Mechanisms of Charge Transfer at the n ‐ GaAs / Room Temperature Molten Salt Electrolyte Interface

Abstract

Photoelectrochemical (PEC) characterization was carried out on electrodes in contact with a room temperature molten salt electrolyte comprising mixtures of and n‐butyl pyridinium chloride (BPC) in varying molar ratios. The ferrocene/ferricenium ion couple was used as the redox system in these studies. The barrier height for charge transfer, which determines the ultimate efficiency of any photovoltaic system, was determined from dark current‐voltage measurements. These data were in good agreement with previously determined values from capacitance measurements. The residual barrier heights in the illuminated PEC system were determined from open‐circuit voltage and short‐circuit current densities using Schottky barrier theory. A simple method for estimating the reorganization energy (λ) of a redox system from dark current‐voltage measurements is presented. A value of was thus deduced for the ferrocene/ferricenium ion couple in the electrolyte. The acidic electrolyte showed larger dark currents and inferior photoresponse relative to the case of electrodes in contact with basic electrolytes. Using the measured values of λ and the semiconductor bandedges, energy band diagrams were developed for the present PEC system as a function of electrolyte composition. The direct correlation which was seen between PEC performance and electrolyte composition is rationalized on the basis of these diagrams. The larger dark currents in the acid electrolytes were shown to result from greater overlap of the empty redox levels with the conduction bandedge in . On the other hand, in basic electrolyte systems, the filled redox levels overlap better with the valence bandedge leading to more efficient charge transfer on illumination. The role of surface states in mediating charge transfer and degrading PEC performance is discussed in this regard. A key finding of the present work is that the electrical behavior of semiconductor/electrolyte junctions is well described by Schottky barrier theory once the differences in the energy distribution in metals vis‐à‐vis electrolytes, are rationalized.