Effect of structural modification of surface bound dyes on electron transfer kinetics at dye / TiO2 / liquid electrolyte interface

Electron transfer is a fundamental mechanism involving redox active molecules (donor and acceptor) in photoelectrochemical systems. Dye Sensitised Solar Cells is regarded as one such example of photoelectrochemical device which involves series of electron transfer steps at a dye / TiO2 / liquid electrolyte interface. Efficient conversion of photoelectrochemical energy is influenced by fast reduction of surface immobilised dye cations by redox mediators dissolved in solution. However, the competing back electron transfer between the injected electrons on the TiO2 surface and the oxidised dyes needs to be slower as well. The parameters from classical Marcus theory which includes driving force (-ΔG°), electronic coupling (HDA) and reorganization () energy all in principle could be tuned by the molecular structure of the electron donating and accepting molecules. Increasing the size of the molecule could result in lesser external reorganization energy with increased coupling however it also provides lesser electron probability density. While dissolving the redox active molecules in solution, the interaction between the dyes and mediator could get better as they can move freely in solution. However, while anchoring the dyes to semiconductor surface the exposed area of the dyes to the redox couple gets limited. The aim of this Ph.D. thesis was to systematically evaluate how the structural modification of surface bound dyes (both donor moiety and π-conjugated backbone) could increase the exposure of the surface area of the dye towards the redox couple, contributing on enhanced forward electron transfer. In addition, the thesis has also investigated ways to control the recombination between the injected electron and the oxidised dyes in the presence of redox mediator through using dye designs having small to bulky structure with modulated alkyl chains ranging from low to high potentials.