Nanostructure designs for solar energy harvesting devices

Abstract

The optical devices containing nanophotonics have received a lot of attention because they bring a huge improvement of efficiencies through the introduction of nanostructures without major modifications of existing systems. Solar energy reaches the earth's surface in the form of heat and light which can be used as a renewable energy source using various optical harvesting devices such as photovoltaic cell, photocatalytic water splitting devices, etc. Through these optical devcies, solar energy can be converted and stored into various renewable energy such as electricity or hydrogen energy. To maximize solar energy conversion efficiency, it is necessary to design optical structures tailored to their applications. In this dissertation, the nano-photonic structure for improving the efficiency of the optical devices which were analyzed by the rigorous coupled wave analysis (RCWA) and finite element time domains (FDTD) for 1) photovoltaic solar cells, 2) water splitting devices, and 3) transparent plastics. The nanostructures were fabricated in consideration of the cost of fabrication, scalable production and ease of fabrication processing, and the nanostructures were characterized by optical and electrical analyses. In the first part of dissertation, photocatalytic water splitting was reported to produce the hydrogen fuel by utilizing sun light. Despite the importance of gallium nitride (GaN) nanostructures for photocatalytic activity, relatively little attention has been paid to their geometrical optimization on the basis of wave optics. Here, we present GaN truncated nanocones to provide a strategy for improving solar water splitting efficiencies, compared to the efficiency provided by the conventional geometries (i. e. flat surface, cylindrical, and cone shapes). Through the formation of gallium nitride nanostructures, the light trapping characteristics on surface compared to the flat GaN were investigated with reduced reflectance. Computational results with a FDTD method and a RCWA reveal important aspects of truncated nanocones, which effectively concentrate light in the center of the nanostructures. The introduction of nanostructures is highly recommended to address the strong light reflection of photocatalytic materials and carrier lifetime issues. To fabricate the truncated nanocones at low cost and with large-area, a dry etching method was employed with thermally dewetted metal nanoparticles, which enables controllability of desired features on a wafer scale. Experimental results exhibit that the photocurrent density of truncated nanocones is improved about three times higher compared to that of planar GaN. In the second part, we report a diffraction grating encapsulant polymer for photovoltaic application. In streamlined multipurpose applications for light management and protection, encapsulants are merged with photonic crystal structures into solar modules. We present an edge-located 1D grating, attachable polymer on the top of a photovoltaic module to provide a strategy for capturing solar light and improving cell efficiency. Large-area solar arrays suffer from space utilization problems due to non-active area. The introduction of periodically patterned gratings with specific geometric range is highly preferred to redirect the light toward photovoltaic active areas. To realize optimized broadband light diffraction for solar devices, the theoretical analysis of one dimensional line patterned diffraction gratings was performed through wave-optic-based simulation. Based on the experimental results, the replica molding-based patterning method was adopted to fabricate the grating polymer for low-cost thin-film production. Also, we demonstrated enhanced light collection by grating patterned encapsulants with improved current density in comparison to the performance of a flat surface. In the last part, we report on the moth-eye, a typical antireflective structure used to improve the efficiency of optical devices. Bioinspired moth-eye surface provides broadband antireflection features, which significantly enhance performances in optical components/devices. However, their practical uses are strictly limited due to poor mechanical stability of nano-patterns. In this study, we artificially engineered moth-eye structures on polycarbonate substrate through a thin-film coating of mechanically stable dielectric materials (i.e., Al2O3, Cr2O3, ZrO2, and TiO2 etc). The geometry of Al2O3-coated moth-eye surface is designed by considering the effective medium theory and confirmed by calculation of diffraction efficiency based on a rigorous coupled-wave analysis method. The tailored Al2O3-coating on moth eye surface exhibit the improved hardness while maintaining high optical transmittance.