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the free-base porphyrin derivative-cellulose:C
60
and Zn porphyrin derivative-cel-
lulose:C
60
in a nitrogen-saturated 0.1 M Na
2
SO
4
aqueous solution containing
hydroquinone as a sacrifice electron donor. The action spectra of free-based
porphyrin derivative-cellulose:C
60
in a ratio 1:2 showed the best response, and the
photoaction spectrum was very similar to the profile of the porphyrin moiety. As it
is well known, it indicates that the porphyrin was the main molecule responsible for
the photocurrent generation. Another interesting behavior pointed out by the
authors was that photocurrent density increased until a thickness of 5 layers was
reached: the direct electron transfer took place from the excited porphyrins toward
the ITO electrode, and, probably as a consequence of electron and energy traps, this
phenomenon was quenched for film with more than five layers.
5 Systems Without Fullerene Derivatives
Investigations by Desormeaux et al. [
36
] were prompted by previous studies on the
photovoltaic properties of chlorophyll
[
37
,
38
]. The knowledge of the effect of
small variations in molecular structure on quantum efficiency of charge generation
of the solar cell devices [
39
] has stimulated the use of chlorophyll
a
in order to
appreciate its photovoltaic properties. Therefore, the photovoltaic peculiarities of
sandwich cells of LB layers of chl
b
, and Zn porphyrin derivative, Zn(II) 12-
carboethoxy-2,7,17-trihexyl-2,8,13,18-tetramethylporphyrin, have been explored
(Fig.
12
).
A layer of cadmium arachidate was deposited on the top aluminum electrode in
order to hydrophobize the solid support. The dark short-circuit currents in all cases
(cells containing chl
a
, chl
b
, and the porphyrin derivative) vary in the range 0.04-
0.60 nA/cm
2
, while the dark open-circuit current-voltage in the range 0.03-0.50 V.
The same cells show short-circuit photocurrents that extend from 1.0 to
18.0 nA/cm
2
and open-circuit photovoltage up to 11 V. The internal resistance
for all these cells range from 30 to more than 1,000 M
a
, chl
b
. The authors propose that
the weak photovoltaic characteristics of such sandwich cells probably depend on
the generation of an insulating layer of aluminum oxide and of the monolayer of the
fatty acid, while the influence of the molecular structure of pigments is negligible.
These conclusions were substantiated by several observations such as photocurrent
vs. photovoltage graphs or the light intensity dependence of the photocurrents.
Gervaldo et al. synthesized metallated and unmetallated porphyrins, Zn(II)-,
Cu(II)-, Pd(II)-, Ni(II)-, Co(II)-, and H
2
-5-(4-carboxyphenyl)-10,15,20-tris
(4-methylphenyl) porphyrin, in order to use them in photoelectrochemical devices
as spectral sensitizers with the goal of broadening their response to the visible
region (Fig.
13
)[
40
].
Two approaches were followed: in the first one, the macrocycles were adsorbed
on SnO
2
nanocrystalline large band gap semiconductor (ITO/SnO
2
electrodes have
been functionalized with the porphyrins by immersion in their solution in a mixture
of hexane and dichloromethane as the solvent), while in the second one, ITO
electrodes were covered with LB films of the dyes. The LB layers were also
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