Molecular Engineering of a Fluorene Donor for Dye-Sensitized Solar Cells

To improve their efficiency beyond the state-of-the-art, D−π–A dyes must display increased spectral breadth and account for the physical limitations observed in the dye-sensitized solar cells. In particular, they should be designed to control the electron-transfer processes that ensure efficient dye-regeneration and prevent undesired electron recombination. In this article, the electronic and steric properties of a fluorene donor are engineered to meet all these requirements. This elegant donor is featured along with a cyclopentadithiophene bridge and a cyanoacrylic acid acceptor in <b>JF419</b>. A thorough comparison with <b>Y123</b> and <b>C218</b> demonstrates the relevance of the design. Relative to conventional donors, the fluorene construct described here enhances the light-harvesting properties, because of its exceptional electron-donating character. The functionalities used to induce the electronic push through the D−π–A structure also provide the dye with favorable steric properties. Indeed, the substitution around the fluorene core adequately insulates the TiO₂ surface from the electrolyte, which prevents back-recombination and prolongs the electron lifetime in the semiconductor. Furthermore, compared to analogous dyes, <b>JF419</b> maintains nearly quantitative regeneration efficiency, despite the lower regeneration driving force. The root of this observation is contributed to a significantly more delocalized hole in the photo-oxidized <b>JF419</b>^{• <b>+</b>}, which is highlighted through transient absorption spectroscopy and quantum chemical calculations. The design principles established are relevant to the development of more comprehensive sensitizers, as evidenced by the 10.3% efficiency obtained in cobalt-based liquid dye-sensitized solar cells