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Ionic liquids (ILs) have gained popularity as “green” and safe replacements for conventional organic solvents. They are defined as ionic salts displaying a melting point below 100 °C. Some of their unique characteristics also include negligible vapour pressure, good electrical conductivity as well as good thermal and chemical stability. While their “green” nature has since been disputed, they can be used and applied in many additional fields, such as solar energy production, new lighting technology and much more. In this thesis, the aim is to gain fundamental knowledge on ILs, specifically their structures and behaviour, in order to design materials tailored for specific applications. We also aim to use ILs to access otherwise difficult to synthesize materials and study their properties and applications. The thermal properties of ILs are one of their most important characteristics. However, it is still poorly understood how the structural aspects of ILs influence their particular thermal behaviour. By studying different systems, we derived relationships between the structure and the thermal behaviour of ILs. Hydrogen bonding and other supramolecular interactions play a major role in controlling both the melting temperature and the IL's ability to support a liquid crystalline mesophase. This control was shown both in a series of ILs based on 1-alkyl-3-dodecylimidazolium bromide and in a series of ILs based on azobenzene-imidazolium compounds. The stability issues associated with the electrolytes used in dye-sensitized solar cells (DSSCs) present a major disadvantage. We tested using ILs as electrolytes to avoid this problem. In our study, we used 1,3-dialkyltriazolium ILs as electrolytes in combination with the iodide redox couple, and not only was the stability of the DSSC improved but also the performance of IL-based DSSCs. Efficient luminescent materials are always sought after. Using ILs in combination with lanthanides, we achieved highly luminescent compounds as well as some magnetic ones. ILs can also be used to access anhydrous forms of otherwise hydrophilic species, such as ions of the lanthanides. We have used acetate ILs to attain water free complexes of the ions from the whole lanthanide series, starting from the hydrated species. This simple process could be applied to more species of hydrophilic metals that are otherwise known to form hydrates. Finally, the ligand obtained through ILs, 1,3-diethylimidazole-2-thione was used to aid in the studying of phase transitions when combined with zinc chloride (ZnCl2). It helped to reveal a yet unseen amorphous step in the solid-solid phase transition from a single crystal into another one, where morphology of the particle was preserved. I forsee that more fundamental structural studies can be conducted by forcing the coordination of the soft-donor nitrogen onto lanthanides by using dicyanamide ILs in the future.

Perovskites-photovoltaic cells are a type of photovoltaic cells which include a chemical compound having perovskite structure, most often a hybrid organic-inorganic lead or a tin halide, in its light-converting active layer. The efficiency of photovoltaic cells used in these materials is increasing constantly since the beginning of the new millennium. It went from 3.8% in 2009 [6] to 22.1% [7] in early 2016 [8].Up until today, this is the fastest development in the history of the photovoltaic history. To this day, some stability problems unfortunately still remain unsolved. However, this technology still exhibits a significant margin to improve performance and low production costs. This means that perovskite cells have become commercially attractive, and start-up companies already announce modules on the market by 2019. This study concluded that the addition of halogenated bidentate additives such as 1,8 Diiodooctane (DIO) not only influenced the performances of perovskite solar cells but also their stability over time. By fine elemental analysis, it was concluded that the addition of chlorine in the solution did not imply the substitution of iodide by chlorine in the structure. Chlorine is therefore believed to play a role in getting rid of the excess of methylamine, thus helping stabilizing the cell and enhancing its performance. As requested by the industry, this work demonstrated the feasibility of replacing the electron transport layer (ETL) of TiO2 by a materials obtained by liquid low-temperature process (