Frequency-Dependent Terahertz Transient Photoconductivity of Mesoporous SnO2 Films

The transient photoconductive properties of tin(IV) oxide (SnO2) mesoporous films have been studied by time-resolved terahertz (THz) spectroscopy. We gain insight into carrier dynamics by measuring overall injection and trapping lifetimes using optical pump−THz probe spectroscopy, as well as the frequency-dependent complex conductivity at various pump−probe delay times. It is found that the method of charge generation, either direct above band gap excitation (at 267 nm) or dye-sensitized electron injection (at 400 nm), has a dramatic effect on the overall injection and trapping dynamics of mobile charge carriers on the picosecond to nanosecond time scale. In the presence of aqueous electrolyte, direct band gap excitation of nonsensitized SnO2 films results in instrument response limited subpicosecond injection lifetimes, while dye-sensitized films require tens of picoseconds for interfacial electron transfer to complete. On the other hand, the rate for trapping of mobile charges is at least 2 orders of magnitude faster in the nonsensitized films compared to the dye-sensitized films, which is likely due to photoinduced charges being more highly concentrated in the nonsensitized films. Finally, we find that the transient photoconductivity deviates from the behavior described by standard conductivity models such as the Drude and Drude−Smith models. This is due to the contribution from a photoinduced change in the permittivity of the SnO2 films.