Optical Properties and Aggregation of Phenothiazine-based Dye-Sensitisers for Solar Cells Applications: A Combined Experimental and Computational Investigation

Combining computational modeling and experimental optical analyses, we investigate two prototypical phenothiazine-based organic solar cells sensitizers with the aim to understand the individual effects of solvation and aggregation on the dyes optical properties. Dye solvation and aggregation play a crucial role in determining the photo-electrochemical properties of these systems and the interplay of these two factors can lead to a misinterpretation of the underlying phenomenology due to their similar spectroscopic signals. In particular, upon adsorption of the dye onto the metal oxide surface, the dye UV-vis absorption spectrum may attain either a blue or a red shift compared to the dye in solution, which can either be originated from aggregation of surface-adsorbed dye and/or solvatochromism in the initial dye solution. Understanding the origin of these spectral changes along with their possible effect on charge-transfer properties is important for the further improvement of Dye-sensitized Solar Cells. Based on our results, we show that the optical properties of phenothiazine-based dyes are much more sensitive to the type of explicit interactions with the solvent than to aggregation on the TiO2 surface. Therefore, this study gets important new insights into the understanding of these properties and may assist the molecular engineering of new and more efficient dyes sensitizers.