Fused five-membered porphyrin for dye-sensitized solar cells

Article abstract of DOI:10.1246/cl.2008.846

Fused five-membered zinc porphyrin carboxylic acid has been synthesized to improve light-harvesting capability in the visible and near-infrared regions. The fused porphyrin-sensitized TiO₂ cell exhibited the photocurrent generation extending ov

Full text of DOI:10.1246/cl.2008.846

846
Chemistry Letters Vol.37, No.8 (2008)
Fused Five-membered Porphyrin for Dye-sensitized Solar Cells
Shinya Hayashi,1 Yusuke Matsubara,1 Seunghun Eu,1 Hironobu Hayashi,1 Tomokazu Umeyama,1
Yoshihiro Matano,1 and Hiroshi Imahoriꢀ1;2;3
1Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510
2Institute for Integrated Cell-Material Sciences, Kyoto University, Nishikyo-ku, Kyoto 615-8510
3Fukui Institute of Fundamental Chemistry, Kyoto University, 34-4 Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103
(Received April 18, 2008; CL-080408; E-mail: imahori@scl.kyoto-u.ac.jp)
Fused five-membered zinc porphyrin carboxylic acid has
porphyrin (fused-H2P) was obtained by the treatment of H2P-Br
with Pd0 catalysis.6b Zinc(II)-metallation and hydrolysis of
fused-H2P gave fused-ZnP. ZnP-ref was also prepared for
comparison.3b The synthetic details and characterization are
provided in Supporting Information.8
been synthesized to improve light-harvesting capability in the
visible and near-infrared regions. The fused porphyrin-sensitized
TiO2 cell exhibited the photocurrent generation extending over
800 nm, although the power conversion efficiency was found
to be moderate (0.30%).
UV–vis absorption spectra of fused-ZnP and ZnP-ref were
measured in CH2Cl2. The peak positions and molar absorption
coefficients (") are listed in Table S1.8 Compared with ZnP-
ref, fused-ZnP reveals broadening and red shift of Soret and Q
bands due to expansion of the ꢁ system with low symmetry
(Figure 1), as seen in fused six-membered porphyrins.5a It is
noteworthy that the absorption edge of fused-ZnP is extended
to ca. 800 nm. Steady-state fluorescence spectra of fused-ZnP
and ZnP-ref were also measured in CH2Cl2 by exciting at the
Soret band wavelength (Figure S1).8 The fluorescence spectrum
of fused-ZnP also reveals red-shifted emission with a maximum
of 772 nm, relative to ZnP-ref, which is consistent with the
results obtained from the UV–vis absorption spectra. The first
oxidation potentials (Eox) of fused-ZnP and ZnP-ref were deter-
mined by differential pulse voltammetry in CH2Cl2 containing
Bu4NPF6 as a supporting electrolyte (Table S1).8 Owing to
the elongation of the ꢁ system, the Eox value of fused-ZnP
(0.93 V vs. NHE) is negatively shifted by 0.07 V in comparison
Recently, dye-sensitized solar cells (DSSC) have attracted
much attention because of their relatively high power conversion
efficiency (ꢀ) and potential low-cost production.1 Ruthenium(II)
bipyridyl complexes have proven to be the most efficient TiO2
sensitizers (ꢀ ¼ 9{11%),2 whereas it is highly desirable to
develop other organic dyes using inexpensive metal or without
metal, which exhibit high cell performance.3 Porphyrins have
strong absorption at 400–450 nm (Soret band) and relatively
weak absorption at 500–700 nm (Q bands), thus they have been
regarded as promising sensitizers in DSSC.4 The drawback
of porphyrins is the insufficient light-harvesting ability in the
visible and near-infrared (NIR) regions relative to Ru-based
dyes. To surmount this problem, we have synthesized unsym-
metrically ꢁ-elongated porphyrins for DSSC.5 As the results
of the elongation of the porphyrin ꢁ-system with low symmetry,
the absorption of the porphyrins was broadened and red-shifted,
leading to improvement of the ꢀ value up to 5.2%.5 Neverthe-
less, in terms of the light-harvesting properties of porphyrins
in the NIR region there is still room for further improvement.
Here, we report the first application of fused five-membered
porphyrin to DSSC. Although fused five-membered porphyrins
have been known to reveal broadening and red shift of the
absorption, they have yet to be used in DSSC.6 The synthetic
route to fused-ZnP is shown in Scheme 1. Starting porphyrin
(H2P) was treated with N-bromosuccinimide (NBS) to yield
monobrominated porphyrin (H2P-Br). The intramolecular fused
with ZnP-ref (1.00 V vs. NHE). The excited-state oxidation
ꢀ
potentials (Eox
)
of fused-ZnP (ꢁ0:69 V) and ZnP-ref
(ꢁ1:10 V) were estimated from the respective Eox values and
the zeroth–zeroth energy (E0ꢁ0). Taking into account the energy
levels of the conduction band (CB) of TiO2 (ꢁ0:5 V vs. NHE)5
ꢁ
and Iꢁ/I3 couple (0.5 V vs. NHE),5 electron injection from the
porphyrin excited singlet state to the CB of TiO2 and charge shift
from Iꢁ to the resulting porphyrin radical cation are thermody-
namically feasible both in the fused-ZnP and ZnP-ref cells
4
Ar
M
Ar
M
Ar
3
N
N
N
N
N
N
N
N
N
N
N
Ar
Ar
Ar
Ar
Ar
Ar
M
x 3
N
a
b
Br
2
Ar = mesityl
COOR
COOR
H2P-Br
COOR
fused-H2P
ε
H2P
1
(M = H2, R = Me)
(M = H2, R = Me)
(M = H2, R = Me)
c, d
c, d
x 30
fused-ZnP
ZnP-ref
(M = Zn, R = H)
(M = Zn, R = H)
0
400
500
600
700
800
900
Scheme 1. Reagent and conditions: a) NBS, CHCl3, reflux,
.
Wavelength / nm
4 h; b) K2CO3, Pd2(dba)3 CHCl3, DMF, reflux, 30 min; c)
Zn(OAc)2, CHCl3, MeOH, rt, 2 h; d) NaOH, H2O, THF, reflux,
1 day.
Figure 1. UV–vis absorption spectra of fused-ZnP (solid line)
and ZnP-ref (dashed line) in CH2Cl2.
Copyright ꢀ 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.8 (2008)
847
(Table S1).8 Geometry optimization and vibrational frequency
analysis of fused-ZnP and ZnP-ref were performed by DFT
methods. Both of the optimized geometries have no negative
frequencies. The optimized geometries show that fused-ZnP
and ZnP-ref possess planar porphyrin rings (Figure S2).8 The
electron densities of HOMO and LUMO in fused-ZnP and
ZnP-ref are delocalized over the fused five-membered porphyrin
and the porphyrin, respectively (Figure S3).8
The porphyrin-modified TiO2 electrodes were prepared by
immersing the mesoporous TiO2 electrodes with 10-mm thick-
ness into 0.2 mM porphyrin MeOH solution at room temperature
for 1 h. Given the surface area of P25 (54 m2ꢂgꢁ1),5 the porphy-
rin densities (ꢂ) on the actual surface area are determined to be
1:1 ꢃ 10ꢁ10 molꢂcmꢁ2 for TiO2/fused-ZnP and 1:2 ꢃ 10ꢁ10
molꢂcmꢁ2 for TiO2/ZnP-ref. Assuming that the porphyrin
molecules are densely packed onto the TiO2 surface to make
a monolayer, the ideal ꢂ values are calculated to be 1:2 ꢃ
10ꢁ10 molꢂcmꢁ2 both for fused-ZnP and ZnP-ref. The experi-
mental ꢂ values are in good agreement with the calculated
ꢂ values, implying that a well-packed porphyrin monolayer
is formed on the TiO2 surface. UV–vis absorption spectra
of TiO2/fused-ZnP and TiO2/ZnP-ref electrodes were also
measured. The light-harvesting properties of TiO2/fused-ZnP
at around 400–500 and 600–800 nm are remarkably improved
compared to those of TiO2/ZnP-ref (Figure 2) considering the
solar energy distribution on the earth.
the IPCE value of the fused-ZnP cell (9.5% at 435 nm) is much
smaller than that of the ZnP-ref cell (76% at 420 nm). Neverthe-
less, owing to the elongation of the porphyrin ꢁ system, the pho-
tocurrent response is extended over 800 nm in the fused-ZnP
cell, which is the longest wavelength among porphyrin-sensi-
tized solar cells.4,5 According to a previous paper7 at least, more
than 0.2 eV of driving force is necessary for efficient electron
injection. Accordingly, insufficient driving force for the electron
injection (0.19 eV) for the fused-ZnP cell may be responsible
for the moderate photovoltaic properties of the fused-ZnP cell.
Moreover, the decreased VOC for the fused-ZnP cell may result
from fast charge recombination from the electron in the CB of
ꢁ
ꢄ
TiO2 to ZnPþ and/or I3 relative to the ZnP-ref cell.
In conclusion, we have successfully prepared novel
fused five-membered porphyrin for DSSC. Although the light-
harvesting properties of the fused porphyrin in the visible
and NIR regions are remarkably improved relative to the typical
tetraphenylporphyrin reference, the photovoltaic properties
are moderate. Improvement of the cell performance in fused
five-membered porphyrins may be possible by modulating the
electronic structure of the porphyrins to optimize the electron
injection process.
This work was supported by Grant-in-Aid (No. 19350068
for H.I.) from the Ministry of Education, Culture, Sports, Sci-
ence and Technology (MEXT), Japan and NEDO. Computation
time was provided by the Supercomputer Laboratory, Institute
for Chemical Research, Kyoto University, and Academic Center
for Computing and Media Studies, Kyoto University.
Cell performances of the TiO2/fused-ZnP and TiO2/ZnP-
ref electrodes were evaluated under standard AM 1.5 conditions
(100 mWꢂcmꢁ2).5 The current–voltage characteristics are given
in Figure S4. The ꢀ value of the fused-ZnP cell (JSC
¼
0:88 mAꢂcmꢁ2, VOC ¼ 0:51 V, ff ¼ 0:67, ꢀ ¼ 0:30%) is much
References and Notes
smaller than that of the ZnP-ref cell (JSC ¼ 9:4 mAꢂcmꢁ2
,
1
2
3
B. O’Regan, M. Gratzel, Nature 1991, 353, 737.
¨
M. Gratzel, Inorg. Chem. 2005, 44, 6841.
¨
VOC ¼ 0:76 V, ff ¼ 0:64, ꢀ ¼ 4:6%). Action spectra of incident
photon-to-current efficiency (IPCE) for the porphyrin-sensitized
cells are depicted in Figure 2. The action spectra and absorption
spectra are virtually similar for TiO2/fused-ZnP and TiO2/
ZnP-ref, implying the involvement of the porphyrins for the pho-
tocurrent generation. In accordance with the cell performances,
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x 100
0
400
500
600
700
800
900
6
7
Wavelength / nm
Figure 2. Action spectra (with circles) and UV–vis absorption
spectra (without circles) of TiO2/fused-ZnP (solid line) and
TiO2/ZnP-ref (dashed line) electrodes. Thickness of the TiO2
films used for the absorption measurements was adjusted to
be 0.7–1.0 mm. The absorption spectra are normalized for com-
parison.
8
Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
Published on the web (Advance View) July 5, 2008; doi:10.1246/cl.2008.846