On the Flat-Band Potential and the Frequency Dispersion of Capacitance and Its Removal in an Normal-Cadmium Telluride in Alkaline Selenide, Polyselenide Photoelectrochemical Cell

Abstract

When seepage of the electrolyte between the semiconductor crystal and its insulation was prevented, the equivalent series capacitance C_S of an n-CdTe|Se^2-, Se_n^2-,OH^-|Pt cell was almost independent of frequency (<2% increase of C_S per decade decrease of frequency) in the range 10⁴-2 Hz. Attributing C_S wholly to the space-charge capacitance resulted in a linear Mott- Schottky plot and this yielded a flat-band potential of -2.0±0.1 V v . s.c.e . and a built-in potential of 1.08±0.06 V. This was consistent with the observed current-voltage curves in both dark and light.

Two conditions under which frequency dispersion was observed were (a) electrolyte seepage and (b) surface damage. The insulating techniques which allowed seepage were similar to those of other workers. The effects of these imperfections on the Mott- Schottky plot are considered. Attempts to extract useful information from frequency dispersion data showed that it is difficult or impossible to obtain a unambiguous equivalent circuit solely on the basis of goodness-of-fit of calculated to observed impedances. The ambiguity can be removed only when sufficient information beyond impedance-frequency data is available. For electrodes with seepage, a model due to Tomkiewicz led to the correct value of the built-in potential.