On the Flat-Band Potential of n-Type Cadmium Telluride in Alkaline Selenide, Polyselenide Electrolytes

The flat-band potential Efb of n-CdTe in alkaline K2Se solution was determined by the Mott-Schottky technique as a function of properties of the crystal and electrolyte. The experimental value, c.- 2.0 V v. s.c.e., is more negative than the value calculated by assuming the absence of localized surface charge, -0.35 V v. s.c.e. For a given electrolyte, Epb depended on the crystal orientation, and became more negative in the order (111), (110), (111). These results are consistent with specific adsorption of anionic selenide species on surface cadmium atoms. For a freshly prepared CdTe electrode, Efb depended on the selenide ion concentration in a Nernstian manner, consistent with sub-monolayer adsorption of HSe-. In contrast, when electrodes had been exposed to selenide electrolytes for several days, Efb was less dependent on the selenide ion concentration, in some cases being constant over four orders of magnitude of [Se2-]. Such results were consistent either with saturation of the surface with adsorbed ions, or with pinning. As the electrolyte was oxidized, Ub increased linearly with Eredox, reached a maximum value at -0.81 V v. s.c.e., and then decreased, so that neither the ideal model nor the Fermi level pinning model applied. Models which assumed adsorption only of HSe- were inconsistent with the observed behaviour, but the results were explained quantitatively by a model which assumed the competitive adsorption of HSe- and Se on the semiconductor surface. Such a model was consistent also with the independence of Efb and the total selenide concentration observed in some experiments. .The largest built-in potential observed was 1.23 V. An improved method of preparing CdTe electrodes is described.