Molecular Orbital Engineering of a Panchromatic Cyclometalated Ru(II) Dye for p-Type Dye-Sensitized Solar Cells

We report a panchromatic cyclometalated Ru(II) complex, denoted as O18, which shows intense metal-to-ligand charge transfer transitions in the visible to near-IR region with the absorption tail extending to 800 nm. The panchromatic spectra response and enhanced molar extinction coefficient (epsilon = 1.9 x 10(4) M-1 cm(-1) at 593 nm in solution) of O18 are attributed to the stabilization of the lowest unoccupied molecular orbital and the increased absorption cross section via rationally extending the pi-conjugated system of 2,2'-bipyridyl (bpy) ligands. As a result, NiO solar cells sensitized with O18 show short-circuit currents up to 3.43 mA cm(-2) and efficiencies up to 0.104%, which are the best among all cyclometalated-Ru(II)-sensitized p-type dye-sensitized solar cells. Femtosecond transient absorption spectroscopy reveals decreased geminate recombination across the O18-NiO interface by 1 order of magnitude compared to our previous report.