Ultrafast Recombination Dynamics in Dye-Sensitized SnO₂/TiO₂ Core/Shell Films

Interfacial dynamics are investigated in SnO₂/TiO₂ core/shell films derivatized with a Ru(II)-polypyridyl chromophore ([Ru^II(bpy)₂(4,4'-(PO₃H₂)₂bpy)]²⁺, RuP) using transient absorption methods. Electron injection from the chromophore into the TiO₂ shell occurs within a few picoseconds after photoexcitation. Loss of the oxidized dye through recombination occurs across time scales spanning 10 orders of magnitude. The majority (60%) of charge recombination events occur shortly after injection (τ = 220 ps), while a small fraction (≤20%) of the oxidized chromophores persists for milliseconds. The lifetime of long-lived charge-separated states (CSS) depends exponentially on shell thickness, suggesting that the injected electrons reside in the SnO₂ core and must tunnel through the TiO₂ shell to recombine with oxidized dyes. While the core/shell architecture extends the lifetime in a small fraction of the CSS, making water oxidation possible, the subnanosecond recombination process has profound implications for the overall efficiencies of dye-sensitized photoelectrosynthesis cells (DSPECs).