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.
Hybrid rylene arrays have been prepared via a combination of Stille coupling and C-H transformation. The ability to extend the π system along the equatorial axis of rylenes not only leads to broadened light absorption but also increases the electron affinity, which can facilitate electron injection and transport with ambient stability.
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