The photoassisted electrical transport is investigated in porous TiO2 (anatase) layers by impedance spectroscopy (IS) and time-of-flight (TOF) at different temperatures and pressures of oxygen. The experiments were carried out under the conditions of zero bias potential (no electron injection at the contacts takes place) and of relatively high bias potential when electrons are injected from the contact into the porous TiO2 layer. Illumination with photons having energies below the band gap leads to a strong increase of the conductivity while the conductivity is practically unchanged for illumination with ultra violet light independent of the applied bias potential. Besides the light induced excess carrier generation, TOF measurements show an increase of the drift mobility under illumination with blue light. The results were explained by the lowering of the barriers between sintered TiO2 nanocrystals due to separation of photoexcited charge carriers.

The influence of adsorbed water, oxygen, air and vacuum on the photoluminescence (PL) and electron paramagnetic resonance (EPR) has been investigated in situ and quasi in situ for por-TiO2 (anatase). The broad PL signal in the visible spectral region decreases with increasing partial pressure of oxygen and vanishes at pressures higher than 1 mbar. Adsorption of water leads (i) to a fast quenching and (ii) to a subsequent increase of the PL signal. The concentration of the O2−, O−, O3− anion-radicals depends sensitively on the surface conditioning and on the illumination of the por-TiO2. Ti3+ centers could be observed only in vacuum treated samples. The concentrations of the Ti3+ and oxygen anion radicals are in the range of 1015 and 1017 cm−3, respectively.

The thermal activation of the electronic transport in sintered nanoporous TiO2 (anatase, rutile) layers has been investigated by current voltage characteristics, impedance spectroscopy and time of flight techniques. The thermal activation energy of the electrical conductivity and drift mobility is about 0.8 eV independent of the phase of the titanium dioxide, the diameter of the TiO2 nanocrystals and of the absolute value of the electrical conductivity. We propose a model which describes the electron transport in a sintered nanoporous TiO2 network as limited by traps located in the small contact regions of the grains. Dielectric screening is discussed as a possible reason for the increase of the energy of defect levels near the conduction band which determine the Fermi-level position.

Oxygen- and water-related surface defects on porous TiO2 (anatase) can be well controlled by the oxygen and water partial pressures and therefore such defects are of technological relevance for dye sensitized TiO2 solar cells. We investigated the action of oxygen and water-related surface defects in situ by impedance spectroscopy, photoconductivity, photoluminescence, and optical transmission as well as by characterizing solar cells which were prepared under respective conditions. Oxygen loss from the TiO2 surface leads to electrical doping by Ti3+/oxygen donor states. Such defects create recombination paths for injected electrons back into the electrolyte. Pre-treatment of porous TiO2 by chemisorption of water increases the open circuit voltage of the solar cells without altering the short circuit current. Water-related surface defects decrease the saturation current of the diode, probably by raising the barrier height at the TiO2/electrolyte interface.

The dependence of the electrical conductivity (σ) of sintered nanoporous TiO2 (rutile and anatase) layers on temperature (T) and partial pressure of oxygen (pO2) has been studied by impedance spectroscopy for T up to 450 °C and pO2 up to 10−5 mbar. The diameter of the TiO2 nanoparticles was changed between 5 and 60 nm. σ is thermally activated with EA=0.85 eV independent of the absolute value of σ and depends on pO2 by power law for pO2<1–10 mbar. The electrical properties of reduced nanoporous TiO2 are determined by surface chemical reactions which lead to the formation of shallow donor and deep trap states.

physica status solidi (a)Volume 167, Issue 1 p. R9-R9 ErratumFree Access Erratum to Electron Drift Mobility in Porous TiO2 (Anatase) Th. Dittrich, Th. Dittrich Physik Department E16, Technische Universität München, D-85748 Garching, GermanySearch for more papers by this authorE. A. Lebedev, E. A. Lebedev A.F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorJ. Weidmann, J. Weidmann INAP GmbH, Munscheidstr. 14, D-45886 Gelsenkirchen, GermanySearch for more papers by this author Th. Dittrich, Th. Dittrich Physik Department E16, Technische Universität München, D-85748 Garching, GermanySearch for more papers by this authorE. A. Lebedev, E. A. Lebedev A.F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorJ. Weidmann, J. Weidmann INAP GmbH, Munscheidstr. 14, D-45886 Gelsenkirchen, GermanySearch for more papers by this author First published: 29 January 1999 https://doi.org/10.1002/(SICI)1521-396X(199805)167:13.0.CO;2-ICitations: 4AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article.Citing Literature Volume167, Issue1May 1998Pages R9-R9 RelatedInformation

The current–voltage characteristics of dye-sensitized porous-TiO2(por-TiO2) solar cells are investigated in the dark and under illumination at light intensities up to 1500 W/m2 and temperatures between −5 and 80 °C. In the dark, the barrier height and the ideality factor of the por-TiO2/electrolyte contact are 0.67 eV and 1.05, respectively. The very low effective Richardson constant indicates the importance of diffusion for transport. A current-dependent effective barrier height has been established under illumination of dye-sensitized por-TiO2 solar cells. The barrier lowering effect should be caused by the low neutralization rate of the positively charged dye radicals in the electrolyte.