Kinetics and thermodynamics of dye (group VIII metal)-sensitized nanocrystalline titanium dioxide photoelectrodes

alternative redox couples;bromide tribromide;direct reduction of triiodide;dye sensitized;iridium

Mayo, Elizabeth Idonia ; Gray, Harry B. (advisor)

University:California Institute of Technology

Department:Chemistry and Chemical Engineering

关键词: alternative redox couples;    bromide tribromide;    direct reduction of triiodide;    dye sensitized;    iridium;   

Others  :  https://thesis.library.caltech.edu/2442/1/Elizabeth_I_Mayo.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

This thesis reports a comprehensive series of experiments involving complementary kinetics and thermodynamic measurements directed at isolating the important individual reactions in dye-sensitized nanocrystalline titanium dioxide solar cells (DSSCs).These experiments were done in conjunction with steady-state photoelectrochemical measurements; a combination which allowed a greater understanding of the overall mechanisms and driving forces of these systems.Alternative two-electron redox couples were studied and efficiency increases of >40% were achieved when compared to similar systems using iodide/triiodide.Surface treatment with carboxylic acids minimized direct reduction of the redox couple by electrons in the titanium dioxide, and interestingly, the photocurrent also increased resulting in overall efficiency increases as high as 20%.Bridging ligands were used in an attempt to minimize recombination of the injected electrons with the resulting oxidized dyes, but DSSCs with these sensitizers showed poor conversion efficiencies and no distance dependence for injection or recombination was observed.The lack of distance dependence was attributed to the flexible single carboxyl anchoring group.To further investigate the effect of binding mode, a series of carboxyl-modified ruthenium bipyridyl sensitizers were studied.A single carboxyl anchoring group resulted in unstable DSSCs due to enhanced desorption as well as poor photon-to-current conversion efficiencies.These dyes injected efficiently into TiO2 on the nanosecond timescale, and regeneration of the oxidized sensitizers competed effectively with recombination.Consequently, individual kinetics measurements could not explain the decreased steady-state performance.The regeneration rates of these dyes in solution were found to rapid, approaching the diffusion controlled limit.The regeneration rate was dependent on the number and electron-withdrawing nature of the pendant groups, with the rate decreasing with increasing number of electron withdrawing substituents.Iridium dyes with cyclometalating ligands were shown to be efficient sensitizers in DSSCs, with quantum yields on the order of a ruthenium analogue having similar spectral overlap.Overall, the repeated inconsistencies between the steady-state behavior and the measured individual kinetics processes indicate that the current kinetic model is insufficient to accurately predict photoelectrochemical behavior.