Oligomeric Ruthenium Polypyridyl Dye for Improved Stability of Aqueous Photoelectrochemical Cells

Water-splitting dye-sensitized photoelectrochemical cells rely on molecular sensitizers to harvest light energy and drive the catalytic reactions necessary to generate hydrogen and oxygen from water. The desorption of sensitizer molecules from the semiconductor–aqueous electrolyte interface is a significant barrier to the practical implementation of these cells. To address this problem, we synthesized an oligomeric ruthenium dye ([RuP]ₙ) that has dramatically improved stability as a photosensitizer for TiO₂ electrodes over the pH range of interest (4–7.8) for DSPECs. Additionally, the efficiency of photoelectrochemical charge separation is known to depend on the rate of cross-surface hole diffusion between dye molecules. The oligomeric dye ([RuP]ₙ) shows an order of magnitude faster cross-surface hole diffusion than the commonly used monomeric [Ru(bpy)₂(4,4-PO₃H₂)₂bpy]²⁺ (RuP) sensitizer. The enhanced stability of the polymeric dye also enables the use of intensity-modulated photovoltage spectroscopy to measure the recombination rate of photogenerated electrons and holes as a function of electrolyte pH.