Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 189
monolayer of dye molecules. The breakthrough in the field was brought about
by the introduction of mesoscopic films made of sintered nanoparticles of
a semiconductor metal oxide with a large surface area, which allowed the
adsorption, at monolayer coverage, of a much larger number of sensitizer
molecules leading to absorbance values, of thin films of a few microns, well
above unity [407].
Wide-bandgap semiconductor materials such as TiO2 show a separation
between the energy levels of the valence and conduction bands of the order
of 3 eV, which means that the electron–hole pair needs to be produced by irradiation
with light having a wavelength shorter than 400 nm, i.e., UV light.
In order to use sunlight, mainly visible and near-IR, two general approaches
have been developed: doping and molecular sensitization. The doping approach
is the preferred choice for conventional photovoltaic devices [405].
Dye-sensitized photelectrochemical cells rely on photosensitization. In this
process, a photoexcited species, the sensitizer (S), is capable of injecting an
electron into the conduction band (CB) or a hole into the valence band (VB)
of the semiconductor (Fig. 18).
Fig. 18 Schemes for sensitized charge injection in the photoelectrochemical solar cells:
a electron injection, b hole injection
In fact, when the excited-state energy level of the sensitizer is higher with
respect to the bottom of the conduction band, an electron can be injected
with no thermal activation barrier in the semiconductor, leaving the sensitizer
in its one-electron oxidized form (Fig. 18a). When the excited state
is lower in energy with respect to the top of the valence band, an electron
transfer (formally a hole transfer) between the semiconductor and the sensitizer
can take place, leaving the molecule in its one-electron reduced form
(Fig. 18b) [408].
The operation of a dye-sensitized solar cell is schematized in Fig. 19 [403,
405]. The system is comprised of two facing electrodes, a photoanode and
a counter electrode, with an electrolyte in between. The transparent conductive
photoanode is covered with a thin film (7–10 µm) of a mesoporous