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Photoelectrochem. measurements have been performed on film electrodes consisting of linked nano-sized TiO2 colloids. The film thickness ranged from 1-40 μm. The film network was attached to a thin transparent conducting layer of SnO2 allowing for photogenerated electrons to be collected in an outer elec. circuit. By illuminating electrodes of different thicknesses with monochromatic light from either sides, it was possible to induce charge sepn. in different regions of the film network. In this way, it was proved that electrons have different probabilities of reaching the back contact depending on the location in the film where they are created. The results also illustrate the importance of the redox species in the cavities of these porous electrodes. By adding acceptors to the electrolyte it was possible to alter the conditions for charge transfer in the nanocryst. film. It was shown that electron acceptors such as oxygen or iodine in the soln. strongly affects the rate of charge transfer at the particle-electrolyte interface and the transport of electrons throughout the TiO2 film-network. Modification of the semiconductor-electrolyte interface with surface adsorbed pyridine, induced major changes in the charge transfer events at the interface. The photocurrent yields was greatly improved by this surface treatment. The effect of pH in soln. was also investigated. The rate of charge transfer at the particle-electrolyte interface was changed at high surface d. of OH--ions. This was explained due to the change of the surface energy causing different driving forces for redox reactions, but also due to the more neg. electrostatic surface potential of the particles preventing the encounter of neg. charged redox species with the colloid surface. Phototransient measurements indicated a depletion of redox species in the pores of the film. Thereby it was pointed out that the dynamics of the redox species in the confined cavities of the film is a limiting factor for the charge sepn. efficiency in nanocryst. film. The photovoltage in anaerobic solns. sustained for very long periods, indicating that the linked particles may work as reservoirs for photoexcited electrons if the access to electron acceptors in the soln. is choked. It was concluded that surface processes are favored at the small semiconductor particles used in this study.

Dye-sensitized micro -or, to be more precise, nanoporous - Ti02 film electrodes for solar cells1' 2 are currently under investigation in our laboratory35. We have earlier presented methods to gain information about fundamental properties of semiconducting thin film electrodes from front-side (EE) and backside (SE) illumination of Ti02 electrodes by analysis of the action spectra3' 6 (For the experimental set up and the definition of SE and EE, see Fig. 1.) We are here extending our experimental work into the visible region and give preliminary result from an investigation of the action spectra of dye-sensitized Ti02-film electrodes.

The photoelectrochem. properties of colloidal film electrodes are compared with solid transparent thin films of TiO2. In particular the information that can be derived from anal. of the action spectra for front and back side illumination of the films is given. The mechanism of charge sepn. in the colloidal film is qual. discussed in terms of the kinetics at the electrode-electrolyte interface, and some effects of the presence of O2 in the electrolyte are demonstrated. The solar energy efficiency from measurements on dye-sensitized TiO2 film electrodes in solar cells is given.