Remarkable improvement in sensitizing property of tetraphenylporphyrincarboxylic acids as sensitizers for dye-sensitized solar cells by the introduction of trimethylsilyl groups as substituents

Article abstract of DOI:10.1246/cl.2010.1063

The effects of the introduction of alkylsilyl groups to porphyrin dyes on their sensitizing property in dye-sensitized solar cells were investigated by using a newly synthesized tetraphenylporphyrincarboxylic acid containing trimethylsilyl groups. The cell with the porphyrin showed higher photovoltaic performance than that with a ierf-butyl-containing analog, and the zinc complex of the porphyrin exhibited remarkably high sensitizing performance comparable to N3 dye.

Full text of DOI:10.1246/cl.2010.1063

doi:10.1246/cl.2010.1063
Published on the web August 28, 2010
1063
Remarkable Improvement in Sensitizing Property of Tetraphenylporphyrincarboxylic Acids
as Sensitizers for Dye-sensitized Solar Cells by the Introduction
of Trimethylsilyl Groups as Substituents
Kenji Kakiage,1 Emi Fujimura,1 Hiroyuki Mine,1 Takenori Izumizawa,2 Takashi Katoh,2
Kimio Yoshimura,1 Soichiro Kyushin,1,3 Toru Kyomen,1,3 and Minoru Hanaya*1,3
1Department of Chemistry and Chemical Biology, Graduate School of Engineering,
Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515
2Goi Research Center, Research Laboratory II, Chisso Petrochemical Corporation,
5-1 Goikaigan, Ichihara, Chiba 290-8551
3International Education and Research Center for Silicon Science, Graduate School of Engineering,
Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515
(Received June 17, 2010; CL-100568; E-mail: hanaya@chem-bio.gunma-u.ac.jp)
The effects of the introduction of alkylsilyl groups to
porphyrin dyes on their sensitizing property in dye-sensitized
1 : M = 2H+, Ar =
Ar
Ar
Ar
solar cells were investigated by using a newly synthesized
tetraphenylporphyrincarboxylic acid containing trimethylsilyl
groups. The cell with the porphyrin showed higher photovoltaic
performance than that with a tert-butyl-containing analog, and
the zinc complex of the porphyrin exhibited remarkably high
sensitizing performance comparable to N3 dye.
N
M
N
CH3
2 : M = 2H+, Ar =
3 : M = 2H+, Ar =
C
CH3
N
N
CH3
CH3
Si CH3
CH3
COOH
4 : zinc complex of 2 (M = Zn2+
)
)
Tetraphenylporphyrin-
carboxylic acids
A dye-sensitized solar cell (DSSC) is a promising device as
a practical solar cell of the next generation because of its high
possibility for lowering energy costs, and the research and
development task is now energetically pushed forward.1 How-
ever, the light-to-electric energy conversion efficiencies of
DSSCs have been limited to 11%,2 and there is still room for
a substantial improvement especially in the sensitizing dyes.3
Porphyrins which are well-known light-harvesting moieties
show strong absorptions with ¾ > 104 in the visible light region
due to the conjugated macrocycle,4 and can be controlled in
oxidation and reduction potentials by changing central metal
ions.5 Therefore, porphyrincarboxylic acids have been actively
studied as sensitizing dyes for DSSCs.6 In the examination of
porphyrincarboxylic acids as sensitizing dyes, the introduction
of alkyl substituents such as tert-butyl (t-Bu) and methyl groups
to the porphyrins has been performed to suppress intermolecular
interaction producing rapid thermal relaxation of the light-
excited porphyrin molecules and to increase the solubility of the
porphyrins in the solvents used in their adsorption on TiO2
electrodes.
Alkylsilyl groups are bulkier substituents than the corre-
sponding alkyl groups and possess higher electron-donating
properties resulting from the characteristics of Si.7 By introduc-
ing alkylsilyl substituents to the porphyrins, therefore, the
sensitizing ability of the porphyrins is expected to be improved
and the DSSCs using them will show higher performance in
light-to-electric energy conversion. However, porphyrins con-
taining alkylsilyl substituents have never been examined as
sensitizing dyes for DSSCs. In this work, we synthesized a
derivative of tetraphenylporphyrincarboxylic acid possessing
trimethylsilyl (TMS) groups as substituents in order to inves-
tigate the effects of the introduction of alkylsilyl groups to the
sensitizing properties of the porphyrin. The TMS-containing
5 : zinc complex of 3 (M = Zn2+
Scheme 1. Molecular structures of tetraphenylporphyrincarbox-
ylic acids 1-5 used in this work as sensitizing dyes.
porphyrin showed higher performance than the t-Bu-containing
analog, and the zinc complex of the porphyrin exhibited
remarkably high sensitization.
Tetraphenylporphyrincarboxylic acid (1), tetraphenylpor-
phyrincarboxylic acid containing t-Bu groups (2), and tetra-
phenylporphyrincarboxylic acid containing TMS groups (3)
(Scheme 1) were synthesized by the reaction of pyrrole with
terephthalaldehydic acid and benzaldehyde, 4-tert-butylbenzal-
dehyde and 4-trimethylsilylbenzaldehyde, respectively. The zinc
complexes of 2 and 3, which correspond to 4 and 5 respectively,
were obtained by the reactions of 2 and 3 with zinc acetate. The
N3 dye of cis-di(thiocyanato)bis(2,2¤-bipyridyl-4,4¤-dicarboxyl-
ic acid)ruthenium(II), which was used as a reference sensitizing
dye in this work, was purchased from SOLARONIX. The
nanocrystalline TiO2 film electrodes were prepared as reported
previously.8 The thickness of the porous TiO2 film was
estimated to be ca. 1.0 ¯m by SEM observation. Adsorptions
of the porphyrin dyes and the N3 dye on the TiO2 electrodes
were performed by immersing the electrodes in 3.0 © 10¹4 M
toluene solutions of 1-5 at 95 °C for 15 h and an ethanol solution
of N3 dye at 70 °C for 15 h, respectively. Photovoltaic measure-
ments were carried out for electrochemical cells of an open
sandwich type with an aperture area of 1.0 cm2. The structure
of the cell, the equipment for the measurements, and the
measurement conditions were the same as reported in our
previous paper.9
Chem. Lett. 2010, 39, 1063-1065
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1064
0.2
0.1
0
0.08
0.06
0.04
0.02
0
4
3
2
1
0
1
2
3
4
5
1
2
3
4
5
450
500
550
600
650
700
400
500
600
700
800
500
550
600
650
700
λ / nm
λ / nm
Figure 2. Absorption spectra in the visible region of 1-5
adsorbed on the TiO2 electrodes.
Figure 1. UV-visible absorption spectra of 1-5 in chloroform
solutions. Inset shows the enlarged spectra in the Q-band region.
Figure 1 shows the UV-visible absorption spectra of 1-5 in
chloroform solutions. The spectra of 1-5 exhibited a series of
absorption bands due to the ³-³* transition in the conjugated
macrocycle between 380 and 680 nm: The absorption at around
420 nm is assignable to the Soret band and that from 460 to
680 nm to the Q-bands.4 The t-Bu-containing dye 2 and the
TMS-containing dye 3 showed spectra similar to that of
tetraphenylporphyrincarboxylic acid (1) and the difference of
the maximum molar absorption coefficients in the Q-band did
not exceed 1% of the magnitude among them, while in the zinc
complexes 4 and 5 a red shift of the absorption maximum in the
Q-band by 36 nm and a slight increase of the maximum molar
absorption coefficient by 6% were observed.
N3 dye
2.5
2
1
2
3
4
5
1.5
1
Figure 2 shows the absorption spectra in the visible region
of 1-5 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 at around 515 nm are similar to
each other as shown in Figure 1, the maximum absorbances
observed here are considered to be an index of the amount of
dye molecules adsorbed on the electrodes. The amounts were
estimated thus to be 0.95 for 2 and 0.86 for 3 in comparison with
the case of 1 in the order of 1 > 2 > 3. This result shows that the
adsorption of the porphyrin dyes on the TiO2 electrodes would
be slightly hindered sterically by the bulkiness of the substitu-
ents (TMS > t-Bu > H). The amounts of 4 and 5 adsorbed on
the TiO2 electrodes were also estimated from the maximum
absorbances and confirmed to be almost the same as 2 and 3,
respectively.
0.5
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Photovoltage / V
Figure 3. I-V properties of the cells using 1-5 and N3 dye as
sensitizing dyes under simulated sunlight irradiation of AM-
¹2
1.5G with the intensity of 100 mW cm
.
Figure 3, and characteristic parameters are listed in Table 1. The
cell with t-Bu-containing dye 2 exhibited higher short-circuit
photocurrent density (Jsc) and higher open-circuit photovoltage
(Voc) than the cell with 1, showing higher photocurrent density in
the measured photovoltage region. The enhancements of photo-
current density and photovoltage were more significant in the
cell with TMS-containing dye 3. Although the amount of dye
molecules of 3 adsorbed on the TiO2 electrode was smaller than
in the cases of 1 and 2 as mentioned above, the cell with 3
In order to compare the sensitizing properties between the
porphyrin dyes containing TMS groups and t-Bu groups, we
measured I-V properties of the cells using 1-3 as sensitizing
dyes under simulated sunlight irradiation (AM-1.5G, 100
mW cm¹2). The results of the I-V measurements are shown in
Chem. Lett. 2010, 39, 1063-1065
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1065
Table 1. Photovoltaic performances of cells with an aperture
area of 1.0 cm2 using 1-5 and N3 dye as sensitizing dyes under
simulated sunlight irradiation (AM-1.5G, 100 mW cm¹2): short-
circuit photocurrent density (Jsc), open-circuit photovoltage
(Voc), fill factor (FF), and conversion efficiency (©)
¹2
Chem., Int. Ed. 2009, 48, 2474.
2
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Dyes
Jsc/mA cm
Voc/V
FF
©/%
1
2
3
4
5
1.02
1.21
1.32
1.76
2.47
2.59
0.53
0.56
0.57
0.60
0.67
0.70
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0.33
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0.83
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3
N3
showed a higher light-to-electric energy conversion efficiency
(©) than the cells with 1 and 2 by factors of 1.3 and 1.2,
respectively, under the present test conditions.
These results exhibit clearly that the introduction of TMS
groups to tetraphenylporphyrincarboxylic acid is more effective
in the improvement of the sensitizing ability than that of t-Bu
groups. The improvement is thought to be due to the bulkiness
of TMS group which prevents the quenching of light-excited
porphyrin molecules resulting from intermolecular interaction,
and also due to electron donation producing the increment of
electron-injection efficiency from the light-excited porphyrin
molecule to the TiO2 electrode.
The I-V properties of the cells with 4 and 5, which are
the zinc complexes of the t-Bu-containing and TMS-containing
porphyrins 2 and 3 respectively, were also examined in this
study. The results of the I-V measurements are shown in
Figure 3 along with that for the cell using N3 dye, and
characteristic parameters are listed in Table 1. The cells with 4
and 5 showed higher photovoltaic performances than those with
2 and 3, and the remarkable performance of the cell with 5 was
close to that with N3 dye which is known as one of the most
effective sensitizers for DSSCs. Such a high sensitizing ability
of 5 is thought to originate in the higher energy level of the
LUMO of 5 than that of 3,5 and also in the bulkiness and the
electron-donating property of TMS group, which would bring
higher efficiency in the electron-injection from the light-excited
dye to the TiO2 electrode in the cell.
4
5
6
In this study, we examined for the first time the applicability
of dyes containing alkylsilyl groups as sensitizers for DSSCs,
and successfully revealed the effectiveness of the introduction of
TMS groups to the tetraphenylporphyrincarboxylic acid for
improving the photosensitizing ability. The results indicate that
such an improvement is also possible for sensitizing dyes
possessing ³-electron systems by introducing alkylsilyl sub-
stituents, and show the high potential of organosilicon com-
pounds as sensitizing dyes for DSSCs.
7
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Chem. Lett. 2010, 39, 1063-1065
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/