Page 58 - Alternative Energy Systems in Building Design

DYE-SENSITIZED SOLAR CELLS 35

materials and inexpensive production, in the near future they are expected to offer a
price-performance ratio large enough to replace a significant amount of electricity
generated by fossil fuels.

COMPARATIVE ANALYSIS
In order to compare differences between existing solid-state semiconductors and DSCs,
it would be important to review the construction and operational characteristics of
both technologies. As discussed earlier, conventional solid-state semiconductor solar
cells are formed from two doped crystals, one doped with an impurity that forms a
slightly negative bias (which is referred to as an N-type semiconductor and has a free
electron) and the other doped with an impurity that provides a slight positive bias
(which is referred to as a P-type semiconductor and lacks free electrons). When placed
in contact to form a PN junction, some of the electrons in the N-type portion will flow
into the P-type to fill in the gap, or electron hole.
Eventually, enough electrons flow across the boundary to equalize what is called the
Fermi levels of the two materials. The resulting PN junction gives rise to the location
where charge carriers are depleted or accumulated on each side of the interface. This
transfer of electrons produces a potential barrier for electron flow that typically has a
voltage of 0.6–0.7 V.
Under direct exposure to solar rays, photons in the sunlight strike the bound elec-
trons in the P-type side of the semiconductor and elevate their energy, a process that
is referred to as photo-excitation. Figure 3.6 shows a DSC epitaxial configuration.

HIGH-ENERGY CONDUCTION BAND

As a result of impact of photons, the electrons in the conduction band are prompted to
move about the silicon, giving rise to electron flow, or electricity. When electrons flow

−
TRANSPARENT
CONDUCTOR

ELECTROLYTE
+

CATALYTIC
TiO PARTICLES COATED CONDUCTOR
2
WITH DYE MOLECULES
Figure 3.6 DSC epitaxial configuration.