Charge Dynamics following Dye Photoinjection into a TiO₂ Nanocrystalline Network

Abstract

Electrons can be injected into TiO2 nanocrystals by a surface-adsorbed Ru-based dye in a well-known photosensitization process. Here we present time-resolved microwave conductivity measurements of the decay of electrons out of an electrically interconnected network of such TiO2 nanocrystals, following charge injection, and in the presence of an electron donor to the dye cation that prevents direct recombination. The time scale for this decay process is hundreds of milliseconds to seconds and explains the high current collection efficiency in devices based on these materials, since the long time scale allows for slow charge transport through the network.

Exciton dissociation, charge transport, and recombination in ultrathin, conjugated polymer-TiO2nanocrystal intermixed composites

“…17. Briefly, the experimental apparatus consists of a microwave cavity with a slit open at one end, permitting optical excitation by a Nd:YAG ͑yttrium aluminum garnet͒ laser beam ͑532 nm͒ converted to pulses of 10-ns duration and various energies by an optical parametric oscillator ͑photon energy 3.5 eV for band-gap excitation in the TiO 2 nanocrystals͒, and microwave generation and detection electronics.…”

“…However, the crucial factor which underlies the efficiency of the dyesensitized system is that the recombination time is quite long, ranging from hundreds of milliseconds to seconds. 17 Since the device is a photoelectrochemical cell, the recombination is not between holes and electrons directly but rather between electrons and an ionic, electron acceptor in solution which may be advantageous from the point of view of reducing the recombination rate. However, it is reasonable to expect that through control of the interfacial area and morphology of nanocrystal:conjugated polymer composites, comparable efficiencies are possible.…”

“…For the thermal velocity we have the formula m e v th 2 ϭ3/2 kT, with m e the effective mass of the electron, v th the thermal velocity, k Boltzmann's constant, and T the temperature. Taking m e to be 100 times that of a freeelectron mass, 17 we have v th ϭ1.1ϫ10 4 cm/s at 300 K. We calculate the drift velocity from the formula v d ϭE, with v d the drift velocity, the electron mobility in the TiO 2 conduction band, and E the built-in electric field. Taking ϭ0.1 cm 2 V Ϫ1 s Ϫ1 and Eϭ5ϫ10 4 V/cm ͑from 0.7 V across 150 nm͒ gives v d ϭ4ϫ10 3 cm/s.…”

“…The estimate implies an upper limit for the time to hop between nanocrystals of T Х10 s. However, in a network of TiO 2 nanocrystals, the diffusion constant is expected to be significantly lower than the bulk value. From recombination measurements in a network of dye-sensitized TiO 2 nanocrystals 17 and the known quantum yields, we estimate a diffusion constant of order 10 Ϫ7 cm 2 s…”

Time-resolved Infrared Absorption Study of Photochemical Reactions Over Metal Oxides

“…Electron injection from the dye to the oxide substrate is quite fast and efficient, not so dependent on materials [16,17,28,29]. Post-injection processes include diffusion, localization and back (substrate-to-dye) electron transfer [30][31][32]. The efficiency of the photon-to-current conversion is thought to be controlled by the post-injection processes.…”