Engendering Long-Term Air and Light Stability of a TiO₂-Supported Porphyrinic Dye via Atomic Layer Deposition

Organic and porphyrin-based chromophores are prevalent in liquid-junction photovoltaic and photocatalytic solar-cell chemistry; however, their long-term air and light instability may limit their practicality in real world technologies. Here, we describe the protection of a zinc porphyrin dye, adsorbed on nanoparticulate TiO₂, from air and light degradation by a protective coating of alumina grown with a previously developed post-treatment atomic layer deposition (ALD) technique. The protective Al₂O₃ ALD layer is deposited using dimethylaluminum isopropoxide as an Al source; in contrast to the ubiquitous ALD precursor trimethylaluminum, dimethylaluminum isopropoxide does not degrade the zinc porphyrin dye, as confirmed by UV-vis measurements. The growth of this protective ALD layer around the dye can be monitored by an in-reactor quartz crystal microbalance (QCM). Furthermore, greater than 80% of porphyrin light absorption is retained over ∼1 month of exposure to air and light when the protective coating is present, whereas almost complete loss of porphyrin absorption is observed in less than 2 days in the absence of the ALD protective layer. Applying the Al₂O₃ post-treatment technique to the TiO₂-adsorbed dye allows the dye to remain in electronic contact with both the semiconductor surface and a surrounding electrolyte solution, the combination of which makes this technique promising for numerous other electrochemical photovoltaic and photocatalytic applications, especially those involving the dye-sensitized evolution of oxygen.