Synthesis of polymers containing 1,2,4-oxadiazole as an electron-acceptor moiety in their main chain and their solar cell applications

Article abstract of DOI:10.1002/pola.26605

To explore the aptitude of 1,2,4-oxadiazole-based electron-acceptor unit in polymer solar cell applications, we prepared four new polymers (P1-P4) containing 1,2,4-oxadiazole moiety in their main chain and applied them to solar cell applications. Thermal,

Full text of DOI:10.1002/pola.26605

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Synthesis of Polymers Containing 1,2,4-Oxadiazole as an
Electron-Acceptor Moiety in Their Main Chain and Their Solar Cell
Applications
Rajalingam Agneeswari,1 Vellaiappillai Tamilavan,1 Myungkwan Song,2 Jae-Wook Kang,2
Sung-Ho Jin,3 Myung Ho Hyun1
1Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 690-735, Republic
of Korea
2Department of Material Processing, Korea Institute of Materials Science, Changwon 641-831, Republic of Korea
3Department of Chemistry Education and Interdisciplinary Program of Advanced Information and Display Materials, Pusan
National University, Busan 609-735, Republic of Korea
Correspondence to: M. H. Hyun (E-mail: mhhyun@pusan.ac.kr)
Received 4 September 2012; accepted 24 January 2013; published online 19 February 2013
DOI: 10.1002/pola.26605
ABSTRACT: To explore the aptitude of 1,2,4-oxadiazole-based
electron-acceptor unit in polymer solar cell applications, we
prepared four new polymers (P1–P4) containing 1,2,4-oxadia-
zole moiety in their main chain and applied them to solar cell
applications. Thermal, optical, and electrochemical properties
of the polymers were studied using thermogravimetric,
absorption, and cyclic voltammetry analysis, respectively. All
four polymers showed high thermal stability (5% degradation
temperature over 335 ꢀC), and the optical band gaps were cal-
culated to be 2.20, 1.72, 1.37, and 1.74 eV, respectively, from
the onset wavelength of the film-state absorption band. The
energy levels of the polymers were found to be suitable for
bulk heterojunction (BHJ) solar cell applications. The BHJ so-
lar cells were prepared by using the synthesized polymers as
a donor and PC71BM as an electron acceptor with the configu-
ration of ITO/PEDOT:PSS/polymer:PC71BM (1:3 wt %)/LiF/Al.
One of the polymers was found to show the maximum power
conversion efficiency of 1.33% with a Jsc of 4.95 mA/cm2, a
Voc of 0.68 V, and a FF of 40%, measured using AM 1.5 G so-
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lar simulator at 100 mW/cm2 light illumination.
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KEYWORDS: bulk heterojunction solar cells; conjugated poly-
mers; copolymerization; oxadiazole-based polymers; pyrrole-
based polymers; solar cells
enhanced in the range of 8–10.6%.9–11 The theoretical stud-
ies implied that the maximum PCE could be reach up to
17% for single layer PSC and 24% for tandem PSCs.12 In our
attempt to improve the PCE of PSCs, we were interested in
synthesizing new donor polymers that have the appropriate
energy levels with broad absorption band in entire part of
the visible spectrum.
INTRODUCTION Polymer solar cells (PSCs) are considered as
a green energy production technique and PSCs based on the
bulk heterojunction (BHJ) structure are competing with inor-
ganic-based solar cells, which are already commercialized,
due to their advantages in that they can be fabricated at low
cost, light weight, flexibility, and most notably, very fast
modes of production to large area by standard roll-to-roll
(R2R) printing techniques.1,2 So far, the power conversion ef-
ficiency (PCE) of the PSCs was significantly improved by
three different ways such as developing structurally new do-
nor polymer that have the capability to absorb the sunlight
efficiently,3–7 replacing the existing acceptor (PC71BM) with
new acceptor unit that has the increased lowest unoccupied
molecular orbital (LUMO) energy level8 or changing the de-
vice architecture to inverted9 or tandem structured
PSCs.10,11 After many efforts, the PCE of the single layer
PSCs was improved to 7.5%3–7 and that of tandem struc-
tured PSCs containing two different photoactive layers was
The broad absorption polymers were usually synthesized by
copolymerizing the electron rich and electron deficient unit
in alternate fashion, thereby decreasing the band gap due to
the combined electronic transition, such as p–p* electronic
transition in the donor and internal charge transfer (ICT)
between the donor and acceptor moiety.13–20 So far, benzo-
thiadiazole and quinoxaline derivatives were widely used as
an electron deficient unit in the low band-gap polymer prep-
aration and the photovoltaic studies of those polymers
showed maximum PCE of 6%.21–28 Recently, a variety of new
electron-accepting heteroaromatic units were incorporated in
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polymer main chain and the PCE of single layer PSCs were
significantly improved up to 7.5% by using them as a donor
polymer29–34 in PSC studies. The later results indicate that
changing the electron-acceptor unit in polymer main chain
can show big difference in the optoelectrical and photovol-
taic properties of the polymer due to the different electron-
accepting strength and carrier mobility. In this instance, find-
ing new acceptor unit to prepare structurally new polymers
for PSC applications is quite important. In heterocyclic chem-
istry, the planar structured oxadiazoles are known to be a
strong electron-accepting moiety35,36 and, consequently, PSC
applications of polymers containing oxadiazole moiety would
be quite interesting. Recently, polymers containing electron-
withdrawing 1,3,4-oxadiazole side chains were investigated
for their photovoltaic properties.37 However, to the best of
our knowledge, polymers containing oxadiazole moiety in
the polymer main chain have not been studied for any optoe-
lectronic applications so far. In this study, we elucidate the
possible utility of oxadiazole moiety as an electron-acceptor
unit in polymer main chain for polymer solar cell applica-
tions. As an oxadiazole moiety, we selected 1,2,4-oxadiazole
because its derivative can be easily prepared from commer-
cially available materials. In this study, we prepared four
new polymers containing 1,2,4-oxadiazole moiety in their
main chain. Here, we report the synthesis and optical, elec-
trical, and photovoltaic properties of the four new polymers.
used as an external standard. Atomic force microscopy
(AFM) images of blend films were obtained on a Veeco-Mul-
timode AFM operating in the tapping mode.
Device Preparation
The polymer solar cells were prepared and characterized as
we described below. Indium tin oxide (ITO)-coated glass sub-
strates were cleaned stepwise by acetone, deionized water,
and isopropyl alcohol in an ultrasonic bath. Followed by
50-nm poly(ethylenedioxythiophene)-poly(styrenesulfonate)
(PEDOT:PSS) (Clevious P VP Al 4083) buffer layer was spin-
coated on theꢀtop of ITO substrate at 3000 rpm for 30 s and
dried at 150 C for 20 min under vacuum to remove the re-
sidual water. The active layer of polymer and PC71BM blend
(1:3 wt/wt) was spin-coated at 2000 rpm from dichloroben-
zene (DCB) solution on the top of ITO/PEDOT:PSS substrate.
The ITO/PEDOT:PSS/Polymer:PC71BM substrate was allowed
to dry for 2 h ꢀin a glove box and then subjected to prean-
nealing at 150 C for 20 min. The corresponding active layer
thickness was found to be around 80 nm. The ITO/
PEDOT:PSS/Polymer:PC71BM substrate was transferred into
the vacuum chamber and around 0.7-nm-thick layer of LiF
was deposited on the substrate. Subsequently, 120-nm-thick
layer of Al was deposited through a shadow mask on the top
of ITO/PEDOT:PSS/Polymer:PC71BM/LiF substrate under
high vacuum (1.2 ꢁ 10ꢂ6 torr). The top metal electrode
area, comprising the active area of the solar cell, was found
to be 0.36 cm2. The BHJ solar cell performance was meas-
ured using a AM 1.5G solar simulator (Oriel 300 W) at 100
mW/cm2 light illumination after adjusting the light intensity
using Oriel power meter (model No. 70260 which was cali-
brated using laboratory standards that are traceable to the
National Institute of Standards and Technologies). Current–
voltage (J–V) characteristics of the polymer BHJ solar cell
devices were measured using a standard source measure-
ment unit (Keithley 236). All fabrication steps and character-
ization measurements were performed in an ambient envi-
ronment without a protective atmosphere. The thickness of
the thin films was measured using a KLA Tencor Alpha-step
IQ surface profilometer with an accuracy of 61 nm.
EXPERIMENTAL
Materials and Instruments
The commercially available reagents were received from
Aldrich or TCI chemicals and used without further purifica-
tion. The anhydrous grade solvents were purchased from
Aldrich or Fluka and the common organic solvents such as
dichloromethane, tetrahydrofuran, and diethyl ether were
distilled and handled in a moisture-free atmosphere. The
compounds were purified by column chromatography on
70–230 mesh silica gel (Merck Kieselgel 60, 70–230 mesh
ASTM). The nuclear magnetic resonance (NMR) spectra of
the compounds were recorded using Varian Mercury Plus
1
spectrometer (300 and 75 MHz, respectively for H and 13C)
Synthesis
in CDCl3. Gel permeation chromatography (GPC) analyses
were conducted on an Agilent 1200 Infinity Series separation
module using polystyrene as a standard and chloroform as
an eluent to determine the molecular weight and polydisper-
sity (PDI) of the polymers. Thermogravimetric analyses
(TGA) were coꢀnducted with a TA instrument Q500 at a heat-
ing rate of 10 C/min under nitrogen. The absorption studies
of the polymers were performed on JASCO V-570 spectropho-
tometer at 25 ꢀC in chloroform or as thin films on glass.
Cyclic voltammetry (CV) measurements were performed
using a CH Instruments Electrochemical Analyzer with a
standard three-electrode electrochemical cell (Ag/AgCl: refer-
ence electrode, platinum: working electrode, platinum wire:
counter electrode) in a 0.1 M tetrabutylammonium tetra-
fluoroborate (Bu4NBF4)/chloroform as the supporting elec-
trolyte at room temperature. During the CV measurements,
the redox couple ferrocene/ferrocenium ion (Fc/Fcþ) was
The general synthetic strategy for monomers M1 and M2
and polymers P1-P4 are outlined in Schemes 1 and 2,
respectively. Monomer M3, 4,7-bis(5-bromo-3-octyl-2-
thienyl)-2,1,3-benzothiadiazole,38 and monomer M4, 4,7-
dibromo-2,1,3-benzothiadiazole,39 were synthesized via the
reported procedures. Monomer M5, 2.5-bis(tributylstan-
nyl)thiophene, is commercially available from Sigma-Aldrich.
The detailed synthetic procedures for monomers M1 and M2
and polymers P1–P4 were described below.
Synthesis of N-[4-Octylphenyl]dithieno[3,2-b:20,30-
d]pyrrole (1)
A stirred solution of 3,30-dibromo-2,20-bithiophene (5.2 g, 16
mmol), which was prepared via the known procedure,40 in
40 mL of toluene was purged well with argon for 15 min,
and then NaOtBu (3.7 g, 38.5 mmol), Pd2dba3 (0.4 g, 0.44
mmol), and 1,1’-bis(diphenylphosphine)ferrocene (DPPF, 1.0
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SCHEME 1 Synthetic route for the synthesis of monomers M1 and M2 and the structures of monomers M3, M4, and M5.
g, 1.8 mmol) were added in sequence. Then, 4-octylaniline
(4.0 mL, 18 mmol) was added, and the mixture was refluxed
for 12 h. The mixture was allowed to cool down, and the sol-
vent was removed by rotary evaporation. The solid residue
was dissolved in dichloromethane and the insoluble precipi-
tates were filtered off. The organic solution was washed with
water and then brine, and dried over anhydrous Na2SO4. The
solvent was removed by rotary evaporation and the crude
product was purified by column chromatography (silica gel,
hexane:CH2Cl2, 90/10) to afford compound 1 as a white
solid.
evaporated by rotary evaporation and the crude product was
purified by column chromatography (silica gel, hexa-
ne:CH2Cl2, 90/10) to afford pure compound 2 as a yellowish
solid.
Yield: 5.4 g (92%). mp 121–122 ꢀC; 1H NMR (300 MHz,
CDCl3): d (ppm) 7.49 (d, 2H), 7.32 (d, 2 H), 7.16 (s, 2 H),
2.67 (t, 2H), 1.60–1.74 (m, 2 H), 1.22–1.44 (m, 10 H), 0.89
(t, 3H); 13C NMR (75 MHz, CDCl3): d (ppm) 142.1, 141.1,
136.8, 130.0, 123.0, 116.6, 115.7, 110.5, 35.7, 32.1, 31.7,
29.7, 29.6, 29.5, 22.9, 14.4; HRMS (EIþ, m/z) [Mþ] Calcd. for
C22H23Br2NS2 522.9639, found 522.9647.
1
ꢀ
Yield: 5.4 g (92%). mp 79–80 C; H NMR (300 MHz, CDCl3):
d (ppm) 7.49 (d, 2H), 7.33 (d, 2 H), 7.17 (s, 4 H), 2.68 (t,
2H), 1.60–1.74 (m, 2 H), 1.22–1.44 (m, 10 H), 0.90 (t, 3H);
13C NMR (75 MHz, CDCl3): d (ppm) 144.4, 141.2, 137.8,
129.9, 123.5, 122.8, 116.8, 112.5, 35.8, 32.1, 31.7, 29.7, 29.6,
29.5, 22.9, 14.4; HRMS (EIþ, m/z) [Mþ] Calcd. for C22H25NS2
367.1428, found 367.1434.
Synthesis of 2,6-Di(trimethyltin)-N-[4-
octylphenyl]dithieno[3,2-b:20,30-d]pyrrole (M1)
Under argon atmosphere, a solution of compound 2 (2.52 g,
4.8 mmol) in dry diethyl ether (40 mL) was cooled to 0 ꢀC
in ice bath for 20 min. To the stirred solution, n-BuLi (4.2
mL, 10.5 mmol, 2.5 M solution in hexane) was added drop-
wise, resulting in the formation of a white precipitate. The
mixture was allowed to reach room temperature, and after
stirring for 20 min, a solution of Me3SnCl (10.5 mL, 10.5
mmol, 1 M solution in hexane) was added. The initial white
precipitates disappeared, and the clear solution was
obtained, from which another white precipitates were
formed. After stirring at room temperature for 1 h, the sol-
vent was removed. Hexane was added, and the white precipi-
tates were filtered off. The filtrate was passed through the
neutral alumina packed (5 cm in height) on sintered crucible
Synthesis of 2,6-Dibromo-N-[4-octylphenyl]dithieno[3,2-
b:20,30-d]pyrrole (2)
Compound 1 (4.0 g, 10.8 mmol) was dissolved in chloroform
(30 mL) and cooled to 0 ꢀC in ice bath under argon. N-Bro-
mosuccinimide (NBS) (4.25 g, 23.9 mmol) was added in one
portion to the stirred solution. After 15 min, the mixture
was allowed to warm to room temperature and stirred for 5
h. The organic mixture was washed with water, brine, and
dried over anhydrous Na2SO4. The solution was filtered and
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SCHEME 2 Synthetic route for the synthesis of polymers P1ꢂP4.
under vacuum. The filtrate was concentrated by rotary evap-
oration and dried under vacuum to afford monomer M1 as a
yellow solid material.
ꢀC in ice bath. After 15 min, pyridine (2 mL, 24 mmol) and
4-bromobenzoyl chloride (5.3 g, 24 mmol) were added and
stirred for 2 h in the same bath. The ice bath was removed
and the mixture was refluxed for 15 h. The insoluble mate-
rial was filtered off in hot condition, and the filtrate was con-
centrated by rotary evaporation. The crude material was
purified by column chromatography (silica gel, hexane) to
afford pure compound 3 as a white solid material.
Yield: 8 g (90%). mp 179–180 ꢀC; 1H NMR (300 MHz,
CDCl3): d (ppm) 8.07 (d, 2 H), 8.03 (d, 2 H), 7.70 (d, 2 H),
7.65 (d, 2 H); 13C NMR (75 MHz, CDCl3): d (ppm) 175.3,
168.6, 132.8, 132.4, 129.8, 129.2, 128.2, 126.1, 125.9, 123.2;
HRMS (EIþ, m/z) [Mþ] Calcd for C14H8Br2N2O 377.9003,
found 377.9009.
Yield: 2.85 g (86%). mp 85–86 ꢀC; 1H NMR (300 MHz,
CDCl3): d (ppm) 7.52 (d, 2H), 7.34 (d, 2 H), 7.15 (s, 2 H),
2.68 (t, 2H), 1.60–1.74 (m, 2 H), 1.22–1.44 (m, 10 H), 0.90
(t, 3H), 0.39 (s, 18 H); 13C NMR (75 MHz, CDCl3): d (ppm)
147.2, 140.8, 138.2, 136.6, 129.8, 122.9, 122.4, 119.3, 35.8,
32.1, 31.8, 29.7, 29.6, 29.5, 22.9, 14.4, ꢂ7.9; HRMS (EIþ, m/
z) [Mþ] Calcd. for C28H41NS2Sn2 695.0724, found 695.0726.
Synthesis of 3,5-Bis(4-bromophenyl)-1,2,4-oxadiazole (3)
To a stirred solution of 4-bromobenzonitrile (5.0 g, 27.5
mmol) in ethanol (50 mL), 50% aqueous hydroxylamine so-
lution (6 mL) was added and refluxed for 2 h. The comple-
tion of the reaction was confirmed by TLC, and ethanol was
removed by rotary evaporation. The crude product was dis-
solved in ethyl acetate (100 mL) and washed one time with
water (20 mL). The organic layer was separated and dried
over anhydrous Na2SO4. After filtration and removal of the
organic solvent by rotary evaporation, (Z)-4-bromo-N0-
hydroxybenzamidine was obtained as a white solid.
Synthesis of 3,5-Bis(4-(3-octylthiophen-2-yl)phenyl)-
1,2,4-oxadiazole (4)
A stirred solution of compound 3 (3 g, 7.9 mmol) and 2-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-3-octylthiophene
(5.8 g, 18 mmol), which was prepared via the known proce-
dure,36 in toluene (50 mL) was purged with argon for 45
min. To the solution, Pd (PPh3)4 (0.37 g, 4 mol %) and aque-
ous 2 M K2CO3 (7 mL) were added. The whole mixture was
refluxed for 24 h under argon. The solution was cooled to
room temperature and concentrated by rotary evaporation.
The crude product was dissolved in dichloromethane and
Yield: 5.7 g (96%). mp 148–149 ꢀC; 1H NMR (300 MHz,
CDCl3): d (ppm) 7.52 (d, 4 H), 4.83 (brd s, 2 H). (Z)-4-
Bromo-N0-hydroxybenzamidine (5.0 g, 23.3 mmol) was dis-
solved in 60 mL of toluene, and the solution was cooled to 0
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washed with water and then brine. The organic layer was
separated and dried over anhydrous Na2SO4. The organic so-
lution was filtered and concentrated by rotary evaporation,
and then the residue was purified by column chromatogra-
phy (silica gel, hexane:ethyl acetate, 9.5/0.5, v/v) to afford
pure compound 4 as a yellow liquid material.
P2
Black color. Yield (0.36 g, 49%). 1H NMR (300 MHz, CDCl3):
d 8.10–8.28 (m, 4 H), 7.98–8.10 (m, 2 H), 7.54–7.68 (m, 4
H), 7.42–7.54 (m, 5 H), 7.38 (d, 5 H), 7.00–7.26 (m, 6 H),
2.60–2.80 (m, 12 H), 1.60–1.80 (m, 12 H), 1.14–1.50 (m, 60
H), 0.74–1.00 (m, 18 H).
1
Yield: 3.9 g (81%). H NMR (300 MHz, CDCl3): d (ppm) 8.27
P3
(d, 2H), 8.23 (d, 2 H), 7.63 (d, 2H), 7.59 (d, 2 H), 7.31 (d, 1
H), 7.27 (d, 1 H), 7.03 (d, 1 H), 7.01 (d, 1 H), 2.66–2.76 (m,
4 H), 1.58–1.70 (m, 4 H), 1.16–1.38 (m, 20 H), 0.87 (t, 6 H);
13C NMR (75 MHz, CDCl3): d (ppm) 175.7, 169.0, 140.3,
139.8, 138.1, 137.1, 136.5, 132.4, 130.2, 130.0, 129.9, 129.3,
128.6, 127.9, 125.9, 125.1, 124.6, 123.0, 32.1, 31.2, 31.1,
29.7, 29.6, 29.5, 29.1, 29.0, 22.9, 14.3; HRMS (EIþ, m/z) [Mþ]
Calcd. for C38H46N2OS2 610.3052, found 610.3054.
Black color. Yield (0.25 g, 42%). 1H NMR (300 MHz, CDCl3):
d 8.36 (s, 1 H), 8.10–8.28 (m, 4 H), 7.84 (s, 1 H), 7.44–7.64
(m, 8 H), 7.28–7.44 (m, 4 H), 6.86–7.24 (m, 6 H), 2.58–2.82
(m, 8 H), 1.60–1.80 (m, 8 H), 1.14–1.50 (m, 40 H), 0.74–1.00
(m, 12 H).
P4
Dark brown color. Yield (0.27 g, 75%). 1H NMR (300 MHz,
CDCl3): d 8.20–8.32 (m, 4 H), 8.06 (s, 1 H), 7.89 (s, 1 H),
7.56–7.72 (m, 4 H), 7.44–7.56 (m, 2 H), 7.24 (s, 1 H), 7.10
(s, 2 H), 7.04 (s, 1 H), 2.64–2.78 (m, 4 H), 1.60–1.74 (m, 4
H), 1.18–1.44 (m, 20 H), 0.76–0.98 (m, 6 H).
Synthesis of 3,5-Bis(4-(5-bromo-3-octylthiophen-2-
yl)phenyl)-1,2,4-oxadiazole (M2)
Compound (4) (3 g, 4.9 mmol) was dissolved in dimethylfor-
mamide (20 mL) under argon. To the stirred solution was
added NBS (1.78 g, 10.0 mmol) in one portion. The resulting
solution was stirred at room temperature under argon over-
night. The solvent was concentrated by using rotary evapora-
tor and then the crude martial was dissolved in dichlorome-
thane and the organic solution was washed with water and
then brine. The organic solution was dried over anhydrous
Na2SO4, filtered, and the solvent was evaporated by rotary
evaporation. The crude product was purified by column
chromatography (silica, hexane:ethyl acetate, 9.5:0.5, v/v) to
afford monomer M2 as a yellow sticky material.
RESULTS AND DISCUSSIONS
Synthesis and Characterization
The synthetic procedures for the monomers and polymers
are outlined in Schemes 1 and 2, respectively. In our labora-
tory, we have been interested in the utilization of pyrrole-
based polymers in PSC applications.38,39,41–44 Recently, we
reported acyclic N-aryl TPT (thiophene-(N-arylpyrrole)-thio-
phene)-based polymers for PSCs applications and those poly-
mers showed reasonable current density and performance in
their photovoltaic studies.38,39,41–44 These results induced us
to develop cyclic N-aryl pyrrole monomer because cyclic
structure has much better p–p conjugation than acylic struc-
ture. In this instance, we prepared a new cyclic N-aryl TPT
(N-aryl DTP) monomer. The electron rich new N-aryl DTP
monomer M1, 2,6-di(trimethyltin)-N-[4-octylphenyl]dithieno
[3,2-b:20,30-d]pyrrole, was synthesized via the procedure sim-
ilar to that reported by Koeckelberghs et al.40 The reaction
between 3,30-dibromo-2,20-bithiophene and 4-octylphenyl an-
iline in the presence of palladium catalyst afforded com-
pound 1. And then compound 1 was selectively brominated
using NBS to yield compound 2. The final N-aryl DTP mono-
mer (M1) was obtained by treating compound 2 with n-BuLi
followed by Me3SnCl. On the other hand, compound 3 con-
taining the electron-accepting 1,2,4-oxadiazole moiety was
prepared by the reaction between (Z)-4-bromo-N0-hydroxy-
benzamidine, which was prepared by treating 4-bromoben-
zonitirle with 50% hydroxy amine solution, and 4-bromoben-
zoyl chloride. The Suzuki coupling reaction between
compound 3 and 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)-3-octylthiophene afforded compound 4. Bromination of
compound 4 with NBS afforded a new 1,2,4-oxadiazole-based
monomer (M2). The other monomers (M3 and M4) were
synthesized via the known literature procedures.38,39 All
polymers (P1–P4) were prepared by using the Stille cou-
pling reaction between the respective monomers. Polymer
P1 is alternating copolymer, whereas all other polymers P2–
P4 are random copolymers. The copolymerization between
monomers M1 and M2 afforded polymer P1 containing
1
Yield: 3.3 g (88%). H NMR (300 MHz, CDCl3): d (ppm) 8.25
(d, 2H), 8.21 (d, 2 H), 7.56 (d, 2H), 7.52 (d, 2 H), 6.98 (s, 1
H), 6.96 (s, 1 H), 2.58–2.68 (m, 4 H), 1.52–1.66 (m, 4 H),
1.12–1.38 (m, 20 H), 0.87 (t, 6 H); 13C NMR (75 MHz,
CDCl3): d (ppm) 175.6, 168.9, 140.9, 140.4, 138.6, 138.5,
138.0, 136.9, 132.9, 132.7, 132.4, 129.9, 129.8, 129.3, 128.7,
128.0, 126.3, 123.3, 32.1, 31.1, 31.0, 29.6, 29.5, 29.4, 29.0,
28.9, 22.9, 14.4; HRMS (EIþ, m/z) [Mþ] Calcd. for
C38H44Br2N2OS2 766.1262, found 766.1268.
General Procedure for Polymer Synthesis
A solution of respective monomers (shown in Scheme 2) in
THF (60 mL) was purged well with argon for 45 min. Then,
Pd (PPh3)4 (0.02 g, 5 mol %) was added to the stirred solu-
tion and the mixture was heated to reflux under argon
atmosphere. After refluxing for 48 h, the reaction mixture
was cooled to room temperature and then poured into the
mixed solvent of methanol and water (200 mL:100 mL) con-
taining 2 N HCl (50 mL) with vigorous stirring. The precipi-
tate was recovered by filtration, and then extracted with
methanol for 24 h and acetone for 24 h in a Soxhlet
apparatus.
P1
1
Red color. Yield (0.29 g, 74%). H NMR (300 MHz, CDCl3): d
8.16–8.34 (m, 4 H), 7.48–7.70 (m, 6 H), 7.39 (d, 2 H), 7.00–
7.26 (m, 4 H), 2.60–2.80 (m, 6 H), 1.60–1.80 (m, 6 H), 1.10–
1.50 (m, 30 H), 0.74–0.98 (m, 9 H).
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TABLE 1 Polymerization Results, Thermal, Optical and
Electrochemical Properties of Polymers P1–P4
P1
P2
P3
P4
a
Mw
2.01 ꢁ 104 1.06 ꢁ 104 5.38 ꢁ 103
4.89 ꢁ 104
2.44
PDIa
1.83
1.88
1:0.8
361
1.35
0.7:1
338
m:n ratio
TGAb (ꢀC)
Abs (nm)
Solution
Filmc
–
1:1
399
430
257, 441
458
259, 441 261, 397, 613 256, 396, 520
458
1.72
265, 407, 626
1.37
420, 562
1.74
d
Eg (eV)
2.20
HOMOe (eV)
LUMOf (eV)
a
ꢂ5.06
ꢂ2.86
ꢂ5.09
ꢂ3.37
ꢂ5.09
ꢂ5.39
ꢂ3.65
ꢂ3.72
Weight average molecular weight (Mw) and polydispersity (PDI) of the
FIGURE 1 TGA curves for polymers P1ꢂP4.
polymers were determined by GPC using polystyrene standards.
b
5% weight loss temperature measured by TGA under N2.
c
Measurements in thin film were performed onto the quartz substrate.
1,2,4-oxadiazole electron-acceptor unit in the polymer main
chain. The absorption study of polymer P1 shows that the
absorption band is extending only up to 550 nm, indicating
the electron attracting ability of the 1,2,4-oxadiazole-acceptor
unit is not satisfactory. To extend the absorption band of
polymer P1 further, a strong electron-accepting monomer
such as M3 or M4 was incorporated into the main chain of
polymer P1 by polymerizing three comonomers (M1 þ
M2 þ M3 or M1 þ M2 þ M4) at the ratio of 2:1:1 to yield
random copolymers P2 and P3. For the preparation of poly-
mer P4, thiophene derivative (M5) used as an electron rich
comonomer instead of M1 was polymerized with M2 and
M4. The structures of polymers P1–P4 were characterized
by NMR and absorption studies. The ratios of the two differ-
ent repeating units [oxadiazole (m) and benzothiadiazole
(n)-based repeating units] in polymers P2, P3, and P4 were
found to be 1:0.8, 0.7:1, and 1:1, respectively. The solubility
of the polymers was tested in the common organic solvents
such as chloroform, tetrahydrofuran, chlorobenzene, and
dichlorobenzene. All three polymers P1, P2, and P3 exhib-
ited good solubility in all solvents, whereas polymer P4
showed poor solubility in chloroform and tetrahydrofuran.
The weight average molecular weights (Mw) of polymers P1–
P4 were 2.01 ꢁ 104, 1.06 ꢁ 104, 5.38 ꢁ 103, and 4.89 ꢁ
104, and their polydispersities (PDI) were 1.83, 1.88, 1.35,
and 2.44, respectively. The thermal stability of the polymers
P1–P4 was determined by using TGA, and the 5% decompo-
sition temperature was found to be 399, 361, 338, and 430
ꢀC, respectively. The TGA curves of the polymers P1–P4
were presented in Figure 1. Polymers P1 and P4 showed
higher thermal stability than polymers P2 and P3. We expect
that the high molecular weights of polymers P1 and P4
might be responsible for their high thermal stability. The po-
lymerization results and the thermal properties of polymers
P1–P4 are summarized in Table 1.
d
Band gap estimated from the onset wavelength of the optical absorp-
tion in thin film.
e
The HOMO levels of the polymers were estimated from cyclic voltam-
metry analysis.
f
The LUMO levels of the polymers were calculated using the equation
of LUMO ¼ HOMO þ Eg.
of the copolymer films was estimated to be 88, 90, 85, and
88 nm, respectively. The color image of the polymers P1–P4
films on glass was displayed in Figure 3. All of the four poly-
mers display their absorption band in the visible part of the
solar spectrum. Alternating copolymer P1 shows the absorp-
tion band from 300 to 550 nm with maximum absorption
peak at 441 and 458 nm, respectively, in solution and film.
Random copolymer P2 shows the absorption band up to
700 nm in film with the maximum absorption peak at 458
and 441 nm, respectively, in film and solution. Polymers P1
and P2 show the exactly identical absorption maximum but
polymer P2 shows broader absorption band than polymer
P1. We expect that M3 unit in polymer P2 main chain
slightly broadens the absorption band due to the extended
p–p delocalization of the repeating units. On the other hand,
copolymer P3 shows broader absorption band in entire part
of the visible spectrum with two maximum absorption peaks
at 397 and 613 nm in solution and 407 and 626 nm as film.
Here, the introduction of M4 unit in the polymer main chain
significantly broadens the absorption band of polymer P3
and the quite broad absorption is attributed to the combined
electronic transitions such as the p–p* transition in the do-
nor unit and the donor–acceptor ICT transition between M1
and M4 moiety. In the case of polymer P4, monomer M1 of
polymer P3 was replaced with thiophene monomer M5.
Polymer P4 showed the blue shifted absorption maximum
(396, 520 nm and 420, 562 nm, respectively, in solution and
film) compared to those of polymers P1 and P2. The film-
state absorption bands of polymers P1–P4 were found to be
slightly broader and their corresponding absorption maxima
were red shifted compared with those observed in solution
state. The later phenomenon indicates that the polymers are
well oriented (better p–p interchain interaction) in film state
Optical Properties
The UV–vis absorption spectra of the polymers P1–P4 were
measured in chloroform and as thin film on glass, and their
absorption spectra were presented in Figure 2. The thickness
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cations. For the efficient electron injection from polymer to
PC71BM, use of the polymer having higher LUMO energy
level than that of PC71BM is most important. The energy lev-
els of the polymers were determined from CV analysis. Fig-
ure 4 represents the cyclic voltammograms of polymers
P1–P4. The HOMO energy levels of polymers P1–P4 were
calculated from the onset oxidation potentials using the
known equation as follows,38 EHOMO ¼ [ꢂ(Eox,
onset vs. Ag/
– Eferrocene
Ag/AgCl) – 4.8] eV, where 4.8 eV is the
vs.
AgCl
energy level of ferrocene below the vacuum level and Eferro-
is 0.51 eV, and Eox, onset is the onset potential
cene vs. Ag/AgCl
values in volts for oxidation process against Ag/AgCl refer-
ence electrode. The onset oxidation potentials (Eox, onset) of
polymers P1–P4 were estimated from the cyclic voltammo-
grams shown in Figure 4 to be 0.77, 0.80, 0.80, and 1.10 V,
respectively, and the HOMO energy levels of polymers P1–P4
were calculated to be ꢂ5.06, ꢂ5.09, ꢂ5.09, and ꢂ5.39 eV,
respectively. The LUMO energy levels of the polymers P1–P4
were calculated by using the equation of LUMO ¼ HOMO þ
Eg, and the LUMO energy levels of polymers P1–P4 were
determined to be ꢂ2.86, ꢂ3.37, ꢂ3.72, and ꢂ3.65 eV,
respectively. The energy-level diagram of polymers P1–P4
and PC71BM with all other materials used in BHJ solar cell
is presented in Figure 5. The conduction bands of all four
polymers were located above the conduction band of
PC71BM and the energy difference between the LUMO levels
of the polymer and PC71BM is estimated to be 1.44, 0.93,
0.58, and 0.65 eV, respectively. According to the literature,
0.2–0.3 eV energy difference is enough for efficient electron
transfer.38,39,41–44 In this instance, the better electron trans-
fer is expected in the order of P3 > P4 > P2 > P1 for the
synthesized polymers P1–P4. The HOMO and LUMO energy
levels of polymers P1–P4 were included in Table 1.
FIGURE 2 UV–visible absorption spectra of polymers P1ꢂP4 in
chloroform and thin film.
BHJ Solar Cell Properties
The energy conversion abilities of the synthesized polymers
were tested by using them as donor materials in PSCs with
device structure of ITO/PEDOT:PSS/P1–P4:PC71BM (1:3 wt/
wt)/LiF/Al. The active layer was sandwitched between the
hole (PEDOT:PSS) injecting and electron transporting (LiF)
layers and covered with ITO anode and Al cathode. The best
donor:acceptor blend ratio was fixed by making the PSC
devices with each of the synthesized polymers as a donor
and PC60BM as an electron acceptor with three different
weight ratios such as 1:1, 1:2, and 1:3 wt/wt. The best
results were obtained with the PSCs made from 1:3 wt/wt
than those in solution. The optical band gaps of polymers
P1–P4 were calculated from the onset wavelength of the op-
tical absorption in thin film to be 2.20, 1.72, 1.37, and 1.74
eV, respectively. The optical properties of polymers P1–P4
are summarized in Table 1.
Electrochemical Properties
Determination of the conduction (HOMO) and valance
(LUMO) band energy levels of the synthesized polymers is
essential to evaluate their suitability for BHJ solar cell appli-
FIGURE 3 The color image of the polymers P1ꢂP4 films on glass.
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FIGURE 4 Cyclic voltammograms of polymers P1ꢂP4 in chloroform.
blend ratio. Later, the active layer of the PSCs were made
with polymer:PC71BM (1:3 wt/wt) blend solution and their
corresponding characteristic parameters such as open-circuit
voltage (Voc), short-circuit current density (Jsc), fill factor
(FF), and PCE are summarized in Table 2. The current den-
sity–voltage (J–V) characteristics of the PSC devices were
measured under the illumination of AM 1.5 G (100 mW/
cm2) solar simulator and their corresponding J-V curves are
shown in Figure 6. Among the four PSC devices, the PSC
made from P1:PC71BM (1:3 wt/wt) showed the least PCE of
FIGURE 5 The energy level diagram of polymers P1ꢂP4 and PC71BM with all other materials used in BHJ solar cell.
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TABLE 2 Solar Cell Performance of Polymer P12P4 as Electron
Donors with PC71BM as an Electron Acceptor in ITO/
PEDOT:PSS/Polymer:PC71BM(1:3 wt/wt)/LiF/Al Device
Photoactive Active
Layer
Voc
(V)a
Jsc
(mA/cm2)b
FF
(%)c
PCE
(%)d
P1:PC71BM (1:3 wt/wt)
P2:PC71BM (1:3 wt/wt)
P3:PC71BM (1:3 wt/wt)
P4:PC71BM (1:3 wt/wt)
0.61
0.61
0.68
0.72
4.27
4.70
4.95
3.48
28
37
40
32
0.73
1.05
1.33
0.80
a
Open-circuit voltage.
b
Short-circuit current density.
Fill factor.
c
d
Power conversion efficiency.
0.73% with a Voc of 0.61 V, a Jsc of 4.27 mA/cm2, and a FF of
0.28. The PSC made from P2:PC71BM (1:3 wt/wt) showed
higher PCE of 1.05% with a Voc of 0.61 V, a Jsc of 4.70 mA/
cm2, and a FF of 0.37 than the PSC made from P1:PC71BM
(1:3 wt/wt). The PSC made from P3:PC71BM (1:3 wt/wt)
showed the best PCE of 1.33% with Voc of 0.68 V, a Jsc of
4.95 mA/cm2, and a FF of 0.40. Finally, the PSC made from
P4:PC71BM (1:3 wt/wt) showed the PCE value of 0.80%
with a Voc of 0.72 V, a Jsc of 3.48 mA/cm2, and a FF of 0.32.
The Voc values obtained from the PSC devices are in the
range of 0.61–0.72 V. The Voc value is highly influenced by
several factors, such as voltage loss at the ohmic contacts,
FIGURE 6 J–V characteristics of BHJ solar cells prepared from
ITO/PEDOT:PSS/Polymer:PC71BM (1:3 wt/wt)/LiF/Al under AM
1.5 irradiation (100 mW/cm2).
molecular weight, and PDI of the donor polymer, carrier mo-
bility of excitons, surface morphology, and quality of hetero-
junction at donor/acceptor interfaces.45–47 To consider all
those factors, the Voc values achieved in this study are
expected to be reasonable. The Jsc value of the PSC is mainly
correlated with the light harvesting ability of the donor poly-
mer. In this point of view, the Jsc values are expected in the
FIGURE 7 AFM image obtained by tapping-mode on the surface for Polymer:PC71BM (1:3 wt/wt) spin coated thin film.
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following order P3 > P2 > P1 > P4 for the synthesized
polymers, and they are correlated well with the experimental
Jsc values. However, polymer P4-based PSC showed higher
PCE, Voc and FF than polymer P1-based PSC. We expect that
the relatively high molecular weight of polymer P4 might be
responsible for its better photovoltaic performance than
polymer P1.46 In this instance, increasing the molecular
weight of polymers P1–P4 is expected to improve their pho-
tovoltaic performances, because high-molecular-weight poly-
mers, in general, show enhanced photovoltaic performances
compared to those of the low molecular weight polymers.46
The molecular weight of polymers can be increased by the
incorporation of alkyl side chain on the polymer backbone.
In our newly synthesized monomer units, we have many
chances for the alkyl group incorporation and, consequently,
we might improve the molecular weight of the 1,2,4-oxadia-
zole-based polymers. In PSCs, the surface morphology of the
active layer is also important to the photovoltaic perform-
ance, and the surface morphology analysis of the active
layers might provide the answer for the different photovol-
taic performances. Figure 7 shows the AFM images of the
active layers of polymer:PC71BM (1:3 wt/wt). The root-
mean-square (RMS) roughness is 1.449, 1.037, 0.722, and
1.308 nm, respectively. The RMS roughness of the
P1:PC71BM (1:3 wt/wt) active layer was found to be higher
than those of all other polymer blends, indicating that the
surface is relatively more rough than the other active layers
surfaces. The higher roughness of P1-based active layer
might be another reason for the poor photovoltaic perform-
ance of polymer P1 compared to that of polymer P4.
electron-accepting unit would be more widely utilized in the
new polymer synthesis for optoelectronic applications.
ACKNOWLEDGMENTS
M. H. Hyun acknowledges the financial support from the New
and Renewable Energy program of the Korea Institute of
Energy Technology Evaluation and Planning (KETEP) grant
(No. 20103020010050) funded by the Ministry of Knowledge
Economy, Republic of Korea.
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