Investigation of Dispersive Kinetics in the Time-dependent Photoluminescence of Dye-sensitized Nanoparticle Films

Mesoporous titanium dioxide nanoparticle films sensitized with ruthenium polypyridal chromophores provide the main architectural framework of dye-sensitized solar cells. The design of these systems is based on the interfacial photoinduced charge separation that occurs when an excited chromophore transfers an electron to acceptor states within the semiconductor material. This process is also called electron injection. If electron injection cannot occur, the excited chromophore can relax through the emission of a photon. Therefore, observing the time-dependent photoluminescence decay of these systems can provide insight on the efficiency of the electron transfer process. This dissertation investigates the dependence of the time-dependent photoluminescence decay on various factors using time-correlated single photon counting techniques with a home-built two-photon laser scanning setup. Interestingly, the time-dependent photoluminescence follows power-law kinetics when electron injection is most efficient. Power-law kinetics are a specific type of dispersive kinetics and several other dispersive kinetic models are also examined in this work. By adjusting the electrolyte composition of these systems as well as applying an electrical bias, the kinetic drive for electron injection can be adjusted. As the kinetic drive for injection decreases, the observed kinetics become less dispserive. At no point, however, is there an observed unquenched, purely emissive population. It is suggested that cross-surface energy transfer provides an alternative route for excited state relaxation. The current model for electron transfer is based upon electron injection into a continuum of acceptor states in the conduction band of the semiconductor. However, there exists an exponential distribution in energy of sub-bandgap acceptor states due to defects on the nanoparticle surface. The work presented in this dissertation provides evidence for a modification of the current model where chromophores are strongly coupled to only a small number of these localized trap states which lead to the dispersive kinetics we observe.Doctor of Philosoph