Photoelectric conversion properties of four novel carboxylated hemicyanine dyes on TiO2 electrode

Article abstract of DOI:10.1039/b300083b

Four hemicyanine derivatives, 2-[4-(N-ethyl-N-carboxyethyl)amino]phenylethenyl-1,3,3-trimethyl-3H-indolium iodide (BIDC1), 2-[4-(N,N-dicarboxyethyl)amino]phenylethenyl-1,3,3-trimethyl-3H-indolium iodide (BIDC2), 2-[4-(N-ethyl-N-carboxyethyl)amino]phenylethenyl-1,1,3- trimethyl-1H-benz[e]indolium iodide (BIDC3) and 2-[4-(N,N-dicarboxyethyl) amino]phenylethenyl-1,1,3-trimethyl-1H-benz[e]indolium iodide (BIDC4), have been synthesized and identified with regard to their structures and electrochemical properties. It is found that the absorption spectra of BIDC3 and BIDC4 are broader than that of BIDC1 and BIDC2 because the former two have one more benzene ring on the electron acceptor side. Upon adsorption on a TiO₂ electrode, the absorption spectra of the four BIDC dyes are all broadened forward to both red and blue sides compared with their respective spectra in ethanol solution. BIDC2 (or BIDC4) bearing two carboxyl groups, has about two (or three) times more adsorption and a broader absorption spectrum on TiO₂ film compared with BIDC1 (or BIDC3) which consequently leads to a better light-to-electricity conversion property. Among the four hemicyanine dyes, BIDC4 generated the highest photoelectric conversion yield of 4.9%, with a short-circuit photocurrent of 21.4 mA cm^-2, an open-circuit voltage of 424 mV, and a fill factor of 0.49 under irradiation with 90.0 mW cm^-2 white light from a Xe lamp.

Full text of DOI:10.1039/b300083b

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Photoelectric conversion properties of four novel carboxylated
hemicyanine dyes on TiO2 electrode
Qiao-Hong Yao,a Fan-Shun Meng,b Fu-You Li,a He Tian*b and Chun-Hui Huang*a
aState Key Laboratory of Rare Earth Materials Chemistry and Applications,
Peking University, Beijing 100871, P. R. China. E-mail: hch@chem.pku.edu.cn;
Fax: 186-(10) 6275 1708; Tel: 186-(10) 6275 7156
bInstitute of Fine Chemicals, East China University of Science & Technology, Shanghai
200237, P. R. China
Received 3rd January 2003, Accepted 14th March 2003
First published as an Advance Article on the web 28th March 2003
Four hemicyanine derivatives, 2-[4-(N-ethyl-N-carboxyethyl)amino]phenylethenyl-1,3,3-trimethyl-3H-indolium
iodide (BIDC1), 2-[4-(N,N-dicarboxyethyl)amino]phenylethenyl-1,3,3-trimethyl-3H-indolium iodide (BIDC2),
2-[4-(N-ethyl-N-carboxyethyl)amino]phenylethenyl-1,1,3-trimethyl-1H-benz[e]indolium iodide (BIDC3) and
2-[4-(N,N-dicarboxyethyl)amino]phenylethenyl-1,1,3-trimethyl-1H-benz[e]indolium iodide (BIDC4), have been
synthesized and identified with regard to their structures and electrochemical properties. It is found that the
absorption spectra of BIDC3 and BIDC4 are broader than that of BIDC1 and BIDC2 because the former two
have one more benzene ring on the electron acceptor side. Upon adsorption on a TiO2 electrode, the
absorption spectra of the four BIDC dyes are all broadened forward to both red and blue sides compared with
their respective spectra in ethanol solution. BIDC2 (or BIDC4), bearing two carboxyl groups, has about two
(or three) times more adsorption and a broader absorption spectrum on TiO2 film compared with BIDC1 (or
BIDC3), which consequently leads to a better light-to-electricity conversion property. Among the four
hemicyanine dyes, BIDC4 generated the highest photoelectric conversion yield of 4.9%, with a short-circuit
photocurrent of 21.4 mA cm22, an open-circuit voltage of 424 mV, and a fill factor of 0.49 under irradiation
with 90.0 mW cm22 white light from a Xe lamp.
dyes would be a promising type of sensitizer for dye sensitized
solar cells.
Introduction
Dye-sensitized solar cells have been potential competitors
against conventional photovoltaic cells since the report of
highly efficient ruthenium complex-sensitized TiO2 solar cells
by Gra¨tzel and co-workers.1,2 To date, the best performance
has been obtained with ruthenium polypyridyl derivatives.3–5
However, pure organic dyes are also attractive sensitizers6–21
because they have the following advantages16 over metal
complexes. In comparison with metal complexes, pure organic
dyes generally have higher absorption coefficients, which could
reduce the amount of dye to be used and reduce the thickness
of the semiconductor film. More importantly, pure organic
dyes are easier to prepare and cost less. In fact, organic dye
sensitized TiO2 solar cells have made great progress in recent
years, and the highest overall yield of solar cells sensitized by
pure organic dyes has exceeded 5%,15,21 indicating that organic
On the basis of our previous studies on photoelectric
conversion of hemicyanine dyes in LB films,22–28 we introduced
2
-RSO3 as attaching groups into hemicyanine dyes and
systematically studied the steric hindrance effect on the
adsorption and photoelectric conversion properties of the
dye.20 In a very recent report, we found that through treating
TiO2 films with hydrochloric acid, the photocurrent of
2
hemicyanine dye (containing -RSO3 groups) sensitized TiO2
solar cells was greatly enhanced with conversion yields up to
5.1%.21 Such a high value encouraged us to further study
hemicyanine dyes containing -COOH groups as sensitizers for
Gra¨tzel type solar cells.
In this paper, we investigated the photoelectrochemical
properties of a series of hemicyanine dyes (Scheme 1) having
different numbers of attaching groups (-COOH) and different
Scheme 1 Structures of the hemicyanine dyes.
1048
J. Mater. Chem., 2003, 13, 1048–1053
This journal is # The Royal Society of Chemistry 2003
DOI: 10.1039/b300083b

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electron donors. Upon comparison some interesting results
were obtained.
J ~ 8.9 Hz), 8.17 (d, 1H, Ar-H, J ~ 8.1 Hz), 8.23 (d, 1H, Ar-H,
J ~ 9 Hz), 8.38 (t, 2H). Elemental analysis for C28H31N2O2I:
calculated C 60.65, H 5.64, N 5.05; found C 60.72, H 5.58, N
4.98%.
Experimental
BIDC4. 1H-NMR (DMSO-d6, 500 MHz): d 1.99 (s, 6H,
-C(CH3)2), 2.59 (t, 4H, -CH2CH3), 3.78 (t, 4H, -2CH2CH2-),
4.12 (s, 3H, -N-CH3), 6.92 (d, 2H, Ar-H, J ~ 9.0 Hz), 7.34
(d, 1H, -CHLCH-, J ~ 15.9 Hz), 7.66 (t, 1H, Ar-H), 7.77 (t,
1H, Ar-H), 8.00 (d, 1H, Ar-H, J ~ 9.0 Hz), 8.09 (d, 2H, Ar-H,
J ~ 8.8 Hz), 8.17 (d, 1H, Ar-H, J ~ 8.2 Hz), 8.24 (d, 1H, Ar-H,
J ~ 8.9 Hz), 8.37 (d, 1H, Ar-H, J ~ 6.9 Hz), 8.41 (d, 1H,
-CHLCH-, J ~ 15.8 Hz). Elemental analysis for C29H31N2O4I:
calculated: C 58.20, H 5.22, N 4.68; found: C 58.35, H 5.25, N
4.65%.
Materials
Titanium(IV) isopropoxide and propylene carbonate (PC) were
purchased from Acros. All other solvents and chemicals used
were produced by Beijing Chemical Factory, China (reagent
grade) and used as received. Optical transparent conducting
glass (CTO glass, fluorine-doped SnO2 overlayer, transmission
w70% in the visible region, sheet resistance 20 V square21) was
obtained from the Institute of Nonferrous Metals of China.
N-Methyl-4-methylpyridinium iodide (PyI) was synthesized accor-
ding to the literature.22 2-[4-(N-Ethyl-N-carboxyethyl)amino]-
phenylethenyl-1,3,3-trimethyl-3H-indolium iodide (BIDC1),
2-[4-(N,N-dicarboxyethyl)amino]phenylethenyl-1,3,3-trimethyl-
3H-indolium iodide (BIDC2), 2-[4-(N-ethyl-N-carboxyethyl)-
amino]phenylethenyl-1,1,3-trimethyl-1H-benz[e]indolium iodide
(BIDC3) and 2-[4-(N,N-dicarboxyethyl)amino]phenylethenyl-
1,1,3-trimethyl-1H-benz[e]indolium iodide (BIDC4) were
synthesized according to the literature,29,30 and the syntheses
route is briefly illustrated in Scheme 2. Their structure and
purity were confirmed by NMR and elemental analysis.
Preparation of nanocrystalline TiO2 electrodes
The nanocrystalline TiO2 films with different thickness were
prepared according to the procedure described in the litera-
ture.20,31 As the film thickness can strongly affect the action
spectrum and the overall energy conversion efficiency, the
effect of film thickness on the performance parameters of the
dye sensitized solar cell was also investigated, and 5 mm thick
film was found to be optimal. The TiO2 film electrodes thus
obtained were coated with dye in a 3 6 1024 mol dm23 ethanol
solution for 12 h at room temperature. The amount of
adsorbed dye was determined by spectroscopic measurement
of the concentration change of the dye solution before and
after adsorption.
BIDC1. 1H-NMR (DMSO-d6, 500 MHz): d 1.16 (t, 3H,
-CH2CH3), 1.75 (s, 6H, -C(CH3)2), 2.59 (t, 2H, -CH2CH3), 3.58
(m, 2H, -CH2CH2-), 3.74 (t, 2H, -CH2CH2-), 3.97 (s, 3H, -N-
CH3), 6.90 (d, 2H, Ar-H, J ~ 9.1 Hz), 7.26 (d, 1H, -CHLCH-,
J ~ 15.6 Hz), 7.49 (t, 1H, Ar-H), 7.56 (t, 1H, Ar-H), 7.71 (d,
1H, Ar-H, J ~ 8.0 Hz), 7.78 (d, 1H, Ar-H, J ~ 7.4 Hz), 8.07 (d,
2H, Ar-H, J ~ 8.7 Hz), 8.31 (d, 1H, -CHLCH-, J ~ 15.7 Hz).
Elemental analysis for C24H29N2O2I: calculated: C 57.15, H
5.79, N 5.57; found: C 57.23, H 5.85, N 5.52%.
Methods
Film thickness was determined with a DEKTAK 3 profilo-
meter. 1H NMR spectra were recorded on Bruker ARX400
and ARX500 spectrometers. Elemental analyses were per-
formed on a Carlo Erba 1106 elemental analyzer. The UV-Vis
spectra were measured with a Shimadzu mode 3100UV-
VIS-NIR spectrophotometer. The fluorescence spectra were
observed on a Hitachi F-4500 fluorescence spectrophotometer.
Synchrotron radiation photon–electron spectroscopy (SRPES)
BIDC2. 1H-NMR (DMSO-d6, 500 MHz): d 1.75 (t, 6H,
-C(CH3)2), 2.59 (t, 4H, -2CH2CH3), 3.78 (t, 4H, -2CH2CH2-),
3.98 (s, 3H, -N-CH3), 6.91 (d, 2H, Ar-H, J ~ 9.1 Hz), 7.28
(d, 1H, -CHLCH-, J ~ 15.6 Hz), 7.50 (t, 1H, Ar-H), 7.57
(t, 1H, Ar-H), 7.72 (d, 1H, Ar-H, J ~ 8.0 Hz), 7.80 (d, 1H,
Ar-H, J ~ 7.4 Hz), 8.08 (d, 2H, Ar-H, J ~ 8.7 Hz), 8.33 (d, 1H,
-CHLCH-, J ~ 15.7 Hz). Elemental analysis for C25H29N2O4I:
calculated: C 54.75, H 5.29, N 5.11; found: C 54.82, H 5.34, N
5.07%.
was measured on
a photon–electron spectrometer (vsw
Scientific Instruments, Ltd.). Under an ultrahigh vacuum of
(3–4) 6 1028 Pa, photon–electron spectra were recorded under
18.7 eV synchrotron radiation light, from which the first
ionization energy, the HOMO energy level, was derived for
different hemicyanine dyes.
BIDC3. 1H-NMR (DMSO-d6, 400 MHz): d 1.17 (t, 3H,
-CH2CH3), 1.99 (s, 6H, -C(CH3)2), 2.60 (t, 2H, -CH2CH3),
3.58 (m, 2H, -CH2CH2-), 3.74 (t, 2H, -CH2CH2-), 4.10 (s, 3H,
-N-CH3), 6.91 (d, 2H, Ar-H, J ~ 9.1Hz), 7.31 (d, 1H,
-CHLCH-, J ~ 15.8 Hz), 7.65 (t, Ar-H, 1H), 7.77 (t, 1H,
Ar-H), 7.98 (d, 1H, Ar-H, J ~ 9.0 Hz), 8.08 (d, 2H, Ar-H,
The photoelectrochemical experiments were carried out in a
standard two-electrode system.2,32 One drop of redox electro-
lyte solution was put onto the dye loaded TiO2 film (effective
area is 0.188 cm2) and a Pt sputtered ITO glass was used as
counter electrode. The redox electrolyte solution was composed
of 0.5 mol dm23 LiI, 0.05 mol dm23 I2, and 0.6 mol dm23 PyI
Scheme 2 Syntheses route of the hemicyanine dyes. BIDC1, BIDC2: R ~ R’ ~ -C2H5; BIDC3, BIDC4: R ~ -C2H4CN, R’ ~ -C2H4COOH.
J. Mater. Chem., 2003, 13, 1048–1053 1049

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can occur readily because of the presence of carboxyl groups
in the molecules. The absorption spectra of the dyes in ethanol
solutions with different concentrations are shown system-
atically in Fig. 2. The data show that the absorption peaks
of the hemicyanine dyes were all blue shifted with increasing
concentration of the solution, suggesting that BIDC dyes
have the tendency to form H aggregates when the solution
concentrations increase. At the same concentration, BIDC2 or
BIDC4 have larger blue shifts than BIDC1 or BIDC3 because
of their extra carboxyl group.
Fig. 3 gives the absorption spectra of the four dyes on TiO2
films after 12 hours adsorption. Upon adsorption on TiO2
film the spectra of BIDCs are broadened compared with those
in ethanol solution (c.f. Fig. 1), and the interesting thing is that
the spectra of the dyes with two carboxyl groups (BIDC2 or
BIDC4) are broader than their congeners (BIDC1 or BIDC3)
which bear one –COOH. As an example, the spectra of BIDC4
on TiO2 film with different adsorption times are given in Fig. 4.
The phenomenon is observed that the spectra were broadened
on both sides, that is, both blue shifted and red shifted. The
same phenomena were also observed with the other three dye
congeners.
Fig. 1 Absorption spectra of BIDCs in 3 6 1026 M ethanol solutions.
in PC. A 500 W xenon lamp (Ushio Electric, Tokyo, Japan)
served as a white light source in conjunction with an IRA-
25S filter (Schott, USA) and a GG400 cutoff filter (Toshiba,
Japan). Here, an IRA-25S filter was used to cut off infrared
light to protect the electrode from heating, and a GG400
cutoff filter was used to filter off light with wavelength less
than 400 nm, which prevents excitation of the TiO2 films.
Monochromatic light in the region of 400 nm to 800 nm was
produced by setting an IRA-25S filter and a suitable band-
pass filter (Schott, USA) in the path of the light beam. Light
intensities were measured with a Light Gauge radiometer/
photometer (Coherent, Auburn, CA).
The amount of BIDCs dye adsorbed on a TiO2 electrode
per unit area was measured and the results are listed in
Table 1. BIDC1 and BIDC3 have almost the same adsorption
amount (0.53 6 1027 mol cm22 and 0.52 6 1027 mol cm22
,
respectively), while BIDC2 and BIDC4 have larger values of
2.10 6 1027 mol cm22 and 1.50 6 1027 mol cm22 , respec-
tively, which are almost four and three times those of BIDC1
and BIDC3. So the spectra of BIDC2 and BIDC4 are broader
than those of BIDC1 and BIDC3, indicating that they could
convert solar light to electricity in a larger region.
Result and discussion
A. Absorption properties in solutions and on TiO2 film
B. HOMO and LUMO energy levels of the dyes
Fig. 1 shows the absorption spectra of BIDCs in ethanol
solution. BIDCs have characteristic p–p* electron transition
bands around 570 nm in ethanol solution because of similar
conjugation systems. However, the different acceptors (e.g.,
1,3,3-trimethyl-1H-indolium group or 1,1,3-trimethyl-1H-benz-
[e]indolium group) and the different numbers of attaching
groups (e.g., one or two -COOH group(s)) result in different
absorption properties. Compared with BIDC1 and BIDC2,
BIDC3 and BIDC4 have one more benzene ring on the electron
acceptor side, which enhanced the extent of electron deloca-
lization over the whole molecule, so their absorption peak and
the threshold of the longer wavelength side were red shifted a
little. As shown in Table 1, the absorption peaks for BIDC1,
BIDC2, BIDC3 and BIDC4 are 561 nm, 560 nm, 577 nm and
578 nm, respectively. The spectra of BIDC3 and BIDC4 in
ethanol solution are similarly broad and are broader than those
of BIDC1 and BIDC2, indicating that one more carboxylic
group has little influence on the absorption properties in
relatively diluted ethanol solution (3 6 1026 M). Here, we
understood that BIDC dyes were present as the monomeric
form in such diluted solution. The maximum absorption
coefficients (emax) for BIDCs are also illustrated in Table 1,
and are much higher than those of Ru complexes (1.4 6
104 M21 cm21).2 It is reported that the dyes have the tendency
to aggregate in solution or at the solid–liquid interface owing
to strong intermolecular van der Waals-like attractive forces
between the molecules.33 For BIDCs this kind of aggregate
To judge the possibility of electron transfer from the excited
dye to the conduction band of TiO2, the highest occupied
molecular orbital (HOMO) levels of 25.8 eV for BIDC1,
25.8 eV for BIDC2, 25.7 eV for BIDC3 and 25.7 eV for
BIDC4 were measured by synchrotron radiation photon–
electron spectroscopy (SRPES) under vacuum. With reference
to the absorption maxima of 561 nm (1.9 eV), 560 nm (1.9 eV),
577 nm (2.0 eV) and 578 nm (2.0 eV), corresponding to the
gap between the HOMO and LUMO levels for BIDC1,
BIDC2, BIDC3 and BIDC4, respectively, the energy levels of
the LUMO can be estimated to be 23.9 eV, 23.9 eV, 23.7 eV
and 23.7 eV respectively. Obviously, the excited-state energy
levels for the four dyes are higher than the bottom of the
conduction band of TiO2 (24.4 eV), clarifying that electron
injection should be possible thermodynamically.
C. Fluorescence emission properties of the dyes
All four dyes display a strong fluorescence emission at room
temperature. Interestingly, the fluorescence emission bands
of the dyes in the visible region disappear when the dyes
are adsorbed onto the nanocrystalline TiO2 electrodes. The
phenomena can be explained by the efficient electron injection
from the excited singlet state of the dyes to the conduction band
of TiO2, and suggesting that all four dyes are good sensitizers
for nanocrystalline TiO2 electrodes.
Table 1 Absorption properties (in 3 6 1026 M ethanol solution) and energy levels for BIDCs
lmax/nm
e/M21 cm21
Adsorption/mol cm22
HOMO/eV
LUMO/eV
BIDC1
BIDC2
BIDC3
BIDC4
561
560
577
578
9.0 6 104
8.1 6 104
9.1 6 104
7.1 6 104
0.53 6 1027
2.10 6 1027
0.52 6 1027
1.50 6 1027
25.8
25.8
25.7
25.7
23.9
23.9
23.7
23.7
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Fig. 2 Absorption spectra of the four dyes in ethanol solution with different concentrations. The original concentration is 3 6 1024 M. (a) Diluted
100 times, (b) diluted 50 times, (c) diluted 30 times, (d) diluted 20 times, (e) diluted 15 times, (f) diluted 10 times.
Fig. 4 Absorption spectra of BIDC4 on TiO2 with different adsorption
times: (a) 5 min, (b) 15 min, (c) 30 min, (d) 60 min, (e) 105 min and (f)
Fig. 3 Absorption spectra of BIDCs on TiO2 electrode after adsorption
for 12 hours. The absorption of the glass and TiO2 film was corrected.
585 min. The absorption of the CTO glass and TiO2 film was corrected.
D. Photoelectrochemical properties of the TiO2 electrode
sensitized by BIDCs
efficiently convert visible light to photocurrent in the region
from 400 nm to 660 nm. Comparing the dye’s IPCE spectra
with absorption spectra on TiO2 film (c.f. Fig. 3), all the four
dyes have sensitization selectivity for different regions of visible
light, that is, they convert more efficiently light of shorter
wavelength. It can be observed from Fig. 4 that in accordance
with the absorption spectra on TiO2 film, the action spectra
for BIDC2 and BIDC4 are broader than those of BIDC1 and
BIDC3, and BIDC4 has higher IPCE values (w60%) in the
region from 460 nm to 640 nm, indicating that a BIDC4
sensitized TiO2 electrode would generate a higher conversion
yield.
The photoelectrochemical properties of BIDCs sensitized
TiO2 electrodes are listed in Table 2, while the photocurrent–
voltage curves are shown in Fig. 6. The dye-coated TiO2 film,
as working electrode, was placed on top of an ITO glass as a
counter electrode, on which the 200 nm thick Pt was sputtered.
The redox electrolyte was introduced into the interelectrode
Photocurrent action spectra for the dye-coated TiO2 electrodes
are shown in Fig. 5. The monochromatic incident photo-to-
electron conversion efficiency (IPCE), defined as the number
of electrons generated by light in the outer circuit divided by
the number of incident photons, was obtained by means of the
following equation:
À
Á
1240Isc mA cm{2
l(nm)Pin W m{2
À
Á
IPCE(%)~
(1)
where the constant 1240 is derived from unit conversion, Isc
is the short-circuit photocurrent generated by monochromatic
light, and l is the wavelength of incident monochromatic
light, the light intensity of which is Pin. The losses of light
reflection and absorption by the conducting glass were not
corrected. From Fig. 5 one can see that all four dyes can
J. Mater. Chem., 2003, 13, 1048–1053
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E. Relationship between photoelectrochemical properties and
chemical structures
From the adsorption amounts in Table 1 and the short-circuit
photocurrents in Table 2, the contribution of one mole of dye
molecules to the short-circuit photocurrent could be calculated
and the values are also listed in Table 2. Comparing BIDC1 (or
BIDC2) with BIDC3 (or BIDC4), one can find that they have
the same donor but different acceptor. The benzeneindolium
group has one more benzene ring than the indolium group; this
fact makes BIDC3 (or BIDC4) have a broader absorption
spectrum than BIDC1 (or BIDC2) in ethanol solution because
of the larger electron delocalization, and consequently leads
to better photoelectric conversion properties to some extent.
For example, one mole of BIDC1 produced 2.32 6 108 mA
short-circuit photocurrent, while the short-circuit photocurrent
generated by one mole of BIDC3 is 2.79 6 108 mA. For BIDC2
and BIDC4, the values are 7.57 6 107 mA and 1.43 6 108 mA,
respectively. On the other hand, it is easy to find that BIDC1
(or BIDC3) and BIDC2 (or BIDC4) have the same acceptor
but different donor. One more carboxyl group is attached onto
the amino group in the latter, which drastically increased the
adsorption of the dye on the TiO2 electrode, and the spectrum
of the latter greatly expanded to both red and blue sides, and
this is beneficial to photoelectric conversion efficiency. For exam-
Fig. 5 Photocurrent action spectra of the TiO2 electrodes sensitized by
BIDCs. The reflection and absorption of the CTO glass were not
corrected.
Table 2 Parameters of dye-sensitized solar cells
Isc/mAcm22
Voc/mV
FF
g (%)
Isca/mA mol21
BIDC1
BIDC2
BIDC3
BIDC4
12.3
15.9
14.5
21.4
480
446
492
424
0.61
0.58
0.56
0.49
4.0
4.6
4.4
4.9
2.32 6 108
7.57 6 107
2.79 6 108
1.43 6 108
ple, the adsorption amount for BIDC3 is 5.2 6 1028 mol cm22
,
while that for BIDC4 is 1.5 6 1027 mol cm22. That is, the
adsorption amount for the latter is about three times larger
than that of the former. Alternatively, if we focus on the short-
circuit photocurrent of the solar cell sensitized with the dyes,
it is easy to find that the three times larger amount of BIDC4
molecules did not generate a three times larger short-circuit
photocurrent, and the increment is only 48%. This result can be
explained by the contribution of one mole of molecules to
the short-circuit photocurrent. One mole of BIDC3 generate
2.79 6 108 mA short-circuit photocurrent while for BIDC4 the
value is just 1.43 6 108 mA, which is only a half of that of the
former. Obviously, the carboxyl group appears to have two
different effects on the adsorption and photoelectric conversion
properties of the dye. The positive effect is that the adsorbed
amount of dye molecules on the TiO2 film greatly increased,
which is favorable for the overall light to electricity conversion.
However, at the same time, the second carboxyl group impeded
the charge separation of the molecules because it was attached
onto the electron donor side and hence reduced the polarity of
the molecules to some extent. Stronger adsorption of the dye
on the TiO2 film could on the other hand facilitate charge
recombination of the injected electron and the dye cation, and
attaching of the adsorbing groups to the electron donor side
could to some extent impede the electron transfer from the
redox-active electrolyte to the dye cation. The same rule can
be found in the comparison between BIDC1 and BIDC2.
aThe short-circuit photocurrent generated per mole of dye molecules.
Fig. 6 Photocurrent–voltage curve of BIDCs-sensitized TiO2 electrodes.
space by capillary force. The solar cells were illuminated from
the front through the conducting glass substrate. Fill factor
(FF) is defined as:
FF ~ VoptIopt
/VocIsc
(2)
where Vopt and Iopt are voltage and current for maximum
power output, and Voc and Isc are open-circuit photovoltage
and short-circuit photocurrent, respectively. The overall yield
(g) is expressed by the following equation:
Conclusion
In this paper, we designed and synthesized four novel hemi-
cyanine congeners, and investigated systematically the rela-
tionship between the photoelectrochemical properties and
structures. Through investigation of their absorption and
photoelectrochemical properties, among the four hemicyanine
dyes, BIDC4 is found to be the best sensitizer, which has a
conversion yield of 4.9%, with a short-circuit photocurrent of
21.4 mA cm22, an open-circuit voltage of 424 mV and a fill
factor of 0.49 under illumination with 90.0 mW cm22 white
light from a Xe lamp. Our study indicates that through
reasonable design, hemicyanine dyes could perform excellent
spectral sensitization. Furthermore, more efficient hemi-
cyanine dyes as sensitizers could be designed after studying
the structure–efficiency relationship.
g ~ (VocIscFF)/Pin
(3)
where Pin is the power of incident white light. We can see from
Fig. 6 and Table 2 that the short-circuit photocurrent of the
BIDC4-sensitized TiO2 electrode is much higher than those of
BIDC1-, BIDC2- and BIDC3- sensitized systems, respectively.
BIDC1 has the lowest short-circuit photocurrent and overall
yield because of its much lower IPCE values and converts light
to electricity in a relatively narrower light region. BIDC4
exhibits the highest short-circuit photocurrent and overall
yield due to its higher IPCE values in a broad light region.
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Y. Abe, H. Sugihara and H. Arakawa, Chem. Commun., 2000,
1173.
Acknowledgements
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This work was financially supported by the State Key Program
of Fundamental Research (G1998061308), the National Key
Program for Basic Research 2001 CCD 04300, and the
National Nature Science Foundation of China (20023005).
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