In dye-sensitized solar cells (DSSCs) and in photo-catalytic devices designed for hydrogen production of CO₂ reduction, light triggers ultrafast molecular processes, such as electron, energy transfer or singlet fission. Since these processes are at the heart of the function of the devices and of their efficiencies, the design of new molecular photo-sensitizers or catalysts can be optimized rationally if these processes are monitored by ultrafast spectroscopy.
In this talk, I will present our latest results on transparent solar cells designed for the near-IR. Here, femtosecond spectroscopy allows to give evidence for the kinetic competition of electron release and detrimental monomer-to-aggregate energy transfer (W. Naim et al, JACS Au 1, 409 (2021)). The latter is minimised by the recent design of diketo-pyrrolopyrrole dyes, as we could directly and quantitavely evaluate from ultrafast fluorescence spectroscopy (T. Baron et al., Angew. Chem. 61, e202207459 (2022) & M. Kurucz et al., ChemPhotoChem 8, e202300175 (2024)). The molecular design concepts are thus confirmed by ultrafast spectroscopy, providing a rational understanding for the state-of-the-art performances combining high average visible transmission (76%) and power conversion efficiency (3.9%).