Temperature dependent normal and anomalous electron diffusion in porousTiO2studied by transient surface photovoltage

Abstract

sensitized with dye molecules at the outer surface is investigated bytime resolved surface photovoltage SPV at temperatures between −120 °C and 270 °C to get informationabout electron diffusion over more than 10 orders of magnitude in time. The SPV transients increase in timedue to independent electron diffusion and reach a maximum at a certain peak time due to reaching thescreening length. The increasing parts of the SPV transients are characterized by a power law while the SPVpower coefficient amounts to half of the dispersion parameter of anomalous diffusion. Anomalous diffusion isobserved for times down to the duration time of the laser pulse 150 ps . With increasing temperature, the SPVpower coefficient increases to its saturation value of 0.5 corresponding to normal diffusion. At lower tempera-tures, the SPV power coefficients decrease with increasing intensity of the exciting laser pulses. The decay ofthe SPV transients is determined by thermally activated normal diffusion. The minimal charge transfer time ofan electron back to the positively charged dye molecule amounts to 2 ps which is obtained from thermallyactivated logarithmic decays.DOI: 10.1103/PhysRevB.73.045407 PACS number s : 72.40. w, 73.63.Bd