Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 256

particle size of 10 to 20 nm. The blocking contact to these nano-particles is en-
sured by a liquid electrolyte, usually the redox pair J 3 - /J - . The photovoltaic activity
of this kind of solar cell is given due to a monomolecular layer of a rubidium dye
adsorbed at the TiO 2 particle surface. Due to the porous sponge-type structure of
the titan oxide (TiO 2 ), its surface is about 1,000 times bigger than the cell surface.
Absorption of sunlight by the dye is only possible due to this area enlargement.
The photon irradiated on the surface of this cell lifts one electron inside the dye
from the basic state into an excited status. The linkage between the adsorbed dye
to TiO 2 is that strong that the excited electron is injected into TiO 2 within only a
few pico seconds, while the dye is regenerated by the electrolyte; i.e. one electron
is delivered in addition to the basic state of the dye.
Fig. 6.14 gives an overview on the design of such a solar cell, as well as a sim-
plified energy scheme for the primary photovoltaic activity. Primary charge sepa-
ration thus involves a three-step-process.
1. Excitation of the dye.
2. Injection of the electron from the excited status of the dye into the conduction
band of TiO 2 .
3. Regeneration of the dye from the electrolyte.
Charge separation is eventually accomplished by the diffusion of the photogener-
ated electron through the TiO 2 network to the front contact, while the electrolyte
is regenerated at the opposite platinum (Pt) rear electrode.

(a) (b)

Excited
state

TiO 2 -Nanoparticles

Conductive
window layer 2

Electrolyte Pt Rear contact Conduction

band

Photon

Glass
substrate

1

−

3

Base
state

Incident light

TiO 2

5-10 µ m

Fig. 6.14 (a) Schematic illustration of a dye solar cell of nano-porous TiO 2 (not shown:
single-molecular dye layer adsorbed by TiO 2 nanoparticles of a thickness of approximately
20 nm); (b) simplified energy scheme illustrating the primary charge carrier separation by a
three-step process: 1 excitation of dye; 2 injection of the electron from the excited status of
the dye into the conduction band of TiO 2 ; 3 regeneration of dye from the electrolyte

Dye Electrolyte

On the one hand, this new solar cell technology is attractive as material costs
are low and the production process is very simple, thus allowing for significant
cost saving. On the other hand, the physics of dye solar cells is very different from
that of all other (solid) solar cells and has not yet been exhaustively investigated,
and as a result, has not been fully understood yet. In fact, primary charge carrier
transfer into dye solar cells is similar to the charge carrier transfer processes real-
ised within the photosynthesis process.