Environmental Engineering Reference
In-Depth Information
This so-called Graetzel's cell has reported to give nearly 10% conversion efficiency
under AM 1.5 solar irradiation with high cost-performance, and so is attracting a great deal of
attention as a future commercial solar cell. One of the problems of this cell to be solved for
practical use is that it uses organic liquid. To overcome this problem solidification of the
organic redox electrolyte solution by molten salts and gelator [34] or by polymer film [35]
has been achieved. Another approach is to use water [36,37], but the efficiency and stability
of the cell in an aqueous phase have been low.
As shown in the sections 3 and 4, the present author has reported that polysaccahrides
such as agarose (1) and κ-carrageenan (2) can form a tight and elastic solid containing excess
water, and that electrochemical and photochemical reactions can take place in the solid the
same as in pure water. The hardness of the solid is, for instance, almost the same as a brick
cheese and one third of a conventional rubber eraser for a 2 wt%κ-carrageenan solid
involving excess water. We have expected that such an interesting solid containing a large
excess liquid could offer a solid state medium for the photosensitized cell. We have
succeeded to substitute the water in the solid with organic liquid, and found that this solid
involving organic solvent and I
-
/I
3
-
redox electrolyte works well as a medium for the TiO
2
cell
with a well-known N3 dye (4) sensitizer [11,12].
(4) N3 dye:
cis-
bis(isothiocyanato)bis(4,4'-dicarboxyl-2,2'-bipyridine)- ruthenium(II), Ru(dcbpy)
2
(NCS)
2.
Experimental and Results for Solid-Type Dye-Sensitized Solar Cell
A colloidal aqueous solution of TiO
2
nanoparticles (P-25) was spin-coated on an ITO
electrode (1.0 x 2.0 cm) and heated at 100
℃
for 30 min. This procedure was repeated several
times and then finally the TiO
2
coated ITO was heated at 450
℃
for 30 min to prepare a
nanoporous TiO
2
thin film of 10 μm thickness. This ITO/TiO
2
film was soaked in a 1.5 x 10
-4
M ethanol solution of
cis-
bis(isothiocyanato)bis(4,4'-dicarboxyl-2,2'-bipyridine)-
ruthenium(II), Ru(dcbpy)
2
(NCS)
2
(called N3) to adsorb the complex onto the TiO
2
. A 2
wt%κ-carrageenan was dissolved in water by applying very carefully a high frequency wave
(2.45 GHz). Before solidifying the solution was poured onto the TiO
2
/N3 film and solidified
by cooling down to room temperature. (C
3
H
7
)
4
NI and I
2
(10:1) were dissolved in a mixture of
acetonitrile and 3-methyl-2-oxazolidinone (1:1) so that their concentrations become 0.3 M
and 0.03 M, respectively, and the water in the carrageenan solid on the TiO
2
/N3 film was
substituted by the mixture solution by dipping the TiO
2
/N3/carrageenan solid film into the
mixture solution. As for the counter electrode, a 5 mM H
2
PtCl
6
ethanol solution was spin-
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