Synthesis of Donor-Acceptor polymers through control of the chemical structure: Improvement of PCE by planar structure of polymer backbones

Article abstract of DOI:10.1016/j.polymer.2012.10.056

A new acceptor unit with high solubility octyloxy-diphenylquinoxaline was synthesized with octyloxy-dibenzophenazine. The new D-A type polymers poly(carbazole-octyloxy diphenylquinoxaline) (PCzPDTQ) and octyloxy- dibenzophenazine) (PCzFDTQ), which adopted carbazole as a donor, were polymerized through a Suzuki coupling reaction. PCzPDTQ and PCzFDTQ dissolved in organic solvents and showed high thermal stability. Due to the appropriate LUMO energy levels, an effective charge transport and a low band gap (2.06 and 1.97 eV) were observed in PCzPDTQ and PCzFDTQ. According to X-ray diffraction measurements, a single broad diffraction peak was detected at approximately 20.5°. The π-π stacking distances (d_π) for PCzPDTQ and PCzFDTQ were 4.4 and 4.3 A?, respectively. When PCzPDTQ and PC ₇₁BM were blended in a 1:3 ratio, The open-circuit voltage (V _OC), short-circuit current (J_SC), fill factor (FF) and power conversion efficiency (PCE) were 0.85 V, 6.5 mA/cm², 32.2% and 1.8%, respectively. The efficiency of PCzFDTQ tended to improve as PC ₇₁BM increased. At a 1:6 ratio of PCzFDTQ to PC₇₁BM, V_oc, J_sc, FF and PCE were 0.85 V, 5.5 mA/cm², 44.9% and 2.1%, respectively. In terms of FF, 39% of the increase was observed in PCzFDTQ.

Full text of DOI:10.1016/j.polymer.2012.10.056

Polymer 54 (2013) 1072e1079
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of DonoreAcceptor polymers through control of the chemical structure:
Improvement of PCE by planar structure of polymer backbones
,
Ho Jun Song a, Tae Ho Lee a, Myung Hee Han a, Jang Yong Lee b, Doo Kyung Moon a
*
a Department of Materials Chemistry and Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea
b Energy Materials Research Center, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A new acceptor unit with high solubility octyloxy-diphenylquinoxaline was synthesized with octyloxy-
dibenzophenazine. The new DeA type polymers poly(carbazole-octyloxy diphenylquinoxaline)
(PCzPDTQ) and octyloxy-dibenzophenazine) (PCzFDTQ), which adopted carbazole as a donor, were
polymerized through a Suzuki coupling reaction. PCzPDTQ and PCzFDTQ dissolved in organic solvents
and showed high thermal stability. Due to the appropriate LUMO energy levels, an effective charge
transport and a low band gap (2.06 and 1.97 eV) were observed in PCzPDTQ and PCzFDTQ. According to
Received 30 August 2012
Received in revised form
16 October 2012
Accepted 21 October 2012
Available online 9 November 2012
X-ray diffraction measurements, a single broad diffraction peak was detected at approximately 20.5ꢀ. The
Keywords:
Quinoxaline
OPVs
ꢀ
p
ep
stacking distances (dp) for PCzPDTQ and PCzFDTQ were 4.4 and 4.3 A, respectively. When PCzPDTQ
and PC71BM were blended in a 1:3 ratio, The open-circuit voltage (VOC), short-circuit current (JSC), fill
factor (FF) and power conversion efficiency (PCE) were 0.85 V, 6.5 mA/cm2, 32.2% and 1.8%, respectively.
The efficiency of PCzFDTQ tended to improve as PC71BM increased. At a 1:6 ratio of PCzFDTQ to PC71BM,
Voc, Jsc, FF and PCE were 0.85 V, 5.5 mA/cm2, 44.9% and 2.1%, respectively. In terms of FF, 39% of the
increase was observed in PCzFDTQ.
Conjugated polymer
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
substituents [14,15]. Most DeA polymers, which used quinoxaline
derivatives, demonstrate low efficiency (2e3%) because of the
For several decades, semiconducting polymers have been used
in diverse applications, such as organic light emitting diodes
(OLEDs) [1e4], organic photovoltaic cells (OPVs) [5e11] and organic
thin film transistors (OTFTs) [12,13]. In these applications, OPVs
have drawn great attention with the global technology trend
toward economic feasibility and continuous development, while
preserving the environment. Low power conversion efficiency
(PCE) has been the largest obstacle in developing OPVs [6]. For the
past few years, the DonoreAcceptor (DeA) type low-band gap
polymer has drawn great attention, as its electronic properties can
be easily be changed based on the unique combination of the DeA
unit. This polymer can also increase the absorption spectra with
long wavelengths.
non-planar properties of the two phenyl rings [16,17]. Recently,
the Jen group reported that with the two phenyl rings connected
to the single bond between ortho-positions, the PCE improved
from 5.69% up to 6.24% [18]. These fused-phenyl rings can enhance
the efficiency through a reduction of the energetic disorder of the
polymer after facilitating polymer coplanarity and interchain pep
interactions [7,8,14]. In these fused-phenyl rings, however, it is
difficult to introduce the akyl-chains necessary to become highly
soluble.
Recently, alkoxy-benzothiadizole derivatives, for which flexible
alkoxy-chains were introduced at the 5th and 6th positions, have
been reported [19,20]. These kinds of alkoxy-benzothiadizole deriv-
atives have a high solubility with the absorption spectra and elec-
tronic properties of benzothiadiazole. Alkoxy-chains with good
solubility introduced at the 11th and 12th positions of the dibenzo-
phenazine derivatives, which are quinoxaline derivatives with
a fused-phenyl ring similar to an alkoxy-benzothiadizole, may
produce polymers with a high solubility along with the absorption
spectra and coplanarity properties of the dibenzo-phenazine
derivates.
Among the DeA polymer acceptor units, quinoxaline deriva-
tives have been widely used due to the electro-withdrawing
properties of the two imine nitrogens. Quinoxaline derivatives
can be easily structurally deformed with high solubility. In addi-
tion, its electronic properties can be changed with various
In this study, new acceptor units with a high solubility, 5,8-bis(5-
bromothiophen-2-yl)-6,7-bis(octyloxy)-2,3-diphenylquinoxaline and
* Corresponding author. Tel.: þ82 2 450 3498; fax: þ82 2 444 0765.
E-mail address: dkmoon@konkuk.ac.kr (D.K. Moon).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.10.056

H.J. Song et al. / Polymer 54 (2013) 1072e1079
1073
10,13-bis(5-bromothiophen-2-yl)-11,12-bis(octyloxy)dibenzo[a,c]
phenazine, were synthesized. Two new DeA type polymers,
poly(carbazole-octyloxy diphenylquinoxaline) (PCzPDTQ) and
poly(carbazole-octyloxydibenzophenazine) (PCzFDTQ), in which
carbazole derivatives were introduced as a donor, were successfully
polymerized. Compared with PCzPDTQ, a more effective pep inter-
action is expected in PCzFDTQ because of the superior coplanarity
properties of fused-phenyl rings, which enables high FF in OPVs.
fabrication was completed by depositing thin layers of BaF2
(1 nm), Ba (2 nm), and Al (200 nm) at pressures of less than
10ꢁ6 torr. The active area of the device was 4.0 mm2. Finally, the
cell was encapsulated using a UV-curing glue (Nagase, Japan). In
this study, all of the devices were fabricated with the following
structure: ITO glass/PEDOT:PSS/polymer:PCBM/BaF2/Ba/Al/encap-
sulation glass.
The illumination intensity was calibrated using a standard a Si
photodiode detector that was equipped with a KG-5filter. The
output photocurrent was adjusted to match the photocurrent of the
Si reference cell to obtain a power density of 100 mW/cm2. After the
encapsulation, all of the devices were operated under an ambient
atmosphere at 25 ꢀC. The currentevoltage (IeV) curves of the
photovoltaic devices were measured using a computer-controlled
Keithley 2400 source measurement unit (SMU) that was equip-
ped with a Peccell solar simulator under an illumination of AM 1.5G
(100 mW/cm2). The thicknesses of the thin films were measured
using a KLA Tencor Alpha-step 500 surface profilometer with an
accuracy of 1 nm.
2. Experimental section
2.1. Instruments and characterization
Unless otherwise specified, all reactions were carried out under
a
nitrogen atmosphere. Solvents were dried using standard
procedures. All column chromatography was performed with silica
gel (230e400 mesh, Merck) as the stationary phase. 1H NMR
spectra were performed in a Bruker ARX 400 spectrometer using
solutions in CDCl3 with chemical shifts recorded in ppm units
using TMS as the internal standard. The elemental analyses were
measured with EA1112 using a CE Instrument. Electronic absorp-
tion spectra were measured in chloroform using an HP Agilent
8453 UVeVis spectrophotometer. The cyclic voltammetric waves
were produced using a Zahner IM6eX electrochemical workstation
2.3. Synthesis
All reagents were purchased from Aldrich, Acros or TCI
companies. All chemicals were used without further purification.
The following compounds were synthesized following modified
literature procedures: 4,7-dibromo-5,6-bis(octyloxy)benzo[c]
[1,2,5]thiadiazole [20], 9-(heptadecan-9-yl)-2,7-bis(4,4,5,5-tetram
ethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole [6].
with a 0.1
M acetonitrile (purged with nitrogen for 20 min) solu-
tion containing tetrabutylammonium hexafluorophosphate
(Bu4NPF6) as the electrolyte at a constant scan rate of 50 mV/s.
ITO, a Pt wire, and silver/silver chloride [Ag in 0.1
M KCl] were used
as the working, counter, and reference electrodes, respectively.
The electrochemical potential was calibrated against Fc/Fcþ. The
HOMO levels of the polymers were determined using the oxida-
tion onset value. Onset potentials are values obtained from the
intersection of the two tangents drawn at the rising current and
the baseline changing current of the CV curves. TGA measure-
ments were performed on an NETZSCH TG 209 F3 thermogravi-
metric analyzer. All GPC analyses were made using THF as an
eluent and a polystyrene standard as a reference. Powder X-ray
2.4. 5,8-dibromo-6,7-bis(octyloxy)-2,3-diphenylquinoxaline
Under a nitrogen atmosphere, 4,7-dibromo-5,6-bis(octyloxy)
benzo[c] [1,2,5]thiadiazole (4.0 g, 7.27 mmol) and zinc (5.88 g
90.0 mmol) dust were dissolved in 110 ml acetic acid. The mixture
was refluxed for 3 h at 80 ꢀC. After cooling to room temperature, the
reaction mixture was washed with a NaOH solution. The solids that
resulted after evaporation of the organic solvent and benzil (3.36 g,
15.92 mmol) were dissolved in 110 ml acetic acid. The mixture was
refluxed for 1 day at 60 ꢀC. After cooling to room temperature, an
orange colored mixture was observed. After filtration, the reaction
mixture was purified by column chromatography on silica gel
(dichloromethane as eluent) to obtain the product as a green solid
(3.2 g, 63.4%) 1H NMR (400 MHz; CDCl3; Me4Si): 7.62(d, 4H; Ar)
7.37(m, 6H; Ar) 4.20(t, 4H; CH2) 1.92(m, 4H; CH2) 1.55(m, 4H; CH2)
1.33(m, 16H; CH2) 0.90(t, 6H; CH3) 13C NMR (100 MHz; CDCl3;
Me4Si): 153.81; 152.69; 138.24; 137.14; 130.22; 129.25; 128.30;
117.26; 74.86; 31.88; 30.33; 29.45; 29.31; 26.07; 22.70; 14.13. Anal.
Calcd for: C36H44Br2N2O2: C, 62.07; H, 6.37; N, 4.02; O, 4.59 found:
C, 61.20; H, 6.44; N, 3.92; O, 4.99.
diffraction (XRD) patterns were obtained using a Bruker D2Pha-
ꢀ
ser(LynxEye of 1D detector) with CuK
a
(1.54 A) radiation. Topo-
graphic images of the active layers were obtained through atomic
force microscopy (AFM) in tapping mode under ambient
a
conditions using an XE-100 instrument. Theoretical analyses were
performed using density functional theory (DFT), as approximated
by the B3 LYP functional and employing the 6e31G* basis set in
Gaussian09.
2.2. Fabrication and characterization of polymer solar cells
All of the bulk-heterojunction PV cells were prepared using the
following device fabrication procedure. The glass/indium tin oxide
(ITO) substrates [Sanyo, Japan (10
U
/
g
)] were sequentially litho-
2.5. 6,7-bis(octyloxy)-5,8-di(thiophen-2-yl)-2,3-
diphenylquinoxaline
graphically patterned, cleaned with detergent, and ultrasonicated
in deionized water, acetone, and isopropyl alcohol. The substrates
were then dried on a hot plate at 120 ꢀC for 10 min and treated
with oxygen plasma for 10 min to improve the contact angle
immediately before the film coating process. Poly(3,4-ethylene-
dioxythiophene): poly(styrene-sulfonate) (PEDOT:PSS, Baytron P
5,8-dibromo-6,7-bis(octyloxy)-2,3-diphenylquinoxaline 8 (2.0 g,
2.87 mmol) and tributyl(thiophen-2-yl)stannane (2.73 ml,
8.61 mmol) in toluene (75 ml) was added to PdCl2(PPh3)2 (0.203 g
0.29 mmol) under a nitrogen atmosphere. After refluxing for 48 h at
80 ꢀC, the mixture was cooled to room temperature and then poured
into H2O; the organic layer was extracted by CHCl3 and dried over
anhydrous Na2SO4. The crude product was purified by column
chromatography on silica gel to give the product as an orange solid
(1.5 g, 74.3%) 1H NMR (400 MHz; CDCl3; Me4Si): 8.05 (d, 2H; Ar)
7.65(d, 4H; Ar) 7.55(d, 2H; Ar) 7.33(m, 6H; Ar) 7.20(d, 2H; Ar) 4.03(t,
4H; CH2) 1.77(m, 4H; CH2) 1.39(m, 4H; CH2) 1.29(m,16H; CH2) 0.90(t,
6H; CH3).13C NMR (100 MHz; CDCl3; Me4Si): 152.88; 150.24; 138.93;
4083 Bayer AG) was passed through a 0.45-mm filter before being
deposited onto the ITO at a thickness of ca. 32 nm by spin-coating
at 4000 rpm in air and then dried at 120 ꢀC for 20 min inside
a glove box. Composite solutions with polymers and PCBM were
prepared using 1,2-dichlorobenzene (DCB). The concentration was
controlled adequately in the 0.5 wt% range. The solutions were
then filtered through a 0.45-
mm PTFE filter and then spin-coated
(500e2000 rpm, 30 s) on top of the PEDOT:PSS layer. The device

1074
H.J. Song et al. / Polymer 54 (2013) 1072e1079
136.43; 133.49; 130.88; 130.32; 128.66; 128.14; 127.85; 126.06;
124.19; 74.10; 31.87; 30.41; 29.50; 29.31; 26.07; 22.71; 14.15. Anal.
Calcd for: C44H50N2O2S2: C, 75.17; H, 7.17; N, 3.98; O, 4.55; S, 9.12
Found: C, 73.14; H, 7.35; N, 3.83; O, 5.13; S, 7.85.
2.9. 10,13-bis(5-bromothiophen-2-yl)-11,12-bis(octyloxy)dibenzo
[a,c]phenazine (M2)
Under
a
nitrogen atmosphere, 11,12-bis(octyloxy)-10,13-
di(thiophen-2-yl)dibenzo[a,c]phenazine 12 (0.3 g 0.427 mmol) was
dissolved in 30 ml of THF, and NBS (0.174 g, 0.982 mmol) was then
added in portions. The mixture was stirred for 24 h at room
temperature. Then, the mixture was poured into water and extracted
with chloroform. The combined organic layers were dried over
Na2SO4, and then the solvent was removed. The crude product was
purified with column chromatography to give M2 as a red liquid
(0.24 g 65.4%) 1H NMR (400 MHz; CDCl3; Me4Si): 9.10 (d, 2H; Ar) 8.31
(t, 2H; Ar) 7.90 (t, 2H; Ar) 7.60(d, 4H; Ar) 7.20(m, 2H; Ar) 3.98(t, 4H;
CH2) 1.72(m, 4H; CH2) 1.34(m, 4H; CH2) 1.21(m,16H; CH2) 0.82(t, 6H;
CH3) 13C NMR (100 MHz; CDCl3; Me4Si): 152.75; 140.39; 137.18;
135.21; 131.88; 131.26; 130.03; 029.87; 128.94; 127.83; 127.14;
123.29; 122.62; 115.74; 74.23; 31.91; 30.54; 29.73; 29.55; 29.37;
26.15; 22.75; 14.18.
2.6. 5,8-bis(5-bromothiophen-2-yl)-6,7-bis(octyloxy)-2,3-
diphenylquinoxaline (M1)
Under
a
nitrogen
atmosphere,
6,7-bis(octyloxy)-5,8-
di(thiophen-2-yl)-2,3-diphenylquinoxaline (1.5 g, 2.13 mmol) was
dissolved in 140 ml of THF, and then NBS (0.87 g, 4.9 mmol) was
added in portions. The mixture was stirred for 24 h at room
temperature. The mixture was then poured into water and
extracted with chloroform. The combined organic layers were dried
over Na2SO4, and the solvent was removed. The crude product was
purified with column chromatography to give M1 as a red-orange
solid (1.2 g, 65.5%) 1H NMR (400 MHz; CDCl3; Me4Si): 7.97(d, 2H;
Ar) 7.63(d, 4H; Ar) 7.36(m, 6H; Ar) 7.14(d, 2H; Ar) 4.07(t, 4H; CH2)
1.83(m, 4H; CH2) 1.42(m, 4H; CH2) 1.30(m, 16H; CH2) 0.90(t, 6H;
CH3). 13C NMR (100 MHz; CDCl3; Me4Si): 152.68; 150.57; 138.46;
135.89; 135.19; 131.25; 130.33; 128.95; 128.89; 128.25; 123.35;
116.09; 74.22; 31.85; 30.42; 29.48; 29.30; 26.07; 22.70; 14.13. Anal.
Calcd for: C44H48Br2N2O2S2: C, 61.39; H, 5.62; N, 3.25; O, 3.72; S,
7.45 Found: C, 61.23; H, 5.65; N, 3.13; O, 4.28; S, 7.42.
2.10. Poly[carbazole-alt-dithienyl-diphenylquinoxaline] (PCzPDTQ)
9-(heptadecan-9-yl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9H-carbazole (0.25 g, 0.29 mmol), M1 (0.191 g,
0.29 mmol), Pd(PPh3)4(0) (0.009 g, 0.008 mmol) and Aliquat336,
were placed in a Schlenk tube, purged with three nitrogen/vacuum
cycles, and under a nitrogen atmosphere, added 2
M degassed
2.7. 10,13-dibromo-11,12-bis(octyloxy)dibenzo[a,c]phenazine
aqueous K2CO3 (10 ml) and dry toluene (20 ml). The mixture was
heated to 90 ꢀC and stirred in the dark for 48 h. After the polymer-
ization was complete, it was end-capped with bromothiophene.
After reaction quenching, the whole mixture was poured into
methanol. The precipitate was filtered off, purified with a Soxhlet
extraction in the following order: methanol, acetone and chloro-
form. The polymer was recovered from the chloroform fraction and
precipitated into methanol. The final product was obtained after
drying in vacuum. Dark red solid (0.24 g 73.2%). 1H NMR (400 MHz;
Under a nitrogen atmosphere, 4,7-dibromo-5,6-bis(octyloxy)
benzo[c] [1,2,5]thiadiazole (1.75 g, 3.18 mmol) and zinc (2.58 g
39.4 mmol) dust were dissolved in 110 ml acetic acid. The mixture
was refluxed for 3 h at 80 ꢀC. After cooling to room temperature, the
reaction mixture was washed with a NaOH solution. The solids that
resulted after the evaporation of the organic solvent and 9,10-
phenanthrenequinone (1.46 g, 7.0 mmol) were dissolved in
110 ml acetic acid. The mixture was refluxed for 1 day at 60 ꢀC. After
cooling to room temperature, an orange colored mixture was
observed. After filtration, the reaction mixture was purified by
column chromatography on silica gel (dichloromethane as eluent)
to obtain the product as a light-yellow solid (1.3 g, 58.9%) 1H NMR
(400 MHz; CDCl3; Me4Si): 9.29 (d, 2H; Ar) 8.41 (t, 2H; Ar) 7.68 (m,
CDCl3; Me4Si):
d
¼ 8.24e8.11 (m), 7.93e7.38 (m), 4.70 (s,1H), 4.19 (d,
4H), 2.46 (br, 2H), 2.04e1.93 (m),1.36e1.14 (m), 0.88e0.75 (m). Anal.
Calcd for: C73H91N3O2S2: C, 79.23; H, 8.29; N, 3.80; O, 2.89; S, 5.79.
Found: C, 76.39; H, 8.21; N, 3.58; O, 2.50; S, 5.58.
2.11. Poly[carbazole-alt-dithienyl-dibenzophenazine] (PCzFDTQ)
4H; Ar) 4.27(t, 4H; CH2) 1.96(t, 4H; CH2) 1.59(m, 4H; CH2) 1.42(m,
13C NMR (100 MHz; CDCl
16H; CH2) 0.91(t, 6H; CH3).
3; Me4Si): 153.91;
9-(Heptadecan-9-yl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-
141.80; 137.92; 132.00; 130.49; 129.47; 127.96; 126.71; 122.73;
117.21; 74.94; 31.90; 30.41; 29.50; 29.35; 26.12; 22.72; 14.14. Anal.
Calcd for: C36H42Br2N2O2: C, 62.26; H, 6.10; N, 4.03; O, 4.61 Found:
C, 60.16; H, 5.78; N, 3.79; O, 5.11.
dioxaborolan-2-yl)-9H-carbazole (0.25 g, 0.29 mmol), M2 (0.184 g
0.28 mmol), Pd(PPh3)4(0) (0.009 g, 0.008 mmol) and Aliquat336
were placed in a Schlenk tube, purged with three nitrogen/vacuum
cycles, and 2
M degassed aqueous K2CO3 (10 ml) and dry toluene
(20 ml) were added under a nitrogen atmosphere. The mixture was
heated to 90 ꢀC and stirred in the dark for 48 h. After the poly-
merization was complete, it was end-capped with bromothio-
phene. After reaction quenching, the whole mixture was poured
into methanol. The precipitate was filtered off, purified with
a Soxhlet extraction in the following order: methanol, acetone and
chloroform. The polymer was recovered from the chloroform
fraction and precipitated into methanol. The final product was
obtained after drying under vacuum. Dark black solid (0.23 g 72.1%).
2.8. 11,12-bis(octyloxy)-10,13-di(thiophen-2-yl)dibenzo[a,c]
phenazine
10,13-dibromo-11,12-bis(octyloxy)dibenzo[a,c]phenazine (1.0 g,
1.44 mmol) and trimethyl(thiophen-2-yl)stannane (1.35 ml,
4.3 mmol) in toluene (37 ml) was added to PdCl2(PPh3)2 (0.101 g
0.15 mmol) under a nitrogen atmosphere. After refluxing for 48 h at
80 ꢀC, the mixture was cooled to room temperature and then
poured into H2O; the organic layer was extracted by CHCl3 and
dried over anhydrous Na2SO4. The crude product was purified by
column chromatography on silica gel to give the product as an
orange liquid (0.6 g, 59.3%) 1H NMR (400 MHz; CDCl3; Me4Si): 9.11
(d, 2H; Ar) 8.30 (t, 2H; Ar) 7.98 (t, 2H; Ar) 7.58(d, 2H; Ar) 7.53(m,
4H; Ar) 7.20(t, 2H; Ar) 3.98(t, 4H; CH2) 1.72(m, 4H; CH2) 1.34(m, 4H;
CH2) 1.21(m, 16H; CH2) 0.82(t, 6H; CH3). 13C NMR (100 MHz; CDCl3;
Me4Si): 153.10; 140.34; 138.02; 133.71; 131.89; 130.93; 130.44;
129.71; 127.85; 127.77; 127.10; 126.08; 124.26; 122.67; 74.21;
31.93; 30.51; 29.56; 29.37; 26.14; 22.76; 14.21.
1H NMR (400 MHz; CDCl3; Me4Si):
d
¼ 9.56 (s, 2H), 8.64e8.63 (s,
2H), 8.25e8.05 (m), 7.88e7.69 (m), 4.75 (s, 1H), 4.23e4.22 (d, 4H),
2.50 (br, 2H), 2.08e1.94 (m), 1.36e1.12 (m), 0.88e0.72 (m). Anal.
Calcd for: C73H89N3O2S2: C, 79.37; H, 8.12; N, 3.80; O, 2.90; S, 5.81.
Found: C, 79.03; H, 8.20; N, 3.80; O, 2.09; S, 5.49.
3. Results and discussion
Scheme 1 shows the chemical structure and the synthesis process.
6,7-bis(octyloxy)-5,8-di(thiophen-2-yl)-2,3-diphenylquinoxaline was

H.J. Song et al. / Polymer 54 (2013) 1072e1079
1075
Scheme 1. Scheme of monomer synthesis and polymerization.
synthesized using tributyl(thiophen-2-yl)stannane and PdCl2(PPh3)2.
However, 11,12-bis(octyloxy)-10,13-di(thiophen-2-yl)dibenzo[a,c]
the polymer was end-capped with boromothiophene. The synthe-
sized polymer was purified using a Soxhlet extractor with methanol,
acetone and chloroform. The chloroform fraction was recovered. The
polymerization rates were 73% and 72%, respectively. The polymers
were dissolved with organic solvents, such as THF, chloroform, chlo-
robenzene and o-dichlorobenze, to produce both a homogenous and
semi-transparent red film through spin-coating.
The molecular weights and thermal properties of the synthe-
sized polymers are shown in Table 1. Using GPC with polystyrene as
the standard, the number-average molecular weights (Mn) of
PCzPDTQ and PCzFDTQ were determined to be 31.0 and 12.3 kg/
mol, respectively. The polydispersity indices (PDI) showed very
phenazine wasn’t synthesized using tributyl(thiophen-2-yl)stannane
andvariouscatalysts.Finally,11,12-bis(octyloxy)-10,13-di(thiophen-2-
yl)dibenzo[a,c]phenazine was successfully synthesized using trime-
thyl(thiophen-2-yl)stannane and PdCl2(PPh3)2. This result might be
caused bystrong steric hindranceof fused-ring compared to separated
phenyl-ring. As shown in this scheme, PCzPDTQ and PCzFDTQ were
polymerized through the Suzuki coupling reaction using carbazole
and the monomers M1, M2. The polymerizationwas reacted for 48h at
90 ꢀCusingpalladiumcatalysts(0),a2
aliquot 336 as a surfactant and toluene as a solvent. Upon completion,
M potassiumcarbonatesolution,

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H.J. Song et al. / Polymer 54 (2013) 1072e1079
Table 1
Molecular weight and thermal properties of the polymers.
Polymer
Mn (kg/mol)
Mw (kg/mol)
PDI
Td (ꢀC)
PCzPDTQ
PCzFDTQ
31.0
12.3
52.2
19.6
1.68
1.70
339
345
narrow distribution with 1.68 and 1.70. The degree of polymeriza-
tion for PCzFDTQ was slightly lower than PCzPDTQ because
dibenzophenazine has a rigid structure. With rigid backbone
structure, polymerization becomes less effective [6]. According to
a TGA thermal analysis, shown in Table 1, high thermal stabilities
were observed at 339e345 ꢀC (5 wt% loss). Due to a rigid phenazine
structure, the thermal stability was slightly higher in PCzFDTQ,
indicating a potential use in polymeric solar cells or other opto-
electronic devices [21], which require high thermal stabilities at
temperatures of 300 ꢀC or higher.
Fig. 1 shows the UVeVis spectra of the solution (10ꢁ5
M) and
the thin film (50 nm) after spin coating. The solution maximum
absorption wavelengths (lmax) were 386 and 492 nm for PCzPDTQ
and 395 and 513 nm for PCzFDTQ. Absorption spectra between
300 and 400 nm are caused by the
pep* transition of the
conjugated backbone. Absorption spectra between 500 and
600 nm are caused by an intermolecular charge transfer between
donor acceptors [14].
The maximum absorption wavelengths of the thin film (lmax
)
occurred at 392 and 509 nm for PCzPDTQ and at 410 and 537 nm for
PCzFDTQ. A 6e24 nm red-shift tendency was observed in the thin
film compared to the solution because of an effective intermolec-
ular interaction between the polymer chains at film formation [8].
Compared with PCzPDTQ, a long-wave absorption was observed in
PCzFDTQ for either the solution or the thin film because the fused-
phenyl ring of the dibenzophenazine derivative in PCzFDTQ
increased the effective conjugation length by enhancing the
planarity between the chain backbones [14]. In addition, the
absorption spectra were more intense at 350e450 nm in PCzFDTQ
because of an increase in a
pep* transition of the polymer back-
bone with the fused-phenyl ring structure [14,22]. The optical band
gap energy of PCzPDTQ and PCzFDTQ calculated through the band
edge were 2.06 and 1.97 eV, respectively [19,20].
Fig. 2. Cyclic voltammogram & energy band diagram of polymers.
The cyclic voltammograms of the PCzPDTQ and PCzFDTQ thin
films, measured in 0.1
acetonitrile, are shown in Fig. 2(a). Table 2 contains the optical and
M tetrabutylammonium-hexafluorophosphate
electrochemicalproperties of thepolymers. As shownin Fig. 2 (a)and
onset
Table 2, the oxidation ðE
Þ onset potential values for PCzPDTQ and
ox
PCzFDTQ were þ1.09 V and þ1.13 V, respectively. The HOMO levels of
PCzPDTQ and PCzFDTQ were ꢁ5.44 and ꢁ5.48 eV, respectively. The
LUMO levels of PCzPDTQ and PCzFDTQ, calculated from the optical
band gap energy from the HOMO levels, were - 3.38 and - 3.51 eV,
respectively. Because all of the polymer HOMO levels are lower than
the P3HT HOMO levels (4.9 eV), a relatively high air stability is ex-
pected [23]. In Fig. 2(b), the energy levels for ITO, PEDOT:PSS,
Table 2
Optical & electrochemical properties of the polymers.
(V) EHOMO (eV)c ELUMO (eV)d Eopt (eV)e
onset
Polymer Absorption,
E
ox
lmax (nm)
Solutiona Filmb
PCzPDTQ 386, 492 392, 509 1.09
PCzFDTQ 395, 513 410, 537 1.13
ꢁ5.44
ꢁ5.48
ꢁ3.38
ꢁ3.51
2.06
1.97
a
Absorption spectrum in DCB solution (10ꢁ5
M).
b
c
Spin-coated thin film (50 nm).
Calculated from the oxidation onset potentials under the assumption that the
absolute energy level of Fc/Fcþ was ꢁ4.8 eV below a vacuum.
d
HOMO e Eopt.
e
Fig. 1. Absorption spectra of polymer in solution & film.
Estimated from the onset of UVevis absorption data of the thin film.

H.J. Song et al. / Polymer 54 (2013) 1072e1079
1077
PCzPDTQ, PCzFDTQ, PC71BM and Al are illustrated in a band diagram.
Compared to the Density Functional Theory (DFT) prediction shown
in Fig. 3, the HOMO and LUMO levels were lower. However, the
HOMO and LUMO levels of PCzPDTQ and PCzFDTQ showed a similar
tendency for both theoretical and measured values [15]. The energy
levelof the electrons(HOMOandLUMO)forPCzFDTQwaslowerthan
PCzPDTQ because of the high oxidative stability of the fused-phenyl
ring of PCzFDTQ. For the distribution of electrons in the LUMO level,
in particular, the fused-phenyl ring of PCzFDTQ exhibited a higher
electron distribution than the phenyl ring of PCzPDTQ because the
fused-phenyl ring transports electrons with a higher conjugation
length than the separated-phenyl ring.
As shown in Fig. 2(b), the difference of LUMO levels was the
lowest with 0.49 eV between PCzFDTQ (ꢁ3.51 eV) and PC71BM
(ꢁ4.0 eV), indicating that an effective electron transport is expected
between PCzFDTQ and PC71BM [8,24].
To analyze the ordering structure of the polymers, X-ray
diffraction in thin film was used, and the results are shown in
Fig. 4(a). The out-of-plane peak, observed in all diffraction patterns
with a highly ordered lamellar polymeric structure, did not occur.
In the (010) crystal plane related to
diffraction peak of PCzPDTQ and PCzFDTQ was detected at
approximately 21.7ꢀ, 23.5ꢀ. Using the calculation (
¼ 2dsin ), the
stacking distances (dp) of PCzPDTQ and PCzFDTQ were 4.0 A
pep stacking, a broad
l
q
ꢀ
pep
ꢀ
and 3.7 A, respectively, indicating that the
pep stacking was
somewhat effective in PCzFDTQ than in PCzPDTQ. This result of
PCzPDTQ is similar to the benzene-thiophene aromatic system
polymers that have perform well as OPVs [25].
Fig. 4. X-ray diffraction pattern of polymers in thin film.
According to a comparative analysis of the tilt angle through DFT
calculations, as shown in Fig. 4(b), the tilt angle of the carbazole-
thiophene linkage was 28ꢀ. The thiophene-quinoxaline of
PCzPDTQ and thiophene-dibenzophenazine of PCzFDTQ were 19ꢀ
and 47ꢀ, respectively. In terms of the tilt angle, PCzFDTQ was greater
than PCzPDTQ. However, an effective
pep stacking is enabled in
PCzFDTQ because the fused-phenyl ring (tilt angle: 0ꢀ) has strong
planarity properties between the chain backbones. As shown in the
XRD measurement in Fig. 4(a), PCzFDTQ had a shorter pep stacking
distance than PCzPDTQ. It appears that a high FF would occur in the
Fig. 3. DFT Gaussian simulation of polymers.
Fig. 5. (a) JeV characteristics & (b) EQE spectra of the BHJ solar cells with the device.

1078
H.J. Song et al. / Polymer 54 (2013) 1072e1079
Table 3
ability because their substitution with electron releasing octyloxy
substituents was around polymer backbone.
Photovoltaic performance of the BHJ solar cells with the device.
For PCzFDTQ, the efficiency tended to improve with increasing
Active layer (w/w)
Weight ratio
(P:A, w/w)
VOC
(V)
JSC
FF
(%)
PCE
(%)
(mA/cm2)
PC71BM. At a PCzFDTQ and PC71BM ratio of 1:6, the values of VOC
,
Polymer (P)
PCzPDTQ
Acceptor (A)
JSC, FF and PCE were 0.85 V, 5.5 mA/cm2, 44.9% and 2.1%, respec-
tively. While VOC and JSC were similar, the value of FF for PCzFDTQ
increased by 39% compared to PCzPDTQ. As supported by the XRD
data in Fig. 4, this increase was caused by the effects of the back-
PC71BM
1:3
1:4
1:3
1:4
1:5
1:6
0.85
0.81
0.65
0.71
0.83
0.85
6.5
6.0
6.2
7.2
5.7
5.5
32.2
29.2
36.2
36.9
42.4
44.9
1.8
1.4
1.5
1.9
2.0
2.1
PCzFDTQ
PC71BM
bone planarity of the polymers. In other words, the effective pep
stacking found in PCzFDTQ with a high coplanarity increases the FF.
Both synthesized polymers had greater open circuit voltage
values than P3HT (Voc ¼ 0.5e0.6) because of the lower HOMO
levels. In other words, the HOMO level of the carbazole derivative
introduced into the main chain of the polymers was low [23].
To check the accuracy of the device measurements, the External
Quantum Efficiency (EQE) was measured and the EQE curve with
the ratio of PC71BM shown in Fig. 5(b). In the EQE curve, photons
mostly occurred in the polymer phase, correlating with the
absorption spectra of the polymers [26]. As shown in Fig. 5(b), the
EQE curve was similar to the UVeVis spectra curve in Fig. 1. For
PCzPDTQ, the short-circuit current density obtained from the EQE
against PC71BM (1:3 ratio) was 6.6 mA/cm2. For PCzFDTQ, the
theoretical short-circuit current density obtained from the EQE
against PC71BM (1:6 ratio) was 5.2 mA/cm2. EQE of PCzPDTQ was
more current value compared to that of PCzFDTQ, which is due to
high molecular weight of PCzPDTQ (31.0 kg/mol) compared to
molecular weight PCzFDTQ (12.3 kg/mol).
OPVs due to the close
properties.
pep stacking of PCzFDTQ with its planarity
Fig. 5 and Table 3 show the results of the evaluation of the
characteristics of the OPV devices. The device was structured as
follows: ITO (170 nm)/PEDOT:PSS (40 nm)/active layer (50 nm)/
BaF2 (2 nm)/Ba (2 nm)/Al (100 nm). The active layer had an opti-
mized blending ratio obtained by dissolving the polymer and
phenyl-C71-butyric acid methyl ester (PC71BM) in chlorobenzene
(CB) with a concentration of approximately 0.5e1 wt% [20,26]. The
open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF)
and power conversion efficiency (PCE) were 0.85 V, 6.5 mA/cm2,
32.2% and 1.8%, respectively, for PCzPDTQ with a 1:3 ratio of
PC71BM (50 nm in thickness). However, device of PCzPDTQ: PC71BM
(1:4 ratio) showed low performance, which is due to aggregation of
PC71BM in part (see Fig. 5S). The structure of PCzPDTQ is similar
with other qunoxaline polymer, but PCzPDTQ showed low PCE
value compared to other quinoxaline polymer [14,15]. This result
might be probably caused by a weakening of the electron accepting
The AFM of the polymer/PCBM blend films are shown in Fig. 6.
As observed in Fig. 6(a), a smooth morphology appeared when
PCzPDTQ and PC71BM were blended in a 1:3 ratio with root-mean-
Fig. 6. Topographic AFM images of (a) PCzPDTQ: PC71BM 1:3 (3 ꢂ 3
m
m2) (b) PCzFDTQ:PC71BM 1:6 (3 ꢂ 3
m
m2).

H.J. Song et al. / Polymer 54 (2013) 1072e1079
1079
square roughness (Rms) of 0.45 nm. Compared with PCzPDTQ,
Appendix A. Supplementary data
a large-phase separation was observed when PCzFDTQ and PC71BM
were mixed at a 1:6 ratio with Rms of 0.77 nm, as shown in
Fig. 6(b). In addition, macro p-n channels were formed. The large-
phase separation produces a low photocurrent by reducing the
charge separation and increasing the exciton diffusion length and
the recombination of electric charges [27]. The blend film between
PCzPDTQ and PC71BM should demonstrate good film morphology
with a smaller domain size approaching the ideal domain size of
about 10 nm [28], which showed high EQE value compared to the
blend film between PCzFDTQ and PC71BM. The lowest photocurrent
(5.5 mA/cm2) was observed for a PCzFDTQ/PC71BM ratio of 1:6.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2012.10.056.
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In this study, two new acceptor units, in which octyloxy with
a high solubility was introduced, have been synthesized. In addi-
tion, DeA type polymers, PCzPDTQ, which accepted carbazole
derivatives as a donor, and PCzFDTQ were polymerized. Both
PCzPDTQ and PCzFDTQ showed high solubility, thermal stability
and a low-band gap. According to XRD measurements, the
pep
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ꢀ
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of the fused phenyl rings, the FF of PCzFDTQ was 39% higher than
PCzPDTQ. Continued optimization of the chemical structures and
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This research was supported by a grant from the Fundamental
R&D Program for Core Technology of Materials funded by the
Ministry of Knowledge Economy, Republic of Korea. This work was
supported by the National Research Foundation of Korea Grant
funded by the Korean Government (MEST) (NRF-2009-C1AAA001-
2009-0093526).