The influence of localized electronic states on the photoelectrochemical behavior of passive films

The Poole-Frenkel effect describes the influence of a superimposed electric field on the escape probability of a charge carrier from a localized electronic state. This effect, including the possibility of tunneling through the escape barrier, is applied to photoinduced currents in disordered materials. Model calculations are performed to obtain photocurrents, resulting from sub-band gap adsorption, in a thin passive film. At high fields, which correspond to moderate potential drops in the passive film, the escape probability is increased due to tunneling contributions.A comparison with experimental results show that the Poole-Frenkel effect can be important for describing the photoelectrochemical behavior of passive films. The sub-band gap photocurrent behavior of ion implanted passive films on hafnium indicates that tunneling contributions exist at high fields, which is inferred from the results of the model calculations.