Soeren Alkaersig Jensen , Klaas-Jan Tielrooij , Euan Hendry , Mischa Bonn , Ivan Rychetský , and Hynek Soeren
J.Phys.Chem. C, Just Accepted Manuscript
DOI: 10.1021/jp406897y
Publication Date (Web): December 10, 2013
Films of colloidal TiO2 nanoparticles are widely used in photovoltaic and photocatalytic applications, and the nature of electrical conductivity in such materials is therefore of both fundamental and practical interest. The conductive properties of colloid TiO2 films depend strongly on their morphology and deviate greatly from the properties of the bulk material. We report ultrafast photoconductivity studies of films consisting of sintered TiO2 particles of very different sizes performed using time-resolved Terahertz spectroscopy. Remarkably, identical photoconductivity spectra are observed for films of particles with diameters of tens and hundreds of nm respectively. The independence of photoconductivity on particle size directly demonstrates that the terahertz photoconductive response of colloidal TiO2 films is not affected by carrier backscattering at particle boundaries as has previously been concluded, but rather by depolarization fields resulting from the spatial inhomogeneities in the dielectric function inherent to these types of films. Modelling of the influence of depolarization fields on the terahertz conductivity allows us to explain the measured data and gain insights into the morphology of the film. Specifically, we show that the observed photoconductivity spectra reflect percolated pathways in the colloidal TiO2 nanoparticles films, through which charge carrier diffusion can occur over macroscopic length scales.
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