Impedance and current-voltage behavior of a semiconductor/liquid junction with a small energy barrier

Energy barriers phi(B) < 0.2 eV are traced at rectifying n-InP/V-3+/2+ junctions via differential capacitance measurements. In 0.1 M V2+ reverse-saturation current densities j(o) remain nine orders of magnitude below the values of semiconductor/metal contacts of identical phi(B). Compared to exchange-current densities of metal/V-3+/2+ junctions, j(o) values are smaller by four to five orders of magnitude. Current density vs. phi(B) ratios represent, when extrapolated to a large phi(B) favorable conditions for photoelectrochemical solar cell (PEC) operation because interfacial oxide layers, familiar at solid-state surface-junction cells, are not required for a reduction of j(o). Moreover, charge-transfer parameters are adjustable: at an unchanged phi(B) (from differential capacitance) deposits of a submonolayer amount of metallic Ag at the interface causes a drastic increase in j(o). The junction's j/U and impedance behavior is analyzed and results are compared to reported data from metal/V-3+/2+ junctions, solid-state n-InP Schottky junctions, and p-InP/V-3+/2+ and n-InP/ferrocenium-ferrocene junctions. The comparison indicates that efficiency losses at PECs owing to a slow minority-carrier charge-transfer are frequent.