RESUMEN
Perovskite solar cells and fluorescent collectors formed by a dispersion of quantum dots in a transparent solid are paradigmatic devices for photon capture and utilization that involve the coupling of photon displacement, absorption and regeneration. In order to obtain information about the coupled photonic processes in systems involving photon recycling, we analyze the transfer function for modulated outgoing to incoming photon flux. We show the physical features of light-to-light impedance that reveals a trap-limited diffusion of photons coupled with the nonradiative recombination. The spectral shapes allow one to distinguish readily photonic recombination and diffusion kinetic phenomena and consequently to determine the physical parameters that control the system's quantum yield for photoluminescence.
RESUMEN
The performance of Dye-sensitized solar cells (DSC) and related devices made of nanostructured semiconductors relies on a good charge separation, which in turn is achieved by favoring charge transport against recombination. Although both processes occur at very different time scales, hence ensuring good charge separation, in certain cases the kinetics of transport and recombination can be connected, either in a direct or an indirect way. In this work, the connection between electron transport and recombination in nanostructured solar cells is studied both theoretically and by Monte Carlo simulation. Calculations using the Multiple-Trapping model and a realistic trap distribution for nanostructured TiO2 show that for attempt-to-jump frequencies higher than 10(11)-10(13) Hz, the system adopts a reaction limited (RL) regime, with a lifetime which is effectively independent from the speed of the electrons in the transport level. For frequencies lower than those, and depending on the concentration of recombination centers in the material, the system enters a diffusion-limited regime (DL), where the lifetime increases if the speed of free electrons decreases. In general, the conditions for RL or DL recombination depend critically on the time scale difference between recombination kinetics and free-electron transport. Hence, if the former is too rapid with respect to the latter, the system is in the DL regime and total thermalization of carriers is not possible. In the opposite situation, a RL regime arises. Numerical data available in the literature, and the behavior of the lifetime with respect to (1) density of recombination centers and (2) probability of recombination at a given center, suggest that a typical DSC in operation stays in the RL regime with complete thermalization, although a transition to the DL regime may occur for electrolytes or hole conductors where recombination is especially rapid or where there is a larger dispersion of energies of electron acceptors.
RESUMEN
Numerous experiments have indicated that the fracture front (in three dimensions) and crack lines (in two dimensions) in disordered solids and rocklike materials is rough. It has been argued that the roughness exponent ζ is universal. Using extensive simulations of a two-dimensional model, we provide strong evidence that if extended correlations and anisotropy-two features that are prevalent in many materials-are incorporated in the models that are used in the numerical simulation of crack propagation, then ζ will vary considerably with the extent of the correlations and anisotropy. The results are consistent with recent experiments that also indicate deviations of ζ from its supposedly universal value, as well as with the data from rock samples.