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Johan Liotier

Synthesis and studies of organic photochromic dyes for the fabrication of solar cells with variable optical transmission

Published on 19 October 2021
Thesis presented October 19, 2021

In the current context of depletion of fossil resources and global climate change, the production of renewable energy is becoming a necessity. In this context, solar technologies are of great interest, particularly in France where there is a lot of sunshine throughout the year. Among these technologies, dye-sensitized solar cells offer good photovoltaic performance with efficiencies reported in the literature up to 14.3%. These cells can be designed to be semi-transparent, which makes them good candidates for integration into buildings, as it would be possible to integrate them as a façade or glazing. However, this prospect raises new questions about the trade-off to be found between transparency and the efficiency of the cells, which is determined during their manufacture. Indeed, integration as glazing requires that a sufficient quantity of light be allowed to pass through without altering the quality of colour perception in the interior. To overcome the current limitations of this technology, we considered modifying the classic structure of the dyes used to manufacture these cells by introducing a naphthopyran photochromic pattern. A first study has identified the NPI dye allowing us to obtain solar cells with conversion efficiencies above 4% while keeping its photochromic properties in cells. These photochromic properties, studied by UV-Vis spectroscopy in solution and in solar cells, demonstrate a reversible colour change of this molecule with complete discolouration in 1 hour in solution and 16 hours in cells. Different chemical modifications were then carried out on the different moieties present on this dye in order to establish clear structure-property relationships between the structure of the molecules and their optical, photochromic and photovoltaic properties. This study was completed by the implementation of colour characterisation techniques to study the transparency and CRI of solar cells. Two analytical methods were also developed to understand the details of the colouring and decolourisation process of the molecules. The first is a spectroscopic method with in-situ irradiation of a photochromic solution in an NMR probe and the second is a method of calculating by DFT the transition states of the molecules during reclosure. Finally, the implementation of a Machine Learning Python program coupled with the implementation of design of experiment allowed for the rapid optimisation of electrolytes containing redox mediators of different natures, increasing the cell performances and transparency. This work made it possible to obtain more than twenty dyes with varied optical properties. These molecules have broad absorption domains in the visible part of the solar spectrum with maximum absorption wavelengths varying from 470 nm to 655 nm. Their colours range from green to orange, through violet and red. The photovoltaic efficiencies of these dyes, a pioneer in this field, have been studied and some of them have efficiencies of over 4.1%.

Organic chemistry, Photochromic dyes, Dyes Sensitised Solar Cells