Influence of Dye-Coordinated Metal Ions on Electron Transfer Dynamics at Dye–Semiconductor Interfaces

Maximizing regeneration and minimizing recombination rates at dye–semiconductor interfaces is crucial for the realization of efficient dye-sensitized solar and photoelectrosynthesis cells. Previously it has been shown that simply coordinating the metal ion to the nonsurface bound carboxylate groups of a dye molecule can slow recombination rates and increase open-circuit voltages. However, it was unclear if the additional steric effects or charge of the metal ion were the cause of this behavior. Here we use three different redox mediators, (1) I – /I 3 –, (2) [tris(1,10-phenanthroline)cobalt] 3+/2+, and (3) [Co(4,4′,4″-tri tert -butyl-2,2′:6′,2″-terpyridine)(NCS) 3 ] 0/1– to elucidate the role, if any, of electrostatic interactions between the coordinated metal ion and mediator in dictating these interfacial electron transfer events. Using a combination of spectroscopy, electrochemistry, and solar cell measurements, we demonstrate that while electrostatic interactions may influence dye regeneration rates, for example, increased steric bulk of the metal ion between TiO 2 (e – ) and the oxidized mediator likely has a stronger influence on the overall device performance. Additionally, electrochemical impedance spectroscopy and intensity dependent measurements suggest that the coordination of the metal ion can slow diffusion of the mediators within the mesoporous oxide which could have implications for the use of multilayer assemblies in dye-sensitized devices.