Electron transfer kinetics in water splitting dye-sensitized solar cells based on core-shell oxide electrodes

Photoelectrochemical water splitting occurs in a dye-sensitized solar cell when a [Ru(bpy) ₃] ²⁺-based dye covalently links a porous TiO ₂ anode film to IrO ₂·nH ₂O nanoparticles. The quantum yield for oxygen evolution is low because of rapid back electron transfer between TiO ₂ and the oxidized dye, which occurs on a timescale of hundreds of microseconds, When iodide is added as an electron donor, the photocurrent increases, confirming that the initial charge injection efficiency is high. When the porous TiO ₂ film is coated with a 1-2 nm thick layer of ZrO ₂ or Nb ₂O ₅, both the charge injection rate and back electron transfer rate decrease. The efficiency of the cell increases and then decreases with increasing film thickness, consistent with the trends in charge injection and recombination rates. The current efficiency for oxygen evolution, measured electrochemically in a generator-collector geometry, is close to 100%. The factors that lead to polarization of the photoanode and possible ways to re-design the system for higher efficiency are discussed.