Power-Law Kinetics in the Photoluminescence of Dye-Sensitized Nanoparticle Films: Implications for Electron Injection and Charge Transport

Dye-sensitized solar cells have provided a model to inexpensively harness solar energy, but the underlying physics that limit their efficiency are still not well understood. We probe electron injection in sensitized nanocrystalline TiO2 films using time-correlated single photon counting (TCSPC) to measure time-dependent chromophore photoluminescence quenching. The time-dependent emission exhibits kinetics that become faster and more dispersive with increasing ionic concentrations in both water and acetonitrile; we quantify these trends by fitting the data using several kinetic models. Even more notably, we show that the residual emission under conditions that favor efficient electron injection exhibits a power-law decay in time. We attribute this highly dispersive kinetic behavior to electron injection from the dye into localized acceptor states of the TiO2 nanoparticle film, which exhibits a distribution of injection rate constants that depend on the energetic distribution of sub-band-gap trap states.