Dye sensitised solar cells (DSSCs) mimic charge excitation and transfer processes found in natural photosynthesis to directly convert sunlight into electricity. Combining easy assembly with relatively cheap materials they offer a potentially cost effective solution to our energy requirements. Numerous physical processes are at work within a DSSC and the underlying complexity of these competing processes has meant that, despite considerable research effort, advances in obtaining a viable device efficiency have stagnated. The aim of this thesis is to examine, by density functional theory calculations, some of the processes at work in DSSCs with the motivation being to provide insight that informs the design of more efficient devices by experimentalists. Our calculations study some of the key factors affecting device efficiency, in particular the interaction of binding moieties with titanium dioxide surfaces, the role intrinsic and extrinsic defects have in defining the properties of semiconductors, the molecular design of sensitising dyes and the effect this has on both dye-dye and dye-semiconductor interactions. Finally we implement and test the excited state formalism of time dependent density functional theory (TDDFT) within the linear scaling DFT code CONQUEST, allowing excited state properties of large systems to be examined computationally. Our approach propagates the density matrix in real time (RT-TDDFT), and finally we use our implementation to model the real time response of titanium dioxide clusters and dyes to external electric fields
