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Title: Pyridinium compounds as sensitisers for solar cells
Author: Ta, Yien Trung
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2011
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The work presented in this thesis concerns the synthesis and study of various RuII polypyridyl complexes and organic chromophores which incorporate strongly electron-accepting and π-conjugated pyridinium groups. Several of these compounds have been studied as potential sensitisers for solar cell applications.The N3 dye, cis-RuII(NCS)2(dcbpy)2 (dcbpy = 4,4'-dicarboxy-2,2'-bipyridyl) has been synthesised in pure form and excellent yield via ester hydrolysis of cis-RuII(NCS)2(Et2dcbpy)2 (Et2dcbpy = 4,4'-di(ethoxycarbonyl)-2,2'-bipyridyl). This well known complex is used as a benchmark for comparison purposes with our novel pyridinium compounds. A single-crystal X-ray structure has been obtained for the new complex cis-RuIICl(NCS)(Et2dcbpy)2.Three new 4,4':2',2":4",4""-quaterpyridinium pro-ligand cations along with several known ones have been synthesised and isolated as their chloride salts. Single-crystal X-ray structures have been determined for one of these salts and also for an unexpected protonated derivative. These pro-ligands have been used to prepare two series of RuII polypyridyl complex salts. A series of eleven new complexes of the form cis-[RuII(bpy)2(LA)]4+ and fourteen of the form cis-[RuIIX2(LA)2]4+ (bpy = 2,2'-bipyridyl; LA = quaterpyridinium-based ligand; X = NCS⁻ or Cl⁻) have been synthesised and isolated as their hexafluorophosphate salts. The various characterisation techniques used include 1H NMR spectroscopy and mass spectrometry. The UV-vis spectra show intense intraligand π→π* absorptions in the UV region and low energy metal-to-ligand charge-transfer (MLCT) bands in the visible. Small shifts in the MLCT bands correlate with changes in the electron-accepting strength of the ligand LA. The UV-vis absorption profiles of the salts cis-[RuIIX2(LA)2][PF6]4 cover the entire visible region extending well into the near-IR, with MLCT bands much more red-shifted than those of their cis-[RuII(bpy)2(LA)][PF6]4 relatives and the N3 dye. Cyclic voltammograms show quasi-reversible or reversible RuIII/II oxidation waves and ligand-based reductions. The variations in the redox potentials correlate with changes in the pyridinium substituents, and also with the MLCT energies. Single-crystal X-ray structures have been solved for three of the new cis-[RuII(bpy)2(LA)][PF6]4 salts. Four carboxylate-functionalised RuII polypyridyl compounds have been tested as dye sensitisers on both TiO2 and ZnO electrodes, but only low overall efficiencies are achieved. A series of eight new stilbazolium compounds has been synthesised and fully characterised as their hexafluorophosphate salts. Single-crystal X-ray structures have been determined for a new picolinium precursor and three of the ester-substituted stilbazolium compounds. The UV-vis spectra of these organic chromophores feature intense intramolecular charge-transfer (ICT) bands. Four acid-functionalised compounds have been tested as dye sensitisers on both TiO2 and ZnO surfaces, with the most encouraging preliminary results observed for the latter.Time-dependent density functional theory (TD-DFT) calculations were carried out on six of the new dye sensitisers to clarify their electronic/optical properties. TD-DFT-derived depictions of the lowest unoccupied molecular orbitals (LUMOs) show negligible electron density near the acid-functionalities. This factor, along with possible energy level mismatch of the excited-state donor orbitals with the conduction band edge of the semiconductor, may be the cause of inefficient electron injection and thus poor photovoltaic performance of our new pyridinium-based dyes.
Supervisor: Coe, Benjamin Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available