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.

We have observed the production of strings (disclination lines and loops) via the Kibble mechanism of domain (bubble) formation in the isotropic to nematic phase transition of a sample of uniaxial nematic liquid crystal. The probablity of string formation per bubble is measured to be 0.33 ± 0.01. This is in good agreement with the theoretical value 1/π expected in two dimensions for the order parameter space S 2 /Z 2 of a simple uniaxial nematic liquid crystal.

We propose an experimental procedure for testing the Einstein relation for carrier drift and diffusion in semiconductors exhibiting non-Gaussian or dispersive transport. We present corresponding hole time-of-flight and steady-state photocarrier grating measurements in hydrogenated amorphous silicon (a-Si:H). For a range of mobilities 10 25 10 22 cm 2 ͞V s we find that our estimates of hole diffusion are approximately twice as large as predicted by the Einstein relation and the mobility measurements. We consider the deviation to represent an upper bound to any true failure of the Einstein relation for hole transport in a-Si:H.

We present temperature-dependent measurements and modeling for a thickness series of hydrogenated amorphous silicon nip solar cells. The comparison indicates that the maximum power density (PMAX) from the as-deposited cells has achieved the hole-mobility limit established by valence bandtail trapping, and PMAX is thus not significantly limited by intrinsic-layer dangling bonds or by the doped layers and interfaces. Measurements of the temperature-dependent properties of light-soaked cells show that the properties of as-deposited and light-soaked cells converge below 250 K; a model perturbing the valence band tail traps with a density of dangling bonds accounts adequately for the convergence effect.

We present intensity-modulated photocurrent and infrared transmittance measurements on dye-sensitized solar cells based on a mesoporous titania (TiO2) matrix immersed in an iodine-based electrolyte. Under short-circuit conditions, we show that an elementary analysis accurately relates the two measurements. Under open-circuit conditions, infrared transmittance, and photovoltage measurements yield information on the characteristic depth at which electrons recombine with ions (the “locus of recombination”). For one particular series of samples recombination occurred near the substrate supporting the titania film, as opposed to homogeneously throughout the film.