Trap-limited recombination in dye-sensitized nanocrystalline metal oxide electrodes - art. no. 205321

We use transient and steady-state optical spectroscopies to study the recom bination reaction between electrons and dye cations in a dye-sensitized nan ocrystalline TiO2 electrode in several different chemical environments. Kin etic decay curves are approximately stretched exponential, and the cation h alf-life, t(50%) varies with electron density n as t(50%) proportional to n (-1/alpha) where alpha is a constant in the range 0.2-0.5. We have develope d a model of electron transport in the presence of an energetic distributio n of trap states and consider two regimes. In the first, the continuous-tim e random-walk (CTRW) electrons are free to diffuse through the lattice, by means of multiple trapping events mediated by the conduction band. In the s econd, the hopping regime, trapped electrons are allowed to tunnel to other , vacant trap sites, or to the dye cation, according to a Miller-Abrahams m odel for the transition rate. We carry out Monte Carlo simulations of the r ecombination kinetics as a function of electron density, trap state distrib utions and other parameters. The CTRW reproduces both the dependence of t(5 0%), on n and the shape of the kinetic curves with only one free fitting pa rameter, for the case of an exponential density of trap states. The hopping model is ruled out by subnanosecond measurements. We conclude that multipl e trapping with a broad energetic distribution of electron traps is respons ible for the slow recombination kinetics. When applied to recombination in a nanocrystalline photovoltaic junction at open circuit, the model predicts a sublinear power-law variation of electron density with light intensity G , n proportional to G(alpha), compatible with the observed behavior.