High-Efficiency III-V Semiconductor Device and System Optimization for Photovoltaic Applications

Abstract

This thesis addresses barriers to adopting high-efficiency photovoltaic devices through innovative structured surfaces, device designs, and epitaxial growth. I demonstrate improvements using these techniques, through a combination of experiments and simulations. First, structured surfaces are investigated to enhance the performance of photovoltaic systems under concentrated sunlight. Second, novel photovoltaic designs are explored to improve the conversion of infrared or monochromatic light into electrical power. Third, the properties of III-V epitaxy on nonplanar surfaces are studied for substrate reuse applications. Integrating structured surfaces into photovoltaic systems enhances their performance. High-efficiency photovoltaic devices, based on the epitaxial growth of III-V semiconductor materials, are expensive due to the scarcity of these elements. They are integrated into concentrator photovoltaic systems, which focus sunlight onto the photovoltaic cell, making terrestrial III-V solar cells cost-competitive with silicon-based solar cells. These cells are coated with an encapsulant to shield them from environmental damage. This thesis explores a microstructured encapsulant, comparing it with a standard encapsulant for these systems, and finds up to a 3.4\% improvement in optical performance. Additionally, this thesis shows that the new encapsulant could extend the concentrator system's temperature range operating at high efficiencies. Optimizing devices at the component level enhances photovoltaic performance for infrared to electrical power conversion. Waste heat is an abundant and largely untapped energy source that can be harnessed using high-efficiency photovoltaic devices. Existing photovoltaic devices achieve efficiencies of less than 1.5\% for waste heat applications. This thesis presents a III-V photovoltaic device design optimized for waste heat recovery, predicted to achieve up to 15\% efficiency in converting waste heat at 900\,K into electrical power. A modified version of this optimized device is then fabricated and characterized. Device-level optimization augments photovoltaic performance for monochromatic to electrical power conversion. Power by light offers an attractive alternative to transmitting power over metallic wires, especially when wires are unfavorable or impractical. These systems generally consist of lasers (at the light source) and photonic power converters. Most commercially available photonic power converters are based on GaAs and require optical wavelengths with short-range transmission through optical fibers. This thesis presents the design and characterization of photonic power converters for optical wavelengths within the transmission window of optical fibers, enabling much longer power transmission capabilities. These optimized devices breach the 50\% efficiency barrier, achieving efficiencies of up to 53.6\%. Epitaxial growth provides an effective method to planarize faceted surfaces, facilitating the substrate reuse process. Substrate reuse promises significant cost reductions for III-V photovoltaics. A promising method is to fracture the epitaxial layers from the bulk substrate crystal, known as spalling. Spalling GaAs(100) substrates, commonly used for III-V photovoltaics, generates a rough surface of faceted ridges that need to be planarized for reuse. This thesis explores planarizing this surface using metal-organic vapor phase epitaxy. The optimized process planarizes the surface within 8 minutes of growth, using up to 95\% of the nominally deposited material, which is much faster than the hours required to grow high-efficiency devices. Overall, this thesis aims to increase efficiencies and reduce the costs of III-V photovoltaics.