The
concept of metallo-organic solar cells.
- Classical
solar cells take advantage of the fact that photons interacting with the
semiconductor can create electron-hole pairs producing an electrical potential
difference at the junction between two different materials. In dye-sensitised
solar cells photoanode made of nanoporous titanium dioxide support the dye
on the surface (figure 2). The TiO2 structure with a porosity of 50% has
a surface available for dye sorption over thousand times that of a flat
identical surface. TiO2 nanocrystalline grain sizes are in the range of
10-80 nm. Due to its large band gap TiO2 absorbs only the ultraviolet part
of the solar emission and so induces low conversion efficiency. The role
of the dye is to sensitise the TiO2 surface by extending the photosensitivity
to longer wavelengths. This role is capital because the injection efficiency
should be higher as possible about 85-90%. The rate of electron transport
in dye-sensitised solar cells is a major element of the overall efficiency
of the cells. The cell is composed of a nanocrystalline TiO2 layer on which
carboxylic
groups attached
the dye. When exposed to light, the dye interacts with photons and goes
to an excited state sufficiently energetic to inject an electron into the
TiO2 conduction band. This electron can be collected at the transparent
conducting glass (ideal case) or can return to the Ru(III) centre (charge
recombination reaction- arrow 6-fig.1) or can even be trapped by the redox
mediator (dark currents generated-arrow 5-fig.1).
- In details,
three main reactions are triggered by an incident photon: (a) electron injection
from the dye excited state to conduction band of TiO2, (b) relaxation/cooling
processes of the hot electron in the conduction band and in trap states,
and (c) recombination between electron and dye cation and or caption by
redox mediator species. Electron injection can occur on a fast or ultra
fast time scale (fig.2). Recombination reaction occurs in a hundred of nanosecond
time scale. Regeneration of the dye cation by iodide system is even faster.
However reactions of conduction band electrons with radical anion I2°-
(on average of hundred nanosecond time scale in regime I) remains a viable
way for electron losses. Even if some researchers announced no electron
loss when less than one electron per nanoparticule is injected, it is difficult
to understand why, at low light intensity, we observe IPCE values ranging
from 10 to 90% depending on dye molecular structure. Dark current lowers
the overall solar cell performance