Synthesis and characterization of squaric acid based NIR dyes for their application towards dye-sensitized solar cells

Article abstract of DOI:10.1016/j.jphotochem.2010.04.015

Synthesis of novel cyan-colored sensitizing dyes bearing squaric acid core and 2,3,3-trimethyl-indole as terminal moieties has been conducted in order to fabricate dye-sensitized solar cells based on nanoporous TiO₂. It has been found that position of -COOH functionality utilized for anchoring with TiO₂ surface has a marked effect on solar cell performance. Carboxylic acid group directly substituted to indole ring gave about 5-fold higher conversion efficiency as compared to the dye when it was substituted in alkyl chain of the indole ring. Efficiency has been found to be hampered due to aggregation and enhancement in the efficiency was observed when dyes were used with chenodeoxycholic acid (CDCA). Using CDCA and long alkyl substituent at the N-position of indole ring to prevent aggregation and enhanced TiO₂ surface passivation, respectively, has achieved a solar conversion efficiency of 3.15% with a short circuit current density of 7.26 mA/cm², an open circuit voltage of 0.64 V and a fill factor of 0.68 for SQ-5 under standard AM 1.5 solar irradiation.

Full text of DOI:10.1016/j.jphotochem.2010.04.015

Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 23–29
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis and characterization of squaric acid based NIR dyes for their
application towards dye-sensitized solar cells
Takafumi Inouea, Shyam S. Pandeya,∗, Naotaka Fujikawaa, Yoshihiro Yamaguchib, Shuji Hayasea
a Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu, Kitakyushu, Japan
b Nippon Steel Chemical Company Limited, Nakabaru, Tobata, Kitakyushu, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of novel cyan-colored sensitizing dyes bearing squaric acid core and 2,3,3-trimethyl-indole as
terminal moieties has been conducted in order to fabricate dye-sensitized solar cells based on nanoporous
TiO2. It has been found that position of –COOH functionality utilized for anchoring with TiO2 surface has
a marked effect on solar cell performance. Carboxylic acid group directly substituted to indole ring gave
about 5-fold higher conversion efficiency as compared to the dye when it was substituted in alkyl chain
of the indole ring. Efficiency has been found to be hampered due to aggregation and enhancement in
the efficiency was observed when dyes were used with chenodeoxycholic acid (CDCA). Using CDCA and
long alkyl substituent at the N-position of indole ring to prevent aggregation and enhanced TiO2 surface
passivation, respectively, has achieved a solar conversion efficiency of 3.15% with a short circuit current
density of 7.26 mA/cm2, an open circuit voltage of 0.64 V and a fill factor of 0.68 for SQ-5 under standard
AM 1.5 solar irradiation.
Received 15 December 2009
Accepted 21 April 2010
Available online 29 April 2010
Keywords:
Dye sensitization
Solar cells
Aggregation
Squaraine dyes
CDCA
Nanocrystalline TiO2
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
the hindrance for fabrication of hybrid DSSC with ruthenium based
dyes for their further enhancement in the conversion efficiency.
Dye-sensitized solar cells (DSSCs) are one of the cheap alter-
natives to silicon solar cells for efficient harvesting of solar energy
into electrical energy reaching photoconversion efficiency over 10%
having efficient light absorption in the visible region of the solar
spectrum [1,2]. Further enhancement in the conversion efficiency
is possible by extending the absorption of sensitizing dyes in near
infrared (NIR) regions. One of the most important and challenging
factors attributed to the lower efficiency of the DSSCs as com-
pared to silicon solar cells, is the lower optical absorption window
(400–800 nm) of organic dyes which is smaller than the optical
absorption window of crystalline silicon (500–1100 nm). A number
of organic dyes giving high photoconversion efficiency similar to
inorganic ruthenium based dyes have been reported but their lower
optical absorption window offers stumbling block towards further
increase in the conversion efficiency utilizing hybrid or tandem
DSSC architecture [3,4]. In an attempt towards development of NIR
organic dyes, Campbell et al. [5] have reported efficient porphyrin
based sensitizers giving photoconversion efficiency between 5%
and 7% having photon harvesting window less than 700 nm offers
Recently, organic NIR sensitizing dyes bearing phthalocyanine and
squaric acid core having photon harvesting beyond 700 nm have
attracted a good deal of attention by material scientists owing to
their potentiality towards utilization in the fabrication of tandem
and hybrid DSSC [6–9].
Recently, Inakazu et al. have reported the extension of opti-
cal absorption window by selective adsorption of two dyes on
nanoporous TiO2 under pressurized CO2 atmosphere leading to the
enhancement in photoconversion efficiency [10]. Broadening in the
light absorption in the extended wavelength region has also been
reported using a mixture of organic dyes which do not create any
hindrance with their sensitization properties [11]. It is believed that
in dye cocktail two dyes are supposed to be arranged in a random
distribution of dyes in the nanoporous TiO2 layer. In an interesting
report based on their investigation using confocal laser fluores-
cence microscope, Noma et al. have emphasized that it is possible
to make a dye double layer hybrid DSSC using a dye cocktail by a
judicious selection of two dyes under investigation leading to the
light absorption in extended wavelength region [12]. Such studies
have proved the potentiality of search for the more efficient sensi-
tizers exhibiting light absorption in the near infrared wavelength
region.
∗
Corresponding author at: Kyushu Institute of Technology, Graduate School
Owing to its higher stability, dyes based on squaric acid possess-
ing a sharp and intense absorption band in the near infrared (NIR)
region have attracted their attention towards application as sensi-
of Life Science and Systems Engineering, 2-4 Hibikino, Wakamatsu, Kitakyushu,
Fukuoka 808-0196, Japan. Tel.: +81 93 695 6044; fax: +81 93 695 6005.
E-mail address: shyam@life.kyutech.ac.jp (S.S. Pandey).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.04.015

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T. Inoue et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 23–29
tizer in electrography, optical data storage and non-linear optical
applications [13,14]. This class of dyes are able to sensitize the
large band gap semiconductors leading to their application as sensi-
tizer for photovoltaic applications, however, their lower efficiency
of sensitization in DSSCs [15] need more logical dye design and
synthesis. Das and co-workers [16] have demonstrated that unsym-
metrical squaraine dyes shows better sensitization performance
for DSSC as compared to their symmetrical dye counterpart led to
the increased interest of material science community. Squaric acid
based dyes are well known to form aggregates leading to decreased
efficiency of sensitization resulting into decreased DSSC perfor-
mance [17,18]. Present article deals with synthesis and application
of some novel squaric acid dyes as sensitizer for the DSSC based on
nanoporous TiO2. An attempt has also been made to delineate the
implication of structure of these novel sensitizers on photoconver-
sion efficiency of DSSC based on such dyes.
2. Experimental
2.1. Materials, instruments and methods
All the chemicals for synthesis or solvents are of analytical or
spectroscopic grade and used as received without further purifica-
tion. Synthesized SQ-dyes and dye intermediates under investiga-
tion were analyzed by high performance liquid chromatography
(JASCO) for purity, MALDI-TOF-mass/FAB-mass spectrometry in
positive ion monitoring mode and nuclear magnetic resonance
spectroscopy (NMR, 500 MHz) for structural elucidation. Electronic
absorption spectroscopic investigations in solution and thin film
adsorbed on TiO2 surface was conducted using UV–visible spec-
trophotometer (JASCO model V550). Optimized geometries of final
dyes along with the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) were calculated
without any symmetry restriction at B3LYP/6-311G level of calcu-
lation using Gaussian 03 program package.
Scheme 1. Synthesis of symmetrical squaraine dyes.
acid (80 ml), sodium acetate (5.5 g; 67 mmol) and 3-methyl-2-
butanone (4.45 g; 51.5 mmol) were added. Reaction mixture was
refluxed at 120 ◦C for 8 h leading to brown suspension. Acetic acid
was evaporated followed by addition of 9:1 water methanol mix-
ture on ice-bath leading to precipitation. Residue was filtered and
dried giving 3.7 g of titled compound as off white powder in 56%
2.2. Synthesis of SQ-dyes and dye intermediates
Side chain carboxy functionalized indole derivative 1-(2-
carboxyethyl)-2,3,3-trimethyl-3H-indolium iodide (2a) was syn-
thesized following the method by Chen et al. [19] while aromatic
ring carboxy functionalized indole derivative 2,3,3-trimethyl-
3H-indole-5-carboxylic acid (1b) was synthesized following the
methodology reported by Pham et al. [20]. Squaric acid based
symmetrical squaraine dyes (SQ-1, SQ-2, SQ-3, SQ-5) and unsym-
metrical squaraine dye (SQ-4) used in the present study have been
synthesized following the methodology adopted by Oswald et al.
[21] as shown in Schemes 1 and 2, respectively.
2.2.1. Synthesis of
1-(2-carboxyethyl)-2,3,3-trimethyl-3H-indolium iodide [2a]
In a round bottom flask fitted with condenser, 2,3,3-trimethyl-
3H-indole (1.61 g; 10 mmol) and 3-iodopropanoic acid (4.0 g;
20 mmol) were added followed by 20 ml of 1,2-dichlorobenzene.
Reaction mixture was heated at 110 ◦C for 16 h under N2 atmo-
sphere. Reaction progress was monitored by TLC and HPLC.
Reaction mixture was cooled and then filtered. Precipitate was
finally washed by ample ethyl acetate which finally gave 1.7 g of
pinkish white titled compound in 48% yield. As confirmed by HPLC
product purity was >98%, MALDI-TOF-mass (measured = 232.67,
calculated = 232.13).
2.2.2. Synthesis of 2,3,3-trimethyl-3H-indolium-5-carboxylic acid
[1b]
In a round bottom flask fitted with condenser and N2 purg-
ing, 4-hydrazinobenzoic acid (5.0 g; 32.85 mmol), glacial acetic
Scheme 2. Synthesis of unsymmetrical squaraine dye SQ-4.

T. Inoue et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 23–29
25
yield. HPLC analysis of product suggests that compound was 100%
pure. FAB-mass (measured 203, calculated 203.09) and 1H NMR
(500 MHz, CDCl3): d/ppm = 7.99 (s, H-4), 7.93 (d, J = 9.0 Hz, H-6),
7.59 (d, H = 8.0 Hz, H-7), 2.23 (s, 3H, H-10), 1.28 (s, 6H, H-11+12)
verifies the successful synthesis of the compound. For NMR chart
see Supporting information S5.
NMR). SQ-2, yield 58%, purity 98%, MALDI-TOF-mass (calculated
540.22 and observed 541.49 (M+H)). SQ-3, yield 58%, purity 98%,
MALDI-TOF-mass (calculated 540.22 and observed 541.49 (M+H))
see Fig. S8 for NMR. SQ-5, yield 46%, purity 97%, MALDI-TOF-
mass (calculated 708.41 and observed 708.48 (M+H)) for NMR see
Fig. S10.
Unsymmetrical squaraine dye SQ-4 was synthesized using
semi-squaraine ester (8) and compound (2b) as follows: in a round
bottom flask fitted with condenser, 466 mg (1 mmol) of compound
(8) was dissolved in 10 ml ethanol followed by 0.54 ml (1.5 mmol)
of aqueous NaOH. Reaction mixture was refluxed for 30 min which
was then cooled followed by addition of 1N HCl (3.6 ml) giving
compound (9). Solvent was then removed at rotary evaporator
followed by addition of 359 mg (1 mmol) of compound (2b) and
20 ml of 1-butanol:toluene mixture (1:1, v/v). Reaction mixture
was refluxed for 18 h using Dean-Stark trap. Reaction mixture was
cooled, solvent was evaporated and product was purified by silica
gel column chromatography using chloroform:methanol as eluting
solvent. 350 mg of final titled compound was obtained as blue solid
in 97% purity as confirmed by HPLC in 64% yield. MALDI-TOF-mass
(calculated 497.24 and observed 497.76) confirms the successful
synthesis of the unsymmetrical dye SQ-4. For NMR chart and anal-
ysis see Fig. S9.
2.2.3. Synthesis of
5-carboxy-2,3,3-trimethyl-1-ethyl-3H-indolium iodide [2b]
820 mg (4 mmol) of 2,3,3-trimethyl-3H-indole-5-carboxylic
acid and 3.21 g (20 mmol) of 1-iodoethane were dissolved in 60 ml
of dehydrated acetonitrile and reaction mixture was refluxed for
18 h under nitrogen. After completion of the reaction, solvent was
evaporated and the crude product was washed with ample diethyl
ether giving 1.14 g of titled compound as off white powder in 79%
yield having 98% purity as confirmed by HPLC. FAB-mass (measured
232.0; calculated 232.13).
2.2.4. Synthesis of
5-carboxy-2,3,3-trimethyl-1-octyl-3H-indolium iodide [2c]
617 mg (3 mmol) of 2,3,3-trimethyl-3H-indole-5-carboxylic
acid and 2.16 g (9 mmol) of 1-iodooctane were dissolved in 30 ml of
dehydrated acetonitrile and reaction mixture was refluxed for 72 h
under nitrogen. After completion of the reaction, solvent was evap-
orated and the crude product was washed with ample diethyl ether
giving 956 mg (2.16 mmol) of titled compound as off white powder
in 72% yield having 98% purity as confirmed by HPLC. FAB-mass
(measured 316.0; calculated 316.23).
2.3. DSSC fabrication and measurement of cell performance
DSSC were fabricated using Ti-Nanoxide D paste (Solaronix SA)
which was coated on a Low E glass (Nippon Sheet Glass Co., Ltd.) by
a doctor blade. The substrate was then baked at 450 ◦C to fabricate
TiO2 layers of about 12 ␮m thickness. The substrate was dipped
in the solution containing dye in the presence and/or absence
of CDCA. The dye concentration was fixed to be 0.25 mM while
CDCA concentration was 2.5 mM. A Pt sputtered SnO2/F layered
glass substrate was employed as the counter electrode. Electrolyte
(WWS30) containing LiI (500 mM), iodine (50 mM), t-butylpyridine
(580 mM), MeEtIm-DCA (ethylmethylimidazolium dicyanoimide)
(4:6, wt/wt) (600 mM) in acetonitrile, was used to fabricate the
DSSC. A Himilan film (Mitsui-DuPont Polychemical Co., Ltd.) of
25 ␮m thickness was used as a spacer. The cell area was 0.25 cm2.
Solar cells were evaluated using a photo-mask on the solar cell
in order to remove the effect of the optical reflection under the
irradiation of 100 mW/cm2 at AM 1.5.
2.2.5. Synthesis of 2,3,3-trimethyl-1-ethyl-3H-indolium iodide [7]
3.17 g (20 mmol) of 2,3,3-trimethyl-3H-indole and 4.68 g
(30 mmol) of 1-iodoethane were dissolved in 100 ml of dehydrated
acetonitrile and reaction mixture was refluxed for 24 h under nitro-
gen. After completion of the reaction, solvent was evaporated and
the crude product was washed with ample diethyl ether giving
5.7 g of titled compound as whitish powder in 91% yield having
99% purity as confirmed by HPLC. FAB-mass (measured 188.0; cal-
culated 188.13).
2.2.6. Synthesis of 3-butoxy-4-[(1-ethyl-1,3-dihydro-3,3-
dimethyl-2H-indole-2-ylidene)methyl]-3-cyclobutene-1,2-dione
[8]
In a round bottom flask fitted with condenser, 1.26 g (4 mmol)
of [7], 900 mg (4 mmol) of 3,4-dibutoxy-3-cyclobutene-1,2-dione,
0.8 ml of triethylamine were dissolved in 6 ml butanol. Reaction
mixture was heated at 70 ◦C for 1 h leading to green solution. Sol-
vent was removed at rotary evaporator and product was purified
by column chromatography (silica gel) with ethyl acetate and hex-
ane as eluent giving 920 mg of titled compound in 50% yield and
99% purity as confirmed by HPLC. Compound was confirmed by
MALDI-TOF-mass [calculated 339.18 and measured 340.60 (M+H)].
3. Results and discussion
3.1. Effect of substituents in sensitizing dyes on DSSC performance
Fig. 1 exhibits the photovoltaic performance of DSSC based
on symmetrical squaraine dyes being used as sensitizer for
nanoporous TiO2 electrodes. Linking of the dye through the anchor-
ing group to the TiO2 surface is one of the important requirements
for facile electron injection from photoexcited dye to the conduc-
tion band of TiO2. As clearly evident from this figure that SQ-2
having anchoring functional group –COOH in alkyl side chain sub-
stituted at N-position shows the lowest photoconversion efficiency
as compared to SQ-1 and SQ-3 bearing carboxylic acid function-
ality directly substituted in the indole ring. It has been widely
reported that delocalization and diversion of electron density of
LUMO towards the anchoring group leads to facile photo induced
electron transfer from the dye to TiO2 electrode which ultimately
results in the improved conversion efficiency [22,23]. In order to
have more in depth insight about the photo physical properties
of these sensitizers, TD-DFT calculations were performed using
B3LYP functional and 6-311g basis set as implemented in G03 pro-
gram package. The calculated MO picture as shown in Supporting
2.2.7. Synthesis of squaraine dyes
Symmetrical squaraine dyes SQ-1, SQ-2, SQ-3 and SQ-5 were
synthesized using corresponding trimethyl-indolium iodide salt
(1 equiv.) and squaric acid (0.5 equiv.) in 1-butanol:toluene mixture
(1:1, v/v). Reaction mixture was refluxed for 18 h using Dean-Stark
trap. After completion of reaction, reaction mixture was cooled, sol-
vent was evaporated and product was purified by silica gel column
chromatography using chloroform:methanol as eluting solvent.
Only in the case of SQ-2 bearing carboxylic acid in substituted alkyl
chain, product was confirmed as butyl ester of the dye which was
then hydrolyzed to get the free acid. SQ-2 ester: yield 42%, purity
98%, FAB-mass (calculated 652.25 and observed 652.0). For NMR
chart of SQ-2 ester see Fig. S7. SQ-1, yield 39%, purity 98%, MALDI-
TOF-mass (calculated 484.16 and observed 484.21) (see Fig. S6 for

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T. Inoue et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 23–29
Fig. 1. Photovoltaic performance symmetrical squaraine dyes SQ-1, SQ-2 and SQ-3
Fig. 3. Effect of molecular asymmetry on the photovoltaic performance of symmet-
rical (SQ-3) and unsymmetrical (SQ-4) squaraine dye based on DSSC.
under 100 mA/cm2 simulated solar irradiation at AM 1.5.
that SQ-1 exhibits a pronounced absorption in lower wavelength
region even in solution state which has also been well reflected
in the action spectrum. This could be attributed to the formation
blue shifted H-aggregates in the solid state upon adsorption of dyes
on nanoporous TiO2. Kim et al. [28] have also emphasized that
some squaraine surfactants easily form blue shifted H-aggregates
on SnO2 surfaces absorbing around 500 nm wavelength region. It
has also been reported that squaraine dyes are prone to easily form
H-aggregates and such aggregates result into the decreased sensiti-
zation efficiency of DSSCs [29]. Therefore, such H-aggregates in case
of SQ-1 could be attributed to the decreased efficiency as compared
to SQ-3 where the presence of ethyl group hinders them to some
extent leading to relatively enhanced IPCE in higher wavelength
region (500–700 nm).
informations (S1 and S2) clearly corroborates that HOMO is mainly
delocalized over central squaric acid core while electronic orbital
delocalization patterns are different for SQ-2 as compared to SQ-1
and SQ-3. LUMO is distinctively delocalized over carboxylic func-
tionality in the case of both of the SQ-1 and SQ-3, which is absent
in the SQ-2. Probably this could be responsible for the inefficient
electron transfer from excited dye to TiO2 leading to decreased
conversion efficiency. Similar kind of low conversion efficiency for
sensitizers having alkyl side chain carboxyl functionalized sensi-
tizers have also been observed by Matsui et al. [24] and Li et al.
[25].
Photo action spectra of DSSC along with the solution phase
UV–visible absorption spectra of sensitizers SQ-1, SQ-2 and SQ-
3 have been shown in Fig. 2. Action spectra are symbatic with
the absorption spectra with spectral broadening due to aggre-
gate formation in solid state. Action spectra of SQ-1 and SQ-3
differ only in the presence and absence of alkyl substituent at N-
position of indole ring and introduction of alkyl group leads to
enhanced conversion efficiency for SQ-3 as compared to that of
SQ-1. Organic dyes with extended ␲-conjugation like phthalocya-
nine and squaraines are well known for the aggregate formation
and derivatives of cholic acid have been found to break such aggre-
gates leading to enhanced efficiency [26,27]. It is interesting to note
3.2. Effect of asymmetry of dyes on DSSC performance
To study the role of dye asymmetry on photoconversion effi-
ciency, we designed SQ-3 and SQ-4 having carboxylic anchoring
group directly attached to aromatic ring. Current–voltage (I–V)
characteristics of DSSC based on these sensitizers under standard
global AM 1.5 solar irradiation have been shown in Fig. 3. A perusal
of this figure and Table 1 showing photovoltaic parameters clearly
indicates that unsymmetrical dye SQ-4 shows enhanced power
conversion efficiency (2.43%) as compared to its symmetrical dye
counterpart SQ-3 (1.46%). It is also interesting to note that the main
factor for efficiency enhancement is short circuit current density
(Jsc) since other cell parameters are nearly the same. Yum et al.
[30] have also emphasized that unidirectional flow of electrons
from sensitizer to semiconductor surface is responsible for efficient
electron transfer from photo excited dye to TiO2 conduction band.
Table 1
Current–voltage characteristics for DSSC fabricated with various squaraine dyes
under 100 mA/cm2 simulated solar irradiation at global 1.5 condition.
Dye
Voc (V)
Jsc (mA/cm2)
FF
Efficiency (%)
SQ-1
SQ-2
SQ-3
SQ-4
0.47
0.49
0.56
0.57
0.60
0.55
0.62
0.64
1.44
1.20
4.03
6.57
5.09
2.03
5.79
7.26
0.63
0.62
0.64
0.65
0.69
0.68
0.69
0.68
0.43
0.36
1.46
2.43
2.09
0.76
2.49
3.15
SQ-5
SQ-2 DCA
SQ-3 DCA
SQ-5 DCA
Fig. 2. IPCE curves for DSSC fabricated with SQ-1, SQ-2 and SQ-3 (lines with symbol)
and electronic absorption spectra of these sensitizers in DMF solution (thin line).

T. Inoue et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 23–29
27
Fig. 4. IPCE curves for DSSC fabricated with SQ-3, and SQ-4 (lines with symbol) and
electronic absorption spectra of these sensitizers in DMF solution (thin line).
Fig. 5. Photovoltaic performance symmetrical squaraine dye bearing shorter (SQ-3)
and longer (SQ-5) alkyl substituent under 100 mA/cm2 simulated solar irradiation
at AM 1.5.
Therefore, it seems that unsymmetrical dye SQ-4 provides such a
more effective electron transfer environment as compared to that
of symmetrical SQ-3 leading to enhanced Jsc
.
reasoned with the upward shift of the conduction band edge of
TiO2. Based on surface potential measurement using Kelvin-probe
method, Pandey et al. [33] have reported that the surface potential
of squaraine dyes was also found to increase with the increasing
alkyl chain length and could, therefore, be attributed to the increase
Photocurrent action spectra DSSC along with UV–visible elec-
tronic absorption spectra of SQ-3 and SQ-4 in DMF solution is
shown in Fig. 4. It clearly shows that although these sensitizers are
very similar in electronic absorption but there is a marked improve-
ment in incident monochromatic photon-to-electron conversion
efficiency (IPCE) for unsymmetrical sensitizer SQ-4 as compared
to SQ-3 which could be attributed to the enhanced overall power
conversion efficiency as shown in Fig. 3. A key requirement for the
facile electron transfer is a good electronic coupling between LUMO
of the dye and TiO2 conduction band which can be obtained by
strong electronic conjugation between sensitizer core and anchor-
ing group. Structure of SQ-2 as shown in Scheme 1 indicates that
the lack of such conjugation between dye core and anchoring
group could be responsible for the very small conversion effi-
ciency provided by this sensitizer. Both of sensitizers, SQ-3 and
SQ-4 meet with this requirement leading to enhanced efficiency
as compared to SQ-2. TD-DFT calculated MO pictures (see S3 and
S4 in Supporting information) show distinctively different electron
density distribution for SQ-3 and SQ-4. LUMO picture for unsym-
metrical SQ-4 show a clear unidirectional shift of electron density
from the opposite end of the molecule towards the anchoring car-
boxylic acid group which is bidirectional for the symmetrical dye
SQ-3. Such a unidirectional flow of electron density is expected
for favorable charge separation and facile electron injection
[30,31].
in Voc
.
Enhancement of Jsc was elucidated with IPCE measurement
after DSSC fabrication as shown in Fig. 6 and the extent of dye
adsorption on the TiO2 surface. As clearly evident from Fig. 6
that there is spectral broadening of IPCE along with the increase
of its magnitude could be attributed to the enhanced Jsc for
the SQ-5 having the relatively longer alkyl substituent. Recently,
Ogomi et al. [34] have demonstrated that increase in the extent
of adsorbed dyes on TiO2 surface leads to enhanced surface trap
passivation leading to increased Jsc. It is interesting to note that
the extent of the dye adsorbed on TiO2 surface was found to
be larger (16 nmol cm−2 ␮m−1) for SQ-5 as compared to that
of SQ-3 (10 nmol cm−2 ␮m−1) which supports the increased Jsc
for squaraine dyes having longer alkyl chain length due to the
enhanced TiO2 surface trap passivation.
3.3. Effect of alkyl chain length on DSSC performance
A perusal of the photovoltaic performance of DSSC based on
squaraine dyes as shown in Fig. 5 and Table 1 clearly corrobo-
rates that enhanced conversion efficiency was observed for the
SQ-dye having the long alkyl chain length. This increase in effi-
ciency was associated with the increase in both of the Voc as
well as Jsc. Koumura et al. have also advocated the importance
of the presence of longer alkyl chain in their MK dyes towards
efficiency enhancement [32]. Enhancement of the Voc as function
of alkyl chain length in these squaraine dyes was elucidated by
dark I–V characteristics. Shift of onset of the dark currents towards
higher voltage indicates the suppression of recombination leading
to improved Voc for squaraine bearing longer alkyl chains. Recently,
Sakaguchi et al. [27] have reported the increase in the Voc with
the increase in surface potential for different sensitizer, which was
Fig. 6. Photo action spectra for DSSC fabricated with SQ-3 (filled triangle) and SQ-5
(open circle) and electronic absorption spectra (thin line) of these sensitizers in DMF
solution.

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T. Inoue et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 23–29
to the dye aggregates, monomers can inject electrons to TiO2 more
efficiently [39]. A perusal of dye structures of SQ-dyes as shown
in Scheme 1 clearly indicates that SQ-3 having shorter alkyl sub-
stituent (ethyl) is relatively prone to form ␲-stacked dye aggregates
as compared to that SQ-5 where the long alkyl chain (octyl) resist
the easy dye aggregation. In the presence of CDCA these ␲-stacked
aggregates get broken leading to facile electron injection yield and
thus Jsc
.
A perusal of action spectrum as shown in Fig. 6 (solid line) clearly
indicates that there is a clear enhancement of IPCE maximum upon
the introduction of CDCA for the dyes SQ-2, SQ-3 and SQ-5 as com-
pared to IPCE for these dyes in the absence of CDCA as shown in
Figs. 2 and 6, which could be attributed enhanced Jsc. Following the
similar trend as observed in the I–V characteristics, there was an
enhancement of IPCE for SQ-2 (6–21%), SQ-3 (22–49%) and SQ-5
(25–58%) in the presence of CDCA at 660 nm. The increased IPCE
for SQ-2 even after DCA introduction was much smaller as com-
pared to the SQ-3 and SQ-5 indicates that in spite of ␲-aggregation
diminished electron injection from excited dye molecule to TiO2
conduction band could be attributed to be responsible for its lower
photoconversion efficiency.
Fig. 7. Photovoltaic performances (line with symbol) and action spectra of (thin
line) for symmetrical squaraine dyes SQ-2, SQ-3 and SQ-5 after DSSC fabrication in
the presence of CDCA under 100 mA/cm2 simulated solar irradiation at AM 1.5.
4. Conclusion
3.4. Effect of chenodeoxycholic acid as co-adsorbent on DSSC
performance
Squaraine core based novel organic sensitizers have been syn-
thesized and their sensitization behaviour has been evaluated by
fabricating the DSSC. Various factors such as position of anchor-
ing group, molecular asymmetry, effect of alkyl chain length and
dye aggregation on sensitization behaviour have been addressed.
It has been demonstrated that attachment of anchoring group in
the aromatic ring and creation of molecular asymmetry facilitates
the effective electron injection from photoexcited dye molecule to
the conduction band of TiO2 leading to enhanced photovoltaic per-
formance. Even in the absence of CDCA unsymmetrical dye SQ-4
shows nearly double conversion efficiency (2.43%) as compared to
its symmetrical dye counterpart SQ-3 (1.46%) having similar alkyl
chain length. Long alkyl chain substitution in the squaraine sen-
sitizers not only leads to suppression of dye aggregation but also
leads to better TiO2 surface passivation resulting in to enhanced Voc
and Jsc after DSSC fabrication.
A number of cholic acid derivatives have been frequently
employed as co-adsorbent along with sensitizing dyes for the
enhancement of efficiency of DSSC [35]. Using various cyanine dyes
Sayama et al. [36] have advocated that use of cholic acid as co-
adsorbent along with sensitizing dye leads to the decrease in the
leakage current attributed to the enhancement of their DSSC effi-
ciency. Kopidakis et al. [37] emphasized that incorporation of CDCA
is associated with upward shift of TiO2 conduction band edge lead-
ing to enhancement of Voc and thus DSSC performance based on
their transient photovoltage measurements. I–V characteristics and
DSSC efficiency parameters shown in Fig. 7 and Table 1, respec-
tively, clearly indicates that introduction of CDCA as co-adsorbent
leads to the enhancement of the over all cell efficiency by increas-
ing both the Voc and Jsc in the case of the all of the organic SQ-dyes
under investigation as compared to DSSC fabricated using these
dyes in the absence of CDCA. A similar observation has also been
made by Wang et al. [38] where both of Jsc and Voc were found to
be increased upon increasing the CDCA concentration in the case
of their coumarin based organic dyes.
Acknowledgement
Authors would like to acknowledge the financial support from
the New Energy Industrial Technology Development Organization
(NEDO), Japan.
A
careful perusal of I–V characteristics corroborates that
although both Voc and Jsc are getting improved upon introduction of
CDCA in the case of the SQ-dyes but improvement in the Jsc is much
pronounced for the dyes SQ-3 and SQ-5 having anchoring –COOH
group directly substituted in the aromatic ring as compared to that
of SQ-2 bearing side chain substituted carboxylic acid anchoring
moiety. This could be attributed to the efficient electron injection
from photoexcited SQ-5 and SQ-3 as compared to SQ-2 to the TiO2
conduction banddue to conjugationofCOOHelectrondensitytothe
aromatic ring. Yum et al. [30] have also emphasized that a strong
conjugation across the chromophoric main body and the anchor-
ing group is highly required for a good electronic coupling between
the LUMO of the dye and TiO2 conduction band. It is interesting
to note that lack of electron density on the LUMO of carboxylic
acid anchoring group in SQ-2 could be attributed to its decreased
photosensitization behavior as compared SQ-dyes having ring sub-
stituted –COOH group bearing sufficient electron density at the
anchoring group. Another role of CDCA has been reported to sup-
press the dye aggregation resulting into the suppressi−on of charge
recombination between the injected electrons and I3 ions of the
electrolyte. Khazraji et al. have also emphasized that as compared
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2010.04.015.
References
[1] M. Gratzel, Solar energy conversion by dye-sensitized photovoltaic cells, Inorg.
Chem. 44 (2005) 6841–6851.
[2] M.K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito,
B. Takeru, M. Gratzel, Combined experimental and DFT–TDDFT computational
study of photoelectrochemical cell ruthenium sensitizers, J. Am. Chem. Soc. 127
(2005) 16835–16847.
[3] G.L. Zhang, H. Bala, Y.M. Cheng, D. Shi, X.J. Li, Q.J. Yu, P. Wang, High efficiency
and stable dye-sensitized solar cells with an organic chromophore featuring a
binary ␲-conjugated spacer, Chem. Commun. (2009) 2198–2200.
[4] S. Ito, H. Miura, S. Uchida, M. Takata, K. Sumioka, P. Liska, P. Comte, P. Pechy,
M. Gratzel, High-conversion-efficiency organic dye-sensitized solar cells with
a novel indoline dye, Chem. Commun. (2008) 5194– 5196.
[5] W.M. Campbell, K.W. Jolley, P. Wagner, K. Wagner, P.J. Walsh, K.C. Gordon, L.
Schmidt-Mende, M.K. Nazeeruddin, Q. Wang, M. Gratzel, D.L. Officer, Highly

T. Inoue et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 23–29
29
efficient porphyrin sensitizers for dye-sensitized solar cells, J. Phys. Chem. C
111 (2007) 11760–11762.
[23] D.P. Hagberg, T. Marinado, K.M. Karlsson, K. Nomura, P. Qin, G. Boschloo, T.
Brinck, A. Hagfeldt, L. Sun, Tuning the HOMO and LUMO energy levels of organic
chromophores for dye sensitized solar cells, J. Org. Chem. 72 (2007) 9550–9556.
[24] M. Matsui, Y. Hashimoto, K. Funabiki, J.Y. Jin, T. Yoshida, H. Minoura, Application
of near-infrared absorbing heptamethine cyanine dyes as sensitizers for zinc
oxide solar cell, Synth. Met. 148 (2005) 147–153.
[25] C. Li, W. Wang, X.S. Wang, B.W. Zhang, Y. Cao, Molecular design of squaraine
dyes for efficient far-red and near-IR sensitization of solar cells, Chem. Lett. 34
(2005) 554–555.
[6] K. Takechi, P.V. Kamat, R.R. Avirah, K. Jyothish, D. Ramaiah, Harvesting infrared
photons with croconate dyes, Chem. Mater. 20 (2008) 265–272.
[7] P.Y. Reddy, L. Giribabu, C. Lyness, H.J. Snaith, C. Vijaykumar, M. Chan-
drasekharam, M. Lakshmikantam, J.H. Yum, K. Kalyanasundaram, M. Gratzel,
M.K. Nazeeruddin, Efficient sensitization of nanocrystalline TiO2 films by a
near-IR absorbing unsymmetrical zinc phthalocyanine, Angew. Chem. Int. Ed.
46 (2007) 373–376.
[8] W. Zhao, Y.J. Hou, X.S. Wang, B.W. Zhang, Y. Cao, R. Yang, W.B. Wang, X.R. Xiao,
Study on squarylium cyanine dyes for photoelectric conversion, Sol. Energy
Mater. Sol. Cells 58 (1999) 173–183.
[9] L. Giribabu, C.V. Kumar, V.G. Reddy, P.Y. Reddy, C.S. Rao, S.R. Jang, J.H. Yum, M.K.
Nazeeruddin, M. Gratzel, Unsymmetrical alkoxy zinc phthalocyanine for sen-
sitization of nanocrystalline TiO2 films, Sol. Energy Mater. Sol. Cells 91 (2007)
1611–1617.
[26] M.K. Nazeeruddin, R. Humphry-Baker, M. Gratzel, D. Wohrle, G. Schnurpfeil, G.
Schneider, A. Hirth, N. Trombach, Efficient near-IR sensitization of nanocrys-
talline TiO2 films by zinc and aluminum phthalocyanines, J. Porphyrins
Phthalocyanines 3 (1999) 230–237.
[27] S. Sakaguchi, S.S. Pandey, K. Okada, Y. Yamaguchi, S. Hayase, Probing TiO2/dye
interface in dye sensitized solar cells using surface potential measurement,
Appl. Phys. Exp. 1 (2008) 105001.
[10] F. Inakazu, Y. Noma, Y. Ogomi, S. Hayase, Dye-sensitized solar cells consisting of
dye-bilayer structure stained with two dyes for harvesting light of wide range
of wavelength, Appl. Phys. Lett. 93 (2008) 93304.
[28] Y.S. Kim, K.N. Liang, K.Y. Law, D.G. Whitten, An investigation of photocurrent
generation by squarine aggregates in monolayer modifies SnO2 electrodes, J.
Phys. Chem. 98 (1994) 984–988.
[11] Y.S. Chen, Z.H. Zeng, C. Li, W.B. Wang, X.S. Wang, B.W. Zhang, Highly efficient
co-sensitization of nanocrystalline TiO2 electrodes with plural organic dyes,
New J. Chem. 29 (2005) 773–776.
[29] Z.S. Wang, K. Hara, Y. Dan-Oh, C. Kasada, A. Shinpo, S. Suga, H. Arakawa, H.
Sugihara, Photophysical and (photo)electrochemical properties of a coumarin
dye, J. Phys. Chem. B 109 (2005) 3907–3914.
[12] Y. Noma, K. Iizuka, Y. Ogomi, S.S. Pandey, S. Hayase, Preparation of double
dye-layer structure of dye-sensitized solar cells from cocktail solutions for
harvesting light in wide range of wavelengths, Jpn. J. Appl. Phys. 48 (2009)
020213.
[30] J.H. Yum, P. Walter, S. Huber, D. Rentsch, T. Geiger, F. Nuesch, F. De Angelis,
M. Gratzel, M.K. Nazeeruddin, Efficient far-red sensitization of nanocrystalline
TiO2 films by an unsymmetrical squaraine dye, J. Am. Chem. Soc. 129 (2007)
10320–10321.
[13] K.Y. Law, F.C. Bailey, Squarine chemistry
–
synthesis, characterization
[31] K. Hara, T. Sato, R. Katoh, A. Furube, Y. Ohga, A. Shinpo, S. Suga, K. Sayama,
H. Sugihara, H. Arakawa, Molecular design of coumarin dyes for efficient dye-
sensitized solar cells, J. Phys. Chem. B 107 (2003) 597–606.
[32] N. Koumura, Z.S. Wang, S. Mori, M. Miyashita, E. Suzuki, K. Hara, Alkyl-
functionalized organic dyes for efficient molecular photovoltaics, J. Am. Chem.
Soc. 128 (2006) 14256–14257.
and optical properties of
a class of novel unsymmetrical squarines 4-
(dimethylamino)-phenyl(4ꢀ-methoxyphenyl)-squarine and its derivatives, J.
Org. Chem. 57 (1992) 3278–3286.
[14] K.Y. Law, Organic photoconductive materials – recent trends and develop-
ments, Chem. Rev. 93 (1993) 449–486.
[15] A. Otsuka, K. Funabiki, N. Sugiyama, T. Yoshida, Dye sensitization of ZnO by
unsymmetrical squaraine dyes suppressing aggregation, Chem. Lett. 35 (2006)
666–667.
[16] S. Alex, U. Santhosh, S. Das, Dye sensitization of nanocrystalline TiO2: enhanced
efficiency of unsymmetrical versus symmetrical squaraine dyes, J. Photochem.
Photobiol. A 172 (2005) 63–71.
[17] A. Mishra, R.K. Behera, P.K. Behera, B.K. Mishra, G.B. Behera, Cyanines during
the 1990s: a review, Chem. Rev. 100 (2000) 1973–2011.
[18] A. Mishra, M.K.R. Fischer, P. Bauerle, Metal-free organic dyes for dye-sensitized
solar cells: from structure, property relationships to design rules, Angew. Chem.
Int. Ed. 48 (2009) 2474–2499.
[19] X. Chen, X. Peng, A. Cui, B. Wang, L. Wang, R. Wang, Photostabilities of novel
heptamethine 3H-indolenine cyanine dyes with different N-substituents, J.
Photochem. Photobiol. A 181 (2006) 79–85.
[20] W. Pham, W.F. Lai, R. Weissleder, C.H. Tung, High efficiency synthesis of a
bioconjugatable near-infrared fluorochrome, Bioconjugate Chem. 14 (2003)
1048–1051.
[21] B. Oswald, L. Patsenker, J. Duschl, H. Szmacinski, O.S. Wolfbeis, E. Terpetschnig,
Synthesis, spectral properties, and detection limits of reactive squaraine dyes,
a new class of diode laser compatible fluorescent protein labels, Bioconjugate
Chem. 10 (1999) 925–931.
[22] C. Kim, H. Choi, S. Kim, C. Baik, K. Song, M.S. Kang, S.O. Kang, J. Ko, Molecular
engineering of organic sensitizers containing p-phenylene vinylene unit for
dye-sensitized solar cells, J. Org. Chem. 73 (2008) 7072–7079.
[33] S.S. Pandey, S. Sakaguchi, Y. Yamaguchi, S. Hayase, Influence of nature of surface
dipoles on observed photovoltage in dye-sensitized solar cells as probed by
surface potential measurement, Org. Electr. 11 (2010) 419–426.
[34] Y. Ogomi, Y. Kashiwa, Y. Noma, Y. Fujita, S. Kojima, M. Kono, Y. Yamaguchi,
S. Hayase, Photovoltaic performance of dye-sensitized solar cells stained with
black dye under pressurized condition and mechanism for high efficiency, Sol.
Energy Mater. Sol. Cells 93 (2009) 1009–1012.
[35] A. Kay, M. Gratzel, Artificial photosynthesis; photosensitization of TiO2 solar
cells with chlorophyll derivatives and related natural porphyrines, J. Phys.
Chem. 97 (1993) 6272–6277.
[36] K. Sayama, S. Tsukagoshi, T. Mori, K. Hara, Y. Ohga, A. Shinpou, Y. Abe, S. Suga,
H. Arakawa, Efficient sensitization of nanocrystalline TiO2 films with cyanine
and merocyanine organic dyes, Sol. Energy Mater. Sol. Cells 80 (2003) 47–71.
[37] N. Kopidakis, N.R. Neale, A.J. Frank, Effect of an adsorbent on recombination and
band-edge movement in dye-sensitized TiO2 solar cells: evidence for surface
passivation, J. Phys. Chem. B 110 (2006) 12485–12489.
[38] Z.S. Wang, Y. Cui, Y. Dan-Oh, C. Kasada, A. Shinpo, K. Hara, Thiophene-
functionalized coumarin dye for efficient dye-sensitized solar cells: electron
lifetime improved by coadsorption of deoxycholic acid, J. Phys. Chem. C 111
(2007) 7224–7230.
[39] A.C. Khazraji, S. Hotchandani, S. Das, P.V. Kamat, Controlling dye (merocyanine-
540) aggregation on nanostructured TiO2 films: an organized assembly
approach for enhancing the efficiency of photosensitization, J. Phys. Chem. B
103 (1999) 4693–4700.