Mott−Schottky and Charge-Transport Analysis of Nanoporous Titanium Dioxide Films in Air

We investigate the electrical properties of air-filled nanoporous TiO 2 films through Mott−Schottky (MS) and current−voltage (IV) analyses. Films of varying thickness and varying TiO 2 particle size are examined. Dark IV analysis indicates formation of an asymmetric Schottky-barrier junction between the nanoporous TiO 2 film and a graphite back contact. Upon increasing the film thickness and decreasing the particle size, diode rectification deteriorates while series resistance increases; these phenomena are attributed to resistive hopping charge transport between TiO 2 particles. Dramatic differences in the dark-IV extrapolated resistance of comparable thickness 300 and 9 nc films indicate that the transport physics may be substantially different between these two types of films in the dark. An effective medium model is developed that appropriately describes the dark MS data, indicating that the TiO 2 film behaves as a pure dielectric in the depletion region and that the internal surface area of the film does not contribute to the capacitance. Upon UV illumination, photodoping increases the TiO 2 film conductivity. Fermi-level splitting results in an ohmic graphite contact and current flow is generally limited by carrier transport rather than charge transfer to the contact. IV analysis based on a space charge limited current model provides mobility estimates of ∼1.7−3.4 × 10 -4 cm 2 V -1 s -1 for the 9 nm diameter TiO 2 particle films and ∼1.8−3.5 × 10 -4 cm 2 V -1 s -1 for 300 nm diameter TiO 2 particle films under AM 1.5 illumination. The close agreement between the illuminated-IV extrapolated electron mobility for both the 9 and 300 nm diameter TiO 2 particles suggests that the transport physics are likely similar in both films when illuminated.