A theoretical treatment of charge transfer via surface states at a semiconductor-electrolyte interface: Analysis of the water photoelectrolysis process

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Abstract
Considerable experimental work has recently been reported on semiconductor-electrolyte interfaces in connection with the photoelectrolysis of water by illuminated n-type semiconductors. It is believed that surface states play an important role in charge transfer at the interface. In this paper the nature of that role of surface states is analyzed theoretically in the context of the photoelectrolysis of water. Conditions resulting in the improvement of charge transfer efficiency at the interface are particularly noticed. The semiconductor-electrolyte junction is treated as a metal-insulator-semicondutor (MIS) structure with the metal and insulator replaced, respectively, by the electrolyte and Helmholtz layer, and the interface current is analyzed in some detail using this model. It is shown that efficient charge transfer observed in the O2 evolution process on n-TiO2 photoanodes is ascribed to the existence of almost completely occupied surface states whose occupation is determined by the electrolyte. Then, such surface states act simply as current flow sites for holes transferred across the interface instead of acting as recombination centers for semiconductor carriers. Further, it is found that the requirement of efficient charge transfer via surface states tends to be incompatible with that of a large band bending in the surface barrier region. Charge transfer accompanied by H2 evolution on a p-type GaP photocathode is also discussed in relation to the presence of surface states at a GaP-electrolyte interface.