Nature of the Ultrafast Interligands Electron Transfers in Dye-Sensitized Solar Cells

Charge-transfer dynamics and interligand electron transfer (ILET) phenomena play a pivotal role in dye-sensitizers, mostly represented by the Ru-based polypyridyl complexes, for TiO₂ and ZnO-based solar cells. Starting from metal-to-ligand charge-transfer (MLCT) excited states, charge dynamics and ILET can influence the overall device efficiency. In this letter, we focus on N3^4- dye ( [Ru(dcbpy)₂(NCS)₂]^4-, dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) to provide a first direct observation with high time resolution (<20 fs) of the ultrafast electron exchange between bpy-like ligands. ILET is observed in water solution after photoexcitation in the ∼400 nm MLCT band, and assessment of its ultrafast time-scale is here given through a real-time electronic dynamics simulation on the basis of state-of-the-art electronic structure methods. Indirect effects of water at finite temperature are also disentangled by investigating the system in a symmetric gas-phase structure. As main result, remarkably, the ILET mechanism appears to be based upon a purely electronic evolution among the dense, experimentally accessible, MLCT excited states manifold at ∼400 nm, which rules out nuclear-electronic couplings and proves further the importance of the dense electronic manifold in improving the efficiency of dye sensitizers in solar cell devices.