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Samuel Fauvel

Synthesis and integration of photochromic molecules in dye-sensitized solar cells

Published on 21 November 2023
Thesis presented November 21, 2023

Since the beginning of the industrial revolution, the need for energy has never stopped growing. Historically, fossil fuels have been massively used to meet this energy demand, but their limitations are pushing us towards more sustainable sources such as solar energy. Among the various photovoltaic technologies, dye-sensitized solar cells (DSSCs) have strong advantages; they can achieve efficiencies above 15%, can be made semi-transparent and are aesthetically pleasing. In addition, as the absorption of the cell is largely dependent on the dye used, interesting abilities can be added to the cell through chemical engineering. In 2020, our team presented the first successful use of photochromic dyes in DSSCs, giving the cell a self-adjusting property. Among the dyes used in this study, the most promising was NPI, which allowed efficiencies above 4% in cells. Since this pioneering work, we decided to work on the molecular design of the photochromic dye by introducing new moieties and study their structure-property relationships. We also focused more on the aesthetic aspect of the cells, which is of prime importance for their potential application as smart windows. These concerns led to the design and synthesis of new families of dyes by modifying different parts of the reference dye NPI. We dedicated a strong attention to the donor part of the molecule and substituted the diphenyl-amine used on NPI by carbazoles moieties to tune the push-pull properties of the molecule. We also worked on the acceptor part of NPI, introducing different chemical functions to study their effect on the resulting dye. Among these new dyes, SF4 showed promising abilities in terms of aesthetic and photovoltaic performance. We used a cosensitization approach with SF9, a dye incorporating a benzothiadiazole moiety, to optimize the properties of the cell. We also worked on the electrolyte used to further improve the optical and photovoltaic performances of SF4. In the end, we produced a 5x5 cm mini-module with an active surface area of 14 cm2 and a power output of 14 mW, while maintaining good aesthetics (CRI above 95) in both activated and inactivated form. Although the optical and photovoltaic performances obtained with SF4 suggest a promising future for this type of device, further optimizations are required to obtain a commercially viable cell.

Solar cells, self-adjustable optical transmission, photochromic dyes