Low-bandgap poly(4H-cyclopenta[def]phenanthrene) derivatives with 4,7-dithienyl-2,1,3-benzothiadiazole unit for photovoltaic cells

Article abstract of DOI:10.1016/j.polymer.2009.12.009

A series of conjugated polymer bearing 4H-cyclopenta[def]phenanthrene (CPP) unit have been synthesized and was evaluated in bulk heterojunction solar cell. The alternating copolymers with CPP unit were incorporated with 4,7-dithienyl-2,1,3-benzothiadiazol

Full text of DOI:10.1016/j.polymer.2009.12.009

Polymer 51 (2010) 390–396
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Low-bandgap poly(4H-cyclopenta[def]phenanthrene) derivatives with
4,7-dithienyl-2,1,3-benzothiadiazole unit for photovoltaic cells
Jinwoo Kim a, Sung Heum Park g, Shinuk Cho b, Youngeup Jin e, Jaehong Kim a, Il Kim c, Jin Sook Lee d,
,
Joo Hyun Kim e, Han Young Woo f, Kwanghee Lee g, Hongsuk Suh a
*
a Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea
b Center for Polymer and Organic Solids, University of California at Santa Barbara, Santa Barbara, CA 93106, USA
c Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea
d Department of Chemistry, Yonsei University, Seoul 120-749, Republic of Korea
e Division of Applied Chemical Engineering, Pukyong National University, Busan 608-739, Republic of Korea
f Department of Nanomaterials Engineering, Pusan National University, Miryang 627-706, Republic of Korea
g Department of Material Science & Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of conjugated polymer bearing 4H-cyclopenta[def]phenanthrene (CPP) unit have been synthe-
sized and was evaluated in bulk heterojunction solar cell. The alternating copolymers with CPP unit were
incorporated with 4,7-dithienyl-2,1,3-benzothiadiazole (DTBT) unit by Suzuki conditions. The newly
synthesized copolymers, poly(2,6-((4,4-bis(2-ethylhexyl)-4H-cyclopenta[def]phenanthrene))-alt-(4,7-
((2-thienyl)-2,1,3-benzothiadiazole))) (PCPP-DTBT), and poly(2,6-(4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-
Received 5 August 2009
Received in revised form
1 December 2009
Accepted 6 December 2009
Available online 16 December 2009
4H-cyclopenta[def]phenanthrene)-alt-(4,7-((2-thienyl)-2,1,3-benzothiadiazole)))
(PBEHPCPP-DTBT),
contain dialkyl and bis(alkoxyphenyl) groups in the CPP unit, respectively. The HOMO–LUMO energy
bandgaps of these materials, estimated from UV–vis spectroscopy and cyclic voltammetry (CV), were
2.00 eV for PCPP-DTBT and 1.80 eV for PBEHPCPP-DTBT. Bulk heterojunction solar cells based on the
blends of the polymers with [6,6]phenyl-C71-butyric acid methyl ester (PC71BM) gave power conversion
efficiencies as 1.00% for PCPP-DTBT and 1.12% for PBEHPCPP-DTBT under AM 1.5, 100 mW/cm2.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Polymer solar cell
Cyclopentaphenanthrene
Benzothiadiazole
1. Introduction
2-yl-2,1,3-benzothiadiazole (DTBT) unit has been used effectively as
an acceptor, and copolymerized with many kinds of donor
The most representative configuration of polymer solar cells is
bulk heterojunction device which is composed of a blend of an
electron-donating material (n-type), and an electron-accepting
material (p-type) such as (6,6)-phenyl C61-butyric acid methyl ester
or (6,6)-phenyl C71-butyric acid methyl ester (PCxBM). Many new
conjugated polymers with a small band gaps are being researched
for polymer solar cells (PSCs) to cover the long-wavelength region
for the improvement of total photovoltaic current [1–6].
Several examples have been known to show that utilization of
the donor and acceptor functionalities in conjugate backbone of the
alternative copolymer structures is an efficient way to decrease the
band gaps. In the donor/acceptor (D–A) combinations, 4,7-dithien-
segments, such as fullerene [7,8], silafluorene [9,10], carbazole [11],
dithienosilole [12], and cyclopenta[2,1-b:3,4-b]dithiophene [13].
When these D–A types of copolymers were applied to photovoltaic
solar cells (PSCs), the power conversion efficiencies (PCEs) in the
range of 0.18%–5.4% have been reported [14,15].
Recently, we reported new blue-emitting polymers, dialkyl
substituted poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[def]phen
anthrene)) (PCPP) [16,17], and bis(alkoxyphenyl) substituted poly(2,6-
(4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenanthre
ne)) (BEHPCPP) [18] with a rigid backbone, which generates stabilized
and efficient blue electroluminescence without exhibiting any peak in
the long-wavelength region even after prolonged annealing or oper-
ation of the devices in air. In addition to the stability of the CPP unit, it’s
combination with DTBT unit can generate the larger difference
between the donor HOMO level and the acceptor LUMO level of the
copolymer, which could be an essential key for the enhanced open
circuit voltage (VOC) of the PSCs [19].
* Corresponding author. Tel.: þ82 51 510 2203; fax: þ82 51 516 7421.
E-mail addresses: jinwoo@pusan.ac.kr (J. Kim), shpark@physics.ucsb.edu (S.H.
Park), sucho@physics.ucsb.edu (S. Cho), yjin@pknu.ac.kr (Y. Jin), ilkim@pusan.ac.kr
(I. Kim), jkim@pknu.ac.kr (J.H. Kim), hywoo@pusan.ac.kr (H.Y. Woo), klee@gist.ac.kr
(K. Lee), hssuh@pusan.ac.kr (H. Suh).
In this paper, we report the synthesis and photovoltaic properties
of new CPP-based conjugated copolymers for solar cell, poly(2,6-
0032-3861/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2009.12.009

J. Kim et al. / Polymer 51 (2010) 390–396
391
((4,4-bis(2-ethylhexyl)-4H-cyclopenta[def]phenanthrene))-alt-(4,7-
((2-thienyl)-2,1,3-benzothiadiazole))) (PCPP-DTBT), and poly(2,6-
(4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenanth
80 ꢀC for 30 min. All fabrication processes were carried out in the
glove box filled with N2. Source and drain electrodes using Au
were deposited by thermal evaporation using shadow mask. The
thickness of source and drain electrodes was 50 nm. Channel
rene)-alt-(4,7-((2-thienyl)-2,1,3-benzothiadiazole)))
(PBEHPCPP-
DTBT).
length (L) and channel width (W) were 50 mm and 1.5 mm,
respectively. Electrical characterization was performed using
a Keithley semiconductor parametric analyzer (Keithley 4200)
under N2 atmosphere. Atom force microscopy (AFM) measure-
ments were obtained with a Veeco NanoScope za AFM at room.
Commercial silicon cantilevers (Veeco) with typical spring
constants of 21–78 N mꢂ1 was used to operate the AFM in
taping mode. Images were taken continuously with the scan rate
2.0 Hz.
2. Experimental section
2.1. Instruments
IR spectra were recorded on a Perkin–Elmer 16F PC FTIR
spectrometer with samples prepared as KBr pellet. 1H and 13C
NMR spectra were recorded with a Varian Gemini-300 (300 MHz)
spectrometer and chemical shifts were recorded in ppm units
with TMS as the internal standard. Flash column chromatography
was performed with Merck silica gel 60 (particle size 230–400
mesh ASTM) with ethyl acetate/hexane or methanol/methylene
chloride gradients unless otherwise indicated. Analytical thin
layer chromatography (TLC) was conducted using Merck 0.25 mm
silica gel 60F precoated aluminum plates with fluorescent indi-
cator UV254. UV spectra were recorded with a Varian CARY-5E
UV/vis spectrophotometer. Cyclic voltammetric waves were
produced by using a EG&G Parc model 273 potentiostat/galvano-
stat. The differential scanning calometry analysis was performed
under a nitrogen atmosphere (50 mL/min) on a DSC 822 at
heating rates of 10 ꢀC/min. Thermo gravimetric analysis was
performed with a Dupont 951 TGA instrument in a nitrogen
atmosphere at a heating rate of 10 ꢀC/min to 700 ꢀC. High reso-
lution mass spectra (HRMS) were recorded on a JEOL JMS-700
mass spectrometer under fast atom bombardment (FAB) condi-
tions in the Korea Basic Science Institute Daegu Branch. Solar cells
were fabricated on an indium tin oxide (ITO)-coated glass
substrate with the following structure; ITO-coated glass substrate/
poly(3,4-ethylenedioxythiophene)(PEDOT:PSS)/polymer:PC71BM/
TiOx/Al. The ITO-coated glass substrate was first cleaned with
detergent, ultrasonicated in acetone and isopropyl alcohol, and
subsequently dried overnight in an oven. PEDOT:PSS (Baytron PH)
was spin-cast from aqueous solution to form a film of 40 nm
thickness. The substrate was dried for 10 min at 140 ꢀC in air and
then transferred into a glove box to spin-cast the charge separa-
tion layer. A solution containing a mixture of polymer:PC71BM
(1:4) in dichlorobenzene solvent with concentration of 7 mg/ml
was then spin-cast on top of the PEDOT/PSS layer. The film was
dried for 60 min at 70 ꢀC in the glove box. The TiOx precursor
solution diluted by 1:200 in methanol was spin-cast in air on top
of the polymer:PC71BM layer (5000 rpm for 40 s). The sample was
heated at 80 ꢀC for 10 min in air. Then, an aluminum (Al, 100 nm)
electrode was deposited by thermal evaporation in a vacuum of
about 5 ꢁ 10ꢂ7 Torr. Current density–voltage (J–V) characteristics
of the devices were measured using a Keithley 236 Source
Measure Unit. Solar cell performance utilized an Air Mass 1.5
Global (AM 1.5 G) solar simulator with an irradiation intensity of
100 Wmꢂ2. An aperture (12.7 mm2) was used on top of the cell to
eliminate extrinsic effects such as cross-talk, waveguiding,
shadow effects etc. The spectral mismatch factor was calculated by
comparison of solar simulator spectrum with AM 1.5 spectrum at
room temperature. The FETs were fabricated on heavily doped n-
type silicon (Si) wafers each covered with a thermally grown
silicon dioxide (SiO2) layer with thickness of 200 nm. The active
layer was deposited by spin coating at 2500 rpm. Prior to active
layer deposition, SiO2 surfaces were treated with octyltri-
chlorosilane (OTS) to make surface hydrophobic. All solutions
were of 0.5 wt % concentration in chlorobenzene. The thickness of
the deposited films was about 60 nm. Prior to deposition of source
drain electrodes, the films were dried on hot plate stabilized at
2.2. Materials
All reagents were purchased from Aldrich or TCI, and used
without further purification. Solvents were purified by normal
procedure and handled under moisture-free atmosphere. 2-(4,4-
Bis(2-ethylhexyl)-6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
4H-cyclopenta[def]phenanthren-2-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (6) [20], and 2,6-dibromo-4,4-bis(4-((2-ethyl-
hexyl)oxy)phenyl)-4H-cyclopenta[def]phenanthrene (8) [18] were
synthesized using similar methods reported by us.
2.3. Synthesis of monomer and polymer
2.3.1. Synthesis of 4,7-di-2-thienyl-2,1,3-benzothiadiazole (4) [20]
To a stirred solution of 1 mL (12.2 mmol) of thiophene (1) in
30 mL of anhydrous THF was added 5.50 mL (8.72 mmol) of n-
buthyllithium (1.6 M in Hexane) at ꢂ78 ꢀC. After 2 h at ꢂ78 ꢀC,
the reaction mixture was added 2.58 g (7.90 mmol) of tributyltin
chloride over 30 min. After stirring over 6 h at room temperature,
the mixture was quenched with 20 mL of saturated aqueous
NaHCO3. The organic phase was dried with MgSO4, concentrated
under reduced pressure, and the resulting residue was dried
under vacuum (4 Torr) for 15 h to give 3 g of tributyl(2-thie-
nyl)stannane (2). To a solution of 1.00 g (3.40 mmol) of 4,7-
dibromobenzo-2,1,3-thiadiazole (3) in 50 mL of THF were added
3.00 g (8.16 mmol) of compound 2 and 97.0 mg (2.00 mol%) of
PdCl2(PPh3)2 under nitrogen atmosphere. After 3 h at reflux, the
solvent was removed under reduced pressure, and then the
residue was purified by flash chromatography (60 ꢁ 150 mm
column, SiO2, 50% methylene chloride/n-hexane) to give 750 mg
of compound 4 as red needles.: mp 124–125 ꢀC, Rf 0.5 (SiO2, 50%
methylene chloride/n-hexane). 1H NMR (300 MHz, CDCl3,
d): 8.08
(dd, 2H, J ¼ 3.85 and 1.1 Hz), 7.79 (s, 2H), 7.45 (dd, 2H, J ¼ 5.22
and 1.1 Hz), 7.19 (dd, 2H, J ¼ 5.22 and 3.85 Hz). 13C NMR (75 MHz,
CDCl3,
d): 152.472, 139.261, 127.924, 127.408, 126.710, 125.822,
125.618; HRMS (FABþ, m/z) Calcd for C14H8N2S3 299.9850; found
299.9852.
2.3.2. Synthesis of 4,7-bis(5-bromo-2-thienyl)-2,1,3-
benzothiadiazole (5)
To a solution of 1 g (3.33 mmol) of compound 4 in 50 ml of N,N-
dimethylformamide (DMF) was added 1.24 g (6.99 mmol) of N-
bromosuccinimide (NBS). After being stirred for 12 h at room
temperature, the dark red precipitate was filtered off and recrys-
tallized with DMF to give 1 g of 5, red solid.: mp 251–252 ꢀC, Rf 0.75
(SiO2, 50% methylene chloride/n-hexane). 1H NMR (300 MHz,
CDCl3),
d
(ppm): 7.82 (d, 2H, J ¼ 3.85 Hz), 7.80(s, 2H), 7.17 (d, 2H,
J ¼ 3.85 Hz). HRMS (m/z, FABþ) Cald for C14H6Br2N2S3 455.8060;
Found 455.8055.

392
J. Kim et al. / Polymer 51 (2010) 390–396
2.3.3. Synthesis of 2,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-
cyclopenta[def]phenanthrene (9)
CDCl3), d (ppm): 0.84–0.93 (m, 12H), 1.27–1.50 (m, 40H), 1.64–1.70
(m, 2H), 3.79 (d, 4H, J ¼ 5.7 Hz), 6.77 (d, 4H, J ¼ 8.8 Hz), 7.23 (d, 4H,
J ¼ 9 Hz) 7.86 (s, 2H), 8.00 (s, 2H), 8.37 (s, 2H). 13C NMR (75 MHz,
To a stirred solution of 0.720 g (0.950 mmol) of 2,6-dibromo-
4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenan-
threne (8) in 40 mL of DMF were added 1.20 g (4.80 mmol) of
bis(pinacolato)diboron, 0.470 g (4.80 mmol) of potassium acetate,
and 50.0 mg (0.050 mmol) of Pd(dppf)Cl2. After 12 h at 60 ꢀC, the
reaction mixture was treated with 200 mL of water and 150 mL of
diethyl ether. The organic phase was dried (MgSO4), and concen-
trated under reduced pressure. The residue was purified by column
chromatography (40 ꢁ 150 mm column, SiO2, 5% ethyl acetate/n-
hexane) to give 0.7 g, yellow liquid.: Rf 0.36 (SiO2, 10% ethyl acetate/
n-hexane). FTIR (KBr, cmꢂ1): 3744, 2914, 2852, 1617, 1521, 1469,
1381, 1328, 1245, 1130, 1020, 951, 841, 718, 525. 1H NMR (300 MHz,
CDCl3), d (ppm): 158.29, 150.47, 138.30, 137.23, 131.43, 129.59,
128.55, 127.43, 126.08, 114.30, 84.08, 70.51, 67.59, 39.62, 30.76,
29.31, 25.21, 24.09, 23.29, 14.32, 11.35. HRMS (m/z, FABþ) Cald for
C55H72B2O6 850.5515; Found 850.5507.
2.3.4. Synthesis of poly(2,6-((4,4-bis(2-ethylhexyl)-4H-
cyclopenta[def]phenanthrene))-alt-(4,7-((2-thienyl)-2,1,3-
benzothiadiazole))) (7, PCPP-DTBT)
To a stirred solution of 0.260 g (0.400 mmol) of 2-(4,4-Bis(2-
ethylhexyl)-6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-4H-
cyclopenta[def]phenanthren-2-yl)-4,4,5,5-tetramethyl-1,3,2-diox-
aborolane (6) in 7 mL of toluene were added 0.180 g (0.400 mmol)
S
S
N
N
N
N
1) n-BuLi
S
PdCl2(PPh3)2 / THF
S
S
S
SnBu3
Br
Br
2) Bu3SnCl
2
1
3
4
S
N
N
Br
Br
S
S
NBS
DMF
5
S
N
N
O
O
O
O
K2CO3 / Pd(PPh3)4
H2O / toluene
S
S
B
B
5
n
PCPP-DTBT
7
6
O
O
O
O
O
O
O
O
AcOK/Pd(dppf)Cl2
O
O
O
B
B
Br
Br
B
B
O
9
8
O
O
S
N
N
K2CO3 / Pd(PPh3)4
H2O / toluene
S
S
5
+
9
n
PBEHPCPP-DTBT
10
Scheme 1. Synthetic routes for the monomers and the polymers.

J. Kim et al. / Polymer 51 (2010) 390–396
393
of 4,7-bis(5-bro-mo-2-thienyl)-2,1,3-benzothiadiazole (5), and
14.0 mg (3.00 mol%) of (PPh3)4Pd(0), and 2 mL (4.00 mmol) of
aqueous 2 M K2CO3. The reaction mixture was further heated at
80 ꢀC for 2 days. After cooling to room temperature, the reaction
mixture was poured into the solution of 100 mL of acetone, and
300 mL of methanol. After being stirred for 1 h, the precipitate was
filtered and dried in vacuum. The precipitate was dissolved with
minimum amount of chloroform and poured into the 300 mL of
methanol. After filtration by glass filter, the precipitate was puri-
fied by Soxhlet extraction with methanol and dried in vacuum for
24 h to generate 47 mg of polymer 7, PCPP-DTBT.
PBEHPCPP-DTBT (10) was obtained under Suzuki coupling condi-
tion, using monomer 5, 2,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxabor-
olan-2-yl)-4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta
[def]phenanthrene (9), Pd(PPh3)4 catalyst and aqueous 2 M potas-
sium carbonate.
The PCPP-DTBT and PBEHPCPP-DTBT were soluble in organic
solvents such as chloroform, tetrahydrofuran, dichloromethane,
and chlorobenzene. Molecular weights and polydispersities of the
1.5
a
2.3.5. Synthesis of poly(2,6-(4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-
4H-cyclopenta[def]phenanthrene)-alt-(4,7-((2-thienyl)-2,1,3-
benzothiadiazole))) (10, PBEHPCPP-DTBT)
PCPP-DTBT
PBEHPCPP-DTBT
To a stirred solution of 560 mg (0.650 mmol) of 2,6-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-4,4-bis(4-((2-ethylhexyl)oxy)-
phenyl)-4H-cyclopenta[def]phenanthrene (9) in 7 mL of toluene
were added 0.300 g (0.650 mmol) of 4,7-bis(5-bromo-2-thienyl)-
2,1,3-benzothiadiazole (5), and 23.0 mg (3.00 mol%) of
(PPh3)4Pd(0), and 3.30 mL (6.50 mmol) of aqueous 2 M K2CO3. The
reaction mixture was further heated at 80 ꢀC for 2 days. After
cooling to room temperature, the reaction mixture was poured into
the solution of 100 mL of acetone and 300 mL of methanol. After
being stirred for 1 h, the precipitate was filtered and dried in
vacuum. The precipitate was dissolved with minimum amount of
chloroform and poured into the 300 mL of methanol. After filtration
by galss filter, the precipitate was purified by Soxhlet extraction
with methanol and dried in vacuum for 24 h to generate 50 mg of
polymer 10, PBEHPCPP-DTBT.
1.0
0.5
0.0
300
400
500
600
700
800
Wavelength (nm)
1.5
b
PCPP-DTBT
PBEHPCPP-DTBT
3. Results and discussion
1.0
0.5
0.0
3.1. Synthesis and characterization of polymers
The general synthetic routes towards the monomer and polymers
are outlined in Scheme 1. Commercially available thiophene (1) was
lithiated using n-buthyllithium and then treated with tributyl-
chlorostanne to afford tributyl(2-thienyl)stannane (2), which was
coupled with commercially available 4,7-dibromobenzo-2,1,3-thia-
diazole (3) in THF by Stille coupling reaction to give 4,7-di-2-thienyl-
2,1,3-benzothiadiazole (4). Compound 4 was brominated using
N-bromosuccinimide to generate 4,7-bis(5-bromo-2-thienyl)-2,1,3-
benzothiadiazole (5) [20]. Compound 8 was treated with bis(pina-
colato)diboron, potassium acetate, and Pd(dppf)Cl2 in DMF to get
2,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-4,4-bis(4-((2-e-
thylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenanthrene (9). The
alternating copolymer 7 (PCPP-DTBT) was affected under Suzuki
coupling condition [21], using monomer 5, 2-(4,4-bis(2-ethylhexyl)-
6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-4H-cyclopenta-
[def]phenanthren-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6)
[18], Pd(PPh3)4 catalyst and aqueous 2 M potassium carbonate.
300
400
500
Wavelength (nm)
600
700
800
c
1.2
0.8
0.4
0.0
PCPP-DTBT
PBEHPCPP-DTBT
Table 1
Characterization of the Polymers.
a
a
a
b
c
Copolymers
Mn
Mw
Mw/Mn
Tg (ꢀC)
Td (ꢀC)
PCPP-DTBT
PBEHPCPP-DTBT
8600
5000
17,000
8000
2.0
1.6
101
79
404
432
a
The number-average molecular weight (Mn) and the weight-average molecular
weight (Mw) were determined by GPC in THF using a calibration curvse of poly-
styrene as the standard.
400
500
600
700
Wavelength (nm)
b
Glass transition temperature determined by DSC.
c
Decomposition temperature corresponding to 5% weight loss in N2 determined
Fig. 1. The normalized UV–visible absorption spectra of PCPP-DTBT and PBEHPCPP-
by TGA.
DTBT (a) in THF solution and (b) in thin film and fluorescence spectra (c) in THF.

394
J. Kim et al. / Polymer 51 (2010) 390–396
polymers were determined by gel permeation chromatography
(GPC) analysis with a polystyrene standard calibration. As shown in
Table 1, the number-average molecular weight (Mn) and the
weight-average molecular weight (Mw) of the PCPP-DTBT were
8600 and 17,000 with polydispersity index of 2.0. Mn and Mw of
PBEHPCPP-DTBT were 5000 and 8000 with polydispersity index of
1.6. The thermal properties of the polymer were evaluated by
differential scanning calorimetry (DSC) and thermo gravimetric
analysis (TGA) in nitrogen. PCPP-DTBT shows loses less than 5% of
its weight on heating to 404 ꢀC. Glass transition temperature
measured by DSC under N2 was 101 ꢀC. PBEHPCPP-DTBT shows
loses less than 5% of its weight on heating to 432 ꢀC. Glass transition
temperature measured by DSC under N2 was 79 ꢀC.
PBEHPCPP-DTBT
-3.4 eV
PCBM
-4.3 eV
Al
(-4.3 eV)
-3.5 eV
LUMO
HOMO
ITO
(-4.8 eV)
PEDOT
(-5.0 eV)
-1.8 eV
-2.0 eV
-5.3 eV
-5.4 eV
-6.1 eV
PCPP-DTBT
Fig. 2. Energy diagram of HOMO/LOMO levels of PCPP-DTBT and PBEHPCPP-DTBT in
relation to the work function of the ITO, PEDOT:PSS, PCBM, and Al.
exhibited irreversible p-doping process. HOMO energy level of the
polymer was ꢂ5.40 eV. The absorption onset wavelength is 620 nm,
which corresponds to band gap of 2.00 eV. The LUMO energy level,
calculated from the value of the band gap and HOMO energy level,
was ꢂ3.40 eV. The oxidation onset potential of PBEHPCPP-DTBT
was 0.54 V and exhibited irreversible p-doping process. HOMO
energy level of PBEHPCPP-DTBT was ꢂ5.30 eV. The absorption
onset wavelength was 688 nm, which correspond to band gap of
1.80 eV. As shown in Fig. 2, the bandgap of PBEHPCPP-DTBT was
decreased by 0.20 eV as compared to that of PCPP-DTBT. Such
3.2. Optical properties
All spectroscopic properties were measured both in THF
solutions and as thin films on glass slides. As shown in Fig.1a, PCPP-
DTBT shows absorption maxima at 375 and 527 nm. PBEHPCPP-
DTBT shows similar absorption maxima at 379 and 527 nm. The
short-wavelength absorption peak is attributed to a delocalized
excitonic
p–p* transition in the polymer chains and the long-
wavelength absorption peaks attributed to a localized transition
between the D–A–D charge transfer. As shown Fig. 1b, PCPP-DTBT
in solid state shows the maximum absorption peaks at 387 and
554 nm. PBEHPCPP-DTBT in solid state exhibits the maximum
absorption peaks at 400 and 570 nm. The second absorption
maxima of the solid thin films exhibit larger red shifts (27–43 nm)
as compared to the corresponding peaks in dilute solutions. Fig. 1c
shows that PCPP-DTBT and PBEHPCPP-DTBT emit deep red fluo-
rescence in THF solution with maximum peaks at 621 and 612 nm,
respectively. This suggests that there is a very efficient energy
transfer from the CPP unit to the benzothiadiazole-cored unit.
a decrease in the band gap can be explained by the extension of p-
conjugation system of the polymer main chain to two aromatic
substituents on the PCCP unit.
3.4. Photovoltaic cell characteristics
The current-voltage characteristics of the solar cells, under
simulated 100 mW/cm2 AM 1.5 G white light illumination, based on
the two blends of PCPP-DTBT/PCBM and PBEHPCPP-DTBT/PCBM
are shown in Fig. 3. Table 3 lists the photovoltaic properties
obtained from the J–V curves for the best devices. The PSCs had
a layered structure of ITO/PEDOT:PSS/copolymers:PC71BM (1:4, w/
w)/TiOx/Al where the polymer was used as the electron donor and
PCBM was used as the electron acceptor. The device with PCPP-
DTBT: PCBM layer showed an open circuit voltage (Voc) of 0.71 V,
a short-circuit current density (Jsc) of 4.7 mA/cm2, and a fill factor
(FF) of 0.30, giving a power conversion efficiency of 1.00%. The
device with PBEHPCPP-DTBT:PC71BM blend demonstrated a Voc
value of 0.61 V, a Jsc value of 4.6 mA/cm2, and a FF of 0.40, leading to
the power conversion efficiency of 1.12%, a slightly improved
3.3. Electrochemical properties
The LUMO energy levels of the polymers were determined from
the band gaps which were estimated from the absorption edges,
and the HOMO energy levels which were estimated from the cyclic
voltammetry [22]. The CVs were performed with a solution of
tetrabutylammonium tetrafluoroborate (Bu4NBF4) (0.10 M) in
acetonitrile at a scan rate of 80 mV/s at room temperature. Polymer
films were prepared by dipping platinum working electrodes into
the polymer solution, which was prepared with minimum amount
of THF, and then air-drying. A platinum wire and an Ag/AgNO3
electrode were used as the counter electrode and reference elec-
trode, respectively. The energy level of the Ag/AgNO3 reference
electrode (calibrated by the FC/FCþ redox system) was 4.76 eV
below the vacuum level. The oxidation potentials derived from the
onset of electrochemical p-doping are summarized in Table 2.
1
0
PBEHPCPP-DTBT
-1
-2
-3
-4
-5
-6
Voc : 0.61 V
2
Isc : 4.6 mA/cm
HOMO levels were calculated according to the empirical formula
ox
FF : 0.40
EHOMO ¼ ꢂ([Eonset
]
þ 4.76) (eV) [22,23]. During the anodic scan,
Efficiency: 1.12 %
the oxidation onset potential of PCPP-DTBT was 0.64 V and
PCPP-DTBT
Voc : 0.71 V
Table 2
2
Isc : 4.7 mA/cm
Electrochemical Potentials and Energy Levels of the Polymers.
FF : 0.30
a
Polymers
Eonset (V)
HOMOb (eV)
LUMOc (eV)
Egd (eV)
Efficiency: 1.0 %
PCPP-DTBT
PBEHPCPP-DTBT
0.64
0.54
ꢂ5.40
ꢂ5.30
ꢂ3.40
ꢂ3.50
2.00
1.80
0.0
0.2
0.4
0.6
0.8
a
Onset oxidation potentials measured by cyclic voltammetry.
Calculated from the oxidation potentials.
Calculated from the HOMO energy levels and Eg.
b
c
Potential (V)
d
Energy band gaps were estimated from the onset wavelengths of the optical
Fig. 3. Current-voltage characteristics of polymers:PC71BM (1:4) bulk heterojunction
absorption.
solar cells under white light illumination (AM 1.5 conditions).

J. Kim et al. / Polymer 51 (2010) 390–396
395
Table 3
Photovoltaric performances of the device with the configuration of ITO/PEDOT:PSS/
copolymers:PC71BM/TiOx/Al.
Copolymers
PCPP-DTBT
Acceptor Ratio Jsc (mA/cm2) Voc (V) FF
Eff. (%)
0.30 1.00
0.40 1.12
PC71BM
1:4
1:4
4.70
4.60
0.71
0.61
PBEHPCPP-DTBT PC71BM
performance as compared to the case of PCPP-DTBT. The Voc value
of PCPP-DTBT is higher than that of PBEHPCPP-DTBT, which is
attributed to the larger difference between the HOMO energy levels
of the polymer and LUMO energy level of PC71BM. The Jsc values of
PCPP-DTBT and PBEHPCPP-DTBT based devices were about same.
PBEHPCPP-DTBT exhibited higher FF value than that of PCPP-DTBT,
resulting in the improved the power conversion efficiency of 1.12%.
Fig. 4 shows the incident photon to concerted current efficiency
(IPCE) of the PSCs for the best device as a function of wavelength,
which follows the copolymer’s UV–vis absorption spectrum, indi-
cating that all the absorption of the polymers contributed to the
photovoltaic conversion. The significant absorbance of the conju-
gated polymers up to 700 nm increased the total photovoltaic
current because the solar photon flux is high in this energy range
[24,25]. The IPCE spectra of PCPP-DTBT and PBEHPCPP-DTBT show
the maximum of 38% at 379 nm for PCPP-DTBT and 40% at 380 nm
for PBEHPCPP-DTBT. A broad plateau around the maximum in IPCE
spectrum exists between 430–580 nm and 430–600 nm for PCPP-
DTBT and PBEHPCPP-DTBT, respectively. The shape of IPCE spec-
trum of PCPP-DTBT:PC71BM film is almost identical with that of
PBEHPCPP-DTBT:PC71BM film.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
60
50
40
30
20
10
0
a
PCPP-DTBT:PCBM (1:4)
300
400
500
600
700
800
Wavelength (nm)
Fig. 5. Tapping mode AFM topographies images at 0–10 nm height scale for polymer/
PC71BM (1:4) blend films of PBEHPPCPP-DTBT and PCPP-DTBT.
60
50
40
30
20
10
0
b
0.6
0.5
0.4
0.3
0.2
0.1
0.0
PBEHPCPP-DTBT:PCBM (1:4)
Parts (a) and (b) of Fig. 5 show AFM images of PBEHPPCPP-DTBT/
PC71BM and PCPP-DTBT/PC71BM blend films. The root mean square
surface roughness (rms) of the two polymer films are 0.52 and
0.44 nm, respectively. PCPP-DTBT/PC71BM shows smaller domain
size and more well-penetrated morphology than PBEHPPCPP-DTBT/
PC71BM, which leads to improved Jsc for PCPP-DTBT/PC71BM. But
PBEHPPCPP-DTBT has higher hole mobility (5.79 ꢁ10ꢂ5 cm2Vꢂ1 sꢂ1
)
than that of PCPP-DTBT (8.22 ꢁ 10ꢂ6 cm2Vꢂ1
s
ꢂ1), which lead to
superior photovoltaic cell performance for PBEHPPCPP -DTBT.
300
400
500
600
700
800
4. Conclusion
Wavelength (nm)
In conclusion, the D–A type of alternating copolymers of PCPP-
DTBT and PBEHPCPP-DTBT were synthesized by Suzuki coupling
polymerization and characterized. The CPP unit was combined with
Fig. 4. The IPCE spectra (circled line) of photovoltaric devices with the configuration of
ITO/PEDOT:PSS/polymers:PCBM(1:4)/TiOx/Al and the absorption spectra for the cor-
responding films of the blends of polymers and PCBM (black solid line).

396
J. Kim et al. / Polymer 51 (2010) 390–396
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Acknowledgement
This work was supported by the Korea Research Foundation
Grant funded by the Korean Government (MOEHRD, Basic Research
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