Synthesis of a new conjugated polymer composed of pyrene and bithiophene units for organic solar cells

Article abstract of DOI:10.1166/jnn.2011.3710

An alternating conjugated copolymer composed of pyrene and bithiophene units, poly(DHBT-alt-PYR) has been synthesized. The synthesized polymer was found to exhibit good solution processibility and thermal stability, losing less than 5% of their weight on heating to approximately 370 °C. The synthesized polymer showed its maximum absorption and peak PL emission at 401 and 548 nm, respectively. The optical band gap energy of the polymer was determined by absorption onset to be 2.64 eV. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymer was determined to be-5.48 and-2.84 eV by cyclic voltametry (CV) and the optical band gap. The polymer photovoltaic devices were fabricated with a typical sandwich structure of ITO/PEDOT:PSS/active layer/LiF/Al using poly(DHBT-alt-PYR) as an electron donor and C₆₀-PCBM or C₇₀-PCBM as electron acceptors. The open circuit voltage, short circuit current and fill factor of the device using C₇₀-PCBM as an acceptor were 0.75 V, 3.80 mA/cm ² and 0.28, respectively, and the maximum power conversion efficiency of the device was 0.80%. Copyright

Full text of DOI:10.1166/jnn.2011.3710

Copyright © 2011 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 11, 4367–4372, 2011
Synthesis of a New Conjugated Polymer Composed of
Pyrene and Bithiophene Units for Organic Solar Cells
Sun-Young Lee1, Choong-Hwa Jung2, Jun Kang2, Hee-Joon Kim2, Won Suk Shin3,
Sung Cheol Yoon3, Sang-Jin Moon3, Changjin Lee3, 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
3Korea Research Institute of Chemistry Technology, 100 Jang-dong, Yuseong-gu, Daejeon 305-343, Korea
An alternating conjugated copolymer composed of pyrene and bithiophene units, poly(DHBT-alt-
PYR) has been synthesized. The synthesized polymer was found to exhibit good solution pro-
cessibility and thermal stability, losing less than 5% of their weight on heating to approximately
370 ꢀC. The synthesized polymer showed its maximum absorption and peak PL emission at 401 and
548 nm, respectively. The optical band gap energy of the polymer was determined by absorption
onset to be 2.64 eV. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energy levels of the polymer was determined to be −5.48 and −2.84 eV by cyclic
voltametry (CV) and the optical band gap. The polymer photovoltaic devices were fabricated with a
typical sandwich structure of ITO/PEDOT:PSS/active layer/LiF/Al using poly(DHBT-alt-PYR) as an
electron donor and C -PCBM or C70-PCBM as electron acceptors. The open circuit voltage, short
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circuit current and fill factor of the device using C70-PCBM as an acceptor were 0.75 V, 3.80 mA/cm2
IP: 86.161.220.70 On: Mon, 21 Dec 2015 10:10:28
and 0.28, respectively, and the maximum power conversion efficiency of the device was 0.80%.
Copyright: American Scientific Publishers
Keywords: Conjugated Polymer, Pyrene, Bithiophene, Photovoltaic Device.
1. INTRODUCTION
charge mobilities. And even P3HT/PCBM solar cells can
be expected to reach 8.4% by minimizing the energy loss
in the electron transfer.21 However, the power conversion
efficiency (PCE) of the polymer PV cells is still far away
from that theoretical limit. To improve the PCE in polymer
PV cells, more efforts should be paid to an efficient light
harvest of photoactive layer, an enhanced interface contact
between the metal electrode and photoactive layer, and an
improvement of charge carrier mobility.22
Recently we synthesized a low molecular organic
semiconductor composed of pyrene and bithiphene, 1,6-
bis(5ꢁ-octyl-2,2ꢁ-bithiophen-5-yl)pyrene, as a new p-type
channel material for organic thin film transistors (OTFTs).
The organic molecule showed high hole-mobility (ca.
0.7 cm2/V·s). So we designed polymeric organic semicon-
ductor which has a similar chemical structure with 1,6-
bis(5ꢁ-octyl-2,2ꢁ-bithiophen-5-yl)pyrene as a p-type active
material for organic photovoltaic devices. In this study
we synthesized an alternating conjugated copolymer com-
posed of pyrene and bithiophene units. The synthetic route
and chemical structures of the monomers and polymer are
shown in Schemes 1 and 2.
Organic semiconducting molecules and polymers have
been extensively studied during last decades as active
materials of light-emitting diodes,1 organic transistors,2
organic solar cells3 and organic memory devices. ꢀ-
Conjugated polymers such as poly(p-phenylenevinylene)
(PPV),4–8 poly(p-phenylene) (PPP),9 polythiophene
(PT),10ꢁ11 poly(thienylenevinylene) (PTV),12ꢁ13 poly(p-
phenyleneethynylene) (PPE),14 polyfluorene (PF),15–18 and
their derivatives have been widely studied as active mate-
rials for diverse organic electronic devices because of their
high charge mobility, good processability, and easy band
gap tunability etc.
Recently, the polymer photovoltaic (PV) cells using the
composites of conjugated polymer and soluble fullerene
derivative have attracted much attention due to their pos-
sibilities of low production cost and easy manufacturing
by solution process.19ꢁ20 Theoretically single-layer poly-
mer/fullerene solar cells can reach nearly 11% efficiency
by optimizing active material energy levels, band gaps,
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 5
1533-4880/2011/11/4367/006
doi:10.1166/jnn.2011.3710
4367

Synthesis of a New Conjugated Polymer Composed of Pyrene and Bithiophene Units for Organic Solar Cells
Lee et al.
Br
C6H13MgBr
Hex
NBS
Hex
2.3. Photovoltaic Device Fabrication
2
Mg/THF
Composite solutions (1:3 wt ratio) with poly(DHBT-alt-
PYR) and C60-PCBM (or C70-PCBM) were prepared
using chlorobenzene as a solvent. The concentration
was controlled adequately in a range of 1.0∼2.0 wt%.
For the measurement of optical absorption spectra of
PCBM, the solutions were spun on the pre-cleaned UV-
grade silica substrates. The polymer photovoltaic devices
were fabricated with a typical sandwich structure of
ITO/PEDOT:PSS/active layer/LiF/Al. The ITO coated
glass substrates were cleaned through a routine cleaning
procedure, including sonication in detergent followed by
distilled water, acetone, and 2-propanol. A 45 nm thick
layer of PEDOT:PSS (Baytron P) was spin coated on a
cleaned ITO substrate after exposing the ITO surface to
ozone for 10 ꢀmin. The PEDOT:PSS layer is baked on a hot
plate at 140 C for 10 min. Active layer was spin coated
from the pre-dissolved composite solution after filtering
through 0.45 ꢂm PP syringe filters. The device structure
was completed by depositing 0.6 nm of LiF and 120 nm
Al cathode as top electrode onto the polymer active layer
under 3×10−6 torr vacuum in thermal evaporator. Current
density–voltage (J–V ) characteristics of all polymer pho-
tovoltaic cells were measured under the illumination of
simulated solar light with 100 mW/cm2 (AM 1.5 G) by
Oriel 1000 W solar simulator.
Br
S
S
Ni(dppp)Cl2
S
Ni(dppp)Cl2
1
2
Hex
Hex
O
O
O
B
n-BuLi
S
S
B
S
S
O
3
O
O
4
Hex
Hex
B
O
Scheme 1. Synthetic method of the bithiophene diboronic ester
monomer.
Hex
Br
O
O
O
O
S
+
B
B
S
Br
Hex
Hex
Hex
Pd(PPh3)4
S
S
S
K2CO3/Aliquit
S
n
Poly(DHBT-alt-PYR)
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Scheme 2. Synthetic route and chemical structure of poly(DHBT-
alt-PYR).
Electric data were recorded using a Keithley 236 source-
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measure unit and all characterizations were carried out in
Copyright: American Scientific Publishers
an ambient environment. The illumination intensity used
was calibrated by a standard Si photodiode detector from
PV measurements Inc. which was calibrated at NREL. The
incident photon-to-current conversion efficiency (IPCE)
was measured as a function of wavelength from 360 to
800 nm (PV measurement Inc.) equipped with a halo-
gen lamp as a light source, and calibration was performed
using a silicon reference photodiode. Thickness of the thin
film was measured using a KLA Tencor Alpha-step IQ
surface profilometer with an accuracy of 1 nm.
2. EXPERIMENTAL DETAILS
2.1. Materials
3-Bromothiophene, 1-bromohexane, N-bromosuccinimide,
1,3-bis(diphenylphosphino)propane nickel(II) chloride, 2-
isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, tetra-
kis(triphenylphosphine)palladium were purchased from
Aldrich and used without further purification. [6,6]-Phenyl
C71-butyric acid methyl ester (C70-PCBM) and [6,6]-
phenyl C61-butyric acid methyl ester (C60-PCBM) were
purchased from Nano-C.
2.4. Synthesis of Monomers and Polymers
2.4.1. Synthesis of 3-hexylthiophene (1)
2.2. Measurements
A solution of 1-bromohexane (15 g, 91.95 mmol) in 60 mL
dry THF was added dropwise into magnesium (4.473 g,
183.9 mmol) in 10 mL of dry THF and the mixture
was refluxed at 60 ꢀC under nitrogen gas for 2 hours.
Then the mixture was added slowly into a mixture of
3-bromothiophene (10 g, 61.3 mmol) and Ni(dppp)Cl2
(0.33 g, 0.613 mmol) in 50 ml dry tetrahydrofuran (THF)
at 0 C. The mixture was refluxed at 60 C for 12 hours.
The organic layer was extracted with diethylether three
times. The combined organic layers were washed with
brine and dried over anhydrous MgSO4. The solvent was
then removed by rotary evaporator, and the obtained crude
1H and 13C NMR spectra were recorded using a Bruker
AM-400 spectrometer. The absorption spectra were mea-
sured using a Hitachi spectrophotometer model U-3501
and the steady-state PL spectra were recorded on a Spex
FL3-11. The molecular weights of the polymers were
determined by gel permeation chromatography (GPC)
analysis relative to a polystyrene standard using a Waters
high-pressure GPC assembly Model M590. Thermal anal-
yses were carried out on a Dupont TGA 9900 thermogravi-
metric analyzer under a nitrogen atmosphere at a heating
ꢀ
ꢀ
ꢀ
rate of 10 C/min.
4368
J. Nanosci. Nanotechnol. 11, 4367–4372, 2011

Lee et al.
Synthesis of a New Conjugated Polymer Composed of Pyrene and Bithiophene Units for Organic Solar Cells
product was purified by flash silica gel chromatography
using n-hexane as an eluent. The product yield was 56%.
1H-NMR (400 MHz, CDCl3) ꢃ 0.85 (t, 3H), 1.25 (m, 6H),
1.62 (m, 2H), 2.75 (t, 2H), 6.90 (d, 2H), 7.15 (d, 1H).
(m, 8H), 1.26 (m, 4H), 1.30 (m, 24H), 1.48 (m, 4H), 2.45
(t, 4H), 7.50 (s, 2H).
2.4.5. Synthesis of an Alternating Copolymer Composed
of 3,3ꢁ-dihexylbithiophene and Pyrene
[Poly(DHBT-alt-PYR)]
2.4.2. Synthesis of 2-bromo-3-hexylthiophene (2)
Tetrakis(triphenylphosphine)palladium (0.084 mmol)
catalyst was charged into a two-necked 50 mL round
bottom flask in a glove box. To the reactor was added
a mixture of 1,6-dibromopyrene (0.921 g, 2.588 mmol),
3,3ꢁ-dihexyl-2,2ꢁ-bithiophenyl-5,5ꢁ-di-4,4,5,5-tetramethyl-
[1,3,2]dioxaborolane (1.5 g, 2.56 mmol), 10 mL of
toluene and 10 mL of THF. 2.0 M aqueous sodium
carbonate solution (8 mL, 5 eq) with Aliquat (0.103 g)
as a phase-transfer catalyst were added to the reaction
3-Hexylthiophene (5.8 g, 34.5 mmol) was dissolved in
50 mL of THF and 50 mL of acetic acid in a 250 mL one-
necked flask with a magnetic stirrer. N-Bromosuccinimide
(NBS) (6.45 g, 36.2 mmol) was added into the solution
ꢀ
in one portion, and the solution was stirred at 0 C for
2 hours. The solution was then extracted with diethylether
and brine. The organic layer was separated and dried over
anhydrous MgSO4 and the removal of solvent gave a color-
1
less oil. The product yield was 89%. H-NMR (400 MHz,
ꢀ
mixture, the solution was stirred at 80 C for 48 hours.
CDCl3) ꢃ 0.85 (t, 3H), 1.25 (m, 6H), 1.62 (m, 2H), 2.75
The polymer was then end-capped with 2-bromothiophene
and thiophene-2-boronic acid, and stirred at 80 ꢀC for
12 hours. After the reaction was quenched with methanol,
the crude product was flash-columned on Florisil using
chloroform as an eluent. The obtained solid product was
dissolved in chloroform and precipitated in methanol three
times and finally purified by a Soxhlet extractor using
methanol as a solvent. 1H-NMR (400 MHz, CDCl3) ꢃ
0.85 (t, 6H), 1.25 (m, 8H), 1.45 (m, 4H) 1.75 (m, 4H),
2.85 (t, 4H), 7.32 (s, 2H), 8.02 (d, 2H), 8.10 (d, 2H),
(t, 2H), 6.90 (d, 1H), 7.15 (d, 1H).
2.4.3. Synthesis of 3,3ꢁ-dihexyl-2,2ꢁ-bithiophene (3)
A
solution of 2-bromo-3-hexylthiophene (9.15 g,
37.2 mmol) in 40 mL dry THF was added drop-wise
manner into magnesium (2.26 g, 93.0 mmol) in 10 mL
of dry THF and the mixture was refluxed at 60 C under
ꢀ
nitrogen gas for 2 hours. Then this mixture was added
slowly into a mixture of 2-bromo-3-hexylthiophene (7.6 g,
31.0 mmol) and Ni(dppp) Cl2 (0.17 g, 0.31 mmol) in
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8.25 (d, 2H), 8.45 (d, 2H) 13C-NMR (400 MHz, CDCl3)
ꢃ 17.0, 21.5, 24.5, 25.0, 25.5, 120.5, 125.5, 126.0, 126.5,
128.0, 129.0, 129.5, 130.0, 131.0, 132.0.
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ꢀ
30 mL of dry THF at 0 C. The mixture was refluxed
Copyright: American Scientific Publishers
ꢀ
at 60 C for 12 hours. The organic layer was extracted
with diethylether/brine three times and then was dried
over anhydrous MgSO4. Solvent was then removed under
reduced pressure, and the product was purified by frac-
tional distillation under vacuum. The product yield was
56%. H-NMR (400 MHz, CDCl3ꢄ ꢃ 0.85 (t, 6H), 1.25
(m, 12H), 1.52 (m, 4H), 2.45 (t, 4H), 6.93 (d, 2H), 7.25
(d, 2H).
3. RESULTS AND DISCUSSION
The synthesized polymer was soluble in common organic
solvents such as chloroform, chlorobenzene, toluene, etc.
and formed uniform thin film after spin-coating with the
polymer solution. The weight average molecular weights
(Mw) of poly(DHBT-alt-PYR) was determined by gel per-
meation chromatography using a polystyrene standard, and
was found to be 7,300 with polydispersity indexes of 1.70.
The polymer yield was 65% after purification. The thermal
properties of the polymers were determined with thermal
gravimetric analysis (TGA). The polymer was found to
exhibit good thermal stability, losing lessꢀthan 5% of their
weight on heating to approximately 370 C.
Figure 1 shows the UV-visible absorption and PL emis-
sion spectra of the polymer thin film. The maximum
absorption and peak emission of the polymer film was
401 and 548 nm, respectively. The UV absorption and
PL emission spectra of the polymer film was red-shifted
than those of polymer solution in chloroform (398 and
523 nm, respectively), suggesting that the effective con-
jugation would be extended in solid state due to staking
of flat pyrene and bithiophene segments. Highest occu-
pied molecular orbital (HOMO) energy level of the poly-
mer was measured by oxidation potential of polymer
1
2.4.4. Synthesis of
3,3ꢁ-dihexyl-5,5ꢁ-[di-4,4,5,5-tetramethyl-1,
3,2-dioxaborolanyl]-2,2ꢁ-bithiophene (4)
To a solution of 3,3ꢁ-dihexyl-2,2ꢁ-bithiophene (4.36 g,
13.0 mmol) in THF (60 mL) was added a 2.5 M solu-
tion of n-BuLi (17.1 mL, 27.37 mmol) in n-hexane ꢀat
ꢀ
−70 C. After 30 min, the mixture was heated to −15 C
and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(5.32 g, 28.6 mmol) was added. The mixture was then
heated to room temperature and stirred for 2 hours. The
reaction solution was hydrolyzed and then extracted with
diethylether and brine. Organic layer was separated, dried
over anhydrous MgSO4 and concentrated using a rotary
evaporator. The obtained crude product was purified by
flash silica gel column chromatography using a solvent
(n-hexane/ethyl acetate = 9/1). The product yield was
1
61%. H-NMR (400 MHz, CDCl3) ꢃ 0.85 (t, 6H), 1.20
J. Nanosci. Nanotechnol. 11, 4367–4372, 2011
4369

Synthesis of a New Conjugated Polymer Composed of Pyrene and Bithiophene Units for Organic Solar Cells
Lee et al.
1.0
PL
UV
0.8
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength (nm)
Fig. 1. UV-visible absorption and PL emission spectra of the
poly(DHBT-alt-PYR) thin film.
film using cyclic voltametry (CV). The measured HOMO
energy level of the polymer was −5.48 eV when thefer-
rocene/ferrocenium redox system was used as a reference.
The optical band gap from the absorption onset of the
polymer film was 2.64 eV, therefore the LUMO energy
level of the polymer would be −2.84 eV.
Fig. 3. (a) Structure of the photovoltaic device, (b) energy band dia-
grams for the photovoltaic device, (c) chemical structure of C60-PCBM,
(d) chemical structure of C70-PCBM.
layer was spin-coated (ca 50 nm thick) after cleaning of
ITO using UV ozone treatment. Polymer and PCBM (1:3)
was dissolved in chlorobenzene at 50 ꢀC for 20 hours with
Photovoltaic devices were fabricated using the
poly(DHBT-alt-PYR) as a p-type electron donor, and
[6,6]-phenyl C61-butyric acid methyl ester (C60-PCBM) or
[6,6]-phenyl C71-butyric acid methyl ester (C70-PCBM) as
n-type acceptors. C70-PCBM has a wider absorption spec-
trum than that of C60-PCBM so we expected that more
energy absorption may cause improved power conversion
efficiency of the devices.
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stirring. The PEDOT:PSS coated ITO glass was baked in
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glove box at 140 ꢀC for 10 min, and polymer solution was
Copyright: American Scientific Publishers
over-coated and baked at 120 ꢀC for 10 min. Then the thin
lithium fluoride (ca. 0.6 nm) and aluminum (ca. 150 nm)
was deposited on the top of the polymer film. An AM
1.5 solar simulator and a radiant power meter were used
to characterize the photovoltaic devices. The structure and
band diagram of the devices, and chemical structure of the
C60-PCBM and C70-PCBM are shown in Figure 3.
Figure 2 shows the UV-visible absorption spectra of
C60-PCBM and C70-PCBM in chloroform solution. Two
kinds of photovoltaic devices with ITO/PEDOT:PSS/
Polymer + C60-PCBM (or C70-PCBM)/LiF/Al configura-
tion which have different electron-acceptors were fabri-
cated and compared device performances. PEDOT:PSS
Figure 4 shows the incident photon-to-current con-
version efficiency (IPCE) spectrum of the devices. The
onset wavelength of photon-to-current conversion was
ca. 750 nm, and peaks of the photon-to-current conver-
sion were 428 and 557 nm. The photon-to-current conver-
sions of both devices are relatively low presumably due
to high band gap energy of poly(DHBT-alt-PYR). C70-
PCBM which has wider absorption band would be better
electron-acceptor than C60-PCBM especially when p-type
donor has no enough absorption bands. The open circuit
voltage, short circuit current and fill factor of the device
using C60-PCBM as an acceptor were 0.73 V, 1.67 mA/cm2
and 0.27, respectively, and the maximum power conversion
efficiency of the device was 0.33%. The device using C70-
PCBM showed better results than those of the device using
C60-PCBM. The open circuit voltage, short circuit current
and fill factor of the device using C70-PCBM as an accep-
tor were 0.75 V, 3.80 mA/cm2 and 0.28, respectively, and
the maximum power conversion efficiency of the device
was 0.80%. The performances of the fabricated devices
1.4
C70-PCBM
C60-PCBM
1.2
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
Wavelength (nm)
Fig. 2. UV-visible absorption spectra of C60-PCBM and C70-PCBM in
chloroform solution.
4370
J. Nanosci. Nanotechnol. 11, 4367–4372, 2011

Lee et al.
Synthesis of a New Conjugated Polymer Composed of Pyrene and Bithiophene Units for Organic Solar Cells
Table I. Device performances of poly(DHBT-alt-PYR)/PCBM solar
Poly(DHBT-alt-PYR) + C70-PCBM
cell devices.
Poly(DHBT-alt-PYR) + C60-PCBM
Poly(DHBT-alt-PYR):
C60-PCBM (1:3 wt%)
Poly(DHBT-alt-PYR):
C70-PCBM (1:3 wt%)
20
10
0
Jsc (mA/cm2)
Voc(V)
FF
1.67
0.73
0.27
0.33
3.80
0.75
0.28
0.80
PCE (%)
to high band gap energy (2.64 eV), nevertheless, the
photovoltaic device using C70-PCBM acceptor and the
synthesized polymer showed 0.8% of power conversion
efficiency. Improvement of device efficiency is expected
by lowering band gap energy of conjugated polymers com-
posed of pyrene and bithiophene units through proper syn-
thetic modification.
400
500
600
700
800
Wavelength (nm)
Fig. 4. The incident photon-to-current conversion efficiency (IPCE)
spectra of the photovoltaic devices.
Acknowledgments: This research was financially sup-
ported by the Ministry of Knowledge Economy (MKE)
under the New & Renewable Energy program of the
KETEP grant (No. 2008-N-PV08-02 and No. 2010302
0010050), and Fundamental R&D Program for Core Tech-
nology of Materials funded by the MKE, Korea.
are not comparable to recent best results but both devices
showed relatively high open circuit voltage in spite of low
short circuit currents. When we consider that the poly-
mer could not absorb enough energy due to high band gap
energy (2.64 eV), the results would be promising. If we
modified the polymer structure to have a smaller band gap,
the power conversion efficiency could be highly improved.
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Delivered by Publishing Technology to: Deakin University Library
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Received: 23 May 2009. Accepted: 20 Nov 2009.
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