Enhanced device performance of polymer solar cells by planarization of quinoxaline derivative in a low-bandgap polymer

Article abstract of DOI:10.1039/c1jm10877h

A series of low-bandgap alternating copolymers consisting of electron-accepting quinoxaline derivatives and electron-donating carbazole or fluorene were synthesized via a Suzuki coupling reaction. For the purpose of improving the molecular packing of poly

Full text of DOI:10.1039/c1jm10877h

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 8583
www.rsc.org/materials
PAPER
Enhanced device performance of polymer solar cells by planarization
of quinoxaline derivative in a low-bandgap polymer†
*
Yoonkyoo Lee, Young Min Nam and Won Ho Jo
Received 28th February 2011, Accepted 2nd April 2011
DOI: 10.1039/c1jm10877h
A series of low-bandgap alternating copolymers consisting of electron-accepting quinoxaline
derivatives and electron-donating carbazole or fluorene were synthesized via a Suzuki coupling
reaction. For the purpose of improving the molecular packing of polymer chains and to enhance the
charge carrier mobility in the packing direction, a new quinoxaline derivative, 10,13-bis-(4-octyl-
thiophene-2-yl)-dibenzo[a,c]phenazine, which is expected to have planar polycyclic structure, was
synthesized and introduced as a new building block of alternating copolymers instead of frequently-
used 5,8-dithien-2-yl-2,3-diphenylquinoxaline. Copolymers with the planar quinoxaline derivative
exhibited better optical and structural properties compared to copolymers with a less planar
quinoxaline derivative. A power conversion efficiency was achieved up to 3.8% when the copolymer
with planar quinoxaline derivative was blended with [6,6]-phenyl-C71-butyric acid methyl ester as an
active layer material in bulk heterojunction solar cells.
derivatives, several research groups have synthesized D–A
Introduction
type low-bandgap copolymers and reported the photovoltaic
properties of those copolymers.8–11 Among Qx derivatives,
5,8-dithien-2-yl-2,3-diphenylquinoxaline has been reported to be
the most efficient building block when it was copolymerized with
thiophene, demonstrating a power conversion efficiency (PCE)
up to 6%.12 However, most of the alternating copolymers
composed of electron-accepting 5,8-dithien-2-yl-2,3-diphe-
nylquinoxaline and electron-donating carbazole or fluorene have
shown poorer photovoltaic performance with PCEs less than
2%.13–15 The low efficiency is probably due to the non-planar
nature of the two pendent phenyl groups attached on Qx, leading
to poor interchain packing and thus to suppress hole mobility.
Along with the light-harvesting ability, high charge carrier
mobility of conjugated polymers is also beneficial for improving
the performance of polymer solar cells. In general, high charge
transport requires close packing of polymer chains, facilitating
charge transport through intermolecular hopping. In the present
work, a new quinoxaline derivative, 10,13-bis-(5-bromo-4-octyl-
thiophene-2-yl)-dibenzo[a,c]phenazine (TBPz), which is expected
to have a planar structure and thus to facilitate the molecular
packing of the resulting copolymer, was synthesized and used as
a new building block of alternating copolymers (Scheme 1).
When the mobility and PCE of copolymers with TBPz are
compared with those of 5,8-bis-(5-bromo-4-octyl-thiophene-2-
yl)-2,3-diphenyl-quinoxaline (TPQx), which is not expected to
have planar structure, the copolymers with TBPz shows a higher
charge mobility and better PCE than the copolymers with TPQx.
One of the TBPz-based copolymers, poly(N-90-heptadecanyl-2,
7-carbazole-alt-10,13-bis-(4-octyl-thiophene-2-yl)-dibenzo[a,c]
phenazine) (PCTBPz), showed the best photovoltaic
Bulk heterojunction (BHJ) polymer solar cells have attracted
tremendous attention over the last decade due to their potential
of being fabricated by a high-throughput roll-to-roll process and
providing low-cost solar electricity.1,2 The record high efficiency
has continually increased to its current value of higher than 7%
due to synergetic advancement in the device optimization,
understanding of organic semiconductor physics, and develop-
ment of new materials.3,4 Particularly, significant progress has
been made in designing semiconducting low-bandgap polymers
to maximize the light absorption, especially at longer wave-
lengths where much of the photon flux emitted from the sun is
found.
In recent years, low-bandgap polymers based on the internal
donor–acceptor (D–A) interaction have attracted great interest
because their electronic properties can be easily tuned by proper
combination of D and A units and their absorption ranges can be
extended to longer wavelengths.5–7 Among a wide variety of
acceptor units, quinoxaline units (Qx) have been shown to be an
excellent building block for synthesis of low-bandgap conjugated
polymers because of the electron-withdrawing property of two
imine nitrogens in the Qx unit. The Qx unit has a fascinating
structure for controlling the electronic structure of the resulting
copolymers, because it can provide the versatility of introducing
substituents easily on the 2 and 3 positions of itself. Using Qx
WCU Hybrid Materials Program and Department of Materials Science
and Engineering, Seoul National University, Seoul, 151-742, Korea.
E-mail: whjpoly@snu.ac.kr; Fax: +82 2-876-6086; Tel: +82 2-880-7192
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1jm10877h
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 8583–8590 | 8583

View Online
Scheme 1 Synthetic route for monomers.
performance with a PCE of 3.8% when blended with [6,6]-
phenyl-C71-butyric acid methyl ester (PC71BM) as an active layer
in BHJ solar cells.
5,8-Bis-(5-bromo-4-octyl-thiophene-2-yl)-2,3-diphenyl-quinoxa-
line (TPQx). The compound 3 (1.42 g, 2.16 mmol) was dissolved
in 20 ml of THF, to which N-bromosuccinimide (NBS) (0.79 g,
4.44 mmol) was added in the dark. After stirring the mixture at
room temperature for 3 h, the solvent was removed under
reduced pressure, and then the residue was purified by column
chromatography on silica gel (1 : 4 chloroform–hexane as eluent)
to afford the product as a red solid. Yield: 1.67 g (95%). 1H NMR
(300 MHz, CDCl3): d (ppm) 8.02 (s, 2H), 7.70 (dd, 4H), 7.52 (s,
2H), 7.40 (m, 3H), 2.62 (t, 4H), 1.66 (m, 4H), 1.29–1.42 (m, 20H),
0.88 (t, 6H). 13C NMR (125 MHz, CDCl3): d (ppm) 152.13,
141.49, 138.50, 137.82, 137.00, 130.71, 130.51, 129.34, 128.46,
126.75, 125.82, 114.37, 32.13, 30.06, 29.80, 29.72, 29.61, 29.50,
22.90, 14.33. Elemental Anal. Calcd for C44H48N2S2Br2 (%): C,
63.80; H, 5.79; N, 3.38; S, 7.74. Found (%): C, 63.71; H, 5.62; N,
3.37; S, 7.78.
Experimental
Synthesis
Materials. All reagents were obtained from Aldrich unless
otherwise specified and used as received. Tetrahydrofuran (THF)
was dried over sodium/benzophenone under nitrogen and freshly
distilled before use. 2-Trimethylstannyl-4-octylthiophene (1),16
5,8-dibromo-2,3-diphenylquinoxaline (2),17 10,13-dibromodi-
benzo[a,c]phenazine (4),17 and 2,7-bis(40,40,50,50-tetramethyl-
10,30,20-dioxaborolan-20-yl)-N-900-heptadecanylcarbazole (7)18 were
synthesized by following the methods reported in the literature.
PC71BM was obtained from Nano-C. Poly(3,4-ethylene-
dioxythiophene) : poly(styrene sulfonate) (PEDOT : PSS) (Cle-
vios P VP AI 4083) was purchased from H. C. Stark and passed
through a 0.45 mm PES syringe filter before spin-coating.
10,13-Bis-(4-octyl-thiophene-2-yl)-dibenzo[a,c]phenazine (5). 2-
Trimethylstannyl-4-octylthiophene (1) (3.68 g, 10.3 mmol) and
10,13-dibromodibenzo[a,c]phenazine (4) (1.8 g, 4.1 mmol) were
dissolved in 80 ml of toluene–THF (1 : 1, v/v). The solution was
deaerated under vacuum and backfilled with N2 gas, and this was
repeated three times prior to addition of Pd(PPh3)2Cl2 catalyst
(144 mg, 0.205 mmol). The solution was allowed to react at 80 ꢀC
for 24 h under a N2 atmosphere. The solvent was removed under
reduced pressure, and then the residue was purified by column
chromatography on silica gel (1 : 3 chloroform–hexane as eluent)
to yield a red compound which solidifies upon standing. Yield:
2.33 g (85%). 1H NMR (300 MHz, CDCl3): d (ppm) 9.48 (dd, 2H),
8.50 (d, 2H), 8.13 (s, 2H), 7.72 (m, 6H), 7.21 (s, 2H), 2.73 (t, 4H),
1.76 (m, 4H), 1.25–1.42 (m, 20H), 0.88 (t, 6H). 13C NMR (125
MHz, CDCl3): d (ppm) 143.01, 141.83, 138.87, 138.80, 132.57,
131.76, 130.79, 130.45, 128.83, 128.18, 127.94, 127.02, 124.07,
123.03, 32.04, 30.89, 30.88, 30.15, 29.88, 29.40, 22.91, 14.36.
Elemental Anal. Calcd for C44H48N2S2 (%): C, 79.04; H, 7.19; N,
4.19; S, 9.58. Found (%): C, 79.08; H, 7.13; N, 4.17; S, 9.68.
5,8-Bis-(4-octyl-thiophene-2-yl)-2,3-diphenyl-quinoxaline (3). 2-
Trimethylstannyl-4-octylthiophene (1) (2.26 g, 6.29 mmol) and
5,8-dibromo-2,3-diphenylquinoxaline (2) (1.11 g, 2.52 mmol)
were dissolved in 20 ml of toluene. The solution was deaerated
under vacuum and backfilled with N2 gas, and this was repeated
three times prior to addition of Pd(PPh3)2Cl2 catalyst (90 mg,
0.128 mmol). The solution was allowed to react at 80 ꢀC for 24 h
under a N2 atmosphere. The solvent was removed under reduced
pressure, and then the residue was purified by column chroma-
tography on silica gel (1 : 4 chloroform–hexane as eluent) to yield
a red compound which solidifies upon standing. Yield: 1.30 g
1
(77%). H NMR (300 MHz, CDCl3): d (ppm) 8.09 (s, 2H), 7.74
(m, 6H), 7.37 (m, 6H), 7.10 (s, 2H), 2.68 (t, 4H), 1.70 (m, 4H),
1.25–1.41 (m, 20H), 0.88 (t, 6H). 13C NMR (125 MHz, CDCl3):
d (ppm) 151.66, 143.04, 138.99, 138.67, 137.50, 131.46, 130.70,
129.15, 128.38, 128.29, 127.10, 123.97, 32.14, 30.85, 30.81, 29.74,
29.65, 29.52, 22.90, 14.33. Elemental Anal. Calcd for
C44H50N2S2 (%): C, 78.80; H, 7.46; N, 4.18; S, 9.56. Found (%):
C, 78.80; H, 7.47; N, 4.11; S, 9.68.
10,13-Bis-(5-bromo-4-octyl-thiophene-2-yl)-dibenzo[a,c]phena-
zine (TBPz). The compound 5 (1.0 g, 1.5 mmol) was dissolved in
100 ml of THF, to which N-bromosuccinimide (NBS) (0.56 g,
8584 | J. Mater. Chem., 2011, 21, 8583–8590
This journal is ª The Royal Society of Chemistry 2011

View Online
3.15 mmol) was added in the dark. After stirring the mixture
at room temperature for 3 h, the residue was condensed
under vacuum evaporation. Precipitation from methanol
yielded the product as a shiny brown powder. Yield: 1.19 g
(96%). 1H NMR (300 MHz, CDCl3): d (ppm) 9.22 (d, 2H),
8.38 (d, 2H), 7.83 (s, 2H), 7.67 (m, 4H), 7.37 (s, 2H), 2.62 (t,
4H), 1.70 (m, 4H), 1.25–1.41 (m, 20H), 0.89 (t, 6H). 13C
NMR (125 MHz, CDCl3): d (ppm) 142.09, 141.56, 138.17,
137.89, 132.60, 130.86, 130.70, 130.40, 128.34, 128.07, 126.70,
125.82, 123.05, 114.17, 32.16, 30.16, 29.88, 29.73, 29.66,
Poly(N-90-heptadecanyl-2,7-carbazole-alt-10,13-bis-(4-octyl-
thiophene-2-yl)-dibenzo[a,c]phenazine) (PCTBPz). PCTBPz
was synthesized by following the same procedure as used in the
synthesis of PFTPQx. Compound 7 (100.0 mg, 0.145 mmol)
and TBPz (120.1 mg, 0.145 mmol) were used as monomers.
1
Yield: 108.8 mg (68%). H NMR (300 MHz, CDCl3): d (ppm)
9.61–9.13 (br, 2H), 8.53–8.14 (m, 4H), 7.86–7.56 (m, 12H),
4.71 (s, 1H), 2.91 (s, 4H), 2.42 (s, 4H), 2.03–1.87 (m, 6H), 1.32–
1.12 (m, 42H), 0.90–0.75 (m, 12H). Elemental Anal. Calcd for
C
73H87N3S2 (%): C, 81.94; H, 8.13; N, 3.93; S, 6.00. Found
29.54,
22.92,
14.35.
Elemental
Anal.
Calcd
for
(%): C, 81.92; H, 8.11; N, 3.90; S, 6.07.
C44H46N2S2Br2 (%): C, 63.95; H, 5.57; N, 3.39; S, 7.76.
Found (%): C, 63.96; H, 5.86; N, 3.35; S, 7.72.
General characterization methods
Poly(9,9-dioctylfluorene-alt-5,8-bis-(4-octyl-thiophene-2-yl)-
2,3-diphenyl-quinoxaline) (PFTPQx). 9,9-Dioctylfluorene-2,7-
diboronicacid-bis-(1,3-propanediol)ester (6) (150.7 mg, 0.3
mmol) and TPQx (250.0 mg, 0.3 mmol) were dissolved in
a mixture of THF (24 mL) and aqueous K2CO3 solution (2 M,
6 mL). The solution was flushed with N2 for 20 min, and then
17.3 mg of Pd(PPh3)4 was added. The reaction mixture was
The chemical structures of the materials used in this study were
identified by 1H NMR (Avance DPX-300) and 13C NMR
(Avance DPX-500). Elemental analysis was performed on an
EA1110 (CE Instrument) elemental analyzer. Molecular weight
and its distribution were measured by gel permeation chroma-
tography (Waters) equipped with a Waters 2414 refractive index
detector using THF as an eluent, where the columns were cali-
brated against standard polystyrene samples. The optical
absorption spectra were obtained by a UV-Vis spectrophotom-
eter (Lambda 25, Perkin Elmer). Cyclic voltammetry experi-
ments were carried out on a potentiostat/galvanostat (VMP 3,
Biologic) in an electrolyte solution of 0.1 M tetrabutylammo-
nium hexafluorophosphate (Bu4NPF6) in dichloromethane. A
three-electrode cell was used for all experiments. Platinum wires
(Bioanalytical System Inc.) were used as both counter and
working electrodes, and silver/silver ion (Ag in 0.1 M AgNO3
solution, Bioanalytical System Inc.) was used as a reference
electrode. An X-ray diffraction (XRD) pattern was obtained
from an X-ray diffractometer (M18XHF-SRA, McScience)
ꢀ
stirred at 70 C for 48 h, and then 2-thienylboronic acid (4.9
mg) and 2-bromothiophene (3.8 mg) were added in order to
end-cap the polymer chain. After being cooled to room
temperature, the resulting mixture was poured into methanol.
The crude product was filtered through a Soxhlet thimble, and
then subjected to Soxhlet extraction with methanol, hexane,
acetone, and chloroform. The polymer was recovered from the
chloroform fraction, and precipitated into methanol to afford
the product as a purple solid. Yield: 231.1 mg (70%). 1H NMR
(300 MHz, CDCl3): d (ppm) 8.18 (s, 2H), 7.89–7.80 (m, 8H),
7.56 (m, 4H), 7.41 (m, 6H), 2.83 (s, 4H), 2.08 (s, 4H), 1.78 (s,
4H), 1.29–1.13 (m, 44H), 0.89–0.77 (m, 12H). Elemental Anal.
Calcd for C73H88N2S2 (%): C, 83.05; H, 8.33; N, 2.65; S, 6.07.
Found (%): C, 83.01; H, 8.30; N, 2.64; S, 6.05.
ꢀ
ꢁ1
ꢀ
using Cu-Ka radiation (l ¼ 1.5418 A) at a scan rate of 2 min
.
TEM observations were performed on a JEOL JEM1010 at an
accelerating voltage of 80 kV.
Poly(9,9-dioctylfluorene-alt-10,13-bis-(4-octyl-thiophene-2-yl)-
dibenzo[a,c]phenazine) (PFTBPz). PFTBPz was synthesized by
following the same procedure as used in the synthesis of
PFTPQx. Compound 6 (100 mg, 0.21 mmol) and TBPz (173 mg,
Fabrication and characterization of photovoltaic devices
Polymer solar cells were fabricated on ITO glass cleaned by
stepwise sonication in acetone and IPA, followed by O2 plasma
treatment for 10 min. PEDOT : PSS was spin-coated on the ITO
glass at 4000 rpm for 1 min and annealed at 120 ꢀC for 30 min to
yield a 40 nm thick film. A mixture of polymer and PC71BM were
dissolved in anhydrous o-dichlorobenzene (30 mg mlꢁ1), and
spin-coated on the top of the ITO–PEDOT : PSS film at 700–800
rpm for 60 s. The typical thickness of the active layer was 80–90
nm. Lithium fluoride (1 nm) and aluminum (100 nm) were
evaporated, under a vacuum lower than 10ꢁ6 Torr, on the top of
the active layer through a shadow mask. The effective area of the
cell was ca. 4 mm2. The current–voltage (J–V) curves of the
device were obtained on a computer-controlled Keithley 4200
1
0.21 mmol) were used as monomers. Yield: 145.5 mg (63%). H
NMR (300 MHz, CDCl3): d (ppm) 9.49 (br, 2H), 8.42–8.14 (m,
4H), 7.82–7.67 (m, 12H), 2.86–2.74 (m, 4H), 2.17–1.85 (m, 8H),
1.33–1.14 (m, 44H), 0.90–0.77 (m, 12H). Elemental Anal. Calcd
for C73H86N2S2 (%): C, 83.10; H, 8.15; N, 2.65; S, 6.08. Found
(%): C, 83.11; H, 8.16; N, 2.65; S, 6.10.
Poly(N-90-heptadecanyl-2,7-carbazole-alt-5,8-bis-(4-octyl-thio-
phene-2-yl)-2,3-diphenyl-quinoxaline) (PCTPQx). PCTPQx was
synthesized by following the same procedure as used in the
synthesis of PFTPQx. 2,7-Bis(40,40,50,50-tetramethyl-10,30,20-
dioxaborolan-20-yl)-N-900-heptadecanyl-carbazole (7) (206.3 mg,
0.3 mmol) and TPQx (250 mg, 0.3 mmol) were used as mono-
mers. Yield: 234.3 mg (71%). 1H NMR (300 MHz, CDCl3):
d (ppm) 8.20 (m, 4H), 7.94 (s, 2H), 7.83–7.76 (m, 4H), 7.59 (s,
2H), 7.47–7.39 (m, 8H), 4.63 (s, 1H), 2.88 (s, 4H), 2.38 (s, 2H),
2.00–1.81 (m, 6H), 1.43–1.14 (m, 44H), 0.87–0.78 (m, 12H).
Elemental Anal. Calcd for C73H89N3S2 (%): C, 81.80; H, 8.30; N,
3.92; S, 5.98. Found (%): C, 81.79; H, 8.29; N, 3.85; S, 5.97.
source measurement unit under AM 1.5G (100 mW cmꢁ2
)
simulated by an Oriel solar simulator (Oriel 91160A). The light
intensity was calibrated using a NREL-certified photodiode
prior to each measurement. The external quantum efficiency
(EQE) was measured using a Polaronix K3100 IPCE measure-
ment system (McScience). The light intensity at each wavelength
was calibrated with a standard single-crystal Si cell.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 8583–8590 | 8585

View Online
Optical properties
Results and discussion
The absorption spectra of the polymers in chloroform solution
and thin film are shown in Fig. 1a and 1b, respectively. All of the
polymers showed two absorption peaks, which is a common
feature of donor–acceptor type copolymers.19,20 The absorption
peak at shorter wavelength is attributed to the p–p* transition of
the conjugated backbone, while the absorption peak at longer
wavelength corresponds to the intramolecular charge transfer
from the donor to the acceptor. It is noted that the absorption
spectra of TBPz-based polymers are red-shifted compared to
those of TPQx-based polymers. This is because the cyclization of
the two pendent phenyl groups on the Qx unit induces planarity
along the chain backbone and hence enhances the effective
conjugation length, resulting in a lower bandgap. This result is
consistent with other reports that a large bathochromic shift of
the absorption spectrum is observed for the phenanthrene-fused
planar thienopyrazine derivative.21 Particularly, when the
maximum absorption wavelengths of the polymers in solution
are compared with those in solid state (Fig. 1a vs. Fig. 1b), it
reveals that TBPz-based polymers show a pronounced peak
broadening and red shift of absorption edges, indicating that the
polymer chains are highly aggregated in the solid state. The
optical bandgaps (Eg(opt)), as determined from the onset of the
absorption spectra of the TBPz-based polymers, are ꢂ1.7 eV
which is close to the ideal bandgap (1.5 eV)22 of polymer solar
cells, while those of TPQx-based polymers (ꢂ1.9 eV) are not
sufficiently low for wideband absorption of solar photon.
Synthesis and characterization
The synthetic route for synthesis of the monomers and polymers
are shown in Scheme 1 and Scheme 2, respectively. Considering
that the cyclization of the two pendent phenyl rings on the 2 and
3 positions of the Qx unit may influence the properties of the
resulting polymers, two different Qx derivatives (2 and 4 in
Scheme 1) were synthesized and used as a building block for
alternating copolymers. 3-Octylthiophene was attached to both
sides of monomers 2 and 4 via the Stille coupling reaction in
order to enhance the charge mobility and to increase the solu-
bility of both the monomers and resulting copolymers in organic
solvents. Bromination of monomers 3 and 5 with NBS affords
two final monomers, TPQx and TBPz, respectively. A fluorene or
carbazole derivative was coupled to each of Qx derivatives via the
palladium-catalyzed Suzuki coupling reaction to afford alter-
nating copolymers. All the polymers synthesized in this study are
highly soluble in common organic solvents such as THF, chlo-
roform, chlorobenzene, and dichlorobenzene at room tempera-
ture. The number-average molecular weight (Mn) and
polydispersity index (PDI) of the polymers are listed in Table 1.
The molecular weights of TBPz-based polymers (17–22 kg molꢁ1
)
are slightly lower than those of TPQx-based polymers (24–30 kg
molꢁ1). This is probably because TBPz has a more planar
structure which more effectively induces interchain interaction of
the polymer chains, resulting in a lower solubility.
Scheme 2 Synthetic route for alternating copolymers.
This journal is ª The Royal Society of Chemistry 2011
8586 | J. Mater. Chem., 2011, 21, 8583–8590

View Online
Table 1 Characteristics of polymers
Absorption
Polymer
M
n (kg molꢁ1
)
PDI
lmax(CHCl3) (nm)
lmax(film) (nm)
Eg(opt)a (eV)
HOMO (eV)
LUMO (eV)
Eg(ec)b (eV)
PFTPQx
PFTBPz
PCTPQx
PCTBPz
24.1
17.0
30.7
22.4
2.40
2.10
2.78
2.23
362, 490
376, 532
363, 494
378, 538
372, 511
380, 555
373, 515
396, 561
1.93
1.72
1.90
1.65
ꢁ5.28
ꢁ5.29
ꢁ5.25
ꢁ5.24
ꢁ3.33
ꢁ3.43
ꢁ3.30
ꢁ3.42
1.95
1.86
1.95
1.82
Determined from the onset of UV-Vis absorption spectra. b Determined from the cyclic voltammetry.
a
than TPQx-based ones. Since the open circuit voltage (Voc) of
polymer solar cells is linearly dependent on the difference
between the HOMO level of the electron donor and the LUMO
level of the electron acceptor, the low-lying HOMO level of the
donor polymer is expected to afford a high Voc for the resulting
polymer solar cells. Furthermore, considering that a LUMO–
LUMO offset of 0.3–0.4 eV is necessary for efficient electron
transfer from donor polymer to PCBM,23 it is expected that
excitons can be easily dissociated at the interface between the
polymers synthesized in this study and PCBM, because the
LUMO levels of the polymers (ꢁ3.30 to ꢁ3.43 eV) are suffi-
ciently higher than that of PCBM (ꢁ4.0 eV). Judging from the
electrochemical characteristics, it is concluded that the polymers
synthesized in this study are promising donor materials for
photovoltaic devices.
Structural properties
When the molecular organization of the copolymers was exam-
ined by XRD, all the polymers showed two diffraction peaks at
around 6ꢀ and 23ꢀ, as shown in Fig. 3. The weak diffraction peak
around 6ꢀ can be assigned as the interchain distance between
polymer chains, where alkyl groups are expected to self-organize
with an interdigitating manner. The broad peaks around 23ꢀ can
be assigned as the face-to-face (p–p stacking) distance of
aromatic groups in the polymer chain. Particularly, the p–p
stacking distances of the two planar TBPz-based polymers,
ꢀ
ꢀ
PFTBPz and PCTBPz, are 3.79 A and 3.83 A, respectively, while
Fig. 1 UV-Vis absorption spectra of copolymers in chloroform solution
(a) and solid state (b).
Electrochemical properties
The electrochemical data of all the polymers are obtained from
the oxidation and reduction cyclic voltammograms, as shown in
Fig. 2, and are summarized in Table 1. The HOMO energy levels
of the polymers were calculated using the equation: HOMO ¼
ꢁ[Eox ꢁ E1/2(ferrocene) + 4.8] V, where Eox is the onset oxidation
potential of the polymer and E1/2(ferrocene) is the onset oxida-
tion potential of ferrocene vs. Ag/Ag+. The LUMO energy levels
were also estimated by using the equation: LUMO ¼ ꢁ[Ered
ꢁ
E1/2(ferrocene) + 4.8] V, where Ered is the onset reduction
potential of the polymer. All the polymers have similar HOMO
energy levels while the LUMO energy levels of the TBPz-based
polymers are about 0.1 eV lower than those of the TPQx-based
polymers, and thus TBPz-based polymers have lower bandgaps
Fig. 2 Cyclic voltammograms of polymers.
J. Mater. Chem., 2011, 21, 8583–8590 | 8587
This journal is ª The Royal Society of Chemistry 2011

View Online
P3HT-based devices (ꢂ0.60 V), which is in accordance with the
low-lying HOMO levels of our polymers. The Voc of a BHJ solar
cell is governed mostly by the difference between the LUMO
level of the acceptor and the HOMO level of the donor, although
other factors should also be taken into account, such as geminate
recombination and resistance due to the thickness and
morphology of the active layer. The devices of PCTBPz and
PFTBPz exhibit PCEs of 3.80% and 2.61%, respectively, while
their TPQx-based counterparts, PCTPQx and PFTPQx, show
PCEs of 1.85% and 1.57%, respectively. The difference in PCE
between the two types of polymers is attributed largely to the
difference in the short-circuit current density (Jsc): The Jscs of the
TBPz-based devices are higher than those of the TPQx-based
devices. The Jsc difference may arise from two sources: first,
TBPz-based polymers have a broader absorption of the solar
spectrum than TPQx-based polymers, as shown in Fig. 1.
Secondly, TBPz-based polymers may have a higher charge
mobility than those of TPQx-based polymers due to an enhanced
p–p stacking.
Fig. 3 XRD diffractograms of polymer thin films.
those of the less planar TPQx-based polymers, PFTPQx and
To quantitatively investigate the effect of interchain packing
on the charge transport property, hole mobility was measured
from the space charge limited current (SCLC) J–V curves as
obtained in the dark using hole-only devices (ITO–PEDOT :
PSS–polymer or polymer : PC71BM–Au). The SCLC behavior is
analyzed using the Mott–Gurney law:24 J ¼ (9/8)3m(V2Lꢁ3),
where 3 is the static dielectric constant of the medium, m is the
charge carrier mobility, V ¼ Va ꢁ Vbi (Va, the applied bias; Vbi,
the built-in potential due to the difference in electrical contact
work functions), and L is the layer thickness. The mobility is
obtained from the slope of the plot of J1/2 vs. V (Fig. 6). When the
hole mobilities of pure polymers are compared, it has been
observed that hole mobilities of the two TBPz-based polymers,
PCTBPz and PFTBPz (4.13 ꢃ 10ꢁ4 cm2 Vꢁ1 s and 3.02 ꢃ 10ꢁ4
cm2 Vꢁ1 s, respectively), are higher than those of the two TPQx-
ꢀ
ꢀ
PCTPQx, are 3.97 A and 4.01 A, respectively. The decrease in the
p–p stacking distance of the TBPz-based polymers is attributed
to the cyclization of the two pendent phenyl groups which makes
the polymer backbone more planar and hence facilitates close-
packing of polymer chains.
The effect of cyclization of the Qx derivative on the structural
property of the copolymers is further examined by comparing the
optimized molecular geometries of the copolymers obtained
from calculation of the density functional theory. To simplify the
calculation, only one repeating unit of each polymer was sub-
jected to the calculation, and all alkyl chains are replaced by
methyl groups. When the dihedral angles between the pendent
phenyl groups and Qx unit are calculated, as shown in Fig. 4,
a highly twisted molecular conformation, with dihedral angles of
around 40ꢀ, is observed for PCTPQx, whereas these angles must
be null for PCTBPz. The twisted conformation due to the
pendent phenyl groups is expected to disrupt the close-packing of
the polymer chains, causing the p–p stacking distances of TPQx-
based polymers to increase.
based polymers, PCTPQx and PFTPQx (7.26 ꢃ 10ꢁ5 cm2 Vꢁ1
s
and 5.87 ꢃ 10ꢁ5 cm2 Vꢁ1 s, respectively). The enhancement of
hole mobility for TBPz-based polymers may arise from the
denser packing of polymers with the more planar Qx derivative,
as evidenced from XRD analysis (Fig. 3). Likewise, higher hole
mobility was also observed for the blends of TBPz-based
Photovoltaic properties
The polymer solar cells were fabricated with a layered configu-
ration of glass–ITO–PEDOT : PSS– polymer : PC71BM–LiF–
Al. The J–V characteristics of photovoltaic devices prepared
from polymer : PC71BM (1 : 3 w/w) using o-dichlorobenzene as
a processing solvent are represented in Fig. 5, and their photo-
voltaic parameters are summarized in Table 2. It should be noted
here that the Vocs of all the devices are higher than those of
Fig. 4 Optimized molecular geometries of repeating unit of PCTPQx (a)
Fig. 5 Current–voltage (J–V) characteristics of polymer : PC71BM (1 : 3
and PCTBPz (b).
w/w) BHJ solar cells under AM 1.5 condition.
8588 | J. Mater. Chem., 2011, 21, 8583–8590
This journal is ª The Royal Society of Chemistry 2011

View Online
Table 2 Photovoltaic parameters of devices tested under standard AM
1.5G condition
Polymer : PC71BM
(1 : 3 w/w)
Jsc
(mA cmꢁ2
Voc (V)
)
FF
PCE (%)
PFTPQx
PFTBPz
PCTPQx
PCTBPz
0.87
0.84
0.82
0.81
4.39
7.06
5.38
9.38
0.41
0.44
0.42
0.50
1.57
2.61
1.85
3.80
Fig. 7 External quantum efficiency spectra of polymer–PCBM solar
cells.
of TBPz-based polymer blends are decreased by 1 order of
magnitude compared to those of pure polymers. This is probably
because the morphologies of the blend films show phase sepa-
ration with very small and discontinuous domains to some extent
(Fig. S9†).
The external quantum efficiency (EQE) of the devices under
monochromatic light is shown in Fig. 7. Compared with the
TPQx-based polymers, the TBPz-based polymers show higher
EQE values and a broader spectral response in the range of 300–
760 nm. The device from PCTBPz : PC71BM shows a consider-
ably high incident photon to current efficiency of 65% at 480 nm.
When the Jsc value was calculated from integration of the EQE
spectrum of PCTBPz : PC71BM device, it was 8.95 mA cmꢁ2
which is close to the Jsc value (9.38 mA cmꢁ2) obtained from the
J–V measurement (Fig. 5). Further device optimization such as
interface engineering between the active layer and cathode, and
control of film morphology using a mixed solvent is needed to
improve the device performance.
Conclusions
We have synthesized a series of D–A low-bandgap alternating
copolymers composed of electron-donating carbazole or fluorene
and electron-accepting quinoxaline derivatives by the palladium-
catalyzed Suzuki coupling reaction. It has been found that the
optical, structural, and photovoltaic properties of the copoly-
mers are strongly dependent upon the planarity of the building
block for the alternating copolymers. The copolymers with
planar quinoxaline derivatives (TBPz) have a lower bandgap and
higher hole mobility than the corresponding copolymers with less
planar quinoxaline derivatives (TPQx), mainly due to the
enhanced intermolecular packing characteristics of TBPz-based
copolymers in the solid state. A blend of PCTBPz and PC71BM
exhibits a PCE of 3.80% with an open-circuit voltage of 0.81 V,
a short-circuit current density of 9.38 mA cmꢁ2, and a fill factor
of 50% under AM 1.5 condition (100 mW cmꢁ2). This result
demonstrates that a new planar quinoxaline derivative, TBPz, is
a promising building block that can be combined with many
other electron-rich units to develop D–A type alternating
copolymers for achieving high performance solar cells.
Fig. 6 Dark J–V characteristics of polymer (a) and polymer : PC71BM
(b) with hole-only devices. The solid lines are the best linear fit of the data
points.
polymers and PC71BM compared with the blends of TPQx-based
polymers and PC71BM :PCTBPz, 3.51 ꢃ 10ꢁ5 cm2 Vꢁ1 s vs.
PCTPQx, 1.27 ꢃ 10ꢁ5 cm2 Vꢁ1 s; PFTBPz, 2.65 ꢃ 10ꢁ5 cm2 Vꢁ1
s
vs. PFTPQx, 1.13 ꢃ 10ꢁ5 cm2 Vꢁ1 s. However, the hole mobilities
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 8583–8590 | 8589

View Online
10 D. Kitazawa, N. Watanabe, S. Yamamoto and J. Tsukamoto, Appl.
Phys. Lett., 2009, 95, 053701.
Acknowledgements
11 E. Zhou, J. Cong, K. Tajima and K. Hashimoto, Chem. Mater., 2010,
22, 4890.
The authors thank the Ministry of Education, Science and
Technology (MEST), Korea for financial support through the
Global Research Laboratory (GRL) and the World Class
University (WCU) programs.
€
€
12 E. Wang, L. Hou, Z. Wang, S. Hellstrom, F. Zhang, O. Inganas and
M. R. Andersson, Adv. Mater., 2010, 22, 5240.
13 M.-H. Lai, C.-C. Chueh, W.-C. Chen, J.-L. Wu and F.-C. Chen, J.
Polym. Sci., Part A: Polym. Chem., 2009, 47, 973.
14 J.-H. Tsai, C.-C. Chueh, M.-H. Lai, C.-F. Wang, W.-C. Chen,
B.-T. Ko and C. Ting, Macromolecules, 2009, 42, 1897.
15 Y.-L. Yang, Y.-H. Lee, C.-J. Chang, A.-J. Lu, W.-C. Hsu, L. Wang,
M.-K. Leung and C.-A. Dai, J. Polym. Sci., Part A: Polym. Chem.,
2010, 48, 1607.
16 E. Bundgaard and F. C. Krebs, Macromolecules, 2006, 39, 2823.
17 M. J. Edelmann, J.-M. Raimundo, N. F. Utesch and F. Diederich,
Helv. Chim. Acta, 2002, 85, 2195.
18 N. Blouin, A. Michaud and M. Leclerc, Adv. Mater., 2007, 19, 2295.
19 J. Yang, Q. Hou, W. Yang, C. Zhang and Y. Cao, Macromolecules,
2005, 38, 244.
20 C. Kanimozhi, P. Balraju, G. D. Sharma and S. Patil, J. Phys. Chem.
B, 2010, 114, 3095.
21 M. H. Petersen, O. Hagemann, K. T. Nielsen, M. Jørgensen and
F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2007, 91, 996.
22 M. C. Scharber, D. Muhlbacher, M. Koppe, P. Denk, C. Waldauf,
A. J. Heeger and C. J. Brabec, Adv. Funct. Mater., 2006, 18, 789.
23 A. P. Zoombelt, M. Fonrodona, M. G. R. Turbiez, M. M. Wienk and
R. A. J. Janssen, J. Mater. Chem., 2009, 19, 5336.
References
1 F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2009, 93, 394.
2 E. Bundgaard, O. Hagemann, M. Manceau, M. Jørgensen and
F. C. Krebs, Macromolecules, 2010, 43, 8115.
3 H.-Y. Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G. W. Yang,
Y. Yang, L. P. Yu, Y. Wu and G. Li, Nat. Photonics, 2009, 3, 649.
4 Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray and L. Yu,
Adv. Mater., 2010, 22, E135.
5 F. C. Tsai, C. C. Chang, C. L. Liu, W. C. Chen and S. A. Jenekhe,
Macromolecules, 2005, 38, 1958.
6 N. Blouin, A. Michaud, D. Gendron, S. Wakim, E. Blair, R. Neagu-
Plesu, A. Belletete, G. Durocher, Y. Tao and M. Leclerc, J. Am.
Chem. Soc., 2008, 130, 732.
7 Y. Lee, T. P. Russell and W. H. Jo, Org. Electron., 2010, 11, 846.
€
8 A. Tsimi, T. W. Bunnagel, T. Farrell, M. Scharber, S. A. Choulis,
C. J. Brabec and U. Scherf, J. Mater. Chem., 2007, 17, 1353.
9 J.-Y. Lee, W.-S. Shin, J.-R. Haw and D. K. Moon, J. Mater. Chem.,
2009, 19, 4938.
24 P. N. Murgatroyd, J. Phys. D: Appl. Phys., 1970, 3, 151.
8590 | J. Mater. Chem., 2011, 21, 8583–8590
This journal is ª The Royal Society of Chemistry 2011