The highest efficiency ever reported for an iron-sensitized solar cell has been obtained using a Fe(ii) pyridylNHC-carboxylic heteroleptic complex. Mg2+ cations and NBu4I in the electrolyte, substantially boosted the photocurrent.

We report a joint experimental and computational work on new organic donor-acceptor dye sensitizers in which a carbazole (CZ) and a phenothiazine (PTZ) units are linked together by an alkyl C6H13, while two different anchoring groups are employed: the cyanoacrylic acid (CS1A, CSORG1) and the rhodanine-3-acetic acid (CS4A, CSORG4). The CZ moiety has multiple roles of (i) acting as an extra-electron donor portion, providing more electron density on the PTZ; (ii) suppressing the back-electron transfer from TiO2 to the electrolyte by forming a compact insulating dye layer; (iii) modulating dye aggregation on the semiconductor surface; and (iv) acting as an antenna, collecting photons and, through long-range energy transfer, redirecting the captured energy to the dye sensitizer. We show that the introduction of the CZ donor remarkably enhances the photovoltaic performances of the rhodanine-based dye, compared to the corresponding simple PTZ dye, with more than a two-fold increase in the overall efficiencies, while it does not bring beneficial effects in the case of the cyanoacrylic-based sensitizer. Based on quantum mechanical calculations and experimental measurements, we show that, in addition to a favored long-range energy transfer, which increases the light absorption in the blue region of the spectrum, the presence of the CZ unit in the CSORG4 dye effectively induces a beneficial aggregation pattern on the semiconductor surface, yielding a broadened and red-shifted light absorption, accounting for the two-fold increase in the generated photocurrent.

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 TiO 2 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.

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