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theses

The conversion of solar energy into chemical energy using renewable feedstocks such as water continues to expand at an accelerating rate. One device capable of producing hydrogen, a renewable chemical fuel, is the photoelectrochemical (PEC) cell which “splits water” by breaking it into its constituent elements at two distinct electrodes connected by an ion conducting medium. However, achieving both high efficiency and device stability at low-cost remain major challenges to the practical use of PEC devices at any scale. The operation of PEC devices can be better understood by examination of the three key components: (i) the catalysts upon which the water is split, (ii) the photoabsorbing semiconductors which capture energy from light, and (iii) passivation layers between catalyst and photoabsorber. We examined each component using lowcost earth-abundant materials and found that a key aspect of creating long-term stable and energy-efficient PEC devices is a full understanding of how best to integrate crystalline catalysts and photoabsorbers, using an appropriate interfacial passivation layer. The two key roles of the passivation layer are first to ensure that the catalyst and photoabsorber layers do not chemical modify each other, and second to ensure that the chemically unstable photoabsorber is protected by the corrosive solution.

In chapter 2, thin-films of nickel phosphide compounds (NixPy) are proposed and developed as low-cost catalysts for the hydrogen evolution reaction (HER). The optimal structure studied in this work involved a tri-layer photocathode structure consisting of cubic-NiP2 catalyst, a TiN ultrathin-film passivation layer, and a Si photoabsorber. We found that an ultrathin interfacial layer of TiN can effectively hinder atomic diffusion during high temperature fabrication, while also protecting the underlying photoabsorber against corrosion during illumination in electrolytes. The thin-film deposition process developed resulted in a cubic-NiP2 film to be grown normal to the planar direction; this film results in hydrogen production with a comparable turnover frequency to that of other state-of-the-art transition metal phosphides (TMPs) for the HER (1.04 H2 s-1). This interfacial fabrication method maintains a stable photocurrent density without loss for at least 125 hours, the duration of the test.

In Chapter 3, we extend this fabrication technology, demonstrating crystalline nickel phosphide catalysts on high efficiency III-V semiconductors in place of Si as photoabsorber, increasing the overall device efficiency. Application of an ultra-thin TiN layer on a highperformance buried junction photocathode comprised of n+p-GaInP2 allows processing at elevated temperatures that is needed for growth of crystalline Ni5P4. The resulting Ni5P4 catalyst on a TiN protection layer results in nano-islands of Ni5P4 that produce negligible absorption loss in the visible spectrum. The light saturated photocurrent density is equivalent to the same junction using a PtRu decorated benchmark photocathode. We demonstrated that the resulting catalyst-protection layer-GaInP2 photocathode structure is stable for over 120 h, the limit of testing, exceeding all previous benchmarks.

Finally, in chapter 4, we investigate methods to create thin films of SrNbO2N as a lowcost wide-bandgap photoabsorber in a tandem photoabsorber structure with crystalline silicon. By introducing a thin film of TaN as diffusion layer on crystalline silicon, we could synthesize SrNbO2N by high temperature ammonolysis of the oxide precursor film without atomic diffusion across the interface. We experimentally determined the electronic band edge positions of thinfilm SrNbO2N and related these to the understanding of efficient charge injection. Furthermore, we determined how oxynitride thickness, crystallite size, and electrochemical surface roughness contribute to optimal performance of the photoanode across a wide range of variables.

ETD_10409

Ph.D.

Includes bibliographical references

doi:10.7282/t3-q3mz-0g72

ETD doctoral

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