Synthesis and characterization of poly(N-alkyloxyarylcarbazolyl-2,7- vinylene) derivatives and their applications in bulk-heterojunction solar cells

Article abstract of DOI:10.1016/j.orgel.2010.03.024

The novel π-conjugated polymers, poly(N-(3,4-bis(decyloxy)phenyl) carbazolyl-2,7-vinylene) (PCzV) and poly[N-(3,4-bis(decyloxy)phenyl)carbazolyl- 2,7-vinylene)-co-{2-methoxy- 5-(2-ethylhexyloxy)-1,4-phenylenevinylene}] (PCzV-co-MEH-PPV) were synthesized b

Full text of DOI:10.1016/j.orgel.2010.03.024

Organic Electronics 11 (2010) 969–978
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Synthesis and characterization of poly(N-alkyloxyarylcarbazolyl-2,7-vinyl-
ene) derivatives and their applications in bulk-heterojunction solar cells
Myungkwan Song a, Jin Su Park a, Kyung Jin Yoon a, Chul-Hyun Kim a, Min Joung Im a,
d
a,
*
Jang Soo Kim a, Yeong-Soon Gal b, Jae Wook Lee c, , Jun Hee Lee , Sung-Ho Jin
*
a Department of Chemistry Education, Interdisciplinary Program of Advanced Information and Display Materials, and Center for Plastic Information System,
Pusan National University, Busan 609-735, Republic of Korea
b Polymer Chemistry Lab., Kyungil University, Hayang 712-701, Republic of Korea
c Department of Chemistry, Dong-A University, Busan 604-714, Republic of Korea
d Department of Advanced Materials Engineering, Dong-A University, Busan 604-714, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The novel
p-conjugated polymers, poly(N-(3,4-bis(decyloxy)phenyl)carbazolyl-2,7-vinyl-
Received 10 October 2009
Received in revised form 7 March 2010
Accepted 30 March 2010
Available online 4 April 2010
ene) (PCzV) and poly[N-(3,4-bis(decyloxy)phenyl)carbazolyl-2,7-vinylene)-co-{2-meth-
oxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene}] (PCzV-co-MEH-PPV) were synthesized
by using the Gilch polymerization method and their photovoltaic properties were investi-
gated. The newly designed and highly branched polymers, formed from PCzV and PCzV-co-
MEH-PPV, are soluble in common organic solvents and easily spin-coated onto indium-tin
oxide (ITO) coated glass substrates.
The weight-average molecular weights (Mw) and the polydispersity of the polymers were
determined to be in the ranges of 3.86–7.9 ꢀ 104 and 1.60–2.05, respectively. Bulk-hetero-
junction solar cells, with ITO/PEDOT:PSS/polymer:PC61BM/TiOx/Al configurations, were
fabricated. The solar cell based on PCzV-co-MEH-PPV:PC61BM (1:6 wt/wt) displays a higher
photovoltaic performance as compared to that produced from PCzV:PC61BM (1:6 wt/wt).
The bulk-heterojunction solar cell with PCzV-co-MEH-PPV:PC61BM (1:6 wt/wt) has a
power conversion efficiency (PCE) of 2.31% (Jsc = 6.43 mA/cm2, Voc = 0.82 V, FF = 44%), mea-
sured using an AM 1.5 G solar simulator at 100 mW/cm2 light illumination.
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Poly[N-(3,4-bis(decyloxy)phenyl)-
carbazolyl-2,7-vinylene]
Bulk-heterojunction solar cell
Gilch polymerization
p-Conjugated polymers
1. Introduction
[2], poly(fluorenes) [3,4], poly(thiophenes) [5,6], and
poly(carbazoles) [7,8], have been designed, prepared and
Over the last two decades, new
p-conjugated polymers,
investigated. Among these substances, p-conjugated poly-
which have a large number of potential applications in the
mers based on poly(carbazoles) have several unique and
attractive properties [9,10]. As a result, the hole-transport-
ing [11] and photovoltaic [12,13] properties of poly(carbaz-
oles) have been probed. In these materials, the carbazole
nitrogen serves as a site for ready introduction of function-
ality that can be used to modulate the properties. Moreover,
the carbazole starting material is inexpensive and carbazole
units can be easily linked to one another through either 2,7-
or 3,6-bonds. Since poly(2,7-carbazoles), in contrast to
poly(3,6-carbazoles), can have linear structures that pos-
field of electronic materials, have been developed. -Conju-
p
gated polymers have many advantages over competitive
materials that include low cost, easy modification, and solu-
tion processability. Various polymers of this type, such as
poly(p-phenylenes) [1], poly(p-phenylenevinylene) (PPV)
* Corresponding author. Tel.: +82 51 510 2727; fax: +82 51 581 2348.
E-mail addresses: jlee@donga.ac.kr (J.W. Lee), shjin@pusan.ac.kr
(S.-H. Jin).
sess high degrees of
p-conjugation along the backbone,
1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2010.03.024

970
M. Song et al. / Organic Electronics 11 (2010) 969–978
they have received particular attention as in the context of
bulk-heterojunction solar cells. For example, Morin and co-
workers described the synthesis of a variety of new
poly(2,7-carbazoles) [14]. In a later effort, the first photo-
voltaic device derived from poly(N-alkyl-2,7-carbazole),
which has a power conversion efficiency (PCE) of 0.6%,
was described by Müllen and co-workers [15].
In recent years, considerable interest has grown in the
area of low-band gap polymers owing to their applications
in organic solar cell devices as an active layer for light
absorption and charge generation [16–18]. High power
conversion efficiencies (PCEs) are key goal for solar appli-
cations since these values determine how much electrical
energy is produced by absorbed photons. One strategy to
improve PCE is to employ well designed and readily syn-
thesized low-band gap polymers, which have improved
overlap of the polymer absorption spectrum with the stan-
dard solar spectrum under AM 1.5G. One successful ap-
proach that has been used to design these polymers
2. Experimental
2.1. General information
1H NMR spectra were recorded on a Varian Mercury
Plus 300 MHz spectrometer and the chemical shifts were
recorded in ppm units with chloroform as the internal
standard. UV–visible and the emission spectra were re-
corded with a JASCO V-570 and Hitachi F-4500 fluores-
cence spectrophotometers. Measurements of solid state
emissions were carried out by supporting each film on a
quartz substrate, which was mounted to receive front-face
excitation at an angle of less than 45°. Each polymer film
was excited with several portions of the visible light from
a xenon lamp. The molecular weight and polydispersity of
the polymer were determined by using gel permeation
chromatography (GPC) using
a PLgel 5 lm MIXED-C
column on an Agilent 1100 series liquid chromatography
system with THF as an eluent and calibration with polysty-
rene standards. Thermal analyses were carried out on a
Mettler Toledo TGA/SDTA 851, DSC 822 analyzer under
N2 atmosphere at a heating rate of 10 °C/min. Cyclic vol-
tammetry (CV) was carried out in a 0.1 M solution of tetra-
butylammonium hexafluorophosphate (TBPAPF6) in
benzene:acetonitrile (1.5:1 v/v) [29] using a CHI600C vol-
tammetric analyzer at a potential scan rate of 100 mV/s.
Each polymer film was applied to a Pt disc electrode
(0.2 cm2) by dipping the electrode into the polymer solu-
tion. A platinum wire was used as the counter electrode,
and Ag/AgNO3 electrode was used as the reference
electrode.
takes advantage of vinylene linkages in p-conjugated sys-
tems. Based on this design principal, a number of polymers
have been prepared which, in comparison to conventional
p-conjugated polymers, have low the band gaps in the
range of 0.3–0.5 eV [19]. Another advantage arising from
the introduction of vinylene units is that it serves as a good
method to create planar molecules with extended p-conju-
gation lengths. This is a consequence of the relatively small
dihedral angle that exists between vinylene and aryl
groups that leads to an increase in the absorption maxima
of the material [20]. Finally, different monomer units can
be easily introduced into the polymer in order to fine tune
electronic properties.
We have previously described the preparation of
2.2. Materials
novel vinylene based,
p-conjugated polymers, including
the alkyloxyphenoxy and 1,3,4-oxadiazole-substituted
PPVs, poly(9,9-dioctylfluorenyl-2,7-vinylene) (PFV), PFV-
co-MEH-PPV, highly luminescent poly(m-SiPh-PV)
derivatives, highly branched poly(PDOT-PV), poly(PDOT-
PV-co-m-SiPh-PV), poly(3OC1OC10-PV), and poly(3OC1O-
C10-PV-co-m-SiPh-PV) [21–27].
Sodium nitrite, copper cyanide, potassium cyanide, cat-
echol, iodine, iodic acid, copper powder, RED-Al, methane-
sulfonyl chloride, triethylamine, potassium tert-butoxide,
4-tert-butylbenzyl bromide, tetrahydrofuran (THF), and
N,N-dimethylacetamide were purchased from Aldrich Co.
and used without further purification unless otherwise
noted. Solvents were dried and purified by fractional distil-
lation over sodium/benzophenone and handled in a mois-
ture-free atmosphere. Column chromatography was
performed using silica gel (Merck, 250–430 mesh).
In this effort, repeating vinylene units were introduced
between the 2- and 7-positions of the carbazole main
chain by using the Gilch polymerization procedure [28].
The substances prepared in this effort include poly(N-
(3,4-bis(decyloxy)phenyl)carbazolyl-2,7-vinylene) (PCzV)
and its copolymer with MEH-PPV segment, poly[N-(3,
4-bis(decyloxy)phenyl)carbazolyl-2,7-vinylene)-co-{2-meth-
oxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene}] [PCzV-
co-MEH-PPV]. These polymers possess many attractive
properties, such as high molecular weights, low polydis-
persities and ready purification. To enhance the solubilities
and electronic properties of the derived poly(carbazolyl-
2,7-vinylene), a meta and para bis-decyloxyphenyl groups
were incorporated on the carbazole nitrogen atoms. The
addition of vinylene units along the 2,7-carbazole back-
bone is expected to tune the highest occupied molecular
2.3. Synthesis of 2,7-dicyanocarbazole (4)
To
a
solution of 2,7-diaminocarbazole (3) (2 g,
10 mmol) and hydrochloric acid (10 mL) in distilled water
(100 mL) was added dropwise an aqueous solution of so-
dium nitrite (1.66 g, 24 mmol). The resulting solution was
stirred for 2 h at 0 °C. Aqueous sodium carbonate was
added at 0 °C to bring the pH to 7 (solution A). To a solution
of copper cyanide (3.63 g, 40.56 mmol) in distilled water
was added
a solution of potassium cyanide (2.64 g,
orbital (HOMO) level of
p
-conjugated polymers as well as
40.56 mmol) in distilled water at room temperature (solu-
tion B). To this mixture in an ice bath was added solution A
at 0 °C. After being stirred overnight at room temperature,
the formed precipitate was removed by filtration. The pre-
cipitate was washed with distilled water before being
improve the electro-optical properties. The preparation of
these polymers along with an analysis of their thermal,
optical, electrochemical, and photovoltaic properties are
described below.

M. Song et al. / Organic Electronics 11 (2010) 969–978
971
repeatedly triturated with acetone. The triturate was con-
centrated in vacuo to give a solid which was dissolved in
ethyl acetate. Filtration followed by concentration of the
filtrate gave a crystalline solid which was recrystallized
from ethanol to give 0.54 g. Yield: 25%, 1H NMR (acetone-
d6, d ppm): 11.3 (br, 1H), 8.46–8.43 (d, 2H, aromatic pro-
tons), 8.06 (s, 2H, aromatic protons), 7.62–7.59 (d, 2H,
aromatic protons). 13C NMR (acetone-d6, d ppm): 141.01,
126.10, 123.48, 123.16, 120.16, 116.85, 110.75.
ppm): 8.26–8.23 (d, 2H, aromatic protons on carbazole),
7.68 (s, 2H, aromatic protons on carbazole), 7.61–7.58 (d,
2H, aromatic protons on carbazole), 7.13–7.08 (d, 1H, aro-
matic proton on benzene), 7.02–6.96 (dd, 1H, aromatic
proton on benzene), 6.94–6.91(d, 1H, aromatic proton on
benzene), 4.16–4.11 (dt, 4H,
a-protons of decyloxy posi-
tion), 1.97–1.80 (m, 4H, b-protons of decyloxy position),
1.57–1.22 (br, 28H, aliphatic protons), 0.92–0.83 (m, 6H,
aliphatic protons). 13C NMR (CDCl3,
d ppm): 149.81,
148.05, 141.09, 135.71, 127.53, 122.84, 120.77, 116.91,
116.80, 114.08, 112.82, 105.59, 98.32, 69.48, 32.05, 29.61,
29.30, 26.26, 22.73, 14.30.
2.4. Synthesis of 1,2-bis(decyloxy)benzene (5)
To a solution of catechol (15 g, 135 mmol) in 1-bromo-
decane (83.7 mL, 404.7 mmol) and DMF (150 mL) was
added potassium carbonate (74.58 g, 539.6 mmol). The
mixture was stirred at 60 °C for 24 h. and concentrated in
vacuo to give a residue which was subjected to flash chro-
matography on silica gel using ethyl acetate:hexane (1:10)
as an eluent. This yielded 50.6 g (96%) of 5. 1H NMR (CDCl3,
2.7. Synthesis of 9-(3,4-bis(decyloxy)phenyl)carbazole-2,7-
dicarboxylic acid (8)
A mixture of 9-(3,4-bis(decyloxy)phenyl)carbazole-2,7-
dicarbonitrile (7) (2 g, 3 mmol) in ethanol (50 mL) contain-
ing sodium hydroxide (30%, 30 mL) was stirred at 110 °C
for 24 h. After hydrochloric acid was added to bring the
pH below 7, a portion of the water and ethanol were re-
moved by concentration in vacuo. The resulting mixture
was stirred at 120 °C for 3 h, cooled and filtered. The pre-
cipitate was dried under reduced pressure to yield 1.99 g
(94%) of 8. 1H NMR (DMSO-d6, d ppm): 8.43–8.40 (d, 2H,
aromatic protons), 7.91–7.87 (d, 4H, aromatic protons),
7.27–7.21 (t, 2H, aromatic protons), 7.16–7.12 (d, 1H, aro-
d ppm): 6.92 (s, 4H, aromatic protons), 4.05 (t, 4H, a-pro-
tons of decyloxy position), 1.93–1.78 (m, 4H, b-protons of
decyloxy position), 1.61–1.25 (br, 28H, aliphatic protons),
0.92–0.89 (m, 6H, aliphatic protons). 13C NMR (CDCl3, d
ppm): 149.25, 121.02, 113.97, 69.21, 32.07, 29.68, 29.47,
26.20, 22.84, 14.24.
2.5. Synthesis of 1,2-bis(decyloxy)-4-iodobenzene (6)
matic proton), 4.12–3.96 (dt, 4H, a-protons of decyloxy po-
To a solution of 1,2-bis(decyloxy)benzene (5) (20 g,
52 mmol) in acetic acid (600 mL) was added water
(200 mL) at 40 °C. Sulfuric acid (16 mL) and iodine
(6.768 g, 26.67 mmol) were added and the resulting mix-
ture was stirred at 44 °C. Iodic acid (1 g, 5.68 mmol) was
added three times during a 40 min period. After cooling,
the mixture was extracted with diethyl ether, washed with
aqueous sodium carbonate and saturated brine, dried over
anhydrous MgSO4 and concentrated in vacuo to give a the
crystalline residue which was recrystallized from ethanol
to yield 18.65 g (71%) of 6. 1H NMR (CDCl3, d ppm): 7.20–
7.12 (s, 2H, aromatic protons), 6.64–6.60 (d, 1H, aromatic
sition), 1.83–1.65 (m, 4H, b-protons of decyloxy position),
1.52–1.09 (br, 28H, aliphatic protons), 0.91–0.0.81 (q, 6H,
aliphatic protons). 13C NMR (DMSO-d6, d ppm): 177.60,
162.22, 149.53, 147.48, 141.97, 137.67, 130.18, 121.41,
114.56, 109.57, 108.64, 103.25, 96.12, 71.66, 38.16, 28.33,
25.82, 22.58, 18.64, 13.62.
2.8. Synthesis of (9-(3,4-bis(decyloxy)phenyl)carbazole-2,7-
diyl)dimethanol (9)
To a solution of 9-(3,4-bis(decyloxy)phenyl)carbazole-
2,7-dicarboxylic acid (8) (2 g, 3 mmol) in benzene
(90 mL) was added sodium bis(2-methoxyethoxy)alumi-
num hydride (RED-Al) (18.7 mL, 62.2 mmol). The mixture
was stirred at 100 °C for 24 h. Ice-water was added after
hydrochloric acid was added to make the solution acidic.
Extraction with chloroform gave an extract that was dried
over anhydrous MgSO4 and concentrated in vacuo to give a
residue which was subjected to flash chromatography on
silica gel using chloroform:ethyl acetate (9:1) as an eluent.
This gave 1.30 g (68%) of 9. 1H NMR (CDCl3, d ppm): 8.11–
8.09 (d, 2H, aromatic protons on carbazole), 7.34 (s, 2H,
aromatic protons on carbazole), 7.29–7.26 (d, 2H, aromatic
protons on carbazole), 7.05–7.04 (m, 2H, aromatic protons
on benzene), 6.99–6.98 (d, 1H, aromatic proton on ben-
protons), 3.97–3.88 (t, 4H, a-protons of decyloxy position),
1.83–1.75 (m, 4H, b-protons of decyloxy position), 1.60–
1.27 (br, 28H, aliphatic protons), 0.91–0.86 (m, 6H, ali-
phatic protons). 13C NMR (CDCl3, d ppm): 149.80, 129.86,
123.68, 122.54, 115.64, 82.63, 69.45, 32.07, 29.52, 29.17,
26.10, 22.86, 14.31.
2.6. Synthesis of 9-(3,4-bis(decyloxy)phenyl)carbazole-2,7-
dicarbonitrile (7)
To a solution of 2,7-dicyanocarbazole (4) (1 g, 5 mmol)
and 1,2-bis(decyloxy)-4-iodobenzene (6) (7.13 g, 13.8
mmol) in N,N-dimethylacetamide (60 mL) was added cop-
per powder (2 g, 31 mmol) and potassium carbonate (2.1 g,
15 mmol) under a N2 atmosphere. The mixture was stirred
at 160 °C for 12 h, diluted with distilled water, and ex-
tracted with methylene chloride. The methylene chloride
extracts were dried over anhydrous MgSO4 and concen-
trated in vacuo giving a residue, which was subjected to
flash chromatography on silica gel using chloroform as an
eluent. This gave 0.56 g (20%) of 7. 1H NMR (CDCl3, d
zene), 4.83–4.81 (d, 4H, –CH2OH), 4.13–3.94 (dt, 4H, a-pro-
tons of decyloxy position), 1.94–1.77 (m, 4H, b-protons of
decyloxy position), 1.58–1.19 (br, 28H, aliphatic protons),
0.92–0.83 (q, 6H, aliphatic protons). 13C NMR (CDCl3, d
ppm): 167.88, 162.92, 161.15, 142.61, 139.19, 124.08,
122.54, 120.06, 119.12, 112.50, 109.43, 94.32, 66.48,
65.18, 32.10, 29.55, 29.04, 26.15, 22.92, 14.33.

972
M. Song et al. / Organic Electronics 11 (2010) 969–978
2.9. Synthesis of 9-(3,4-bis(decyloxy)phenyl)-2,7-
2.11. Solar cell fabrication and measurements
bis(chloromethyl)carbazole (10)
Bulk-heterojunction solar cells used to measure photo-
voltaic properties were constructed as follows. Each glass
substrate was coated with a transparent ITO electrode
To a solution of (9-(3,4-bis(decyloxy)phenyl)carbazole-
2,7-diyl)dimethanol (9) (1 g, 2 mmol) in methylene chlo-
ride (100 mL) was added methanesulfonyl chloride
(0.50 mL, 6.48 mmol) and triethylamine (0.90 mL,
6.48 mmol). The resulting mixture was stirred for 48 h at
room temperature, diluted with water and extracted with
chloroform. The extracts were dried over anhydrous
MgSO4 and concentrated in vacuo giving a residue which
was subjected to flash chromatography on silica gel using
chloroform as an eluent to yield 0.51 g (48%) of 10. 1H
NMR (CDCl3, d ppm): 8.12–8.09 (d, 2H, aromatic protons
on carbazole), 7.387.26–8.09 (m, 4H, aromatic protons on
carbazole), 7.11–7.08 (q, 2H, aromatic protons on ben-
zene), 7.0 (s, 1H, aromatic proton on benzene), 4.80–4.67
(110 nm thick, 10–20
X/sq sheet resistance). The ITO-
coated glass substrates were ultrasonically cleaned with
detergent, deionized water, acetone, and isopropyl alcohol.
The layer of 40 nm thick PEDOT:PSS (H.C. Stack, PH500)
was spin-coated onto the pre-cleaned and UV-ozone trea-
ted ITO substrates. The spin-coated film was baked in air
at 150 °C for 30 min. For fabrication of the active layers
composed of interconnected networks of electron donor
and acceptor, 1,2-dichlorobenzene and chloroform (1:1
wt.%) solutions of the synthesized polymer (10 mg/mL)
and PC61BM (15 mg/mL) were shaken at room temperature
for 12 h. The polymer and PC61BM blends was then pre-
pared by mixing the two solutions and subsequent shaking
(s, 4H, CH2Cl), 4.14–3.97 (dt, 4H, a-protons of decyloxy po-
sition), 1.93–1.82 (m, 4H, b-protons of decyloxy position),
for 12 h. Filtration using a 0.45 lm PTFE (hydrophobic) syr-
1.57–1.19 (br, 28H, aliphatic protons), 0.99–0.86 (q, 6H,
inge filter gave the polymer blends with a ratio of the elec-
tron donor to the PC61BM as an electron acceptor of 1:3,
1:4, 1:5, and 1:6 wt/wt, respectively. The TiOx precursor
solution (1 wt.%) was spin-casted (4000 rpm) onto the ac-
tive layer with thickness of ca. 10 nm and heated at 80 °C
for 10 min in air. After being subjected to vacuum
(5 ꢀ 10ꢁ6 Torr), the Al electrodes with thickness around
100 nm were deposited. The top metal electrode area,
comprising the active area of the solar cells, was found to
be 4 mm2. Performance of the solar cells were measured
aliphatic protons). 13C NMR (CDCl3,
d ppm): 150.11,
149.02, 142.04, 135.71, 129.66, 122.84, 120.85, 120.77,
119.80, 114.08, 112.82, 110.29, 69.49, 47.33, 32.08, 29.63,
29.31, 26.26, 22.86, 14.31.
2.10. Synthesis of poly[N-(3,4-bis(decyloxy)phenyl)-
carbazolyl-2,7-vinylene)-co-{2-methoxy-5-(2-ethyl-
hexyloxy)-1,4-phenylenevinylene}] [PCzV-co-MEH-PPV] (13)
using
a AM 1.5G solar simulator (Oriel 300 W) at
A potassium tert-butoxide solution (1.84 mL of a 1.0 M
solution in THF) was added dropwise by using a syringe
pump over a 15 min period to a 0 °C solution of 9-(3,4-
bis(decyloxy)phenyl)-2,7-bis(chloromethyl)carbazole (10)
(100 mg, 0.15 mmol) and 1,4-bis(chloromethyl)-2-(2-eth-
ylhexyloxy)-5-methoxybenzene (12) (51 mg, 0.15 mmol)
in freshly distilled THF (12 mL) under N2 atmosphere.
The solution was stirred at 0 °C for 50 min and cooled.
End capping was performed by the addition of 3–4 drops
of 4-tert-butylbenzyl bromide followed by stirring for
15 min at room temperature. The solution was poured
into methanol and the precipitate was removed by filtra-
tion. After carrying out successive Soxhlet extractions, the
polymer was obtained by pouring the solution into meth-
anol. The resulting product 13 (49 mg, 38%) was obtained
by drying reduced pressure. 1H NMR (CDCl3, d (ppm):
8.19–6.70 (br, 13H, aromatic and vinylic protons), 4.26–
100 mW/cm2 light illumination after adjusting the light
intensity using Oriel power meter (model No. 70260 which
was calibrated using laboratory standards that are trace-
able to the National Institute of Standards and Technolo-
gies, USA). Current density–voltage (I–V) curves were
recorded using
a standard source measurement unit
(Keithley 236). All fabrication steps and characterization
measurements were performed in an ambient environ-
ment without a protective atmosphere. Thickness of the
thin films was measured using a KLA Tencor Alpha-step
IQ surface profilometer with an accuracy of 1 nm.
3. Results and discussion
3.1. Synthesis and characterization
3.29 (br, 9H,
a
-protons of alkoxy position), 2.03–0.86
The synthetic routes used for preparation of the mono-
mer and the polymers are shown in Scheme 1. Syntheses of
the poly(carbazolyl-2,7-vinylene) derivatives, although not
often straightforward [30], were accomplished starting
(br, 53H, aliphatic protons). Anal. Calcd. for PCzV-co-
MEH-PPV: C, 78.72; H, 9.32; N, 0.21. Found: C, 75.70; H,
9.13; N, 0.17.
The homopolymer, poly(N-(3,4-bis(decyloxy)phenyl)-
carbazolyl-2,7-vinylene) (PCzV) was synthesized by using
a method that is similar to that used to prepare PCzV-co-
MEH-PPV.
with 4,40-dinitro-2-biphenylamine and employing
a
sequence involving the known intermediates 4,40-dinitro-
2-azidobiphenyl (1), 2,7-dinitrocarbazole (2), and 2,7-
diaminocarbazole (3) [31].
PCzV: 1H NMR (CDCl3, d ppm): 8.12–7.94 (br, 2H, aro-
matic protons), 7.57–7.36 (br, 4H, aromatic protons),
7.18–6.79 (br, 5H, aromatic and vinylic protons), 4.23–
In order to introduce chloromethyl units into the 2- and
7-position of carbazole, the dicyano derivative 4 is gener-
ated by using a diazotation-cyanide substitution route. To
improve the processibility of the poly(carbazolyl-2,7-
vinylene) derivatives, the bulky didecyloxyphenyl pendant
group is introduced at the nitrogen atom of carbazole
3.48 (br, 4H,
a-protons of decyloxy position), 1.95–0.68
(br, 38H, aliphatic protons). Anal. Calcd. for PCzV: C,
82.85; H, 9.21; N, 2.42. Found: C, 77.96; H, 9.12; N, 2.22.

M. Song et al. / Organic Electronics 11 (2010) 969–978
973
NO2
O2N
NO2
O2N
NO2
O2N
N
H
NH2
N3
(1)
(2)
1)HCl, NaNO2
2)Na2CO3
CN
NH2
NC
H2N
N
H
N
H
3)CuCN, KCN, H2O
(4)
(3)
OH
OH
n-C10Br, K2CO3
DMF,
I
OC10H21
OC10H21
I2,HIO3
AcOH,H2SO4,
OC10H21
OC10H21
OC10H21: n-decyloxy
(6)
(5)
O
O
CN
1)EtOH, NaOH,
2)HCl,
NC
OH
N
Cu, K2CO3
DMAc
HO
(6)
N
+
(4)
OC10H21
OC10H21
OC10H21
OC10H21
(8)
(7)
∗
n
Cl
OH
Cl
HO
∗
N
N
N
MsCl,Et3N
MC
t-BuOK
THF
RED-Al
benzene,
OC10H21
OC10H21
OC10H21
OC10H21
OC10H21
OC10H21
(11)
(9)
(10)
PCzV
O
Cl
co
Cl
*
*
m
O
N
O
N
n
+
t-BuOK
Cl
THF
Cl
OC10H21
OC10H21
OC10H21
O
OC10H21
(12)
PCzV-co-MEH-PPV
(10)
(13)
Scheme 1.
backbone. For this purpose, the ortho-hydroquinone deriv-
ative 5 is produced by using the Williamson ether synthe-
sis method through reaction of n-decyl bromide and
catechol. Iodination of 5 yields the iodo-arene 6, whose
coupling reaction with carbazole 4 produces 7, which con-
tains two decyloxy groups at the m- and p-positions of the
N-phenyl position. Hydrolysis of the dicyano compound (7)
affords the dicarboxylic acid 8, which undergoes reduction
with RED-Al in benzene to yield diol 9. The corresponding
dichloride 10 is then produced from 9 by treatment with

974
M. Song et al. / Organic Electronics 11 (2010) 969–978
methane sulfonyl chloride. PCzV and PCzV-co-MEH-PPV
are synthesized by using the Gilch polymerization method.
Homopolymerization of 9-(3,4-bis(decyloxy)phenyl)-2,7-
bis(chloromethyl)carbazole (10) and copolymerization
with 1,4-bis(chloromethyl)-2-(2-ethylhexyloxy)-5-meth-
oxybenzene (12) [32] are carried out with an excess of
potassium tert-butoxide in dry THF under a N2 atmo-
sphere. This is followed by the addition of 3–4 drops of
4-tert-butylbenzyl bromide in order to end-cap the poly-
mer chain. The polymers, formed in this manner, were
found to be completely soluble in organic solvents such
as chloroform, 1,2-dichlorobenzene, and THF. The high sol-
ubility of these polymers is an important requirement for
device fabrication. To improve photovoltaic performance,
the resulting polymers were purified by using Soxhlet
extraction with different solvents, including methanol,
acetone and finally chloroform, in order to remove the
unreacted monomers, impurities, and oligomers.
Implementation of this procedure results in the produc-
tion of highly purified and the narrowly polydispersed
polymers. Molecular structures of the monomer and the
resulting polymers were characterized by using 1H-, and
13C NMR spectroscopy, and elemental analysis. The chloro-
methyl protons of monomer 10 at 4.80–4.67 ppm disap-
pear during the polymerization reaction and new vinylic
proton resonances appear at 7.18–6.79 ppm together with
the carbazole protons in PCzV and PCzV-co-MEH-PPV. The
composition ratio of PCzV-co-MEH-PPV is calculated from
the integration ratios of the 8.19–6.70 ppm and 4.26–
3.29 ppm peaks in the 1H NMR. 1H NMR peak area
calculations show that the actual composition of PCzV-
co-MEH-PPV is 30:70 wt.%. Table 1 contains a summary
of the polymerization results in the form of molecular
weight, polydispersity, and thermal characteristics of PCzV
and PCzV-co-MEH-PPV.
3.2. Thermal, optical and electrochemical properties of the
polymers
The thermal behaviors of the polymers were investi-
gated by using DSC and TGA thermograms. PCzV and
PCzV-co-MEH-PPV have glass transition temperatures (Tg)
of 95 and 78 °C, respectively, and these polymers have
good thermal stabilities with a 5% weight loss temperature
at 407 and 377 °C under N2 atmosphere. PCzV exhibits a
higher thermal stability than the PCzV-co-MEH-PPV and
both polymers have higher thermal stabilities than dialkyl-
oxy-substituted PPVs or poly(9,9-dialkylfluorene)s [33,34].
The high Tg values and thermal stabilities of PCzV and
PCzV-co-MEH-PPV indicate that these polymers can be
used for the fabrication of bulk-heterojunction solar cells
without complications associated with deformation or
degradation during operation of the solar cells.
Fig. 1 is shown the normalized UV–visible absorption
spectra (Fig. 1a) of the PCzV and PCzV-co-MEH-PPV in
chloroform solutions and in thin films. The absorption
spectrum of the PCzV contains two broad peaks at
381 nm and 403 nm in the solution state and at 386 nm
and 417 nm in the thin film state. The peaks at ca.
400 nm correspond to the carbazole chromophore. The
absorption spectrum of the PCzV-co-MEH-PPV has a max-
imum at 489 nm in the solution state and at 499 nm in
the thin film state. Both absorption bands in thin film
states are slightly red-shifted by ca. 10 nm compared to
the solution state owing to interactions between the poly-
mer chains. The optical band gaps (Eg), calculated from the
onset of the absorption of the PCzV and PCzV-co-MEH-PPV,
are 2.50 eV and 2.07 eV, respectively. The wavelength of
the absorption peaks and the optical band gaps of the PCzV
and PCzV-co-MEH-PPV are summarized in Table 1. Photo-
luminescent (PL) spectra of PCzV in chloroform solution
and in the thin film state are shown in Fig. 1b. The spec-
trum of homopolymer PCzV has PL peaks at ca. 467 nm
and 492 nm in the solution state and at 482 nm and
510 nm in the thin film state. The PL spectrum of the
PCzV-co-MEH-PPV has maxima at 556 nm and 596 nm in
the solution state and at 585 nm and 631 nm in the thin
film state. The disappearance of PL emission from PCzV
in the PCzV-co-MEH-PPV copolymer indicates that efficient
energy transfer takes place from the wide band gap PCzV
segment to the small band gap MEH-PPV segment. Thus,
the transfer of exciton energy from the PCzV segment to
the MEH-PPV segment occurs mainly along the polymer
chain and intermolecular energy transfer occurs in the
PCzV-co-MEH-PPV.
The weight-average molecular weight (Mw) and
the polydispersity of the PCzV and PCzV-co-MEH-PPV
were found to be 38,600, 1.60 and 79,900, 2.05,
respectively.
Table 1
Polymerization results, thermal and electro-optical properties of PCzV and
PCzV-co-MEH-PPV.
PCzV
PCzV-co-MEH-PPV
Mw ꢀ 104a
PDIa
3.86
1.60
95
407
7.99
2.05
78
377
DSC (Tg)b
TGAc
To investigate the charge injection and transport prop-
erties of PCzV and PCzV-co-MEH-PPV and to determine
the energies of their highest occupied (HOMO) and lowest
unoccupied (LUMO) molecular orbitals, redox measure-
ments were carried using CV. The HOMO energies of PCzV
and PCzV-co-MEH-PPV for the standard ferrocene/ferroce-
nium (4.8 eV) are found to be 5.29 eV and 5.34 eV, respec-
tively. The respective LUMO energies, calculated based on
the HOMO energies and the band gaps determined by
using UV spectroscopic analysis spectra, for PCzV and
PCzV-co-MEH-PPV are found to be 2.79 eV and 3.27 eV,
respectively. The energy band diagrams of PCzV, PCzV-co-
Abs (nm)
Solution
Filmd
381,403
386,417
467,492
482,510
2.50
489
499
556,596
585,631
2.07
PL (nm)
Solution
Filmd
e
Eg (eV)
a
Mw and PDI of the polymers were determined by GPC using poly-
styrene standards.
Determined by DSC at
atmosphere.
b
a heating rate of 10 °C/min under N2
c
TGA was measured at temperature of 5% weight loss for the polymers.
Measured in the thin film onto the quartz substrate.
Band gap estimated from the onset wavelength of the optical
d
e
absorption.

M. Song et al. / Organic Electronics 11 (2010) 969–978
975
PCzV(sol.)
PCzV(film)
PCzV-co-MEH-PPV(sol.)
PCzV-co-MEH-PPV(film)
(a)
300
400
500
600
700
800
Wavelength (nm)
PCzV(sol.)
PCzV(film)
PCzV-co-MEH-PPV(sol.)
PCzV-co-MEH-PPV(film)
(b)
500
600
700
800
Wavelength (nm)
Fig. 1. UV–visible absorption (a) and PL emission (b) spectra of PCzV and PCzV-co-MEH-PPV.
Fig. 2. Energy level diagram of the components of the bulk-heterojunction solar cell.
MEH-PPV and PCBM are shown in Fig. 2. The energies of
these electron donors are well-matched to those of PC61BM
as an electron acceptor. Photogeneration of charge in most
of p-conjugated polymers is not very efficient since there
recombination of two charge carriers is always involved.
However, a consideration of the high LUMO energies of

976
M. Song et al. / Organic Electronics 11 (2010) 969–978
PCzV and PCzV-co-MEH-PPV and the corresponding low
HOMO energy level of PC61BM leads to the prediction that
charge transfer will take place at the interface of these
materials.
tovoltaic performance using a 1,2-dichlorobenzene and
chloroform mixed solvent system. The current density–
voltage (J–V) characteristics of solar cells, fabricated with
1:3, 1:4, 1:5 and 1:6 ratios of polymer to PC61BM, under
AM 1.5G illumination, are shown in Fig. 3 and summarized
in Table 2.
Fig. 3a is shown the J–V characteristics of PCzV with
different ratios of PC61BM. Open-circuit voltages (Voc) of
bulk-heterojunction solar cells are closely related to the
energy difference between the HOMO level of the electron
donor and the LUMO level of the electron acceptor. The Voc
of all solar cells we constructed are almost identical, but
higher short-circuit current densities (Jsc) and a higher fill
factors (FF) are observed with increasing PC61BM content
in the blended active layer. The best photovoltaic perfor-
mance of 0.57% was seen for a solar cell fabricated using
a 1:6 ratio of PCzV:PC61BM. For reference, the PCE of a cell
made using a 1:3 ratio of PCzV:PC61BM (1:3 wt/wt) is
0.26%.
3.3. Solar cell properties of the polymers
Bulk-heterojunction solar cells were fabricated using
PCzV and PCzV-co-MEH-PPV as electron donors and
PC61BM as an electron acceptor. The photovoltaic cell
structures are as follow: ITO/PEDOT:PSS/polymer:PC61BM/
TiOx (10 nm)/Al (100 nm). We also used solution process-
able TiOx [35–37] between the active layer and the metal
cathode as a hole blocking and electron transporting layer
[38,39]. The energy level of TiOx matches the work function
of Al well. Many research groups have obtained best pho-
tovoltaic performances through optimization of the ratios
of electron donor polymers to the electron acceptor
PC61BM [40,41]. The charge balance generally depends on
the thickness of the active layer and the amount of PC61BM
in the polymer blended active layer. In this effort, we
probed the effect of the ratios of PCzV and PC61BM on pho-
Table 2
Solar cell performance of PCzV and PCzV-co-MEH-PPV as electron donor
sand PC61BM as the electron acceptor in ITO/PEDOT:PSS/polymer:PC61BM/
TiOx/Al devices.
Active layer
(polymer:PC61BM)
Voc
(V)
Jsc
FF
PCE
(%)d
a
(mA/cm2)b
(%)c
0
PCzV:PC61BM (1:3 wt/wt)
PCzV:PC61BM (1:4 wt/wt)
PCzV:PC61BM (1:5 wt/wt)
PCzV:PC61BM (1:6 wt/wt)
PCzV-co-MEH-PPV:PC61BM
(1:3 wt/wt)
0.70
0.73
0.73
0.73
0.81
1.47
1.72
2.22
2.31
5.16
25
24
31
34
38
0.26
0.30
0.51
0.57
1.60
(a)
-1
PCzV-co-MEH-PPV:PC61BM
(1:4 wt/wt)
PCzV-co-MEH-PPV:PC61BM
(1:5 wt/wt)
PCzV-co-MEH-PPV:PC61BM
(1:6 wt/wt)
0.81
0.80
0.82
5.46
6.52
6.43
41
39
44
1.81
2.04
2.31
PCzV : PCBM(1:3)
PCzV : PCBM(1:4)
PCzV : PCBM(1:5)
PCzV : PCBM(1:6)
-2
-3
a
b
c
Voc, open-circuit voltage.
Jsc, short-circuit current density.
FF, fill factor.
0.0
0.2
0.4
0.6
0.8
d
PCE, power conversion efficiency.
Bias [V]
PCzV : PCBM (1:6)
0
50
40
30
20
10
0
PCzV-co-MEH-PPV : PCBM (1:3)
PCzV-co-MEH-PPV : PCBM (1:4)
PCzV-co-MEH-PPV : PCBM (1:5)
PCzV-co-MEH-PPV : PCBM (1:6)
(b)
-2
-4
-6
-8
PCzV-co-MEH-PPV:PCBM(1:3)
PCzV-co-MEH-PPV:PCBM(1:4)
PCzV-co-MEH-PPV:PCBM(1:5)
PCzV-co-MEH-PPV:PCBM(1:6)
0.0
0.2
0.4
0.6
0.8
1.0
400
500
600
700
Bias [V]
Wavelength (nm)
Fig. 3. Current density–voltage characteristic of PCzV (a) and PCzV-co-
MEH-PPV (b) bulk-heterojunction solar cells containing different ratios of
PC61BM.
Fig. 4. IPCE spectra of PCzV and PCzV-co-MEH-PPV bulk-heterojunction
solar cells containing different ratios of PC61BM.

M. Song et al. / Organic Electronics 11 (2010) 969–978
977
40
30
20
10
0
7
6
5
4
3
2
1
0
(a)
(b)
,
,
,
,
,
PCzV : PCBM (1:6)
PCzV-co-MEH-PPV : PCBM (1:3)
PCzV-co-MEH-PPV : PCBM (1:4)
PCzV-co-MEH-PPV : PCBM (1:5)
PCzV-co-MEH-PPV : PCBM (1:6)
400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 5. (a) Photocurrents obtained using data from Fig. 4. (b) The total solar photocurrent obtained by integrating the curves shown in plots a.
Owing to the higher energy difference (1.04 eV) be-
tween the HOMO of PCzV-co-MEH-PPV and LUMO of
PC61BM relative to that of PCzV and PC61BM (0.99 eV),
the Voc of PCzV-co-MEH-PPV:PC61BM solar cells are higher
than those of the PCzV:PC61BM solar cells. The PCzV-co-
MEH-PPV:PC61BM derived solar cells have similar photo-
voltaic performances as the PCzV:PC61BM cells (see
Fig. 3b). The solar cell fabricated with 1:6 ratio of PCzV-
co-MEH-PPV and PC61BM, with TiOx with Voc of 0.82 V, Jsc
of 6.43 mA/cm2, FF of 44%, and PCE of 2.31%, displays the
highest photovoltaic performance when compared with
the solar cells arising from PCzV and PC61BM.
(wt/wt) of PCzV-co-MEH-PPV:PC61BM/TiOx. This finding
may be a result of an increase of the charge-carrier mobil-
ity in the active layer and, thus, an increase in light absorp-
tion in the solar cell active layer promoted by a high
amount of PC61BM. This factor also contributes to the in-
creased PCE of PCzV-co-MEH-PPV:PC61BM solar cells.
Integrated photocurrent curves were obtained by inte-
grating IPCE spectra in the wavelength region between
400 nm and 750 nm (see Fig. 5). The Jsc value, determined
from the overlap integral of the IPCE curve and the stan-
dard AM 1.5 G solar emission spectrum, is in excellent
agreement with the measured photocurrent density. The
error of Jsc and the integrated photocurrent is 5–7%, which
is in the 5–10% range often seen in these values [42].
Therefore, a negligible spectral mismatch exists between
the solar simulator used in this study and standard AM
1.5 G sunlight.
Incident photon to current conversion efficiency (IPCE)
spectra of the bulk-heterojunction solar cells fabricated
with PCzV-co-MEH-PPV and PC61BM as active layers, re-
corded in the wavelength region between 380 nm and
750 nm, are shown in Fig. 4. In IPCE spectra of bulk-hetero-
junction solar cells, the shape of peak around 400 nm was
enhanced with increasing amount of the PC61BM. In order
to easily understand, we measured absorption spectra of
the PCzV-co-MEH-PPV, PC61BM and PCzV-co-MEH-
PPV:PC61BM solutions (see Fig. S1). PC61BM showed a
strong absorption peak at 340 nm and small peaks located
at 400–600 nm. However as increasing the blending ratio
of PC61BM content in PCzV-co-MEH-PPV and PC61BM solu-
tions, the strong absorption peak of PC61BM showed at
340 nm and the absorption peak of PCzV-co-MEH-PPV
was gradually reduced. It should be indicated that this
makes it possible for the PC61BM molecules to penetrate
into the PCzV-co-MEH-PPV polymer chains. Thus, the peak
in IPCE spectra around 400 nm arises from the absorption
of PC61BM and PCzV-co-MEH-PPV. As increasing the
PC61BM content, more PC61BM molecules are embedded
into the PCzV-co-MEH-PPV polymer chains, which was in
favor of the generation and transport of charge carriers in
the PC61BM phase. A maximum IPCE of 17% was observed
in the case of the pristine PCzV homopolymer bulk-hetero-
junction solar cell at a wavelength of 410 nm. As increasing
the MEH-PPV content in PCzV-co-MEH-PPV copolymers,
IPCE also increases considerably and reaches a maximum
of 52% at 410 nm and broadened up to 650 nm for a
bulk-heterojunction solar cell comprised of a 1:6 ratio
4. Conclusions
A new class of high molecular weight poly(N-(3,4-
bis(decyloxy)phenyl)carbazolyl-2,7-vinylene) (PCzV) and
its copolymers with an MEH-PPV have been prepared by
using the Gilch polymerization procedure. PCzV and
PCzV-co-MEH-PPV exhibit high glass transition tempera-
tures and are easily spin casted onto the ITO substrates.
Bulk-heterojunction solar cells with ITO/PEDOT:PSS/poly-
mer:PC61BM/TiOx/Al configurations were fabricated with
PCzV and PCzV-co-MEH-PPV as electron donors and
PC61BM as the electron acceptor. The solar cells are com-
prised of 1:3, 1:4, 1:5 and 1:6 ratios of polymer to PC61BM.
The PCE values of the solar cells were found to increase
with increasing PC61BM content and a maximum PCE of
2.31% (Jsc = 6.43 mA/cm2, Voc = 0.82 V, FF = 44%) is observed
for the cell prepared from a 1:6 ratio (wt/wt) of PCzV-co-
MEH-PPV:PC61BM.
Acknowledgements
This work was supported by the Korea Science and
Engineering Foundation (KOSEF) Grant funded by the

978
M. Song et al. / Organic Electronics 11 (2010) 969–978
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