24.90, 22.95,
22.85, 19.93. Anal. Calcd for C48H56Br2N2O2S2: C, 62.88; H,
6.16; N, 3.06; S, 6.99. Found: C, 62.81; H, 6.16; N, 3.03; S, 6.99.
General Procedure for the Synthesis of Polymers with the
Suzuki Reaction. Synthesis of Poly[N-900-heptadecanyl-2,7-car-
bazole-alt-5,5-(50,80-di-2-thienyl-2,3-bis(4-octyloxyl)phenyl)quinoxaline]
(P2). This is presented in detail as a representative example. 2,7-
Bis(40,40,50,50-tetramethyl-10,30,20-dioxaborolan-20-yl)-N-900-hepta-
decanylcarbazole (0.263 g, 0.400 mmol), 2,3-bis(4-octyloxyphe-
nyl)-5,8-bis(50-bromodithien-2-yl)quinoxaline (8a) (0.344 g,
0.400 mmol), tricyclohexylphosphine (3.4 mg, 0.0120 mmol),
and palladium(II) acetate (0.0080 mmol, 5.4 mg) were dissolved
in 4 mL of anhydrous toluene. To the solution, tetraethylammonium
hydroxide solution (1.30 g) degassed by sonicator was added
under argon flow. The mixture was heated under reflux for 24 h,
and then, the reaction mixture was cooled to about 50 °C and
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: (I.-N.K.) inamkang@catholic.ac.kr; (S.-J.M.) moonsj@
krict.re.kr.
’ ACKNOWLEDGMENT
This study was supported by the Converging Research
Center Program through the Ministry of Education, Science
and Technology (2010K000970) and by the Korea Energy
Management Corporation (KEMCO) under a New and Renew-
able Energy R&D Grant (2008-N-PV08-02), Republic of Korea.
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