Exploring ruthenium dye synthesis and TiO2-dye-I-/I3- electron transfer reactions in a dye-sensitised solar cell.
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Octahedral, six co-ordinate ruthenium complexes containing acid substituted polypyridyl ligands have proved particularly successful as dyes for Dye-Sensitised Solar Cells (DSSCs); thus there have been hundreds, if not thousands of these types of complexes synthesised and studied. Ruthenium dyes are now incorporated into commercial DSSCs; yet there is limited understanding of the interactions between the dye and the liquid I-/I3- electrolyte, which can both facilitate and hinder the generation of current and voltage within the cell. Monodentate –NCS ligands incorporated into ruthenium dyes appear to interact strongly with I- and I2 within the electrolyte. The analogous –NCO containing dye has therefore been synthesised to study the effect of the chalcogen atom on these interactions. Substituting –NCS for -NCO resulted in a significant change in the spectroscopic and electronic properties of the molecular dye, with destabilisation of the HOMO of the dye causing a red shift in dye absorption. Possibly due to this change in properties, the nature of the chalcogen atom was shown to have a significant impact on the performance of the dye in a DSSC. The effect of the nature of substituents on ancillary ligands of Ru(H2-dcbpy)(4,4`-Y2- bpy)(NCS)2 dyes on recombination across the TiO2 – electrolyte interface within the cell has been proven significant due to changes in the strength of binding of I2 to the substituent groups. However, few substituent groups have been investigated; therefore a series of halogenated dyes, where Y = Cl, Br, were synthesised. The effect of the nature of the halogen on dye recombination was significant, although the trends observed were not consistent with the reported data for iodine binding. Similar trends were observed for the analogous series, Ru(H2-dcbpy)(5,5’-Y2- bpy)(NCS)2 where Y = F, Cl, Br. By comparison of this series of dyes with the 4,4` dyes, it was discovered that the position of the substituent had a significant effect on the rate of recombination within the solar cell, as well as the electrochemical and spectroscopic properties of the dyes themselves. Such isomeric effects have not been previously reported. In the synthesis of these dyes, and in attempting the synthesis of five other ruthenium dyes, many barriers to efficient dye synthesis were discovered. Therefore, an investigation into the synthesis of ruthenium dyes has been conducted. By analysis of the breakdown products formed a number of avoidable side reactions, including decarboxylation and ruthenium catalysed nucleophilic substitution of the bpy ligands, were shown to occur. Problems associated with the high lability of the ruthenium centre at high temperatures have also been explored, and the use of UV/vis monitoring to aid optimisation of the reaction conditions was implemented. Thus, the development of two novel synthetic procedures allowed the synthesis of the dyes investigated during the course of this thesis.