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)(2)bPY)(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 (tau = 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).