Fluctuation-Induced Tunneling Conductivity in TiO$_{2}$ Nanoparticle Thin Films

We integrate temperature-dependent dark DC conductivity measurements and theoretical modeling to elucidate the mechanism of electron transport in nanoporous TiO$_{2}$, a common photoanode material for dye-sensitized solar cells (DSSCs) and solar photocatalysis. We show that fluctuation-induced tunneling conduction through contact junctions between sintered TiO$_{2}$ nanoparticles can account for the temperature dependence over the entire temperature range studied. We find quantitative agreement between experimental and calculated conductivities, which span over four orders of magnitude and change with decreasing temperature from thermally activated to temperature-independent. The reported results suggest that efforts to optimize charge transport in nanoporous TiO$_{2}$ thin films as a means of improving the overall efficiency of DSSCs and solar photocatalysis should focus on fabrication conditions that optimize the properties of the contact junctions between sintered TiO$_{2}$ nanoparticles.