Semiconducting systems, space charge

Fig. 40. Electron transfer from redox systems within the electrolyte across a semiconducting passive layer (n-type) (1) direct tunnelling to CB (2) tunnelling through space charge layer (3) transfer via surface states (4) hopping mechanism via interband states (5) transfer via sub-band and (6) transfer via valence band.

There are some further aspects which must be considered when photochemical diodes are used. When a metal (catalyst) is deposited on a semiconductor, then frequently a Schottky barrier instead of an ohmic contact is formed at the semiconductor-metal interface. In this case, the latter junction behaves as a photovoltaic system by itself, which may determine or essentially change the properties of the photochemical diode. The consequences have been discussed in detail in [14, 27]. Frequently, colloidal semiconducting particles have been used, their size being much smaller than the thickness of the space charge region expected. 

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