Charge Transfer in Photoelectrochemical Devices via Interface States: Unified Model and Comparison with Experimental Data

Abstract

A simple, unified model is presented for the mediation of charge transfer across the semiconductor/electrolyte interface by states localized in the bandgap of the semiconductor. These states are interpreted to arise from specific adsorption of anionic species from the electrolyte. The adsorbed ions could be either one of the components of a regenerative redox couple or comprise the constituent ions of the supporting electrolyte. The role of the interphasial layer in photoelectrochemical (PEC) devices is examined in the light of the above model. The key factor in determining the efficacy of energy conversion in the PEC system is identified as the competition between tunneling of photogenerated holes across the interphasial layer and their recombination with the majority carriers in the semiconductor conduction band. The extent of matching between the interface state and the reduced (filled) energy levels in the electrolyte is shown to be important in this regard. Experimental data on the temperature molten salt electrolyte and the interfaces are discussed in the light of the present model. The poor charge transfer characteristics previously observed in the former system are attributed to hindrance to the tunneling process arising from nonoptimum location of the interface state vis‐à‐vis the redox species in the bulk electrolyte. Consequently, recombination processes are enhanced with a net reduction in the output current from the PEC system. Transient photoresponse measurements on the molten salt electrolyte interface are consistent with these ideas. Similar behavior is shown by the thin film electrolyte system although differences exist in the chemistry at the two interfaces. The general applicability of the present model is finally discussed with reference to data reported recently in the literature on and p‐Si/aqueous electrolyte interfaces.