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Abstract
Artificial photosynthesis on semiconductor photoelectrodes is a clean and eco-friendly method for the generation of solar fuels, including hydrogen and hydrocarbons directly from sunlight, water, and carbon dioxide, which can address the challenges associated with energy demands and storage. Simultaneously achieving high efficiency (solar-to-hydrogen efficiency > 15%) and stability (> 1000 h) for unassisted photoelectrochemical water splitting is the “holy grail” in the field of clean, renewable energy. III-nitride semiconductors are promising materials to realize high-efficiency photoelectrodes: their energy bandgap can be varied across nearly the entire solar spectrum by changing the alloy compositions, and the energy band edge positions straddle water oxidation and reduction potentials under deep visible and near-IR light irradiation. In this study, we report the development of high quality (In)GaN nanostructures on Si, using molecular beam epitaxy, for high efficiency and ultrahigh stable photoelectrochemical water splitting photoelectrodes. We have designed InGaN alloy photoanodes having indium content ~ 50%, corresponding to an energy bandgap of ~1.7 eV, for high-efficiency solar water oxidation. This study lays a solid foundation for the development of a tandem device with InGaN as top light absorber stacked on Si bottom absorber to achieve solar-to-hydrogen efficiency > 25%.
Furthermore, we demonstrated the use of multifunctional N-terminated GaN nanowires protection layer on Si photocathode with Pt catalyst, which enhances light absorption and reduces interfacial charge transfer losses, to achieve high half-cell conversion efficiency ~ 12% and the longest stability of 3000 h, for any photoelectrode operating at a similar efficiency level, under AM 1.5G one-sun illumination for solar water splitting. We further showed the first demonstration of low-cost, earth-abundant and few monolayers thick MoSe2 as a protection layer on Si photoanode for hydrogen production under AM 1.5G one-sun illumination. In the end, we have also presented the growth of tunnel junction nanowires to monolithically integrate the p+-InGaN nanowires (top cell) on Si solar cell (bottom cell) to form a double-junction photocathode. This photocathode, with optimized surface modifications, can achieve a high solar-to-hydrogen efficiency of ~ 10.1% and high stability of 100 h for unassisted water splitting under AM 1.5G one-sun illumination. These results are significantly superior compared to other state-of-the-art photoelectrodes for unassisted solar water splitting. The III-nitride nanostructures presented in this work bring us one step closer in achieving high efficiency, long-term stability and low-cost photoelectrochemical water splitting systems required for large-scale applications.