Fast Transporting ZnO–TiO₂ Coaxial Photoanodes for Dye-Sensitized Solar Cells Based on ALD-Modified SiO₂ Aerogel Frameworks

A doubly coaxial photoanode architecture based on templated SiO₂ aerogels was fabricated on transparent conducting oxides for use in dye-sensitized solar cells (DSSCs). These templates were coated with ZnO <i>via</i> atomic layer deposition (ALD) to yield an electronically interconnected, low-density, high-surface-area, semiconductor framework. Addition of a thin conformal layer of a second metal oxide (alumina, zirconia, or titania) <i>via</i> ALD was found to suppress the dissolution of ZnO that otherwise occurs when it is soaked in alcohol solutions containing acidic dyes used for sensitization or in acetonitrile solutions containing a pyridine derivative and the iodide/tri-iodide (I^–/I^–₃) redox shuttle. Electron transport in SiO₂–ZnO–TiO₂ photoelectrodes was found to be nearly 2 orders of magnitude faster than in SiO₂–TiO₂ structures, implying that the interior ZnO sheath serves as the primary electron conduit. In contrast, rates of electron interception by the oxidized form of the redox shuttle were observed to decrease when a TiO₂ shell was added to SiO₂–ZnO, with the decreases becoming more significant as the thickness of the titania shell increases. These effects lead to improvements in efficiency for DSSCs that utilize I^–/I^–₃, but much larger improvements for DSSCs utilizing ferrocene/ferrocenium, a notoriously fast redox shuttle. For the former, overall energy conversion efficiencies maximize at 4.0%. From a variety of experiments, the primary factor limiting aerogel-based DSSC performance is light loss due to scattering. Nevertheless, variants of the doubly coaxial structure may prove useful in devising DSSCs that can achieve excellent energy conversion efficiencies even with fast redox shuttles