Novel Ruthenium complexes and their application in dye sensitised solar cells
McCall, Keri Laura
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This work focuses on the design, synthesis and characterisation of novel ruthenium bipyridyl complexes and their use in dye sensitised solar cells (DSSCs). Four series of dyes have been synthesised with the general formula Ru(R-bpy)2L, where R = H, CO2Et, CO2H and L represents four different bidentate ligands; cyanodithioimidocarbonate (L1), ethyl xanthate (L2), 2,3-bis(2-cyanoethylthio)-6,7-bis(methylthio) tetrathiafulvalene (L3) and Cu(exoO2-cyclam) (L4). These have been chosen to investigate two key aims: firstly, the increase of the light-harvesting ability of the dye and secondly, the retardation of the recombination of the injected electron with the oxidised dye. Each complex was characterised using electrochemistry, absorption spectroscopy, spectroelectrochemistry and hybrid-DFT calculations. In addition the performance of the acid derivatives in a DSSC was also investigated using IPCE and IV measurements, as well as transient absorption spectroscopy. Two different S-donor ligands, L1 and L2, have been used to investigate the effect of these strongly electron-donating systems on the light-harvesting ability of the dye. Complexes utilising the di-anionic L1 were shown to exhibit significantly increased absorption range and higher extinction co-efficients, relative to the high efficiency dye N719. This dye series also showed a large degree of S-donor ligand character to the HOMO, deduced via spectroelectrochemical and computational studies. Despite these advantageous features the dye performed very poorly in a DSSC, which was attributed to fast recombination. This was a result of the cyano group of L1 coordinating to the TiO2 in addition to the acid groups of the bipyridine ligands. The complexes synthesised with the mono-anionic L2 on the other hand showed only a slightly increased lightharvesting ability relative to N719 and no significant ligand character to the HOMO. However, the performance of this dye in a DSSC was more promising, with efficiencies of up to ~ 2 % achieved. The control of the loss mechanism via recombination of the injected electron with the oxidised dye in a DSSC was investigated by incorporating redox-active ligands, L3 and L4. The series of dyes synthesised with L3 showed significant ligand character to the HOMO orbital, as deduced by spectroelectrochemical, emission and computational studies. Upon adsorption of the acid derivative to TiO2 an extremely long-lived chargeseparated state of 20 ms was observed via transient absorption spectroscopy. Despite this unique long-lived charge-separated state, the dye yielded extremely low DSSC efficiencies. This was attributed to the poor regeneration of the neutral dye by iodide, which in turn was thought to be the result of a stable intermediate formation between the dye cation and the iodide anion. The complexes synthesised with L4 showed the highest light-harvesting efficiency of all the series studied with a wide absorption range and large molar extinction co-efficients. Whilst the maximum efficiency of the dye in a DSSC was nearly 3 %, the performance was found to vary under prolonged irradiation. This was attributed to the degradation of the dye by either exchange of the counter ions with the electrolyte or loss of L4.