Page 61 - Alternative Energy Systems in Building Design

38 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

anode located on top). In the meantime, each dye molecule loses an electron, which could
result in decomposition of the substance if another electron is not provided.
In the process, the dye strips one electron from iodide in the electrolyte below the
TiO , oxidizing it into a substance called triiodide. This reaction takes place very
2
quickly when compared with the time it takes for the injected electron to recombine
with the oxidized dye molecule, thus preventing a recombination reaction that would
cause the solar cell to short-circuit. The triiodide then recovers its missing electron by
mechanically diffusing to the bottom of the cell, where the counterelectrode reintro-
duces the electrons after they flow through the external circuit load. Figure 3.8 shows
a flexible nanotechnology solar PV module.
Power output measurement, is expressed as the product of short-circuit current I sc
and open voltage V . Another solar cell efficiency measurement, defined as quantum
oc
efficiency, is used to compare the chance of one photon of impacted solar energy
resulting in the creation of a single electron.
In quantum efficiency terms, DSCs are extremely efficient. Owing to their nanostruc-
tured configurations, there is a high chance that a photon will be absorbed. Therefore,
they are considered highly effective in converting solar rays into electrons.
Most of the power conversion losses in DSC technology result from conduction
losses in the TiO and the clear electrode, as well as optical losses in the front elec-
2
trode. Their overall quantum efficiency is estimated to be approximately 90 percent.
The maximum voltage generated by DSCs is simply the difference between the
Fermi level of the TiO and the potential of the dye electrolyte, which is about 0.7 V
2
total (V ). This is slightly higher than semiconductor-based solar cells, which have a
oc
maximum voltage of about 0.6 V.
Even though DSCs are highly efficient in turning photons into electrons, it is only
those electrons with enough energy to cross the TiO bandgap that result in current
2

Figure 3.8 A nanotechnology solar PV cell. Photo courtesy
of Nano Solar.