Figure 32 One important example of such a structure is provided by mesoscopic dye-sensitized solar
cells (Grätzel 2000; Grätzel 2001), which generally involve use of a highly porous film of randomly
ordered nanoparticles of a transparent nanocrystalline oxide, such as TiO 2 , coated with an ultra-thin layer
of light absorber (e.g., dye molecules or semiconductor quantum dots).
or solid medium that permeates the porous structure; this regenerates the absorber and completes
the cycle. Another example of such nanostructured devices is the use of semiconductor
nanowires or nanorods to absorb light and transfer the charge carriers over a very short distance
to the collecting phase, which can be a conducting polymer, an electrolyte (a liquid charge
conductor), or an inorganic conductor. Yet another example is an interpenetrating network of
n-type and p-type organic semiconductors that form heterojunction-type solar cells.
An exciting aspect of this approach is that the generic concept of the nanostructured cell can be
extended to a range of novel configurations involving different light absorbers and electron-hole
conducting phases. A key property of the thin nanostructured film is that since charge carrier
pairs are generated only near the interfaces and are separated rapidly into two different phases,
bulk recombination and semiconductor instability are avoided. Junction recombination does,
however, have to be minimized, and the surfaces of such systems need to be controlled to ensure
that they have a relatively low level of defect-driven electrical recombination sites, to allow
carriers to actually be collected from such devices. Fabrication of these types of cells can be
remarkably simple, and efficiencies over 11% have already been reported for some dye-
sensitized nanostructured systems. There is considerable potential for increasing this
performance to 20% by imaginative approaches that exploit the rapidly growing field of