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Title: Intermolecular charge transport in dye monolayers
Author: Moia, Davide
ISNI:       0000 0004 5371 9297
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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This thesis reports the experimental investigation of intermolecular charge transport in dye monolayers anchored to the surface of nanocrystalline oxides. I use electrochemistry and transient spectroscopy to measure diffusion of holes within dye monolayers and interpret my observations on the basis of the non-adiabatic Marcus theory of charge transfer. I observe thermally activated hole diffusion for dyes used in dye sensitized solar cells (DSSCs) anchored to TiO2 and immersed in an inert acetonitrile based electrolyte. The corresponding values of reorganization energy of charge transfer between the dyes range between 700 and 1500 meV. Assuming negligible contribution from energetic disorder, this shows agreement with previously reported calculations of reorganization energy. Low outer sphere and low inner sphere reorganization energies correlate with delocalization of the HOMO and with rigid molecular structures showing extended conjugation. I show that hole diffusion in the monolayer can be controlled both at the μm and at the nm scale by varying the fraction of TiO2 surface covered with dyes. I present the effect of decreasing the dye surface coverage and consequently stopping hole diffusion on photo-electrochemical device structures. First, I observe a slowdown of the photo-induced recombination reaction of holes in the dye monolayer to electrons in the TiO2 when decreasing the dye loading. This result is consistent with the hypothesis that hole diffusion in the dye monolayer contributes to faster recombination. Second, I show that hole transport in the dye monolayer is responsible for increased dye regeneration efficiency in solid state DSSCs. I quantify improved regeneration yield by between 50% and 5% depending on the degree of the pore filling by the hole transporting material spiro OMeTAD. Finally I demonstrate that effective photo-conversion can occur in solar cell structures where dye monolayers function as the only hole transport phase.
Supervisor: Barnes, Piers ; Nelson, Jenny Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral