A generalised model for dynamic photocurrent responses at dye-sensitised liquid|liquid interfaces

The heterogeneous photoinduced electron transfer involving dyes adsorbed at the interface between two immiscible electrolyte solutions and redox molecules located in the adjacent phase manifests itself as photocurrent responses under potentiostatic conditions. Photocurrent transients as functions of the light intensity and bias potential allow the extraction of insightful information on the kinetics of the various processes associated with the photoinduced reaction. Previous analyses of this type of responses were based on phenomenological models that did not consider mass transport. In the present paper, we develop a generalised model for photocurrent transients taking into account the diffusion of reacting species to the interface. Comparison with the experimental data confirms that the responses can be described adequately by applying stationary conditions to the surface concentration of the photoactive species. Mechanistic aspects associated with the nature of the photocurrent relaxation on the microsecond time scale are examined. In particular, the dependence of the transient response on the light intensity indicates that charge recombination proceeds mainly as a first order reaction from an interfacial geminate ion pair. Coupled ion transfer reactions involving the photoproducts can also contribute to the photocurrent, depending on the formal ion transfer potential of the corresponding species.