Molecular modification of dye constituents through grafting anchoring groups and pi-spacers: towards DSSCs application

A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Materials Science and Engineering of the Nelson Mandela African Institution of Science and TechnologyTriphenylamine based dyes molecular modification by grafting anchoring groups and spacers was performed within computational framework aimed at potential materials for dye- sensitized solar cells (DSSCs) application. For the modification, benzothiadiazole-like spacers, heterocyclic anchoring groups and heteroatom doping have been taken into account. The reported structural and optoelectronic properties of the dyes were realized through density functional theory and time-dependent density functional theory using B3LYP and CAM-B3LYP functionals coupled with 6-31G(d,p) and 6-31G+(d,p) basis sets. The findings show that presence of atoms of nitrogen and sulphur in the -spacers has a beneficial impact on the material‟s properties, whereas the branched -spacer impairs the electronic light absorption characteristics of the sensitizers. The heterocyclic anchoring units result in a bathochromic shift with maximum absorption within 450 − 600 nm. The computed light harvesting efficiencies, excited-state lifetimes, electron injection and regeneration abilities prove that dyes with heterocyclic anchoring groups to be potential candidates for DSSC applications. The heteroatom doping demonstrates that the chalcogens enhance the absorption and fluorescent emission spectra of the isolated dyes in the visible and near infra-red regions with maximum wavelength 504 – 556 nm and 637 – 732 nm, respectively. Simulation of the dyes attachment to the TiO 2 surface was undertaken; two models of the crystal surface considered, TiO 2 slab and hydrogenated (TiO 2 ) 6 cluster. Among possible adsorption modes of the dye@TiO 2 complexes, monodentate, bidentate and tridentate, the bidentate mode was found thermodynamically more favourable. In both, individual dyes and dye@TiO 2 complexes, less electronegative dopants contributed to the improvement of the UV-Vis spectra and redistribution of electron density. The calculated energies of the dye@TiO 2 attachment relate to dopant heteroatoms; stronger binding is observed in the complexes with heavier heteroatoms – selenium and tellurium. The adsorption energy magnitudes range between 0.11 − 1.75 eV for the TiO 2 slab and 7.61 − 9.48 eV for (TiO 2 ) 6 cluster. Energy difference between the two binding models was found to correspond to the enthalpy of sublimation of TiO 2 from the TiO 2 anatase. One can anticipate that systematic modification of dyes‟ building blocks may lead to novel materials with suitable characteristics for application in DSSCs