Synthesis and properties of new low band gap semiconducting polymers

Article abstract of DOI:10.1166/jnn.2014.8426

Low band gap organic semiconducting polymers were prepared as p-type donors for organic photovoltaic devices. A novel dibrominated monomer composed of phenothiazine, thiophene, and benzothiadiazole (DPDTBT) was synthesized as a low band gap core block. DP

Full text of DOI:10.1166/jnn.2014.8426

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 5187–5191, 2014
Copyright © 2014 American Scientific Publishers
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Printed in the United States of America
Synthesis and Properties of New Low Band Gap
Semiconducting Polymers
Ji-Hoon Kim1, Jun Kang2, Dongbo Mi1, Fei Xu1, Sung-Ho Jin3,
Ho-Hwan Chun4, and Do-Hoon Hwang1ꢀ∗
1Department of Chemistry, and Chemistry Institute for Functional Materials, Pusan National University,
Busan 609-735, Republic of Korea
2Department of Applied Chemistry, Kumoh National Institute of Technology, Kumi 730-701, Korea
3Department of Chemistry Education and Interdisciplinary Program of Advanced Information and
Display Materials, Pusan National University, Busan 609-735, Korea
4Global Core Research Center for Ships and Offshore Plants (GCRC-SOP), Pusan National University,
Busan 609-735, Korea
Low band gap organic semiconducting polymers were prepared as p-type donors for organic
photovoltaic devices. A novel dibrominated monomer composed of phenothiazine, thiophene, and
benzothiadiazole (DPDTBT) was synthesized as a low band gap core block. DPDTBT was copoly-
merized with three different boronic esters of dithiophene, fluorene, and phenothiazine by the Suzuki
coupling polycondensation reaction. The band gap energies of the synthesized polymers ranged
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between 2.05 and 2.11 eV, depending on the polymer structure. Bulk heterojunction solar cells
Copyright: American Scientific Publishers
fabricated using the polymers and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) as an accep-
tor were characterized. The best power conversion efficiency obtained from the fabricated devices
under simulated AM 1.5 G solar irradiation of 100 mW/cm2 was 0.46%.
Keywords: Low Band Gap, Organic Semiconducting Polymer, Solar Cell.
1. INTRODUCTION
interface contact between the metal electrode and the
photoactive layer, and improved charge carrier mobility.
For application in bulk heterojunction OPVs, conjugated
polymers should have good solubility which would facil-
itate solution-processing and film-forming, show a strong
and broad absorption band in the visible near-IR region to
enable efficient sunlight harvesting, and have high charge
carrier mobility and purity.
Recently, we designed a new low band gap monomer
unit composed of phenothiazine, thiophene, and ben-
zothiadiazole (DPDTBT). 4,7-Bis(5-bromothiophene-2-yl)
benzo[c][1,2,5]thiadiazole was coupled with two phenoth-
iazine boronic esters by the Suzuki coupling reaction.
Phenothiazine is a well-known electro-active heterocyclic
compound with electron-rich sulfur and nitrogen het-
eroatoms. Molecules and polymers containing phenoth-
iazine moieties have recently attracted much research
interest.8–10 We expected that conjugation of phenothiazine
with the electron-withdrawing benzothiadiazole moiety
would result in improved hole mobility and a narrow
In the last few decades, organic semiconducting materi-
als have attracted considerable attention in science and
technology for use in diverse applications such as tran-
sistors, photovoltaic devices, and polymer light-emitting
diodes (PLEDs). This wide range of applicability of PLED
is due to their excellent electrical and optical proper-
ties and good processibility comparable to that of inor-
ganic semiconductors.1–3 Recently, organic photovoltaic
(OPV) cells fabricated using conjugated polymer and sol-
uble fullerene derivative composites have attracted much
attention because of their numerous advantages such as
low production cost and ease of manufacture by solution
processes.4–7 However, the power conversion efficiencies
(PCEs) of the polymer PV cells are lower than those of
silicon based solar cells. To improve the PCE of poly-
mer PV cells, efforts should be made to achieve efficient
light harvesting by the photoactive layer, and enhanced
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 7
1533-4880/2014/14/5187/005
doi:10.1166/jnn.2014.8426
5187

Synthesis and Properties of New Low Band Gap Semiconducting Polymers
Kim et al.
spectrophotometer, and a Shimadzu RF-5301PC fluorom-
eter, respectively. Thermo-gravimetric analysis (TGA) was
carried out using a Q500 analyzer at a heating rate of
S
1
O
B
(1) Pd(PPH3)4/K2CO3
N
N
S
O
+
Br
Br
(2) Br2/dichloromathane
S
S
N
R
2
ꢀ
10 C/min in a nitrogen atmosphere. The polymer molec-
S
N
N
ular weights were determined by gel permeation chro-
matography (GPV) using a GPCV 2000 instrument, with
THF as the eluent and polystyrene as the standard. Cyclic
voltammetry (CV) measurements were performed using
an AUTOLAB potentiostat/galvanostat at room tempera-
ture with a three-electrode cell in a solution of TBABF4
(0.10 M) in acetonitrile at a scan rate of 50 mV/s.
Br
S
S
Br
S
S
N
R
N
R
DPDTBT
(R:2-ethylhexyl)
O
B
R
O
O
B
S
O
O
O
S
B
O
O
B
B
O
S
B
O
O
N
R
R
R
R
O
(R:hexyl)
(R:2-octyl)
(R:2-ethylhexyl)
2.3. Fabrication of Polymer Solar Cells
Pd(0)
DPDTBT
Polymer solar cells with the structure ITO/PEDOT:PSS/
Polymers:PC70BM (1:5, w/w)/TiOx/Al (120 nm) were
fabricated as follows. A 40-nm-thick layer of poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) was spin-
coated onto a precleaned, indium tin oxide (ITO)-coated
glass substrate. This was followed by spin coating of the
polymer/PC70BM blend solution on to the substrate. The
concentration of the polymer:PC70BM blend solution used
for spin coating was 10 mg/mL, and dichlorobenzene and
CHCl3 were used as the solvents. The thickness of the
active layer was ∼80 nm. Then, the TiOx precursor solu-
tion was spin-cast in air on top of the polymer:PC70BM
composite layer. Subsequent heating of the composite for
10 min at 80 ꢀC in air resulted in hydrolysis of the precur-
S
N
R
2
N
S
S
S
*
*
S
n
S
S
N
R1
N
R1
R
2
PBTPTBT
S
(R1:2-ethylhexyl)
(R2:hexyl)
N
N
S
S
*
*
*
n
S
S
N
R1
N
R1
R
R
2
2
PFPTBT
S
(R1:2-ethylhexyl)
(R2:octyl)
N
N
S
S
S
*
S
S
N
R
N
R
N
R
n
PPPTBT
(R:2-ethylhexyl)
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sor and conversion to TiOx. Finally, the device fabrication
Synthetic route of low bandIgPa:p 4po6l.y1m6e1rs..61.111 On: Thu, 23 Jun 2016 12:09:36
Scheme 1.
was completed by deposition of Al metal electrodes by
Copyright: American Scientific Publishers
evaporation. The devices were illuminated using a solar
simulator (Oriel 300 W) equipped with an air mass (AM)
1.5 G filter at a power of 100 mW/cm2. The current
density–voltage (J–V ) characteristics were recorded with
a Keithley 236 source meter.
band gap. DPDTBT was copolymerized with three counter
boronic ester monomers of dithiophene, fluorene and
phenothiazine by the Suzuki coupling polycondensation
reaction to give PBTPTBT, PFPTBT and PPPTBT, respec-
tively. Depending on their structure, the synthesized
polymers showed band gap energies between 2.05 and
2.11 eV. Photovoltaic devices were fabricated using the
polymers and characterized. The synthetic route and poly-
mer structures are shown in Scheme 1.
2.4. Synthesis of Monomers and Polymers
4,7-Bis (5-bromothiophen-2-yl) benzo [c][1,2,5] thiadiazole
(1). 4,7-Di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (4.0 g,
13.3 mmol) was dissolved in CHCl3 (100 mL) and a cat-
alytic amount of acetic acid. Then, N-bromosuccinimide
(NBS) (5.5 g, 30.6 mmol) was added portion wise, and
the mixture was stirred for 36 h at room temperature with
light protection. The precipitate was filtered through a
Büchner funnel, and then washed with copious amounts
of MeOH, water, and acetone. The product was collected
2. EXPERIMENTAL DETAILS
2.1. Materials
Tetrakis(triphenylphosphine)-palladium, potassium car-
bonate, and bromine (Br2) were purchased from Aldrich
and used without further purification. All the sol-
vents were dried and purified under nitrogen. 4,7-
Bis(5-bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole11 and
10-(2-ethylhexyl)-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-10H-phenothiazine12 were synthesized according to
the reported methods.
1
as a red solid (4.6 g, 76%). H NMR (400 MHz, CDCl3ꢀ:
ꢁ 7.80 (dd, 4 H), 7.13 (d, 2 H). 13C NMR (400 MHz,
CDCl3ꢀ: ꢁ 154.2, 140.8, 131.1, 129.6, 129.1, 128.0,
111.6.
3-(4,4,5,5-Tetramethyl-1,3,2-di-oxaborolan-2-yl)-10-(2-
ethylhexyl)-phenothiazine (2). 3-Bromo-10-(2-ethylhexyl)-
phenothiazine (3.0 g, 7.7 mmol) was dissolved in
THF (150 mL) at −78 ꢀC. To the solution, 4.6 ml of
n-butyllithium (2.0 M in pentane) was added by a syringe.
2.2. Measurements
1H and 13C NMR spectra were recorded using a Bruker
AM-400 spectrometer. UV-vis and photoluminescence
(PL) spectra were recorded using a Shimadzu UV-3600
ꢀ
The mixture was stirred at −78 C for 2 h. 2-Isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3.1 mL) was
5188
J. Nanosci. Nanotechnol. 14, 5187–5191, 2014

Kim et al.
Synthesis and Properties of New Low Band Gap Semiconducting Polymers
added to the solution, and the resulting mixture was
stirred at −78 C for 1 h, warmed to room temperature,
3. RESULTS AND DISCUSSION
3.1. Characterization of the Polymers
ꢀ
and stirred for 40 h. The mixture was poured into water,
extracted with dichloromethane, and dried over MgSO4.
The solvent was removed by evaporation, and the residue
was purified by column chromatography using ethylac-
etate/hexane (1:9) as the eluent. The product was collected
as a green liquid. (2.7 g, 79%). 1H NMR (400 MHz,
CDCl3ꢀ: ꢁ 7.50 (m, 4 H), 6.91 (d, 1 H), 6.58–6.55 (m,
2 H), 3.80 (d, 2 H), 1.75 (m, 1 H), 1.32 (s, 12 H),
1.48–1.20 (m, 8 H), 0.85 (m, 6 H). 13C NMR (400 MHz,
CDCl3ꢀ: ꢁ 145.1, 138.2, 133.2, 132.8, 130.1, 129.5, 127.1,
123.2, 122.5, 120.1, 118.2, 115.0, 88.3, 68.1, 38.5, 33.2,
30.2, 26.9, 24.7, 23.5, 15.2, 12.1.
All the synthesized polymers were completely soluble in
common organic solvents such as THF and CHCl3. The
polymer solutions were spin-coated onto glass or ITO sub-
strates to form transparent, homogeneous thin films. The
number-average molecular weights (Mnꢀ of the polymers
as determined by GPC using a polystyrene standard, were
found to range from 7,700 to 15,100, with polydispersity
indices ranging from 2.3 to 2.9. The thermal properties
of the polymers were determined by TGA. All the poly-
mers were found to exhibit good thermal stability, with
weight lossꢀ of less than 5% upon heating to temperature
above 320 C. The molecular weights and decomposition
temperatures of the polymers are summarized in Table I.
Synthesis of 4,7-bis(5-(7-bromo-10-(2-ethylhexyl)-
10 H-pheno-thiazin-3-yl)thiophen-2 yl)benzo[c][1,2,5]thia-
diazole (DPDTBT). To a 100-mL two-neck flask were
added 1.0 g (2.2 mmol) of 1 and 2.0 g (4.6 mmol) of 2 in
20 mL of anhydrous toluene. Tetrakis(triphenylphosphine)
palladium (0.076 g, 0.07 mmol) was transferred to the
mixture in an inert atmosphere. Aqueous potassium car-
bonate (2.0 M) and several drops of Aliquat336 in toluene
were transferred to the reaction mixture via a cannula.
3.2. Optical and Electrochemical Properties
The optical properties of the polymers were investigated
using UV-visible absorption spectroscopy. Spectra were
obtained in solution and thin-film states. Figure 1 shows
the absorption spectra of the polymer solutions in CHCl3.
The polymers exhibited two absorption peaks at approx-
imately 350 and 460 nm, which is a common feature
of internal donor/acceptor polymers. PBTPTBT exhibited
absorption at a longer wavelength than did other the poly-
mers. In the spectra of the polymer films, the absorption
ꢀ
The reaction mixture was stirred at 80 C for 24 h. The
obtained product was further reacted with two equivalents
of Br2 in dichloromethane for 2 h at room temperature.
The resulting mixture was extracted with CHCl3 and dried
peaks shifted to longer wavelength, and the absorption
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over MgSO4. The solvent was removed by evaporation,
IP: 46.161.61.111 On: Thu,in2t3enJsuitny 2in0c1r6ea1se2d:09in:3t6he region where the wavelength
and the residue was purified by column chromatography.
Copyright: American Scientific Publishers
exceeded 600 nm as shown in Figure 2. The absorption
1
The product was obtained as a red solid. (1.0 g, 52%). H
peaks in the spectra of the polymer films at short wave-
lengths (347–363 nm) originated from the ꢂ–ꢂ∗ transi-
tions of electron donor units, while the absorption peaks
at longer wavelengths (466–469 nm) are due to the ꢂ–ꢂ∗
transitions of the benzothiadiazole moiety.
Electrochemical CV has been widely employed to inves-
tigate the redox behavior of polymers and to estimate
their HOMO and LUMO energy levels. In this study,
CV was performed in solutions of Bu4NBF4 (0.10 M) in
dichloromethane at a scan rate of 100 mV/s, at room tem-
perature in an argon atmosphere.
A platinum plate, a platinum wire, and a Ag/AgNO3
electrode were used as the working electrode, counter elec-
trode, and reference electrode, respectively. The HOMO
energy level was calculated using Eox. The LUMO energy
levels of the polymers were estimated from the onset of
the absorption peak in the spectra of the copolymer films.
NMR (400 MHz, CDCl3ꢀ: ꢁ 8.23 (d, 2 H), 8.18 (q, 4 H),
8.03 (d, 2 H), 7.97 (m, 2 H), 7.69 (m, 2 H), 7.57 (t, 2 H),
7.39 (t, 2 H), 7.17 (s, 2 H), 4.20 (2, 4 H) 2.07 (m, 2 H),
1.42 (m, 16 H), 0.89 (m, 12 H). 13C NMR (400 MHz,
CDCl3ꢀ: ꢁ 155.3, 148.6, 145.3, 139.7, 138.1, 129.9, 128.7,
128.1, 127.5, 126.4, 125.9, 125.5, 125.1, 125.0, 124.9,
124.1, 123.1, 122.8, 115.7, 49.52, 30.0, 20.3, 13.8. Calcd
for C46H36Br2N2S5: C, 57.26; H, 3.76; N, 5.81; S, 16.62.
Found: C, 56.96; H, 3.72; N, 5.75; S, 15.54.
Synthesis of PBTPTBT, PFPTBT and PPPTBT.
PBTPTBT, PFPTBT and PPPTBT were prepared via simi-
lar procedures by coupling DPDTBT with the three corre-
sponding boronic esters. DPDTBT (1.0 mmol), the boronic
ester (1.0 mmol), and toluene (20 mL) were placed in a
two-necked flask. The solution was flushed with argon for
10 min, and then, 23 mg of Pd(PPh3ꢀ4 was added into
the flask. The solution was flushed again for 20 min. The
ꢀ
oil-bath temperature was raised to 110 C, and the reac-
Table I. Molecular weights and thermal properties of the polymers.
tion was stirred for 48 h in an argon atmosphere. Then,
the reaction mixture was cooled to room temperature, and
the product was precipitated by adding the reaction mix-
ture to 300 mL of MeOH. The resulting polymers were
purified by successive extractions with methanol, hexane,
and acetone using a Soxhlet extractor. The polymer yields
were 52%, 36% and 20% for PBTPTBT, PFPTBT, and
PPPTBT, respectively.
Polymers
Mna
Mwa
PDIa
Tdb ꢃꢀC)
PBTPTBT
PFPTBT
PPPTBT
15ꢄ100
9ꢄ400
7ꢄ700
37,300
25,900
17,600
2.47
2.76
2.29
327.1
341.8
349.4
Notes: aMn, Mw, and PDI of the polymers were determined by gel permeation
chromatography using polystyrene standards in CHCl3; bTemperature at 5% weight
loss by a heating rate of 10 ꢀC/min under nitrogen.
J. Nanosci. Nanotechnol. 14, 5187–5191, 2014
5189

Synthesis and Properties of New Low Band Gap Semiconducting Polymers
Kim et al.
Table II. Summary of photophysical and electrochemical properties of
polymers.
1.2
1.0
0.8
0.6
0.4
0.2
0.0
PBTPTBT
PFPTBT
PPPTBT
ꢆmaxꢄabs (nm)
Polymers
Sol.a Filmb HOMO (eV) LUMO (eV) Egopt (eV)c
PBTPTBT
PFPTBT
PPPTBT
357
351
324
363
354
347
−5ꢅ30
−5ꢅ45
−5ꢅ33
−3ꢅ23
−3ꢅ34
−3ꢅ28
2.07
2.11
2.05
Notes: a1×10−5 M in anhydrous chloroform; bPolymer film on a quartz plate by
spin-casting from a solution in chloroform at 1500 rpm for 30 s; cCalculated from
the absorption band edge of the copolymer films, Eg = 1240/ꢆedge
.
300
400
500
600
700
Wavelength (nm)
Figure 1. UV-vis absorption spectra of polymers in chloroform
solution.
To obtain accurate redox potentials, the reference elec-
trode was calibrated against the ferrocene/ferrocenium
(Fc/Fc+ꢀ redox couple, the redox potential of which is
assumed to have an absolute energy level of −4ꢅ80 eV in
vacuum. Thus, the HOMO energy values were calculated
using the equation EHOMO = ꢃEonsetꢄoxꢅ + 4ꢅ4) eV, where
Eox is the onset oxidation potential versus Ag/Ag+. The
HOMO energy levels obtained for PBTPTBT, PFPTBT,
and PPPTBT were −5ꢅ30, −5ꢅ34, and −5ꢅ33 eV, respec-
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tively. The absorption properties, HOMO and LUMO
energy levels, and band gap energies of the polymers are
listed in Table II and Figure 3.
IP: 46.161.61.111 On: Thu, 23 Jun 2016 12:09:36
Figure 3. Energy band diagram for the devices.
Copyright: American Scientific Publishers
(> 0.6 V), but the PCE were low because of their low
short-circuit current densities.
3.3. Photovoltaic Properties
The device fabricated using PBTPTBT produced the
best results among the devices fabricated in this study. The
open-circuit voltage, short-circuit current density, and fill
factor of the device were 0.56 V, 2.79 mA/cm2, and 0.30,
respectively. The calculated PCE of the device containing
PBTPTBT was 0.46%. The solar cells fabricated using the
synthesized polymers showed relatively high open-circuit
Bulk heterojunction polymer solar cells were fabri-
cated with the structure ITO/PEDOT:PSS (40 nm)/
polymer:PC70BM(1:5, w/w) (80 nm)/TiOx (20 nm)/Al
(120 nm) where the polymers were used as electron donors
and PC70BM as an electron acceptor. Figure 4 shows
the J–V curves of the devices. The devices comprising
PFPTBT and PPPTBT showed high open-circuit voltages
1.2
0
PBTPTBT
1.0
0.8
0.6
0.4
0.2
0.0
PFPTBT
PPPTBT
–2
PBTPTBT:PC70BM
PPPTBT:PC70BM
–4
PFPTBT:PC70BM
0.0
0.2
0.4
0.6
0.8
300
400
500
600
700
Wavelength (nm)
Bias (V)
Figure 2. UV-visible absorption spectra of polymers solid films.
Figure 4. J–V characteristics of the fabricated solar cells.
J. Nanosci. Nanotechnol. 14, 5187–5191, 2014
5190

Kim et al.
Synthesis and Properties of New Low Band Gap Semiconducting Polymers
Table III. Summary of performances of the fabricated solar cells.
Energy Program of the Korea Institute of Energy Technol-
ogy Evaluation and Planning (KETEP) funded by the Min-
istry of Knowledge Economy (MKE) (20113010010030),
and a grant from the cooperative R&D Program (B551179-
08-03-00) funded by the Korea Research Council Indus-
trial Science and Technology, Republic of Korea.
Active layer
Jsac (mA/cm2ꢀ Voac (V) FFa (%) PCEa (%)
PBTPTBT:PC70BM
PFPTBT:PC70BM
PPPTBT:PC70BM
2.79
0.82
1.54
0.56
0.60
0.61
0.30
0.26
0.31
0.46
0.13
0.29
Notes: aPhotovoltaic properties of copolymers/PCBM-based devices spin-coated
from a dichlorobenzene solution for polymers (1:5 w/w).
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4. CONCLUSIONS
Three new low band-gap organic semiconducting poly-
mers were synthesized as p-type donor materials for
organic photovoltaic devices. The synthesized polymers
had number-average molecular weights between 7,700 and
15,100. The band-gap energies of the synthesized poly-
mers ranged between 2.05 and 2.11 eV depending on the
polymer structure.
Bulk-heterojunction solar cells fabricated using the
polymers and PC70BM as an acceptor were characterized.
One of the devices showed a PCE of 0.46% under simu-
lated AM 1.5 G solar irradiation of 100 mW/cm2.
10. J.-H. Kim, D. Mi, I. N. Kang, W. S. Shin, S. C. Yoon, S. J. Moon,
Delivered by Ingenta to: McMaster University
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Acknowledgment: This work was supported by the
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National Research Foundation of Korea (NRF) grant
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Received: 18 May 2011. Accepted: 8 August 2011.
J. Nanosci. Nanotechnol. 14, 5187–5191, 2014
5191