Time Domain Simulation of Novel Photovoltaic Materials

Thin-film silicon-based solar cells have operated far from the Shockley- Queisser limit in all experiments to date. Novel light-trapping structures, however, may help address this limitation. Finite-difference time domain simulation methods offer the potential to accurately determine the light-trapping potential of arbitrary dielectric structures, but suffer from materials modeling problems. In this thesis, existing dispersion models for novel photovoltaic materials will be reviewed, and a novel dispersion model, known as the quadratic complex rational function (QCRF), will be proposed. It has the advantage of accurately fitting experimental semiconductor dielectric values over a wide bandwidth in a numerically stable fashion. Applying the proposed dispersion model, a statistically correlated surface texturing method will be suggested, and light absorption rates of it will be explained. In future work, these designs will be combined with other structures and optimized to help guide future experiments

Simulation Design for Photovoltaics Using Finite Difference Time Domain and Quadratic Complex Rational Function Methods

Photovoltaics (PV) can in principle supply enough renewable energy to offset a great deal of fossil fuel usage. To achieve this transition, it is critical to develop improved PV cells with decreased material costs and improved efficiencies. This goal can be greatly facilitated by a tool simulating the absorption and efficiency of experimentally relevant 3-D PV designs made of realistic materials, including those that have not yet been discovered. By incorporating the quadratic complex rational function algorithm (QCRF) with the finite difference time domain methods (FDTD), simulations can include frequency response and optical properties, while allowing full customization of tandem or single junction photovoltaic cell designs. FDTD models how electro-magnetic waves travel through materials and vacuum, while QCRF allows for more realistic material dispersion. This tool allows users to easily incorporate commonly-used photovoltaic materials. By incorporating the QCRF-FDTD method, the simulation provides increased material & design customization with a shorter runtime than most current tools. nanoHUB.org—an open-access science gateway for cloud-based simulation tools and resources in nanoscale science and technology. This tool will predict PV absorption spectra, external quantum efficiencies, short-circuit currents, and power conversion efficiencies, to help guide future experiments toward higher efficiencies and lower costs

Designing an Experimentally Feasible Selective Emitter For a Thermophotovoltaic System

More than 60% of the raw energy used in the US is dissipated as waste heat. Thermophotovoltaics (TPV) provide a means to capture this waste heat into electricity. Inefficiencies in TPV systems are due to various loss mechanisms, particularly a lack of spectral matching between the emission spectrum of the emitter and the absorption spectrum of the photovoltaic cell. This study aims to design a simple structure emitting thermal photons mostly at high energies, which could allow for efficient generation of electricity through a photovoltaic cell. Optical data for the different materials obtained using ellipsometry and previous research is incorporated into a nanoHUB tool, known as the Thermophotonic Selective Emitter Simulator (TPXsim), to compute the expected enhancement of the TPV system efficiency. Changes have been made to the TPXsim tool to incorporate customized top dielectric mirror layers, samarium doped glass cavity and bottom metallic back reflectors. It is seen that a TPV system consisting of a rare-earth wafer emitter at 1573 K plus a cold-side rugate filter at 300 K shows an overall efficiency of around 18%. Previous research on emitter designs with top and bottom layers of dielectric mirror is seen to increase this efficiency at a large number of layers while degrading the performance for a small number of layers. Our research shows that using aperiodic customized multilayer structures and metallic back reflectors improves the efficiency over a bare wafer while maintaining the ease of fabrication of the selective emitter

Inverse Design of High-NA Metalens for Maskless Lithography

We demonstrate an axisymmetric inverse-designed metalens to improve the performance of zone-plate-array lithography (ZPAL), one of the maskless lithography approaches, that offer a new paradigm for nanoscale research and industry. First, we derive a computational upper bound for a unit-cell-based axisymmetric metalens. Then, we demonstrate a fabrication-compatible inverse-designed metalens with 85.50\% transmission normalized focusing efficiency at 0.6 numerical aperture at 405nm wavelength; a higher efficiency than a theoretical gradient index lens design (79.98\%). We also demonstrate experimental validation for our axisymmetric inverse-designed metalens via electron beam lithography. Metalens-based maskless lithography may open a new way of achieving low-cost, large-area nanofabrication

Inverse Designed WS2 Planar Chiral Metasurface with Geometric Phase

Increasing attention is being paid to chiral metasurfaces due to their ability to selectively manipulate right-hand circularly polarized light or left-hand circularly polarized light. The thin nature of metasurfaces, however, poses a challenge in creating a device with effective phase modulation. Plasmonic chiral metasurfaces have attempted to address this issue by increasing light-matter interaction, but they suffer from metallic loss. Dielectric metasurfaces made from high index materials enable phase modulation while being thin. Very few materials, however, have high refractive index and low loss at visible wavelengths. Recently, some 2D materials have been shown to exhibit high refractive index and low loss in the visible wavelengths, positioning them as promising platform for meta-optics. This study introduces and details a planar chiral metasurface with geometric phase composed of WS2 meta-units. By employing adjoint optimization techniques, we achieved broadband circular dichroism.Comment: 9 pages, 5 figure

Computational Design for Next Generation Solar Cells

Although photovoltaic technology has improved tremendously over the past several decades, there is still signicant scope for improvements which can be systematically investigated through advanced simulation techniques, particularly in the electromagnetic domain. However, accurately simulating the detailed performance of emerging light trapping and current-harvesting harvesting structures still requires a tremendous amount of detailed calculations. For these reasons, without any simplications, 3-D electromagnetic computation of a single photovoltaic unit cell easily exceeds the computing limit of a single core machine or even that of a computing cluster. Thus, building a more efficient and accurate simulation framework for solar cells can provide a deep understanding of solar cell physics, generate new conceptual designs, and enable breakthrough next generation solar cells