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 JF419. A thorough comparison with Y123 and C218 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, JF419 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 JF419• +, 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.