Absorption Spectra and Excited State Energy Levels of the N719 Dye on TiO2 in Dye-Sensitized Solar Cell Models

We have investigated the absorption spectrum and the alignment of ground and excited state energies for the prototypical N719 Ru(II) sensitizer adsorbed on an extended TiO2 model by means of high level DFT/TDDFT calculations. The calculated and experimental absorption spectra for the dye on TiO2 are in excellent agreement over the explored energy range, with an absorption maximum deviation below 0.1 eV, allowing us to assign the underlying electronic transitions. We find the lowest optically active excited state to lie ca. 0.3 eV above the lowest TiO2 state. This state has a sizable contribution from the dye pi* orbitals, strongly mixed with unoccupied TiO2 states. A similarly strong coupling is calculated for the higher-lying transitions constituting the visible absorption band centered at ca. 530 am in the combined system. An ultrafast, almost instantaneous, electron injection component can be predicted on the basis of the strong coupling and of the matching of the visible absorption spectrum and density of TiO2 unoccupied states. Surprisingly, this "almost direct" injection mechanism, corresponding to excitation from the dye ground state to an excited state largely delocalized within the semiconductor, is found to give rise to almost exactly the same absorption profile as for the dye in solution, despite the drastically different nature of the underlying excited states. On the basis of our calculations it seems therefore that no sizable lower bound to an "injection time" exists, rather the timings of electron injection are mainly ruled by electron dephasing in the semiconductor.