Long-Range p-Conjugation in Phenothiazine-containing Donor – Acceptor Dyes for Application in Dye-Sensitized Solar Cells Mini

Dye sensitized solar cells (DSCs) have attracted considerable attention in recent years owing to low cost and high efficiency. The DSCs based on ruthenium sensitizers have reached overall power conversion efficiency (PCE) of over 11 % under standard AM 1.5G illumination. Many efforts have been devoted to develop efficient metal-free sensitizers for practical use owing to their high molar absorption coefficient, ease of synthesis, scalability, and structural modifications. The PCE of donor–pbridge–acceptor (D-p-A) organic sensitizers has reached more than 12 % making them attractive candidates for use as sensitizers for DSC. A zinc porphyrin donor–acceptor dye already reached 13 % power conversion efficiency in conjunction with a cobalt-based redox electrolyte. In search of efficient D-p-A organic sensitizers, several donor groups such as diphenylamine, carbazole, triphenylamine, indoline, coumarin, pyrrolidine, and phenothiazine were used. However, little is reported on the use of phenothiazine as a p-bridge in D-p-A sensitizers. Organic dyes containing oligo-phenothiazine were used as both the electron donor and as the p-conjugated spacer in sensitizers for DSC. It is additionally reported that long pbridges can adversely affect photovoltaic device performance. In this study we combine these concepts to systematically varied the p-bridge length by introducing 0, 1, or 2 Four organic donor–p-bridge–acceptor dyes containing phenothiazine as a spacer and cyanoacrylic acid as an acceptor were synthesized and tested as sensitizers in dye-sensitized solar cells (DSCs). The influence of iodideand cobalt-based redox electrolytes on the photovoltaic device performance was investigated. In these new dyes, systematic p-conjugation was extended by inserting one or two phenothiazine moieties and investigated within the context of the resulting photoinduced charge-transfer properties. A detailed investigation, including transient absorption spectroscopy and quantum chemical methods, provided important information on the role of extended p-conjugation on the photophysical properties and photovoltaic device performance. Overall, the results showed that the extension of p-conjugation by one phenothiazine unit resulted in the best device performance owing to reduced recombination rates, whereas extension by two phenothiazine units reduced dye adsorption on TiO2 probably owing to the increase in molecular size. The performance of the dyes in DSCs was found to be a complex interaction between dye structure and size.