Low temperature thermally stimulated current characterization of nanoporous TiO sub(2) films

• Published in: Physica status solidi. A, Applications and materials science Vol. 211; no. 8; pp. 1691 - 1697
• Main Authors: Bruzzi, Mara, Mori, Riccardo, Carnevale, Ennio, Scaringella, Monica, Bogani, Franco
• Format: Journal Article
• Language: English
• Published: 01.08.2014
• Subjects: Carriers; Hopping (conductivity); Mathematical models; Nanostructure; Resistivity; Solar cells; Thermally stimulated current; Titanium dioxide
• ISSN: 1862-6300; 1862-6319
• DOI: 10.1002/pssa.201330608

Nanoporous films of TiO sub(2) have been obtained by deposition on an alumina substrate of a commercial colloid and Au contacts have been put on surface for in-plane conductivity measurements. Samples have been characterized by means of a series of sequential thermally stimulated currents scans, from nearly liquid He temperature up to 200K, in order to get information on: conduction mechanisms, distribution of intragap density of states and recombination times of free an localized carriers. A model for hopping conductivity has been considered to properly take into account of localized states filling; it has been used to fit the experimental data, without a-priori assumptions on the nature of the conduction mechanisms. It is found that hopping conductivity prevails over free-carrier, in quasi-equilibrium regime, in the investigated temperature range. The role of localized states in electrical conductivity on nanocrystalline TiO sub(2) for dye sensitized solar cells (DSSCs) has been investigated by thermally stimulated current (TSC) measurements. The theoretical analysis of TSCs in a heavily disordered medium is a quite complex phenomenon. The authors have outlined a procedure of numerical simulation that allows for the extraction of significant parameters, as density of defect states and value of the energy constant of the exponential band-tail distribution. The study shows that in a quasi-equilibrium regime, main conduction mechanism in nc-TiO sub(2) is due to hopping, rather than to free carriers that undergo multi-trapping processes.