Temperature dependence of the electron diffusion coefficient in electrolyte-filled ${\mathrm{TiO}}_{2}$ nanoparticle films: Evidence against multiple trapping in exponential conduction-band tails

Temperature dependence of the electron diffusion coefficient in electrolyte-filled ${TiO}_{2}$ nanoparticle films: Evidence against multiple trapping in exponential conduction-band tails

Nikos Kopidakis, Kurt D. Benkstein, Jao van de Lagemaat, Arthur J. Frank, Quan Yuan, and Eric A. Schiff

Phys. Rev. B 73, 045326 – Published 25 January 2006

Abstract

The temperature and photoexcitation density dependences of the electron transport dynamics in electrolyte-filled mesoporous ${TiO}_{2}$ nanoparticle films were investigated by transient photocurrent measurements. The thermal activation energy of the diffusion coefficient of photogenerated electrons ranged from 0.19–0.27 eV, depending on the specific sample studied. The diffusion coefficient also depends strongly on the photoexcitation density; however, the activation energy has little, if any, dependence on the photoexcitation density. The light intensity dependence can be used to infer temperature-independent dispersion parameters in the range 0.3–0.5. These results are inconsistent with the widely used transport model that assumes multiple trapping of electrons in an exponential conduction-band tail. We can also exclude a model allowing for widening of a band tail with increased temperature. Our results suggest that structural, not energetic, disorder limits electron transport in mesoporous ${TiO}_{2}$ . The analogy between this material and others in which charge transport is limited by structural disorder is discussed.