Abstract
Impedance measurements were used to complement previously described electrolyte electroreflectance studies in an attempt to understand the improvement in photoelectrochemical performance of single‐crystal 
in contact with aqueous polyiodide electrolyte, due to etching and oxidation. We have found that etching removes most of the fast surface states from the interface, retaining less than 1% of a monolayer of states of two different energies: one centered at 0.17 eV below the conduction band and another one is located around the middle of the gap, 0.45 eV below the conduction band. The flatband potential of the etched sample was found to be −0.69V vs. the solution potential, in agreement with the electroreflectance results. Only one of the surface states, the one close to the conduction band, was found to be responsible for surface recombination. Fitting the light‐induced I‐V characteristics to a simple extension of the Gärtner model leads to a lower limit of 0.04 μm for the effective minority carrier diffusion length and to a ratio of 23.2 between the rates of surface recombination and charge transfer to the electrolyte. Controlled oxidation of the electrode produces large changes in the impedance response. We find that the oxide layer is transparent to the incoming illumination in the visible portion of the spectrum and causes a small increase in the series resistance of the cell. We were able to investigate the effects of the oxidized layer on the potential distribution of the 
at the interface. We have found that the flatband potential is not affected by the oxidation, but that the effective doping level is reduced by more than an order of magnitude. The reduction in the doping level is such that all the incoming light is absorbed within the space‐charge layer, thus eliminating all the recombination mechanisms that take place in the bulk of the semiconductor. By fitting the experimental light‐induced current‐voltage curve, we show that all of the improvement in performance can be accounted for by this reduction in the effective doping level.