Band-gap states in unfilled mesoporous nc-TiO2: measurement protocol for electrical characterization

Abstract

The characterization of the intrinsic electrical properties of large surface area materials such as mesoporous nanocrystalline (nc-TiO2), requires one to avoid sensor-like responses to external agents. Both an appropriate sample configuration and a suitable measurement protocol are mandatory. In this work, both the stack and planar contacts configurations were studied, the latter giving evidence which is potentially useful for space charge limited (SCL) current investigations in order to study the band-gap states (BGSs) properties in nc-TiO2. Moreover, in the absence of a suitable measurement protocol, standard dye sensitized solar cells (DSSCs) electrode films show apparent SCL current behaviour, as a consequence of the surface electrical transport contribution due to the physisorbed water on the hydroxylated metal oxide large surface area. This feature recalls the typical results of electrolyte filled systems, in which an exponential distribution of trap states is reported; it is not expected to be an intrinsic feature of the nc-TiO2 trap states distribution. In the absence of adsorbates, no deviation from an ohmic regime is observed in standard electrodes, while in planar configuration samples, the SCL regime is accessible and shows a different BGS signature. The nature of electrical contacts, ohmic in this situation, is also discussed.