Asymmetrical DπA 2,2'-diimines for homoleptic copper(I)-based dyes in dye-sensitised solar cells

We live in times of grave precariousness for our environment. It is a proven fact that global warming is mainly caused by the uncontrolled human emissions of CO2 in the atmosphere. The Earth as we know it may irreparably change if we as humans do not take action. The prospects for the future are in rapid change and the temperatures increase must be contained at all costs. In order to do so, new policies must be enforced to promote a conversion of our primary energy sources away from fossil fuels and greatly develop non-emissive sources. Hydroelectric, wind and solar are at the forefront of the renewable energies. The solar sector has been expanding quite rapidly in the last twenty years and will continue to do so. However, the high production costs of silicon solar panels are yet a limiting factor.
As described in Chapter 1, the invention of the dye-sensitised solar cell (DSC) paved the way to the development of a rising technology. The DSC is easy to manufacture and its inexpensive production costs are appealing. State-of-the-art devices are sensitised with organic or ruthenium-based dyes. However, ruthenium is a noble metal, its low abundance is responsible of the prohibitive costs, and together with its general toxicity it is unsuitable for commercial applications.
Copper(I) is a metal with similar photophysical properties to ruthenium(II), and it is widely available thanks to a greater abundance on the Earth’s crust. In Chapter 2, heteroleptic bis(diiminine) copper(I)-based dyes are initially investigated in the classical push-pull architecture, where two 2,2'-bipyridine ligands are functionalised with an anchoring and electron-donor units. The focus of this chapter is the effects of introducing alkynyl spacers in the ancillary ligand.
In Chapters 3–5, an attempt has been made to overcome known issues such as ligand dissociation and to improve the photoconversion efficiencies, by making a shift from the heteroleptic copper(I)-based dyes with the push-pull design to homoleptic copper(I)-based dyes with the alternative [Cu(DπA)2]+ design. Organic dyes bearing a coordinating domain are presented as new asymmetrical DπA 2,2'-bipyridine ligands. The old and the new design are compared thanks to the study of pairs of structural isomers. It is demonstrated that the properties of the [Cu(DπA)2]+ dyes are superior through a thorough analysis of the DSC performances, corroborated by DFT and TD-DFT studies. The properties of these dyes are investigated by structural changes at the 2,2'-bipyridine scaffold. Finally, the properties of the [Cu(DπA)2]+ design are further studied by broadening the scope with 2,2'-biquinoline ligands. Furthermore, one of the presented [Cu(DπA)2]+ dye nears 50% of relative photoconversion efficiency to the reference N719 ruthenium dye, being among the best efficiencies recorded for copper(I)-based dyes.