Minority Carrier Dynamics and Interfacial Kinetics at the ZnO/Electrolyte Interface Studied by Combined Photocurrent/Microwave Photoconductivity Measurements

Abstract

A pronounced peak of microwave photoconductivity (PMC), observed in the region of onset of photocurrents through a ZnO (0001)/aqueous electrolyte interface, is studied in dependence of electrolyte composition, electrochemical pretreatments, and light intensity. The amplitude, position, and shape of the peak were found to depend on interfacial kinetics, and the peak converts into a plateau with limiting PMC in the presence of an organic electrolyte. The PMC peak can be understood as an accumulation of minority carriers, the surface lifetime of which has increased due to limiting interfacial reactivity in charge transfer or recombination. The theory allows the determination of interfacial rate constants from parallel potentiodynamic PMC and photocurrent measurements. An exponential dependence of the charge transfer rate constant (kr) on the applied voltage was found for ZnO in contact with different aqueous electrolytes at low electrode bias. This is in contradiction with classical models on photoelectrochemical charge-transfer which assume potential independent rate constants (Gärtner model as applied to electrodes: kr → ∞; Marcus−Gerischer model, kr = const). The conclusion is that these models cannot be applied and that the condition of weak interaction during electron transfer is not fulfilled. The latter condition can however be approached with an organic electrolyte containing ferrocene/ferrocenium+.