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Title: Multiscale modelling of intermolecular charge transfer in dye sensitised solar cells
Author: Vaissier, Valérie
ISNI:       0000 0004 5367 4252
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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Quantum chemistry based simulations allow us to explore the length and time scales which are experimentally inaccessible. In particular, these simulations bring a unique perspective on processes governed at the nanoscale by electronic interactions such as charge transfer. In this thesis, I present a framework for the multiscale simulation of hole transfer between dye molecules tethered on (101) TiO2 surfaces as in Dye Sensitized Solar Cells (DSSC). At the molecular level, I use methods derived from ground state density functional theory to calculate the reorganization energy (λ_tot) including ionic solvent effects, and electronic coupling (J_ij) distributions representing the conformational disorder of a dye monolayer. At the nanoscale, I use the semi-classical non adiabatic Marcus's equation to calculate the rate of hole transfer in the high temperature limit from λ_tot and J_ij. At the macroscopic scale, I calculate hole diffusion coefficients from kinetic Monte Carlo (KMC) simulations and validate my results by comparing with experimental data, when available. I find that the polar electrolytes used in DSSC contribute to 80% of the total reorganization energy of hole exchange. By including the effect of structural rearrangement of the dyes on various timescales, I show that large amplitude fluctuations of the tethered dyes at the microsecond timescale may enable charges to escape configurational traps. However, the analysis of Quasi Elastic Neutron Scattering (QENS) data on dye sensitised TiO2 nanoparticles suggests that the dyes are immobile between tens of picoseconds and few nanoseconds. This implies that the hypothesised dynamical rearrangement of the dye monolayer at the microsecond timescale originates from the collective motion of the molecule and its neighbours. These findings suggest that charge transport across disordered dye monolayers is enabled by the structural rearrangement of the molecules while the low measured diffusion coefficients (~10^-8 cm^2.s^-1) arise from the high polarity of the medium.
Supervisor: Nelson, Jenny ; Barnes, Piers Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral