Exploring Thiophene Oligomers and Ruthenium(II) complexes for their use in dye-sensitised solar cells
Despite offering relatively high conversion efficiencies, dye-sensitised solar cells using liquid electrolytes containing either I-/I3 - redox couple or Co2+/Co3+ redox couple suffer from durability problems, such as electrode corrosion and electrolyte leakage. Replacements for liquid electrolytes have been extensively studied, but the efficiencies of the resulting devices remain low. One of the factors that limit the efficiency is the sensitising dye. Large sized hole-transport material results in poor pore-filling and thus leads to a fast back electron recombination that reduces the effective electron diffusion length to few micrometeres. The optimal TiO2 layer thickness (2 μm) for maximal power conversion in solid-state dye-sensitised solar cell is much smaller than the 6-10 μm layer thickness required for quantitative light absorption by many dye molecules. Thus, dyes that can absorb in both visible and near-IR region with high extinction coefficient are needed. In order to achieve this, novel oligomers and ruthenium (II) complexes are designed, synthesized and studied as sensitisers for both liquid state and solid state dye-sensitised solar cells in this thesis. Series of ‘donor-free’ dyes including oligo(3-hexylthiophene) (oligo-3HT) (Chapter 3) and oligo(4,4-dihexyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophene) (oligo-CPDTs) (Chapter 4) functionalized with cyanoacrylic end groups are easily synthesized using cross-coupling. They were fully characterised through electrochemical, spectroscopic and computational techniques, showing versatile colour-tuning, as well as outstanding absorption coefficients up to 75000 M-1cm-1. Liquid and solid-state DSSCs device performances are studied and discussed in terms of the dye structures. These dyes are effective sensitisers for liquid-state and solid-state dye-sensitised solar cells, although they do not contain a typical donor group, thus open a new strategy of designing dyes in the future. New dyes containing different azo ligands as an additional chromophore moiety to enhance light harvesting of Ru complexes (Chapter 5) have been prepared using a protection/deprotection strategy that allows for convenient purification. The absorption spectrum of the dyes showed an enhanced light harvesting compared to the N719 dye that lacks the azo ligand and electrochemical study also showed properties suitable for application as sensitisers in DSSCs. Following hydrolysis, the complexes were investigated in DSSCs, with performance investigated using I-V measurements. Poor performance was observed and we attribute this as mostly likely due to poor charge injection due to short excited-state lifetime. Although the application of these current dyes in DSSCs is not feasible due to their poor performance, this study allowed us to determine the positions of the HOMO and LUMO orbitals and correlate it to the π-acidity of the dyes.