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)(PF 6 ) 2 (abbreviated Rux, where x = 0, 1 or 2 xylyl groups; bpy = 2,2‘-bipyridine) dyes can act as sensitizers for nanocrystalline TiO 2 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 (10 6 −10 8 s -1 ) was slightly faster for the x = 0 dye, but both recombination (10 -15 −10 -13 cm 3 s -1 ) and regeneration (10 4 −10 6 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−TiO 2 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 TiO 2 and the I 3 - /I - electrolyte solution is as favorable for the Rux sensitizers as for unmodified TiO 2 electrodes.