M o d u L e 2 : a p p L i c a t I o n s a n d I m p L i c a t I o n s

photoelectrochemical cells 
(PEC). The complex dye molecules (called sensitisers) are attached to the surface of a mesoporous titan-
ium oxide. These are different from a classic thin-film PV in that light is absorbed in a semiconductor 
layer, whereas in 
the Gräztel cell, absorption occurs in the dye molecules.
The dye molecules act somewhat like an antenna (mimicking chlorophyll), meaning that more of the 
light of a particular colour can be captured but also that a wider range of colours of light can be 
absorbed compared to pure TiO
2
, thus increasing the efficiency of the device (
Figure 3).
Figure 3:
Schematic representa-
tion of the operation of the 
dye-sensitised electrochemical 
photovoltaic cell (Gräztel cell)
Image: L. Filipponi, iNANO, Aarhus 
University, own work
In the Gräztel cell, the anode is made 
of mesoporous dye-sensitised TiO
2
and receives electrons 
from the photo-excited dye which is thereby oxidised. The oxidised dye in turn oxidises the mediator, 
a redox species that is dissolved in the electrolyte. The mediator is regenerated by reduction at the 
cathode by the electrons circulated through the external circuit. The mesoporous nature of the titanium 
oxide provides an enormous internal surface area, thereby reducing the amount of material needed in 
the cell. The titanium oxide films are produced from a nanoparticle suspension (which is synthesised to 
form a stable porous material). Specific synthetic dyes are under development to increase light absorp-
tion. These cells are extremely promising because they are made of low-cost materials and do not need 
elaborate apparatus to manufacture. Current scale-up of production utilising polymer materials and 
roll-to-roll continuous production has the potential to produce the large areas of solar cells required to 
capture significant amounts of solar energy. Some companies, such as DyeSol, are already producing 
and selling these types of solar cells, with conversion efficiencies of about 12 % (
Figure 4).

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N A N O T E C H N O L O G I E S : P R I N C I P L E S , A P P L I C A T I O N S , I M P L I C A T I O N S A N D H A N D S - O N A C T I V I T I E S
Quantum-dot-sensitised solar cells
Another possibility is to use semiconductor nanocrystals (known as quantum dots, QDs) instead of the 
photosensitive dyes used in the Gräztel cell. For example, CdS, CdSe, InP and InAs quantum dots have 
been combined with mesoscopic networks of TiO
2
nanoparticles to obtain