INTERFACE ENGINEERING OF SILICON NANOWIRES FOR SOLAR ENERGY APPLICATIONS

Silicon p-i-n superlattices are an innovative new photodiode that combines the enhanced properties of nanomaterials with the mechanisms and function of p-n junction photovoltaics. These materials, referred to as multijunction silicon nanowires (MJ SiNWs), can absorb light across the solar spectrum, produce tunable photovoltages, and have been demonstrated to perform in both device and suspension architectures. Thus, these materials enable a variety of next generation solar energy and solar fuel products. A key challenge to realizing viable NW-based solar technology is achieving high solar conversion efficiency. In this work, we aim to explore the mechanisms governing solar power conversion efficiency in MJ SiNWs in order to further develop these materials as photovoltaics and photocatalysts.Solar power conversion and solar-to-fuel conversion efficiencies are a product of the individual efficiencies of light absorption, charge carrier generation and separation, and, in the case of fuel generation, redox reactions. Here, we utilize a combination of experimental and computational methods to investigate charge carrier generation and separation efficiency in SiNWs with single and multiple p-i-n junctions. We explore the effects of doping, growth polarity, surface charge, and surface processing on charge carrier recombination and photovoltaic performance. We reveal the magnitude of material defects which makes studying these parameters challenging and find that charge carrier recombination is highly spatially dependent within the p-i-n junction.We explore methods to manipulate the internal and external composition of the NW via doping profiles, surface passivation, and VLS catalyst. Single NW photovoltaic devices, ultrafast pump-probe measurements, and electron microscopy reveals the origin and impact of defects resulting from the Au VLS catalyst. Through simulation, we construct gradient doping profiles that engineer the electric field to improve internal quantum efficiency and photovoltaic performance of a p-i-n junction. Extensive single NW analysis provides insight into quantitative and qualitative impacts of defect states on interfacial process development and charge carrier efficiency. This work develops a deeper understanding and appreciation of the intricacies of bottom-up synthesis of high-quality NW photodiodes. The results presented herein enable the design and fabrication of synthetically tunable, efficient p-i-n SiNW superlattices as photovoltaic and photocatalytic materials.