Novel unsymmetrically π-elongated porphyrin for dye-sensitized TiO 2 cells

Article abstract of DOI:10.1039/b702501g

A novel naphthyl-fused zinc porphyrin carboxylic acid has been synthesized and employed successfully in a dye-sensitized TiO₂ solar cell, with a power conversion efficiency of 4.1%, which is improved by 50% relative to the unfused porphyrin ref

Full text of DOI:10.1039/b702501g

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Novel unsymmetrically p-elongated porphyrin for dye-sensitized TiO2
cells{
Masanobu Tanaka,a Shinya Hayashi,b Seunghun Eu,b Tomokazu Umeyama,b Yoshihiro Matanob and
Hiroshi Imahori*b
Received (in Cambridge, UK) 16th February 2007, Accepted 11th April 2007
First published as an Advance Article on the web 26th April 2007
DOI: 10.1039/b702501g
to the conduction band (CB) of a TiO2 electrode, a carboxyl group
is attached to the fused naphthyl moiety in the porphyrin ring.
Bulky mesityl groups are also introduced at the three meso
positions of the porphyrin ring to reduce the aggregation of the
porphyrin molecules on the TiO2 surface.6,7
A novel naphthyl-fused zinc porphyrin carboxylic acid has been
synthesized and employed successfully in a dye-sensitized TiO2
solar cell, with a power conversion efficiency of 4.1%, which is
improved by 50% relative to the unfused porphyrin reference
cell.
The synthetic route to fused-Zn-1 is shown in Scheme 1.
5-(4-Carbomethoxynaphth-1-yl)-10,15,20-tris(2,4,6-trimethylphenyl)
porphyrin (H2-1) was synthesized by the cross-condensation of
mesityldipyrromethane with 4-carbomethoxynaphthylalde-
hyde and 2,4,6-trimethylbenzaldehyde. Ni(II)-metallation was
carried out by the treatment of H2-1 with Ni(acac)2 at 110 uC
to yield Ni-1. Ring-closure was achieved by the treatment of
Ni-1 with FeCl3.5e Subsequent demetallation, hydrolysis, and
Zn(II)-metallation gave fused-Zn-1. Porphyrin reference Zn-1
was also prepared from H2-1. Their structures were verified by
Since the first report on a high power conversion efficiency (g) in
dye-sensitized solar cells by Gra¨tzel and coworkers, they have been
extensively investigated.1 To date, ruthenium(II) bipyridyl com-
plexes have proven to be the most efficient TiO2 sensitizers (g 5 9–
11%).2 On the other hand, the use of porphyrins as light harvesters
on solar cells is particularly attractive given their primary role in
photosynthesis.3 However, the g values of porphyrin-sensitized
nanocrystalline TiO2 cells are much smaller than those of
ruthenium dye-sensitized nanocrystalline TiO2 cells because of
their insufficient light-harvesting properties in the visible region.3
Porphyrins have very strong absorption in the 400–450 nm
region (Soret band) and weak absorption in the 500–700 nm
region (Q bands).4 Both Soret and Q bands arise from p–p*
transitions and can be explained in terms of a linear combination
of transitions from slightly splitting HOMO and HOMO-1 to a
degenerate pair of LUMOs. The configuration interaction leads to
the intense short-wavelength Soret band and the weak long-
wavelength Q bands. The elongation of p-conjugation and loss of
symmetry causes splitting in the p and p* levels and reduces the
HOMO–LUMO gap, resulting in broadening of the absorption
and an increasing intensity of Q bands. More importantly, the
Soret and Q bands are red-shifted, which improves the overlap
between the absorption and solar energy distribution on the earth.
Although there have been several reports on the synthesis of
aromatic ring-fused, p-extended porphyrins with low symmetry,5
they have never been applied to dye-sensitized solar cells.
1
spectroscopic analyses including H NMR and MALDI-TOF
mass spectra (see Supporting Information).
Fig. 1 displays UV–visible absorption spectra of fused-Zn-1 and
Zn-1 in CH2Cl2. As we expected, the light-harvesting properties of
fused-Zn-1 are improved by the expansion of the p-system and loss
of symmetry. The absorption of fused-Zn-1 (lmax 5 482 nm,
e 5 1.24 6 105; 682 nm, e 5 2.46 6 104) is red-shifted and
broadened compared to that of Zn-1 (422 nm, e 5 4.33 6 105;
551 nm, e 5 2.07 6 104). The molar absorptivity at the Soret band
Here we report the first application of aromatic ring-fused,
p-extended porphyrins with low symmetry (fused-Zn-1) to dye-
sensitized solar cells. We designed a novel naphthyl-fused zinc
porphyrin carboxylic acid (Scheme 1). The unsymmetrically
p-elongated porphyrin is expected to collect visible light efficiently,
leading to the improvement of the photovoltaic properties. To
facilitate electron injection from the porphyrin excited singlet state
aKyoto University International Innovation Center, Nishikyo-ku, Kyoto
615-8520, Japan
bDepartment of Molecular Engineering, Graduate School of
Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan.
E-mail: imahori@scl.kyoto-u.ac.jp; Fax: +81-75-383-2571;
Tel: +81-75-383-2566
{ Electronic supplementary information (ESI) available: Experimental,
electrochemical data. See DOI: 10.1039/b702501g
Scheme 1 Reagents and conditions: a) BF3(OEt2), CHCl3, r.t., 2 h; b)
DDQ, 1 h, 9% (2 steps); c) Ni(acac)2, toluene, reflux, 20 h, 82%; d) FeCl3,
CH2Cl2, CH3NO2, r.t., 30 min; e) TFA, H2SO4, 16% (2 steps); f) KOH,
H2O, THF, EtOH, reflux, 4 h; g) Zn(OAc)2, CHCl3, r.t., 4 h, 77% for
fused-Zn-1 (2 steps).
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Zn-1 for 1 h at room temperature. The total amounts of the
porphyrins adsorbed on the TiO2 films were determined by
measuring the absorbance of the porphyrins, which were dissolved
from the porphyrin-adsorbed TiO2 films into DMF containing
0.1 M NaOH.7 Taking into account the surface area of P25
(54 m2 g21),7,9 the porphyrin densities (C) on the actual surface
area were determined from the total amount of the porphyrins
adsorbed on the TiO2 films (fused-Zn-1, 7.8 6 10211 mol cm22
;
Zn-1, 8.2 6 10211 mol cm22). Assuming that the porphyrin
molecules are densely packed onto the TiO2 surface to make a
monolayer where the single bond between the carboxyl group and
the naphthyl moiety is perpendicular to the TiO2 surface, the ideal
C values are estimated to be 9.1 6 10211 for fused-Zn-1 and 1.0 6
10210 for Zn-1, which largely agree with the experimental values.
Reflecting the distorted structure of fused-Zn-1, the ideal C value
of fused-Zn-1 is smaller than that of Zn-1 (see Fig. S2). Immersion
of the TiO2 electrode into the porphyrin solutions for a longer time
(.1 h) did not increase the C value, showing saturated behavior of
the porphyrin adsorption on the TiO2 surface.
Fig. 1 UV–visible absorption spectra of fused-Zn-1 (solid line) and Zn-1
(dashed line) in CH2Cl2.
The current–voltage characteristics were measured using the
10 mm-thick TiO2 electrode modified with each porphyrin and a Pt
counter electrode under AM 1.5 conditions (100 mW cm22).7 The
g value is derived from the equation g 5 JSC 6 VOC 6 ff, where
JSC is short circuit current density (mA cm22), VOC is open circuit
potential (V), and ff is fill factor. For the fused-Zn-1-sensitized cell,
JSC of 10.6 mA cm22, VOC of 0.62 V, and ff of 0.62 yield g of 4.1%,
while for the Zn-1-sensitized cell, JSC of 6.7 mA cm22, VOC of
0.61 V, and ff of 0.68 yield g of 2.8% (see Fig. S3). Thus, the g
value of the fused-Zn-1-sensitized cell is larger by 50% than that of
the Zn-1-sensitized cell. Action spectra of incident photon-to-
current efficiency (IPCE) for the fused-Zn-1- and the Zn-1-
sensitized cells are depicted in Fig. 2. The action spectra are similar
to the corresponding absorption spectra on the TiO2 electrode
(Fig. S4) as well as in CH2Cl2 (Fig. 1). It is noteworthy that the
fused-Zn-1-sensitized cell reveals high IPCE values of up to 55%,
of fused-Zn-1 is lower than that of Zn-1, however, the integrated
molar absorptivity with respect to wavenumber at the Soret band
region (400–530 nm) is rather comparable for fused-Zn-1 (3.3 6
108) and Zn-1 (4.2 6 108). More importantly, the integrated value
of the molar absorptivity at the Q band region (530–750 nm) of
fused-Zn-1 (5.0 6 107) is two times larger than that of Zn-1 (2.5 6
107). These results ensure that the incident light is absorbed
intensively in the visible and near-infrared regions by fused-Zn-1.
The fluorescence spectra of fused-Zn-1 and Zn-1 were also
measured in CH2Cl2 excited at the peak position of the Soret
band (see Fig. S1). The emission maximum of fused-Zn-1 (716 nm)
is red-shifted relative to that of Zn-1 (601 nm), which is consistent
with the results on the UV–visible absorption spectra. From the
intercept of the normalized absorption and emission spectra,
the zeroth–zeroth energies (E0-0) are determined (see Table S1).
The difference in E0-0 values of fused-Zn-1 (1.80 eV) and Zn-1
(2.15 eV) implies a small HOMO–LUMO gap of the p-elongated
porphyrin. Cyclic voltammetry was used to determine the first
oxidation potentials of the porphyrins. All porphyrins exhibited
reversible waves for a one-electron oxidation in CH2Cl2 containing
0.1 M Bu4NPF6. The first oxidation potential of fused-Zn-1 (0.99 V
vs. NHE) is shifted by 0.05 V in the negative direction compared to
that of Zn-1 (1.04 V vs. NHE) as a result of the p-elongation. The
energy levels of the porphyrin excited singlet state for fused-Zn-1
(20.78 V vs. NHE) and Zn-1 (21.11 V vs. NHE) are higher than
the CB of TiO2 (20.5 V vs. NHE),7 whereas the energy levels of
the porphyrin radical cation for fused-Zn-1 (0.99 V vs. NHE) and
2
Zn-1 (1.04 V vs. NHE) are lower than those of the I2/I3 couple
(0.5
V
vs. NHE).7 Thus, charge separation from the
porphyrin excited singlet state to the CB of TiO2 and charge shift
2
from the I2/I3 couple to the porphyrin radical cation are
thermodynamically feasible.8
Nanoporous films were prepared from a colloidal suspension of
TiO2 nanoparticles (P25) dispersed in distilled water.9 The
suspension was deposited on a transparent conducting glass by
using a doctor blade technique. The films were annealed at 673 K
for 10 min, followed by similar deposition and annealing (723 K,
2 h) for the 10 mm-thick TiO2 films. The TiO2 electrodes were
immersed in MeOH containing 0.2 mM fused-Zn-1 for 0.5 h or
Fig. 2 Action spectra of fused-Zn-1 (solid line) and Zn-1 (dashed
line)-sensitized solar cells. Conditions: electrolyte 0.1 M LiI, 0.05 M I2,
0.6 M 2,3-dimethyl-1-propylimidazolium iodide, and 0.5 M 4-t-butyl-
pyridine in CH3CN; input power: AM 1.5 under simulated solar light
(100 mW cm22).
2070 | Chem. Commun., 2007, 2069–2071
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4.1%, which was improved by 50% relative to the unfused
porphyrin reference cell. These results clearly show that the
elongation of a porphyrin p-system with low symmetry is a useful
tactic for collecting solar light in the visible and near infrared
regions, leading to improved cell performance of porphyrin-
sensitized solar cells. Further improvement of the power conver-
sion efficiency in porphyrin-sensitized TiO2 cells will be possible by
designing a planar fused porphyrin in which a larger aromatic ring
is used as a fused moiety instead of a naphthalene ring.
This work was supported by the Strategic University/Industry
Alliance of the International Innovation Center, Kyoto University.
We gratefully acknowledge Prof. Susumu Yoshikawa (Kyoto
University) and Prof. Shozo Yanagida (Osaka University) for the
use of equipment for photovoltaic measurements. Computation
time was provided by the Supercomputer Laboratory, Institute for
Chemical Research, Kyoto University, and the Academic Center
for Computing and Media Studies, Kyoto University.
Notes and references
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Fig. 3 Molecular orbitals of fused-Zn-1 and Zn-1 calculated at the
B3LYP/3-21G* level.
extending the response of the photocurrent generation close to
800 nm. The improved photocurrent generation of the fused-Zn-1-
sensitized cell relative to that of the Zn-1-sensitized cell parallels the
results on the g values. To shed light on the electronic structures of
fused-Zn-1 and Zn-1, DFT calculations were performed.
Geometry optimization and vibration frequency analysis were
carried out at the B3LYP/3-21G* level. The optimized structures
have no negative frequencies. The electron densities of HOMO
and LUMO in fused-Zn-1 are delocalized over the elongated p
system (Fig. 3), which is in good agreement with the UV–visible
absorption and fluorescence spectra and the electrochemical
studies. It should be noted here that there are significant electron
densities on the carboxyl group in LUMO of fused-Zn-1, whereas
no apparent electron densities are seen on the carboxy group in
LUMO of Zn-1. The comparison supports efficient electron
injection from the porphyrin excited singlet state to the CB of the
TiO2 electrode in the fused-Zn-1-sensitized cell owing to the strong
electronic coupling between the porphyrin and the TiO2 surface
through the carboxyl group. Thus, the strong electronic coupling
also rationalizes the higher g value of the fused-Zn-1-sensitized cell
in comparison with the Zn-1-sensitized cell. On the other hand, the
porphyrin plane of fused-Zn-1 is strained due to the steric
hindrance between the b-proton and that of the fused naphthalene
ring compared to that of Zn-1. The strain of the porphyrin ring is
likely to cause fast nonradiative relaxation in the porphyrin excited
singlet state, resulting in moderate improvement of the power
conversion efficiency.
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In conclusion, we have successfully synthesized a novel
unsymmetrically p-elongated porphyrin to apply it to a dye-
sensitized TiO2 solar cell for the first time. The porphyrin-
sensitized TiO2 cell exhibited a power conversion efficiency of
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