What Limits Photoconductance in Anatase TiO2 Nanostructures? A Real and Imaginary Microwave Conductance Study

The macroscopic electron transport in porous films of sintered 9 nm sized nanocrystals (NC-TiO2) is known to be 4-6 orders of magnitude worse than in polycrystalline (Pol-TiO2) dense films. To obtain fundamental knowledge regarding this large difference, we investigated the effects of spatial confinement and electron trapping processes on the charge transport in these samples. We determined the time-resolved real and imaginary microwave photoconductance (TRMC) on pulsed excitation. Large amounts of the photoexcited electrons are readily immobilized in deep traps in NC-TiO2 as concluded by comparing the TRMC decay kinetics with previously published transient absorption measurements. Our results show that trapped electrons do not give rise to a microwave photoconductance response, nor do they affect the motion of conduction band electrons Additionally, by comparing a bare NC-TiO2 film with a dye sensitized NC-TiO2 sample, the influence of the photogenerated holes on the photoconductance as a function of their locus is investigated. The positive charges either inside or outside the TiO2 nanocrystals contribute insignificantly to the photoconductance. On a nanosecond time scale only a minor fraction (maximum 2%) of the photoexcited electrons resides in the conduction band of NC-TiO2. Importantly, in both NC-TiO2 and Pol-TiO2 these electrons have the same intraparticle microwave mobility of 13 cm(2)/(V s) due to frequent backscattering events at a mean time interval of 85 fs. This mobility value represents the upper limit for the trap-free dc electron mobility in anatase TiO2 irrespective of the crystallite size. Hence, the photoconductance across a NC-TiO2 layer can be strongly enhanced by reducing the electron trap density and by eliminating the relatively inefficient electron hopping steps between adjacent nanocrystals.