Ambipolar Diffusion of Photocarriers in Electrolyte-Filled, Nanoporous TiO₂

Abstract

We report transient photocurrent measurements on solar cell structures based on dye-sensitized, porous TiO2 films filled with a liquid electrolyte. The measurements are interpreted as ambipolar diffusion; under most measurement conditions, the ambipolar diffusion coefficient is dominated by electrons diffusing in the TiO2 matrix. We report a strong dependence of the ambipolar diffusion coefficient upon the photoexcitation density, as has been proposed previously. The coefficients vary from 10-8 cm2 s-1 at low density to 10-4 cm2 s-1 for densities of 1018 cm-3. At a specified photoexcitation density, ambipolar diffusion coefficients measured using weak laser pulses and optical bias are about 10 times larger than coefficients measured using large-intensity laser pulses. We describe trapping models for these effects based on an exponential distribution (T 0 = 650 K) of electron trap levels in TiO2. We infer an electron recombination cross section less than 2 × 10-27 cm2; this value is nearly 10 orders of magnitude smaller than typical values in compact semiconductors and indicates the extraordinarily effective separation of electrons in the TiO2 matrix from electrolyte ions only nanometers distant.

PbSe Quantum Dot Field-Effect Transistors with Air-Stable Electron Mobilities above 7 cm2 V–1 s–1

“…For example, the carrier mobility in nanocrystalline TiO 2 films increases by orders of magnitude upon ∼1 sun illumination as a result of trap filling. 51,55 A similar effect should occur in illuminated PbX QD films. The density of photogenerated charges (n) can be estimated from the product of the generation rate (G) and the carrier lifetime (τ), n = Gτ.…”

Chemical Diffusion Coefficient of Electrons in Nanostructured Semiconductor Electrodes and Dye-Sensitized Solar Cells

“…Indeed, measurements of TiO 2 and ZnO nanoparticulate networks by small perturbation techniques (both time transient and frequency methods) show clearly the dependence of the electron diffusion coefficient on the steady-state conditions. [5][6][7][8][9][10][11][12][13][14][15] We remark that the diffusion coefficient is a magnitude of central importance in nanostructured semiconductors and dye-sensitized solar cell because it is measured directly.…”

Temperature dependence of the electron diffusion coefficient in electrolyte-filledTiO2nanoparticle films: Evidence against multiple trapping in exponential conduction-band tails

“…However, for both classes, the porosity is associated with significant disorder that, for example, greatly reduces charge carrier mobilities or diffusion coefficients from the values in the underlying crystals. [5][6][7] This type of disorder is an interesting if little explored aspect of the enormous subject of transport in highly disordered, nominally homogeneous systems. Research on electronic properties of porous materials includes theoretical work on quantum percolation, [8][9][10] which applies when porosity has an atomic length scale, and studies of Coulomb-blockaded semiclassical transport on porous lattices that reveals analogies with phase transitions.…”

“…The electrolyte, which fills the pores, is not only essential to the application of this material in solar cells, but also appears to passivate defects that greatly retard electron transport in "dry" mesoporous titania. 14 Technically, diffusion of electrons in the electrolyte-filled material is ambipolar, 6,15 meaning that the mobile electrons in titania carry a cloud of countercharges ͑cations͒ in the electrolyte. This is a well-known phenomenon in the diffusion theory of electrolyte solutions.…”