Effects of bridging ligands on the current - Potential behavior and interfacial kinetics of ruthenium-sensitized nanocrystalline TiO₂ photoelectrodes

We have shown that Ru^II(bpy)₂(bpy-4-(xylyl)_x-≡-phenyl-COOH)( PF₆)₂ (abbreviated Rux, where x = 0, 1 or 2 xylyl groups; bpy = 2,2′-bipyridine) dyes can act as sensitizers for nanocrystalline TiO₂ in functional photoelectrochemical cells under simulated solar illumination, albeit with low efficiencies. Both the short-circuit photocurrent density and the open-circuit voltage decreased as x was increased. Electron injection (10⁶-10⁸ s^-1) was slightly faster for the x = 0 dye, but both recombination (10^-1--10^-13 cm³ s^-1) and regeneration (10⁴-10⁶ s^-1 for 10 mM I^-) were slightly faster for the x = 2 dye. We suggest that the lack of distance dependence is due to the flexible one-carboxyl attachment to the surface resulting in the Ru-TiO₂ electron-tunneling distance being very similar for x = 0, 1, and 2. For all of the Rux sensitizers, a relatively small potential was needed for generation of current in the dark, indicating that the reaction between electrons in TiO₂ and the I₃^-/I^- electrolyte solution is as favorable for the Rux sensitizers as for unmodified TiO₂ electrodes.