www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 481 (2000) 42 – 51 Photoelectrochemical sensitisation of ZnO – tetrasulfophthalocyaninatozinc composites prepared by electrochemical self-assembly D. Schlettwein a, *, T. Oekermann a , T. Yoshida b , M. Tochimoto b , H. Minoura b a Institut fu ¨r Angewandte und Physikalische Chemie, Fachbereich 2, Uniersita ¨t Bremen, Postfach 330440, D-28334 Bremen, Germany b Department of Chemistry, Faculty of Engineering, Gifu Uniersity Yanagido 1 -1, Gifu 501 -11, Japan Received 14 May 1999; received in revised form 27 September 1999; accepted 23 November 1999 Abstract 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. Homoge- neously 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. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Zinc oxide; Substituted phthalocyanine; Electrodeposition; Interface; Photoelectrochemistry; Transient methods 1. Introduction As highly efficient solar cells can be prepared from dye-sensitised TiO 2 [1] that also show remarkable long- term stability, sensitisation of wide bandgap oxide semi- conductors by organic dye molecules, in general has regained considerable interest in experiments as well as in modelling and theory [2–4]. The general goal in the ongoing and future work [2] is the task of preparing highly porous electrodes of high structural order covered by a monolayer of strongly absorbing dyes. Ultrafast electron transfer from the dye to the electrode and suppression of back transfer to the dye (recombination) or the electrolyte as compared to hole transfer from the dye to the electrolyte are further prerequisites for efficient and chemically stable elec- trodes [5–14]. Apart from TiO 2 , ZnO is one of the most promising semiconductor materials for nanocrystalline cells [15 – 18]. Its higher bandgap as well as the higher position of its conduction band compared with that of TiO 2 would in principle allow higher photovoltages in a given sys- tem. Until now, however, electrodes based on ZnO were found to be considerably lower in light harvesting and charge transfer quantum yields when compared to TiO 2 . On the way towards improved electrodes, new ways of preparing semiconductor-dye composites are sought and ZnO offers a new perspective in this respect. Crystalline films are obtained electrochemically on the cathode during electrolysis of aqueous Zn(NO 3 ) 2 . Re- cently we reported the successful preparation of a dye- loaded ZnO semiconductor thin film by electro- deposition in the presence of water soluble dyes such as tetrasulfophthalocyaninatozinc(II) (TSPcZn) [19] or te- trabromophenol blue [20]. Well-crystallised ZnO was found to hinder adsorption of the dye during an at- tempt at stepwise processing of the film by elec- * Corresponding author. Fax: +49-421-2184918. E-mail address: k18c@zfn.uni-bremen.de (D. Schlettwein) 0022-0728/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0022-0728(99)00475-1