Pyrazino[2,3-g]quinoxaline-based conjugated copolymers with indolocarbazole coplanar moieties designed for efficient photovoltaic applications

Article abstract of DOI:10.1039/c1jm10433k

A series of low band gap copolymers consisting of electron-accepting pyrazino[2,3-g]quinoxaline (PQx) and an electron-donating indolo[3,2-b]carbazole and thiophene units have been designed and synthesized by Stille coupling polymerization. Their optical and electrical properties could also be facilely fine-modulated for photovoltaic application by adjusting the donor/acceptor ratios. UV-vis measurements showed that increasing the content of PQx units led to enhanced absorption. The band gaps obtained from UV-vis spectra, CV scanning, and DFT modeling all indicated a narrowing band gap with increasing the PQx content in the copolymer structure. The photovoltaic solar cells (PSCs) based on these copolymers were fabricated and tested with a structure of ITO/PEDOT:PSS/copolymer:PCBM/Ca/Al under the illumination of AM 1.5G, 100 mW cm^-12. The best performance was achieved using P3/[70]PCBM blend (1:3) with J_sc = 9.55 mA cm^-2, V_oc = 0.81 V, FF = 0.42, and PCE = 3.24%, which is the highest efficiency for the PQx and indolo[3,2-b]carbazole based devices. The present results also indicate that the efficient photovoltaic materials with suitable electronic and optical properties can be achieved by just fine-tuning the ratios of the strong electron-deficient accepters and large-π planar donors.

Full text of DOI:10.1039/c1jm10433k

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PAPER
Pyrazino[2,3-g]quinoxaline-based conjugated copolymers with
indolocarbazole coplanar moieties designed for efficient photovoltaic
applications†
a
a
a
a
a
Qiang Peng, Xiangju Liu, Yuancheng Qin, Jun Xu, Mingjun Li and Liming Dai
b
*
*
Received 28th January 2011, Accepted 20th March 2011
DOI: 10.1039/c1jm10433k
A series of low band gap copolymers consisting of electron-accepting pyrazino[2,3-g]quinoxaline (PQx)
and an electron-donating indolo[3,2-b]carbazole and thiophene units have been designed and
synthesized by Stille coupling polymerization. Their optical and electrical properties could also be
facilely fine-modulated for photovoltaic application by adjusting the donor/acceptor ratios. UV-vis
measurements showed that increasing the content of PQx units led to enhanced absorption. The band
gaps obtained from UV-vis spectra, CV scanning, and DFT modeling all indicated a narrowing band
gap with increasing the PQx content in the copolymer structure. The photovoltaic solar cells (PSCs)
based on these copolymers were fabricated and tested with a structure of ITO/PEDOT:PSS/copolymer:
PCBM/Ca/Al under the illumination of AM 1.5G, 100 mW cmꢀ12. The best performance was achieved
using P3/[70]PCBM blend (1 : 3) with Jsc ¼ 9.55 mA cmꢀ2, Voc ¼ 0.81 V, FF ¼ 0.42, and PCE ¼ 3.24%,
which is the highest efficiency for the PQx and indolo[3,2-b]carbazole based devices. The present results
also indicate that the efficient photovoltaic materials with suitable electronic and optical properties can
be achieved by just fine-tuning the ratios of the strong electron-deficient accepters and large-p planar
donors.
In general, the power conversion efficiency (PCE) relies on the
open-circuit voltage (Voc), the short-circuit current density (Jsc),
and the fill factor (FF) of the devices. In PVCs, Jsc is controlled
by the matchment between the absorption of the conjugated
polymers and the solar spectrum.12 The Voc is determined by the
difference between the energy levels of lowest unoccupied
molecular orbital (LUMO) of the fullerene derivatives (as elec-
tron acceptor, A) and the highest occupied molecular orbital
(HOMO) of conjugated polymers (as electron donor, D).13
Therefore, further improvement in the PCE demands the devel-
opment of novel conjugated polymers with appropriate energy
levels and broader absorption with the solar spectrum. On the
other hand, the high charge carrier mobility of polymer semi-
conductors should also be taken into account.14
Introduction
The sunlight is a clean, abundant and virtually limitless energy
source which can be used to address the growing global energy
needs. Recently, polymer photovoltaic cells (PVCs) have received
much attention because of their flexibility, facile processibility,
low weight and low production cost.1–5 Since the 1990s, poly-
(p-phenylene vinylene)s (PPVs) and the polythiophenes (PTs)
were most studied conjugated polymers with high power
conversion efficiencies (PCE).6,7 However, their relatively large
bandgaps limited the short circuit current (Jsc), reducing the
PCE. In order to further improve the efficiency, low band gap
conjugated polymers were developed to better match the solar
spectrum and thereby produce higher Jsc.8,9 Great progress has
been made in PVCs based on bulk-heterojunction (BHJ)
networks made of low band gap conjugated polymers and
fullerene derivatives in the past few years, and efficiency as high
as more than 7% has been reported.10,11
The powerful strategy to achieve a low band gap conjugated
polymer is to incorporate alternating donor (D) and acceptor (A)
moieties segments in the polymer main-chain. Base on this type
of push-pull structure, efficient photoinduced intramolecular
charge transfer (ICT) can easily take place from the donor to the
acceptor upon photoexcitation and generate an absorption band
at the lower energy.15 Very recently, series of conjugated poly-
mers containing quinoxaline (Qx) derivatives were reported for
the production of low band gap conjugated polymers for efficient
PVCs.16–20 Qx heterocycles were chosen as electron-acceptor
segments in these work because of their high electron affinity
attributed to symmetric unsaturated imine nitrogen (C]N)
aSchool of Environmental and Chemical Engineering, Nanchang Hangkong
University, Nanchang, 330063, P. R. China. E-mail: qiangpengjohnny@
yahoo.com; Fax: +86-0791-3953369; Tel: +86-0791-3953369
bDepartment of Chemical Engineering, Case Western Reserve University,
Cleveland, Ohio, 44106
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1jm10433k
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structures.15 In comparison with Qx, the pyrazino[2,3-g]qui-
noxaline (PQx) unit is relatively more electron-deficient and
more rigid. Therefore, replacing Qx units in conjugated polymers
with PQx moieties can give rise to the corresponding polymers
with lower band gap, higher ionization potential (IP) and more
rigid structures. Up to now, only a few molecular or polymeric
materials based on PQx were explored for PVCs.21,22 In addition,
the introduction of a planar p–p system in the polymer back-
bone allows the polymer to form long-range intermolecular p–p
stacking arrangements, which can lead to a high charge carrier
mobility. Therefore, a new series of copolymers consisting of an
electron-accepting pyrazino[2,3-g]quinoxaline (PQx) and an
electron-donating coplanar indolo[3,2-b]carbazole and thio-
phene units have been designed (Scheme 1). Recently, Krebs and
co-workers studied the stability of carbazole units and generally
found the N-alkyl bond as a weak point in air.23,24 However,
indolo[3,2-b]carbazole derivatives showed relatively high hole
mobilities of more than 0.02 cm2 Vꢀ1 sꢀ1 in organic field-effect
transistors (OFETs).25,26 Also, low band gap conjugated
copolymers of indolo[3,2-b]carbazole with benzothiadiazole (BT)
exhibited the high power conversion efficiency in PVCs.27,28 So, it
is reasonable to expect that these polymers would have narrower
band gaps and higher charge mobility. Their optical and elec-
trical properties also can be facilely fine-modulated by adjusting
the donor/acceptor ratios.29,30
Results and discussion
Synthesis and characterization
The synthetic route of the monomers and copolymers is outlined
in Scheme 1. Bromination of 2,1,3-benzothiadiazole with Br2 and
HBr (47%) gave 4,7-dibromo-2,1,3-benzothiadiazole (1),28 which
was subsequently nitrificated a 1 : 1 (vol/vol) of fuming nitric
acid and fuming sulphuric acid to afford 4,7-dibromo-5,6-
dinitro-2,1,3-benzothiadiazole (2) in 28% yield after recrystalli-
zation in ethanol.31 The Stille coupling reaction between
compound 2 and 5-tri-n-butylstannyl-3-n-hexylthiophene in the
presence of PdCl2(PPh3)2 afforded 4,7-di-(3-n-hexylthiophen-2-
yl)-5,6-dinitro-2,1,3-benzothiadiazole (3) in 77% yield. 2,3,7,
8-Tetraethyl-5,10-di-(3-n-hexylthiophen-2-yl)-pyrazino[2,3-g]-
quinoxaline (4) was then synthesized in 56% yield using a one-pot
reaction. 3 was first reduced to its corresponding diamine by
heating at 60 ꢁC in acetic acid with a excess of zinc powder
without separation. After 1.5 h, 3,4-hexanedione was added to
above reactant solution and condensed with the diamine deri-
vative to yield 4. Monomer 5 was finally prepared by dibromi-
nation of 5 with N-bromosuccinimide (NBS) in a 1 : 1 (vol/vol)
mixture of acetic acid and chloroform in 79% yield. 3,9-
Dibromo-6-hexyl-5,11-dihydroindolo[3,2-b]carbazole (6) was
synthesized in one-pot method by the reaction of 5-bromoindole
with n-heptaldehyde using iodine as a catalyst and subsequently
Scheme 1 Synthetic route for monomers and random conjugated copolymers.
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treated with triethyl orthoformate in methanol in the prescence
of a catalyst amount of methanesulfonic.32 The N-hexylation
reaction of 6 was carried out in a DMF solution with n-bro-
mohexane using sodium hydride as an alkali, which was used to
give a high solubility to the monomer 7 containing indolo[3,2-b]
carbazole segment. Random copolymers P1–P4 that consist of
indolo[3,2-b]carbazole and thiophene as electron-donating units
and pyrazino[2,3-g]quinoxaline (PQx) as electron-accepting
moieties were synthesized by Pd(0)-catalyzed Stille coupling
polymerization in chlorobenzene, where the monomer 5 reacted
with 2,5-bis(tributylstannyl) thiophene and 7 at different molar
ratios (see Experimental). The structures of the copolymers were
confirmed with NMR spectroscopy and elemental analysis.
All of the copolymers show good solubility at room temper-
ature in organic solvents such as chloroform, tetrahydrofuran,
toluene, xylene and chlorobenzene. The incorporation of hexyl
or ethyl side chains on indolo[3,2-b]carbazole, thiophene and
PQx segments enables them to have a good solubility. The
weight-average molecular weight (Mw) and polydispersity index
(Mw/Mn) were measured by gel permeation chromatography
(GPC) using THF as the eluent and polystyrenes as the internal
standards. The results showed that P1–P4 have a Mn of 7200–
8500 with polydispersity indices of 1.76–1.98 (Table 1). More-
over, the resulting copolymers exhibited a good thermal stability
with decompositiꢁon onset temperatures (Td) (about 5% weight-
loss) of 290–343 C as measured by thermogravimetric analysis
(TGA). The glass transition temperatures (Tg) of these copoly-
mers were also investigated by differential scanning calorimetry
(DSC), and the glass transition temperatures were determined to
be 72–90 ꢁC, respectively. The combinations of such good
thermal properties are adequately suitable for PVCs and other
optoelectronic devices.
Fig. 1 Optical absorption spectra of P1–P4 in THF solution (a) and thin
film on glass (b).
Optical properties
wavelength range of 350–450 nm. This absorbance as well as the
identical absorption from benzic cycle (ca. 300 nm) enhanced
from P1 to P4 indicating the increasing the content of
indolo[3,2-b]carbazole moieties in the polymer chains, which
agreed with the molar ratios (see Experimental).
All electronic absorption properties of the obtained copolymers
were measured both in THF solutions (Fig. 1a) and thin films on
quartz slides (Fig. 1b). The spectroscopic data are also listed in
Table 2. As shown in Fig. 1a, all absorption spectra of these
copolymers in THF solutions have one broad distinct peak in the
wavelength range of 450–1000 nm, arising from the n–p* tran-
sition from PQx heterocyclic units, which redshifted with the
increase of PQx content. This indicated that better absorbance
could be achieved with incorporation of more PQx units.
Another peak attributed to p–p* electronic transitions of the
conjugated polymer backbones were also observed in the
A similar behavior was observed for the absorption spectra of
these copolymers in thin solid films (Fig. 1b). As compared to
their counterparts in the solution state, the broad peaks in the
wavelength range of 450–1000 nm showed different red-shifts
from 7, 16, 29, 34 nm for P1, P2, P3 and P4, respectively. The
reason can be explained by the formation of p-stacked structures
in the solid state30 which could facilitate charge transportation
for photovoltaic applications. The level of red-shift obviously
relied on the content of cofacial indolo[3,2-b]carbazole groups.
The energy band gaps calculated from the absorption band edges
of the optical absorption spectra were about 1.31, 1.38, 1.47, and
1.51 eV for polymers P1–P4, respectively, indicating an effective
narrowing in the band gap energy by increasing the PQx content
in the polymer backbone.
Table 1 Molecular weights and thermal properties of the copolymers
Copolymers Yield (%) Mn (ꢂ 103) PDIa (Mw/Mn) Td (ꢁC) Tg (ꢁC)
a
b
c
P1
P2
P3
P4
58
60
61
63
7.6
8.0
8.5
7.9
1.85
1.92
1.98
1.86
320
343
305
301
72
75
77
85
a
Molecular weights and polydispersity indices were determined by GPC
Electrochemical properties
b
in THF using polystyrene as standards.
temperature measured by TGA under N2.
temperature measured by DSC under N2.
Onset decomposition
c
Glass transition
The redox behavior of the copolymer films on Pt plate electrodes
was studied by cyclic voltammetry (CV), which is used to
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Table 2 Optical and electrochemical data of the copolymers
b
Eg (eV)
Ega (eV)
E (V)/HOMO (eV)
E (V)/LUMO (eV)
THF
Abs. (nm) l
max
film
max
Copolymers
Abs. (nm) l
P1
P2
P3
P4
381, 588
378, 574
374, 551
372, 541
409, 595
401, 590
383, 580
379, 575
1.31
1.38
1.47
1.51
0.62/ꢀ5.02
0.70/ꢀ5.10
0.74/ꢀ5.14
0.75/ꢀ5.15
ꢀ0.67/ꢀ3.73
ꢀ0.70/ꢀ3.70
ꢀ0.76/ꢀ3.64
ꢀ0.89/ꢀ3.51
1.29
1.40
1.50
1.64
Optical band gap was estimated from the wavelength of the optical absorption edge of the copolymer film. b Electrochemical band gap was calculated
a
from the LUMO and HOMO energy levels.
determine the energy levels of the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO). The CV measurements were performed using a three-
electrode cell in an anhydrous CH3CN solution of 0.1 M tetra-
butylammonium perchlorate (n-Bu4NClO4). A Pt wire was used
as the counter electrode and Ag/AgCl (0.1 M) as the reference
electrode. The energy level of the Ag/AgCl reference electrode
(calibrated against the FC/FC+ redox system) was 4.40 eV below
the vacuum level.33 The cell was purged with pure argon prior to
each scan. The scans toward the anodic and cathodic directions
were performed separately at a scan rate of 50 mV sꢀ1 at room
temperature. As shown in Fig. 2, all the copolymers show good
reversibility in the n-doping processes due to the electron-rich
indolo[3,2-b]carbazole and thiophene units and p-doping
processes due to the electron-poor pyrazino[2,3-g]quinoxaline
units.
close to those obtained by the optical method described above.
The electrochemical data of the copolymers are summarized in
Table 2.
Theoretical calculations
DFT/B3LYP/6-31G* has been found to be an accurate method
for calculating the optimal geometry and electronic structures of
many molecular systems. To gain insight into the nature of the
excited states of the copolymers, we performed density functional
theory (DFT) calculations at the B3LYP/6-31G* level on a D–A
model compounds and optimized their geometrical structures
followed by calculation of the lowest vertical excitations using
the Gaussian 09 program suite.34 The alkyl groups were not
included in the calculation because they do not significantly
affect the equilibrium geometry and the electronic properties.
Fig. S1† shows the molecular structures and molecular orbitals
(HOMO and LUMO) of the model compounds. Ab initio
calculations on the model compounds for the resulting copoly-
mers show that they are planar molecules, which enable the
electrons to be delocalized within the molecule systems by
conjugation to get higher carrier mobility.35 As shown in
Fig. S1,† the results indicate that the electron density of LUMO
is mainly localized on the pyrazino[2,3-g]quinoxaline unit, while
the electron density of HOMO is distributed over the entire
conjugated molecule. Calculated results show that the D–A
model molecules have HOMO energy levels from ꢀ4.75 eV to
ꢀ4.46 eV and a LUMO energy level from ꢀ2.98 eV to ꢀ3.22 eV.
The band gap is determined to be about 1.77 eV to 1.24 eV.
Obviously, the HOMO energy levels increase with the content of
pyrazino[2,3-g]quinoxaline units, however, the LUMO energy
levels and band gaps decrease. The results are in good agreement
with those obtained by CV and UV-vis measurements. However,
these values from calculations are somewhat difference
compared to experimental data due to the part extended p-
conjugation system or just several repeat sections chosen from
copolymers. Anyway, these calculated values for LUMO,
HOMO levels and band gaps of model compounds are still
important to predict the trend of experimental results allowing
a rapid screening of the most promising polymeric structures.
On anodic sweep, the onset potentials of the p-doping process
for polymer P1, P2, P3 and P4 are determined to be 0.62, 0.70,
0.74, and 0.75 V, which correspond to a HOMO energy level
(EHOMO) of 5.02, 5.10, 5.14 and 5.15 eV, respectively. Obviously,
the increase in oxidation potentials of these copolymers can be
attributed to the higher content of indolo[3,2-b]carbazole units
incorporated into the polymer main chains. On sweeping the
polymers cathodically, the onset potentials of the n-doping
process for P1, P2, P3 and P4 occur at about ꢀ0.67, ꢀ0.70,
ꢀ0.77 and ꢀ0.89 V. The lowest unoccupied molecular orbital
(LUMO) energy levels of the corresponding copolymers are
estimated to be ꢀ3.73, ꢀ3.70, ꢀ3.64 and ꢀ3.51 eV, respectively.
From the onset potentials of the oxidation and reduction
processes, the band gaps (Eg) of the P1, P2, P3 and P4 were
calculated to be about 1.31, 1.38,1.47 and 1.51 eV. The values are
Photovoltaic properties
As described above, the resulting coplanar copolymers are
promising materials for polymer photovoltaic applications. The
bulk-heterojunction (BHJ) polymer photovoltaic cells (PVCs)
were fabricated using a similar method to our previous work19,30
with a conventional sandwich structure of ITO/PEDOT:PSS/
Fig. 2 Cyclic voltammograms of the copolymers P1–P4.
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active layer/Ca/Al by spin-coating from a chlorobenzene solution
of copolymer/PCBM blend as the active layer. The devices were
optimized by changing some conditions, such as the ratio of the
copolymer and PCBM, the thickness of active layer, the thermal
annealing temperature. For this work, using Ca/Al as the
cathode and annealing the blend films at 110 ꢁC for 5 min
afforded the best device performance. Better fabrication condi-
tions were obtained by spin-coating from a 10 mg mLꢀ1 chlo-
robenzene solution of copolymer/[60]PCBM ratio of 1 : 3 (w/w).
Current–voltage (I–V) curves of the PVCs based on P1–P4 are
also shown in Fig. 3a. The open-circuit voltage (Voc), short-
circuit current (Jsc), fill factor (FF), and power conversion effi-
ciency (PCE) values of these devices under the illumination of
AM1.5 (100 mW cmꢀ2) were measured to be 0.70 V, 3.18 mA
cmꢀ2, 0.41, and 0.91% for P1; 0.79 V, 4.09 mA cmꢀ2, 0.41 and
1.32% for P2; 0.83 V, 5.70 mA cmꢀ2, 0.45 and 2.13% for P3; and
0.82 V, 5.16 mA cmꢀ2, 0.42, and 1.78% for P4, respectively. Table
3 summaries the corresponding data of the devices. Obviously,
increasing the content of indolo[3,2-b]carbazole units in the
polymer main chains has enhanced the efficiency of PVCs. After
the ratio reaching to some extent of 0.7 : 0.3, the carrier mobility
may not play a preponderant role compared to the absorption
and led to decreased efficiency. Anyway, considering the rela-
tively low FF of these devices (0.41–0.45), there is a plenty room
for future improvement in performance of PVCs based on the
resulting copolymer/PCBM system.
Fig. 3b shows the external quantum efficiency (EQE) curves of
the optimized PVCs with copolymer:[60]PCBM under the illu-
mination of monochromatic light. As shown in Fig. 3b, the shape
of the EQE curves is similar to the absorption spectra, indicating
that all the absorption wavelengths of the polymers contributed
to photocurrent generation. That is to say the excitons are mainly
generated in copolymer phases. However, the broader profile and
high values of EQE were observed which consistent with the high
JSC measured in these PVCs.
[70]PCBM has similar electronic properties as [60]PCBM, but
a relatively higher absorption coefficient in the visible region,36
which can be used to compensate for the poor absorption of
copolymer/[60]PCBM blend in the range of 400–500 nm. For the
purpose of direct comparison, the copolymer [70]PCBM based
PVCs were further fabricated to get better efficiencies instead of
[60]PCBM. To understand the role of the P3/[70]PCBM blend
composition on the photovoltaic properties, PVCs with P3:[70]
PCBM ratios of 1 : 1, 1 : 2, 1 : 3, and 1 : 4 were fabricated and
measured. All the photovoltaic parameters including Jsc, Voc,
FF, and PCE for these devices are also summarized in Table 3.
The I–V characteristics of the devices from P3/[70]PCBM are
described in Fig. 4a. The Jsc first increased from 4.66 mA cmꢀ2 at
a P3/[70]PCBM ratio of 1 : 1 to 9.55 mA cmꢀ2 at 1 : 1 compo-
sition, but decreased to 6.48 mA cmꢀ2 at a P3/[70]PCBM ratio of
1 : 4. The PVC from P3/[70]PCBM ¼1 : 3 has the highest PCE of
3.24% under the illumination of AM1.5 (100 mW cmꢀ2), with
a Jsc of 9.55 mA cmꢀ2, an Voc of 0.81 V, and an FF of 0.42. The
efficiency increased significantly in comparison with that
obtained from [60]PCBM based cells, which was mainly attrib-
uted to the enhanced photoresponse in the visible region. As
shown in Fig. 4b, it is obvious that the valley in the range of 400–
500 nm was efficiently compensated for and gives rise to higher
Jsc. The enhanced absorption and thus improved light harvesting
in the PVCs using [70]PCBM is also consistent with the observed
enhanced device efficiency. The Voc in these four devices changed
just a little without improvement. Generally, the Voc in polymer/
PCBM BHJ solar cells is related to the energy difference between
the HOMO energy level of the donor and the LUMO energy level
of the acceptor. So the obtained values in this work have almost
achieved the limits for the Voc. The FF reflects the fraction of
photogenerated charge carriers that reached the electrodes.
However, the FFs in this work are not so high. Nevertheless,
further work to identify the optimized electrodes and device
performance is still ongoing.
Conclusions
A new series of low band gap copolymers based on electron-
accepting pyrazino[2,3-g]quinoxaline (PQx) and an coplanar
electron-donating indolo[3,2-b]carbazole and thiophene units
have been synthesized by Stille coupling polymerization. Their
optical and electrical properties can be facilely fine-modulated by
adjusting the donor/acceptor ratios. All these copolymers exhibit
good light conversion efficiency with low band gaps. UV-vis
spectra showed that increasing the number of PQx units led to
Fig. 3 (a) Current–voltage curves of photovoltaic cells based on
copolymers together with [60]PCBM under AM 1.5 illumination, (b)
EQE curves of photovoltaic cells based on copolymers together with [60]
PCBM.
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Table 3 Characteristic current–voltage parameters from device testing at standard AM 1.5G conditions
Copolymers
copolymer/PCBM (w/w)
VOC (V)
JSC (mA cmꢀ2
)
FF
PCE (%)
P1
P2
P3
P3
P3
P3
P3
P4
1 : 3a
1 : 3a
1 : 3a
1 : 1b
1 : 2b
1 : 3b
1 : 4b
1 : 3a
0.70
0.79
0.83
0.78
0.83
0.81
0.80
0.82
3.18
4.09
5.70
4.66
6.84
9.55
6.48
5.16
0.41
0.41
0.45
0.44
0.44
0.43
0.45
0.42
0.91
1.32
2.13
1.60
2.52
3.24
2.36
1.78
a
Using [60]PCBM as acceptors. b Using [70]PCBM as acceptors.
of 0.7 : 0.3, the efficiency would decrease because the carrier
mobility does not play a preponderant role compared to the
absorption. The best performing copolymer in a device using [70]
PCBM as accepters was P3 with Jsc ¼ 9.55 mA cmꢀ2, Voc
¼
0.81 V, FF ¼ 0.42, and PCE ¼ 3.24%, which is the highest effi-
ciency for the PQx and indolo[3,2-b]carbazole based on PVCs.
The present results indicate that the efficient photovoltaic
materials with suitable electronic and optical properties can be
achieved by just fine-tuning the ratios of the strong electron-
deficient accepters and large-p planar donors.
Experimental
Materials and instruments
NMR spectra were recorded on a Bruker Avance-400 spec-
trometer with d-chloroform as the solvent and tetramethylsilane
as the internal standard. Cyclic voltammetry measurements were
made on an CHI660 potentiostat/galvanostat electrochemical
workstation at a scan rate of 50 mV sꢀ1, with a platinum wire
counter electrode and a Ag/AgCl reference electrode in an
anhydrous and nitrogen-saturated 0.1 mol Lꢀ1 acetonitrile
(CH3CN) solution of tetrabutylammonium perchlorate (Bu4N-
ClO4). The copolymers were coated on the platinum plate
working electrodes from dilute chloroform solutions. UV-vis
spectra were obtained on a Carry 300 spectrophotometer.
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurements were conducted on a TA
Instrument Model SDT Q600 simultaneous TGA/DSC analyzer
at a heating rate of 10 ꢁC minꢀ1 and under a N2 flow rate of
90 mL minꢀ1. Polymer photovoltaic cells were fabricated with
ITO glass as an anode, Ca/Al as a cathode, and the blend film of
the copolymer and 1-(3-methoxycarbonyl) propyl-1-phenyl-6,6-
methanofullerence (PCBM) as a photosensitive layer. The
photosensitive layer was prepared by spin-coating a blend solu-
tion of the copolymer and PCBM in chlorobenzene on the ITO/
PEDOT:PSS electrode. The current–voltage (I–V) characteriza-
tion of the devices was carried out on a computer-controlled
Keithley 236 Source Measurement system. A solar simulator was
used as the light source, and the light intensity was monitored by
a standard Si solar cell. The thickness of films was measured
using a Dektak 6 M surface profilometer.
Fig. 4 (a) Current–voltage curves of photovoltaic cells based on
copolymer P3 together with [70]PCBM under AM 1.5 illumination, (b)
EQE curves of photovoltaic cells based on copolymer P3 together with
[70]PCBM.
a red-shift in the optical absorption. The band gaps calculated
based on UV-vis spectra, CV scanning, and DFT modeling all
indicated a narrowing band gap with increasing the PQx content
in the copolymer structures. The PSCs based on these copoly-
mers were fabricated with a structure of ITO/PEDOT:PSS/
copolymer:PCBM/Ca/Al under the illumination of AM 1.5G,
100 mW cmꢀ12. The results showed that increase the content of
indolo[3,2-b]carbazole units in the copolymer chains had
enhanced the efficiencies of PVCs. If the ratio reached the value
All reagents were purchased from aladdin Co., Alfa Aesar Co.
and Aldrich Chemical Co., unless stated otherwise. Commercial
chemicals were used without further purification. 4,7-Dibromo-
benzo[2,1,3]thiadiazole (1) and 4,7-dibromo-5,6-dinitro-benzo
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 7714–7722 | 7719

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[2,1,3]thiadiazole (2) were synthesized according to the
literature.37
dropwise as catalyst. The reaction mixture was stirred for 14 h at
room temperature under a nitrogen atmosphere. After that, the
excess saturated sodium sulfite solution was added into the
mixture and extracted with ethyl acetate. The filtrate was dried
with anhydrous magnesium sulfate and the solvent was removed
off by vacuum distillation to afford intermediate compound. The
crude product was dissolved in a mixture of triethyl ortho-
formate (2.67 g, 18 mmol), methanesulfonic acid (0.36 g, 3.7
mmol) and methanol (20 ml) without purification. The solution
was stirred overnight at room temperature. The precipitates were
filtered and washed with methanol to give a yellow solid 6 (1.64 g,
17.8%). mp 223 ꢁC. 1H NMR (CDCl3, 400MHz, d (ppm)): 11.39
(1H, s, N–H), 11.19 (1H, s, N–H), 8.46 (1H, s, Ph-H), 8.20 (1H, s,
Ph-H), 8.08 (1H, s, Ph-H), 7.54–7.50 (2H, m, Ph-H), 7.47–7.45
(2H, m, Ph-H), 3.46–3.37 (2H, m, a-CH2), 1.79–1.76 (2H, m,
CH2), 1.62–1.57 (2H, m, CH2), 1.42–1.31 (4H, m, CH2), 0.91–
0.87 (3H, t, CH3). 13C NMR (CDCl3, 100 MHz, d (ppm)): 140.46,
136.51, 134.91, 128.43, 127.76, 125.14, 124.64, 123.47, 122.37,
120.09, 31.65, 29.39, 29.07, 28.62, 22.54, 14.36.
4,7-Di(4-hexyl)thiophen-2-yl-5,6-dinitro-benzo[2,1,3]thiadia-
zole (3). A solution of 4,7-dibromo-5,6-dinitro-benzo[2,1,3]thia-
diazole (2) (5.50 g, 14.4 mmol) and 3-hexyl-5-(tribytylstannyl)
thiophene (16.03 g, 35.10 mmol) in freshly distilled THF (70 ml)
was degassed. The mixture was heated to reflux under a nitrogen
atmosphere and then dichlorobis(triphenylphosphine)palladium
(II) [PdCl2(PPh3)2] (203mg, 0.29 mmol) was added. After 12 h, the
THF was removed off by vacuum distillation. The residues were
purified by column chromatography (eluant: petroleum ether:
CHCl3 ¼ 2 : 1) to give pure compound 3 (6.20 g, 77%) as an
orange solid. mp 53–56 ꢁC. 1H NMR (CDCl3, 400 MHz,
d (ppm)): 7.25 (1H, s, thiophene-H), 7.19 (1H, s, thiophene-H),
2.60 (4H, t, a-CH2), 1.58 (4H, m, CH2), 1.30–1.18 (12H, m, CH2),
0.85–0.81 (6H, m, CH3). 13C NMR (CDCl3, 100 MHz, d (ppm)):
144.40, 132.17, 129.20, 126.40, 121.42, 31.63, 30.30, 30.25, 28.88,
22.59, 14.07.
2,3,6,7-Tetraethyl-9,10-di(4-hexyl)thien-2-ylpyrazino[2,3-g]qui-
noxaline (4). A suspension mixture of zinc dust (4.86g, 74.41
mmol) and 4,7-di(4-hexyl)thiophen-2-yl-5,6-dinitro-benzo[2,1,3]
thiadiazole (3) (2.0 g, 3.58 mmol) in acetic acid (35 ml) was kept
3,9-Dibromo-5, 6,11-trihexyl-indolo[3,2-b]carbazole (7). To
a solution of 3,9-dibromo-11-hexyl-indolo[3,2-b]carbazole (6)
(0.36 g, 0.72 mmol) in DMF (6 ml), NaH (0.048 g, 1.99 mmol)
was added slowly. The reaction solution was stirred for 30
minutes and 1-bromohexane (0.36 g, 2.17 mmol) was added by
syringe. The mixture was stirred vigorously for 36 h and poured
into excess water. The mixture was then extracted with CHCl3
and the organic phase was dried and ꢁconcentrated to give
ꢁ
at 60 C for one and a half hours. The reaction mixture turned
pale green slowly. The solution was cooled to room temperature
and 3,4-hexanedione (2.45 g, 21.48 mmol) was added. The
mixture was stirred vigorously for 24 h at room temperature. The
solvent and excess 3,4-hexanedione were removed off by vacuum
distillation. The residues were purified by column chromato-
graphy (eluant: petroleum ether: CHCl3 ¼ 1 : 1) to give pure
1
a yellow solid 7 (0.38 g, 78.9%). mp 171 C. H NMR (CDCl3,
400 MHz, d (ppm)): 8.25–8.20 (2H, d, Ph-H), 7.74 (1H, s, Ph-H),
7.49 (2H, t, Ph-H), 7.24–7.21 (2H, d, Ph-H), 4.38–4.27 (4H, m,
N-CH2), 3.51 (2H, s, a- CH2), 1.86–1.77 (6H, m, CH2), 1.65 (2H,
s, CH2), 1.45–1.22 (16H, m, CH2), 0.96–0.79 (9H, m, CH3). 13C
NMR (CDCl3, 100 MHz, d (ppm)): 140.95, 136.56, 133.78,
128.56, 127.60, 125.36, 124.57, 123.49, 122.58, 120.85, 45.88,
43.19, 31.71, 31.60, 30.21, 29.76, 28.79, 28.57, 27.02, 26.70, 22.63,
14.05.
ꢁ
1
compound 4 (1.25 g, 56%) as a red solid. mp 102 C. H NMR
(CDCl3, 400 MHz, d (ppm)): 8.26 (2H, s, thiophene-H), 7.16 (2H,
s, thiophene-H), 3.10–3.05 (8H, q, a-CH2), 2.67 (4H, t, a-CH2),
1.73–1.65 (4H, m, CH2), 1.51–1.47 (12H, q, CH3), 1.37–1.26
(12H, m, CH2), 0.84 (6H, t, CH3). 13C NMR (CDCl3, 100 MHz,
d (ppm)): 156.35, 141.61, 136.32, 135.41, 134.25, 127.87, 124.35,
31.81, 30.77, 30.70, 29.18, 28.36, 22.64, 14.14, 11.52.
Copolymers. In a typical experiment, 1.0 mmol of the
compound 5 and 7 mixture was put into a 100 mL three-necked
flask. The feed ratio for each of the components in the mixture was
controlled at different values for the preparation of different
copolymers (vide infra). To the mixture solution, 20 mL of
degassed toluene was added under the protection of argon, fol-
lowed by flushing the solution with argon for 10 min. A total of
50 mg of Pd(PPh3)4 and 1.0 mmol of 2,5-bis(tributylstannyl)
thiophene were then added into the mixture solution. After
another flushing with argon for 20 min, the reactant mixture was
heated and refluxed for 24 h. 79 mL 2-tributylstannyl thiophene
was added to the reaction and then after two hours, 25 mL 2-
bromothiophene was added. The mixture was stirred overnight to
complete the end-capping reaction. Thereafter, the reaction
solution was cooled to room temperature and precipitated by the
addition of 500 mL of methanol. The crude copolymer was filtered
and extracted in a Soxhlet apparatus with methanol, hexane, and
acetone. Finally, the polymer was extracted with chloroform. The
polymer solution was condensed to about 20 mL and slowly
poured in 500 mL methanol. The solid was filtered and a dark
purple solid was obtained with a yield of 69–72%.
2,3,6,7-Tetraethyl-9,10-di(5-dibromo-4-hexyl)thien-2-ylpyr-
azino[2,3-g]quinoxaline (5). To a solution of 2,3,6,7-tetraethyl-
9,10-di(4-hexyl)thien-2-ylpyrazino[2,3-g] quinoxaline (4) (1.60 g,
2.55 mmol) in chloroform (60 ml) and of acetic acid (25 ml), N-
bromosuccimide (NBS) (1.82 g, 10.20 mmol) was added. This
mixture was then stirred for 4 h at room temperature in darkness.
The solvent was removed off by vacuum distillation. The residues
were purified by column chromatography (eluant: petroleum
ether: CHCl3 ¼ 2 : 1) to give pure compound 5 (2.57 g, 79%) as
ꢁ
1
a red solid. mp 118 C. H NMR (CDCl3, 400 MHz, d (ppm)):
8.54 (1H, s, thiophene-H), 8.45 (1H, s, thiophene-H), 3.14–2.91
(8H, m, a-CH2), 2.73–2.60 (4H, m, a-CH2), 1.66–1.60 (4H, m,
CH2), 1.53–1.49 (8H, q, CH2), 1.37–1.26 (16H, m, CH2, CH3),
0.87–0.82 (6H, q, CH3). 13C NMR (CDCl3, 100 MHz, d (ppm)):
157.29, 140.29, 133.96, 135.75, 134.52, 129.28, 126.56, 31.74,
30.30, 29.86, 28.90, 28.36, 22.60, 14.12, 11.89.
3,9-dibromo-11-hexyl-indolo[3,2-b]carbazole (6). To a solution
of 5-bromoindole (7.25 g, 37 mmol) and n-heptaldehyde (2.06 g,
18 mmol) in acetonitrile (50 ml), a few drops of HI was added by
7720 | J. Mater. Chem., 2011, 21, 7714–7722
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P1: Compound 5 (0.7063 g, 0.9 mmol), compound 7 (0.0667 g,
0.1 mmol), 2,5-bis(tributylstannyl)thiophene (0.6622 g, 1.0
mmol), toluene (20 mL), and Pd(PPh3)4 (50 mg, 0.043 mmol)
were used, and the general procedure described previously for the
copolymer syntheses was followed (cf. Scheme 1). The obtained
solid was a dark purple powder with a yield of 68%. Mn ¼ 7600,
Mw/Mn ¼ 1.85. 1H NMR (CDCl3, 400 MHz, d (ppm)): 8.60–8.46
(m, thiophene-H), 8.33–8.28 (m, Ph-H), 7.80–7.78 (m, Ph-H),
7.58–7.41 (m, thiophene-H), 7.33–7.23 (m, Ph-H), 4.767–4.23 (m,
N-CH2), 3.43–2.91 (m, a-CH2), 1.78–1.18 (m, CH2, CH3), 0.90–
0.82 (m, CH3). Anal. calcd for (C41.8H49.8S2.8N3.8)x C, 72.21; H,
7.22; N, 7.66; S, 12.91. Found: C, 72.34; H, 7.18; N, 7.62; S,
12.98.
(No: 2008DQ00700), Aviation Science Fund of China (No:
2008ZF56014) and Natural Science Foundation of Jiangxi
Province (Grant No: 2010GZH0110).
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The work was financially supported by the Natural Science
Foundation of China (NSFC, No: 20802033), Program for New
Century Excellent Talents in University (No: NCET-10-0170),
Yong Scientist of Jing Gang Zhi Xing Project of Jiangxi Province
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J. Mater. Chem., 2011, 21, 7714–7722 | 7721

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