Alkyloxy substituted organic dyes for high voltage dye-sensitized solar cell: Effect of alkyloxy chain length on open-circuit voltage

Article abstract of DOI:10.1016/j.dyepig.2011.10.014

Three novel organic dyes (SB1, SB2, and SB3) containing 4-(hexyloxy)-N-(4-(hexyloxy)phenyl)-N-phenylaniline as electron donor and cyanoacrylic acid as electron acceptor bridged by alkyloxy (methyl = SB1, propyl = SB2 and hexyl = SB3) substituted p-phenylenevinylene linkers have been synthesized. Density functional theory (DFT) has employed to study electron distribution and intramolecular charge transfer. Increase in alkyl chain length in alkyloxy substituent leads to increase in open-circuit voltage (V _OC), which is found to be related to the increased electron lifetime at open-circuit condition. Under AM 1.5 G 1 sun light illumination (100 mW/cm²), an optimized SB3-sensitized cell show a short-circuit photocurrent density (J_SC) of 12.83 mA/cm², an open-circuit voltage (V_OC) of 0.745 V and a fill factor (FF) of 0.64, corresponding to an overall conversion efficiency (η) of 6.12%. Little degradation in η observed over 40 days is indicative of long-term stability of the SB-series dyes.

Full text of DOI:10.1016/j.dyepig.2011.10.014

Dyes and Pigments 94 (2012) 88e98
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Alkyloxy substituted organic dyes for high voltage dye-sensitized solar cell:
Effect of alkyloxy chain length on open-circuit voltage
*
Soo-Byung Ko, An-Na Cho, Mi-Jeong Kim, Chang-Ryul Lee, Nam-Gyu Park
School of Chemical Engineering and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Three novel organic dyes (SB1, SB2, and SB3) containing 4-(hexyloxy)-N-(4-(hexyloxy)phenyl)-N-phe-
nylaniline as electron donor and cyanoacrylic acid as electron acceptor bridged by alkyloxy
(methyl ¼ SB1, propyl ¼ SB2 and hexyl ¼ SB3) substituted p-phenylenevinylene linkers have been
synthesized. Density functional theory (DFT) has employed to study electron distribution and intra-
molecular charge transfer. Increase in alkyl chain length in alkyloxy substituent leads to increase in open-
circuit voltage (VOC), which is found to be related to the increased electron lifetime at open-circuit
condition. Under AM 1.5 G 1 sun light illumination (100 mW/cm2), an optimized SB3-sensitized cell
show a short-circuit photocurrent density (JSC) of 12.83 mA/cm2, an open-circuit voltage (VOC) of 0.745 V
Received 29 July 2011
Received in revised form
14 October 2011
Accepted 19 October 2011
Available online 2 November 2011
Keywords:
Organic dyes
and a fill factor (FF) of 0.64, corresponding to an overall conversion efficiency (
h) of 6.12%. Little
p-Phenylenevinylene
Alkyloxy chain length
Dye-sensitized solar cell
Open-circuit voltage
Recombination suppression
degradation in observed over 40 days is indicative of long-term stability of the SB-series dyes.
h
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
coumarin [8], indoline [9], squaraine [10], polyene [11], hemi-
cyanine [12], oligothiophene [13], perylene [14] porphyrin [15],
carbazole [16], benzothidizole [17] and truexene [18] have been
developed and attained high efficiency up to 9% [9b,19]. Another
derivatives containing triarylamine unit [20] or additional methoxy
groups [19b,21] also have been widely studied due to their prom-
inent electron donating ability and hole-transport properties. As
part of our efforts to investigate the structural modifications of
triarylamine based organic dye, p-phenylenevinylene units instead
During the foregone two decades, the dye-sensitized solar cell
(DSSC) composed of a dye-adsorbed mesoporous TiO2 film, a redox
electrolyte, and a platinum-coated counter electrode has attracted
remarkable attention by virtue of its giant potential as a low-cost
solar-to-electrical energy conversion device [1,2]. Promoted by
continuous material development as well as device engineering,
the cells using polypyridyl ligand coordinated ruthenium complex
dyes such as N3 [1,3], N719 [4], and N749 [5] have achieved solar-
to-electrical energy conversion efficiency as high as 11% under
the standard solar illumination. Although these ruthenium deriv-
atives exhibited high efficiency and good stability [6], they seem to
be expensive due to ruthenium metal and hard to purify from the
synthetic mixture. Meanwhile, metal-free organic dyes have
explored for DSSC because of their wide variety of the structure
versatility, and facile modifications for the absorption wavelength
range, molar extinction coefficient and HOMOeLUMO energy levels
of thiophene chain have been introduced by
p-conjugated linker
[22]. In particular, organic molecules including oligo(p-phenyl-
enevinylene) unit were applied for materials in light-emitting
diodes [23] and solar cells [24] owing to their stability and high
luminescent efficiency. We previously reported the methoxy
substituted phenylenevinylene dye (TA-DM-CA), which was
developed to enhance the electron donating character in
conjugated donoreacceptor skeleton, showed an efficiency (
p-
) of
h
6.49% with short-circuit current density (JSC) of 13.83 mA/cm2,
open-circuit voltage (VOC) of 0.677 V and fill factor (FF) of 0.692
[19be22b]. However, increase in the conjugation length (TA-TM-CA
and TA-HM-CA) led to relatively lower efficiency (5.02% and 5.21%)
although their absorption bands shift to longer wavelength, which
[7]. Generally,
p-conjugated bridge structure with electron
donoreacceptor moieties has been known to be effective for
intramolecular charge transfer, which is suitable for high efficient
DSSC applications. To date, various organic dyes including
was due to the decreased VOC
.
The performances of DSSCs based on the organic dyes, however,
have not exceeded those on the polypyridyl ligand coordinated
* Corresponding author. Tel.: þ82 31 290 7241.
E-mail address: npark@skku.edu (N.-G. Park).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.10.014

S.-B. Ko et al. / Dyes and Pigments 94 (2012) 88e98
89
ruthenium complexes yet because of their shorter electron lifetime
r) and intermolecular charge transfer due to dye aggregation,
(CV) was measured with a three-electrode electrochemical cell on
a Autolab (AUT128N, FRA2) electrochemical analyzer. The
(s
associated with relatively low VOC [7,25]. For overcoming these
limitations, our further efforts have been made to increase sr and
reduce intermolecular charge transfer by structural modification of
our previous TA-DM-CA dye. We herein report three novel organic
dyes (SB1, SB2, and SB3) containing 4-(hexyloxy)-N-(4-(hexyloxy)
phenyl)-N-phenylaniline as electron donor and cyanoacrylic acid as
electron acceptor bridged by alkyloxy substituted p-phenyl-
measurements were carried out in CH2Cl2 solution with tetrabu-
tylammonium hexafluorophosphate (0.1 M) at a scan rate 100 mV/
s. Au, Pt and Ag/AgCl were employed as working, counter, and
reference electrodes. Under the present experimental conditions,
the redox potentials are reported with a ferrocenium/ferrocene
(Fcþ/Fc) redox couple as an internal reference (0.46 V vs SCE) [26].
enevinylene
p-conjugated spacers (Fig. 1). Optical and electro-
2.3. Fabrication and characterization of dye-sensitized solar cells
chemical characterization are performed and effect of alkyl chain
length in the alkyloxy substituent on photovoltaic performance is
investigated. Long-term stability for the SB-series dyes is also
reported.
FTO glasses (Pilkington, TEC-8, 8 U/sq) were washed in ethanol
using an ultrasonic bath for 10 min. The FTO layer was first covered
with 0.1 M Ti(IV) bis(ethyl acetoacetato)diisopropoxide (Aldrich) in
1-butanol (Aldrich) solution by spin-coating method, on which the
nanocrystalline TiO2 paste was deposited using a doctor-blade
technique. After the TiO2 films were heated at 550 ꢀC for 1 h, the
sides of TiO2 films was trimmed to 9 mm wide and 5 mm long. The
thickness of TiO2 films was measured by Alpha step profiler
(KLA-Tencor). The annealed TiO2 electrodes were immersed in
ethanol containing 0.5 mM photosensitizer (SB1, SB2 or SB3) for 2 h
at 40 ꢀC. Pt counter electrodes were prepared by spreading
a droplet of 7 mM H2PtCl6 in 2-propanol on top of a FTO substrate
and heating heated at 400 ꢀC for 20 min. The two electrodes were
2. Experimental
2.1. Materials
All reagents were obtained from Aldrich unless otherwise
specified as received without further purification. Flash column
chromatography was carried out on silica gel (Merck; silica gel 60,
230e400 mesh). Anatase TiO2 nanoparticles were synthesized by
acetic acid via the catalyzed hydrolysis of titanium isopropoxide
(97%, Aldrich), followed by autoclaving at 230 ꢀC for 12 h. An
aqueous solvent in the autoclaved TiO2 colloid solution was
replaced by ethanol for preparation of non aqueous TiO2 paste.
Ethyl cellulose (Aldrich), lauric acid (Fluka) and terpineol (Aldrich)
were added into the ethanol solution of the TiO2 particles, and then
ethanol was removed from the solution using a rotary evaporator to
obtain viscous pastes. For homogeneous mixing, the paste was
further treated with three-roll mill. The nominal composition of
TiO2/terpineol/ethylcellulose/lauric acid was 1.25/4/0.3/0.1. N719
(TBA2[RuL2(NCS)2], where L is 4-carboxylic acid-40carboxylate-2,20-
bipyridine and TBA is tetrabutylammonium) dye was obtained from
Esolar. Compound TA-DM-CA was prepared according to the liter-
ature procedures [19be22b].
sealed with 60 mm thick Surlyn (Solaronix). Redox electrolyte was
introduced through a small hole drilled in the counter electrode.
The electrolyte employed was a solution of 0.7 M 1-methyl-3-
propylimidazolium iodide (MPII), 0.03 M iodine (I2), 0.10 M
lithium iodide (LiI) and 0.5
M 4-tert-butylpyridine (TBP) in
a mixture of acetonitrile and valeronitrile (volume ratio, 85:15).
Photocurrent and voltage were measured from a solar simulator
(Oriel Sol 3A class AAA) equipped with 450 W Xenon lamp (New-
port 6279NS) and a Keithley 2400 source meter. Light intensity was
adjusted with the NREL-calibrated Si solar cell having KG-2 filter for
approximating one sun illumination (100 mW/cm2). The cell was
covered with an aperture black mask to measure photocurrent and
voltage accurately. The incident photon-to-current conversion
efficiency (IPCE) spectra was measured as a function of wavelength
from 300 to 800 nm under DC measurement mode using a specially
designed IPCE system (PV Measurements, Inc.), where 75 W Xe
light source was used for monochromatic light. The absorbance of
photosensitizers was recorded by a UVeVis spectrophotometer
(Agilent 8453) in the wavelength range of 300e800 nm. Impedance
spectra were recorded using a Autolab (AUT128N, FRA2) electro-
chemical station under one sun illumination with a frequency
range of 0.1 Hze100 kHz.
2.2. General characterization methods
The chemical structures of the materials used in this study were
identified by nuclear magnetic resonance (NMR) spectra were
recorded on a Varian Unity inova 300 MHz spectrometer. 1H NMR
spectra were referenced to tetramethylsilane (TMS, 0.00 ppm)
using CDCl3 as solvent. 13C NMR spectra were referenced to solvent
carbons (77.16 ppm for CDCl3). High resolution mass spectra which
were recorded on a JEOL Ltd JMS-700 mstation obtained by FAB and
reported in units of mass to charge (m/z). The absorption and
photoluminescence spectra were recorded on an Agilent 8453
UVevisible spectroscopy and a Riken-Keiki fluorescence spec-
trometer, respectively. The electrochemical cyclic voltammetry
2.4. Synthesis of photosensitzres
Compounds 1be7 were prepared according to the modified
literature procedures [27e34]. 1H and 13C NMR spectra of all
compounds are available in the Supplementary material.
2.4.1. 1,4-Dipropoxybenzene (1b)
A suspension of KOH (2.16 g, 0.038 mol) in ethanol (10 mL) was
stirred at room temperature. Hydroquinone (2.00 g, 0.018 mol) was
added dropwise. 1-Bromohexane (3.8 mL, 0.038 mol) was added to
the stirred mixture. After stirring for 24 h with heating at reflux, the
ethanol was evaporated at reduced pressure. The residue was then
extracted with chloroform. The combined organic layer was
washed with water and dried over anhydrous magnesium sulfate.
The white solid product 1b (3.01 g, 0.015 mol) was obtained in 86%
yield after column chromatography on silica gel (eluent n-
O
R
O
NC
CO OH
N
O
R
O
SB1 (R = Me)
SB2
(R = -Pr)
n
SB3
(R = -Hx)
n
Fig. 1. Molecular structures of the SB1, SB2, and SB3 dyes.
Hx:CH2Cl2 ¼ 5:1, Rf ¼ 0.50). 1H NMR (300 MHz, CDCl3):
d 6.77 (s, 4H,

90
S.-B. Ko et al. / Dyes and Pigments 94 (2012) 88e98
Ar), 3.77 (t, J ¼ 6.6 Hz, 4H, CH2), 1.76e1.70 (m, 4H, CH2), 0.99
(t, J ¼ 7.5 Hz, 6H, CH3).
Ar), 7.19 (s, 1H, Ar), 4.04e3.98 (m, 4H, CH2), 1.86e1.77 (m, 4H, CH2),
1.59e1.44 (m, 4H, CH2), 1.37e1.34 (m, 8H, CH2), 0.91 (t, J ¼ 6.9 Hz,
6H, CH3).
2.4.2. 1,4-Bis(hexyloxy)benzene (1c)
The synthetic method resembles that of compound 1b, and
compound 1c was purified by column chromatography on silica gel
(eluent n-Hx:CH2Cl2 ¼ 5:1, Rf ¼ 0.50) to obtain in 88% yield as white
2.4.9. 1-(Hexyloxy)-4-iodobenzene (4)
To a suspension of 4-iodophenol (5.0 g, 22.7 mmol) and K2CO3
(4.4 g, 31.8 mmol) in ethanol (30 mL) was added 1-bromohexane
(2.9 mL, 20.4 mmol) at room temperature. After being stirred
under reflux for 24 h, the mixture was filtered to remove the solid.
The crude product was extracted into methylene chloride, washed
with water, and dried over anhydrous magnesium sulfate. After
removing solvent under reduced pressure, the residue was purified
by column chromatography on silica gel (eluent n-Hx:CH2Cl2 ¼ 5:1,
Rf ¼ 0.50) to obtain compound 4 (6.2 g, 20.4 mmol) in 90% yield as
solid. 1H NMR (300 MHz, CDCl3):
d
6.82 (s, 4H, Ar), 3.90 (t, J ¼ 6.6 Hz,
4H, CH2), 1.80e1.73 (m, 4H, CH2), 1.47e1.30 (m, 12H, CH2), 0.90
(t, J ¼ 5.7 Hz, 6H, CH3).
2.4.3. 1,4-Diiodo-2,5-dimethoxybenzene (2a)
A solution of H5IO6 (3.20 g, 0.014 mol) in methanol (25 mL) was
stirred for 10 min, and I2 (6.97 g, 0.028 mol) was added. After the
solution was stirred for another 10 min, 1,4-dimethoxybenzene
(3.00 g, 0.022 mol) was added and the mixture heated at 70 ꢀC
for 4 h. The resulting solution was poured into a solution of K2S2O5
(6.20 g, 0.028 mol) in water (50 mL). The precipitate was washed
with methanol and dissolved in methylene chloride. After filtration,
the filtrate was collected and evaporated under vacuum to afford
a white solid (8.12 g, 0.021 mol) in 90% yield. 1H NMR (300 MHz,
viscous colorless oil. 1H NMR (300 MHz, CDCl3):
d
7.54 (d, J ¼ 8.4 Hz,
2H, Ar), 6.67 (d, J ¼ 8.4 Hz, 2H, Ar), 3.91 (t, J ¼ 6.6 Hz, 2H, CH2),
1.81e1.55 (m, 2H, CH2), 1.47e1.25 (m, 6H, CH2), 0.90 (t, J ¼ 6.6 Hz,
3H, CH3).
2.4.10. 4-(Hexyloxy)-N-(4-(hexyloxy)phenyl)-N-phenylaniline (5)
To a stirred solution of compound 4 (5.0 g, 16.4 mmol), cuprous
chloride (65 mg, 0.6 mmol), 1,10-phenanthroline (236 m g,
1.312 mmol), potassium hydroxide (1.5 g, 26.2 mmol) in anhydrous
CDCl3):
d 7.19 (s, 2H, Ar), 3.82 (s, 6H, CH3).
2.4.4. 1,4-Diiodo-2,5-dipropoxybenzene (2b)
toluene (12 mL), aniline (598 mL, 6.6 mmol) was added under argon
The synthetic method resembles that of compound 2a, and
atmosphere. The reaction mixture was refluxed for 12 h and then
water added. The crude product was extracted into dichloro-
methane, and the organic layer was washed with water and dried
over anhydrous magnesium sulfate. After removing solvent under
reduced pressure, the residue was purified by column chromatog-
raphy silica gel (eluent n-Hx:CH2Cl2 ¼ 1:1, Rf ¼ 0.50) to obtain
compound 5 (1.8 g, 4.0 mmol) in 60% yield as viscous yellowish oil.
compound 2b was obtained in 92% yield as white solid. 1H NMR
(300 MHz, CDCl3):
1.86e1.79 (m, 4H, CH2), 0.90 (t, J ¼ 7.2 Hz, 6H, CH3).
d
7.17 (s, 2H, Ar), 3.90 (t, J ¼ 6.3 Hz, 4H, CH2),
2.4.5. 1,4-Bis(hexyloxy)-2,5-diiodobenzene (2c)
The synthetic method resembles that of compound 2a, and
compound 2c was obtained in 90% yield as white solid. 1H NMR
1H NMR (300 MHz, CDCl3):
d
7.17 (d, J ¼ 7.5 Hz, 2H, Ar), 7.03
(300 MHz, CDCl3):
d
7.17 (s, 2H, Ar), 3.93 (t, J ¼ 6.3 Hz, 4H, CH2),
(d, J ¼ 9.0 Hz, 4H, Ar), 6.93 (d, J ¼ 9.0 Hz, 2H, Ar), 6.82 (t, J ¼ 7.2 Hz,
1H, Ar), 6.80 (d, J ¼ 9.0 Hz, 2H, Ar), 3.91 (t, J ¼ 6.6 Hz, 4H, CH2),
1.81e1.55 (m, 4H, CH2), 1.46e1.45 (m, 4H, CH2), 1.36e1.31 (m, 8H,
CH2), 0.91 (t, J ¼ 6.9 Hz, 6H, CH3).
1.82e1.75 (m, 4H, CH2), 1.53e1.32 (m, 12H, CH2), 0.91 (t, J ¼ 6.6 Hz,
6H, CH3).
2.4.6. 4-Iodo-2,5-dimethoxybenzaldehyde (3a)
A solution of compound 2a (2.00 g, 5.130 mmol) in diethyl ether
(60 mL) was cooled to ꢁ10 ꢀC. To this solution was added dropwise
n-BuLi (1.60 M in hexane, 3.20 mL) over 20 min. The reaction
mixture was stirred in an ice/brine bath at ꢁ8 to ꢁ5 ꢀC for 1 h. DMF
(0.60 mL, 7.690 mmol) was added in one portion. The reaction
mixture was allowed to warm to room temperature over 6 h and
then poured into water in a separatory funnel. The organic layer
was washed with H2O and brine once and dried over magnesium
sulfate. After removed the solvent under vacuum, the residue was
purified by column chromatography on silica gel (eluent n-
Hx:EA ¼ 8:1, Rf ¼ 0.30) to obtain compound 3a (1.20 g, 3.591 mmol)
2.4.11. 4-(Bis(4-(hexyloxy)phenyl)amino)benzaldehyde (6)
To a cold solution of compound 5 (1.50 g, 3.36 mmol) in N,N-
dimethylformamide (15 mL) at 0 ꢀC was added a Vilsmeier reagent,
which was prepared by phosphorus oxychloride (345
mL
3.70 mmol) and N,N-dimethylformamide (5 mL) mixture. The
reaction mixture was stirred at room temperature for 48 h and then
water added. The crude product was extracted into dichloro-
methane, washed with water, and dried over anhydrous magne-
sium sulfate. After removing solvent under reduced pressure, the
residue was purified by column chromatography on silica gel
(eluent n-Hx:EA ¼ 8:1, Rf ¼ 0.33) to obtain compound 6 (1.4 g,
2.86 mmol) in 85% yield as viscous yellowish oil. 1H NMR (300 MHz,
in 70% yield as white solid. 1H NMR (300 MHz, CDCl3):
d 10.40 (s,1H,
CHO), 7.47 (s, 1H, Ar), 7.23 (s, 1H, Ar), 3.90 (s, 3H, CH3), 3.88 (s, 3H,
CH3).
CDCl3):
d
9.74 (s, 1 H, CHO), 7.61 (d, J ¼ 9.0 Hz, 2H, Ar), 7-12-7.09
(m, 4H, Ar), 6.90e6.81 (m, 6H, Ar), 3.94 (t, J ¼ 6.6 Hz, 4H, CH2),
1.80e1.75 (m, 4H, CH2), 1.48e1.44 (m, 4H, CH2), 1.36e1.31 (m, 8H,
CH2), 0.91 (t, J ¼ 6.9 Hz, 6H, CH3).
2.4.7. 4-Iodo-2,5-dipropoxybenzaldehyde (3b)
The synthetic method resembles that of compound 3a, and
compound 3b was purified by column chromatography on silica gel
(eluent n-Hx:CH2Cl2 ¼ 5:1, Rf ¼ 0.15) to obtain in 62% yield as white
2.4.12. 4-(Hexyloxy)-N-(4-(hexyloxy)phenyl)-N-(4-vinylphenyl)
aniline (7)
solid. 1H NMR (300 MHz, CDCl3):
d
10.42 (s, 1H, CHO), 7.45 (s, 1H,
Methyltriphenyl-phosphonium bromide (634 mg, 1.774 mmol)
and compound 6 (700 mg, 1.478 mmol) were dissolved in 10 mL of
anhydrous THF under argon. A solution of potassium tert-butoxide
(1.0 M THF; 2.2 mL, 2.217 mmol) was slowly dropwised to the
resulting mixture at room temperature. The mixture was stirred for
8 h and then water added. The crude product was extracted into
dichloromethane, washed with water, and dried over anhydrous
magnesium sulfate. After removing solvent under reduced pres-
sure, the residue was purified by column chromatography on silica
Ar), 7.17 (s, 1H, Ar), 4.00e3.94 (m, 4H, CH2), 1.90e1.78 (m, 4H, CH2),
1.06 (t, J ¼ 6.6 Hz, 6H, CH3).
2.4.8. 2,5-Bis(hexyloxy)-4-iodobenzaldehyde (3c)
The synthetic method resembles that of compound 3a, and
compound 3c was purified by column chromatography on silica gel
(eluent n-Hx:CH2Cl2 ¼ 5:1, Rf ¼ 0.15) to obtain in 60% yield as white
solid. 1H NMR (300 MHz, CDCl3):
d 10.42 (s, 1H, CHO), 7.45 (s, 1H,

S.-B. Ko et al. / Dyes and Pigments 94 (2012) 88e98
91
Scheme 1. Synthetic procedure of organic dyes SB1, SB2, and SB3.
gel (eluent n-Hx:CH2Cl2 ¼ 5:1, Rf ¼ 0.27) to obtain compound 7
methylene chloride, washed with water, and dried over anhydrous
magnesium sulfate. After removing solvent under reduced pres-
sure, the residue was purified by column chromatography on silica
gel (eluent n-Hx:EA ¼ 8:1, Rf ¼ 0.23) to obtain compound 8a
(732 mg, 1.151 mmol) in 84% yield as viscous orange oil. 1H NMR
(523 mg, 1.109 mmol) in 75% yield as viscous yellowish oil. 1H NMR
(300 MHz, CDCl3):
d
7.17 (d, J ¼ 9.0 Hz, 2H, Ar), 7.00 (d, J ¼ 9.0 Hz,
4H, Ar), 6.85 (d, J ¼ 8.7 Hz, 2H, Ar), 6.77 (d, J ¼ 8.7 Hz, 2H, Ar), 6.58
(dd, J ¼ 17.4 Hz, 1H, CH), 5.53 (d, J ¼ 17.7 Hz, 1H, CH]CH2), 5.04 (d,
J ¼ 10.8 Hz,1H, CH]CH2), 1.79e1.70 (m, 4H, CH2), 1.49e1.40 (m, 4H,
CH2), 1.35e1.30 (m, 8H, CH2), 0.88 (t, J ¼ 6.9 Hz, 6H, CH3). 13C NMR
(300 MHz, CDCl3):
d 10.40 (s, 1 H, CHO), 7.37e6.80 (m, 16H, Ar),
3.95e3.90 (m, 7H, CH2 and CH3), 3.86 (s, 3H, CH3), 1.79e1.72 (m, 4H,
CH2), 1.49e1.44 (m, 4H, CH2), 1.36e1.31 (m, 8H, CH2), 0.91 (t,
(75 MHz, CDCl3):
d 155.5, 148.6, 140.7, 136.4, 129.9, 126.9, 126.6,
126.4, 120.4, 115.3, 111.0, 68.2, 31.7, 29.8, 29.4, 25.9, 22.7, 14.2. ESI-
J ¼ 6.9 Hz, 6H, CH3). 13C NMR (75 MHz, CDCl3):
d 188.9, 156.9, 155.9,
HRMS (positive) calcd for [C32H42NO2] þ 472.3216 found 472.3170.
151.2, 149.2, 140.4, 135.1, 132.4, 129.1, 127.9, 127.0, 123.6, 112.0, 119.6,
115.4, 109.3, 109.0, 68.4, 56.3, 56.2, 31.7, 29.4, 25.9, 22.7, 14.1. ESI-
HRMS (positive) calcd for [C41H50NO5] þ 635.3689 found 635.4802.
2.4.13. 4-(4-(Bis(4-(hexyloxy)phenyl)amino)styryl)-2,5-
dimethoxybenzaldehyde (8a)
Trimethylamine (286 ml, 2.055 mmol) was added to a solution
of compound 7 (775 mg 1.63 mmol), aldehyde 3a (400 mg,
2.4.14. 4-(4-(Bis(4-(hexyloxy)phenyl)amino)styryl)-2,5-
dipropoxybenzaldehyde (8b)
1.370 mmol) and trans-di(
m
-aceto)bis[o-(di-o-tolyphosphino)
The synthetic method resembles that of compound 8a, and
compound 8b was purified by column chromatography on silica gel
(eluent n-Hx:EA ¼ 15:1, Rf ¼ 0.23) to obtain in 84% yield as viscous
benzyl]-dipalladium(II) (30 mg, 0.032 mmol) in 5 mL of N,N-
dimethylacetamide, DMAc under argon atmosphere. The resulting
mixture was stirred at 80 ꢀC for 72 h. The reaction mixture was
cooled to room temperature and then saturated sodium chloride
aqueous solution added. The crude product was extracted into
orange oil. 1H NMR (300 MHz, CDCl3):
d 10.43 (s, 1 H, CHO),
7.35e6.81 (m, 16H, Ar), 4.05 (t, J ¼ 6.6 Hz, 2H, CH2), 4.03e3.90
(m, 6H, CH2), 1.90e1.72 (m, 8H, CH2), 1.46e1.41 (m, 4H, CH2),

92
S.-B. Ko et al. / Dyes and Pigments 94 (2012) 88e98
1.36e1.34 (m, 8H, CH2), 1.10e1.05 (m, 6H, CH2), 0.91 (t, J ¼ 6.6 Hz,
6H, CH3). 13C NMR (75 MHz, CDCl3):
189.0, 156.5, 155.9, 150.6,
(m, 8H, CH2), 1.46e1.26 (m, 28H, CH2), 0.92 (m, 12H, CH3). 13C NMR
(75 MHz, CDCl3): 168.2, 155.8, 153.5, 150.2, 148.8, 140.6, 133.4,
d
d
149.1, 140.4, 135.2, 132.2, 129.2,127.8, 127.0, 123.8, 119.9,115.4,110.2,
110.1, 70.8, 70.7, 68.4, 31.7, 29.8, 29.4, 25.9, 22.8, 22.7, 14.1, 10.8, 10.7.
ESI-HRMS (positive) calcd for [C45H58NO5]þ 692.4315, found
692.4310.
129.2, 115.4, 111.4, 109.6, 69.5, 69.0, 68.4, 31.9, 31.8, 31.6, 29.8, 29.5,
29.0, 26.2, 25.9, 25.6, 22.7, 14.2. ESI-HRMS (positive) calcd for
[C54H71N2O6]þ 843.5312 found 843.5335.
3. Results and discussion
2.4.15. 4-(4-(Bis(4-(hexyloxy)phenyl)amino)styryl)-2,5-
bis(hexyloxy)benzaldehyde (8c)
SB1, SB2, and SB3 were synthesized by the stepwise synthetic
procedure (Scheme 1). First, compounds of 3ae3b such as dia-
lkyloxy substituted p-phenylenevinylene spacer were prepared in
three steps from p-hydroquinone [27]. A Williamson etherification
of p-hydroquinone [28] followed by iodination [29] gave 2ae2c.
Monolithiation of 2ae2c and quenching the aryllithium by using
anhydrous DMF obtained dialkyloxy substituted benzaldehyde
3ae3c. 1-Bromo-4-hexyloxybenzene 4 was prepared from 4-
iodophenol [30]. After that, 4-(hexyloxy)-N-(4-(hexyloxy)phenyl)-
N-phenylaniline 5 was obtained from the reaction of aniline with
compound 4 via the Ullmann condensation [31]. Compound 5 was
converted into its corresponding 4-(bis(4-(hexyloxy)phenyl)
amino)-benzaldehyde 6 by means of the VilsmeiereHaack reaction
The synthetic method resembles that of compound 8a, and
compound 8c was purified by column chromatography on silica gel
(eluent n-Hx : EA ¼ 15:1, Rf ¼ 0.33) to obtain in 72% yield as viscous
orange oil. 1H NMR (300 MHz, CDCl3):
d 10.42 (s, 1 H, CHO),
7.35e6.80 (m, 16H, Ar), 4.07 (t, J ¼ 6.6 Hz, 2H, CH2), 3.98
(t, J ¼ 6.0 Hz, 2H, CH2), 3.91 (t, J ¼ 6.6 Hz, 4H, CH2),1.85e1.71 (m, 8H,
CH2), 1.48e1.41 (m, 8H, CH2), 1.35e1.33 (m, 16H, CH2), 0.93e0.89
(m, 12H, CH2 and CH3). 13C NMR (75 MHz, CDCl3):
d 188.9, 156.4,
155.9, 150.6, 149.1, 140.3, 135.1, 132.1, 129.1, 127.7, 126.9, 123.8, 119.9,
115.4, 110.1, 110.0, 69.2, 69.1, 68.3, 31.7, 31.6, 29.3, 25.9, 25.8, 22.7,
14.1. ESI-HRMS (positive) calcd for [C51H70NO5]þ 776.5254, found
776.5209.
[32].
4-(Hexyloxy)-N-(4-(hexyloxy)phenyl)-N-(4-vinylphenyl)
2.4.16. 4-(Bis(4-(hexyloxy)phenyl)amino)styryl)-2,5-
dimethoxyphenyl)-2-cyanoacrylic acid (SB1)
A mixture of compound 8a (240 mg, 0.377 mmol) and cyano-
acetic acid (42 mg, 0.490 mmol) was vacuum dried, and acetonitrile
aniline 7 was prepared from the reaction of compound 6 with
methyltriphenyl-phosphonium bromide [20f]. Coupling reaction of
aldehydes 3ae3c with compound 7 under the Heck reaction led to
its corresponding aldehydes 8ae8c in good yields (72e87%) [33].
The aldehydes 8ae8c, upon reaction with cyanoacetic acid in the
presence of piperidine as base via the Knoevenagel condensation,
produced SB1, SB2 and SB3 in moderate yields (52e54%).
UVeVis absorption and emission spectra of SB1, SB2, and SB3 in
ethanol are shown in Fig. 2 and their absorption data are listed in
Table 1. In UVeVis spectra, the absorption of SB1 w SB3 exhibits
two major bands at 305e306 nm and at 447e448 nm. The former is
(40 mL) and piperidine (37
mL, 0.377 mmol) were added under
argon atmosphere. The reaction mixture was refluxed for 10 h. The
reaction mixture was cooled to room temperature and then satu-
rated sodium chloride aqueous solution added. The crude product
was extracted into methylene chloride, washed with water, and
dried over anhydrous magnesium sulfate. After removing solvent
under reduced pressure, the residue was purified by column
chromatography on silica gel (eluent CH2Cl2:MeOH
Rf ¼ 0.13) to obtain compound SB1 (138 mg, 0.196 mmol) in 54%
yield as red solid. 1H NMR (300 MHz, CDCl3):
8.68 (s, 1 H, CH), 7.96
¼
10:1,
attributed to a localized aromatic
ascribed to charge-transfer (CT) transitions. The absorption coeffi-
pep* transitions and the later is
d
cient (
3
) aꢁt 1absorption maximum of SB1, SB2 and SB3 in ethanol is
(s, 1H, CH), 7.33e6.79 (m, 15H, Ar), 3.95e3.91 (m, 10H, CH2 and
CH3), 1.80e1.75 (m, 4H, CH2), 1.48e1.44 (m, 4H, CH2), 1.36e1.28
(m, 8H, CH2), 0.91 (t, J ¼ 6.9 Hz, 6H, CH3). 13C NMR (75 MHz, CDCl3):
33,637 M cmꢁ1 at 447 nm, 29,703 Mꢁ1 cmꢁ1 at 448 nm and
40,922 Mꢁ1 cmꢁ1 at 448 nm, respectively. Structural difference
d
168.9, 155.9, 154.9, 149.2, 140.6, 140.2, 135.6, 132.9, 128.7, 128.1,
127.1, 126.8, 119.6, 118.9, 116.6, 115.4, 110.3, 107.7, 68.4, 56.3, 56.1,
31.7, 29.8, 29.4, 25.9, 22.7, 14.2. ESI-HRMS (positive) calcd for
[C44H50N2O6]þ 703.3747 found 703.3702.
2.4.17. 4-(Bis(4-(hexyloxy)phenyl)amino)styryl)-2,5-
dipropoxyphenyl)-2-cyanoacrylic acid (SB2)
The synthetic method resembles that of compound SB1, and
compound SB2 was purified by column chromatography on silica
gel (eluent CH2Cl2:MeOH ¼ 5:1, Rf ¼ 0.33) to obtain in 52% yield as
red solid. 1H NMR (300 MHz, CDCl3):
d 8.59 (s, 1 H, CH), 7.73 (s, 1H,
CH), 7.23e6.78 (m, 15H, Ar), 3.91 (t, J ¼ 5.3 Hz, 8H, CH2), 1.80e1.73
(m, 8H, CH2), 1.48e1.42 (m, 4H, CH2), 1.37e1.27 (m, 8H, CH2), 0.92
(t, J ¼ 6.6 Hz, 12H, CH3). 13C NMR (75 MHz, CDCl3):
d 164.8, 155.7,
153.4, 150.1, 148.6, 140.5, 137.5, 133.1, 131.0, 129.6, 127.8, 126.8, 120.1,
115.3, 110.3, 109.0, 70.9, 70.3, 68.3, 31.7, 29.8, 29.5, 26.0, 22.7, 14.2,
10.9, 10.6. ESI-HRMS (positive) calcd for [C48H59N2O6]þ 759.4373
found 759.4328.
2.4.18. 4-(Bis(4-(hexyloxy)phenyl)amino)styryl)-2,5-bis(hexyloxy)
phenyl)-2-cyanoacrylic acid (SB3)
The synthetic method resembles that of compound SB1, and
compound SB3 was purified by column chromatography on silica
gel (eluent CH2Cl2:MeOH ¼ 5:1, Rf ¼ 0.60) to obtain in 52% yield as
red solid. 1H NMR (300 MHz, CDCl3):
d 8.63 (s, 1 H, CH), 7.74 (s, 1H,
CH), 7.25e6.79 (m, 15H, Ar), 3.92 (t, J ¼ 6.3 Hz, 8H, CH2), 1.80e1.75
Fig. 2. Absorption and emission spectra of SB1, SB2 and SB3 dyes in ethanol.

S.-B. Ko et al. / Dyes and Pigments 94 (2012) 88e98
93
Table 1
Absorption and electrochemical properties of SB1, SB2 and SB3 dyes.
a
a
b
c
d
Dye
lmax (nm)a
3
(Mꢁ1 cmꢁ1
)
lmax (nm) a
3
(Mꢁ1 cmꢁ1
)
EHOMO (V)
E0e0 (V)
Ered (V)
SB1
SB2
SB3
305
306
306
22,119
18,878
25,611
447
448
448
33,637
29,703
40,922
1.23
1.19
1.14
2.27
2.27
2.25
ꢁ1.04
ꢁ1.08
ꢁ1.11
a
Absorption spectra were measured in ethanol solution.
b
c
EHOMO was calculated by Ered ꢁ E0e0
.
E0e0 was determined from intersection of absorption and emission spectra.
d
Ered were measured in CH2Cl2 with 0.1 M (Bu4N)PF6 at scan rate of 100 mV sꢁ1
.
between the previously reported TA-St-CA and TA-DM-CA dyes
[22] and the SB-series dyes is that the SB-series dyes have two
hexyloxy groups attached to the triphenylamine on the electron
donor moiety as well as two alklyoxy groups in p-phenyl-
enevinylene spacer unit. These substituted alkyloxy groups
enhance the extent of electron delocalization over the whole
molecule, so their maximum absorption peaks are red shifted by
about 60 nm and 15 nm than that of TA-St-CA and TA-DM-CA. SB1,
SB2, and SB3 show luminescence maxima at 604 nm, 596 nm, and
is expected from LUMO to the conduction band of TiO2. The HOMO
levels are calculated from Ered and E0e0 determined by the inter-
section of absorption and emission spectra listed in Table 1. The
HOMO values of SB1, SB2, and SB3 are determined to be 1.23 V,
1.19 V, and 1.14 V vs. SCE. As depicted in Fig. 4, these levels are more
positive than the Iꢁ=I3ꢁ redox potential (0.20 V vs. SCE) [34] so that
the oxidized dyes can be thermodynamically accept electrons from
I
ꢁ ion in iodide/triiodide electrolyte for regeneration.
To understand the electron distribution at ground and excited
606 nm when they are excited within their
p
e
p
* band at 298 K.
states in SB-series dyes, the electronic property of SB1, SB2 and SB3
are studied via density functional theory (DFT). The computational
calculations were performed using Gaussian09 software package
[35]. The DFT was applied B3LYP exchange-correlation functional
[36] and 6-31G(d) basis set [37]. Fig. 5 shows the resulting electron
distributions of SB1, SB2 and SB3. Molecular orbital calculations on
the SB1-SB3 indicated that electrons in the HOMO levels spread
With respect to the conduction band edge of TiO2 and the redox
potential of electrolyte, proper energy levels of lowest unoccupied
molecular orbital (LUMO) and highest occupied molecular orbital
(HOMO) of dyes are required to separate electrons from LUMO and
holes from HOMO. Fig. 3 shows the cyclic voltammograms of SB1,
SB2 and SB3 dyes in the potential range of 2.0 to ꢁ2.0 V. In the
negative potential region, the reduction potential (Ered) of SB1
exhibits a quasi-reversible behavior at ꢁ1.04 V, which corresponds
to LUMO energy level. Under the same conditions, LUMO energy
levels of SB2 and SB3 are determined to be ꢁ1.08 V and ꢁ1.11 V,
respectively. These levels are higher than the conduction band edge
energy level (ꢁ0.74 V vs. SCE) [2c] of the TiO2, so electron injection
over the p-framework from 4-(hexyloxy)-N-(4-(hexyloxy)phenyl)-
N-phenylaniline to p-phenylenevinylene spacers. At the LUMO
levels excited electrons are shifted from the electron donating 4-
(hexyloxy)-N-(4-(hexyloxy)phenyl)-N-phenylaniline units to the
electron accepting cyanoacrylic acid unit due to the intramolecular
charge transfer along the p-conjugated skeleton. Therefore, we can
expect that the spatial orientation of HOMO and LUMO levels for
the dyes is suitable for DSSCs, which can facilitate the interfacial
electron injection from the excited dye to the conduction band of
TiO2.
The photovoltaic performance of SB1, SB2 and SB3 is evaluated
by employing a thin (5.0 mm) transparent TiO2 film and a redox
electrolyte of 0.7 M MPII, 0.03 M I2, 0.10 M LiI and 0.5 M TBP in
a mixture of acetonitrile and valeronitrile (volume ratio, 85:15),
which is compared with that of conventional ruthenium complex
dye coded N719 and TA-DM-CA. Fig. 6 shows photocurrent densi-
tyevoltage curves of SB1, SB2, SB3 and N719 along with TA-DM-CA.
Fig. 3. Cyclic voltammograms of SB1, SB2, and SB3 measured in CH2Cl2 solution
containing 0.1 M (Bu4N)PF6 at scan rate of 100 mV sꢁ1. Pt and saturated calomel
electrode (SCE) were used as the counter electrode and as the reference electrode,
respectively.
Fig. 4. LUMO and HOMO energy levels of SB1, SB2 and SB3 dyes with respect to the
conduction band (CB) edge and the valence band (VB) edge energies of TiO2 and the
Iꢁ3 =Iꢁ redox potential.

94
S.-B. Ko et al. / Dyes and Pigments 94 (2012) 88e98
Fig. 5. Frontier molecular orbitals of SB1, SB2 and SB3.
The photovoltaic characteristic parameters of JSC, VOC, FF and
h
are
discussed in detail with impedance spectra. Fig. 7 shows IPCE
spectra for the DSSCs sensitized with SB1, SB2 and SB3 and those
are compared with that of N719 and TA-DM-CA-sensitized DSSC.
The SB1, SB2 and SB3 dyes show higher IPCE in the region from
350 nm to 600 nm than N719. The IPCE maximum values reach 78%
at around 480 nm for all three organic dyes, while N719 has
maximum IPCE of about 70% at 540 nm. The SB1 sensitizer shows
slightly higher IPCE at wavelength ranging from 550 to 700 nm
summarized in Table 2. Under standard AM 1.5G 1 sun (100 mW/
cm2) illumination condition, VOC increases from 0.706 V (SB1) to
0.731 V (SB2) and to 0.757 V (SB3), while little change in JSC
(11.07 mA/cm2 for SB1, 10.71 mA/cm2 for SB2 and 10.90 mA/cm2 for
SB3) is observed. Due to the increased VOC, the overall conversion
efficiency increases from 4.76% (SB1) to 4.83% (SB2) and to 5.21%
(SB3). Compared to the SB series, the efficiency of N719 is slightly
higher due to slightly higher VOC and FF. For the case of TA-DM-CA,
JSC of 9.37 mA/cm2 is lower than those of the alkyloxy substituted
SB series, which is due to larger HOMOeLUMO gap energy, asso-
ciated with shorter absorption wavelength (see IPCE in Fig. 7). It is
noted that VOC increases from 0.706 V to 0.757 V as the substituted
alkyloxy chain length increase from methyl to hexyl and VOC of SB3
is close to that of N719. The enhancement of VOC are probably due to
reduced recombination ability at TiO2/electrolyte interface by
substitution of insulating alkyloxy chains on the dyes, which will be
than SB2 and SB3, which is responsible for slightly higher JSC
.
Although IPCE of N719 is lower than those of SB series in the
wavelength ranging from 350 nm to 600 nm, its JSC is similar to
those of SB dyes due to higher IPCE in the wavelength region from
600 nm to 750 nm.
Electrochemical impedance spectroscopy (EIS) analysis was
performed to elucidate the correlation between VOC and the
interfacial charge-transfer kinetics. Fig.
8 shows impedance
spectra of SB1, SB2, and SB3-sensitized solar cells measured under
one sun illumination at open-circuit condition. Typical Nyquist
diagram of DSSC is known to be composed of three semicircles for
angular frequency (
semicircle (u1: higher than 103 Hz) is attributed to charge transfer
at the Pt/electrolyte interface, the second large semicircle (u2
u) in the impedance function: the first small
:
around 10 Hz) to charge transfer at the TiO2/dye/electrolyte
interface, and the third small semicircle (u3: around 0.1 Hz) to the
carrier transport by ions within the electrolyte [38]. As shown in
Fig. 8, little difference is observed in the small u1 semicircles,
which is due to the fact that the same Pt and electrolyte are used.
Table 2
Open-circuit voltage (VOC), short-circuit photocurrent density (JSC), fill factor (FF)
and efficiency (h
) of the DSSCsa based on SB1, SB2, SB3, N719 and TA-DM-CA.
Dye
VOC (V)
JSC (mA/cm2)
FF
h (%)
SB1
SB2
SB3
N719
TA-DM-CA
0.706
0.731
0.757
0.771
0.730
11.07
10.71
10.90
10.89
9.37
0.61
0.62
0.63
0.69
0.75
4.76
4.83
5.21
5.79
5.10
Fig. 6. Photocurrent densityevoltage characteristics of the DSSCs based on SB1, SB2,
a
SB3 N719 and TA-DM-CA. TiO2 film thickness was 5.0
m
m. Measurements were per-
Active area was 0.45 cm2. An aperture black mask was used during measure-
formed under AM 1.5 G 1sun light illumination (100 mW/cm2) and the cells were
covered with aperture masks during measurement. Electrolyte used was composed of
0.7 M 1-methyl-3-propylimidazolium iodide (MPII), 0.03 M iodine (I2), 0.10 M lithium
iodide (LiI) and 0.5 M 4-tert-butylpyridine (TBP) in a mixture of acetonitrile and
valeronitrile (volume ratio, 85:15).
ment at AM 1.5G 1 sun light illumination (100 mA/cm2). TiO2 film thickness was
5.0 mm. Electrolyte used was composed of 0.7 M 1-methyl-3-propylimidazolium
iodide (MPII), 0.03 M iodine (I2), 0.10 M lithium iodide (LiI) and 0.5 M 4-tert-
butylpyridine (TBP) in a mixture of acetonitrile and valeronitrile (volume ratio,
85:15).

S.-B. Ko et al. / Dyes and Pigments 94 (2012) 88e98
95
Table 3
Electrochemical impedance data for DSSCs based on SB1, SB2 and SB3.a
Sensitizer
Rr
(U
)
fp (Hz)
sr (ms)b
SB1
SB2
SB3
14.6
15.3
16.8
12.07
9.10
6.87
13.19
17.49
23.17
a
Impedance was measured under one sun illumination with a frequency range of
0.1 Hze100 kHz at open-circuit voltage. TiO2 film thickness for impedance study
was 5.0
m
m.
b
sr was calculated by 1/2pfp. Rr, fp and sr denote the charge-transfer resistance at
the TiO2/electrolyte interface, the peak frequency at the maximum imaginary
resistance of the u2: and the time constant for recombination, respectively.
Table 4
Photovoltaic performance of DSSCs based on the SB1eSB3.a
Sensitizer
VOC (V)
JSC (mA/cm2)
FF
h (%)
SB1
SB2
SB3
N719
TA-DM-CA
0.707
0.714
0.745
0.724
0.707
12.62
13.22
12.83
15.74
10.96
0.61
0.64
0.64
0.67
0.73
5.43
6.02
6.12
7.60
5.65
Fig. 7. Incident photon-to-current conversion efficiency (IPCE) as
wavelength for the DSSCs based on SB1, SB2, SB3 N719 and TA-DM-CA. TiO2 film
thickness was 5.0 m.
a function of
m
a
Performance of DSSCs with 0.45 cm2 working area were obtained under AM 1.5
G 1 sun light illumination (100 mA/cm2). A black aperture mask was used during
measurement. Double-layer TiO2 film with thickness of 11.5 mm was used. Elec-
trolyte used was composed of 0.7 M MPII, 0.03 M I2, 0.10 M LiI, and 0.5 M TBP in
a mixture of acetonitrile and valeronitrile (volume ratio, 85:15).
On the other hand, there is substantial difference in the large u2
semicircles, which indicates that charge-transfer behavior
between TiO2 and electrolyte is significantly altered and this is
likely to be due to surface modification with different dyes. The
resistances at the TiO2/electrolyte interface (Rr) are obtained by fit
the measured data to the equivalent circuit and the time constants
for recombination (
data at the maximum imaginary resistance of the u2, which are
listed in Table 3. Rr increases from SB1 (14.6 ), SB2 (15.3 ) to SB3
(16.8 ), which indicates that charges are not readily transferred
sr) are evaluated using the peak frequency (fp)
U
U
U
from TiO2 to electrolyte as alkyloxy chain length increases. The fp
value decreases in the order SB1 (12.07 Hz) > SB2 (9.10 Hz) > SB3
(6.87 Hz), which implies that time for recombination increases in
the same order because sr is inversely proportional to fp. The sr
values of SB1, SB2 and SB3 are determined to be 13.19 ms,
17.49 ms and 23.17 ms from the relation sr ¼ 1/2p p
f [39], which
can explain the gradual increase in VOC going from SB1 to SB3. As
shown in Fig. 8b, the differences in the maximum frequency,
associated with lifetime, of SB series sensitized DSSCs are
Fig. 8. (a) Nyquist plots and (b) Bode phase plots for the DSSCs based on SB1 (-), SB2
(C), and SB3 (:) measured under one sun illumination. TiO2 film thickness for
Fig. 9. Photocurrent densityevoltage characteristics of DSSCs based on SB3. TiO2 film
impedance study was 5.0
mm. The inset in (a) shows an equivalent circuit for fitting
thickness was 11.5 mm. The inset shows the IPCE spectra of the DSSCs as a function of
(ꢁ). Rs is the series resistance, Rr is the charge-transfer resistance at the TiO2/elec-
trolyte interface, Cm is a constant phase element representing the chemical capacitance,
RPt is the charge-transfer resistance at the counter electrode/electrolyte interface, CPt is
the interfacial capacitance at the counter electrode/electrolyte interface.
the wavelength of the light. Measurements were performed under AM 1.5 G light
illumination (100 mW/cm2) and the cells were covered with aperture black masks
during measurement. Electrolyte: 0.7 M MPII, 0.03 M I2, 0.10 M LiI, and 0.5 M TBP in
a mixture of acetonitrile and valeronitrile (volume ratio, 85:15).

96
S.-B. Ko et al. / Dyes and Pigments 94 (2012) 88e98
a double-layer TiO2 film with a light scattering overlayer yielded up
to 6.12%.
Acknowledgment
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Ministry of Education, Science
and Technology (MEST) of Korea under contracts No. 2011-0016441,
2010-0028821, 2011-0001055 (ERC program) and R31-2008-10029
(WCU program) and the Korea Institute of Energy Technology
Evaluation and planning (KETEP) grant funded by the Ministry of
Knowledge Economy under contract No. 20103020010010.
Appendix. Supplementary material
Supplementary data related to NMR spectroscopy data for
compound 1-SB3 can be found online at http://www.sciencedir-
ect.com/science/article/pii/, doi:10.1016/j.dyepig.2011.10.014.
Fig. 10. Variation of photovoltaic parameters (JSC, VOC, FF and h) with aging time for the
devices sensitized with SB1, SB2 and SB3. A double-layer TiO2 film was employed and
an electrolyte used was composed of 0.7 M MPII, 0.03 M I2, 0.10 M LiI, and 0.5 M TBP in
a mixture of acetonitrile and valeronitrile (volume ratio, 85:15).
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5
mm-thick TiO2 layer in Table 2, the light scattering overlayer
contained TiO2 film improves JSC by about 14e23% without dete-
rioration of VOC. As a result, the conversion efficiency increases
from 4.76% to 5.43% for SB1, from 4.83% to 6.02% for SB2 and from
5.21% to 6.12% for SB3. In Fig. 9, photocurrent-voltage curve of the
SB3-sensitized DSSC with double-layer TiO2 film is depicted along
with IPCE.
The stability of the synthesized organic dyes was investigated,
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