Photochemistry and Photophysics of Coordination Compounds

190 S. Campagna et al.
Fig. 19 Schematic operation principle of a dye-sensitized solar cell
semiconductor oxide obtained via a sol–gel procedure. Dye coverage of semiconductor
nanoparticles is generally obtained from alcoholic solutions of the
sensitizer, in which the sintered film is left immersed for a few hours. Sensitizers
are usually designed to have functional groups such as – COOH, – PO3H2,
or – B(OH)2 for stable adsorption onto the semiconductor substrate. The dyecovered
film is in intimate contact with an electrolytic solution containing
a redox couple dissolved in a suitable solvent. The electron donor member of
the redox couple must reduce quickly and quantitatively the oxidized sensitizer,
so closing the circuit. A variety of solvents with different viscosity and of
redox mediators have been the object of intense studies, the most commonly
used being the couple I – 3 /I– in acetonitrile or methoxypropionitrile solution.
The counter electrode is a conductive glass covered with a few clusters of
metallic platinum, which has a catalytic effect in the reduction process of the
electron mediator. Further details on the cells and on their preparation can be
found in the literature [403–405].
The complete photoelectrochemical cycle of the device can be outlined as
follows. The adsorbed sensitizer molecules (S) are brought into their excited
state (S∗ ) by photon absorption and inject one electron into the empty conduction
band of the semiconductor in a timescale of femtoseconds. Injected
electrons percolate through the nanoparticle network and are collected by the
conductive layer of the photoanode electrode, while the oxidized sensitizer
(S + ) in its ground state is rapidly reduced by I – ions in solution. Photoinjected
electrons flow in the external circuit where useful electric work is produced
and are available at the counter electrode for the reduction of the electron
mediator acceptor I – 3 . The entire cycle consists in the quantum conversion of
photons to electrons.
S+hν → S∗ photoexcitation (25)
S∗ +TiO2→S + +(e – ,TiO2) electron injection (26)
2S + +3I – → 2S+I3 – sensitizer regeneration (27)
I3 – +2e – → 3I – electron donor regeneration . (28)