New moderate bandgap polymers containing alkoxysubstituted-benzo[c][1,2,5] thiadiazole and thiophene-based units

Article abstract of DOI:10.1002/pola.24879

Alkoxysubstituted benzo[c][1,2,5]thiadiazole electron accepting units were prepared and copolymerized with various thiophene-based electron donating monomers to produce new low bandgap polymers P1-4. The materials showed broad absorption in the range from

Full text of DOI:10.1002/pola.24879

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New Moderate Bandgap Polymers Containing Alkoxysubstituted-
Benzo[c][1,2,5]thiadiazole and Thiophene-Based Units
Zheng Bang Lim,1 Bofei Xue,2 Swarnalatha Bomma,1 Hairong Li,1 Shuangyong Sun,1
Yeng Ming Lam,1 Warwick J. Belcher,2 Paul C. Dastoor,2 Andrew C. Grimsdale1
1School of Materials Science and Engineering and Energy Research Institute at NTU, Nanyang Technological University, 50
Nanyang Avenue, Singapore 639798, Singapore
2Centre for Organic Electronics, University of Newcastle, New South Wales 2308, Australia
Correspondence to: P. C. Dastoor (E-mail: Paul.Dastoor@newcastle.edu.au) or A. C. Grimsdale (E-mail: acgrimsdale@ntu.edu.sg)
Received 18 May 2011; accepted 7 July 2011; published online 28 July 2011
DOI: 10.1002/pola.24879
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ABSTRACT: Alkoxysubstituted benzo[c][1,2,5]thiadiazole electron
accepting units were prepared and copolymerized with various
thiophene-based electron donating monomers to produce new
low bandgap polymers P1–4. The materials showed broad
absorption in the range from 300 to 700 nm with bandgaps below
2 eV in solution. Efficiencies of over 1% were obtained from pho-
tovoltaic cells using P4 with PCBM as acceptor. 2011 Wiley Peri-
odicals, Inc. J Polym Sci Part A: Polym Chem 49: 4387–4397, 2011
KEYWORDS: atomic force microscopy; conjugated polymers;
copolymerization; heteroatom-containing polymers; organic
solar cells
INTRODUCTION In recent years, considerable efforts have
been devoted to the synthesis of conjugated polymers for
applications in organic solar cells,1 organic light emitting
diodes (OLEDs),2 and organic field-effect transistors
(OFETs)3 due to their potentially low production costs as
they can be processed from solution, and the possibility of
making light weight, flexible large-area devices. Organic solar
cells, although still inferior to silicon-based solar cells in
terms of power conversion efficiency (PCE) and stability, are
still an attractive alternative due to the advantages of
low cost and ability to make large-area devices. Among
conjugated polymers for use in solar cells, low bandgap
(Eg < 2 eV) polymers are of particular interest as electron
donor materials for solar cell applications as they allow
better harvesting of solar energy since they absorb across a
wide section of the solar spectrum. Theoretical models
suggest that organic solar cells with efficiencies above
10% are attainable using polymers with bandgaps between
1.3 and 1.8 eV as donors when combined with electron-
accepting molecules such as fullerenes in bulk heterojunction
(BHJ) devices.4,5 While such high efficiencies have yet to be
obtained, already very promising PCEs of 5–8% have been
achieved from BHJ devices using electron donating conju-
gated polymers with bandgaps below 2 eV.6–15
mainly to the electron donor materials as PCBM absorbs
weakly in the visible range,7 though we have recently shown
that the PCBM does contribute significantly to the overall
photocurrent.16 Hence, the design and synthesis of low
bandgap polymer is important to improve light harvesting,
and so increase the PCE. However, too low a bandgap may
result in increased recombination losses and consequent
lower power conversion efficiencies even though the device
may harvest more photons with lower energy.17,18
Low bandgap polymers can be prepared by alternating
copolymerization of electron-donating donor (D) and elec-
tron-withdrawing acceptor (A) units. The interaction
between these units in the resulting donor-acceptor (D–A)
copolymers raises the highest occupied molecular orbital
(HOMO) energy level and lowers the lowest unoccupied mo-
lecular orbital (LUMO) energy level so reducing the bandgap
of the polymers.19,20 The bandgap of a polymer may also be
further modified by the attachment of electron-donating or
accepting groups as substituents on the chain.
Benzo[c][1,2,5]thiadiazole (BT, 2) is
a commonly used
acceptor unit in low bandgap conjugated polymers, but the
lack of solubilizing units, reduces the solubility and potential
molar masses of the polymers containing it. This low solubil-
ity can be overcome by adding alkoxy groups at the 5- and
6-positions as in 3.9
The fullerene derivative, [6,6]-phenyl-C61 butyric acid methyl
ester (PCBM, Figure 1, 1), is the most widely used electron
acceptor material in polymer solar cells. The absorption of
polymer: PCBM solar cells has usually been attributed
As the donor moieties in our intended D-A copolymers we
chose to use thiophenes and fused thiophene units. Polymers
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column chromatography was done on the silica gel 60
Geduran (Merck). Nuclear magnetic resonance (NMR) spec-
tra (1H and 13C) were recorded on a spectrometer (Bruker
Advance 400), using magnetic field 400 MHz. Common sol-
vent used in the NMR characterization is chloroform-deuter-
ated (CDCl3). Chemical shifs are d in units (ppm) referenced
to residual proton signals in CDCl3. Coupling constants (J)
are reported Hertz (Hz). NMR splitting patterns are desig-
nated as s, singlet; d, doublet; t, triplet; q, quartet; AB, pair
of doublet which is mirror image to each other; m, multiplet;
and br, broad. Mass spectra were recorded using Shimadzu
Axima matrix-assisted laser desorption/ionization time-of-
flight (MALDI-TOF) mass spectrometer. Molecular weight of
polymers was determined by gel permeation chromatogra-
phy (GPC) method in chloroform, which was performed on
Agilent GPC 1100 series model G1311A. In this method, col-
umn of PL gel 5 lm, MIXED-C was used and polystyrene was
used as a standard. Absorption spectra were taken on a UV-
Vis recording spectrophotometer (Shimadzu UV-2501PC).
Fluorescent measurements were taken on a spectrofluoro-
photometer (Shimadzu RF-5301PC). Thermogravimetric anal-
ysis (TGA) experiments were performed on TA TGA-Q500
FIGURE 1 PCBM and units used to make polymers P1–4.
containing fused thiophenes have been extensively investi-
gated as materials for OFETs due to their high charge carrier
mobilities assisted by their close solid-state packing.3 In par-
ticular polymers containing thieno [3,2-b]thiophene (TT, 4)
have been shown to have charge carrier mobilities as high as
0.2–0.6 cm2 Vꢀ1 sꢀ1 21
Such high charge carrier mobilities
.
are advantageous in materials for solar cells also as they will
assist the collection of charges generated within the cell.
Owing to the larger resonance stabilization energy of the TT
compared to thiophene, delocalization of electrons from TT
into the polymer backbone may be reduced so lowering the
polymer’s HOMO which enhances stability and also raises
the potential Voc.
ꢁ
with a dynamic heat rate (10 C/min) under nitrogen ꢁatmos-
phere (50 mL/min) in the temperature range 25–800 C. Dif-
ferential scanning calorimetry (DSC) measurements were
performꢁed on TA DSC-Q10 with dynamic heating and cooling
rate (5 C/min) under nitrogen atmosphere (50 mL/min) in
the temperature range 0–200 ꢁC. The electrochemical cyclic
voltammetry was conducted on a frequency response ana-
lyzer (Solartron 1255B) with a sweep rate 100 mV/s by a
platinum working electrode. Gold was used as counter elec-
trode. Ag/Agþ was used as reference electrode and DCM
containing 0.1 M tetra-n-butylammonium hexafluorophos-
phate (Bu4NPF6) was used as electrolyte in electrochemical
measurements. Thin films were made by Spin Coater model
P6700 series with 1250 rpm for 1.5 min.
Siloles have been investigated as charge transporting materi-
als due to their good charge carrier mobilities and relatively
low bandgaps.22 The silicon-fused bithiophene (dithieno[3,2-
b:20,30-d]silole, DTS, 5) was chosen as a donor unit since it
has a notably low bandgap due to the newly formed
extended planar p-conjugated tricylic system and r*–p* con-
jugation between the r*-orbit of silicon and p*-orbit of the
bithiophene unit.23–25 DTS-containing polymers have been
used in organic solar cells with high efficiencies (around
5%).7 The silole units in these polymers significantly enhan-
ces their hole mobilities, and as a result, the fill factor (FF)
and short circuit current (Jsc) of these polymer-based device
are also enhanced. It is also reported that silole units impart
high photochemical stability to conjugated polymers.26
Solar Cells Fabrication and Characterization
PEDOT:PSS (Clevios P, filtered by 0.45 lm PVDF filter) films
were spin-coated (4000 rpm) on pre-cleaned patterned ITO
glass slides. The PEDOT:PSS films were transferred into a
glove box with dry nitrogen atmosphere and annealed at
120 ꢁC for 20 min to eliminate water in the films. Different
weight ratios were used for P4 and PCBM (Luminescence
Tech., Taiwan) blends. The P4:PCBM blend solution (different
concentration in chloroform) was sonicated for 30 min and
then used to spin-coat the active layers in an inert atmos-
phere glove box at 2000 rpm for 1 min. Thickness of these
active layers were measured by a KLA Tencor profilometer
These active layers were then transferred into a vacuum
chamber for electrode evaporation. The aluminum (Al) elec-
trodes were evaporated on the active layers in vacuum (2 ꢂ
10ꢀ6 Torr). The thickness of Al layers was measured to be
about 80 nm by a KLA Tencor profilometer and the area of
each cell was 5 mm2.
Herein, we report the synthesis of new copolymers P1–4
based on alkoxysubstituted BT with different thiophene-
based monomers using Stille cross-coupling reactions, the
characterization of their optoelectronic properties and some
preliminary results from solar cells using them as electron
donors.
EXPERIMENTAL
General
All starting materials (chemicals) used in this project were
purchased from different well-known companies, for exam-
ple, Sigma Aldrich, Alfa Aesar, and others. All reactions were
carried out under inert N2 atmosphere, unless otherwise
stated. Analytical thin layer chromatography (TLC) was per-
formed on aluminum sheets pre-coated with silica gel (with
fluorescent indicator 254 nm). Visualization was accom-
plished with UV light. Purification of reaction mixture using
The photocurrent density-voltage (J-V) measurements were
conducted using a Newport Class A solar simulator with an
AM 1.5 spectrum filter to illuminate the full cells. The light
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intensity was measured to be 100 mW cmꢀ2 by a silicon ref-
erence solar cell (Newport) and the J-V data were recorded
by a Keithley 2400 source meter. The cells were tested after
Ca-Al evaporation. Post-evaporation annealing at different
temperature and duration was also carried out to study the
J-V performance of these cells after annealing.
solution of the impure 11 (2.2 g, 4.35 mmol) in a mixture of
DCM (100 mL) and acetic acid (52 mL) was added bromine
(1.7 mL, 33.15 mmol). The resulting mixture was stirred in
the dark for about 48 hrs at 40 ꢁC. The mixture was then
poured in water, extracted with DCM, sequentially washed
with water, saturated NaHCO3 (aq), and 1 M Na2S2O3 (aq)
and the organic layer was then dried over MgSO4, concen-
trated, and subjected to column chromatography with a sol-
vent mixture of 5% ethyl acetate in hexane to obtain 12 as a
white solid (2.6 g, 90%).
Synthesis and Characterization
1,2-bis(dodecyloxy)benzene (8)
KOH (19 g, 340.57 mmol) was added to a solution catechol
(15 g, 136.23 mmol) in EtOH (150 mL). After stirring at 60
ꢁC for 30 mins, 1-bromododecane (82 mL, 340.57 mmol)
was added dropwise into the reaction mixture and the reac-
tion mixture was stirred at 60 ꢁC overnight. After cooling
down to room temperature, EtOH was removed on a rotary
evaporator and the residue was extracted with DCM and
water. The organic layer was then dried over MgSO4 and
concentrated. The residue was washed with MeOH to com-
pletely remove remaining 1-bromododecane and dried to
give 8 as a white solid (56 g, 92%).
1H NMR (CDCl3): d 4.157 (t, J ¼ 6.8 Hz, 4 H), 1.844-1.915
(m, 4 H), 1.494-1.563 (m, 4 H), 1.267 (br, 32 H), 0.878 (t, J
¼ 6.8 Hz, 6 H). 13C NMR (CDCl3): d 154.52, 150.38, 106.27,
75.16, 31.94, 30.28, 29.70, 29.67, 29.64, 29.62, 29.45, 29.38,
26.00, 22.71, 14.13. MALDI TOF MS: m/z 666.46 (Mþ4H).
5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-
thiadiazole (12)
A round bottomed flask containing 6 (2.6 g, 3.94 mmol), thi-
ophene-2-boronic acid (1.3 g, 9.8 mmol) and Pd(PPh3)4
(0.48 g, 0.394 mmol) was degassed with N2. Two molar
Na2CO3 (80 mL) aqueous solution and THF (120 mL),
degassed with N2, were added and the reaction mixture was
stirred in the dark at 80 ꢁC. After 48 h of reaction, another
0.7 g of thiophene-2-boronic acid and 0.48 g of Pd(PPh3)4
were added into the mixture and the reaction was continued
to run for another 24 h. The reaction mixture was then
cooled to room temperature, extracted with chloroform and
water, and THF was removed using rotary evaporator. Chlo-
roform layer was dried over MgSO4, concentrated and sub-
jected to column chromatography with a solvent mixture of
20% DCM in hexane to purify the mixture to obtain 12 as a
yellow-green oily-solid (2.2 g, 83%).
1H NMR (CDCl3): d 6.887 (s, 4 H), 3.991 (t, J ¼ 6.8 Hz, 4 H),
1.776-1.847 (m, 4 H), 1.432-1.501 (m, 4 H), 1.267-1.351 (br,
32 H), 0.882 (t, J ¼ 6.4 Hz, 6 H). 13C NMR (CDCl3): d 149.91,
121.66, 114.79, 69.95, 32.60, 30.38, 30.34, 30.32, 30.12,
30.04, 30.02, 26.73, 23.37, 14.79.
1,2-bis(dodecyloxy)-4,5-dinitrobenzene (9)
8 (12 g, 27 mmol) was added in portions slowly to a mix-
ture of 65% HNO3 (80 mL) and 100% HNO3 (50 mL) in an
ice bath. The reaction mixture was then stirred overnight at
room temperature. The reaction mixture was then poured
over an ice-water mixture. The precipitate was collected by
vacuum filtration, washed with water, and recrystallized
using EtOH to afford 9 as a yellow solid (13.14 g, 92%).
1H NMR (CDCl3): d 8.466 (d, J ¼ 3.2 Hz, 2 H), 7.504 (d, J ¼
4.8 Hz, 2 H), 7.234 (t, J ¼ 4.6 Hz, 2 H), 4.106 (t, J ¼ 7.2 Hz,
4 H), 1.881-1.954 (m, 4 H), 1.432-1.452 (m, 4 H), 1.271 (br,
32 H), 0.884 (t, J ¼ 6.8 Hz, 6 H). 13C NMR (CDCl3): d
151.983, 151.016, 134.127, 130.548, 127.310, 126.764,
117.634, 74.38, 31.95, 30.34, 29.72, 29.68, 29.65, 29.57,
29.39, 25.97, 22.71, 14.13. MALDI TOF MS: m/z 671.51
(Mþ2H).
1H NMR (CDCl3): d 7.289 (s, 2 H), 4.092 (t, J ¼ 6.4 Hz, 4 H),
1.829-1.898 (m, 4 H), 1.434-1.505 (m, 4 H), 1.240-1.259 (br,
32 H), 0.892 (t, J ¼ 6.4 Hz, 6 H). 13C NMR (CDCl3): d 151.79,
136.48, 107.88, 70.20, 31.93, 29.68, 29.66, 29.58, 29.55,
29.37, 29.24, 29.08, 28.69, 25.82, 22.70, 14.12.
4,7-dibromo-5,6-didodecyloxy-benzo[c][1,2,5]thiadiazole (11)
Sn(II)Br2 (5.7 g, 20.4 mmol) and 9 (1.5 g, 2.79 mmol) in
ethanol (40 mL) and conc. HBr (21 mL) were stirred at 100
ꢁC for 18 hrs. After cooling to room temperature, the precipi-
tate was collected by vacuum filtration and rinsed with hex-
ane to give crude 4,5-bis(dodecyloxy)benzene-1,2-diamine
dihydrobromide 10 as an air-sensitive purple solid. This
solid was then transferred into an ice-cooled round-bot-
tomed flask, into which was injected anhydrous chloroform
(15 mL) and thionyl bromide (2.16 mL, 27.9 mmol) followed
by anhydrous pyridine (3.6 mL, 41.85 mmol). The tempera-
ture of reaction mixture was then gradually raised ꢁto room
temperature, followed by heating and stirring at 70 C, over-
night. After cooling down to room temperature, the reaction
mixture was then counter-extracted with 5% NaHCO3 aque-
ous solution till neutralization, the chloroform layer was
then dried over MgSO4, and concentrated, to give crude 11
which was contaminated with brominated derivatives. To a
4,7-bis(5-bromothiophen-2-yl)-5,6-bis(dodecyloxy)
benzo[c][1,2,5]thiadiazole (7)
12 (0.31 g, 0.47 mmol) was dissolved in THF (20 mL). The
reaction mixture was then placed in ice-bath, and NBS
(0.21 g, 1.175 mmol) was added. The reaction mixture was
then stirred for 18 hrs, with the temperature being gradually
raised to RT. The reaction mixture was then extracted with
DCM and excess of water (to remove NBS and THF). The
DCM layer was dried over MgSO4 and concentrated to give 7
as a reddish orange solid (0.38 g, 97%).
1H NMR (CDCl3): d 8.364 (d, J ¼ 4.4 Hz, 2 H), 7.172 (d, J ¼
4.4 Hz, 2 H), 4.120 (t, J ¼ 7.2 Hz, 4 H), 1.92-1.957 (m, 4 H),
1.422-1.442 (m, 4 H), 1.262-1.275 (br, 32 H), 0.884 (t, J ¼ 6
Hz, 6 H). 13C NMR (CDCl3): d 151.497, 150.411, 135.708,
131.015, 129.689, 117.010, 115.466, 74.59, 31.95, 30.28,
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SCHEME 1 Synthesis of monomers 6 and 7.
29.73, 29.69, 29.65, 29.53, 29.45, 29.40, 25.94, 22.72, 14.14.
MALDI TOF MS: m/z 828.43 (Mþ2H).
room temperature (white suspension gradually formed),
then stirred at ꢀ78 ꢁC again for 10 mins. Tributyl tin chlo-
ride (7.2 mL, 26.62 mmol) of was injected dropwise into the
reaction mixture and it was stirred at ꢀ78 ꢁC for 30 mins.
The reaction mixture was then stirred at room temperature
overnight. The reaction mixture was extracted with ether
(100 mL) and DI water (800 mL) and the ether layer was
concentrated. 13 was obtained as a light brown liquid and
was immediately put inside fridge for storage and used for
polymer synthesis without purification.
2,5-bis(tributylstannyl)thiophene (13)
2,5-dibromothiophene (1 mL, 8.88 mmol) was injected drop-
wise into a solution of dry THF (15 mL) and 1.6 M BuLi in
hexane (17 mL, 26.62 mmol) at ꢀ78 ꢁC. The reaction mix-
ture was then stirred at ꢀ78 ꢁC for 10 mins. The tempera-
ture of the reaction mixture was then gradually raised to
SCHEME 2 Synthesis of polymers P1–4.
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