Interfacial electron transfer yields in dye-sensitized NiO photocathodes correlated to excited-state dipole orientation of ruthenium chromophores

Interface dynamics of nanocrystalline NiO thin films sensitized with two ruthenium polypyridyl chromophores have been investigated to examine the influence that excited-state dipole orientation and the position of the bipyridine radical formed in the charge-separated state have on interfacial electron transfer yields. In ultrafast transient absorption experiments, the charge separated state is observed on the nanosecond timescale for the trifluoromethyl-substituted chromophore, [Ru(flpy)2(dcb)]2+ (flpy = 4,4′-bis(trifluoromethyl)-2,2′-bipyridine, dcb = 4,4′-dicarboxy-2,2′-bipyridine), but not for [Ru(bpy)2(dcb)]2+ (bpy = 2,2′-bipyridine). Differences are attributed to the positioning of the bipyridine radical formed in the charge separated state; for [Ru(flpy)2(dcb)]2+, the electron is localized on the flpy ligand distal to the surface, whereas for [Ru(bpy)2(dcb)]2+, the electron is localized on the dcb ligand, proximal to the NiO surface. Enhanced photovoltaic performance is observed for dye-sensitized solar cell devices prepared with [Ru(flpy)2(dcb)]2+, demonstrating that enhanced charge separation can be correlated with device efficiency.