Silicon Microwire Arrays for Photoelectrochemical and Photovoltaic Applications

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Si microwire (Si MW) arrays grown by the vapor-liquid-solid (VLS) process are promising materials for next-generation solar energy devices. High-aspect-ratio semiconductor structures have attracted recent interest as solar absorber materials because their radial geometry decouples the direction of light absorption and carrier collection, enabling the use of materials with shorter minority-carrier diffusion lengths than would be acceptable in a planar geometry. The VLS growth process is a low-cost deposition technique, which can be used to fabricate flexible, high-performance semiconductor materials. Si MW arrays have been investigated as an inexpensive alternative to wafer-based solar photovoltaics for low- cost electricity generation. Another potential application is to use these vertically oriented wire arrays as photocathodes of a solar fuel conversion devices, where instead of producing electricity, sunlight is used to directly drive a fuel-forming reaction (e.g., splitting water to form O2 and H2). The high aspect ratio of the Si MW arrays provides a large surface area for the integration of fuel-forming catalysts, allowing for the development of a low-cost, scalable, energy storage technology. This thesis discusses the fabrication and photoelectrochemical characterization of Si MWs grown by the VLS process, focusing on the use of these wire arrays as hydrogen- evolving photocathodes for solar water-splitting. To optimize such a device it is important to balance all of the factors that will affect performance: light absorption, band energetics, attainable open circuit voltage, and catalysis. First, we characterize the electrical performance of the wire arrays using regenerative photoelectrochemistry to understand the material quality and band energetics at the Si/water interface. We demonstrate the fabrication of H2-evolving photocathodes using p-n junction Si MW arrays and earth-abundant Ni–Mo alloy hydrogen evolution catalysts. We then investigate modifying the fabrication techniques for wire growth to lower the cost and processing time required to grow arrays of Si MWs. Finally, we study the band energetics of Si decorated with Pt to build a preliminary model of the heterogeneous catalyst/semiconductor/liquid interface.