Role of surface states and adsorbates in time-resolved photocurrent measurements and photovoltage generation at phthalocyaninatozinc(II)-photocathodes

The role of surface states in the photoelectrochemistry of phthalocyaninato zinc(II) (PcZn) has been investigated by time-resolved photocurrent measure ments in the millisecond range and by photovoltage measurements. Plots of t he photovoltage against the redox potential of the electrolyte yielded slop es a with - 1 < a < 0 indicating partial Fermi-level pinning due to the pre sence of surface states. The charging and discharging of the surface states could be seen clearly in the photocurrent transients. The amount of chargi ng and discharging was found to be determined by the recombination rate as a function of the applied electrode potential and by the charge-transfer ra te, which was studied in dependence upon the nature and the concentration o f the redox couple in the electrolyte (ferri/ferrocyanide, hydroquinone/ben zoquinone) in comparison to oxygen-saturated and deaerated KCl electrolytes . The charge-transfer rate was found to be limited by an adsorption step of the electroactive species. The number of surface states was found to incre ase with exposure to air. This is interpreted as a modification of the PcZn surface due to reaction with oxygen. The surface state densities as derive d from the extent of the Fermi-level pinning corresponded well with the dis charges seen in the photocurrent transients and show that a considerable fr action of the PcZn surface molecules represents surface states after longer exposure to air. (C) 1999 Elsevier Science S.A. All rights reserved.