First row transition metal complexes for application to dye-sensitised solar cells
Linfoot, Charlotte Louise
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Ruthenium (II) complexes are used extensively in photoelectrochemical and photophysical devices, such as Dye-Sensitized Solar Cells (DSSCs). The use of Cu(I) as a possible replacement for Ru(II) has to date had limited exploration, but has obvious advantages in terms of low cost and high abundance. However, Cu(I) typically undergoes conformational change from tetrahedral towards square planar upon oxidation or MLCT excitation, often leading to reduced stability, reduced electron transfer rates and reduced excited state lifetime, thus impairing useful function. Typically, steric constraints are used to prevent this; however these can often be synthetically intensive, involving multi-step and low yielding synthetic pathways. In this work, we explore “blocking” functionality using two different ligands combined with a range of bipyridyl ligands with varying substituent groups. The study has looked into the synthesis of heteroleptic Cu(I) complexes of the general formula: [Cu(POP)(bipyridyl)][BF4], where POP = bis[2-(diphenylphosphanyl)phenyl] ether, and [Cu(pmppE)(bipyridyl)], where pmppE = hydrazono pyrazol-5-thiones(one). The work presented in this thesis focuses on the synthesis, and subsequent photoelectrochemical and photophysical characterisation of Cu(I) complexes, yielding results that open new avenues for design of functional Cu(I) systems. Solar cell testing also revealed photovoltages comparable to those of existing Cu(I) DSSC sensitisers. An extensive spectroscopic study of [Cu(POP)(dmbpy)]+ and [Cu(POP)(tmbpy)]+ has revealed the latter to have the significantly larger quantum yield: 65 % and 4% respectively in PMMA at 300 K. A complimentary computational investigation was carried out in order to gain a better understanding of how structural rigidity affects emission properties.