Kumada Coupled Anti-Aggregation Moieties in Phenothiazine Dyes for Dye-Sensitized Solar Cells

Abstract

In the continuous search for renewable energy sources, dye-sensitised solar cells (DSSCs) have emerged as a very promising, low-cost solution. In the quest for higher efficiency and stability, the DSSC components are continuously optimised, and none has received more focus than the all-important dye molecules. Common mechanisms limiting the dye performance are recombination and aggregation. The traditional approach has been antiaggregation additives such as chenodeoxycholic acid (CDCA), breaking up the undesired dye-dye interactions within the DSSC.

Controlling the CDCA-dye ratio is of high importance, and the goal of this thesis was to attach the CDCA covalently to a dye molecule, ensuring a 1:1 ratio. The properties of the Kumada coupling made this a good candidate for coupling the cholesterol derivative to the commonly used thiophene pi-spacer of the sensitiser. The two target sensitisers were T2 (with the CDCA) and the reference T1 (with a hexyl chain).

A convergent synthesis route was followed, where the donor and pi-spacer moieties were prepared separately before being joined in the penultimate synthesis step. Sensitiser T1 was prepared successfully in a 4% yield over four steps, while the crucial Kumada coupling failed in the synthesis towards T2.

The photovoltaic performance of T1 compared to a reference dye without the anti-aggregation chain was surprisingly low. A possible explanation could be the increased size of the pi-spacer causing an unfavourable twist of the conjugated system, decreasing the photovoltaic performance.