Effect of the Particle Size on the Electron Injection Efficiency in Dye-Sensitized Nanocrystalline TiO₂ Films Studied by Time-Resolved Microwave Conductivity (TRMC) Measurements

The efficiency of electron injection from photoexcited dye molecules into TiO 2 nanoparticles with two different sizes (20 or 300 nm) was studied using the time-resolved microwave conductivity (TRMC) technique. For the ruthenium dye (N719) adsorbed on 20 nm TiO 2 particles, we have found a near 100% injection efficiency, which is consistent with previous transient optical absorption studies. In contrast, the electron injection efficiency for N719 sensitized on 300 nm TiO 2 particles was found to be less than 50%. A similar behavior was found for coumarine-sensitized TiO 2 nanoparticle films. The difference is tentatively attributed to differences in the adsorption of dye molecules on the TiO 2 surface. Interestingly, only for 20 nm TiO 2 particles, a trap filling effect has been observed, that is, at low illumination intensities, the TRMC signal increases more than proportional with the illumination intensity. This is attributed to an increase in electron mobility due to occupation of deep traps. The absence of a significant trap filling effect for 300 nm particles points toward a considerably lower trap density.