Organoruthenium Complexes for Solar Energy Harvesting

Abstract

One of the greatest challenges of this time is providing the world with the energy it needs to sustain human kind's current standard of living. Solar energy is the most abundant and ubiquitous renewable energy source available, and as such it holds great promises. Traditionally, the field of solar energy conversion has been dominated by solid-state semiconductor technology. However, the need for high-purity materials makes these single junction solar cells expensive and unlikely to reach cost parity with fossil energy in the near future. The dependence on high purity materials can be substantially decreased by physically separating the charge separation and transport functions of the solar cell. This vital insight stands at the cradle of dye-sensitized solar cells (DSSCs), in which a large bandgap semiconductor is sensitized by a sensitizer molecule. Producing a DSSC is a relatively simple and cheap process compared to low bandgap solid-state semiconductor cells based on silicon. Demonstration kits for home-built DSSCs are commercially available which use harmless components and colorants such as berry juices. However, the highest efficiencies, up to 11 % power conversion, are typically reached using polypyridine complexes of ruthenium as sensitizers. Unfortunately, issues related to the efficiency for the conversion of red light as well as long term stability are still associated with these complexes. This thesis describes our efforts towards using cyclometalation as a new design paradigm to address these issues. A two-step upconversion would allow the use of photons below the operational threshold set by the bandgap energy but requires new types of sensitizer. Our efforts in the design and characterization of potential dyes for this process are also described. Cyclometalated complexes contain at least one covalent carbon-to-metal bond in a multidentate ligand. The interaction between the strong sigma-donor carbon atom and the metal center affects the electronic properties of the resulting complexes, red-shifting the absorption characteristics. The red-shift of the absorption features in the standard dyes for DSSCs is typically achieved with monodentate thiocyanate ligands, which are sensitive towards decomposition processes associated with dissociation. In contrast, the polydentate nature of cyclometalated ligands make these complexes inherently long-term stable. This thesis uses experimental and theoretical tools to study the fundamental influence of cyclometalation in polypyridine-type ruthenium complexes on the electronic and photophysical properties, as well as their chemical consequences. It is demonstrated that cyclometalation is a very promising technique in the design of new sensitizer molecules. It is also concluded that extensive knowledge of the complexes' photophysical properties is a prerequisite to the de novo design of efficient systems. Finally, the possibility to use photons of energy below the operational threshold of the DSSC is explored using a two-step upconversion scheme. To study this upconversion scheme several dinuclear redox unsymmetrical ruthenium complexes have been studied in solution as well as incorporated in a DSSC, demonstrating the necessary individual processes.