Small band gap polymers synthesized via a modified nitration of 4,7-dibromo-2,1,3-benzothiadiazole

Article abstract of DOI:10.1021/ol1020724

The nitration of 4,7-dibromo-2,1,3-benzothiadiazole was modified by using CF₃SO₃H and HNO₃ as the nitrating agent, and the related yield was improved greatly. On the basis of this improvement, two new small band gap polymers, P1TPQ and P3TPQ, were developed. Bulk heterojunction solar cells based on P3TPQ and [6,6]-phenyl-C₇₁- butyric acid methyl ester exhibit interesting results with a power conversion efficiency of 2.1% and photoresponse up to 1.1 μm.

Full text of DOI:10.1021/ol1020724

ORGANIC
LETTERS
2010
Vol. 12, No. 20
4470-4473
Small Band Gap Polymers Synthesized
via a Modified Nitration of
4,7-Dibromo-2,1,3-benzothiadiazole
Ergang Wang,*,† Lintao Hou,‡ Zhongqiang Wang,‡ Stefan Hellstro¨m,†
Wendimagegn Mammo,†,§ Fengling Zhang,‡ Olle Ingana¨s,‡ and
Mats R. Andersson*,†,‡
Department of Chemical and Biological Engineering/Polymer Technology, Chalmers UniVersity
of Technology, SE-412 96 Go¨teborg, Sweden, Biomolecular and Organic Electronics, IFM,
Center of Organic Electronics, Linko¨ping UniVersity, SE-581 83 Linko¨ping, Sweden, and
Department of Chemistry, Addis Ababa UniVersity, P.O. Box 1176, Addis Ababa, Ethiopia
ergang@chalmers.se; mats.andersson@chalmers.se
Received July 2, 2010
ABSTRACT
The nitration of 4,7-dibromo-2,1,3-benzothiadiazole was modified by using CF3SO3H and HNO3 as the nitrating agent, and the related yield was
improved greatly. On the basis of this improvement, two new small band gap polymers, P1TPQ and P3TPQ, were developed. Bulk heterojunction
solar cells based on P3TPQ and [6,6]-phenyl-C71-butyric acid methyl ester exhibit interesting results with a power conversion efficiency of
2.1% and photoresponse up to 1.1 µm.
Great progress has been achieved in the performance of
polymer solar cells,1 especially after the introduction of the
bulk heterojunctions (BHJ) concept2 with conjugated poly-
mers as electron donors and fullerene analogues, such as
[6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM) or
[6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM), as
electron acceptors.1 Yet, the commercialization of polymer
solar cells is mainly limited by low-power conversion
efficiencies (PCEs). One of the main reasons for the limited
PCEs is the mismatch between the absorption spectra of
polymers and the solar irradiation.1 High band gap polymers
allow only a small fraction of the solar irradiation to be
utilized. The development of small band gap polymers is
critical to harvest more solar irradiation. An effective strategy
is the combination of electron-rich and electron-deficient
units to form alternating donor-acceptor structures, which
are widely used to obtain small band gap polymers.1,3
Recently, 4,7-dibromo-5,6-dinitro-2,1,3-benzothiadiazole
(2) has become an important intermediate compound for the
synthesis of pyrazino[2,3-g]quinoxalines and thiadiazolo-
† Chalmers University of Technology.
‡ Linko¨ping University.
§ Addis Ababa University.
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Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649. (c) Park,
S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.;
Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297. (d)
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10.1021/ol1020724  2010 American Chemical Society
Published on Web 09/15/2010

quinoxalines, which are highly electron-deficient units used
for achieving small band gap polymers for solar cells,4 along
with developing small molecules for near-infrared (NIR)
applications.5 However, the widely used method for prepar-
ing 2 developed by Uno et al.6 was marred by very low yields
(20-30%).4,5 We have now developed a more efficient
method of preparing 2 by using nitronium trifluoromethane-
sulfonate (NTMS) as a nitrating agent. By taking advantage
of the improved yield of 2, two new small band gap polymers
were developed, and their photophysical, electrochemical,
and photovoltaic properties were investigated.
mononitro compound, and the purification of the product
mixture by chromatography was quite difficult.6 NTMS was
shown to have excellent nitrating properties especially for
the nitration of toluene.7 Thus, the nitration of 1 with NTMS
as nitrating agent was carried out as follows: NTMS was
obtained by mixing fuming HNO3 with fuming CF3SO3H
carefully at 0 °C, and then 1 was added into the mixture.
First, the mixture was stirred at 0 °C for 5 h, then thin layer
chromatography (TLC) showed only a small portion of 1
turned to 2. The reaction temperature was increased to room
temperature and stirred overnight, but TLC still showed that
a lot of 1 was left unreacted. Finally, the reaction temperature
was increased to 50 °C, and after 12 h, TLC showed that 1
was completely converted. The pure 2 was obtained with a
high yield of 85% after simple purification by recrystalliza-
tion from ethanol, which is much simpler compared to the
previously reported method.6 Compound 2 was reduced with
zinc in glacial acetic acid and several drops of water4b to
yield the tetraamine 3, which was condensed with diketone
4 immediately since the tetraamine is not so stable. Monomer
5 was obtained in moderate yield and was then combined
with tributylstannyl thiophene to yield 6, followed by
bromination with N-bromosuccinimide to yield monomer 7
in high yield. As shown in Scheme 2, the polymerization of
The synthetic routes toward the two monomers are
depicted in Scheme 1. As previously reported,4,6 the nitration
Scheme 1. Synthesis of the Two Monomers
Scheme 2. Synthesis of the Two Polymers P1TPQ and P3TPQ
of 4,7-dibromo-2,1,3-benzothiadiazole (1) was carried out
in a mixture of fuming H2SO4 and fuming HNO3. The yield
of 2 was however very low. The main byproduct was the
5 and 7 with 2,5-bis(trimethylstannyl)thiophene (8) via Stille
coupling reaction with Pd2(dba)3 and P(o-Tol)3 as catalyst
resulted in the green polymers P1TPQ and P3TPQ, respec-
tively. Both polymers exhibited quite high molecular weights
with Mn ) 75 000 and Mw ) 208 000 for P1TPQ and Mn )
100 000 and Mw ) 537 000 for P3TPQ. High molecular
weights proved to be favorable to achieve better morphology
and higher PCE of the resulting polymer solar cells.8
The absorption spectra of the two polymers in chloroform
solution and in film are plotted in Figure 1. The optical
(4) (a) Steckler, T. T.; Abboud, K. A.; Craps, M.; Rinzler, A. G.;
Reynolds, J. R. Chem. Commun. 2007, 4904. (b) Zhang, F. L.; Bijleveld,
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M. M.; Sieval, A. B.; Hummelen, J. C.; Janssen, R. A. J. Org. Lett. 2009,
11, 903. (d) Perzon, E.; Zhang, F. L.; Andersson, M.; Mammo, W.; Ingana¨s,
O.; Andersson, M. R. AdV. Mater. 2007, 19, 3308. (e) Cai, T.; Zhou, Y.;
Wang, E.; Hellstro¨m, S.; Zhang, F.; Xu, S.; Ingana¨s, O.; Andersson, M. R.
Sol. Energy Mater. Sol. Cells 2010, 94, 1275. (f) Perzon, E.; Wang, X.;
Admassie, S.; Ingana¨s, O.; Andersson, M. R. Polymer 2006, 47, 4261.
(5) (a) Qian, G.; Zhong, Z.; Luo, M.; Yu, D.; Zhang, Z.; Wang, Z. Y.;
Ma, D. AdV. Mater. 2009, 21, 111. (b) Qian, G.; Li, X.; Wang, Z. Y. J.
Mater. Chem. 2009, 19, 522. (c) Karikomi, M.; Kitamura, C.; Tanaka, S.;
Yamashita, Y. J. Am. Chem. Soc. 1995, 117, 6791.
(7) Coon, C. L.; Blucher, W. G.; Hill, M. E. J. Org. Chem. 1973, 38,
4243.
(8) (a) Schilinsky, P.; Asawapirom, U.; Scherf, U.; Biele, M.; Brabec,
C. J. Chem. Mater. 2005, 17, 2175. (b) Bijleveld, J. C.; Zoombelt, A. P.;
Mathijssen, S. G. J.; Wienk, M. M.; Turbiez, M.; de Leeuw, D. M.; Janssen,
R. A. J. J. Am. Chem. Soc. 2009, 131, 16616.
(6) Uno, T.; Takagi, K.; Tomoeda, M. Chem. Pharm. Bull. 1980, 28,
1909.
Org. Lett., Vol. 12, No. 20, 2010
4471

Table 1. Photovoltaic Parameters of the Devices
weight ratio
Jsc (mA cm-2
) Voc (V) FF PCE (%)
P1TPQ/[60]PCBM ) 1:3
P3TPQ/[60]PCBM ) 1:3
P1TPQ/[70]PCBM ) 1:3
P3TPQ/[70]PCBM ) 1:1
P3TPQ/[70]PCBM ) 1:2
P3TPQ/[70]PCBM ) 1:3
P3TPQ/[70]PCBM ) 1:4
0.8
3.5
1.5
2.5
8.7
7.3
4.5
0.71 0.58
0.51 0.51
0.66 0.50
0.51 0.34
0.50 0.40
0.52 0.54
0.51 0.58
0.3
0.9
0.5
0.4
1.7
2.1
1.3
Figure 1. Absorption spectra of P1TPQ and P3TPQ in chloroform
solution and in film.
[70]PCBM has similar electronic properties to [60]PCBM
but a considerably higher absorption coefficient in the visible
region because of its asymmetric chemical structure.12
Photovoltaic devices were made by using [70]PCBM as an
acceptor instead of [60]PCBM. Although the performance
improvement for P1TPQ was not obvious when [70]PCBM
was used, the PCEs of the solar cells fabricated from blends
of P3TPQ/[70]PCBM in o-dichlorobenzene (ODCB) solution
displayed a significant increase. The weight ratios of P3TPQ
and [70]PCBM were optimized from 1:1, 1:2, 1:3, to 1:4,
and the related performance parameters are summarized in
Table 1. A maximum PCE of 2.1% with an open-circuit
voltage (Voc) of 0.52 V, a Jsc of 7.3 mA cm-2, and a fill
factor (FF) of 0.54 was obtained from the solar cells of
P3TPQ/[70]PCBM ) 1:3 under illumination from an AM
1.5G solar simulator (100 mW cm-2). Low Voc is one of
drawbacks for small band gap polymers. It is worth noting
that the Voc of 0.52 V is rather high for a polymer with a
band gap as small as 1.1 eV. The Voc has achieved the upper
limit of its theoretical value according to the empirical
formula eVoc ) Eg - 0.6 eV.13 Compared to the Jsc (3.5 mA
cm-2) from the device of P3TPQ/[60]PCBM, the dramati-
cally increased Jsc (7.3 mA cm-2) from the device of P3TPQ/
[70]PCBM indicates that the charge transfer between P3TPQ
and [70]PCBM is effective, which means the offset (0.22
eV) between the LUMO levels of P3TPQ and [70]PCBM
should be sufficient for electron transfer.
bandgaps (Eg), defined by the onset of the absorption spectra
of the films, are 1.2 eV for P1TPQ and 1.1 eV for P3TPQ,
which demonstrate the two polymers to be rather small band
gap polymers. Unlike the absorption maximum (λmax) of a
similar polymer PTBEHBQ reported by Zoombelt et al. that
showed a small blue shift when going from solution to the
solid state,9 the λmax at long wavelength of P3TPQ showed
a quite large red shift of ca. 90 nm and an increase in
intensity. This phenomenon indicates good intermolecular
ordering (π-stacking/aggregates) in the solid state, which
should benefit charge transport, and then higher performance
can be expected in the resulting polymer solar cells.
To investigate if the HOMO and LUMO levels of the
polymers match well with the acceptors [60]PCBM or
[70]PCBM, the electrochemical behaviors of all the materials
were measured under the same conditions by square wave
voltammetry (SWV).10 The HOMO and LUMO levels
determined from oxidation and reduction peaks are -5.73
and -3.96 eV, respectively, for P1TPQ. The corresponding
values for P3TPQ are -5.64 and -3.90 eV, respectively.
As shown in Figure S13 (Supporting Information), the
reduction potentials of the two polymers are quite close to
that of [60]PCBM, with an offset of 0.1-0.2 V, which
indicates that the electron transfer between polymers and
[60]PCBM may not be so effective since it is generally
considered that a minimum offset of ∼0.3-0.4 eV between
LUMO levels of electron donor and acceptor is necessary
for efficient electron transfer.11
The current density-voltage (J-V) curves of the solar
cells from P3TPQ/[70]PCBM ) 1:3 both in the dark and
under illumination are shown in Figure 2a. The dark J-V
curve shows a typical diode characteristic with very small
leakage current at -1 V. To confirm the accuracy of the
measurement of the devices, the corresponding external
quantum efficiency (EQE) of the solar cell was measured
under illumination of monochromatic light. The Jsc calculated
from integration of the EQE with an AM 1.5G reference
spectrum agrees well with the Jsc obtained from the J-V
measurement. As shown in Figure 2b, the photoresponse of
the solar cell from P3TPQ/[70]PCBM ) 1:3 in the visible
region was significantly enhanced when going from [60]PCBM
to [70]PCBM. This is consistent with the aborption spectra
Bulk heterojunction solar cells were fabricated from
P1TPQ and P3TPQ with a device architecture of glass/ITO/
PEDOT:PSS/active layer/LiF/Al. As summarized in Table
1, the device results from the blends of polymers/[60]PCBM
(optimized ratio is 1:3 for both polymers) in chloroform
solution are not great with maximum PCE of 0.3% for
P1TPQ and 0.9% for P3TPQ. The low short-circuit current
density (Jsc) can be partially attributed to the insufficient
driving force for charge separation mentioned above.
(9) Zoombelt, A. P.; Fonrodona, M.; Turbiez, M. G. R.; Wienk, M. M.;
Janssen, R. A. J. J. Mater. Chem. 2009, 19, 5336.
(10) Hellstro¨m, S.; Zhang, F. L.; Ingana¨s, O.; Andersson, M. R. Dalton
Trans. 2009, 10032.
(12) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.;
Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed.
2003, 42, 3371.
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Waldauf, C.; Heeger, A. J.; Brabec, C. J. AdV. Mater. 2006, 18, 789. (b)
Koster, L. J. A.; Mihailetchi, V. D.; Blom, P. W. M. Appl. Phys. Lett. 2006,
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Org. Lett., Vol. 12, No. 20, 2010

active layer, atomic force microscopy (AFM) was used. As
shown in Figure 3, the blend from P3TPQ/[70]PCBM ) 1:3
Figure 3
.
AFM height images (5 µm × 5 µm) of the films spin-
coated from (a) P3TPQ/[60]PCBM ) 1:3 in chloroform solution
and (b) P3TPQ/[70]PCBM ) 1:3 in ODCB solution.
in ODCB solution has a smaller domain size compared to
the blend from P3TPQ/[60]PCBM ) 1:3 in chloroform
solution, which indicates that there is a more optimal
morphology in P3TPQ/[70]PCBM ) 1:3 film. This is an
important reason for the improved performance of the devices
based on P3TPQ/[70]PCBM ) 1:3.
In conclusion, the preparation of 2, which is the key
compound for achieving strong acceptors, was modified by
using the mixture of fuming HNO3 and fuming CF3SO3H as
the nitrating agent. The great improvement in yield (from
30 to 85%) makes the important imtermediate compound 2
readily available. Accordingly, two new small band gap
polymers P1TPQ and P3TPQ were synthesized with high
molecular weights. P3TPQ showed interesting photovoltaic
properties with Voc of 0.52 V, PCE over 2%, and a
photoresponse up to 1.1 µm in BHJ polymer solar cells. This
work will be of importance in guiding the design and
development of very small band gap polymers for polymer
solar cells and NIR detectors.
Figure 2. (a) J-V characteristics of P3TPQ/[70]PCBM ) 1:3 based
solar cells under an AM 1.5G illumination (100 mW cm-2) and in
the dark. (b) EQE profiles from the devices of P3TPQ/[70]PCBM
) 1:3 and P3TPQ/[60]PCBM ) 1:3 as well as the absorption
spectra of the films from P3TPQ/[70]PCBM ) 1:3 and P3TPQ/
[60]PCBM ) 1:3 (normalized at 900 nm for comparison).
of the films from the blends of P3TPQ/[60]PCBM ) 1:3
and P3TPQ/[70]PCBM ) 1:3 measured under similar
conditions. This is the main reason for the improvement of
the PCE when [70]PCBM was used as the acceptor instead
of [60]PCBM in the solar cells. The photoresponse at long
wavelength region originates from P3TPQ with a peak at
860 nm and an edge at 1.1 µm. To our knowledge, this is
one of the highest PCEs for small band gap polymers with
a band gap as small as 1.1 eV and a photoresponse up to
1.1 µm reported to date.4c,d,14
Acknowledgment. This work was financed by the Swed-
ish Energy Agency through projects Polarge. We thank the
Knut and Alice Wallenberg Stiftelse for instrument grants.
The morphology of the active layer is very important for
BHJ polymer solar cells and can influence charge separation
and transport.15 To gain insight into the morphology of the
Supporting Information Available: Experimental pro-
cedures, electrochemical measurement, and device fabrica-
tion. This material is available free of charge via the Internet
at http://pubs.acs.org.
(14) (a) Wen, L.; Duck, B. C.; Dastoor, P. C.; Rasmussen, S. C.
Macromolecules 2008, 41, 4576. (b) Zhou, E.; Wei, Q.; Yamakawa, S.;
Zhang, Y.; Tajima, K.; Yang, C.; Hashimoto, K. Macromolecules 2010,
43, 821
.
(15) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45.
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