Sensitized planar electrodes

A practical disadvantage of the sensitized planar electrodes described in the previous section was their low solar energy conversion efficiencies. An experimental drawback was that characterization was limited to photoelectrochemical or luminescence techniques. Both of these issues arose from the poor light harvesting by monolayers on flat surfaces or inefficient interfacial electron transfer yields from thick films or concentrated solutions. In 1990, Gratzel and co-workers reported experimental studies of dye sensitized colloidal Ti02 films that eliminated both of these problematic issues [112]. The effective surface area for sensitizer binding... 

It is surprising that Kamat, O Regan and co-workers found a decreased injection yield at potentials near the flat-band condition. In the standard Gerischer model for sensitized planar electrodes, the low photocurrent near the flat band results because the injected carriers rapidly recombine with the oxidized sensitizer owing to the lack of a substantial depletion layer. Gerischer theory would not predict a decreased injection yield near the flat band, but this behavior can clearly be realized at sensitized nanocrystalline semiconductor films. 

Metal-based receptors which are able to form preorganised monolayers on planar electrodes can be expected to do the same on the surface of nanoparticles, leading to a surface sensing amplification effect. The very large surface area of nanoparticles could also allow greater overall sensitivity to anions. 

In the remainder of this section we briefly overview fundamental concepts, vocabulary, photoelectrochemical characterization techniques and the dye-sensitization mechanisms developed for planar electrodes. We end the section with a short literature review of dye-sensitization studies. 

The long effective pathlength and high surface area afforded by these colloidal semiconductor materials allow spectroscopic characterization of interfacial electron transfer in molecular detail that was not previously possible. It is likely that within the next decade photoinduced interfacial electron transfer will be understood in the same detail now found only in homogeneous fluid solution. In many cases the sensitization mechanisms and theory developed for planar electrodes" are not applicable to the sensitized nanocrystalline films. Therefore, new models are necessary to describe the fascinating optical and electronic behavior of these materials. One such behavior is the recent identification of ultra-fast hot injection from molecular excited states. Furthermore, with these sensitized electrodes it is possible to probe ultra-fast processes using simple steady-state photocurrent action spectrum. 

---