Photoelectrochemical Cell Band Model

A common photoelectrolysis cell structure is that of a semiconductor photoanode and metal cathode, the band diagrams of which are illustrated in Fig. 3.15 together with that of electrolyte redox couples. In Fig. 3.15(a) there is no contact between the semiconductor anode and metal cathode (no equilibrium effects communicated through the electrolyte). As seen in Fig. 3.15(b), contact between the two electrodes (no illumination) results in 

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Before discussing the photoelectrochemical cells, a brief review of the nature of semiconductors is perhaps desirable. The electronic properties of solids can be described in terms of a band model which treats the behavior of electrons moving in the field of the atomic nuclei. When a solid is formed, the isolated atoms, which are characterized by filled and vacant atomic orbitals are assembled into a lattice containing about 5 X 10 atoms cm This leads to the formation of new molecular orbitals which are so closely spaced that they form essentially continuous bands. 

The promise of photoelectrochemical devices of both the photovoltaic and chemical producing variety has been discussed and reviewed extensively.Cl,, 3,4) The criteria that these cells must meet with respect to stability, band gap and flatband potential have been modeled effectively and in a systematic fashion. However, it is becomirg clear that though such models accurately describe the general features of the device, as in the case of solid state Schottky barrier solar cells, the detailed nature of the interfacial properties can play an overriding role in determining the device properties. Some of these interface properties and processes and their potential deleterious or beneficial effects on electrode performance will be discussed. 

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