Guided-mode Resonant Solar Cells And Flat-top Reflectors: Analysis, Design, Fabrication And Characterization

Abstract

This dissertation addresses the guided-mode resonance (GMR) effect and its applications. In particular, this study presents theoretical analysis and corresponding experiments on two important GMR devices that can be broadly described as GMR-enabled thin-film solar cells and flat-top reflectors.The GMR-induced enhanced absorption of input light is observed and quantified in a fabricated nano-patterned amorphous silicon (a-Si) thin-film. Compared to a reference homogeneous thin-film of a-Si, approximately 50% integrated absorbance enhancement is achieved in the patterned structure. This result motivates the application of these resonance effects in thin-film solar cells where enhanced solar absorbance is a crucial requirement. Light trapping in thin-film solar cells through the GMR effect is theoretically explained and experimentally demonstrated. Nano-patterned solar cells with 300-nm periods in one-dimensional gratings are designed, fabricated, and characterized. Compared to a planar reference solar cell, around 35% integrated absorption enhancement is observed over the 450-750-nm wavelength range. This light-management method results in enhanced short-circuit current density of 14.8 mA/cm2, which is a ~40% improvement over planar solar cells. The experimental demonstration proves the potential of simple and well-designed guided-mode resonant features in thin-film solar cells. In order to complement the research on GMR thin-film solar cells, a single-step, low-cost fabrication method for generating resonant nano-grating patterns on poly-methyl-methacrylate (PMMA; plexiglas) substrates using thermal nano-imprint lithography is reported. The imprinted structures of both one and two dimensional nano-grating patterns with 300 nm period are fabricated. Thin films of indium-tin-oxide and silicon are deposited over patterned substrates and the absorbance of the films is measured. Around 25% and 45% integrated optical absorbance enhancement is observed over the 450-nm to 900-nm wavelength range in one- and two-dimensional patterned samples, respectively. In addition, two types of GMR flat-top reflectors have been designed, analyzed, fabricated and experimentally demonstrated. The first one is GMR broadband reflector in the spectral domain whereas the second is a Rayleigh reflector in the angular domain. The designed broadband reflector exhibits more than 99% reflectance over a spectral width of 380 nm ranging from 1440 to 1820 nm wavelength. Experimental reflectance greater than 90% is achieved over a ~360-nm bandwidth. The reported reflector bandwidth exceeds comparable published results for two-part periodic structures working in transverse electric polarization. In the Rayleigh reflector, the interaction of GMR and Rayleigh anomaly creates an extraordinary photonic response and results in a flat-top angularly delimited optical filter. The physical process of the rapid energy exchange between the reflected zero-order wave and a propagating substrate wave across a small angular change is investigated with numerical computations. An experimental proof of the Rayleigh reflector concept is presented. The combined GMR-Rayleigh anomaly effect holds the potential to portend a new research area of novel photonic devices with interesting and useful attributes.