Kevin P. Regan, John R. Swierk, Jens Neu, and Charles A. Schmuttenmaer
http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.7b02318
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 timescale. In the presence of aqueous electrolyte, direct band gap excitation of non-sensitized SnO2 films results in instrument response limited sub-picosecond 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 two orders of magnitude faster in the non-sensitized films compared to the dye-sensitized films, which is likely due to photoinduced charges being more highly concentrated in the non-sensitized films. Finally, we find that the transient photoconductivity deviates from the behavior described by standard conductivity models such as the Drude and Drude-Smith model. This is due to the contribution from a photoinduced change in the permittivity of the SnO2 films.
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