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Maxime Godfroy

Modulation of the optoelectronic properties of organic dyes for applications in hybrid solar cells

Published on 14 October 2016
Thesis presented October 14, 2016

During one hour, the Earth receives solar energy which is equivalent to one year of the world energy consumption. For this reason, photovoltaic cells that convert photons to electricity, have a key role to play in the energetic transition imposed by climate change. Dye-sensitized solar cells are one of the emergent technologies that have already been used at the industrial scale in a few examples of building integrating. They represent an esthetic and low-cost alternative compared to silicon solar cells. These hybrid cells also named « Grätzel cells » use a nanostructured inorganic semi-conductor where a dye is grafted onto the surface and acts as a sensitizer. This dye injects electrons after photo-excitation in the oxide. The dye is regenerated by a redox couple present in a liquid electrolyte or a hole transport material that are themselves regenerated by the counter electrode. In this context, this work presents studies about some of the cell constituents (from the semi-conductor to the dye regenerating system). The major part of this thesis concerns the synthesis and the advanced characterization of organic semi-conductors, dyes or hole transport materials, and the study of the structure/properties relations. In particular, the replacement, the substitution, or the rigidification of some functional groups in these structures were achieved and their influence on the properties of the new molecules were studied. The synthesized dyes present maxima of the absorption band at the lowest energy between 440 nm and 610 nm. Energy levels of the new organic materials were determined by cyclic voltammetry and also calculated and localized using the quantum chemistry. Some of the compounds were studied by X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry. After a complete characterization, these materials were integrated in dye-sensitized photovoltaic devices using a liquid electrolyte to achieve high efficiencies up to 9.78 % using a single dye and up to 10.90 % in the case of the co-sensitization of TiO2 with two dyes. Certain dyes have demonstrated state-of-the-art efficiencies at 7.81 % by replacing the liquid electrolyte by an ionic liquid electrolyte. Moreover, the use of some of the dyes in these last devices was carried out and found to have an excellent stability with a loss of initial efficiency included between 7 % and 38 % after 7000 hours of continuous illumination at 1000 W.m-2 at 65 °C. Finally, first tests were also realized in solid state devices that showed an efficiency of 4.5 % with a reference hole transport material opening new application perspectives after optimizations. In parallel, the new synthesized hole transport materials in this work were effective in perovskite-based cells.

Organic dyes, Solar cells, Semi-conductor, Photovoltaic, Hybrid, Synthesis

On-line thesis.