Influence of Dye Sensitizers on Charge Dynamics in SnO2 Nanoparticles Probed with THz Spectroscopy

Using dye-sensitized water-splitting photoelectrochemical cells is a promising way to satisfy an increasing demand for energy without significant production of greenhouse gases. These cells commonly utilize a large band gap nanoparticulate semiconductor photoelectrode that is sensitized with a photoactive dye. In this paper, we study SnO2 nanoparticles (NPs) that are sensitized with either a ruthenium-based dye (RuP) or a porphyrin-based dye (Por). These dyes have a significantly different molar extinction coefficient, resulting in either a high carrier density (Por) or a moderately low density (RuP) within the SnO2 NPs. The influence of the dye on charge carrier dynamics and photoconductivity in the NPs has been explored. We present experimental results on samples of five different SnO2 film thicknesses, sensitized with either RuP or Por. We find that the injection dynamics are very similar for both dyes, but the trapping dynamics strongly depend on the dye. We conclude that this dependence is linked to dye-induced surface states, electrostatic environment, and defects which increase carrier-trapping at the surface. Frequency-dependent measurements confirm that the dye influences the carrier dynamics in the NPs. This can be seen by the significant change in the Drude-Smith parameter c, which describes backscattering/localization effects at the surface. This parameter is approximately -0.9 for RuP and -0.8 for Por, showing a dependence on the chemical environment and the dye cation after injection.