CHARGE-TRANSFER AT PHOTOELECTROCHEMICAL SOLAR-CELLS - CONCLUSIONS FROM THE OPEN-CIRCUIT IMPEDANCE OF P-INP IN V-3+/2+

Two resistance-capacitance (RC) time constants are detectable in the open-circuit impedance of illuminated (bare and Ag-treated) p-InP in contact with V-3+/2+ + HCl solution. The dependence of the time constants on illumination intensity and concentration of V3+ and V2+ is used to identify processes related to recombination and electrochemical charge transfer, respectively. Charge transfer (CT) rates at metal-treated electrodes [p-InP(Ag)] turn out to be somewhat smaller than at metallic Ag electrodes. For 0.3M V-3+/2+, CT at p-InP(Ag) is sufficiently fast for solar cell operation under solar irradiation intensities (exchange current density i(o) approximate to 40 mA cm(-2)). CT at bare p-InP is much slower (i(o) approximate to 0.2 to 2 mA cm(-2)). impedance data are fitted with two transformable equivalent circuits (differential models) which both yield similar values for recombination and CT elements. Thereby charging/discharging processes at the interface can be attributed either to a charging/discharging of the differential Helmholtz capacity C-HH or an interfacial defect capacity. Measured values for either element are equal or smaller than the C-HH value of bulk metal electrodes [about 2 to 20 mu F cm(-2) at pInP(Ag), < 1 mu F cm(-2) at bare p-InP].