On the diffusion impedance at semiconductor electrodes

It is well known that at metal electrodes, mass transport limitations introduce a Warburg impedance in the electrochemical impedance of an electrode process. Semiconductor electrodes, however, react differently from metal electrodes to variations of the applied potential, so that the influence of diffusion on the electrochemical impedance is less straightforward. In this paper, we propose a general approach of this problem, allowing to describe both the metal and semiconductor electrode behavior by using appropriate kinetical models. When discussing the diffusion impedance at ideal semiconductor/electrolyte contacts. distinction must be made between direct capture reactions and direct injection reactions. Whereas the former result in a Randles-like equivalent circuit, no Warburg component is present in the impedance spectrum of the latter if the reverse reaction is negligible. Thus, the electrochemical impedance provides a clear distinction between both reaction types. At nonideal semiconductor/electrolyte contacts, the situation is different because of the unpinning of the band edges, This may result in a Warburg impedance appearing in the electrochemical impedance of an injection reaction as well, i.e. if the shift of the band edges influences the rate constant of the injection reaction.