Low-bandgap donor-acceptor copolymers with 4,6-bis(3′-(2-ethylhexyl) thien-2′-yl)thieno[3,4-c][1,2,5]thiadiazole: Synthesis, optical, electrochemical, and photovoltaic properties

Article abstract of DOI:10.1002/pola.24780

Two new low-bandgap alternating copolymers (CEHTF and CEHTP) consisting of 4,6-bis(3′-(2-ethylhexyl)thien-2′-yl)thieno[3,4-c][1,2,5] thiadiazole and 9,9-bis(2-ethylhexyl)fluorene or 2,5-bis(isopentyloxy)benzene were synthesized by Suzuki coupling reaction of corresponding comonomers. Their optical, electrochemical, and photovoltaic (PV) properties were studied and are reported. Both the copolymers exhibited long-wavelength absorption covering the whole visible spectral region, which is in CEHTP thin films extended up to near infrared region, ambipolar redox properties, and electrochromism. High-electron affinities and low-optical bandgap values, 1.37 and 1.15 eV, were determined for CEHTF and CEHTP, respectively. PV devices with bulk heterojunction made of blends of copolymers and fullerene derivative [6,6]-phenyl-C⁶¹- butyric acid methyl ester ([60]PCBM) were prepared and characterized. Effects of intramolecular charge transfer strength and side-chain nature and length on photophysical properties are discussed.

Full text of DOI:10.1002/pola.24780

Low-Bandgap Donor–Acceptor Copolymers with
4,6-Bis(30-(2-ethylhexyl)thien-20-yl)thieno[3,4-c][1,2,5]thiadiazole:
Synthesis, Optical, Electrochemical, and Photovoltaic Properties
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VERA CIMROVA, IVAN KMINEK, PETRA PAVLACKOVA, DRAHOMIR VYPRACHTICKY
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, v.v.i.,
Heyrovsky´ Sq. 2, 162 06 Prague 6, Czech Republic
Received 29 March 2011; accepted 14 May 2011
DOI: 10.1002/pola.24780
Published online 8 June 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Two new low-bandgap alternating copolymers
(CEHTF and CEHTP) consisting of 4,6-bis(30-(2-ethylhexyl)thien-
20-yl)thieno[3,4-c][1,2,5] thiadiazole and 9,9-bis(2-ethylhexyl)-
fluorene or 2,5-bis(isopentyloxy)benzene were synthesized by
Suzuki coupling reaction of corresponding comonomers. Their
optical, electrochemical, and photovoltaic (PV) properties were
studied and are reported. Both the copolymers exhibited long-
wavelength absorption covering the whole visible spectral
region, which is in CEHTP thin films extended up to near infra-
red region, ambipolar redox properties, and electrochromism.
High-electron affinities and low-optical bandgap values, 1.37
and 1.15 eV, were determined for CEHTF and CEHTP, respec-
tively. PV devices with bulk heterojunction made of blends of
copolymers and fullerene derivative [6,6]-phenyl-C61-butyric
acid methyl ester ([60]PCBM) were prepared and characterized.
Effects of intramolecular charge transfer strength and side-
chain nature and length on photophysical properties are dis-
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cussed.
2011 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 49: 3426–3436, 2011
KEYWORDS: conjugated
polymers; electrochemistry; low-
bandgap; photophysics; photovoltaics; UV–vis spectroscopy
INTRODUCTION Polythiophenes and thiophene-containing
polymers are extensively studied due to their potential use
in organic photovoltaics (PVs).1–3 Low-bandgap polymers
with broad absorption up to longer wavelengths are required
to improve harvesting of the terrestrial solar radiation. The
most successful strategy to achieve low-bandgap polymers is
copolymerization of an electron-donor and electron-acceptor
monomers, where fine-tuning of the bandgap is enabled by a
choice of building blocks due to the intramolecular charge
transfer (ICT) from the donor to the acceptor moiety. The
thiadiazole nucleus is becoming widely used in benzothiadia-
zole or benzothiadiazole-based forms.4
derivatives and 9,9-bis(2-ethylhexyl)fluorene possessing
higher molecular weights and better solubility.6,7
In this article, we extended our synthetic work and prepared a
new T derivative 4,6-bis(30-(2-ethylhexyl)thien-20-yl)thieno[3,4-
c][1,2,5]thiadiazole (EHT) with branched 2-ethylhexyl chains
attached to its thiophenes. Using this T derivative, we suc-
ceeded in the preparation of two new copolymers (CEHTF and
CEHTP) consisting of the EHT and 9,9-bis(2-ethylhexyl)fluor-
ene or 2,5-bis(isopentyloxy)benzene unit (Fig. 1). We report
on the synthesis of EHT comonomer unit and two soluble low-
bandgap copolymers (CEHTF and CEHTP) and their optical
and electrochemical properties. In addition, the new donor–
acceptor copolymers were tested in simple PV devices with
active layer from polymer blend with fullerene derivative
[6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM). The
results are compared with those obtained for corresponding
copolymers CHTF and CDTF with linear alkyl chains on the
thiophene units. Effects of branched side-chain nature and
length on photophysical properties are discussed.
Recently, we used the 4,6-di(thien-20-yl)thieno[3,4-c][1,2,5]
thiadiazole as an interesting building block due to its
extended absorption at longer wavelengths and the electron-
acceptor nature of its central thienothiadiazole moiety. At
first, we prepared a soluble low-bandgap donor–acceptor
copolymer, CTF (Fig. 1), consisting of 4,6-di(thien-20-yl)-
thieno[3,4-c][1,2,5]thiadiazole (T) and 9,9-bis(2-ethylhexyl)-
fluorene.5 The soluble fractions of the copolymer were typi-
cally prepared with lower molecular weights, Mw ꢀ 5000. To
improve the solubility of the final copolymers, we introduced
linear alkyl chains (hexyls or dodecyls) to the thiophene
units of the T, and we succeeded in the synthesis of the
copolymers (CHTF and CDTF) containing 30,30-dialkyl T
EXPERIMENTAL
Materials
All chemicals were obtained from Sigma-Aldrich and used as
purchased. Solvents were dried with Na/LiAlH4 (THF), KOH
(pyridine), or molecular sieves 4A (DMF) and distilled.
Correspondence to: V. Cimrova´ (E-mail: cimrova@imc.cas.cz)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 3426–3436 (2011) 2011 Wiley Periodicals, Inc.
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ARTICLE
particle size 5 lm, and UV detection at 254 nm. THF was the
mobile phase, and polystyrene standards were used for cali-
bration. Thermogravimetric analysis (TGA) was performed
either in air or in nitrogen flow (50 mL min–1) at a heating
rate of 10 K min–1, using Perkin-Elmer TGA 7 Thermogravi-
metric Analyzer, operated with Pyris 1 software.
UV–vis spectra were measured on a Perkin-Elmer Lambda
35 UV/VIS spectrometer. Absorption spectra of thin films
were also recorded in the glove box using fiber optics con-
nected to the spectrophotometer. Layer thicknesses were
measured using a KLA-Tencor P-10 profilometer. Cyclic vol-
tammetry (CV) was performed with a PA4 polarographic an-
alyzer (Laboratory Instruments, Prague, CZ) with a three-
electrode cell. Platinum (Pt) wire electrodes were used as
both working and counter electrodes, and nonaqueous Ag/
Agþ electrode (Ag in 0.1 M AgNO3 solution) was used as the
reference electrode. CV measurements were made in solu-
tion of 0.1 M tetrabutylammonium hexafluorophosphate
(TBAPF6) in anhydrous acetonitrile in nitrogen atmosphere.
Typical scan rates were 20, 50, and 100 mV sꢁ1
.
Device Preparation and Measurements
FIGURE 1 Chemical structure of the copolymers under study.
Thin films of copolymers and their blends with [60]PCBM
were prepared by spin coating from 1,2-dichlorobenzene sol-
utions. Thin films for optical studies were spin-coated onto
fused silica substrates or coated on Pt wire electrode by dip-
ping for electrochemical measurements. For polymer PV
devices, polymer layers were prepared on indium-tin oxide
(ITO) substrates covered with a thin layer of poly[3,4-(ethyl-
enedioxy)thiophene]/poly(styrenesulfonate) (PEDOT:PSS). All
polymer films were dried in vacuum (10ꢁ3 Pa) at 353 K for
2 h. The ITO glass substrates were purchased from Merck
(Germany) and PEDOT:PSS (Baytron PH500) from HC Starck
AG (Germany). The 50-nm thick PEDOT:PSS layers were pre-
pared by spin coating and dried in air at 396 K for 15 min.
The calcium (20 nm), and subsequently, 60–80 nm thick alu-
minum electrodes were vacuum-evaporated on the top of
polymer films to form PV device. Typical active areas of the
PV devices were 12 mm2. Current–voltage characteristics of
PV devices were measured using the Keithley 237 source
measure unit. A 300-W xenon lamp (Oriel) with a filter
WG35 was used for illumination. The light intensity was
checked by radiometer Rs 3960 (Laser precision). All thin
film preparations and the device fabrication were done in a
glove box under nitrogen atmosphere.
2,5-Dibromo-3,4-dinitrothiophene, 3-hexyl-2-(tributylstannyl)-
thiophene, and 3-(2-ethylhexyl)-2-(tributylstannyl)thiophene
were prepared according to literature methods.8–10
4,6-Di(thien-20-yl)thieno[3,4-c][1,2,5]thiadiazole was also
prepared according to literature.11,12 Fullerene derivative
[60]PCBM was purchased from SolenneBV (Netherlands).
Copolymers CTF, CHTF, and CDTF were prepared by the
Suzuki coupling. Detailed syntheses of monomers and poly-
mers CTF, CHTF, and CDTF are described in our previous
articles.5–7 CTF: Mw ¼ 4400, Mn ¼ 3100, CHTF: Mw ¼ 8900,
Mn ¼ 3050, and CDTF: Mw ¼ 73,700, Mn ¼ 29,000.
Methods
High-resolution 1H NMR and 13C NMR spectra were meas-
ured (in a CDCl3 or DMSO-d6 solution) with an upgraded
Bruker Avance DPX-300 spectrometer at 300.13 and 75.45
MHz, respectively, using hexamethyldisiloxane as an internal
standard. The spectra were measured with a broadband
probe, taking 32 and 12,000 scans for 1H and 13C, respec-
tively. 13C CP/MAS NMR spectra were recorded on a Bruker
AVANCE 500 spectometer (Larmor frequencies m(13C) ¼
125.783 MHz) using a 4-mm MAS probe. The number of
scans for accumulation of 13C CP/MAS NMR spectra was
1200, repetition delay of 5 s, and spin lock pulse of 2 ms.
During detection, a high-power dipolar decoupling was used
to eliminate strong heteronuclear dipolar coupling. The iso-
tropic chemical shift of 13C scale was calibrated with glycine
external standard (176.03 ppm to carbonyl signal). The spin-
ning speed of rotor sample was 11 kHz. FTIR spectra were
measured on a Perkin-Elmer Paragon 1000 PC Fourier trans-
form infrared spectrometer. SEC measurements were per-
formed using Liquid chromatograph CALC 100 (LABIO,
Prague, Czech Republic), with one column PL gel MIXED-C,
Synthesis of Monomers
3,300-Bis(2-ethylhexyl)-30,40-dinitro-2,20:50,200-terthiophene (3)
2,5-Dibromo-3,4-dinitrothiophene (16 g, 48.2 mmol), 3-(2-
ethylhexyl)-2-(tributylstannyl)thiophene (60 g, 123.6 mmol),
(Ph3P)2PdCl2 (1.1 g, 1.56 mmol), and CuI (0.3 g, 1.58 mmol)
were dissolved in anhydrous 1,4-dioxane (500 mL) under ar-
gon and refluxed for 3 h. The solvent was evaporated in vac-
uum, and a crude product was purified by column chroma-
tography on silica gel in toluene/heptane (1:5 by vol.). The
product was obtained as yellow oil.
Yield: 20 g (74%). 1H NMR (CDCl3, 296 K): d (ppm) ¼ 7.46
(d, J ¼ 5.1 Hz, 2H), 6.96 (d, J ¼ 5.8 Hz, 2H), 2.52 (d, J ¼ 7.2
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SYNTHESIS AND PROPERTIES OF LOW-BANDGAP COPOLYMERS, CIMROVA ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Hz, 4H), 1.62 (m, 2H), 1.37–1.30 (m, 8H), 1.23–1.19 (m, 8H),
0.92–0.78 (m, 12H). 13C NMR (CDCl3, 296 K): d (ppm) ¼
145.58, 138.69, 135.04, 129.78, 128.95, 120.92, 40.58, 33.48,
32.67, 28.86, 25.78, 22.98, 14.15, 10.82 (each signal repre-
sents 2C).
130.77, 113.33, 111.82, 39.77, 34.58, 32.54, 28.70, 25.76,
23.08, 14.15, 10.74 (each signal represents 2C). Anal. Calcd.
for C28H36Br2N2S4 (688.67): C, 48.83; H, 5.27; N 4.07; Found:
C, 48.97; H, 5.00; N 3.93.
2,5-Bis(isopentyloxy)benzene-1,4-diboronic acid (6)
To a solution of 1,4-dibromo-2,5-bis(isopentyloxy)benzene
(5; 5.00 g, 12.3 mmꢂol) in absolute diethyl ether (110 mL)
under argon at ꢁ78 C, the n-butyllithium (2.5 M solution in
hexane, 25 mL, 62.5 mmol) was added slowly. The reaction
mixture was stirred at ꢁ78 ꢂC (1 h) and at laboratory tem-
perature (1 h), and then it was poured into a solution of tri-
methyl borate (18 mL, 160 mmol) dissolved in diethyl ether
(100 mL) at ꢁ78 ꢂC. This mixture was allowed to warm up
slowly to room temperature, and it was further stirred for
12 h. Hydrochloric acid (100 mL, 2 M HCl) was then added
and mixed for 24 h. The white precipitate was filtered off,
and the ether part was evaporated. Both parts were com-
bined and dissolved in hot acetone (250 mL) and filtered.
Addition of 2 M HCl (150 mL) to the filtrate causes the pre-
cipitation of compound 6.
3,300-Bis(2-ethylhexyl)-30,40-diamino-2,20:50,200-terthiophene
(4)
Terthiophene 3 (5.00 g, 8.88 mmol) was dissolved in boiling
dichloromethane (300 mL), and activated zinc dust (5 g, 77
mmol) was added. Zinc was activated with 5% HCl (5 min),
then washed with MeOH and diethylether, and dried in vac-
uum. The formic acid (10 mL, 12.2 g, 265 mmol) was added
dropwise (5 min) to the boiling stirred mixture of 3 and zinc
dust. After additional 30 min, the mixture was cooled
quickly, zinc dust was filtered off, and the obtained light-
brown solution was washed with 5 wt % aqueous K2CO3
(slight excess of carbonate in respect to formic acid was
used) Organic phase was dried with MgSO4, and the solvent
was removed in vacuum. Light brown oil resulted as the
product. Yield: 4.0 g (90%). The crude diamine-free base 4
was immediately used in the next reaction step.
Yield: 3.15 g (76%). M. p. 133–134 ꢂC. 1H NMR (DMSO-d6,
296 K): d (ppm) ¼ 7.78 (s, 4H), 7.19 (s, 2H), 4.01 (t, J ¼
13.2 Hz, 4H), 1.80–1.71 (m, 2H), 1.63 (q, J ¼ 18 Hz, 4H),
0.92 (d, J ¼ 6 Hz, 12H). Anal. Calcd. for C16H28B2O6
(338.01): C, 56.85; H, 8.35; Found: C, 56.87; H, 8.55.
4,6-Bis(30-(2-ethylhexyl)thien-20-yl)
thieno[3,4-c][1,2,5]thiadiazole (EHT)
Crude diamine 4 (4.00 g, 7.95 mmol) was dissolved in dry
pyridine (200 mL) under argon, N-thionylaniline was added
(5.0 g, 35.9 mmol), and the mixture was quickly heated up
to 80 ꢂC. After 5 min, the chlorotrimethylsilane (12 mL, 9.7
g, 89 mmol) was syringed into the solution and reacted for
1 h. The most of pyridine and chlorotrimethylsilane was
evaporated in vacuum, and the residue was twice extracted
with MeOH (2 ꢃ 100 mL). Resulting dark blue oil was dried
and chromatographed on silica gel in toluene/heptane (1:5
by vol.). The product was obtained as blue oil.
2,5-Bis(isopentyloxy)-1,4-bis(1,3,2-dioxaborinan-2-yl)
benzene (7)
The esterification of diboronic acid 6 (3.00 g, 8.88 mmol)
with 1,3-propanediol (2 mL, 1.42 g, 18.64 mmol) was per-
formed (24 h) in a mixed solvent toluene/1,1,2-trichloro-
ethane (200 mL þ 50 mL) using Soxhlet apparatus contain-
ing MgSO4 in a thimble for water capture. The mixed solvent
was evaporated, and the product was obtained as colorless
crystals after double recrystalization from hexane.
1
Yield: 2.0 g (47%). H NMR (CDCl3, 296 K): d (ppm) ¼ 7.38
(d, J ¼ 6 Hz, 2H), 7.00 (d, J ¼ 6 Hz, 2H), 2.86 (d, J ¼ 6 Hz,
4H), 1.71 (m, 2H), 1.33–1.27 (m, 8H), 1.23–1.19 (m, 8H),
0.84–0.79 (m, 12H). 13C NMR (CDCl3, 296 K): d (ppm) ¼
156.83, 139.59, 130.79, 129.13, 125.84, 112.84, 39.84, 34.41,
32.59, 28.72, 25.82, 23.10, 14.15, 10.76 (each signal repre-
sents 2C). Anal. Calcd. for C28H38N2S4 (530.87): C, 63.35; H,
7.21; N, 5.28; Found: C, 62.90; H, 6.93; N, 5.10.
1
ꢂ
Yield: 2.12 g (57%). M. p. 62–64 C. H NMR (DMSO-d6, 296
K): d (ppm) ¼ 6.93 (s, 2H), 4.03 (t, J ¼ 10.5 Hz, 8H), 3.83 (t,
J ¼ 12.9 Hz, 4H), 2.00–1.93 (m, 4H), 1.86–1.79 (m, 2H), 1.56
(q, J ¼ 15.6 Hz, 4H), 0.92 (d, J ¼ 6 Hz, 12H). Anal. Calcd. for
C22H36B2O6 (418.14): C, 63.19; H, 8.68; Found: C, 63.18; H,
8.64.
Synthesis of Polymers
Copolymer CEHTF
4,6-Bis(50-bromo-30-(2-ethylhexyl)thien-20-yl)
thieno[3,4-c][1,2,5] thiadiazole (EHTBr)
The EHTBr (0.891 g, 1.294 mmol) and 9,9-bis(2-ethylhexyl)-
2,7-bis(1,3,2-dioxaborinan-2-yl)fluorene (2.58 mL of 0.5 M
solution, 1.29 mmol) were dissolved in THF (40 mL) and
aqueous 15 wt % NaHCO3 was added (40 mL). The reaction
mixture was flushed with argon (15 min) and the Pd(PPh3)4
(0.060 g, 0.052 mmol) was added. The reaction apparatus
was then several times degassed repeating the vacuum/ar-
gon cycles, and the mixture was refluxed for 23 h under ar-
gon. Phenylboronic acid pinacol ester (0.30 g, 1.47 mmol)
was then added, and the mixture was refluxed for another
2 h. End-capping of polymer was finished by refluxing with
bromobenzene (0.5 mL, 0.74 g, 4.61 mmol) for 2 h. The reac-
tion mixture was then cooled, water phase was discarded,
Terthiophene EHT (2.0 g, 3.76 mmol) was dissolved in pyri-
dine (50 mL) at 40 ꢂC, and N-bromosuccinimide (NBS; 1.39
g, 7.8 mmol) was added in several portions during 30 min
under the absence of light. Reaction mixture was stirred for
another 30 min, pyridine was evaporated in vacuum, and a
crude product was chromatographed on silica gel in tolu-
ene/heptane (1:5 by vol.). The product was obtained as blue
crystals.
1
ꢂ
Yield: 2.1 g (81%). m.p. 59–63 C. H NMR (CDCl3, 296 K): d
(ppm) ¼ 6.91 (s, 2H), 2.77 (d, J ¼ 6 Hz, 4H), 1.68 (m, 2H),
1.37–1.32 (m, 8H), 1.26–1.20 (m, 8H), 0.83 (m, 12H). 13C
NMR (CDCl3, 296 K): d (ppm) ¼ 156.72, 139.93, 133.26,
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SCHEME 1 Synthesis of mono-
mers.
organic phase reduced in volume, and the polymer was pre-
cipitated into MeOH and collected on a frit. After washing
with water, acetone, hexane, boiling diethylether, and drying,
the polymer was extracted in Soxhlet apparatus with chloro-
form. This solution was reduced in volume and precipitated
into MeOH. Finally, obtained material after filtration and dry-
ing was reprecipitated from THF into MeOH, filtered, and
vacuum dried to constant weight. The CEHTF was obtained
as a black-green powder.
and the mixture was refluxed for 3 h. End-capping of copoly-
mer was finished by refluxing with bromobenzene (1 mL,
1.495 g, 9.23 mmol) for 3 h. The reaction mixture was
cooled, water phase was discarded, organic phase reduced in
volume and precipitated into MeOH, and dried. After washing
(acetone, diethylether) and drying, the polymer was dissolved
in 1,2-dichlorobenzene (50 mL) at 100 ꢂC under argon. The
solution was then cooled to room temperature (Ar) and 2 h
later filtered through glass frit S3. Finally, the filtered poly-
mer solution was precipitated into MeOH, collected, and dried
to a constant weight. The CEHTP was obtained as black-green
flakes.
Yield: 0.880 g (74%). GPC(THF): Mw ¼ 95,000, Mn ¼ 31,700.
1H NMR (CDCl3, 330 K, broad signals observed): d (ppm) ¼
7.69 (6H), 7.23 (2H), 2.95 (4H), 2.10 (2H), 1.84 (2H), 1.43
(8H), 1.29 (8H), 0.93–0.87 (32H), 0.68 (6H), 0.58 (6H). 13C
NMR (CDCl3, 330 K): d (ppm) ¼ 157.20 (2C), 151.92 (2C),
145.30 (2C), 140.99 (2C), 132.80 (2C), 129.24 (2C), 126.87
(2C), 125.10 (2C), 121.50 (2C), 120.28 (2C), 113.31 (2C),
113.23 (2C), 55.49 (1C), 45.04 (2C), 40.18 (2C), 35.30 (2C),
34.31 (2C), 33.06 (2C), 29.08 (4C), 28.62 (2C), 27.53 (2C),
26.34 (2C), 23.21 (2C), 22.91 (2C), 14.13 (2C), 14.04 (2C),
10.99 (2C), 10.57 (2C). Anal. Calcd. for C57H76N2S4 structural
unit: C, 74.63; H, 8.36; N, 3.06; Found: C, 74.31; H, 8.05; N,
2.84.
Yield: 1.070 g (71%). GPC(THF): Mw ¼ 5100, Mn ¼ 4300.
13C CP/MAS NMR (very broad signals observed): d (ppm) ¼
156.47 (2C), 149.11 (4C), 137.17 (4C), 133.16 (2C), 126.06
(2C), 121.66 (2C), 111.98 (2C), 107.93 (2C), 66.89 (2C),
38.27 (6C), 34.42 (2C), 29.03 (2C), 23.26 (8C), 14.34 (2C),
11.66 (2C). Anal. Calcd. (for C44H60N2O2S4 structural unit): C,
68.00; H, 7.78; N, 3.60. Found: C, 67.50; H, 7.24; N 3.45.
RESULTS AND DISCUSSION
Synthesis of Monomers
Copolymer CEHTP
Synthesis of terthiophene comonomer EHTBr is shown in
Scheme 1. 2,5-Dibromo-3,4-dinitrothiophene and 3-(2-ethyl-
hexyl)-2-tributylstannylthiophene were prepared after pub-
lished procedures.6 Preparation of dinitro terthiophene 3
was done using Stille coupling as was described in our ear-
lier article for its nonalkylated analog, as well as for dihexyl
and didodecyl derivatives.6 A subsequent reduction of both
nonalkylated and alkylated dinitroterthiophenes to unstable
diamines was carried out according to the published
The EHTBr (1.340 g, 1.946 mmol) and 7 (0.814 g, 1.946
mmol) were dissolved in THF (70 mL), and aqueous 15 wt %
NaHCO3 was added (50 mL). The reaction mixture was
flushed with argon (15 min), and the Pd(PPh3)4 (0.090 g,
0.077 mmol) was added. The reaction apparatus was then
several times degassed repeating the vacuum/argon cycles,
and the mixture was refluxed for 16 h under argon. Phenyl-
boronic acid pinacol ester (0.30 g, 1.47 mmol) was added,
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SYNTHESIS AND PROPERTIES OF LOW-BANDGAP COPOLYMERS, CIMROVA ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 2 Synthesis of the copolymers.
procedure using Sn or SnCl2 in HCl/EtOH mixtures.11
Although this method worked well for nonalkylated com-
pound, a reduction of hydrophobic dihexyl and didodecyl de-
rivative in the highly polar reaction system was tedious and
slow, accompanied with major side reactions. Therefore, we
adopted for this purpose a quick and efficient reduction of
aromatic nitro compounds using formic acid and activated
zinc dust,13 with a boiling dichloromethane as a suitable
nonpolar solvent. Resulting diamine 4 was immediately used
in the next reaction step (a closure of thienothiadiazole ring,
leading to EHT). Bromination of EHT to EHTBr in pyridine
proceeded quickly anꢂd with a high yield, when pyridine was
slightly heated to 40 C.
alkyl chains results in the high-molecular weight polymer
with average molar mass similar or slightly higher than that
reported in our previous paper6 for CDTF copolymer with
long linear dodecyl chains.
Alternating copolymer CEHTP was prepared by Suzuki
cross-coupling reaction of EHTBr with 7 in boiling THF using
the Pd(PPh3)4 and aqueous NaHCO3 as above. Optimal reac-
tion time was found by means of test polymerizations. The
obtained copolymer was partly soluble in common solvents
such as THF, toluene, and chloroform, but the best solvents
were found to be chlorobenzene and 1,2-dichlorobenzene.
The copolymer was characterized by elemental analysis and
13C CP/MAS NMR. Molar mass of THF-soluble fraction of
CEHTP was Mw ¼ 5100 and Mn ¼ 4300. The low values are
due to the limited solubility of the CEHTP in THF. Higher av-
erage relative molecular weights are expected for 1,2-
dichlorobenzene soluble CEHTP fraction. This expectation
was proved by absorption measurements of THF-soluble and
1,2-dichlorobenzene-soluble CEHTP fractions in 1,2-dichloro-
benzene. Absorption maximum corresponding to the lowest
p–p* transition (backbone conjugation) in absorption spec-
trum of THF-soluble fraction was 19-nm blueshifted com-
pared with that in absorption spectrum of 1,2-dichloroben-
zene-soluble CEHTP fraction. For the following photophysical
studies, we used the high-molecular weight CEHTP soluble
in 1,2-dichlorobenzene.
Synthesis of 2,5-bis(isopentyloxy)benzene-1,4-diboronic acid
(6) and its 1,3-propanediol diester 2,5-bis(isopentyloxy)-1,4-
bis(1,3,2-dioxaborinan-2-yl)benzene (7) was performed
according to modified published14,15 procedure (Scheme 1).
In comparison with the published procedure we used a
higher excess of n-BuLi for lithium-halogen exchange in the
first step to avoid a formation of monolithioaromatic inter-
mediate. Of course, an adequate amount of trimethyl borate
was necessary to use in the second step. For the esterifica-
tion reaction, we applied both the Dean-Stark and Soxhlet
apparatus. With Soxhlet apparatus equipped with a thimble
containing a drying agent (MgSO4), we obtained a better
yield of 7.
Synthesis of Polymers
Thermal Properties
Alternating copolymer CEHTF was synthetized by Suzuki
coupling reaction of EHTBr with 9,9-bis(2-ethylhexyl)-2,7-
bis(1,3,2-dioxaborinan-2-yl)fluorene in THF, using the
Pd(PPh3)4 as a catalyst and aqueous NaHCO3 as a source of
necessary base (Scheme 2). Copolymer soluble in common
solvents such as THF or chloroform was obtained in a high
yield with a high molar mass. The average relative molecular
weights, Mw ¼ 95,000 and Mn ¼ 31,700, were determined
by GPC measurement in THF using polystyrene standards. It
indicates that using the EHTBr comonomer with branched
The thermal properties of the copolymers were investigated
by thermogravimetric analysis (TGA) at a heating rate of 10
K/min. Typical TGA curves of the copolymers in air are
shown in Figure 2. The TGA analysis data are summarized in
Table 1, where the data for the CTF, CHTF, and CDTF are
introduced for comparison. The copolymers exhibited very
good thermal stability even in the air. The 10% weight loss
temperatures were detected at 428 and 416 ꢂC in nitrogen
(at 419 and 410 ꢂC in air) for CEHTF and CEHTP, respec-
tively. The residual weight percentage in nitrogen at 780 ꢂC
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ARTICLE
uted to the conformational changes due to the attachment of
more bulky branched alkyl chains.
Optical properties of the copolymers were studied in solu-
tions and thin films. Absorption spectra of the new CEHTF
copolymer, measured in toluene and 1,2-dichlorobenzene sol-
utions (c ꢀ 10ꢁ5 M) are shown in Figure 4(a), where also
the absorption spectrum of CDTF in 1,2-dichlorobenzene is
displayed for comparison. Three maxima were present, simi-
larly to the CTF, CHTF, and CDTF absorption spectra. The
shortest wavelength maximum is located nearly at the same
position as in the EHT unit, whereas the other two maxima
are significantly redshifted due to the backbone conjugation.
The absorption band at about 440–450 nm can be assigned
to the lowest p–p* transition, whereas the lowest energy
absorption band to the ICT-type transition between the thio-
phene donor and thienothiadiazole acceptor moiety. The
positions of the long-wavelength maxima in solution depend
on the solvent. Redshifts are observed in 1,2-dichlobenzene
when compared with the spectra in toluene, THF, or chloro-
form. The change in the spectra of polymer solutions can be
explained by different conformational modifications in vari-
ous solvents influencing also the strength of ICT. The absorp-
tion maxima are summarized in Table 2, where the data for
the copolymers CHTF and CDTF with linear alkyl chains are
also given. It is evident that the absorption maxima of
CEHTF with the branched alkyl chains are blueshifted com-
pared with the absorption maxima of CHTF and CDTF
copolymers with linear alkyls. The blue shift could be in the
consequence of lower CEHTF backbone planarity. On the
other hand, the new CEHTP copolymer exhibits even more
extended absorption up to near infrared (NIR) [see Fig.
4(b)]. Absorption spectra measured in 1,2-dichlorobenzene
FIGURE 2 TGA curves of CEHTF and CEHTP copolymers meas-
ured at scan rate of 10 K/min in nitrogen (solid) and in air
(dashed).
was 48 and 38% for CEHTF and CEHTP, respectively. This is
a high value due to the high aromatic content. The decrease
is a result of an additional decomposition of alkyl chains on
thiophene rings and fluorene or alkoxy chains in CEHTP.
Weight loss about 3% was observed at high temperatures, at
ꢂ
ꢂ
367 and 328 C in nitrogen and at 351 and 312 C in air for
CEHTF and CEHTP, respectively. It indicates a very good
thermal and/or oxidation stability of the new polymers; bet-
ter than those for CHTF and CDTF with linear alkyls.
Optical Properties
Absorption spectra of the new EHT unit measured in various
solvents are shown in Figure 3. The shape of the spectra is
similar as reported in our previous article6 for the unit T
and its derivatives (HT and DT) with linear hexyl and do-
decyl chains. The absorption shoulder at about 350–366 nm
can be assigned to the lowest p–p* transition and the lowest
energy absorption with maxima at 592–615 nm to the ICT
type transition between the thiophene donor and thienothia-
diazole acceptor moiety. The long wavelength absorption
maximum was located 592 nm in THF, at 600 nm in chloro-
form and toluene, in 1,2-dichlorobenzene redshifted to 615
nm and 5–6 nm blueshifted compared with its position in
HT and DT absorption spectra. The blueshift can be attrib-
TABLE 1 Thermal Properties of Copolymers
Wres (%)
Td5% (ꢂC)
Td10% (ꢂC)
at 780 ꢂC
Polymer
N2
Air
N2
Air
N2
CEHTF
CHTF
402.7
355.4
365.6
389.4
384.6
318.8
294.5
364
427.9
405
419.3
369
48
45
43
38
CDTF
389.2
416.4
338.1
409.9
CEHTP
FIGURE 3 UV–vis absorption spectra of the EHT unit measured
in 1,2-dichlorobenzene(solid), toluene(dashed), chloroform
(dash and dot) and THF (dotted) solution.
Td5% and Td10% are decomposition temperatures at which 5 and 10%
weight loss was recorded, respectively, Wres residual weight percentage
at 780ꢂC under nitrogen.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
loxy)benzene unit lead to much stronger ICT state due to the
stronger donor character of 2,5-bis(isopentyloxy)benzene
unit than that of 9,9-bis(2-ethylhexyl)fluorene. In addition,
an absorption shoulder at about 915–920 nm is present in
the absorption spectrum of CEHTP solution, and its absorb-
ance was concentration dependent. The absorbance
increased with the increasing concentration indicating an ag-
gregate formation due to the intermolecular interactions.
The absorption spectra of CEHTF thin films (Fig. 5) are simi-
lar compared with those of its 1,2-dichlorobenzene solution;
there is no redshift. This is a difference compared with the
results obtained for the CHTF and CDTF copolymers with lin-
ear alkyls, where significant redshifts in positions of long
wavelength maxima were observed in thin film spectra com-
pared with the spectra in solutions due to strong intermolec-
ular interactions in solid state due to the close SAN intermo-
lecular contacts. The largest red shift was detected for CDTF
thin films with the linear dodecyl chains (see Table 2). Our
results indicate that linear alkyl chains improve the stabiliza-
tion of the supramolecular structure, whereas the branched
more bulky ethylhexyl chains hinder the strong intermolecu-
lar interactions. The intermolecular contacts due to S–N
interactions improve structural order. It seems that the
strength of the stacking is more shielded by bulky branched
chains, therefore similar spectra are observed for CEHTF sol-
utions and thin films. It should be also noticed that in fluo-
rene–thienothiadiazole copolymers, the longest wavelength
absorption maxima (assigned to ICT-type transition and to
the lowest p–p* transition) depended on the film thickness
(see Table 2). An increase in the film thickness indicates a
blueshift of the long wavelength maxima and also a decrease
in absorption coefficient.
FIGURE 4 UV–vis absorption spectra of the copolymers: (a)
CEHTF in toluene (dashed) and in 1,2-dichlorobenzene (solid),
and CDTF in 1,2-dichlorobenzene (dotted) (b) CEHTP in 1,2-
dichlorobenzene of various molar concentrations 1.15 ꢃ 10ꢁ5
(solid), 3.15 ꢃ 10ꢁ5 (dashed), and 4.2 ꢃ 10ꢁ5 (dotted).
The absorption of CEHTP thin films is extended up to NIR
with the long-wavelength maxima located at 496 and 918
nm. The lowest p–p* transition is significantly redshifted
compared with that in corresponding solution spectrum. The
long-wavelength absorption region 750–1100 nm consists of
the contribution of ICT-type transition and also aggregate
absorption, which prevails in solid state giving the rise of
the maximum at 918 nm. This indicates strong intermolecu-
lar stacking important for charge transport properties.
solution exhibited maxima at 488 and 820 nm, correspond-
ing to the lowest p–p* transition and ICT-type transition,
respectively. Compared with CEHTF, the first long-wavelength
maximum at 488 nm is by about 30 nm shifted to longer
wavelength, whereas the second one at 820 nm is even
more shifted by about 86 nm. It demonstrates that the
change of 9,9-bis(2-ethylhexyl)fluorene by 2,5-bis(isopenty-
TABLE 2 Optical Properties of Copolymers Measured in Diluted Solutions and Thin Films
Toluene
1,2 Dichlobenzene
Thin Film
kmax (nm)
Copolymer
CEHTF
kmax (nm)
kmax (nm)
d (nm)
316, 442, 713
316, 450, 734
315, 441, 734
315, 442, 727
315, 466, 772
315, 453, 755
315, 473, 785
314, 465, 771
314, 463, 759
314, 496, 918
104
168
50
CHTF
CDTF
314, 447, 721
314, 448, 724
316, 455, 744
316, 456, 747
158
44
97
173
99
CEHTP
–
316, 480, 820
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ARTICLE
ured are displayed in Figure 6. The copolymers exhibited re-
versibility in both oxidation and reduction. Electrochromic
changes of the color on reduction and oxidation were
observed. The CV curve of CEHTF copolymer shows two
clear p- and n-doping peaks followed by corresponding
undoping peaks similarly as CV curves of copolymers CHTF
and CDTF,6 which CV curve is also shown in Figure 6(a)
(dashed) for comparison. The CV curves of new CEHTP co-
polymer [Fig. 6(b)] show broader p-doping peaks and two
clear n-doping peaks followed by corresponding undoping
peaks. The ionization potential (HOMO level), EIP, and elec-
tron affinity (LUMO level), EA, were estimated from onset
potentials Eonset from the first oxidation and reduction peaks
on the basis of the reference energy level of ferrocene
(4.8 eV below the vacuum level) using the equation
EIP(EA) ¼ – (Eonset – Eferr) – 4.8 (eV), where Eferr is the value
for ferrocene versus the Ag/Agþ electrode.16 The onset val-
ues differed in the first scan from the other ones, and there-
fore, the first scan was not used for evaluation of the elec-
tronic properties. The EIP and EA values were evaluated as
the average from CV curves measured on several samples at
scan rates 20 and 50 mV sꢁ1 and are given in the Table 3,
where also the values for copolymers CHTF and CDTF with
linear alkyls are introduced. The new CEHTF copolymer pos-
sesses a similar ionization potential at about 5 eV as was
determined for the copolymers CHTF and CDTF with linear
alkyl chains. The ionization potential of new CEHTP copoly-
mer is 4.56 eV, that is, nearly 0.5 eV lower than in the
CEHTF due to the stronger donor character of the 2,5-bis-
(isopentyloxy)benzene unit in CEHTP compared with the
9,9-bis(2-ethylhexyl)fluorene in CEHTF backbone. The simi-
lar electron affinity values 3.56 and 3.52 eV were determined
for CEHTF and CEHTP, respectively, which are also similar to
those determined for CHTF and CDTF. It is not surprising
because the electron affinity is determined by EHT unit. As it
was already reported in our previous article,6 the introduc-
tion of the alkyl chains on the thiophene rings lead only to a
small decrease in electron affinity resulting in a small
increase in the bandgap value of CHTF, CDTF, and CEHTF
copolymers compared with CTF. In fluorene-thienothiadiazole
copolymers, there is no significant dependence of electro-
chemical properties on the nature of the alkyl chain. The
value of the electrochemical bandgap (Eeglc ¼ 1.5 eV) for
CEHTF is similar to that determined for CDTF. The Eeglc val-
ues are in good agreement with the optical bandgap values
evaluated from the absorption edge of thin film spectra
(Eogpt). The change of 9,9-bis(2-ethylhexyl)fluorene unit by
2,5-bis(isopentyloxy)benzene unit lead to a decrease of
FIGURE 5 UV–vis absorption spectra of the copolymers of
CEHTF (solid), CDTF (dotted), and CEHTP (dashed) thin films
(the thickness about 100 nm).
Electrochemical Properties
CV was used to obtain the information on electronic struc-
ture of the copolymers. Representative CV curves of thin
CEHTF and CEHTP copolymer films coated on Pt wire meas-
TABLE 3 Electronic Properties of Copolymrs
HOMO
LUMO
Eeglc
Eogpt
Copolymer
EIP (eV)
EA(eV)
(eV)
(eV)
FIGURE 6 Representative cyclic voltammograms of thin films
coated on Pt wires made of (a) CEHTF (solid) and CDTF
(dashed) recorded at the scan rate of 50 mV sꢁ1, and (b) CEHTP
recorded at the scan rates of 50 mV sꢁ1 (solid) and 20 mV sꢁ1
(dashed).
CEHTF
CHTF
5.04
4.94
5.02
4.56
3.56
3.54
3.52
3.52
1.50
1.40
1.50
1.05
1.37
1.36
1.36
1.15
CDTF
CEHTP
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SYNTHESIS AND PROPERTIES OF LOW-BANDGAP COPOLYMERS, CIMROVA ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
fill factor, FF, 25–30%. PV devices with CEHTP þ [60]PCBM
(1:1) active layer exhibited better performance compared
with those made of CEHTF þ [60]PCBM (1:1) blend. The
power efficiency was at about 1.5%, Voc in the range 0.3–
0.34 V, short-circuit currents 8–9 mA cmꢁ2, and fill factor
30–40%. Examples of dark and PV current–voltage charac-
teristics for ITO/PEDOT:PSS/CEHTF þ [60]PCBM(1:1)/Ca/Al
and ITO/PEDOT:PSS/CEHTP þ [60]PCBM(1:1)/Ca/Al devices
are shown in Figure 8(a,b), respectively.
The lower efficiency of CEHTF þ [60]PCBM PV devices were
gained when compared with those of the devices made of
blends CHTF þ [60]PCBM and CDTF þ [60]PCBM also in
weight ratio 1:1, which exhibited the power conversion effi-
ciency 2–3% as reported in our previous article.7 The lower
efficiency of CEHTF þ [60]PCBM PV devices is attributed to
lower short-circuit currents and also slightly to the lower fill
factor. Voc values were in the same range 0.5–0.6 V. The exci-
ton dissociation and charge carrier generation and also
charge carrier mobility could be affected by the alkyl chain
nature. A similar result, a lower PV device performance, was
obtained in the study of a series of anthracene-containing
poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinyl-
ene)s (PPE-PPV) copolymers for polymers with branched
FIGURE 7 Absorption spectra of 109-nm-thick film CEHTF þ
[60]PCBM (1:1 wt; dashed) 105-nm-thick film made of CEHTP þ
[60]PCBM (1:1 wt; solid).
ionization potential, which resulted in the low bandgap of
the copolymer CEHTP (Eeglc ¼ 1.05 eV). The 2,5-bis(isopenty-
loxy)benzene unit has stronger donor character than 9,9-
bis(2-ethylhexyl)fluorene unit and therefore the ICT is much
stronger in CEHTP than CEHTF as was already discussed
above.
Photovoltaic Properties
PV devices with bulk heterojunction made of blends of new
copolymers and fullerene derivative [60]PCBM in weight ra-
tio 1:1 were prepared and characterized. ITO/PEDOT:PSS
and Ca/Al electrodes were used. Absorption spectra of blend
layers corresponding to the active layers in CEHTF þ
[60]PCBM (1:1 wt) and CEHTP þ [60]PCBM (1:1 wt) PV
devices are shown in Figure 7. Compared with the absorp-
tion of thin films made of neat polymers an increase in UV
absorption in the blend layers is observed due to the
[60]PCBM content and a decrease in absorption in visible
region. The long-wavelength absorption maxima of CEHTF þ
[60]PCBM layer located at 443 and 744 nm are redshifted
compared with maxima at 441 and 734 nm for the neat
CEHTF thin film. The absorption maxima CEHTP
þ
[60]PCBM layer at 499 and 938 nm are also redshifted com-
pared with the maxima (496 and 918 nm) for the neat
CEHTP thin film. Similar redshifts were observed in absorp-
tion spectra of P3HTþ [60]PCBM blend films compared with
neat polymer films, and related to an order of the polymer
phase and formation of crystalline nanodomains.17 This ex-
planation could be partly adopted also in our case including
nanodomain aggregate formation influencing ICT-type transi-
tion. The higher tendency to aggregation in CEHTP was evi-
dent already from the observed absorption concentration de-
pendence in its solution.
FIGURE 8 Current–voltage characteristics PV devices (a) ITO/
The power conversion efficiency of the devices with the
active layer made of CEHTF þ [60]PCBM (1:1 wt) blend was
in the range 0.2–0.3%. Typical open-circuit voltages, Voc,
about 0.5–0.6 V, short-circuit currents, Isc, 1–2 mA cmꢁ2, and
PEDOT:PSS/CEHTF
þ
[60]PCBM (1:1)/Ca/Al and (b) ITO/
PEDOT:PSS/CEHTP þ [60]PCBM (1:1)/Ca/Al measured in dark
(solid circles) and under illumination 80 mW cmꢁ2 (open
circles).
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ARTICLE
good thermal stability and ambipolar electrochemical behav-
ior. Their absorption in thin films covers whole visible spec-
tral region. CEHTP thin films absorption is extended up to
NIR and redshifted compared with CEHTF absorption due to
the stronger donor character of 2,5-bis(isopentyloxy)benzene
unit than that of 9,9-bis(2-ethylhexyl)fluorene. In CEHTF thin
film absorption, the redshift was not detected compared
with solution absorption. This indicates that bulky branched
side chains protect strong intermolecular interactions
observed in analogous copolymers with linear alkyl chains.
The optical bandgap values of 1.37 and 1.15 eV for CEHTF
and CEHTP, respectively, corresponded well with the electro-
chemical bandgap values of 1.5 and 1.05 eV. The CEHTF co-
polymer possesses a similar ionization potential at about 5
eV as was determined for the copolymers CHTF and CDTF
with linear alkyl chains. On the other hand, the ionization
potential of CEHTP copolymer is lower (4.56 eV). The similar
electron affinity values 3.56 and 3.52 eV were determined
for CEHTF and CEHTP, respectively. PV devices with bulk het-
erojunction made of blends of new copolymers and fullerene
derivative [60]PCBM in weight ratio 1:1 were prepared and
characterized. PV devices made of CEHTP þ [60]PCBM
exhibited higher photoconversion efficiency than those made
of CEHTF þ [60]PCBM. Performance of the PV devices using
CEHTF copolymer with branched alkyls was worse than
those using CHTF and CDTF copolymers with linear alkyls.
FIGURE 9 3D profilometer surface images of thin films made
of (a) CEHTF þ [60]PCBM (1:1 wt) and (b) CEHTP þ [60]PCBM
(1:1 wt).
The authors acknowledge the support of the Ministry of Edu-
cation, Youth and Sports of the Czech Republic (grant No.
1M06031).
bulky alkoxy chains compared with polymers with linear
side chains although the polymers with the branched chains
exhibited the higher charge carrier mobilities.18,19
REFERENCES AND NOTES
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¼
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