Environmental Engineering Reference
In-Depth Information
where the diffusion lengths can be many
m, as outlined at the end of Chapter 3. It
seems that lowmobility and short diffusion lengthsmust necessarily limit the ef
ciency
of dye-sensitized and other organic types of solar cells.
m
6.5.2
Questions of Efficiency
State-of-the-art performance of such a dye-sensitized cell is shown in Figure 6.18.
The dye-sensitized solar cell represented in Figure 6.18, under simulated AM1.5G
irradiation, exhibited short-circuit photocurrent 20.5mA/cm
2
, open-circuit voltage
720mV, and ef
ciency 10.4%. This ef
ciency is close to the best value ever obtained
in this kind of cell. The dyes represented in Figure 6.18 are termed black dyes
because of their wide absorption range, and are complexes based on ruthenium. The
structure of a slightly different dye, N945, is shown in Figure 6.19.
Cells using this dye have been characterized by short-circuit current 18.8mA/cm
2
,
open-circuit voltage 783.2mV, and ef
ciency 10.8%.
6.6
Polymer Organic Solar Cells
Probably, the easiest type of cell to produce is the polymer organic cell, which involves
coating a conductive substrate with two polymers and covering this with a transparent
electrode.
Figure 6.18 Principle of dye sensitization.
Plots [84] of incident photon to current efficiency,
for pure TiO
2
(left trace) and titania (anatase)
coated with two different dyes (right two traces).
It is evident that photoelectrons generated in
the dyes are efficiently transferred to the titania,
to flow in the external circuit. Dyes that are
optimum have high absorption that extends to
long wavelength, and many of the best
performing dyes are based on the metal
ruthenium. (Although the conversion of
photons to electrons is efficient, these devices
as solar cells exhibit relatively low
efficiencies.) [84].
