Fluctuation-Induced Tunneling Conductivity in Nanoporous TiO2 Thin Films

N TiO2 thin films have attracted considerable attention due to their pivotal role in energy conversion and environmental applications, including photovoltaics, 3 photocatalysis and remediation of hazardous waste, 5 electrochromic windows and displays, and chemical sensors. However, the underlying mechanisms responsible for electron transport through nanoporous TiO2 remain only partially understood even though they often determine a limiting factor in device performance. Here, we address the mechanism responsible for dark DC conductivity in TiO2 thin films under vacuum conditions. We find that a fluctuation-induced tunneling conductivity (FITC) mechanism is supported by experimental data for a wide temperature range. In recent years, there have been significant contributions toward understanding charge transport in nanoporous titania thin films. 19 The most popular models include variable-range hopping (VRH) and/or multiple trap and release (MTR) of electrons in an electrically homogeneous medium containing a distribution of traps. These models have been particularly successful when applied to the conductivity of photogenerated electrons at temperatures experimentally accessible in a native device environment. However, they predict a temperature (T) dependence of the dark DC conductivity of the form ln σ T , where R = 1 in the MTR model and R = 1/4 for VRH. As shown in Figure 1, such a dependence does not account for the observed saturation of dark conductivity at low temperature, as reported here and elsewhere. 25 In contrast, the FITC model offers a proper description of conductivity over the entire temperature range with a single set of structural parameters, predicting not only theArrhenius high-temperature behavior but also the temperature-independent tunneling regime at low temperature. Because the model can be closely tied to the nanoporous film microstructure, it should provide valuable insight for the development of high-performance electrode materials. FITC models have been extensively applied to a variety of systems with heterogeneous microstructures, including carbon Figure 1. Arrhenius plot of the dark DC conductivities of nanoporous TiO2 films, made by sintering Sigma Aldrich (sample A; filled circles) and Ishihara (sample B; open squares) nanoparticles. The solid lines are obtained using the fluctuation-induced tunneling conduction (FITC) model. The short-dashed and long-dashed lines are the results when using the variable-range hopping (VRH) and multiple trap and release (MTR) models, respectively. Temperatures in K are shown on the top axis for convenience.