Diodes for Optical Rectennas

Two types of ultra-fast diode are fabricated, characterized, and simulated for use in optical rectennas. A rectenna consists of an antenna connected to a diode in which the electromagnetic radiation received by the antenna is rectified in the diode. I have investigated metal/insulator/metal (MIM) tunnel diodes and a new, geometric diode for use in rectenna-based infrared detectors and solar cells. Factors influencing the performance of a rectenna are analyzed. These include DC and optical-frequency diode-characteristics, circuit parameters, signal amplitude, and coherence of incoming radiation. To understand and increase the rectification response of MIM-based rectennas, I carry out an in-depth, simulation-based analysis of MIM diodes and design improved multi-insulator tunnel barriers. MIM diodes are fundamentally fast. However, from a small-signal circuit model the operating frequency of a rectenna is found to be limited by the diode\u27s RC time constant. To overcome this limitation, I have designed and simulated a distributed rectifier that uses the MIM diode in a traveling-wave configuration. High-frequency characteristics of MIM diodes are obtained from a semiclassical theory for photon-assisted tunneling. Using this theory, the dependence of rectenna efficiency on diode characteristics and signal amplitude is evaluated along with the maximum achievable efficiency. A correspondence is established between the first-order semiclassical theory and the small-signal circuit model. The RC time constant of MIM diodes is too large for efficient operation at near-infrared-to-visible frequencies. To this end, a new, planar rectifier that consists of an asymmetrically-patterned thin-film, is developed. The diode behavior in this device is attributed to the geometric asymmetry of the conductor. Geometric diodes are fabricated using graphene and measured for response to infrared illumination. To model the I(V) curve of geometric diodes, I have implemented a quantum mechanical simulation based on the tight-binding Hamiltonian. The simulated and the measured current-voltage characteristics are consistent with each other. I have also derived a semiclassical theory, analogous to the one for MIM diodes, for analyzing the optical response of geometric diodes

Optimization of the Antireflection Coating of Thin Epitaxial Crystalline Silicon Solar Cells

AbstractIn this work we use an effective weighting function to include the internal quantum efficiency (IQE) and the effective thickness, Te, of the active cell layer in the optical modeling of the antireflection coating (ARC) of very thin crystalline silicon solar cells. The spectrum transmitted through the ARC is hence optimized for efficient use in the given cell structure and the solar cell performance can be improved. For a 2-μm thick crystalline silicon heterojunction solar cell the optimal thickness of the Indium Tin Oxide (ITO) ARC is reduced by ∼8nm when IQE data and effective thickness are taken into account compared to the standard ARC optimization, using the AM1.5 spectrum only. The reduced ARC thickness will shift the reflectance minima towards shorter wavelengths and hence better match the absorption of very thin cells, where the short wavelength range of the spectrum is relatively more important than the long, weakly absorbed wavelengths. For this cell, we find that the optimal thickness of the ITO starts at 63nm for very thin (1μm) active Si layer and then increase with increasing Te until it saturates at 71nm for Te > 30μm