XIX. An Investigation of S/Se Substitution in Single Crystal CdSe and CdS Photoelectrodes by Electron Spectroscopy

Photoelectron spectroscopy studies verify the occurrence of S/Se substitution in single crystal CdSe and CdS photoanodes when used in photoelectrochemical cells containing S2or Se 2solutions. A possible mechanism of the S /Se exchange is discussed as well as its consequences on the electrode stabi l i ty and the output parameters of the photoelectrochemical cells. A number of studies have shown that cadmium chalcogenide photoelectrodes can be stabilized in aqueous solutions by the addition of sulfide, selenide, or tel luride ions (1-3). This stabilization is at tr ibuted to the favorable redox potential of the X/X 2couple (X --S, Se, Te) (4, 5) compared to the potential where electrode oxidation can occur or the preferential adsorption of sulfide ions for the case of CdS. Recently (6), the behavior of mixed CdS/CdSe solid solution photoelectrodes in polysulfide solutions was described, where changes in the flatband potential and bandgap with variations in the S/Se ratio were observed. Although CdS and CdSe show quite stable operation in sulfide and selenide electrolytes, changes in the photocurrent with time are observed and indeed one would expect from thermodynamic reasoning that exchange of solution chalcogenide ions with semiconductor lattice ions would occur. Recent reports (7-9) indeed have provided evidence for such substitution. Thus Cahen et al. (7) showed by x ray photoelectron spectroscopy (XPS or ESCA) that sulfide substitution occurs on polycrystall ine CdSe. Similar ly Heller et al. (8) found evidence of S substitution in CdSe by Auger electron spectroscopy (AES) and electron beam luminescence. Gerischer and Gobrecht (9) also proposed that changes occur in the surface of CdS and CdSe single crystals under il lumination in sulfide electrolytes based on changes in the photocurrent spectra and Mott-Schot tky plots with operation time. We report here detailed AES and XPS studies of the surface changes which occur in single crystal CdS and CdSe photoanodes when used in photoelectrochemical cells (PEC). The results show that Se substitution occurs in the CdS/Se 2system and S substitution occurs in the CdSe/S ~-system and provide information about the mechanism of stabilization and its effect on the output parameters of the PEC.