Dye-sensitized solar cells based on donor-π-acceptor fluorescent dyes with a pyridine ring as an electron-withdrawing-injecting anchoring group

Article abstract of DOI:10.1002/chem.201101923

A new-type of donor-acceptor π-conjugated (D-π-A) fluorescent dyes NI3-NI8 with a pyridine ring as electron-withdrawing-injecting anchoring group have been developed and their photovoltaic performances in dye-sensitized solar cells (DSSCs) are investigate

Full text of DOI:10.1002/chem.201101923

FULL PAPER
DOI: 10.1002/chem.201101923
Dye-Sensitized Solar Cells Based on Donor-p-Acceptor Fluorescent Dyes
with a Pyridine Ring as an Electron-Withdrawing-Injecting Anchoring Group
Yousuke Ooyama, Tomoya Nagano, Shogo Inoue, Ichiro Imae, Kenji Komaguchi,
Joji Ohshita, and Yutaka Harima*[a]
Abstract: A new-type of donor–accept-
or p-conjugated (D-p-A) fluorescent
dyes NI3–NI8 with a pyridine ring as
electron-withdrawing-injecting anchor-
ing group have been developed and
their photovoltaic performances in dye-
sensitized solar cells (DSSCs) are in-
vestigated. The short-circuit photocur-
rent densities and solar energy-to-elec-
tricity conversion yields of DSSCs
based on NI3–NI8 are greater than
those for the conventional D-p-A dye
sensitizers NI1 and NI2 with a carboxyl
group as the electron-withdrawing an-
choring group. The IR spectra of NI3–
NI8 adsorbed on TiO2 indicate the for-
mation of coordinate bonds between
the pyridine ring of dyes NI3–NI8 and
the Lewis acid sites (exposed Tin+ cat-
ions) of the TiO2 surface. This work
demonstrates that the pyridine rings of
D-p-A dye sensitizers that form a coor-
dinate bond with the Lewis acid site of
a TiO2 surface are promising candi-
dates as not only electron-withdrawing
anchoring group but also electron-in-
jecting group, rather than the carboxyl
groups of the conventional D-p-A dye
sensitizers that form an ester linkage
with the Brønsted acid sites of the TiO2
surface.
Keywords: donor–acceptor
sys-
tems · dyes/pigments · fluorescence ·
sensitizers · solar cells
Introduction
donor, and a carboxylic acid, cyanoacrylic acid, or rhoda-
nine-3-acetic acid moiety that acts as electron acceptor as
well as anchoring group for attachment on a TiO2 surface.
The carboxyl group enables good electron communication
between the dye and TiO2 by forming a strong ester linkage
with Brønsted acid sites (surface-bound hydroxyl groups) of
the TiO2 surface. Recently, as a successful molecular design
of D-p-A dyes to improve the performances of DSSCs, a
new series of donor–acceptor substituted squaraine dyes and
porphyrin dyes providing good absorption in the red/near-
IR region of the solar spectrum have been designed and de-
veloped.[12,13] DSSCs based on the D-p-A dyes have reached
power conversion efficiencies as high as 10–11%, compara-
ble to those of Ru complexes. However, the fact is that the
photovoltaic performances of DSSCs based on D-p-A dyes
have tended to stagnate for the last few years. To achieve a
breakthrough in the development of new and efficient D-p-
A dyes for DSSCs, an epoch-making molecular design, such
as formation of a strong interaction between the electron-ac-
cepting moiety of sensitizers and the TiO2 surface, is re-
quired.
On the other hand, very recently, as a new-type of D-p-A
dye sensitizers for DSSCs, we have designed and synthesized
fluorescent dyes NI3–NI6 with a pyridine ring as electron-
withdrawing anchoring group (Scheme 1).[14] The FTIR spec-
tra of NI3–NI6 adsorbed on TiO2 nanoparticles indicate
strong coordinate bonding between the pyridine ring of the
dyes and the Lewis acid sites of the TiO2 surface. The short-
circuit photocurrent densities (Jsc) and solar energy-to-elec-
tricity conversion yields (h) of DSSCs based on NI3–NI6 are
greater than those of the conventional D-p-A dye sensitizers
NI1 and NI2 with a carboxyl group as the electron-with-
During the last two decades, dye-sensitized solar cells
(DSSCs) have received considerable attention because of
their high incident solar light-to-electricity conversion effi-
ciency and low cost of production.[1–11] To increase the
power conversion efficiency, much research has focused on
the development of new dye sensitizers and electrolytes and
the improvement of nanocrystalline TiO2 electrodes. In par-
ticular, as the most promising organic dye sensitizers for
DSSCs, many kinds of donor–acceptor p-conjugated (D-p-
A) dyes with both electron-donating (D) and electron-ac-
cepting (A) groups linked by a p-conjugated bridge, which
exhibit broad and intense absorption, have been devel-
oped.[2–11] The spectral features of the D-p-A dyes are asso-
ciated with the intramolecular charge transfer (ICT) excita-
tion from the donor to acceptor moiety of the dye, which
leads to efficient electron transfer from the excited dye
through the acceptor moiety into the conduction band (CB)
of TiO2. Most of the D-p-A dyes, such as coumarin, polyene,
thiophene, and indoline dyes, for DSSCs developed so far
have a dialkyl amine or diphenylamine moiety as electron
[a] Dr. Y. Ooyama, T. Nagano, S. Inoue, Dr. I. Imae, Dr. K. Komaguchi,
Prof. Dr. J. Ohshita, Prof. Dr. Y. Harima
Department of Applied Chemistry
Graduate School of Engineering
Hiroshima University
Higashi-Hiroshima 739-8527 (Japan)
Fax : (+81)82-424-5494
E-mail: harima@mls.ias.hiroshima-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101923.
Chem. Eur. J. 2011, 17, 14837 – 14843
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
14837

pose the use of a pyridine ring as not only electron-with-
drawing anchoring group but also electron-injecting group
in place of the carboxyl group in conventional D-p-A dye
sensitizers.
Results and Discussion
Spectroscopic properties of NI1–NI8 in solution and ad-
sorbed on TiO2 nanoparticles: The absorption and fluores-
cence spectra of NI1–NI8 in 1,4-dioxane are shown in
Figure 1 and the spectral data are summarized in Table 1.
All the dyes show two absorption maxima: one band ap-
pears at around 300–315 nm ascribed to a p!p* transition,
and the other band occurs at around 370–400 nm assigned to
the ICT excitation from D (diphenylamino group) to A (car-
boxyphenyl group for NI and NI2 and pyridine ring for
NI3–NI8). The ICT bands of NI5, NI6, and NI8 occur at a
longer wavelength by approximately 20 nm than those of
NI1–NI4 and NI7. Furthermore, the molar extinction coeffi-
cients (e) for the ICT bands of NI5, NI6, and NI8 are
around
50000mꢀ1 cmꢀ1,
higher
than
the
30000–
35000mꢀ1 cmꢀ1 for NI1–NI4 and NI7. These results show
Scheme 1. Chemical structures of D-p-A fluorescent dyes NI1–NI8.
that the introduction of a thiophene unit on the carbazole
drawing anchoring group. Consequently, it was demonstrat-
ed that the formation of coordinate bonds between the pyri-
dine ring of dyes NI3–NI6 and the Lewis acid sites of the
TiO2 surface leads to efficient electron injection owing to
the good electron communication between them, rather
than the formation of an ester linkage between dyes NI1
and NI2 and the Brønsted acid sites of the TiO2 surface.
In this work, to ensure the usefulness of the pyridine ring
as an efficient electron-injecting group of new-type D-p-A
dye sensitizers in DSSCs, we have designed and synthesized
D-p-A fluorescent dyes NI7 and NI8 with a pyridine ring as
electron-accepting group and carboxyl group as anchoring
group. Dyes NI7 and NI8 have a nonconjugated alkyl chain
containing a carboxyl group at
Figure 1. Absorption (g) and fluorescence (c) spectra of NI2, NI4,
and NI6–NI8 in 1,4-dioxane. Inorm =normalized fluorescence intensity.
the end position, so that the
carboxyl group as an anchoring
group is separated from the
Table 1. Optical and electrochemical data, HOMO and LUMO energy levels, and DSSC performance param-
eters of NI1–NI8.
Dye labs [nm]
lem [nm]
Eo1 x
HOMO LUMO Molecules Jsc
Voc
FF[f]
h
=
2
(e
A
[V][c] [V][d]
[V][d]
[cmꢀ2
5.3ꢁ1016
10.4ꢁ1016 2.96
4.8ꢁ1016
1.80
]
G
]
[mV][f]
[%][f]
electron-acceptor
moiety
[e]
[f]
(Scheme 1; detailed synthetic
procedures for the dyes are
given in the Supporting Infor-
mation). The IR spectra of NI7
and NI8 adsorbed on TiO2
nanoparticles indicate the for-
mation of coordinate bonds be-
tween the pyridine ring of dyes
NI7 and NI8 and the Lewis
acid sites of the TiO2 surface.
Herein, we report the photo-
voltaic performances of DSSCs
based on new-type D-p-A fluo-
rescent dyes NI3–NI8 and pro-
NI1 374 (34900)
NI2 376 (34300)
438 (0.89) 0.30 0.93
ꢀ2.12
516
503
517
520
524
522
540
548
568
568
0.59 0.60
0.61 0.91
0.60 0.56
0.61 0.97
0.63 1.04
0.62 1.15
0.60 1.89
0.60 1.84
0.62 1.81
0.59 2.35
442 (0.87) 0.37 1.00
ꢀ2.03
NI3 372 (30200)
NI4 375 (33000)
NI5 394 (48100)
NI6 396 (49600)
NI7 375 (33000)
NI8 396 (48700)
423 (0.83) 0.34 0.97
423 (0.84) 0.39 1.02
465 (0.60) 0.30 0.93
464 (0.58) 0.34 0.97
425 (0.85) 0.38 1.01
467 (0.58) 0.34 0.97
ꢀ2.15
ꢀ2.07
ꢀ1.93
ꢀ1.87
ꢀ2.08
ꢀ1.87
4.9ꢁ1016
4.7ꢁ1016
7.9ꢁ1016
8.0ꢁ1016
3.16
3.35
5.80
5.63
11.7ꢁ1016 5.16
11.4ꢁ1016 7.04
[a] In 1,4-dioxane. [b] In 1,4-dioxane. Fluorescence quantum yields (Ff) were determined by using a calibrated
integrating sphere system (lex =370 nm). [c] Half-wave potentials for oxidation (Eo1 x) versus ferrocene/ferroce-
=
nium (Fc/Fc+) were recorded in CH2Cl2/Bu4NClO4 (0.1m) solution. [d] Versus the 2normal hydrogen electrode
(NHE). [e] The adsorption amount per unit area of TiO2 film was controlled by the immersion time of the
TiO2 electrode in the dye solution. [f] The photocurrent–voltage characteristics were measured under simulat-
ed solar light (air mass (AM) 1.5, 100 mWcmꢀ2). FF=fill factor.
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Dye-Sensitized Solar Cells Based on Donor-p-Acceptor Fluorescent Dyes
FULL PAPER
skeleton expands the p conjugation in the dye and thus re-
sults in a redshift of the absorption maximum and enhance-
ment of e. The corresponding fluorescence maximum (lem
)
occurs at around 420–470 nm. The fluorescent dyes NI1–NI4
and NI7 (Ff =ca. 0.85) exhibit a higher fluorescence quan-
tum yield than NI5, NI6, and NI8 (Ff =ca. 0.6).
The absorption spectra of the dyes adsorbed on TiO2
nanoparticles are shown in Figure 2. The absorption peak
Figure 3. Cyclic voltammogram of NI8 in CH2Cl2 containing Bu4NClO4
(0.1m) at a scan rate of 50 mVsꢀ1. The arrow denotes the direction of the
potential scan.
1.02 V with respect to a normal hydrogen electrode (NHE),
which indicated that all the dyes have similar HOMO
ꢀ
energy levels that are more positive than the I3 /Iꢀ redox
potential (0.4 V). This assures an efficient regeneration of
the oxidized dyes by electron transfer from Iꢀ in the electro-
lyte. The LUMO energy levels of the dyes were estimated
from Eo1 x and an intersection of absorption and fluorescence
Figure 2. Absorption spectra of a) NI2, NI4, and NI6 and b) NI7 and NI8
adsorbed on TiO2 nanoparticles with (c) and without (g) CDCA as
coadsorbent. The y axis is expressed in terms of the Kubelka–Munk
equation K/S=(1ꢀR)2/2R, in which K is the absorption coefficient, S is
the scattering coefficient, and R is the fractional reflectance.
=
2
spectra (407 and 409 nm (3.05 and 3.03 eV) for NI1 and
NI2, respectively, 398, 401, and 401 nm (3.12, 3.09, and
3.09 eV) for NI3, NI4, and NI7, respectively, and 434, 436,
and 436 nm (2.86, 2.84, and 2.84 eV) for NI5, NI6, and NI8,
respectively). The LUMO energy levels of NI1–NI8 were
ꢀ2.12, ꢀ2.03, ꢀ2.15, ꢀ2.07, ꢀ1.93, ꢀ1.87, ꢀ2.08, and
ꢀ1.87 V, respectively. The LUMO levels of NI5, NI6, and
NI8 are lower than those of NI1–NI4 and NI7, which lead
to the decrease of the energy gap between the HOMO and
LUMO responsible for the redshift of ICT absorption bands
for NI5, NI6, and NI8 relative to NI1–NI4 and NI7. Evi-
dently, the LUMO energy levels of all the dyes are higher
than the energy level of the CB of TiO2 (ꢀ0.5 V), so that
these dyes can efficiently inject electrons into the TiO2 elec-
trode.
wavelengths (labs) are redshifted by about 10 nm for NI1
and NI2, about 25 nm for NI3, NI4, and NI7, about 30 nm
for NI5 and NI6, and about 40 nm for NI8 compared with
those in 1,4-dioxane. Chenodeoxycholic acid (CDCA) was
employed as coadsorbent to prevent dye aggregation on the
TiO2 surface. When CDCA is coadsorbed with NI3–NI8 on
TiO2, the absorption peak wavelengths are blueshifted by
about 10 nm for NI3, NI4, and NI7, about 20 nm for NI5
and NI6, and about 10 nm for NI8, although the peak wave-
lengths are still redshifted compared with those in 1,4-diox-
ane. In contrast, the absorption peak wavelengths of NI1
and NI2 adsorbed on TiO2 with coadsorption of CDCA are
similar to those in 1,4-dioxane. These results show that the
redshifts of NI3–NI8 by adsorption on TiO2 are due to the
strong interaction between the dyes and the TiO2 surface,
which will be discussed later along with the FTIR spectra of
the dyes adsorbed on TiO2.
Semiempirical MO calculations (AM1, INDO/S) of NI1–
NI8: The photophysical properties of NI1–NI8 were ana-
lyzed by using semiempirical molecular orbital (MO) calcu-
lations. The molecular structures were optimized by using
the MOPAC/AM1 method,[15] and then the INDO/S
method[16] was used for spectroscopic calculations. The cal-
culated absorption wavelengths and the transition characters
of the absorption bands are collected in Table 2. These cal-
culated values for NI1–NI8 are comparable to the observed
spectra in 1,4-dioxane. The calculations indicate that the
longest excitation bands were mainly assigned to the transi-
tion from HOMO to LUMO, for which HOMOs were
mostly localized on the diphenylaminocarbazole moiety for
NI1–NI4 and NI7 and the diphenylaminothiophenylcarba-
zole moiety for NI5, NI6, and NI8, and LUMOs were
mostly localized on the carboxyphenylcarbazole moiety for
NI1 and NI2, the pyridinylcarbazole moiety for NI3, NI4,
and NI7, and the pyridinylthiophenylcarbazole moiety for
NI5, NI6, and NI8. The HOMO and LUMO of NI2, NI4,
Electrochemical properties of NI1–NI8 and their HOMO
and LUMO energy levels: The electrochemical properties of
all the dyes were determined by cyclic voltammetry (CV) in
CH2Cl2 containing 0.1m tetrabutylammonium perchlorate
(Bu4NClO4). The potentials were referred to ferrocene/fer-
rocenium (Fc/Fc+) as the internal reference. A typical CV
curve of NI8 is shown in Figure 3. The electrochemical prop-
erties of NI1–NI8 are summarized in Table 1. The oxidation
peaks of NI1–NI8 were observed at 0.34–0.42 V versus Fc/
Fc+ and the corresponding reduction peaks appeared at
0.26–0.35 V, thus showing that the oxidized states of all the
dyes are stable. The HOMO and LUMO energy levels of all
the dyes were evaluated from the spectral analyses and the
half-wave potentials for oxidation of NI1–NI8 (Eo1 x =0.30–
=
2
0.39 V). The HOMO energy levels for NI1–NI8 were 0.93–
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14839

Y. Harima et al.
Table 2. Calculated absorption spectral data for NI1–NI8.
Compound
Absorption (calcd)
lmax [nm]
CI component[b]
f[a]
NI1
NI2
NI3
NI4
NI5
NI6
NI7
NI8
348
348
345
345
378
378
344
377
1.13
1.08
0.96
0.87
1.84
1.83
0.84
1.84
HOMO!LUMO (41%)
HOMO!LUMO (40%)
HOMO!LUMO (65%)
HOMO!LUMO (61%)
HOMO!LUMO (65%)
HOMO!LUMO (65%)
HOMO!LUMO (59%)
HOMO!LUMO (65%)
[a] Oscillator strength. [b] The transition is shown by an arrow from one
orbital to another, followed by its percentage configuration interaction
(CI) component.
and NI6–NI8 are shown in Figure 4a and b. The changes in
the calculated electron density accompanied by the first
electron excitation for NI2, NI4, and NI6–NI8 are shown in
Figure 4c, which reveal a strong ICT nature from the diphe-
nylamino moiety to the carboxyphenyl group or pyridine
ring upon illumination by light.
FTIR spectra of NI1–NI8 adsorbed on TiO2 nanoparticles:
To elucidate the adsorption states of dyes NI1–NI8 on TiO2
nanoparticles, we measured the FTIR spectra of the dye
powders and the dyes adsorbed on TiO2 nanoparticles. The
examples of NI2, NI4, and NI6–NI8 are shown in Figure 5.
For the powders of dyes NI1, NI2, NI7, and NI8, the C=O
stretching band of the carboxyl group was observed at
1680 cmꢀ1. When the dyes were adsorbed on the TiO2 sur-
face, the C=O stretching band at 1680 cmꢀ1 disappeared.
These observations indicate that the carboxyl groups of the
dyes form an ester linkage with the TiO2 surface.[17] On the
other hand, the characteristic stretching bands for C=N or
C=C were clearly observed at around 1590, 1490, and
1460 cmꢀ1 for all the dye powders. In the FTIR spectra of
Figure 5. FTIR spectra of the dye powders and dyes adsorbed on TiO2
nanoparticles for a) NI2, b) NI4, c) NI6, d) NI7, and e) NI8.
NI3–NI8 adsorbed on TiO2, a new band appeared at around
1615 cmꢀ1, which can be assigned to a pyridine ring coordi-
nated to the Lewis acid sites of the TiO2 surface.[18,19] This
indicates that the dyes NI1 and
NI2 are adsorbed on the TiO2
surface by the ester linkage
alone at Brønsted acid sites
(surface-bound
hydroxyl
groups), whereas the dyes
NI3–NI6 are predominantly
adsorbed on the TiO2 surface
by coordinate bonding at
Lewis acid sites (exposed Tin+
cations), and the dyes NI7 and
NI8 are adsorbed on the TiO2
surface by both the ester link-
age and coordinate bonding.
The strong coordinate bonding
is responsible for the large red-
shift of the absorption peak for
NI3–NI8 adsorbed on TiO2
(Figure 2).
Figure 4. a) LUMO and b) HOMO of NI2, NI4, and NI6–NI8. The red and blue lobes denote the positive and
negative phases, respectively, of the coefficients of the MOs. The size of each lobe is proportional to the MO
coefficient. c) Calculated electron density changes accompanying the first electronic excitation of NI2, NI4,
and NI6–NI8. The black and white lobes signify the decrease and increase in electron density accompanying
the electronic transition, respectively. Their areas indicate the magnitude of the electron density change.
(Light blue, green, blue, red, and gold balls correspond to hydrogen, carbon, nitrogen, oxygen, and sulfur
atoms, respectively.)
Photovoltaic performances of
DSSCs based on NI1–NI8: The
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Dye-Sensitized Solar Cells Based on Donor-p-Acceptor Fluorescent Dyes
FULL PAPER
DSSCs were prepared by using the dye-adsorbed TiO2 elec-
trode, Pt-coated glass as a counter electrode, and an acetoni-
trile solution with iodine (0.05m), lithium iodide (0.1m), and
1,2-dimethyl-3-propylimidazolium iodide (0.6m) as electro-
lyte. The photocurrent–voltage (I–V) characteristics were
NI4 and NI7 in Figure 7. The Jsc values increase almost line-
arly with the increase in the adsorption amounts of dyes, al-
though a salient feature to be noted in Figure 7 is the differ-
ence of the slope values: the slopes became steep in the
measured
under
simulated
solar
light
(AM 1.5,
100 mWcmꢀ2). The I–V curves and the incident photon-to-
current conversion efficiency (IPCE) spectra are shown in
Figure 6. The photovoltaic performance parameters of
Figure 7. Plots of Jsc values against amounts of dye adsorbed on TiO2 for
NI1–NI4 and NI7.
order NI1=NI2<NI7<NI3=NI4. As shown in Figure 8,
this result indicates that the formation of strong coordinate
bonds between the pyridine ring of dyes NI3 and NI4 and
Figure 6. a) IPCE spectra and b) I–V curves of DSSCs based on NI1–NI8.
The amounts of adsorbed dyes on TiO2 film are 10.4ꢁ1016, 10.8ꢁ1016,
4.9ꢁ1016, 4.7ꢁ1016, 7.9ꢁ1016, 8.0ꢁ1016, 11.7ꢁ1016, and 11.4ꢁ
1016 moleculescmꢀ2 for NI1–NI8, respectively. TiO2 electrodes of thick-
ness 9 mm were used. CDCA was not employed.
Figure 8. Configurations of NI2, NI4, and NI7 on the TiO2 surface.
DSSCs based on the dyes NI1–NI8 are collected in Table 1.
The short-circuit photocurrent density (Jsc) and solar
energy-to-electricity conversion yield (h) increase in the
order NI1 (2.96 mAcmꢀ2, 0.91%)<NI2 (3.07 mAcmꢀ2,
0.97%)<NI3 (3.16 mAcmꢀ2, 1.04%)<NI4 (3.35 mAcmꢀ2,
1.15%)<NI7 (5.16 mAcmꢀ2, 1.81%), when comparisons
are made of the maximum adsorption amounts of dyes ad-
sorbed on TiO2 (10.4ꢁ1016, 10.8ꢁ1016, 4.9ꢁ1016, 4.7ꢁ1016,
and 11.7ꢁ1016 moleculescmꢀ2 for NI1–NI4 and NI7, respec-
tively) under the adsorption condition of 1ꢁ10ꢀ4 m dye solu-
tion in THF. The open-circuit photovoltages (Voc) for NI1–
NI4 and NI7 are 503, 520, 524, 552, and 568 mV, respective-
ly, which were slightly different among the five dyes. The
maximum IPCE values of NI1–NI4 (47–55%) resemble
each other very well. On the other hand, the maximum
IPCE value (ca. 80%) of NI7 is higher than those of NI1–
NI4.
the Lewis acid sites of the TiO2 surface leads to an efficient
electron injection owing to good electron communication
between them, rather than the formation of an ester linkage
between the dyes NI1 and NI2 and the Brønsted acid sites
of the TiO2 surface. On the other hand, the slope for NI7 is
smaller than those for NI3 and NI4. It is reasonable to pre-
sume that the relatively less efficient electron injection for
NI7 is ascribable to weaker coordinate bonding between the
pyridine ring of NI7 and the Lewis acid sites of the TiO2 sur-
face because of the flexibility of the alkyl chain. Thus, our
results demonstrate that the dyes NI3, NI4, and NI7 can
inject electrons efficiently from the pyridine ring as elec-
tron-withdrawing anchoring group to the CB of the TiO2
electrode through strong coordinate bonding with the Lewis
acid site of the TiO2 surface. However, the maximum ad-
sorption amounts of dyes adsorbed on TiO2 for NI3 and NI4
are smaller than those for NI1, NI2, and NI7, because for
the TiO2 nanoparticles commonly used in DSSCs, the Lewis
acid sites on the TiO2 surface are fewer than the Brønsted
To see explicitly the probability of electron injection from
the dye to the CB of TiO2, the adsorption amounts of dyes
adsorbed on TiO2 are plotted against the Jsc value for NI1–
Chem. Eur. J. 2011, 17, 14837 – 14843
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14841

Y. Harima et al.
acid sites. Thus, further studies on the development of TiO2
nanoparticles having Lewis acid sites enriched by treatment
of the TiO2 surface or hydrolysis of TiCl4 with ammonium
hydroxide are now in progress, to increase the adsorption
amounts of the D-p-A dye sensitizers with pyridine rings as
electron-withdrawing anchoring groups on TiO2.
Experimental Section
General: IR spectra were recorded on a Perkin–Elmer Spectrum One
FTIR spectrometer by the attenuated total reflectance (ATR) method.
Absorption spectra were observed with a Shimadzu UV-3150 spectropho-
tometer and fluorescence spectra were measured with a Hitachi F-4500
spectrophotometer. The fluorescence quantum yields in solution and in
the solid state were determined by a Hamamatsu C9920-01 instrument
equipped with a charge-coupled device (CCD) by using a calibrated inte-
grating sphere system (lex =370 nm). Cyclic voltammetry (CV) curves
were recorded in CH2Cl2/Bu4NClO4 (0.1m) solution with a three-elec-
trode system consisting of Ag/Ag+ as reference electrode, a Pt plate as
working electrode, and a Pt wire as counter electrode by using a Hokuto
Denko HAB-151 potentiostat equipped with a function generator.
On the other hand, the Jsc and h values for NI5
(5.80 mAcmꢀ2, 1.89%), NI6 (5.63 mAcmꢀ2, 1.84%), and
NI8 (7.04 mAcmꢀ2, 2.35%) are larger than those for NI3,
NI4, and NI7. The maximum IPCE values were 65–70% in
the range from 410 to 470 nm for NI5 and NI6 and about
80% in the range from 420 to 500 nm for NI8. The relatively
high photovoltaic performances of NI5, NI6, and NI8 are at-
tributed to both the redshift of the absorption band and the
good balance between the LUMO level of the dye and the
energy level of the CB of TiO2 by the introduction of a thio-
phene unit to the p-conjugation system of the dye. Finally,
DSSCs based on the new-type D-p-A fluorescent dyes NI3–
NI8 show a good light-soaking stability comparable to those
of the conventional D-p-A dye sensitizers NI1 and NI2
under simulated solar light (AM 1.5, 100 mWcmꢀ2). After
10 h of light soaking, there is little change in the Jsc, Voc, FF,
and h values (see Figure S2 in the Supporting Information
for the device stability of DSSCs).
Computational methods: Semiempirical calculations were carried out
with the WinMOPAC Version 3.9 package (Fujitsu, Chiba, Japan). Ge-
ometry calculations in the ground state were made by using the AM1
method. All geometries were completely optimized (keyword PRECISE)
by the eigenvector following routine (keyword EF). Experimental ab-
sorption spectra of the compounds were compared with their absorption
data by the semiempirical method INDO/S (intermediate neglect of dif-
ferential overlap/spectroscopic). Dipole moments of the compounds were
also evaluated from INDO/S calculations. All INDO/S calculations were
performed by using single excitation full SCF/CI (self-consistent field/
configuration interaction), which included the configuration with one
electron excited from any occupied orbital to any unoccupied orbital, for
which 225 configurations were considered [keyword CI (15 15)].
Preparation of DSSCs based on dyes NI1–NI6: The TiO2 paste (JGC
Catalysts and Chemicals Ltd., PST-18NR) was deposited on a fluorine-
doped tin oxide (FTO) substrate by doctor-blading, and sintered for
50 min at 4508C. The 9 mm thick TiO2 electrode (0.5ꢁ0.5 cm2 in photoac-
tive area) was immersed in a 0.1 mm dye solution in tetrahydrofuran for
a number of hours, enough to adsorb the photosensitizer. DSSCs were
fabricated by using the TiO2 electrode thus prepared, with Pt-coated
glass as a counter electrode and a solution of iodine (0.05m), lithium
iodide (0.1m), and 1,2-dimethyl-3-propylimidazolium iodide (0.6m) in
acetonitrile as electrolyte. The photocurrent–voltage characteristics were
Conclusion
As a new-type of D-p-A dye sensitizers for DSSCs, we have
designed and synthesized fluorescent dyes NI3–NI8 with a
pyridine ring as electron-withdrawing-injecting anchoring
group. The FTIR spectra of NI3–NI8 adsorbed on TiO2
nanoparticles indicate the formation of strong coordinate
bonding between the pyridine ring of the dyes and the
Lewis acid sites of the TiO2 surface. The Jsc and h values of
DSSCs based on NI3–NI8 are greater than those of the con-
ventional D-p-A dye sensitizers NI1 and NI2 with a carbox-
yl group as electron-withdrawing anchoring group. Conse-
quently, it was demonstrated that the formation of coordi-
nate bonds between the pyridine ring of dyes NI3–NI8 and
the Lewis acid sites of the TiO2 surface leads to efficient
electron injection owing to good electron communication
between them, rather than the formation of an ester linkage
between the dyes NI1 and NI2 and the Brønsted acid sites
of the TiO2 surface. Thus, we propose the use of a pyridine
ring as not only electron-withdrawing anchoring group but
also electron-injecting group in a new-type of D-p-A dye
sensitizers for DSSCs. Furthermore, it is expected that D-p-
A fluorescent dyes NI3–NI8 are Type-II sensitizers with a
direct electron injection mechanism from the ground state
of the dye to the CB of TiO2 because of coordinate bonding
between the pyridine ring of dyes NI3–NI8 and the Lewis
acid sites of the TiO2 surface. Further studies to elucidate
whether the fluorescent dyes NI3–NI8 are Type-I or Type-II
sensitizers are now in progress based on transient absorption
spectroscopy and the transient photovoltage techniques.
measured with
100 mWcmꢀ2). IPCE spectra were measured under monochromatic irra-
diation with tungsten–halogen lamp and monochromator. The
a potentiostat under simulated solar light (AM 1.5,
a
a
amount of adsorbed dye on TiO2 nanoparticles was determined by ab-
sorption spectral measurement of the concentration change of the dye so-
lution before and after adsorption. Absorption spectra of the dyes ad-
sorbed on TiO2 nanoparticles were recorded on the dye-adsorbed TiO2
film (thickness of 9 mm) in the diffuse-reflection mode with a calibrated
integrating sphere system.
Acknowledgements
This work was supported by Grants-in-Aid for Young Scientists (B)
(22750179) from the Japan Society for the Promotion of Science (JSPS)
and by the Fujii Research Foundation from Hiroshima University. Y.O.
also acknowledges the Nissan Chemical Industries Award in Synthetic
Organic Chemistry, Japan.
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Received: June 22, 2011
Revised: August 4, 2011
Published online: November 22, 2011
Chem. Eur. J. 2011, 17, 14837 – 14843
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