Localizing the initial excitation – A case study on NiO photocathodes using Ruthenium dipyridophenazine complexes as sensitizers

We report on the localization of the initially excited electronic state within the molecular framework of a series of [Ru(bpy)₂dppz]²⁺ derivatives (bpy:2,2′-bipyridine, dppz: dipyrido-phenazine) as sensitizers in NiO based photocathodes. The introduction of conjugated linkers with phenylene and triazole moieties in the bpy ligand sphere separates the NiO surface from the metal center and hence is considered to stabilize the charge separated state, which results from light-driven hole injection. However, introduction of the conjugated linkers also alters the localization of the excess electron density in the excited state within the ligand sphere and impacts the extent to which the charge-separated state is formed. The study emphasizes that tuning the ligand with the lowest-energy π* orbital distal or proximal to the NiO surface significantly affects the initial charge-separation and the solar cell performance. The stability of the charge-separated state correlates with the observed photocurrents in dye-sensitized solar cells. Furthermore, the study challenges the widely accepted concept that the introduction of extended anchoring groups, i.e. increasing Ru – NiO distance, stabilizes the charge-separated state and suppresses charge recombination at the metal-oxide molecule interface.