Hierarchical Nanostructures for Photo-Electro-Chemical Cells - Hierarchical Nanostructures for Energy Devices - page 177

The working principle of the electrical energy-generating PEC cell is well
documented 12,15,16 and can be summarized as follows based on a cell with
an n-type semiconductor with which most of this type of PEC cells are de-
veloped. As illustrated in Figure 8.2, 12 dyes or photosensitizers (S) with broad
absorption spectra that are adsorbed at the surface of a semiconductor such
as TiO 2 and ZnO absorb photons and generate excited electrons (S 0 þ hn

d n 3 r 4 n g | 4

-

S 1 þ e ). The excited electrons travel through the conduction band
of the semiconductor to the conducting back plate, and then flow to the
external load and re-enter the cell on a metal electrode (counter electrode)
to reduce the redox relay molecules in the electrolyte (e.g. I 3 þ 2e

S*, S* -

3I in
I 3 /I redox couples). For the ecient injection of electrons from the dye to
the semiconductor, the conduction band edge of the semiconductor should
be placed lower than excited-state energy level of the dye. Finally, the oxidized
dye molecules are regenerated, accepting electrons and oxidizing the redox
relay molecule (e.g. S 1 þ e

-

I 3 þ 2e in I 3 /I redox couples).
Therefore, there is no net chemical change behind this cycle. Many kinds of
redox mediators such as vanadium( II )/vanadium( III ), 17,18 sulfide/poly-
sulfide 19,20 and cobalt( II )/cobalt( III ) 21,22 have been reported and analogous
considerations applied to the chemical reactions. For high power conversion
(i.e. photoconversion or light conversion) eciency, maximizing photo-
absorption while minimizing energy loss due to recombination of electrons
with either the oxidized dye molecules or other chemical species in the
electrolyte is required. To meet these conditions, the three most critical re-
quirements for the semiconductor photoanode are (1) a large surface area
for higher dye loading, (2) high electronic mobility for rapid charge transport
and (3) high stability in acidic dye solution for sucient dye adsorption over
an extended period of time. The photoconversion eciency, one of the key

S 0 , 3I

- -

.

Figure 8.2 Illustration of the working mechanism of the dye-sensitized solar cell in

the electrolyte containing I 3 /I redox couples.
Reproduced with permission from ref. 12. (Copyright 2001, NPG.)