(623d) Electron Transfer and Transport Measured in Nanotube and Nanoparticle Photoanodes by Ultrafast THz Spectroscopy

Dye sensitized solar cells (DSSC's) make use of TiO2 nanoparticle films as a high surface area support for light harvesting dye molecules. It has been proposed that TiO2 nanotubes could be an alternative high surface area dye support that may have superior electron transport properties to random nanoparticle networks. However, macroscopic photocurrent measurements of fully functional solar cells suggest electron transport rates in titania nanotube and nanoparticle films to be more or less comparable. However, photovoltage measurements reveal that nanotubes have significantly lower energy loss due to electron recombination. Through its interaction with free carriers far-infrared (FIR) light can be used as a non-contact probe for observing free carriers in nanostructures directly. Using ultra-short FIR laser pulses the free electron mobility and population can be tracked with a temporal resolution on the order of 10 to 100 femtoseconds. These considerations render time-resolved terahertz spectroscopy (TRTS) an ideal probe with which to confirm, and obtain a fundamental understanding of, the macroscopic electron transport measurements mentioned above. Charge injection into TiO2 nanotubes and nanoparticles was monitored on sub-picosecond to nanosecond times scales by time-resolved terahertz spectroscopy (TRTS). Dye molecules injected electrons into both the semiconductor nanoparticles and nanotubes after photoexcitation with a 400 nm pump pulse, causing an increase in the semiconductor's electron density that can be detected and monitored using a THz probe pulse. To study free electron mobility and dynamics within nanotubes and particles the time-resolved frequency-dependent photoconductivity was measured. The nanotube and nanoparticle photoconductivity was obtained by measuring the transmitted far-infrared electric field. Electron mobility, electron scattering times, and electron lifetimes within nanotubes and particles were extracted from the data. The variation of these electron transport properties was studies as a function of nanostructure (nanotubes vs. nanoparticles), nanotube morphology (wall thickness, diameter etc.) and crystallinity (annealing temperature and time). The implications of these findings for understanding and optimizing the functioning of DSSC's and electrical transport in disordered media will be discussed.