Nonradiative excitation decay of dyes on insulators and semiconductors

One of the practical problems associated with dye-sensitized semiconductors in liquid-junction solar cells has been the very low conversion efficiencies that have been reported. To better understand the causes of this problem, picosecond time-correlated photon counting fluorescence profiles were collected from dye molecules on different substrates. In one experiment cresyl violet (CV) molecules were adsorbed on fused quartz substrates at submonolayer coverages. It was found that the best description of the energy transfer between the monomeric and dimeric forms of CV was given by a model having an exponential term and a two-dimensional Foerster term. Another set of experiments examined the distance dependence of the lifetime of emitting CV molecules spaced at various distances from a TiO2 surface by means of Cd-arachidate monolayers. Nonradiative excitation transfer is found to be an important mechanism for this situation. The measured lifetimes are compared to the theoretical lifetime vs. distance curves for TiO2 that are calculated from the Chance, Prock, and Silbey theory. In the final set of experiments that are discussed, the fluorescence decay profiles for rhodamine 3B (Rh3B) molecules adsorbed directly on fused quartz, TiO2, and ZnO substrates are collected and analyzed. It was found that the two-dimensional Foerster model fits the data from all three substrates and gives reasonable values of the natural lifetime, tau, and reduced trap concentration, C/sub T/. This suggests that the energy acceptors are located in the surfaces of the substrates. 150 refs