The relation between performance and stability of Cd-chalcogenide/polysulfide photoelectrochemical cells. The effect of potential

The stability and relative quantum yields of n-CdSe/polysulfide photoelectrochemical cells under different applied potentials, light intensities and electrolyte concentrations, and after different photoelectrode surface preparation techniques, were investigated. It was deduced that the availability at the electrode surface of polysulfide ions, which serve as hole acceptors, decreases with forward bias. On the other hand, the flow of the photogenerated holes towards the surface of the semiconductor is affected to a lesser extent by increasing (forward) bias up to the point of maximum power, beyond which the hole flux drops sharply with the bias. Consequently, the polarization of the cell (as given by the ratio between the peak and the steady-state photocurrents) is maximal near the point of maximum power and there the stability is lowest at moderate light intensities (ca. AM1). Under strong illumination, depletion of the polysulfide ions from the surface of the electrode due to photooxidation, together with increasing sulfur to sulfide ratio at the surface, leads to considerable band flattening (decrease in the flat-band potential), and consequently electrodes under forward bias show considerably smaller photocurrents and thus greater stability than electrodes under short-circuit (and reverse bias) conditions (at constant illumination). Increasing the surface area of the electrode by photoetching or increasing the concentration of the electrolyte leads to the behavior which is observed under moderate light intensities, which shows the interdependency of all these factors.