High performance of Si-O-Ti bonds for anchoring sensitizing dyes on TiO2 electrodes in dye-sensitized solar cells evidenced by using alkoxysilylazobenzenes

Article abstract of DOI:10.1246/cl.2010.260

Applicability of alkoxysilyl group was examined for the first time as the anchor moiety of sensitizing dyes for dye-sensitized solar cells by using alkoxysilylazobenzenes. Alkoxysilylazobenzenes adsorbed efficiently onto TiO₂ electrodes by the formation of Si-O-Ti bonds, and the electrodes exhibited much higher durability to water and better photovoltaic performance than in the case of a conventional carboxy dye.

Full text of DOI:10.1246/cl.2010.260

doi:10.1246/cl.2010.260
260
Published on the web February 16, 2010
High Performance of Si-O-Ti Bonds for Anchoring Sensitizing Dyes on TiO2 Electrodes
in Dye-sensitized Solar Cells Evidenced by Using Alkoxysilylazobenzenes
Kenji Kakiage,1 Masaki Yamamura,1 Emi Fujimura,1 Toru Kyomen,1,2 Masafumi Unno,1,2 and Minoru Hanaya*1,2
1Department of Chemistry and Chemical Biology, Graduate School of Engineering,
Gunma University, 1-5-1 Tenjin-cho, Kiryu 376-8515
2International Education and Research Center for Silicon Science, Graduate School of Engineering,
Gunma University, 1-5-1 Tenjin-cho, Kiryu 376-8515
(Received December 22, 2009; CL-091133; E-mail: hanaya@chem-bio.gunma-u.ac.jp)
Applicability of alkoxysilyl group was examined for the first
time as the anchor moiety of sensitizing dyes for dye-sensitized
solar cells by using alkoxysilylazobenzenes. Alkoxysilylazo-
benzenes adsorbed efficiently onto TiO2 electrodes by the
formation of Si-O-Ti bonds, and the electrodes exhibited much
higher durability to water and better photovoltaic performance
than in the case of a conventional carboxy dye.
N
N
N
OEt
Si OEt
OEt
N
N
N
1
2
3
Ph
Si OEt
OEt
Dye-sensitized solar cells (DSSCs) have been actively
studied since Grätzel and co-workers reported high solar-cell
performance with DSSCs based on Ru-complex photosensitiz-
ers.1 The sensitizing dye plays a key part in the light-to-electric
energy conversion, and the development of new sensitizing
dyes could bring a breakthrough for the application of DSSC
technology. However, the study of the sensitizing dyes has been
mostly limited to the dyes including carboxy groups as the
anchor moiety for chemical binding to the surface of the TiO2
electrodes.1,2 The adsorption of the carboxy dye molecules is
understood to occur by the formation of an ester-like C(=O)O-
Ti bond between the carboxy group and the hydroxy group on
the surface of the TiO2 electrodes,3 and the instability of the
bond to water has been one of the major problems preventing
practical use of DSSCs.4
On the other hand, alkoxysilanes are known to possess high
bonding ability to metal oxides by forming strong Si-O-metal
bonds on the metal-oxide surface.5 By using the dyes including
alkoxysilyl groups as the sensitizing dyes, TiO2 electrodes can
be coated effectively and the dye-adsorbed TiO2 electrodes
would obtain durability to water based on the high stability of
the Si-O-Ti bond. In this work, we synthesized ethoxysilyl-
azobenzenes as model compounds of alkoxysilyl dyes in order
to examine, for the first time, the applicability of alkoxysilyl
group as the anchor moiety of sensitizing dyes for DSSC, and
tested the durability of the formed Si-O-Ti bonds and inves-
tigated precisely their performance in DSSC.
4-(Triethoxysilyl)azobenzene (1) and 4-(diethoxyphenyl-
silyl)azobenzene (2) were synthesized by lithiation of 4-
iodoazobenzene followed by substitution with ClSi(OEt)3 and
ClSiPh(OEt)2, respectively (Scheme 1). 4-(Phenylazo)benzoic
acid (3), which was used as a reference to the alkoxysilyl-
azobenzenes, was purchased from Aldrich.
The nanocrystalline TiO2 film electrodes were prepared by
spin-coating TiO2 paste on a F-doped SnO2 (FTO) coated glass
plate (25 © 50 mm2, 15-20 ³ sq.¹1; Asahi Glass) followed by
sintering the TiO2 layer at 450 °C for 30 min, and then by spin-
coating TiO2 paste again and sintering it at 500 °C for 30 min.
The TiO2 paste was prepared by mixing P-25 (Nippon Aerosil)
O
C
OH
Scheme 1. Molecular structures of azobenzene dyes 1-3.
TiO2 particles with water, acetylacetone, and Triton X-100
and grinding them in an agate mortar. The thickness of the TiO2
film with porosity was estimated to be ca. 2.1 ¯m by the SEM
observation of the cross section of the TiO2 electrode.
Adsorption of the dyes on the TiO2 electrodes was performed
by immersing the electrode in the 3.0 © 10¹4 M toluene solu-
tions of 1-3 at 100 °C for 15 h. The UV-visible absorption
spectra of the dyes were recorded on a Hitachi U-3010
spectrometer, and an integrating sphere was equipped to the
spectrometer for the measurements of the dyes adsorbed on the
TiO2 electrodes.
Photovoltaic measurements were performed for an electro-
chemical cell consisting of the dye-adsorbed TiO2 electrode, a
counter electrode, a polyethylene film spacer (100 ¯m thick), and
an organic electrolyte. A Pt-sputtered FTO-coated glass plate
was used as the counter electrode, and a solution of 0.3 M
LiI and 0.015 M I2 in acetonitrile/ethylene carbonate (2:8 in
volume) was used as the electrolyte. The photovoltaic perform-
ance of the cells was assessed from the I-V properties of the cells
measured with a solar simulator of OTENTO-SUN III (Bunkoh-
Keiki) and a source meter of R6240A (Advantest). The aperture
area of the cells was maintained at 1.0 cm2 using a shading mask
and the I-V properties were measured under irradiation of AM-
1.5G global-one sun condition (100 mW cm¹2) at 25 « 2 °C.
Figure 1 shows the UV-visible absorption spectra of 1-3
in acetonitrile solutions. Alkoxysilylazobenzenes (1 and 2)
showed spectra similar to that of 4-(phenylazo)benzoic acid (3):
The spectra exhibited two absorption bands assignable to the n-
³* transition in the azobenzene group between 370 and 550 nm
Chem. Lett. 2010, 39, 260-262
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261
0.08
0.06
0.04
0.02
0
100
80
60
40
20
0
1
2
1
2
1
0
3
2
3
400
500
600
400
600
800
0
20 40 60 80 100
2000
λ / nm
Water Soaked Time / h
Figure 1. UV-visible absorption spectra of 1-3 in acetonitrile
solutions: Solid line represents the result for 1, dotted line for 2,
and broken line for 3. Inset shows the enlarged spectra in
visible-light region.
Figure 3. Changes of the absorbances due to the dyes 1-3
adsorbed on the TiO2 electrodes with the soaked time in water at
25 °C in the dark: Open circles represent the results for 1, open
triangles for 2, and closed squares for 3. The absorbance ratios
were evaluated by the normalization of absorbance at the
maxima around - = 415 nm with respect to those before soaked
in water.
3
0.12
0.08
0.04
0
observed here are considered as the index of the amount of dye
molecules adsorbed on the TiO2 electrode. The amounts were
estimated thus to be in the order of 3 ² 1 > 2. This result
shows that triethoxysilyl group is highly adsorptive compared to
carboxy groups on TiO2 electrodes, and that the adsorption
would be slightly hindered sterically by the substituent on the
silicon atom as in the case of 2.
1
2
The adsorption of the carboxy dye molecule 3 is believed to
occur through formation of an ester-like linkage between the
carboxy group and the hydroxy group on the surface of the TiO2
electrodes.3 In the case of dyes bearing ethoxysilyl groups, the
adsorption is thought to proceed from the formation of Si-O-Ti
bonds.5,7 The adsorption of 1 and 2 onto the surface of TiO2 in
such a manner was confirmed by IR spectroscopy. In DSSCs
with conventional carboxy dyes, instability of the dye-adsorbed
electrodes to water is a serious problem from a practical point of
view, and the instability results from the weakness of the ester-
like bond formed between the dye and the TiO2 electrode.4 In the
case of alkoxysilyl dyes, on the other hand, the formed Si-O-Ti
bond is expected to be more stable than the ester-like bond. To
establish the durability of the Si-O-Ti bond, we soaked the dye-
adsorbed TiO2 electrodes into water at 25 °C in the dark and
measured the changes of the absorption due to the dyes at the
absorbance maxima around - = 415 nm (Figure 2). The results
are shown in Figure 3. In the cases of the alkoxysilyl dyes of 1
and 2 more than 80% dyes were kept on the electrode even after
2000-h soaking in water, while almost all the dye molecules
were eliminated from the electrode for the carboxy dye of 3. The
confirmed tightly bonding property of the alkoxysilyl dyes to
TiO2 electrode could be a great advantage in the utility of the
dyes as sensitizing dyes for DSSC.
400
600
800
λ / nm
Figure 2. Absorption spectra in visible-light region of 1-3
adsorbed on TiO2 electrodes: Solid line represents the result for
1, dotted line for 2, and broken line for 3.
and to the ³-³* transition in the phenyl moieties at around
320 nm.6 Among 1-3, the difference of the molar absorption
coefficients at around -max in the n-³* band did not exceed 5%
of the magnitude.
Figure 2 shows the absorption spectra in the visible-light
region of 1-3 adsorbed on the TiO2 electrodes, which were used
in the cells for photovoltaic measurements. The spectra were
obtained by subtracting the absorption due to the TiO2 electrode
from those of dye-adsorbed TiO2 electrodes. Since the molar
absorption coefficients of 1-3 are similar to each other in the
visible-light region as shown in Figure 1, the absorbances
Chem. Lett. 2010, 39, 260-262
© 2010 The Chemical Society of Japan
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262
sensitizing dyes for DSSC, and proves that the electron transfer
from the light-excited dye to the TiO2 electrode through the Si-
O-Ti bonds is more efficient than that through the ester-like
bond.
0.8
0.6
0.4
0.2
0
2
The results obtained here clearly show the advantage of
the Si-O-Ti bond over the conventional ester-like bond for
anchoring sensitizing dye on TiO2 electrode not only in
durability to water but also in electron-transfer properties.
Alkoxysilyl dye is proving to be a promising candidate for a
sensitizer of DSSC, and has an advantage of easiness in its
synthesis and handling over silanol dye.5 An alkoxysilyl dye
possessing light-harvesting moieties with high absorption
coefficient in visible-light region would bring a remarkable
advance in practical DSSC technology.
1
3
no dye
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In order to compare the sensitizing properties quantitatively
between the dyes containing alkoxysilyl and carboxy groups, we
measured the I-V properties of the cells using 1-3 as sensitizing
dyes. The results of the I-V measurements are shown in Figure 4
along with the result for the cell containing no dye. The
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Chem. Lett. 2010, 39, 260-262
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/