(a)
(b)
porphyrin units. The corresponding absorption peak of Zn-ZnA-
Zn on TiO2 was broadened to both sides from the peak position
in solution. On the other hand, the corresponding absorption of
ZnA-ZnA-ZnA on TiO2 was mainly broadened toward the
shorter-wavelength direction. This feature indicates the increas-
ed ratio of the trimer with high coplanarity between the adjacent
porphyrins, where the transition dipole moments are arranged
in parallel, leading the blue-shifted absorption. In the case of the
coplanar conformation, π conjugation is effectively extended,
making the Q band absorption of ZnA-ZnA-ZnA red-shifted.
This tendency is explained by assuming that each porphyrin
unit is arranged longitudinally with the carboxy group directing
to the surface, as shown in Figure 1 insets. In the case of Zn-
ZnA-Zn, the central porphyrin unit is anchored onto the surface,
while the side porphyrin units are not anchored. Therefore, the
side porphyrin units can rotate, adopting various dihedral angles
between the central porphyrin. Because of the steric effect,
the side porphyrin units tend to be parallel to the surface. The
horizontal arrangement of a porphyrin unit increases the
occupying area compared to the vertical arrangement. Therefore,
Zn-ZnA-Zn molecules occupy a relatively large area of the
surface, adopting various dihedral angles between adjacent
porphyrin units. Such arrangement is supported by the fact that
the absorbance of Zn-ZnA-Zn on TiO2 did not increase under the
long dipping time condition. In the case of ZnA-ZnA-ZnA, all
the porphyrin units can be anchored onto the surface. Therefore,
the porphyrin units in ZnA-ZnA-ZnA should adopt the coplanar
conformation with each other. This is in accordance with
the absorption spectra of ZnA-ZnA-ZnA on TiO2. If a planar
molecule is arranged perpendicular to the surface, the molecule
occupies a smaller area of the surface. Therefore, the absorbance
of ZnA-ZnA-ZnA on TiO2 increased along with the increase in
the dipping time. As seen here, the experimental results indicate
the alteration of the ratio of coplanar and nonplanar conforma-
tions upon the adsorption of trimers onto the TiO2 surface.
To suppress the aggregate formation on the TiO2 surface,
deoxycholic acid (DCA) as coadsorbents was added to the dye
solution during the immersion process. Figure 2a shows the
DCA concentration dependence of the absorbance of the trimers
on 3-¯m-thick TiO2 films. The absorbance of the trimers
monotonically decreased along with the increase in the amount
of DCA, indicating the increase of the coverage of DCA on the
TiO2 surface. Compared to the trimer with single-anchoring
group, the absorbance of the trimers with plural-anchoring
groups decreased moderately along with the increase in the
amount of DCA. This result indicates that the increase in the
number of anchoring groups enhances the adsorbability of the
trimers onto the TiO2 surface. DSSCs were prepared with ca. 10-
¯m-thick TiO2 electrode. The IPCE values of the DSSCs using
the trimers at the corresponding wavelength were plotted with
respect to various DCA concentrations, as shown in Figure 2b.
In spite of the decrease in the absorbance, the IPCE values of
these DSSCs increased by the addition of DCA, indicating that
the charge-separation efficiency of the trimers were improved by
the addition of DCA.
1.0
0.8
0.6
0.4
0.2
0
50
40
30
20
10
0
0
100 200 300
DCA concentration / mM
0
100 200 300
DCA concentration / mM
Figure 2. DCA concentrations dependence of (a) absorbance
at 500 nm and (b) IPCE values of Zn-ZnA-Zn (triangles), ZnA-
Zn-ZnA (circles), or ZnA-ZnA-ZnA (squares).
12
10
8
0.8
0.6
0.4
6
4
2
0
3.0
2.0
1.0
0.0
0.4
η
0.3
0
100 200 300
DCA concentration / mM
0
100 200 300
DCA concentration / mM
Figure 3. DCA concentration dependence of the solar cell
performance parameters of the DSSCs using Zn-ZnA-Zn
(triangles), ZnA-Zn-ZnA (circles), or ZnA-ZnA-ZnA (squares).
cally increased under this condition. These tendencies were in
accord with the results of the IPCE values. The open-circuit
photovoltage (VOC) of the trimers increased by the addition
of DCA. The fill factor (FF) was almost constant, regardless of
the DCA concentrations. Reflecting the tendencies of JSC and
V
OC, the photoelectric conversion efficiency (©) of ZnA-Zn-ZnA
showed the highest value.
As shown in Figure 4, the IPCE maximum of the DSSC
using Zn-ZnA-Zn with a large amount of DCA in the Q band
region appeared at the shorter wavelength than that without
DCA. This tendency is opposite from the absorption maxima,
which appeared at the longer wavelengths along with the
increase in the amount of DCA. In the case of ZnA-ZnA-ZnA,
the IPCE maximum in the Q band region appeared at the longer
wavelength than the absorption maximum. These results are
interpreted as follows. The change in the absorption of Zn-ZnA-
Zn upon the addition of DCA is caused by the suppression of
the formation of H-type aggregates and/or the increased portion
Figure 3 shows the solar cell performance parameters of the
DSSCs using the trimers, plotted versus the DCA concentra-
tions. The short-circuit photocurrent density (JSC) of ZnA-Zn-
ZnA reached the maximum value at the higher concentration
than that of Zn-ZnA-Zn, while that of ZnA-ZnA-ZnA monotoni-
© 2014 The Chemical Society of Japan | 797