DREAM · Design Rules for Efficient Photogeneration in Metal Oxides

Photoelectrochemical (PEC) water splitting is an attractive route for green hydrogen production. Despite nearly half a century of research efforts, no material has successfully met the stringent requirements for a photoelectrode material, the light harvesting semiconductor within the PEC cell. Metal-oxides are widely viewed as the most promising photoelectrode materials for their exceptional stability in aqueous electrolytes, but those with suitable band gaps for visible light absorption typically have open d shell configurations, and suffer from low photoconversion efficiencies. I hypothesize that the underperformance of such materials is related to their electronic configuration which reduces the photogeneration yield of mobile charge carriers, an overlooked yet critical loss mechanism in metal-oxides. Thus, unlike conventional semiconductors where all absorbed photons generate electrons and holes, in metal-oxides with open d shell configuration, many of the photons give rise to localized electronic transitions that do not contribute to the photocurrent. In addition, polaronic transport and charge carrier recombination reduce the charge carrier collection efficiency. DREAM will address these challenges and provide a leap forward in understanding the photogeneration processes in metal-oxide photoelectrodes and their effect on photoconversion efficiency. To achieve these goals, we will couple systematic control of crystallographic structure, d orbital occupancy, and local cation environment using heteroepitaxial thin film growth together with wavelength and temperature-resolved characterization of the photogeneration yield spectrum. The knowledge gained by these fundamental investigations will lead to new design rules, which we will employ to engineer new metal-oxides with near unity photogeneration yield, and integrate them into novel device architectures, enabling highly efficient PEC-PV tandem cells for unassisted solar water splitting.