Electrical characterisation of emerging photo anodes suited to water dissociation with an applied bias

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Walsh, Adrian
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University College Cork
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One of the pivotal challenges of the 21st century is to develop alternative energy sources to replace the inevitable depletion of fossil fuels. One candidate for a non-polluting, abundant and renewable source of energy is sunlight. The significant challenge for the large-scale uptake of solar energy is overcoming the intermittent nature of solar radiation using energy storage methods. Synthesis of fuels from sunlight is one potential storage approach, providing the need for optimized photo-electrochemical devices and materials. The direct photovoltage of water to produce hydrogen and oxygen represents the most direct route to the synthesis of clean fuel. Recently it has been showing that technology from the microelectronics industry can be used to fabricate metal oxide semiconductors for use as photoanodes. Given that the protective oxide needs to be transparent to allow light to the photoactive material, two possible solutions are available. To have a thicker transparent conducting oxide, such as indium-tin-oxide, which would create a Schottky junction with the silicon below, or to use very thin oxides such as titanium dioxide, which when a catalyst is deposited on top creates a MOS structure. Of interest to the first option are the electrical properties of the Schottky junction in particular the barrier height, as this will dictate the current flow through to the water and drive the electrochemical reaction. For the second option to be successful, the thin oxide must not have any pinholes, i.e. to isolate the photoactive material below from the water. Atomic layer deposition is able to provide this level of conformality. A conduction mechanism study was performed and an investigation into the relationship between the stoichiometry of the titanium dioxide and the leakage current. It was shown that the level of oxygen vacancies played a key role in the level of leakage current, but a dominant conduction mechanism was not proved. Next a study into how the thickness affects the conduction through the films was performed, where results show that an increased thickness results in an increased leakage current, opposite of what would be expected. Some ellipsometry data shows a definite change in the films as they get thicker. The relationship between these electrical properties and the electrochemical properties of interested in a water splitting cell were also studied and shown that it is not always possible to compare the two, especially when, the electrochemical measurement includes a light source. In addition, an investigation into deposition of mixed oxides based on titanium dioxide, was undertaken, using a novel ALD method whereby the titanium precursor was not pulsed in saturation. In this way a number of reaction sites were left unfilled, meaning that when a second metal precursor was pulsed, a single cycle of growth contained two different types of metal atoms, as opposed to the more traditional method of the supercycle detailed in chapter 1. The electrical properties of these films were measured, and a definitive trend showed that, creating mixed oxides in this way can change the electrical properties but that the incorporated amount of the second metal is an important factor. An anneal study was found to vastly change the electrical properties of these mixed oxides samples, where the leakage current fell by a number of orders of magnitude. For a water splitting application, this is not a desirable effect.
Atomic layer deposition , Thin films , MOS characterisation , Ellipsometry , Metal oxide
Walsh, A. 2018. Electrical characterisation of emerging photo anodes suited to water dissociation with an applied bias. PhD Thesis, University College Cork.