This PDF file contains the front matter associated with SPIE Proceedings Volume 11121, including the title page, copyright information, table of contents, and author and conference committee lists.

Thermophotovoltaics (TPVs) are solid-state devices that may enable scalable electricity generation from a variety of high-temperature heat sources in applications such as grid-scale electricity storage and distributed co-generation of heat and power. These systems consist of a thermal emitter and a photovoltaic (PV) cell in close proximity. Spectrally selective techniques, categorized as either emission control or absorption control, have led to improved performance in TPVs. In particular, suppression of sub-bandgap radiative transfer is essential for improving efficiency. However, the spectral-selectivity of absorption control strategies in conventional cells has been limited by parasitic absorption of sub-bandgap radiation due to a variety of possible mechanisms including absorption in the growth substrate, the thickest layer of the cell. Thin-film TPV cells have the potential to enable selective radiative transfer by reducing the optical path length through the cell and leveraging thin-film interference. Here, we demonstrate high spectral-selectivity in thin-film InGaAs-based TPV cells with back-surface-reflectors and optimized dielectric coatings. Selective absorption using thin semiconductor layers has been investigated for solar absorbers to minimize thermal re-radiation, but has not been demonstrated in the context of TPV cells. The fabricated TPV cells exhibit high absorption of radiation above the semiconductor bandgap and high reflectance below the bandgap, particularly when dielectric layers surrounding the InGaAs are optimized. Furthermore, thin-film devices have the potential for significant economic improvements over conventional TPVs that use expensive growth substrates in the operating device. Fabrication of thin-film group III-V semiconductor cells through non-destructive epitaxial lift-off has enabled wafer reuse. Considering the high cost of III-V wafers, reuse can be expected to significantly reduce the cost of TPV generators.

A thermophotovoltaic (TPV) device design based on multiple quantum well (MQW) materials composed of Pb_0.81Sn_0.19Se wells and Pb_0.80Sr_0.20Se barriers, which are nanostructured materials that can be gown on low-cost silicon, was modeled to predict electrical power generation densities. MQW materials with intersubband energy gaps of 343 meV and 450 meV in a dual junction configuration were studied. For a thermal radiator at a temperature of 1364ºC the short circuit current density was estimated to be 12.1 A/cm² for each junction. Open circuit voltages for each junction ranged from 130 mV to 262 mV depending on bandgap and temperature. Power generation densities for this dual junction device increased from 2.7 W/cm² to 3.4 W/cm² as temperature decreased from 50ºC to 7ºC. Using a conservative value of $1/cm2 for the manufacturing of this silicon-based TPV device technology, the costs for this novel electrical power generation technology are projected to be less than $0.30/W.

We develop a general and self-consistent formalism to describe the thermal radiation from arbitrary optical resonators made by lossy and dispersive materials like metals and graphene-based on quasi-normal modes (QNM). Our formalism derives the fundamental limit of the spectral thermal emission power from an optical resonator and proves that this limit can be achieved when the mode losses to the emitter and the absorber (or far-field background) are matched, and meanwhile, the predominant resonant mode is electrically quasi-static. We also extend our theory to optical resonators with higher order symmetry, where degenerate and spectrally-adjacent modes are taken into consideration. With our formalism serving as a general principle of designing the thermal emitters with maximized emission in both near and far-fields, we propose a metamaterial-based structure consisting of patterned doped silicon nanorod emitters that exhibits tunable narrow-band thermal emission. Direct numerical simulation based on the Wiener chaos expansion (WCE) method is performed to accurately investigate the heat transfer mechanism of metamaterials in the near field. By applying group theory to the geometry of thermal emitter, we identify the existence and the upper limit to the resonance mode degeneracy and its influence on the far-field thermal emission. The existence of the degeneracy proves to be harmful to the far-field thermal. The upper limit of far-field thermal radiation is derived in terms of the coupling strength between degenerate modes. By building up the thermal emitter with higher-order symmetry group, the far-field thermal radiation intensity at certain resonance frequencies turns out to be stronger compared to the single emitter when normalized to the emitting volume. It demonstrates great potential to design the meta-surface with perfect absorption.

Vapor condensation plays a crucial role in solar water-purification technologies. Conventional condensers in solar water-purification systems do not provide sufficient cooling power for vapor condensation, limiting the water production rate to 0.4 L m^-2 hour^-1. On the other hand, radiative dew condensation, a technique used by existing radiative dew condensers, only works at nighttime and is incompatible with solar water-purification technologies. Here, we develop daytime radiative condensers that reflect almost all solar radiation, and can thus create dew water even in direct sunlight. Compared to stateof- art condensers, our daytime radiative condenser doubles the production of purified water over a 24-hour period.

Thermal radiation with a narrow-band emission spectrum is of great significance in various applications such as infrared sensing, thermophotovoltaics, radiation cooling, and thermal circuits. Although resonant nanophotonic structures such as metamaterials and nanocavities have been demonstrated to achieve the narrow-band thermal emission, tuning their radiation power toward perfect emission still remains challenging. Here, based on the recently developed quasi-normal mode theory, we prove that thermal emission from nanoscale transmission line resonators can always be controlled by tuning the size and geometry of single resonator and the density of the resonator array. By use of nanoscale transmission line resonators as basic building blocks, we experimentally demonstrate a new type of macroscopic perfect and tunable thermal emitters. The transmission line resonator arrays are fabricated by standard E-beam lithography techniques and subsequent lift-off process. The emissivity of the samples is measured by using a FTIR spectrometer combined with an infrared microscope. Our experimental demonstration in conjunction with the general theoretical framework lays the foundation for designing tunable narrowband thermal emitters with applications in thermal infrared light sources, thermal management, and infrared sensing and imaging.

Radiative cooling is a uniquely compact and passive cooling mechanism. Significant applications can be found in energy generation, particularly concentrating photovoltaics (CPV) and thermophotovoltaics (TPV). Both rely on low-bandgap PV cells that experience significant reductions in performance and lifetime when operating at elevated temperatures. This issue creates a significant barrier to widespread adoption. To address this challenge, we demonstrate enhanced radiative cooling for low-bandgap PV cells under concentrated sunlight for the first time. A composite material stack is used as the radiative cooler. Enhanced radiative cooling reduces operating temperatures by 10 degrees C, translating into a relative increase of 5.7% in open-circuit voltage and an estimated increase of 40% in lifetime at 13 suns. Using a model, we also estimate the same setup could achieve an improvement of 34% in open-circuit voltage for 35 suns, which could reduce levelized costs of energy up to 33% for high activation energy failure modes. The radiative cooling enhancement demonstrated here is a simple and straightforward approach, which can be generalized to other optoelectronic systems.

The Luminescent Solar Concentrator (LSC) is a photovoltaics concept that can easily be used for integration in buildings. As a window component the transmitted spectrum of the LSC affects the color quality of daylight in the interior of a building. We evaluate eight state of the art nanocrystal luminophores as candidates for LSC windows applications, using ray-trace Monte Carlo simulations. LSCs are assessed with respect to different color quality criteria, using transparency ranging from 90% to 50%. We find that LSC device efficiency is inversely proportional to transparency. Also, luminophores with a wide absorption bandwidth in the visible have higher color temperatures and high color rendering index, leading to good visual comfort at appreciable device efficiency.

In this paper a spectrum-splitting photovoltaic system is proposed that uses bifacial silicon solar cells to maximize total energy yield. The system is unique in its ability to convert direct sunlight with high-efficiency (<30%) while simultaneously converting diffuse and rear-side irradiance. A volume holographic lens array is used to divide the solar spectrum into spectral bands optimized for conversion by wide-bandgap and bifacial silicon solar cells. An approach for simulating the energy yield, optimizing the holographic lens array, and analyzing the effect of concentration ratio, aspect ratio, and illumination characteristics is described. Design examples for two different solar cell combinations are provided. A GaAs and bifacial silicon combination achieves an energy conversion efficiency of 32.0% and a MgCdTe and bifacial silicon combination achieves a 31.0% energy conversion efficiency. Additional solutions are provided when constraints on concentration ratio and aspect ratio are applied, allowing the designer to balance energy yield with cost and size considerations. The performance of the proposed system is compared to conventional monofacial silicon, bifacial silicon, and monofacial spectrum-splitting modules, and show that improvements in energy yield of over 45%, 25%, and 10% can be achieved, respectively.

Increasing the efficiency and cost effectiveness of solar energy generation allowed them to compete with traditional carbon-based energy sources in many energy markets worldwide. However, a major problem facing the proliferation of solar energy generation is energy storage. Photovoltaic (PV) generators enjoy relatively high efficiency but suffer from high electric energy storage costs. On the other hand, solar-thermal energy conversion enables storing heat and dispatch electricity at lower storage costs but with less efficiency compared to photovoltaics. Hybridizing both solar energy conversion can address the energy storage problem. Furthermore, single junction PVs are unable to convert a large portion of the solar spectrum to energy which eventually lead to PV thermalization. Spectral beam splitting is a promising method to achieve high efficiency solar energy conversion while hybridizing electric and thermal solar energy generation. Here, we use novel thin-film based optical coatings to develop single element selective light reflector/absorber that reflects within the wavelength range corresponding to a PV cell absorption band, while absorbing the remaining solar spectrum. We show that reflecting solar light on a PV cell using a silver mirror is less efficient and has higher temperature compared to using selective light reflector/absorber when using optical concentration exceeding 2 suns. We demonstrate hybrid PV generation and water sanitation/desalination using an Aluminum sheet with thin-film selective light reflector /absorber deposited on one side, while the other side is treated with femtosecond laser to become superwicking.

Windows provide an attractive platform for building integrated photovoltaics due to the presence of an existing glass substrate and their widespread deployment. A challenge in implementing integrated photovoltaics within windows are the competing design criteria between window transparency and solar cell efficiency. A common approach to address this conflict is to use a solar cell device with partially transparency in the visible range and strong light absorption in the UV and/or IR ranges. Recent research has also attempted to circumvent this trade-off using switchable materials that can transform from opaque and efficient to transparent but with poor photovoltaic performance. Here, we present a new alternative switchable device approach that couples a semiconductor solar absorber to a Polymer Dispersed Liquid Crystal (PDLC) cell. By applying an electric field these PDLC devices can be dynamically tuned from opaque, reflective and light diffusing in the off state to clear and transparent in the on state. By enabling light-trapping to greatly enhance solar cell efficiency, this off-state scattering makes PDLCs a particularly attractive option for the creation of solar windows. As an added benefit, the system provides control over haze for privacy and window reflectivity for reducing the lighting and heating costs. Enabled by the very low power requirements for maintaining the window’s transparent on-state, we also demonstrate potential for self-powering this switchable solar window.

The aim of these investigations was to identify and evaluate appropriate degradation indicators for PV encapsulation materials in order to qualify new and emergent polymeric components and to predict the lifetime of materials and modules. Therefore, the influence of the relevant stress parameters like ultraviolet radiation, corrosion environment and temperature-humidity cycles on the degradation behavior of EVA selected as encapsulant material had to be determined. Therefore, the material properties and the aging behavior were characterized by optical and thermal analysis techniques by infrared (IR) spectroscopy in attenuated total reflection mode (ATR), UV/VIS spectroscopy, photoluminescence spectroscopy analysis, Raman analysis and differential scanning calorimetric (DSC) analysis. Different degradation indicators were derived from the characterization methods and explicitly discussed considering the required property profile of polymers for PV encapsulation materials. Here, we describe a fast and non-destructive optical method to determine the EVA indoor aging.

This work focused on the technology of luminescent down shift (LDS), with a primary aim to identify and investigate a methodology to introduce the luminescent quantum dots (LQD) into PVB polymer encapsulant as emergent material for photovoltaic application. For this goal, we propose to study the feasibility to implement the LDS functionality and to identify suitability of available luminescent to be incorporated into the host polymer encapsulant material. The first step to this direction was through a comprehensive optical study of LQD dyes in ethanol solvent. The methodology and experimental conditions such as laboratory polymer preparation and luminescence dye concentration were presented. Also, the emergent polymer encapsulant sheets were characterized by using optical analysis techniques. The absorption spectrum of the prepared PVB material shifts towards longer wavelengths, depending on the nature of the LQD.