Tailoring the Dielectric Contrast of Hierarchically Structured Photoelectrodes via Sol-Gel Synthesis of Ternary Metal Oxides

A common problem in photoelectrochemistry is that attractive thin film semiconductors for use as light absorbing photoelectrodes often have long characteristic absorption length and short minority carrier diffusion length. This is a particularly common combination of properties for chemically synthesized, nanoparticulate films formed via chemical synthesis. Strategies for increasing the incident photon-to-electron conversion efficiency (IPCE) in photoelectrodes often involve the use of multiscale porosity. The optical advantage of hierarchical nanostructure in these electrodes is that it induces light scattering to increase the effective path length in thin film light absorbers. More complex schemes induce modes of resonant light trapping (via photonic crystals, for example) to confine and concentrate incident light to small volumes of the electrode. The transition from a weak scattering structure to a strong one depends on the refractive index contrast between the electrode material and the surrounding electrolyte. The optical properties of these structures are predictable by electromagnetic simulations such as finite element methods. However, the continuously definable parameters in simulations do not directly translate to experimental systems due to the limitations of physically realizable materials. Here, we describe the experimental synthesis of ternary metal oxide composites (Ta-Ti-O, for example) via sol-gel synthesis as an experimental approach to tailor the refractive index contrast in hierarchically structured photonic glass electrodes. The difference in refractive index between TiO₂ (n = 2.5) and Ta₂O₅ (n = 2.1) or Ga₂O₃ (n = 1.9) enables nearly continuous control over the <i>effective</i> refractive index in that range. Sol gel infiltration in colloid-templated structure allows this control to be integrated into a three-dimensional electrode structure. We will show how simulation and experimental results can elucidate the connection between the synthesis and optical properties of thin film and three-dimensional structures. These materials can then serve as hierarchically structured transparent conductive oxide current collectors for photoelectrochemical and optoelectronic applications.