To improve the solar response of a photoelectrode, a proper match between the solar spectrum and the bandgap of the semiconductor should be maintained. When a single – bandgap semiconductor is used, a bandgap of ~1.4 eV is the most desirable from the standpoint of optimum solar conversion efficiency. An important criterion is that the minority carrier that is driven towards the semiconductor/ electrolyte interface should not participate in a photocorrosion reaction, because this would be detrimental to the long-term stability of the photoelectrode. Photocorrosion can arise from either kinetic or thermodynamic causes, or a mixture of both. From a thermodynamic perspective, a photoanode is susceptible to corrosion if the Fermi level for holes is at a positive potential with respective to the semiconductor corrosion potential (Bard and Wrighton, 1977; Gerischer, 1977). The corrosion can be prevented, or at least inhibited, by choosing a redox couple of more negative redox potential than that of the corrosion process (Hodes et al, 1976; Heller et al, 1978). The kinetic approach has been to allow another desired redox process to occur at a much faster rate than the photocorrosion reaction (Bolts et al., 1979). Other methods used to minimise photocorrosion include coating the photoelectrode surface with layers such as Se (Frese, 1982), ITO (Hodes et al., 1983) or protective conductive polymer films (Fan, et al., 1981), or using alternate low-bandgap semiconductors (Sharon, 1988) such as CdTe or CdSe rather than CdS (Cahen et al., 1980; Hodes, 1983). Etching of photoelectrode surfaces has also been recognised and widely used as an important treatment to achieve high conversion efficiency (Heller et al., 1977). The efficacy of this is mostly attributed to removal of surface states that may act as trapping centres for photogenerated carriers. A related procedure called photoetching (Tenne and Hodes, 1980), initially developed for CdSe, improves the photoelectrode performance and preferentially removes the surface defects acting as recombination centres.
In addition to a variety of etching procedures, several other surface treatments have been used to improve photoelectrode performance. Examples include a Ga3+ ion dip for CdSe (Tomkiewicz et al., 1982), a ZnCl2 dip for thin-film CdSe (Hodes et al., 1980; Reichman and Russak, 1982), and the deposition of Ru on GaAs (Parkinson, et al., 1979) and on InP (Heller, 1982), and Cu on CdSe (Flaisher et al., 1984). Reasons advanced for the effectiveness of these treatments range from a suppression of dark current to electrocatalysis by surface-deposited metal atoms.