Photoelectrochemical sensitisation of ZnO–tetrasulfophthalocyaninatozinc composites prepared by electrochemical self-assembly

Electrodes prepared by a one-step electrochemical synthesis of a dye-loaded ZnO thin film from an aqueous mixture of zinc nitrate and a water soluble phthalocyanine, 2,9,16,23-tetrasulfophthalocyaninatozinc(II) (TSPcZn) have been studied. Homogeneously blue transparent particulate thin films were formed. Monomeric or aggregated TSPcZn species could be obtained dependent on the deposition conditions. Visible light absorbed in TSPcZn led to sensitisation of ZnO. Negative shifts of the rest potential and anodic photocurrents were obtained in contact with an iodide-containing electrolyte in accordance with the n-type conduction of ZnO. Time-resolved measurements indicated charging and discharging of surface states, allowing discussion of the alignment of energy levels in the contact as well as of charge transfer and recombination in the cases of monomeric or aggregated TSPcZn. Electrons were injected into ZnO from the first excited singlet state of TSPcZn. Monomeric TSPcZn showed a considerably larger quantum efficiency in sensitisation of ZnO, whereas a larger rate of charge transfer to the electrolyte was derived for the aggregated form.