Photovoltaic polymers based on difluoroqinoxaline units with deep HOMO levels

Article abstract of DOI:10.1002/pola.29014

We report the synthesis of low bandgap polymers with a difluoroquinoxaline unit by Stille polymerization for use in polymer solar cells (PSCs). A new series of copolymers with 2,3-didodecyl-6,7-difluoro quinoxaline as the electron-deficient unit and alkyloxybenzo[1,2-b:4,5-b′]dithiophene and thiophene as the electron-rich unit were synthesized. The photovoltaic properties of the devices based on the synthesized polymers revealed that the fluorine atoms at the quinoxaline units aid in decreasing the highest occupied molecular orbital (HOMO) energy levels; this in turn increased the open circuit voltage of the devices. The polymers with long alkyl chains exhibited good solubility that increased their molecular weight, but the power conversion efficiency was low. Efficient polymer solar cells were fabricated by blending the synthesized copolymers with PC₇₁BM, and the PCE increased up to 5.11% under 100 mW cm^?2 AM 1.5 illumination. These results demonstrate that the importance of having a control polymer to be synthesized and characterized side by side with the fluorine analogues.

Full text of DOI:10.1002/pola.29014

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Photovoltaic Polymers Based on Difluoroqinoxaline Units
With Deep HOMO Levels
1
,2
3
3
1
1
Suhee Song, Sangha Keum, Ji-Hyun Lee, Jihoon Lee, Seungmin Kim,
3
2
1
2
Seong Soo Park, Jin Young Kim, Sung Heum Park, Youngeup Jin
1
Department of Physics, Pukyong National University, Busan, 48513, Korea
2
Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 44919, South Korea
Department of Industrial Chemistry, Pukyong National University, Busan, 48513, Korea
3
Correspondence to: Y. Jin (E-mail: yjin@pknu.ac.kr) or S. H. Park (E-mail: shpark@pknu.ac.kr)
Received 7 February 2018; accepted 26 March 2018; published online 00 Month 2018
DOI: 10.1002/pola.29014
ABSTRACT: We report the synthesis of low bandgap polymers
with a difluoroquinoxaline unit by Stille polymerization for use
in polymer solar cells (PSCs). A new series of copolymers with
power conversion efficiency was low. Efficient polymer solar
cells were fabricated by blending the synthesized copolymers
with PC71BM, and the PCE increased up to 5.11% under 100
22
2,3-didodecyl-6,7-difluoro quinoxaline as the electron-deficient
mW cm AM 1.5 illumination. These results demonstrate that
the importance of having a control polymer to be synthesized
and characterized side by side with the fluorine analogues.
0
unit and alkyloxybenzo[1,2-b:4,5-b ]dithiophene and thiophene
as the electron-rich unit were synthesized. The photovoltaic
properties of the devices based on the synthesized polymers
revealed that the fluorine atoms at the quinoxaline units aid in
decreasing the highest occupied molecular orbital (HOMO)
energy levels; this in turn increased the open circuit voltage of
the devices. The polymers with long alkyl chains exhibited
good solubility that increased their molecular weight, but the
V
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2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym.
Chem. 2018, 00, 000–000
KEYWORDS: alkyl chain; fluorine atom; polymer; solar cells;
synthesis
1
5
INTRODUCTION Organic photovoltaics (OPVs) have become a
hot topic of research in the last two decades because of their
high performance, reliance on potentially nontoxic and inex-
pensive materials, and ease of solution processing on light-
band gap of the donor material. Many low band gap conju-
gated polymers have been designed with electron deficient
“acceptor (A)” type unit and electron rich “donor (D)” units
to enhance the p-p stacking between polymer main chains,
which was attributed intramolecular charge transfer (ICT)
1
weight and cost-effective flexible substrates. Extensive
research has been carried out to increase the power conver-
sion efficiency (PCE) of OPVs by the synthesis of new materi-
16,17
from electron donor to the electron acceptor.
2
–4
2,5,6
als,
morphology control,
and design of new device
Many of the low-bandgap conjugated polymers have an
electron-withdrawing building block (e.g., benzothiadiazole
2
,7,8
structure.
A critical feature of polymer solar cells (PSCs),
1
8,19
20,21
22
which are a class of OPVs, is the interface between an
electron-donating polymer and an electron-accepting fuller-
(BT),
2H-benzimidazole,
diketopyrrolopyrrole and
2
3
6,7-difluoroquinoxaline ) and an electron-donating moiety
1
,9
24
25
ene derivative.
Bulk heterojunction (BHJ) structure have
(e.g., such as carbazole, indenoindene, cyclopentaphenan-
2
0
26–28
attracted significant interest, due to their potential advan-
tages: (1) controlled HOMO energy levels, (2) ameliorated
absorption spectrum, (3) improved mechanical/thermal
properties, and (4) a high hole mobility from planar struc-
threne, and benzodithiophene.
Benzodithiophene is a
widely used electron donating materials because it readily
undergoes polymerization and polymers of dialkoxy benzodi-
2
9
thiophene provide high PCEs. The quinoxaline moiety has
been widely used as an electron acceptor in PSCs because the
1
0
ture. The PCE of solar cell devices has been improved to
over 10% by designing a new low-bandgap material and
two N atoms at the 1,4-positions make the quinoxaline unit an
and the two alkyl chains at its
2- and 3-positions lead to good solubility and tenability.
1
1–14
30,31
device optimization.
In order to realize high photovol-
electron-deficient molecule,
3
2
taic performance, development of a low bandgap copolymer
is very important to control the energy levels and optical
Introduction of fluorine atom exhibits the effect on the energy
Additional Supporting Information may be found in the online version of this article.
2018 Wiley Periodicals, Inc.
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of the charge transfer state caused by electron withdrawing
effect of the fluorine atom. The hygregen bond (CAHꢀꢀꢀFAC)
reduces the distance between polymer chains, which induce
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
3
3
improvement of charge transport and ordering. The absorp-
tion of polymer with 6,7-difluoroquinoxaline was red-shifted
compared to non-fluorinated analogue, attributed to the aggre-
organic layer were dried over MgSO . The organic phase was
4
concentrated under reduced pressure and the residue was
purified by flash column chromatography to give 3.7 g of
3
4
1
gated domain in solution. Two fluorine atoms at the 6- and
-positions of the quinoxaline moiety leads to lower highest
occupied molecular orbital (HOMO) energy levels and good
compound 4 as white solid. H NMR (CDCl , 400MHz, d):
3
7
3.03 (t, 4H, J 5 7.66 Hz), 1.88 (m, 4H, J 5 7.52 Hz), 1.24-1.44
1
3
(m, 36H), 0.86 (t, 6H, J 5 6.84 Hz).
C NMR (CDCl3,
1
5,35
device performance.
Conjugated polymers with deeper
1
3
00MHz, d): 158.17, 149.71 (d of d), 135.75, 109.23 (d of d),
4.62, 31.92, 29.68, 29.65, 29.58, 29.51, 29.45, 29.37, 29.31,
HOMO energy levels are conductive for achieving a higher
3
6
open-circuit voltage (VOC) and better PCE. In addition, selec-
tion of a suitable alkyl chain to ensure good solubility is imper-
ative because the alkyl chain influences the molecular weights,
intermolecular interactions, morphology, and charge transport
27.66, 22.69, 14.12. MS (m/z): calcd for C H Br F N ,
3
2
50
2 2 2
658.2; found, 658.
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-difluoroquinoxaline
(4) (2.5 g, 3.8 mmol) and tributyl(2-thienyl)stannane (4.23 g,
1
5
of the polymer. Moreover, high thermal stability of the
polymers is crucial for preventing deformation of the polymer
morphology, which is imperative for application in OPVs.
1
1.4 mmol) in 60 mL of THF at room temperature was treated
In this work, we designed and synthesized push-pull conjugated
polymers based on 6,7-difluoroquinoxaline, benzodithiophene
and different bridges such as thiophene (P1 and P2), 3-
hexylbithiophene (P3), and 4-hexylbithiophene (P4). To ensure
good solubility, we introduced octyldodecyl chains in the benzodi-
thiophene unit in P1 and P2. The solubility of P3 and P4 greatly
improved with the introduction of a hexyl chain at the bithiophene
bridges. Conjugated polymers as promising candidate for high
performance PSCs were synthesized by Stille polymerization. The
thermal, optical, and electrochemical properties of the synthesized
polymers were investigated. PSCs with the configuration ITO/
with dichlorobis(triphenylphosphine)palladium(II) (2 mol%).
The reaction mixture was stirred for 12 h at 90 8C, concentrated
under reduced pressure, and the residue was purified by flash
column chromatography to give 2.1 g of compound 5 as a yellow
solid. H NMR (CDCl , 400MHz, d): 7.98 (d, 2H, J 5 2.96 Hz),
1
3
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
1
3
(
t, 6H, J 5 6.86 Hz). C NMR (CDCl , 100MHz, d): 155.21,
3
1
1
2
48.82 (d of d), 134.53, 131.01, 130.41(d), 129.52, 126.33,
17.51, 34.68, 31.92, 29.72, 29.69, 29.66, 29.60, 29.57, 29.38,
9.37, 28.05, 22.69, 14.12. MS (m/z): calcd for C H Br N S ,
4
0
56
2 2 2
PEDOT:PSS/polymer:PC BM/Al were fabricated for evaluating
71
666.4; found, 666.
the photovoltaic properties of the polymers.
Synthesis of 5,8-bis(5-bromo-2-thienyl)-2,3-didodecyl-
,7-difluoro quinoxaline (6)
,3-Didodecyl-6,7-difluoro-5,8-di(2-thienyl) quinoxaline (5)
6
2
EXPERIMENTAL
Characterization
(
2.0 g, 3.0 mmol) was brominated with N-bromosuccinimide
All reagents purchased from Aldrich or TCI were used without
further purification. Solvents were purified by typical proce-
dures and handled under moisture-free atmosphere. 1H and
(
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
13C NMR spectra were recorded on a JNM ECP-400 (400 MHz,
JEOL) spectrometer, and chemical shifts were recorded in ppm
using TMS as the internal standard. Flash column chromatogra-
phy was performed using Merck silica gel 60 (particle size 230–
over MgSO . The organic phase was concentrated under
reduced pressure and the residue was purified by flash col-
4
umn chromatography to give 2.4 g of compound 6 as an
1
400 mesh ASTM) with a gradient elution of ethyl acetate/hex-
orange solid. H NMR (CDCl , 400MHz, d): 7.75 (d, 2H,
3
ane or methanol/dichloromethane unless otherwise indicated.
Analytical thin layer chromatography (TLC) was conducted
using Merck 0.25-mm silica gel 60F pre-coated aluminum plates
with a fluorescent indicator UV254. The molecular weight and
polydispersity of the polymer were determined by gel perme-
ation chromatography (GPC) with calibration using polystyrene
standards. UV–vis absorption spectra were recorded on a Varian
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,
1
3
6
1
1
2
H, J 5 6.84 Hz).
C NMR (CDCl , 100MHz, d): 155.42,
3
48.68 (d of d), 133.58, 132.60, 130.35, 129.03, 118.56,
16.61, 34.75, 31.93, 29.72, 29.70, 29.63, 29.59, 29.41,
9.38, 28.14, 22.70, 14.14. MS (m/z): calcd for
C H Br F N S , 822.2; found, 822.
4
0
54
2 2 2 2
5
E UV–VIS–NIR spectrophotometer.
0
Synthesis of 2,3-didodecyl-6,7-difluoro-5,8-bis(3 -hexyl-
[2,2 -bithiophen]-5-yl)quinoxaline (8)
0
Synthesis of 5,8-dibromo-2,3-didodecyl-6,7-
difluoroquinoxaline (4)
A solution of 5,8-bis(5-bromo-2-thienyl)-2,3-didodecyl-6,7-
difluoro quinoxaline (6) (4.13 g, 5.0 mmol) and tributyl(3-
hexylthiophen-2-yl)stannane (7) (6.88 g, 15.0 mmol) in
100 mL of DMF at room temperature was treated with
3,6-Dibromo-4,5-difluorobenzene-1,2-diamine (3) (2.2 g, 7.3
mmol) and 13,14-hexacosanedione (2) (3.48 g, 8.8 mmol)
were dissolved in in acetic acid (60 mL) and heated to 100
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dichlorobis(triphenylphosphine)palladium(II) (2 mol%). The
reaction mixture was stirred for 12 h at 90 8C, concentrated
under reduced pressure, and the residue was purified by
Polymerization of poly[2,6-(4,8-bis(2-butyloctyloxy)
0
0
benzo[1,2-b:4,5-b ]dithiophene)-alt-5,5 -(5,8-(bis-(2-
thionyl))-6,7-difluoro-2,3-dihexylquinoxaline)] (P1)
Carefully purified 2,6-bis(trimethyltin)-4,8-bis(2-butyloctylox-
flash column chromatography to give 4.1 g of compound
1
0
8
as a yellow solid. H NMR (CDCl , 400MHz, d): 7.97 (d, 2H,
y)benzo[1,2-b:4,5-b ]dithiophene (13) (0.62 g, 0.7 mmol),
3
J 5 4.04 Hz), 7.21 (m, 4H, J 5 4.00 Hz), 6.97 (d, 2H,
J 5 5.08Hz), 3.08 (t, 4H, J 5 7.76 Hz), 2.86 (t, 4H, J 5 7.80
Hz), 1.99 (m, 4H, J 5 7.52 Hz), 1.68 (m, 4H, J 5 7.49 Hz),
5,8-bis(5-bromothiophen-2-yl)-6,7-difluoro-2,3-dihexylqui-
noxaline (15) (0.46 g, 0.7 mmol), P(o-tolyl)
DMF (2 mL) and Pd (dba) (3 mol%) were dissolved in
(40 mol%),
3
2
3
1
.49-1.21 (m, 48H), 0.86 (m, 12H).
8 mL of chlorobenzene. The mixture was refluxed with vigor-
ous stirring for 2 days under argon atmosphere. After cool-
ing to room temperature, the mixture was poured into
methanol. The precipitated material was recovered by filtra-
tion. 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 col-
lected by filtration to give 577 mg as violet solid.
0
0
0
Synthesis of 5,8-bis(5 -bromo-3 -hexyl-[2,2 -bithiophen]-
-yl)-2,3-didodecyl-6,7-difluoroquinoxaline (9)
5
0
0
2,3-Didodecyl-6,7-difluoro-5,8-bis(3 -hexyl-[2,2 -bithiophen]-
5-yl)quinoxaline (8) (4.1g, 4.1 mmol) as brominated with N-
bromosuccinimide (NBS) (1.4 g, 8.1 mmol) in THF (12 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 acetate and combined organic layer
were dried over MgSO . The organic phase was concentrated
under reduced pressure and the residue was purified by
Polymerization of poly[2,6-(4,8-bis(2-butyloctyloxy)
0
0
benzo[1,2-b:4,5-b ]dithiophene)-alt-5,5 -(5,8-(bis-(2-
thionyl))-2,3-didodecyl-6,7-difluoroquinoxaline)] (P2)
By following the similar method used for P1, P2 was synthe-
sized with 2,6-bis(trimethyltin)-4,8-bis(2-butyloctyloxy)-
4
flash column chromatography to give 4.4 g of compound 9
as an orange solid. H NMR (CDCl , 400MHz, d): H NMR
1
1
3
0
benzo[1,2-b:4,5-b ]dithiophene (13) (0.62 g, 0.7 mmol) and
(CDCl , 400MHz, d): 7.95 (d, 2H, J 5 3.76 Hz), 7.14 (d, 2H,
3
5,8-bis(5-bromo-2-thienyl)-2,3-didodecyl-6,7-difluoroquinoxa-
J 5 4.01Hz), 6.93 (d, 2H, J 5 4.32 Hz), 3.07 (s, 4H), 2.78
t, 4H, J 5 2.68 Hz), 1.99 (s, 4H), 1.63 (s, 4H), 1.55-1.22 (m,
8H), 0.86 (m, 12H).
line (6) (0.46 g, 0.7 mmol). The purified polymer was col-
lected by filtration to give 490 mg as violet solid.
(
4
0
Polymerization of poly[2,6-(4,8-bis(2-ethylhexyloxy)
Synthesis of 2,3-didodecyl-6,7-difluoro-5,8-bis(4 -hexyl-
0
0
0
0
benzo[1,2-b:4,5-b ]dithiophene)-alt-5,5 -(5,8-bis(3 -hexyl-
[2,2 -bithiophen]-5-yl)quinoxaline (11)
0
[
2,2 -bithiophen]-5-yl)-2,3-didodecyl-6,7-
A solution of 5,8-bis(5-bromo-2-thienyl)-2,3-didodecyl-6,7-
difluoro quinoxaline (6) (4.1 g, 5.0 mmol) and tributyl(4-hex-
ylthiophen-2-yl)stannane (10) (6.9 g, 15.0 mmol) in 100 mL of
DMF at room temperature was treated with dichlorobis(triphe-
nylphosphine)palladium(II) (2 mol%). The reaction mixture
was stirred for 12 h at 90 8C, concentrated under reduced pres-
sure, and the residue was purified by flash column chromatog-
difluoroquinoxaline)] (P3)
By following the similar method used for P1, P3 was synthesized
0
with (4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b ]dithiophene-
2
5
6
,6-diyl)bis(trimethylstannane) (14) (0.39 g, 0.5 mmol) and
,8-bis(5 -bromo-3 -hexyl-[2,2 -bithiophen]-5-yl)-2,3-didodecyl-
,7-difluoroquinoxaline (9) (0.58 g, 0.5 mmol). The purified
0
0
0
1
polymer was collected by filtration to give 642 mg as violet
solid.
raphy to give 4.3 g of compound 11 as a yellow solid. H NMR
CDCl , 400MHz, d): 7.93 (d, 2H, J 5 4.04 Hz), 7.22 (d, 2H,
(
3
J 5 4.01 Hz), 7.12 (d, 2H, J 5 4.01 Hz), 6.84 (s, 2H) 3.10 (t, 4H,
J 5 7.24 Hz), 2.60 (t, 4H, J 5 7.80 Hz), 2.06 (m, 4H), 1.54 (m,
Polymerization of poly[2,6-(4,8-bis(2-ethylhexyloxy)
0
0
0
benzo[1,2-b:4,5-b ]dithiophene)-alt-5,5 -(5,8-bis(5 -
4H), 1.50-1.17 (m, 48H), 0.86 (m, 12H).
0
0
bromo-4 -hexyl-[2,2 -bithiophen]-5-yl)-2,3-didodecyl-6,7-
difluoroquinoxaline)] (P4)
0
0
0
Synthesis of 5,8-bis(5 -bromo-4 -hexyl-[2,2 -bithiophen]-
-yl)-2,3-didodecyl-6,7-difluoroquinoxaline (12)
2
By following the similar method used for P1, P4 was synthesized
5
0
with (4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b ]dithiophene-
0 0
,3-Didodecyl-6,7-difluoro-5,8-bis(4 -hexyl-[2,2 -bithiophen]-5-yl)
2
,6-diyl)bis(trimethylstannane) (14) (0.39 g, 0.5 mmol) and 5,8-
quinoxaline (11) (4.3 g, 4.3 mmol) was brominated with N-
bromosuccinimide (NBS) (1.5 g, 8.5 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 acetate and combined organic layer were
0
0
0
bis(5 -bromo-4 -hexyl-[2,2 -bithiophen]-5-yl)-2,3-didodecyl-6,7-
difluoroquinoxaline (12) (0.58 g, 0.5 mmol). The purified poly-
mer was collected by filtration to give 554 mg as violet solid.
The Solar Cell Fabrication
dried over MgSO . The organic phase was concentrated under
reduced pressure and the residue was purified by flash column
Solar cells were fabricated on an indium tin oxide (ITO)-coated
glass substrate with the following structure: ITO-coated glass sub-
strate/poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate)
(PEDOT:PSS)/polymer:PCBM/Al. The ITO-coated glass sub-
strate was first cleaned using a detergent and ultrasonicated in
acetone and isopropyl alcohol, followed by drying overnight in
an oven. PEDOT:PSS (Baytron PH) aqueous solution was spin-
cast to form a 40 nm thick film. The substrate was dried for 10
4
chromatography to give 4.4 g of compound 12 as an orange
1
1
solid. H NMR (CDCl , 400MHz, d): H NMR (CDCl , 400MHz,
3
3
d): 7.90 (d, 2H, J 5 3.76 Hz), 7.18 (d, 2H, J 5 4.13Hz), 6.96 (s,
H), 3.09 (t, 4H, J 5 7.8Hz), 2.54 (t, 4H, J 5 7.8 Hz), 2.06 (t, 4H,
J5 7.00 Hz), 1.75 (t, 4H, J 5 7.01 Hz), 1.54-1.21 (m, 48H), 0.86
m, 12H).
2
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SCHEME 1 Synthetic route of the monomers.
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:PCBM in dichlorobenzene as solvent was
then spin-casted on top of the PEDOT/PSS layer. Then, 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,
N-bromosuccinimide (NBS) to generate 5,8-bis(5-bromo-2-
thienyl)-2,3-didodecyl-6,7-difluoro quinoxaline (6). Compound
6 was reacted with tributyl(3-hexylthiophen-2-yl)stannane (7)
or tributyl(4-hexylthiophen-2-yl)stannane (10) through Stille
0
coupling to obtain 2,3-didodecyl-6,7-difluoro-5,8-bis(3 -hexyl-
0
[2,2 -bithiophen]-5-yl)quinoxaline (8) or 2,3-didodecyl-6,7-
0
0
1
00 nm) electrode was deposited by thermal evaporation under
difluoro-5,8-bis(4 -hexyl-[2,2 -bithiophen]-5-yl)quinoxaline (11),
respectively. Next, compound or compound 11 were
brominated with NBS in tetrahydrofuran (THF) to obtain 5,8-
27
a vacuum of approximately 5 3 10 Torr. Current density-
voltage (J-V) characteristics of the devices were measured using
a Keithley 236 source measure unit. Solar cell performance was
measured using an Air Mass 1.5 Global (AM 1.5 G) solar simula-
8
0
0
0
bis(5 -bromo-3 -hexyl-[2,2 -bithiophen]-5-yl)-2,3-didodecyl-6,7-
0
0
0
difluoroquinoxaline (9) and 5,8-bis(5 -bromo-4 -hexyl-[2,2 -
bithiophen]-5-yl)-2,3-didodecyl-6,7-difluoroquinoxaline (12),
respectively. Copolymers with alkyloxybenzo[1,2-b:4,5-
2
2
tor with an irradiation intensity of 1000 Wm . An aperture
2
(12.7 mm ) was used on top of the cell to eliminate extrinsic
0
effects such as cross-talk, and waveguiding, shadow effects. The
spectral mismatch factor was calculated by the comparison of
the solar simulator spectrum with the AM 1.5 spectrum at room
temperature.
b ]dithiophene and thiophene as the electron-donating moiety
and 2,3-dialkyl-6,7-difluoroquinoxaline as the electron-
accepting unit were synthesized through Pd(0)-catalyzed Stille
coupling polymerization in chlorobenzene. Two dodecyl chains
in the quinoxaline moiety and a butyloctyloxy chain in the ben-
zodithiophene moiety were introduced to improve the solubility
of the polymers. All the synthesized polymers showed good sol-
ubility in organic solvents.
RESULTS AND DISCUSSION
Synthesis and Characterization
The synthetic routes to the monomers and polymers are outlined
in Schemes 1 and 2, respectively. In the first step, 13,14-hexacosa-
nedione (2) was prepared from oxalyl chloride via the Grignard
reaction. 3,6-Dibromo-4,5-difluorobenzene-1,2-diamine (3) was
coupled with compound 2 to obtain 5,8-dibromo-2,3-didodecyl-
Table 1 summarizes the including molecular weight, polydis-
persity index (PDI) and thermal stability of the polymers.
The molecular weights and PDIs of the polymers dissolved
in chloroform were determined by gel permeation chroma-
tography (GPC) using polystyrene as the standard. The
6
,7-difluoroquinoxaline (4). Stille coupling of compound 4 with
tributyl(2-thienyl)stannane afforded 2,3-didodecyl-6,7-difluoro-
,8-di(2-thienyl)-quinoxaline (5), which was brominated with
number-average molecular weight (M ) values for the poly-
n
5
mers were 16,000–50,000 with PDI values of 1.2-2.4. P2 has
4
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SCHEME 2 Synthetic route of the polymers.
a higher molecular weight as compared with P1 due to its
better solubility resulting from longer alkyl chain at the qui-
noxaline units. Figure 1 shows the thermogravimetric analy-
sis (TGA) curves, which indicate the thermal properties of
the polymers. TGA was performed on a DuPont 951 TGA
instrument under nitrogen, at a heating rate of 10 8C/min to
quartz plates to obtain a thin film. Figure 2 shows the UV–vis
absorption spectra of the polymers in solution and thin film
states. The maxima wavelength in solutions and in films, and
absorption onsets are summarized in Table 1. P1 and P2
showed similar absorption spectra in solution, with the
absorption maxima at 564 and 568 nm, respectively. The
absorption spectra of P3 and P4 with alkylated thiophene
units in solution showed absorption maxima at about 538
and 547 nm, respectively. The polymers with increasing num-
500 8C. The TGA curves revealed that the copolymers were
thermally stable, with a weight loss of approximately 5% in
the range 335–342 8C, in air.
3
7
bers of thiophene rings show broad absorptions.
The
Optical Properties
For the measurement of optical properties, a solution of each
polymer in dichlorobenzene was prepared and spin-coated on
absorption maxima of P3 and P4 in solution was blue-shifted
as compared with those of P1 and P2 because of a larger
3
7,38
average dihedral angle with longer donor segments,
which
TABLE 1 Polymerization Results, Thermal and Optical properties of Polymers
a
a
w
a
b
Polymer
M
n
(g/mol)
M
(g/mol)
PDI
TGA (T
d
)
in solution (nm)
in thin film (nm)
P1
P2
P3
P4
38000
50000
21000
16000
92000
120000
51000
19000
2.4
2.4
2.4
1.2
336
342
335
336
564
568
538
547
574
573
564
555
a
b
Molecular weight (M
w
) and polydispersity (PDI) of the polymers were
Onset decomposition temperature (5% weight loss) measured by TGA
under N
determined by gel permeation chromatography (GPC) in THF using
polystyrene standards.
2
.
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FIGURE 1 Thermogravimetric analysis of the polymers under
. [Color figure can be viewed at wileyonlinelibrary.com]
FIGURE 3 Electrochemical properties of polymers. [Color figure
N
2
can be viewed at wileyonlinelibrary.com]
3
9
can disruption the planarity of the polymer backbone. The
absorption band of P3 was blue-shifted as compared with
that of P4 caused by more larger dihedral angle of donor seg-
mono thiophene, illustrating that the electron-rich moiety
weaken the intramolecular charge transfer (ICT) transitions.
The spectra of the synthesized polymers in solid films exhib-
ited absorption bands with kmax at 555–574 nm and absorp-
tion onset at 652–671 nm, corresponding to band gaps of
39
3
7
ment from 3-hexyl in thiophene. The absorption band in the
longer wavelength region (500ꢁ750 nm) could be assigned to
the intramolecular charge transfer (ICT) transition from elec-
1.85-1.90 eV. The absorption bands in films exhibited a signifi-
3
8
tron donor to the electron acceptor. The peaks at ꢁ 600 nm
cant red-shift and broader as compared to those in solution
of the polymer with bithiophene were weaker than those with
40,41
caused by interchain p-stacking.
Electrochemical and Theoretical Calculations
The electrochemical properties of the polymers were deter-
mined using the band gap estimated from the absorption
onset and the HOMO energy level estimated by cyclic vol-
tammetry (CV). CV was performed with a solution of tetrabu-
tylammonium tetrafluoroborate (Bu NBF ) (0.10 M) in
4
4
acetonitrile at a scan rate of 100 mV/s at room temperature
2
under argon. A platinum electrode (ꢁ0.05 cm ) coated with
a thin polymer film was used as the working electrode; a Pt
wire and Ag/AgNO electrode were used as the counter and
3
reference electrodes, respectively. The energy level of the
1
Ag/AgNO3 reference electrode (calibrated by the Fc/Fc
redox system) was 4.8 eV below the vacuum level.
Figure 3 shows the CV spectra of the polymers, and Table 2
summarizes their oxidation potentials derived from the onsets
of electrochemical p-doping. The HOMO and lowest unoccu-
pied molecular orbital (LUMO) levels were calculated accord-
ox
ing to the empirical formula (E
5 2([Eonset] 1 4.8) eV)
HOMO
red
and (ELUMO 5 2([Eonset
tion potential onsets of the synthesized polymers ranged from
.80 to 0.87 V, corresponding to the HOMO energy levels from
5.60 to 25.61 eV. Since the length of the substituted alkyl
]
1 4.8) eV), respectively. The oxida-
0
2
chain does not affect the HOMO energy levels, the synthesized
polymers had similar HOMO energy levels. The reduction onset
1
potentials ranged from 20.82 to 21.09 V vs. Ag/Ag . The
LUMO energy levels were calculated to be in the range 23.71
to 23.98 eV. Electrochemical band gaps, calculated from CV
data, were determined to be 1.63-1.95 eV. The HOMO energy
levels of the fluorinated polymers were lower because of the
electron-withdrawing effect of the fluorine atoms, leading to
FIGURE 2 UV–visible absorption spectra of polymers in dichlo-
robenzene solution (a) and the solid state. [Color figure can be
viewed at wileyonlinelibrary.com]
4
2,43
higher VOC values.
6
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TABLE 2 Electrochemical Potentials and Energy Levels of the Polymers
Optical band
Electrochemical
a
b
c
d
d
red
e
Polymers
gap (eV)
HOMO (eV)
LUMO (eV)
E
ox (V)
E
(V)
band gap (eV)
P1
P2
P3
P4
1.89
25.61
25.60
25.60
25.67
23.98
23.97
23.71
23.72
0.81
0.80
0.80
0.87
20.82
20.83
21.09
21.08
1.63
1.63
1.89
1.95
1.87
1.85
1.90
a
d
Optical energy band gap was estimated from the onset wavelength of
the optical absorption.
Onset oxidation and reduction potential measured by cyclic
voltammetry.
e
b
Calculated from the oxidation potentials.
Calculated from the Eox and Ered.
c
Calculated from the reduction potentials.
The optimized geometries were calculated at the B3 LYP/6–
rich unit (6,7-difluoroquinoxaline unit). The introduction of
bithiophene into the polymer decreased the electron density at
the difluoroquinoxaline moiety due to the weak push-pull
effect. Due to the enhanced conjugation length by the inserted
alkyl-thiophene with respect to the monomer of polymer, the
HOMO energy level was raised by about 0.31 eV. The repeating
unit (WO) that had bithiophene without the alkyl chain led to
decreased electron density at the quinoxaline unit, which
was confirmed to exhibit no effect on the alkyl chain at
bithiophene.
4
4
31G (d,p) level of theory using the Gaussian 03 package. The
HOMO and LUMO surfaces were plotted using the GaussView
version 4.1. For simplicity, the alkyl chains were replaced with
methyl groups and only one repeating unit, M1, M3, or M4,
was used for the simulation of P1 (or P2), P3, or P4, respec-
tively. The optimized structures of M1, M3 and M4 are shown
in Figure 4. The HOMO wave functions of the repeating units
are usually delocalized along the entire polymer backbone,
while the LUMO wave function is localized onto the electron-
FIGURE 4 Theoretical calculation of the monomers by density functional theory (DFT) at the B3LYP/6–31G level. [Color figure can
be viewed at wileyonlinelibrary.com]
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FIGURE 5 Current density-potential characteristics of the poly-
2
mer solar cells under the illumination of AM 1.5, 100 mW/cm .
FIGURE 6 IPCE curves of the corresponding polymer solar
[
Color figure can be viewed at wileyonlinelibrary.com]
cells. [Color figure can be viewed at wileyonlinelibrary.com]
Photovoltaic Properties of Synthesized Polymers
Photovoltaic cells based on the synthesized polymers were
fabricated by spin coating a dichlorobenzene solution of the
by the tapping mode) obtained from the polymer:PC BM
7
1
blend films. Although the blend films of P1 and P2 showed
similar morphological properties (aggregation sizes and
features), the more homogeneous surface of P1 led to a higher
PCE.
PC BM polymers. Figure 5 summarizes the photovoltaic
7
1
properties measured for the devices with the following
configuration, ITO/PEDOT:PSS (40 nm)/polymer:PC BM
7
1
(
80 nm)/Al (100 nm), under AM 1.5G irradiation (100 mW/
2
cm ), and Table 3 summarizes the values obtained. The
CONCLUSIONS
device comprising P1:PCBM at a concentration of 1.5 wt% in
We used Stille coupling polymerization to prepare new con-
jugated polymers with benzo[1,2-b:4,5-b ]dithiophene moie-
dichlorobenzene exhibited a V of 0.82, short-circuit current
OC
2
0
density (J ) of 9.66 mA/cm , and fill factor (FF) of 0.65,
SC
ties and 2,3-dialkyl-6,7-difluoroquinoxaline units in the main
backbone. The HOMO energy levels of the synthesized poly-
mers were 25.60 to 25.61 eV, which demonstrated that the
electron-withdrawing effect of the fluorine atoms decreased
the HOMO levels. The device comprising P1:PCBM at a con-
respectively corresponding to a PCE of 5.11%. The corre-
sponding values for the device with P2:PCBM were 0.85 V,
2
8
.52 mA/cm , and 0.63, affording a PCE of 4.54%. The
device based on P3 and P4 with bithiophene units showed
2
VOC of 0.67 and 0.70 V, JSC of 9.83 and 7.63 mA/cm , and FF
centration of 1.5 wt% in dichlorobenzene exhibited a V of
OC
of 0.62 and 0.68, giving a PCE of 4.07% and 3.61%, respec-
tively. Although the polymers with bithiophene units show a
large conjugated area, P3 and P4 exhibit localization electron
density at benzothiophene unit to decrease PCE.
2
0
.82 V, JSC of 9.66 mA/cm , and FF of 0.65, affording a PCE
of 5.11%. This series of synthesized polymers with 2,3-
dialkyl-6,7-difluoroquinoxaline units possess a large potential
for higher performance PSCs.)
The incident-photon-to-current efficiency (IPCE) spectra of the
photovoltaic devices from the polymer:PCBM blends are pre-
sented in Figure 6. The spectra showed maxima of 71.5% at
ACKNOWLEDGMENT
410 nm for P1, 55.9% at 430 nm for P2, 71.1% at 390 nm for
P3 and 53.8% at 390 nm for P4. The enhanced efficiency of P1
was due to the higher IPCE between 300 and 800 nm. The sur-
face morphologies of the photovoltaic films were determined
through atomic force microscopy (AFM) measurements in Sup-
This work was supported by a Research Grant of Pukyong
National University (2017–2018).
2
porting Information. AFM images (surface area of 2 3 2 lm
REFERENCES
1
K. R. Graham, C. Cabanetos, J. P. Jahnke, M. N. Idso, A.
Labban, G. O. N. Ndjawa, T. Heumueller, K. Vandewal, A.
Salleo, B. F. Chmelka, A. Amassian, P. M. Beaujuge, M. D.
McGehee, J. Am. Chem. Soc. 2014, 136, 9608.
TABLE 3 Photovoltaic Properties of the Polymer Solar Cells
2
Polymers
J
SC (mA/cm )
V
OC (V)
FF
PCE (%)
2
J. Zhang, Y. Zhang, J. Fang, K. Lu, Z. Wang, W. Ma, Z. Wei,
J. Am. Chem. Soc. 2015, 137, 8176.
P1
P2
P3
P4
9.66
8.52
9.83
7.63
0.82
0.85
0.67
0.70
0.65
0.63
0.62
0.68
5.11
4.54
4.07
3.61
3
Y. J. Cheng, S. H. Yang, C. S. Hsu, Chem. Rev. 2009, 109,
5
868.
4
P. L. T. Boudreault, A. Najari, M. Leclerc, Chem. Mater. 2010,
23, 456.
8
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2018, 00, 000–000

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
5
G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriarty, K.
Emery, Y. Yang, Nat. Mater. 2005, 4, 864.
J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J.
Heeger, G. C. Bazan, Nat. Mater. 2007, 6, 497.
J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T. Q. Nguyen, M.
Dante, A. J. Heeger, Science 2007, 317, 222.
Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu, L. Chen, S.
Su, Y. Cao, Adv. Mater. 2011, 23, 4636.
D. Beljonne, J. Cornil, L. Muccioli, C. Zannoni, J.-L. Br eꢀ das, F.
Castet, Chem. Mater. 2011, 23, 591.
0 C. Lee, H. Kang, W. Lee, T. Kim, K. H. Kim, H. Y. Woo, C.
Wang, B. J. Kim, Adv. Mater. 2015, 27, 2466.
1 W. Zhao, S. Li, H. Yao, S. Zhang, Y. Zhang, B. Yang, J. Hou,
J. Am. Chem. Soc. 2017, 139, 7148.
2 Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin,
H. Ade, H. Yan, Nat. Commun. 2014, 5, 5293.
3 H. J. Cho, Y. J. Kim, S. Chen, J. Lee, T. J. Shin, C. E. Park,
C. Yang, Nano Energy. 2017, 39, 229.
4 J. Lee, D. H. Sin, B. Moon, J. Shin, H. G. Kim, M. Kim, K.
Cho, Energy Environ. Sci. 2017, 10, 247.
5 H. Hu, K. Jiang, G. Yang, J. Liu, Z. Li, H. Lin, Y. Liu, J. Zhao,
J. Zhang, F. Huang, Y. Qu, W. Ma, H. Yan, J. Am. Chem. Soc.
29 Q. V. Hoang, C. E. Song, S. J. Moon, S. K. Lee, J. C. Lee, B.
J. Kim, W. S. Shin, Macromolecules 2015, 48, 3918.
6
30 K. W. Song, T. H. Lee, E. J. Ko, K. H. Back, D. K. Moon, J.
Polym. Sci. Pol. Chem. 2014, 52, 1028.
7
31 M. Liu, Y. Gao, Y. Zhang, Z. Liub, L. Zhaoa, Polym. Chem.
2017, 8, 4613.
8
32 S. Song, T. Kim, D. Kang, H. Suh, M. H. Hyun, S. S. Park, G.
D. Lee, J. Y. Kim, Y. Jin, Polym. Bull. 2016, 73, 2511.
9
33 R. Kroon, R. Gehlhaar, T. T. Steckler, P. Henriksson, C.
Muller, J. Bergqvist, A. Hadipour, P. Heremans, M. R.
Andersson, Sol. Energ. Mat. Sol. C. 2012, 105, 280.
1
34 A. Iyer, J. Bjorgaard, T. Anderson, M. E. K o€ se, Macromole-
cules 2012, 45, 6380.
1
35 Y. Q. Zheng, Z. Wang, J. H. Dou, S. D. Zhang, X. Y. Luo, Z.
F. Yao, J. Y. Wang, J. Pei, Macromolecules 2015, 48, 5570.
1
36 D. Liu, W. Zhao, S. Zhang, L. Ye, Z. Zheng, Y. Cui, Y. Chen,
J. Hou, Macromolecules 2015, 48, 5172.
1
37 Q. Tao, Y. Xia, X. Xu, S. Hedstr o€ m, O. B a€ cke, D. I. James, P.
Persson, E. Olsson, O. Ingan a€ s, L. Hou, W. Zhu, E. Wang, Mac-
romolecules 2015, 48, 1009.
1
1
38 J. H. Kim, J. B. Park, H. U. Kim, I. N. Kang, D. H. Hwang,
Synth. Met. 2014, 194, 88.
2
015, 137, 14149.
3
9 K. Lu, J. Fang, H. Yan, X. Zhu, Y. Yi, Z. Wei, Organic Elec-
tronics 2013, 14, 2652.
0 Q. Fan, H. Jiang, Y. Liu, W. Su, H. Tan, Y. Wang, R. Yang,
W. Zhu, J. Mater. Chem. C. 2016, 4, 2606.
1 S. Wen, C. Wang, P. Ma, Y. X. Zhao, C. Li, S. Ruan, J. Mater.
Chem. A. 2015, 3, 13794.
2 H. Bronstein, J. M. Frost, A. Hadipour, Y. Kim, C. B. Nielsen,
16 K. Li, Z. Li, K. Feng, X. Xu, L. Wang, Q. Peng, J. Am. Chem.
Soc. 2013, 135, 13549.
7 Z. Zeng, Y. Li, J. Deng, Q. Huang, Q. Peng, J. Mater. Chem.
A. 2014, 2, 653.
8 S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J. S.
4
1
4
1
Moon, D. Moses, M. Leclerc, K. Lee, A. J. Heeger, Nat. Photon-
ics. 2009, 3, 297.
4
R. S. Ashraf, B. P. Rand, S. Watkins, I. McCulloch, Chem. Mater.
2013, 25, 277.
19 N. Blouin, A. Michaud, M. Leclerc, Adv. Mater. 2007, 19,
2
295.
4
3 S. Tang, P. Murto, X. Xu, C. Larsen, E. Wang, L. Edman,
Chem. Mater. 2017, 29, 7750.
4 M. J. Frisch, G. W. Trucks, H. B. chlegel, G. E. Scuseria, M.
20 S. Song, Y. Jin, S. H. Park, S. Cho, I. Kim, K. Lee, A. J.
Heeger, H. Suh, J. Mater. Chem. 2010, 20, 6517.
1 S. Song, J. Kim, J. Shim, J. Kim, B. H. Lee, Y. Jin, I. Kim, K.
Lee, H. Suh, Sol. Energ. Mat. Sol. C. 2012, 98, 323.
4
2
A. Robb, J. R. Cheeseman, J. A. Montgomery Jr., T. Vreven, K.
N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,
V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J.
B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J.
J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M.
C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul,
S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M.
W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, J. A.
Pople, Gaussian 03, Revision E.01, Gaussian Inc., Wallingford,
CT, USA, 2004.
2
2 I. Welterlich, O. Charov, B. Tieke, Macromolecules 2012, 45,
4511.
23 S. Song, H. I. Choi, I. S. Shin, H. Suh, M. H. Hyun, G. D. Lee,
S. S. Park, S. H. Park, Y. Jin, Bull. Korean Chem. Soc. 2014, 35,
2245.
2
2
4 S. Song, S. H. Park, Y. Jin, I. Kim, K. Lee, H. Suh, Polymer
010, 51, 5385.
25 S. Song, Y. Jin, S. H. Kim, J. Moon, K. Kim, J. Y. Kim, S. H.
Park, K. Lee, H. Suh, Macromolecules 2008, 41, 7296.
6 H. H. Cho, T. E. Kang, K. H. Kim, H. Kang, H. J. Kim, B. J.
Kim, Macromolecules 2012, 45, 6415.
7 L. Dou, J. Gao, E. Richard, J. You, C. C. Chen, K. C. Cha, Y.
He, G. Li, Y. Yang, J. Am. Chem. Soc. 2012, 134, 10071.
8 Y. Liang, D. Feng, Y. Wu, S. T. Tsai, G. Li, C. Ray, L. Yu, J.
Am. Chem. Soc. 2009, 131, 7792.
2
2
2
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