Photoelectrochemical water splitting is an attractive route for sustainable generation of renewable, nonpolluting hydrogen fuel. The largest challenge towards production of efficient photoelectrochemical cells is in the design of a suitable photoelectrode, which must absorb visible light, be comprised of abundant materials, and be stable under operation in aqueous solution. Hematite (α-Fe2O3), a leading candidate for use as a photoanode material, meets these requirements. However, despite much progress, the efficiency of state-of-the-art hematite photoanodes remains significantly lower than the theoretical maximum. In this talk, I will present our work towards understanding these efficiency losses, placing particular emphasis on those associated with charge carrier generation and transport, which arise in part due to electron-electron and electron-lattice correlation effects that introduce deviation from conventional semiconductor behavior. Combining the use of model heteroepitaxial thin film photoanodes with optical and photoelectrochemical characterization, we demonstrate that the wavelength dependent photocurrent cannot be explained by traditional semiconductor models. Instead, we find that visible light absorption in hematite is composed of both delocalized excitations, which generate mobile charge carriers, and localized excitations, which do not. We combine these observations with ultrafast time-resolved terahertz and microwave conductivity measurements to study the wavelength dependent photogeneration and charge carrier dynamics. We then develop a new method based on quantum efficiency measurements, optical characterization, and spatial collection efficiency analysis to estimate the mobile carrier photogeneration yield in any thin film photoelectrode material. From these observations, we expand our knowledge on solar cell materials into correlated electron systems and provide a toolbox to study photogenerated charge carrier transport in new photoactive materials and devices for solar energy conversion and storage.

“Rust” challenge: Solar water splitting with hematite photoanodes

2019 Spring National Meeting

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