Metal-oxide-semiconductor photoelectrodes for solar water splitting
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The rapidly increasing global demand for energy, combined with the environmental impact of fossil fuels, has spurred the search for alternative sources of clean energy. One promising approach is to convert solar energy into hydrogen fuel using photoelectrochemical cells. However, the semiconducting photoelectrodes used in these cells typically have low efficiencies and/or stabilities. This dissertation will describe engineering of metal-oxide-semiconductor (MIS) photoelectrodes for application in solar water splitting. First, we show that a silicon-based photocathode with an epitaxial oxide capping layer can provide efficient and stable hydrogen production from water. In particular, we grow a thin epitaxial layer of strontium titanate (SrTiO3) directly on Si (001) by molecular beam epitaxy. Photogenerated electrons can be easily transported through this layer because of the conduction band alignment and lattice match between single crystalline SrTiO3 and silicon. The approach is used to create a metal-insulator-semiconductor photocathode that under broad-spectrum illumination at 100 mW/cm2 exhibits a maximum photocurrent density of 35 mA cm2 and an open circuit potential of 450 mV; there was no observable decrease in performance after 10 hours of operation in 0.5 M H2SO4. Then, we propose and demonstrate a general method to decouple the two roles of the insulator by employing localized dielectric breakdown. This approach allows the insulator to be thick, which enhances stability, while enabling low-resistance carrier transport as required for efficiency. This method can be applied to various-oxides, such as SiO2 and Al2O3. In addition, it is suitable for silicon, III-V, and other optical absorbers for both photocathodes and photoanodes. Finally, the thick metal-oxide layer can serve as a thin-film antireflection coating, which increases light absorption efficiency.