Graduate Thesis Or Dissertation
Investigation of Conformational Control of Photoinduced Electron Transfer in Ruthenium Polypyridyl Dye-Sensitized Solar Cells Public Deposited
To explore the impact of structure on photoinduced electron transfer processes, systematically tuned 4′-aryl-substituted terpyridyl ruthenium(II) complexes are synthesized and then studied in solution, film, and device settings. These complexes are introduced in the framework of dye-sensitized solar cells (DSSCs)—a low cost alternative to conventional silicon-based devices. A monolayer of ruthenium chromophores attached to a mesoporous TiO2 semiconductor is designed to efficiently absorb photons and initiate the separation of charge across a dye/semiconductor interface. Acting as a working electrode in a complete cell, this construction of sensitized TiO2 particles allows for the conversion of photons to current. In our work, structural tuning is performed within the aryl-terpyridine ligands, by systematically introducing methyl substituents at the ortho positions of the aryl moiety in order to explore dynamical electron transfer coupling at ruthenium/TiO2 interfaces.
First, syntheses, electrochemical potentials, static emission, and temperature-dependent excited-state lifetimes of several 4′-aryl-substituted terpyridyl complexes of ruthenium(II) are presented. In these solution phase studies, the power of structural tuning manifests primarily with a 14-fold decrease in the excited-state lifetime of dimethylated species relative to unsubstituted aryl-terpyridine species. Next, an overview of the materials and procedures utilized to make complete DSSCs is presented. In an effort to determine the implications of structural tuning of these complexes within the DSSCs, total solar power conversion efficiency measurements are performed. These are then deconstructed to better understand the fundamental processes and how they impact light-to-current conversion. Injection yields are identified as the primary factor limiting the efficiencies of these ruthenium terpyridyl DSSCs. The final chapter presents an investigation of the kinetics of charge recombination—an energy-wasting back electron transfer process at play in photoinduced charge-separated processes. Transient absorption spectroscopy is applied to sensitized TiO2 films in an electrochemical setup simulating device conditions. The addition of methyl steric bulk is found to inhibit charge recombination, with measured recombination lifetimes increasing by over 12-fold across the series of structurally-tuned complexes. If injection yields can be improved, the structural tuning of recombination rate constants may be an important design rule for improving solar conversion efficiency in solar cells and water splitting devices.
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