Viewing nanocrystalline TiO2 photoelectrodes as three-dimensional electrodes: Effect of the electrolyte upon the photocurrent efficiency

Nanocrystalline TiO₂ films are widely investigated both as photoelectrodes to carry out photocatalytic reactions under band-gap near-UV illumination or, alternatively, as an electronically conducting matrix of dye-sensitised electrodes as a part of a liquid-junction solar photovoltaic cell. The main features of these photoelectrodes are: (i) significant thickness to allow effective absorption of incident light—the films consist typically of 100-1000 superimposed layers of individual nanoparticles; (ii) large porosity—ca. 50% of the film volume is normally filled with electrolyte; (iii) large surface-to-volume ratio resulting from the small size of TiO₂ nanoparticles (with diameters in the range of 10-30 nm) forming the film. This work re-examines the question of charge and mass transport in nanocrystalline titanium dioxide electrodes which are among the major factors affecting the efficiency of TiO₂-based dye-sensitised solar cells and photocatalytic devices. In contrast to most of the reports published in the literature, which analyse of the behaviour of such films under transient conditions, we focus here on the steady-state operation of nanocrystalline TiO₂ electrodes, directly relevant to the efficiency of cells used in real-world applications. Understanding of the charge and mass transport across these nanocrystalline films is greatly facilitated by considering them as three-dimensional electrodes including two coincidental, superimposed continua, i.e., the solid matrix that conducts electrons towards the back contact and the electrolyte—an ionic conductor carrying another part of the current through the pores of the nanostructured film. It is shown that the composition and conductivity of the chosen electrolyte and, in particular, the nature of the ionic species which act as hole scavengers or redox mediators largely affect the current distribution within the film and thus the final photon-to-current conversion efficiency of the photoelectrodes.