Synthesis and photovoltaic properties of copolymers with a fluoro quinoxaline unit

Article abstract of DOI:10.1002/pola.28957

Two novel accepter units, namely, difluoroquinoxaline and monofluoroquinoxaline, were prepared and used for the synthesis of the conjugated polymers containing electron donor–acceptor pairs for use in organic photovoltaics. The introduction of a fluorine atom into the quinoxaline moiety resulted in polymers with lowered highest occupied molecular orbital (HOMO) energy levels; this increased the open circuit voltage of the devices based on the synthesized polymers. The conjugated polymers containing difluoroquinoxaline and monofluoroquinoxaline, namely, thiophene and benzodithiophene, were synthesized using the Stille polymerization reaction to produce PEHBQxF2, PEHBQxF1, PEHBDTQxF2, and PEHBDTQxF1. The HOMO energy levels of PEHBQxF2, PEHBQxF1, PEHBDTQxF2, and PEHBDTQxF1 were determined to be ?5.66, ?5.52, ?5.54, and ?5.39 eV, respectively. The device with PEHBDTQxF2/PC₇₁BM (1:2, w/w) and containing diiodooctane (3 vol %) exhibited the best photovoltaic performance, with its V_OC being 0.79 V, J_SC being 10.44 mA/cm², FF being 68%, and PCE being 5.58%.

Full text of DOI:10.1002/pola.28957

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Synthesis and Photovoltaic Properties of Copolymers with a Fluoro
Quinoxaline Unit
Suhee Song,1,2 Seungmin Kim,1 Wonjun Kim,3 Seong Soo Park,3 Sung Heum Park,1
3
Youngeup Jin
1Department of Physics, Pukyong National University, Busan 48513, South Korea
2Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, South Korea
3Department of Industrial Chemistry, Pukyong National University, Busan 48513, South Korea
Correspondence to: Y. Jin (E-mail: yjin@pknu.ac.kr) or S. H. Park (E-mail: shpark@pknu.ac.kr)
Received 13 December 2017; accepted 2 January 2018; published online 00 Month 2018
DOI: 10.1002/pola.28957
ABSTRACT: Two novel accepter units, namely, difluoroquinoxa-
line and monofluoroquinoxaline, were prepared and used for
the synthesis of the conjugated polymers containing electron
donor–acceptor pairs for use in organic photovoltaics. The
introduction of a fluorine atom into the quinoxaline moiety
resulted in polymers with lowered highest occupied molecular
orbital (HOMO) energy levels; this increased the open circuit
voltage of the devices based on the synthesized polymers. The
conjugated polymers containing difluoroquinoxaline and
monofluoroquinoxaline, namely, thiophene and benzodithio-
phene, were synthesized using the Stille polymerization
reaction to produce PEHBQxF2, PEHBQxF1, PEHBDTQxF2, and
PEHBDTQxF1. The HOMO energy levels of PEHBQxF2,
PEHBQxF1, PEHBDTQxF2, and PEHBDTQxF1 were determined
to be 25.66, 25.52, 25.54, and 25.39 eV, respectively. The
device with PEHBDTQxF2/PC71BM (1:2, w/w) and containing
diiodooctane (3 vol %) exhibited the best photovoltaic perfor-
mance, with its VOC being 0.79 V, JSC being 10.44 mA/cm2, FF
C
V
being 68%, and PCE being 5.58%. 2018 Wiley Periodicals, Inc.
J. Polym. Sci., Part A: Polym. Chem. 2018, 00, 000–000
KEYWORDS: fluorine atom; polymer; solar cells; synthesis
electron donor, such as 2,7-fluorene,10 carbazole,11 or ben-
zodithiophene.1 Among the various donor units available,
benzodithiophene has elicited much interest with respect to
the design of novel-conjugated polymers for use in PSCs,
owing to its planarity and rigidity, which result from the
fusion of its benzene ring with two thiophene units, as well
as its good solubility, which is caused by its various side
chains, and its high hole mobility.1,5,12,13 Quinoxaline-based
copolymers have been used for their ability to withdraw
electrons from the nitrogen atoms in imine moieties and
from their high solubility, which, as stated previously,
results from their side chains.14–17 Recently, an electron-
withdrawing fluorine atom was introduced in quinoxaline
to produce a low-bandgap-conjugated polymer with a rela-
tively low HOMO energy level, to enhance the VOC of
PSCs.18 Compared to other electron withdrawing groups,
the F atom, when introduced in the quinoxaline, results in
the planarization of the conjugated polymer, owing to the
low degree of the steric interactions of the fluorine
atom.5,19,20 Thiophene units are highly promising as elec-
tron rich units for use in PSCs, as they augmentation conju-
gation length and synthetic multiformity.3,21,22 Polymers
with thiophene units as the bridge between D and A
INTRODUCTION In the last two decades, polymer solar cells
(PSCs) have attracted significant attention from both aca-
demia and the industry, owing to their potential value in the
organic photovoltaics (OPVs) market.1,2 Bulk-heterojunction
(BHJ) solar cells are the most successful OPV devices using
conjugated polymers as the electron donors and phenyl
butyric acid methyl ester (PCBM) as the electron acceptor.3
A power conversion efficiency (PCE) as high as 10% had
been reported for single junction PSCs very recently.4 Fur-
thermore, recent studies have focused on designing conju-
gated polymers that exhibit a down-shifted highest occupied
molecular orbital (HOMO) energy level, to increase the open
circuit voltage (VOC) of PSCs, and a low bandgap energy
level, so that they are able to absorb longer wavelength light
and thus increase the efficiency of PSCs.
PSC performance has been improved significantly though
the design and synthesis of novel-conjugated polymers with
a donor–acceptor (D–A) structure, which allows for the tun-
ing of the bandgap, HOMO energy level, charge carrier
mobility, and absorption strength.5,6 Many of the push–pull
type low bandgap-conjugated copolymers use an electron
acceptor, such as quinoxaline7 or benzimidazole,8,9 and an
C
V
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SCHEME 1 Routes for synthesizing the monomers.
segments exhibit a lower bandgap, since the p-conjugated
hexacosanedione (2) to obtain 5,8-dibromo-2,3-didodecyl-6,7-
difluoroquinoxaline (4). The Stille coupling reaction of 5,8-
dibromo-2,3-didodecyl-6,7-difluoroquinoxaline (4) with tribu-
tyl(2-thienyl)stannane was performed to obtain 2,3-didodecyl-
6,7-difluoro-5,8-di(2-thienyl)-quinoxaline (5). Compound 5
was brominated with N-bromosuccinimide (NBS) to generate
system is delocalized to a greater extent.23
In this paper, we report the synthesis of D–A alternating copoly-
mers with novel electron deficient units, namely, 6,7-difluoro-
2,3-dihexylquinoxaline17 and 6-fluoro-2,3-dihexylquinoxaline,
for use in high performance PSCs. It was assumed that the
introduction of an F atoms into the quinoxaline moiety would
lower the HOMO energy levels of the synthesized polymers and
lead to an increase in VOC. To be able to fabricate PSCs whose
absorption spectrum covers a wider range of the solar spec-
trum, these push–pull type conjugated polymers were synthe-
sized by Stille coupling reaction of 2,6-bis(trimethyltin)-4,8-
bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene (as the
electron donor) and 6,7-difluoro-2,3-dihexylquinoxaline and 6-
fluoro-2,3-dihexylquinoxaline (as the electron acceptor). Fur-
thermore, the optical, electrochemical, and photovoltaic prop-
erties of the synthesized polymers were investigated.
5,8-bis(5-bromo-2-thienyl)-2,3-didodecyl-6,7-difluoro
qui-
noxaline (6). The monofluoroquinoxaline monomer, 5,8-
dibromo-2,3-didodecyl-6-fluoroquinoxaline (14), was pre-
pared from 3,6-dibromo-4-fluoro-1,2-benzenediamine (13)25
and 13,14-hexacosanedione (2) using acetic acid. Compound
14 was coupled with tributyl(2-thienyl)stannane though the
Stille coupling reaction to obtain 2,3-didodecyl-6-fluoro-
5,8-di(2-thienyl)-quinoxaline (15). Next, compound 15 was
brominated with NBS in tetrahydrofuran (THF) to obtain
5,8-bis(5-bromo-2-thienyl)-2,3-didodecyl-6-fluoroquinoxaline
(16). Copolymers with 2,6-bis(trimethyltin)-4,8-bis(2-ethyl-
hexyloxy)benzo[1,2-b:4,5-b0]dithiophene (17) for the electron
donating units and various quinoxaline monomers as the
electron-accepting moiety were synthesized through Pd(0)-
catalyzed Stille coupling polymerization in chlorobenzene.
The two dodecyl chains in the quinoxaline moiety and the eth-
ylhexyloxy chain in the benzodithiophene moiety were intro-
duced to improve the solubility of the polymers. All of the
synthesized polymers showed good solubility in organic sol-
vents such as chloroform, THF, chlorobenzene, and o-dichloro-
benzene (DCB), at room temperature.
RESULTS AND DISCUSSION
Synthesis and Characterization
The routes used to synthesize the monomers and polymers
are outlined in Schemes 1 and 2, respectively. In the first
step, 13,14-hexacosanedione (2)24 was prepared from oxalyl
chloride via the Grignard reaction. 3,6-Dibromo-4,5-
difluorobenzene-1,2-diamine (3) was coupled with 13,14-
2
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SCHEME 2 Routes for synthesizing the polymers.
Table 1 lists the results of the polymerization process,
including the molecular weights, polydispersity index (PDI)
values, and thermal stabilities of the synthesized polymers.
Through gel permeation chromatography, performed using
polystyrene as the standard for calibration, it was confirmed
that the number-average molecular weights (Mn), weight-
average molecular weight (Mw), and PDI values (Mw/Mn) of
the synthesized polymers were 34,000–66,000, 59,000–
100,000, and 1.47–1.74, respectively. The thermal properties
of the polymers were investigated through thermal gravimet-
ric analysis (TGA), which was performed on a Dupont 951
TGA system in a nitrogen atmosphere at a heating rate of
10 8C/min; the samples were heated to 600 8C. The TGA
curve showed a decrease in weight of 5% between 312 and
321 8C. The polymers with rigid thiophene, PEHBDTQxF2
and PEHBDTQxF1, show higher Td values; a high Td value is
desirable in the case of the polymers used in OPVs, as it
indicates a good morphology.
Optical Properties
TABLE 1 Polymerization Results and Thermal Properties of
The absorption spectra of the synthesized polymers in solu-
tion and thin film form are shown in Figure 1, while their
absorption properties are listed in Table 2. The solutions
were prepared using DCB as the solvent, while the thin films
were formed by spin-coating the polymer solutions in DCB on
a quartz plate at room temperature. In solution form,
PEHBQxF2 and PEHBQxF1 showed similar absorption spectra,
with their absorption maxima being at 596 and 602 nm,
respectively. Furthermore, the absorption spectra of
PEHBDTQxF2 and PEHBDTQxF1 with thiophene units in solu-
tion contained two maxima each, at approximately 398, 568
and 398, 548, respectively. PEHBQxF2 was blue-shifted than
that of PEHBQxF1 by its degree of steric hindrance owing to
not having thiophene as bridge unit. Although the fluorinated
Polymers
a
a
b
Mn
Mw
Td
Polymer
(g/mol)
(g/mol)
PDIa
(8C)
PEHBQxF2
34,000
66,000
43,000
54,000
59,000
100,000
75,000
89,000
1.72
1.47
1.74
1.65
312
314
320
321
PEHBQxF1
PEHBDTQxF2
PEHBDTQxF1
a
Molecular weight (Mw) and polydispersity (PDI) values of the polymers
were determined by gel permeation chromatography (GPC), which was
performed in THF using polystyrene standards.
b
Onset decomposition temperature (5% weight loss), as measured by
TGA in N2 atmosphere.
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FIGURE 2 Electrochemical properties of the synthesized poly-
mers. [Color figure can be viewed at wileyonlinelibrary.com.]
red shifted with respect to those of the polymers in solution
form. PEHBDTQxF2 and PEHBDTQxF1 with thiophene units
showed broader absorption compared to counterparts with-
out the thiophene bridge; thus, the former should help
improve solar cell efficiency. Furthermore, PEHBQxF2,
PEHBQxF1, PEHBDTQxF2, and PEHBDTQxF1 in the solid thin
film form exhibited absorption onsets at 652, 654, 651, and
660 nm, respectively, which corresponded to bandgaps of
1.90 1.89, 1.90, and 1.88 eV, respectively. The polymers with
the mono-fluoroquinoxaline units showed a slightly smaller
optical bandgap than the polymers with the difluoroquinoxa-
line units.
FIGURE 1 UV–visible absorption spectra of the synthesized
polymers in (a) dichlorobenzene solution and (b) the solid
state. [Color figure can be viewed at wileyonlinelibrary.com.]
Electrochemical Properties and Results of Theoretical
Calculations
The electrochemical properties of the polymers were deter-
mined based on the bandgaps estimated from their
quinoxaline-containing polymer showed a clear blue shift
compared to the polymer without fluorine atoms, owing to
the increased twisting,26,27 that of PEHBDTQxF1 was more
blue-shifted by twist of polymer backbone due to intramolec-
ular dipolar interaction than that of PEHBDTQxF228 because
PEHBDTQxF1 has region irregular structure when polymeri-
zation reaction. The maxima of PEHBQxF2, PEHBQxF1,
PEHBDTQxF2, and PEHBDTQxF1 in solid thin films form were
seen at 601, 606, 578, and 572 nm, respectively. Hence, the
spectra of the synthesized polymers in thin film forms were
TABLE 3 Electrochemical Potentials and Energy Levels of the
Synthesized Polymers
Electro-
Optical
chemical
d
d
Bandgapa HOMOb LUMOc Eox Ered Bandgape
Polymers (eV)
(eV)
(eV)
(V) (V)
(eV)
PEHBQxF2 1.90
PEHBQxF1 1.89
25.66
25.52
25.54
23.40 0.86 21.40 2.26
23.44 0.72 21.36 2.08
23.54 0.74 21.26 2.00
TABLE 2 Characteristic Peaks in the UV–Vis Absorption Spectra
PEHBD
TQxF2
PEHBD
TQxF1
1.90
of the Synthesized Polymers
1.88
25.39
23.42 0.59 21.38 1.97
In
In
Solution
(nm)
Thin Film
(nm)
a
Polymer
Optical energy bandgap was estimated from the onset wavelength for
optical absorption.
b
PEHBQxF2
596
602
568
548
601
606
578
572
Calculated from the oxidation potentials.
c
Calculated from the reduction potentials.
PEHBQxF1
d
Onset oxidation and reduction potential measured by cyclic
PEHBDTQxF2
PEHBDTQxF1
voltammetry.
e
Calculated from Eox and Ered
.
4
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FIGURE 3 Results of the theoretical simulations of the monomers of the four polymers based on density functional theory at the
B3LYP/6–311G1(d,p) level. [Color figure can be viewed at wileyonlinelibrary.com.]
FIGURE 4 Current density-potential characteristics of the PEHBQxF2, PEHBQxF1, PEHBDTQxF2, and PEHBDTQxF1 solar cells under
AM 1.5, 100 mW/cm2 illumination. [Color figure can be viewed at wileyonlinelibrary.com.]
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TABLE 4 Photovoltaic Properties of Solar Cells Based on the
Synthesized Polymers
Ratio
JSC
VOC
PCE
(%)
Polymers
(Polymer:PCBM) (mA/cm2) (V)
FF
PEHBQxF2
1:0.5
1:1
0.24
1.26
1.07
1.19
0.59
2.15
2.92
1.02
1.24
6.72
6.27
3.97
10.41
0.67 0.30 0.05
0.75 0.35 0.33
0.69 0.41 0.30
0.69 0.59 0.49
0.51 0.35 0.11
0.66 0.34 0.48
0.67 0.48 0.94
0.61 0.48 0.30
0.66 0.33 0.27
0.79 0.48 2.51
0.80 0.62 3.12
0.82 0.59 1.91
0.79 0.66 5.45
1:2
1:3
PEHBQxF1
1:0.5
1:1
1:2
1:3
PEHBDTQxF2 1:0.5
1:1
1:2
1:3
1:2 (1 vol
% DIO)
1:2 (2 vol
% DIO)
1:2 (3 vol
% DIO)
1:2 (6 vol
% DIO)
PEHBDTQxF1 1:0.5
10.10
10.44
9.12
0.76 0.64 4.95
0.79 0.68 5.58
0.75 0.61 4.15
1.71
6.66
8.18
4.26
11.20
0.69 0.35 0.41
0.73 0.52 2.55
0.75 0.61 3.75
0.67 0.46 1.29
0.74 0.62 5.11
1:1
1:2
1:3
1:2 (3 vol
% DIO)
FIGURE 5 Current density-potential characteristics of the (a)
PEHBDTQxF2:PCBM(1:2) and (b) PEHBDTQxF1:PCBM(1:2) with
DIO solar cells under AM 1.5 (100 mW/cm2) illumination. [Color
figure can be viewed at wileyonlinelibrary.com.]
PEHBDTQxF1 were determined to be 0.86, 0.72, 0.74, and
0.59 V, respectively, which corresponded to HOMO energy
levels of 25.66, 25.52, 25.54, and 25.39 eV, respectively.
The polymers with the difluoro-quinoxaline units, PEHBQxF2
and PEHBDTQxF2, showed lower HOMO energy levels, owing
to fluorination effect, as compared to the polymers with the
monofluoro-quinoxaline units. When the thiophene units
were introduced, the HOMO energy levels of the polymers
became higher because of the increase in the extent of conju-
gation of the electron donating units parallel to the back-
bone.29 LUMO energy levels of the synthesized polymers
were estimated to range from 23.40 eV to 23.54 eV. The
electrochemical bandgaps of PEHBQxF2, PEHBQxF1,
PEHBDTQxF2, and PEHBDTQxF1 were approximately 2.26,
2.08, 2.00, and 1.97 eV, and somewhat higher than their
respective optical bandgaps. PEHBDTQxF2 and PEHBDTQxF1,
the polymers with thiophene, showed lower electrochemical
bandgaps from higher HOMO energy levels.
respective absorption onset wavelengths, and the HOMO
energy levels, which were estimated by cyclic voltammetry
(CV). The CV measurements were performed using a solution
of
tetrabutylammonium
tetrafluoroborate
(Bu4NBF4)
(0.10 M) in acetonitrile at a scan rate of 100 mV/s at room
temperature in an argon atmosphere. A platinum electrode
(ꢀ0.05 cm2) coated with a thin film of the polymer being
tested was used as the working electrode. A Pt wire and an
Ag/AgNO3 electrode were used as the counter and reference
electrodes, respectively. The energy level of the Ag/AgNO3
reference electrode (calibrated using the Fc/Fc1 redox sys-
tem) was 4.8 eV below the vacuum level. The obtained CV
spectra are shown in Figure 2, while the oxidation potentials
derived from the onsets of electrochemical p-doping are
listed in Table 3. The HOMO and lowest unoccupied molecu-
lar orbital (LUMO) levels were calculated using the empirical
formulas (EHOMO 5 –([Eonset
]
ox 1 4.8) eV) and (ELUMO 5 –
The optimized geometries of the synthesized polymers were cal-
culated at the B3 LYP/6–311G (d,p) level of theory using the
Gaussian 03 package.30 The HOMO and LUMO surfaces were
([Eonset
]
red 1 4.8) eV), respectively. The oxidation potential
onsets of PEHBQxF2, PEHBQxF1, PEHBDTQmxF2, and
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FIGURE 6 AFM (topography) images of films spin coated from PEHBDTQxF2:PC70BM (1:2) (a) without and (b) with DIO. [Color fig-
ure can be viewed at wileyonlinelibrary.com.]
plotted using the GaussView version 4.1. To simplify the calcula-
tions, the alkyl chains were replaced with methyl groups and the
repeating units, BQxF2, BQxF1, BDTQxF2, and BDTQxF1, were
used for the simulations of PEHBQxF2, PEHBQxF1, PEHBDTQxF2,
and PEHBDTQxF1, respectively. The optimized structures of
PEHBQxF2, PEHBQxF1, PEHBDTQxF2, and PEHBDTQxF1 are
shown in Figure 3. The HOMO wave functions of BQxF2, BQxF1,
BDTQxF2, and BDTQxF1, are usually delocalized along the entire
polymer backbone, while the LUMO wave function is highly local-
ized. A greater degree of delocalization of the HOMO waver func-
tions was observed in the case of the polymers with the
thiophene units as compared to BQxF2 and BQxF1.
(1:2 w/w), with 1,8-diiodooctane (DIO) being used as an addi-
tive. DIO was selected as the solvent additive because of its
ability to solvate fullerenes and because it has a high boiling
point.31 The polymers with the difluoroquinoxaline units,
PEHBQxF2 and PEHBDTQxF2, exhibited higher VOC values,
since their HOMO energy levels were lower. The devices with
PEHBDTQxF2/PC71BM and PEHBDTQxF1/PC71BM with DIO
(3 vol %) exhibited the best photovoltaic performances, given
the increase in the miscibility of the blends.32 These devices
showed VOC values of 0.79 and 0.74 V, respectively; JSC values
of 10.44 and 11.2 mA/cm2, respectively; FF values of 68 and
62%, respectively; and PCE values of 5.58 and 5.11%, respec-
tively. Although the device with DIO showed a slightly lower
VOC value, its JSC and FF values were higher; this resulted in a
higher efficiency, owing to improved intermolecular packing.33
The nanoscale phase separation improved carrier collection
efficiency and exciton dissociation to lead increasing short cir-
cuit current density as well as the device efficiency.34 The
HOMO surfaces of BDTQxF2 and BDTQxF1, containing thio-
phene bridge, were delocalized along the entire polymer back-
bone to lead higher PCE.
Photovoltaic Properties of Synthesized Polymers
To demonstrate the suitability of the synthesized polymers in
PSCs, we used the polymers as electron donors and PC71BM as
the electron acceptor and fabricated BHJ PSCs with the config-
uration ITO/PEDOT:PSS/polymer:PC71BM/Al. These devices
were then tested under AM 1.5G irradiation (100 mW/cm2).
Figures 4 and 5 show the current density–voltage spectra of
the devices, while Table 4 lists the open circuit voltage (VOC),
short circuit current density (JSC), fill factor (FF), and the PCE
of the devices. The active layers were spin-coated using the
corresponding DCB solutions. The weight ratio of the synthe-
sized polymers and PC71BM was varied from 1: 0.5 to 1:3, and
the device thickness was controlled based on the spinning
speed, to optimize the device performance. The devices based
on PEHBQxF2:PC71BM (1:3 w/w), PEHBQxF1:PC71BM (1:1.5
w/w), PEHBDTQxF2:PC71BM (1:2 w/w), and PEHBDTQx
F1:PC71BM (1:2w/w) showed VOC values of 0.69, 0.67, 0.80,
and 0.75 V, respectively; JSC values of 1.19, 2.92, 6.27, and 8.18
mA/cm2, respectively; and FF values of 59, 48, 62, and 61%,
respectively; with the PCE values being 0.49, 0.94, 3.12, and
3.75%, respectively. Since the VOC, JSC, and FF values increased
with the introduction of the thiophene unit in the main chain,
the devices based on the polymers with the thiophene units
showed higher efficiencies, owing to the increase in the conju-
gated area with the increase in the extent of p–p stacking and
p-overlapping between the polymer main chains.26 Figure 5
displays the J–V curves of the devices incorporating
PEHBDTQxF2:PC71BM (1:2 w/w) and PEHBDTQxF1:PC71BM
Figure 6 displays the surface morphologies (surface area of
2 3 2 lm2; observations made in the tapping mode) of the
films of PEHBDTQxF2/PC71BM and PEHBDTQxF1/PC71BM
with DIO (3 vol %), as determined through atomic force
microscopy (AFM). The PEHBDTQxF2 blend without DIO
exhibited a grainy surface owing to limited charge transport
by the discontinuous grain structures.28 The root-mean-
square roughnesses (RMS) values of the blend films with
and without DIO were 1.95 and 2.84 nm, respectively. After
the addition of DIO, the roughness decreased, owing to the
formation of phase segregated structures.33 As a result, JSC of
the blend film with 3% DIO was increased caused by more
effective charge transport from minimal phase separation
between the donor polymer and the PCBM acceptor.35,36
CONCLUSIONS
We used Stille coupling polymerization to prepare a series of
new conjugated polymers PEHBQxF2, PEHBQxF1, PEHBDTQxF2,
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and PEHBDTQxF1-containing alternating benzo[1,2-b:4,5-
b0]dithiophene moieties and fluorinated quinoxaline units in
their main backbones. The HOMO energy levels of PEHBQxF2,
PEHBQxF1, PEHBDTQxF2, and PEHBDTQxF1 are 25.66,
25.52, 25.54, and 25.39 eV, respectively. The polymers with
the difluoro-quinoxaline units, PEHBQxF2 and PEHBDTQxF2,
showed lower HOMO energy levels owing to a more pro-
nounced electron withdrawing effect as compared to the poly-
mers with the monofluoro-quinoxaline units. The device
based on PEHBDTQxF2/PC71BM (1:2, w/w) with DIO (3 vol
%) exhibited the best photovoltaic performance, showing a
VOC of 0.79 V, JSC of 10.44 mA/cm2, FF of 68%, and PCE of
5.58%.
intensity of 1000 W m22. An aperture (12.7 mm2) was used
on top of the cell to eliminate extrinsic effects such as cross-
talk, waveguiding, and shadow effects. The spectral mismatch
factor was calculated by comparison of solar simulator spec-
trum with AM 1.5 spectrum at room temperature.
Synthesis of 5,8-Dibromo-2,3-didodecyl-6,7-
difluoroquinoxaline (4)
3,6-Dibromo-4,5-difluorobenzene-1,2-diamine (3) (2.2 g, 7.3
mmol) and 13,14-hexacosanedione (2)24 (3.48 g, 8.8 mmol)
were dissolved in acetic acid (60 mL) and heated to 100 8C
overnight. After the reaction mixture was cooled to room
temperature, water, and ethyl acetate were added. The aque-
ous phase was extracted with ethyl acetate and combined
organic layer were dried over MgSO4. The organic phase was
concentrated under reduced pressure and the residue was
purified by flash column chromatography to give 3.7 g of
compound 4 as white solid. 1H NMR (CDCl3, 400 MHz, d):
3.03 (t, 4H, J 5 7.66 Hz), 1.88 (m, 4H, J 5 7.52 Hz), 1.24–1.44
(m, 36H), 0.86 (t, 6H, J 5 6.84 Hz). 13C NMR (CDCl3, 100
MHz, d): 158.17, 149.71 (d of d), 135.75, 109.23 (d of d),
34.62, 31.92, 29.68, 29.65, 29.58, 29.51, 29.45, 29.37, 29.31,
27.66, 22.69, 14.12. MS (m/z): calcd for C32H50Br2F2N2,
658.2; found, 658.
EXPERIMENTAL
Materials and Methods. All reagents were purchased from
Aldrich or TCI, and used without further purification. Sol-
vents were purified by normal procedure and handled under
moisture-free atmosphere. 1H NMR spectra were recorded
with a JNM ECP-400 (400 MHz, JEOL) spectrometer and
chemical shifts were recorded in ppm units with TMS as the
internal standard. Flash column chromatography was per-
formed with Merck silica gel 60 (particle size 230–400 mesh
ASTM) with ethyl acetate/hexane or methanol/methylene
chloride gradients unless otherwise indicated. Analytical thin
layer chromatography (TLC) was conducted using Merck
0.25 mm silica gel 60F pre-coated aluminum plates with
fluorescent indicator UV254. Molecular weight and polydis-
persity of the polymer were determined by gel permeation
chromatography (GPC) analysis with a polystyrene standard
calibration. The UV–vis absorption spectra were recorded by
a Varian 5E UV/VIS/NIR spectrophotometer, while the Oriel
InstaSpec IV CCD detection system with xenon lamp was
used for the photoluminescence and electroluminescence
spectra measurements.
Synthesis of 2,3-Didodecyl-6,7-difluoro-5,8-di(2-thienyl)-
quinoxaline (5)
A solution of 5,8-dibromo-2,3-didodecyl-6,7-difluoroquinoxa-
line (4) (2.5 g, 3.8 mmol) and tributyl(2-thienyl)stannane
(4.23 g, 11.4 mmol) in 60 mL of THF at room temperature
was treated with dichlorobis(triphenylphosphine)palladiu-
m(II) (2 mol %). The reaction mixture was stirred for 12 h
at 90 8C, concentrated under reduced pressure, and the resi-
due was purified by flash column chromatography to give
2.1 g of compound 5 as a yellow solid. 1H NMR (CDCl3, 400
MHz, d): 7.98 (d, 2H, J 5 2.96 Hz), 7.59 (d, 2H, J 5 4.96 Hz),
7.21 (t, 2H, J 5 4.56 Hz), 3.06 (t, 4H, J 5 7.52 Hz), 1.96 (m,
4H, J 5 7.45 Hz), 1.24–1.45 (m, 36H), 0.86 (t, 6H, J 5 6.86
Hz). 13C NMR (CDCl3, 100 MHz, d): 155.21, 148.82 (d of d),
134.53, 131.01, 130.41 (d), 129.52, 126.33, 117.51, 34.68,
31.92, 29.72, 29.69, 29.66, 29.60, 29.57, 29.38, 29.37, 28.05,
22.69, 14.12. MS (m/z): calcd for C40H56Br2N2S2, 666.4;
found, 666.
Solar cells were fabricated on an indium tin oxide (ITO)-
coated glass substrate with the following structure; ITO-
coated glass substrate/poly(3,4-ethylenedioxythiophene):po-
ly(stylenesulfonate) (PEDOT:PSS)/polymer: PC71BM/Al. The
ITO-coated glass substrate was first cleaned with detergent,
ultrasonicated in acetone and isopropyl alcohol, and subse-
quently dried overnight in an oven. PEDOT:PSS (Baytron PH)
was spin-casted from aqueous solution to form a film of
40 nm thickness. The substrate was dried for 10 min at 140
8C in air and then transferred into a glove box to spin-cast
the charge separation layer. A solution containing a mixture
of polymer:PC71BM in DCB solvent with concentration of 7
wt/mL % was then spin-casted on top of the PEDOT/PSS
layer. The film was dried for 60 min at 70 8C in the glove
box. The sample was heated at 80 8C for 10 min in air. Then,
an aluminum (Al, 100 nm) electrode was deposited by ther-
mal evaporation in a vacuum of about 5 3 1027 Torr. Cur-
rent density–voltage (J–V) characteristics of the devices were
measured using a Keithley 236 Source Measure Unit. Solar
cell performance was measure by using an Air Mass 1.5
Global (AM 1.5 G) solar simulator with an irradiation
Synthesis of 5,8-Bis(5-bromo-2-thienyl)-2,3-didodecyl-
6,7-difluoro quinoxaline (6)
2,3-Didodecyl-6,7-difluoro-5,8-di(2-thienyl) quinoxaline (5)
(2.0 g, 3.0 mmol) was brominated with N-bromosuccinimide
(NBS) (1.1 g, 6.0 mmol) in THF (60 mL) at room tempera-
ture. After stirring for 24 h, water (100 mL) and ethyl ace-
tate (200 mL) were added. The aqueous phase was extracted
with ethyl acetate and combined organic layer were dried
over MgSO4. The organic phase was concentrated under
reduced pressure and the residue was purified by flash col-
umn chromatography to give 2.4 g of compound 6 as an
orange solid. 1H NMR (CDCl3, 400 MHz, d): 7.75 (d, 2H,
J 5 4.00 Hz), 7.14 (d, 2H, J 5 4.04 Hz), 3.08 (t, 4H, J 5 7.66
Hz), 1.97 (m, 4H, J 5 7.46 Hz), 1.23–1.46 (m, 36H), 0.85 (t,
8
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ARTICLE
6H, J 5 6.84 Hz). 13C NMR (CDCl3, 100 MHz, d): 155.42,
148.68 (d of d), 133.58, 132.60, 130.35, 129.03, 118.56,
116.61, 34.75, 31.93, 29.72, 29.70, 29.63, 29.59, 29.41,
29.38, 28.14, 22.70, 14.14. MS (m/z): calcd for
C40H54Br2F2N2S2, 822.2; found, 822.
(CDCl3, 100 MHz, d): 157.11 (d of d), 156.09, 154.46, 137.95,
137.30, 133.63, 133.18, 130.25, 129.65, 128.72, 125.27,
117.99, 117.45, 115.56, 113.93, 113.73, 34.98, 34.78, 31.95,
29.79, 29.73, 29.65, 29.63, 29.40, 27.95, 27.75, 22.71, 14.15. MS
(m/z): calcd for C40H55Br2FN2S2, 804.2; found, 804.
Polymerization of Poly[2,6-(4,8-bis(2-
Synthesis of 5,8-Dibromo-2,3-didodecyl-6-
ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene)-alt-5,8-
(2,3-didodecyl-6,7-difluoroquinoxaline)] (PEHBQxF2)
Carefully purified 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)-
benzo[1,2-b:4,5-b0]dithiophene (17) (0.4624 g, 0.7 mmol), 5,8-
dibromo-2,3-didodecyl-6,7-difluoroquinoxaline (4) (0.5406 g,
0.7 mmol), P(o-tolyl)3 (40 mol %), DMF (2 mL), and Pd2(dba)3
(3 mol %) were dissolved in 5 mL of chlorobenzene. The mixture
was refluxed with vigorous stirring for 2 days under argon atmo-
sphere. After cooling to room temperature, the mixture was
poured into methanol. The precipitated material was recovered
by filtration. The resulting solid material was reprecipitated
using 100 mL of THF/1.0 L of methanol several times to remove
residual amount of catalyst. The purified polymer was collected
by filtration to give 112 mg as violet solid.
fluoroquinoxaline (14)
3,6-Dibromo-4-fluoro-1,2-benzenediamine (13)25 (1.4 g, 4.9
mmol) and 13,14-hexacosanedione (2) (2.32 g, 5.9 mmol)
were dissolved in acetic acid (60 mL) and heated to 100 8C
overnight. After the reaction mixture was cooled to room
temperature, water, and ethyl acetate were added. The aque-
ous phase was extracted with ethyl acetate and combined
organic layer were dried over MgSO4. The organic phase was
concentrated under reduced pressure and the residue was
purified by flash column chromatography to give 2.2 g of
compound 14 as white solid. 1H NMR (CDCl3, 400 MHz, d):
7.80 (d, 1H, J 5 8.08 Hz), 3.04 (m, 4H), 1.88 (m, 4H), 1.24–
1.45 (m, 36H), 0.86 (t, 6H, J 5 6.84 Hz). 13C NMR (CDCl3,
100 MHz, d): 158.86, 158.25 (d of d), 157.36, 139.40 (d),
136.22, 123.75 (d), 121.86, 107.80 (d), 34.75, 34.60, 31.92,
29.67, 29.65, 29.58, 29.52, 29.46, 29.36, 27.72, 22.69, 14.12.
MS (m/z): calcd for C32H51Br2FN2, 640.2; found, 640.
Polymerization of Poly[2,6-(4,8-bis(2-
ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene)-alt-5,8-
(2,3-didodecyl-6-fluoroquinoxaline)] (PEHBQxF1)
By following the similar method used for PEHBQxF2,
PEHBQxF1 was synthesized with 2,6-bis(trimethyltin)-4,8-
Synthesis of 2,3-Didodecyl-6-fluoro-5,8-di(2-thienyl)-
quinoxaline (15)
bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene
(17)
A solution of 5,8-dibromo-2,3-didodecyl-6-fluoroquinoxaline
(14) (3.7 g, 5.7 mmol) and tributyl(2-thienyl)stannane
(6.4 g, 17.2 mmol) in 60 mL of THF at room temperature
was treated with dichlorobis(triphenylphosphine)palladiu-
m(II) (2 mol %). The reaction mixture was stirred for 12 h
at 90 8C, concentrated under reduced pressure, and the resi-
due was purified by flash column chromatography to give
(0.3862 g, 0.5 mmol) and 5,8-dibromo-2,3-didodecyl-6-fluoro-
quinoxaline (14) (0.3213 g, 0.5 mmol). The purified polymer
was collected by filtration to give 291 mg as violet solid.
Polymerization of Poly[2,6-(4,8-bis(2-
ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene)-alt-5,8-
(2,3-didodecyl-6-fluoroquinoxaline)] (PEHBDTQxF2)
By following the similar method used for PEHBQxF2,
PEHBDTQxF2 was synthesized with 2,6-bis(trimethyltin)-
4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene (17)
(0.5406 g, 0.7 mmol) and 5,8-bis(5-bromo-2-thienyl)-2,3-
didodecyl-6,7-difluoro quinoxaline (6) (0.5774 g, 0.7 mmol).
The purified polymer was collected by filtration to give
775 mg as violet solid.
1
2.8 g of compound 15 as a yellow solid. H NMR (CDCl3, 400
MHz, d): 7.82 (m, 3H), 7.5 (m, 2H), 7.19 (m, 2H), 3.04 (m,
4H), 1.97 (m, 4H), 1.24–1.45 (m, 36H), 0.86 (t, 6H, J 5 6.86
Hz). 13C NMR (CDCl3, 100 MHz, d): 156.60 (d of d), 155.92,
154.27, 138.43 (d), 137.55, 134.22, 132.01 (d), 131.58 (d),
129.83 (d), 129.35, 128.58, 126.46 (d), 126.13, 116.32 (d),
115.54 (d), 34.85, 34.67, 31.93, 29.73, 29.67, 29.62, 29.57,
29.38, 27.92, 27.87, 22.70, 14.13. MS (m/z): calcd for
C40H57F2N2S2, 648.4; found, 648.
Polymerization of Poly[2,6-(4,8-bis(2-
ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene)-alt-5,8-
(2,3-didodecyl-6-fluoroquinoxaline)] (PEHBDTQxF1)
By following the similar method used for PEHBQxF2,
PEHBDTQxF1 was synthesized with 2,6-bis(trimethyltin)-
4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene (17)
(0.5406 g, 0.7 mmol) and 5,8-bis(5-bromo-2-thienyl)-2,3-
didodecyl-6-fluoroquinoxaline (16) (0.5648 g, 0.7 mmol).
The purified polymer was collected by filtration to give
759 mg as violet solid.
Synthesis of 5,8-Bis(5-bromo-2-thienyl)-2,3-didodecyl-6-
fluoroquinoxaline (16)
2,3-Diodecyl-6-fluoro-5,8-di(2-thienyl)-quinoxaline (15) (2.5 g,
3.85 mmol) was brominated with N-bromosuccinimide (NBS)
(1.2 g, 7.7 mmol) in THF (60 mL) at room temperature. After
stirring for 24 h, water (100 mL) and ethyl acetate (200 mL)
were added. The aqueous phase was extracted with ethyl ace-
tate and combined organic layer were dried over MgSO4. The
organic phase was concentrated under reduced pressure and
the residue was purified by flash column chromatography to
1
give 2.9 g of compound 6 as an orange solid. H NMR (CDCl3,
ACKNOWLEDGMENTS
400 MHz, d): 7.80 (d, 1H, J 5 13.72 Hz), 7.72 (m, 1H), 7.49 (d,
1H, J 5 4.04 Hz), 7.10 (m, 2H, J 5 4.03), 3.07 (m, 4H), 1.98 (m,
4H), 1.24–1.44 (m, 36H), 0.86 (t, 6H, J 5 6.72 Hz). 13C NMR
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MEST)
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2018, 00, 000–000
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