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We present inverted metamorphic Ga0.3In0.7As photovoltaic converters with sub-0.60 eV bandgaps grown on InP and GaAs substrates. The compositionally graded buffers in these devices have threading dislocation densities of 1.3x10^6 cm^-2 and 8.9x10^6 cm^-2 on InP and GaAs, respectively. The devices generate open-circuit voltages of 0.386 V and 0.383 V, respectively, at a current density of ~10 A/cm^2, yielding bandgap-voltage offsets of 0.20 and 0.21 V. We measured their broadband reflectance and used it to estimate thermophotovoltaic efficiency. The InP-based cell is estimated to yield 1.09 W/cm^2 at 1100 degrees C vs. 0.92 W/cm^2 for the GaAs-based cell, with efficiencies of 16.8 vs. 9.2%. The efficiencies of both devices are limited by sub-bandgap absorption, with power weighted sub-bandgap reflectances of 81% and 58%, respectively, which we assess largely occurs in the graded buffers. We estimate that the thermophotovoltaic efficiencies would peak at ~1100 degrees C at 24.0% and 20.7% in structures with the graded buffer removed, if previously demonstrated reflectance is achieved. These devices also have application to laser power conversion in the 2.0-2.3 micron atmospheric window. We estimate peak LPC efficiencies of 36.8% and 32.5% under 2.0 micron irradiances of 1.86 W/cm^2 and 2.81 W/cm^2, respectively., Comment: 14 pages, 6 figures

Thermophotovoltaics are promising solid-state energy converters for a variety of applications such as grid-scale energy storage, concentrating solar-thermal power, and waste heat recovery. Here, we report the design, fabrication, and testing of large area (0.8 cm$^2$), scalable, single junction 0.74-eV GaInAs thermophotovoltaic devices reaching an efficiency of 38.8$\pm$2.0% and an electrical power density of 3.78 W/cm$^2$ at an emitter temperature of 1850{\deg}C. Reaching such a high emitter temperature and power density without sacrificing efficiency is a direct result of combining good spectral management with a highly optimized cell architecture, excellent material quality, and very low series resistance. Importantly, fabrication of 12 high-performing devices on a two-inch wafer is shown to be repeatable, and the cell design can be readily transferred to commercial epitaxy on even larger wafers. Further improvements in efficiency can be obtained by using a multijunction architecture, and early results for a two-junction 0.84-eV GaInPAs / 0.74-eV GaInAs device illustrate this promise., Comment: 27 pages, 15 figures

Near-field radiative heat transfer (NFRHT) between irregularly shaped dielectric particles made of SiO2 and morphology characterized by Gaussian random spheres is studied. Particles are modeled using the discrete system Green's function (DSGF) approach, which is a volume integral numerical method based on fluctuational electrodynamics. This method is applicable to finite, three-dimensional objects, and all system interactions are defined independent of thermal excitation by a generalized system Green's function. The DSGF method is deemed suitable to model NFRHT between irregularly shaped particles after verification against the analytical solution for chains of two and three SiO2 spheres. The NFRHT results reveal that geometric irregularity in particles leads to a reduction of the total conductance from that of comparable perfect spheres at vacuum separation distances smaller than the particle size, a regime in which NFRHT is a surface phenomenon. At vacuum separation distances larger than the particle size, NFRHT becomes a volumetric process, and the total conductance between irregularly shaped particles converges to that of comparable perfect spheres. Spectral analysis reveals, however, that particle irregularity leads to damping and broadening of resonances at all separation distances, thereby highlighting the importance of the DSGF method for spectral engineering in the near field. The reduced spectral coherence when particle size is larger than the vacuum separation distance is attributed to coupling of surface phonon-polaritons within the randomly generated, distorted particle features. For particle size smaller than the vacuum separation distance, resonance broadening and damping is linked with the multiple localized surface phonon modes supported by the composite spherical harmonic morphologies of the Gaussian random spheres., Comment: 59 pages, 9 figures, 6 supplementary figures

An ideal solar thermal absorber has a sharp transition between high and low absorptance at the wavelength where the blackbody emissive power begins to exceed the solar irradiance. However, most real selective absorbers have a fairly broad transition, leading to both solar absorption and thermal emission losses. Here, we model, fabricate, and characterize a highly selective semiconductor-dielectric-metal (Ga0.46In0.54As - MgF2 - Ag) solar absorber with an extremely sharp transition from high to low absorptance. The thin semiconductor serves as a selective filter, absorbing photons with wavelengths shorter than the bandgap and transmitting those with longer wavelengths. The highly reflective dielectric-metal rear mirror allows the structure to have very low emittance for longer wavelengths. These characteristics provide the absorber with a measured solar absorptance >91% below the bandgap wavelength and infrared emittance <5% at 100 C above the bandgap wavelength. This transition wavelength can be tuned by modifying the semiconductor composition, and modeling indicates that the absorber's optical properties should be stable at high temperatures, making the structure a good candidate for unconcentrated to highly concentrated solar thermal energy conversion., Comment: 16 pages, 6 figures

We report the fabrication and measurement of thermophotovoltaic (TPV) cells with efficiencies of >40%, which is a record high TPV efficiency and the first experimental demonstration of the efficiency of high-bandgap tandem TPV cells. TPV efficiency was determined by simultaneous measurement of electric power output and heat dissipation from the device via calorimetry. The TPV cells are two-junction devices comprising high-quality III-V materials with band gaps between 1.0 and 1.4 eV that are optimized for high emitter temperatures of 1900-2400{\deg}C. The cells exploit the concept of band-edge spectral filtering to obtain high efficiency, using high-reflectivity back surface reflectors to reject unusable sub-bandgap radiation back to the emitter. A 1.4/1.2 eV device reached a maximum efficiency of (41.1 +/- 1)% operating at a power density of 2.39 W/cm2 under an irradiance of 30.4 W/cm2 and emitter temperature of 2400{\deg}C. A 1.2/1.0 device reached a maximum efficiency of (39.3 +/- 1)% operating at a power density of 1.8 W/cm2 under an irradiance of 20.1 W/cm2 and emitter temperature of 2127{\deg}C. These cells can be integrated into a TPV system for thermal energy grid storage (TEGS) to enable dispatchable renewable energy. These new TPV cells enable a pathway for TEGS to reach sufficiently high efficiency and sufficiently low cost to enable full decarbonization of the grid. Furthermore, the high demonstrated efficiency also gives TPV the potential to compete with turbine-based heat engines for large-scale power production with respect to both cost and performance, thereby enabling possible usage in natural gas or hydrogen-fueled electricity production.

Emitted photons stemming from the radiative recombination of electron-hole pairs carry chemical potential in radiative energy converters. This luminescent effect can substantially alter the local net photogeneration in near-field thermophotovoltaic cells. Several assumptions involving the luminescent effect are commonly made in modeling photovoltaic devices; in particular, the photon chemical potential is assumed to be zero or a constant prescribed by the bias voltage. The significance of photon chemical potential depends upon the emitter temperature, the semiconductor properties, and the injection level. Hence, these assumptions are questionable in thermophotovoltaic devices operating in the near-field regime. In the present work, an iterative solver that combines fluctuational electrodynamics with the drift-diffusion model is developed to tackle the coupled photon and charge transport problem, enabling the determination of the spatial profile of photon chemical potential beyond the detailed balance approach. The difference between the results obtained by allowing the photon chemical potential to vary spatially and by assuming a constant value demonstrates the limitations of the conventional approaches. This study is critically important for performance evaluation of near-field thermophotovoltaic systems., Comment: 40 pages, 11 figures

Accurate modeling of charge transport and both thermal and luminescent radiation is crucial to the understanding and design of radiative thermal energy converters. Charge carrier dynamics in semiconductors are well-described by the Poisson-drift-diffusion equations, and thermal radiation in emitter/absorber structures can be computed using multilayer fluctuational electrodynamics. These two types of energy flows interact through radiation absorption/luminescence and charge carrier generation/recombination. However, past research has typically only assumed limited interaction, with thermal radiation absorption as an input for charge carrier models to predict device performance. To examine this assumption, we develop a fully-coupled iterative model of charge and radiation transport in semiconductor devices, and we use our model to analyze near-field and far-field GaSb thermophotovoltaic and thermoradiative systems. By comparing our results to past methods that do not consider cross-influences between charge and radiation transport, we find that a fully-coupled approach is necessary to accurately model photon recycling and near-field enhancement of external luminescence. Because these effects can substantially alter device performance, our modeling approach can aid in the design of efficient thermophotovoltaic and thermoradiative systems., Comment: 21 pages, 8 figures

We propose a solar thermal energy conversion system consisting of a solar absorber, a thermoradiative cell or negative illumination photodiode, and a photovoltaic cell. Because it is a heat engine, this system can also be paired with thermal storage to provide reliable electricity generation. Heat from the solar absorber drives radiative recombination current in the thermoradiative cell, and its emitted light is absorbed by the photovoltaic cell to provide an additional photocurrent. Based on the principle of detailed balance, we calculate a limiting solar conversion efficiency of 85% for fully concentrated sunlight and 45% for one sun with an absorber and single-junction cells of equal areas. Ideal and nonideal solar thermoradiative-photovoltaic systems outperform solar thermophotovoltaic converters for low bandgaps and practical absorber temperatures. Their performance enhancement results from a high tolerance to nonradiative generation/recombination and an ability to minimize radiative heat losses. We show that a realistic device with all major losses could achieve increases in solar conversion efficiency by up to 7.9% (absolute) compared to a solar thermophotovoltaic device under low optical concentration. Our results indicate that these converters could serve as efficient heat engines for low cost single axis tracking systems., Comment: 20 pages, 7 figures

Systems of many nanoparticles or volume-discretized bodies exhibit collective radiative properties that could be used for enhanced, guided, or tunable thermal radiation. These are commonly treated as assemblies of point dipoles with interactions described by Maxwell's equations and thermal fluctuations correlated by the fluctuation-dissipation theorem. Here, we unify different theoretical descriptions of these systems and provide a complete derivation of many-dipole thermal radiation, showing that the correct use of the fluctuation-dissipation theorem depends on the definitions of fluctuating and induced dipole moments. We formulate a method to calculate the diffusive radiative thermal conductivity of arbitrary collections of nanoparticles; this allows the comparison of thermal radiation to other heat transfer modes and across different material systems. We calculate the radiative thermal conductivity of ordered and disordered arrays of SiC and SiO2 nanoparticles and show that thermal radiation can significantly contribute to thermal transport in these systems. We validate our calculations by comparison to the exact solution for a one-dimensional particle chain, and we demonstrate that the dipolar approximation significantly underpredicts the exact results at separation distances less than the particle radius., Comment: 20 pages, 6 figures

Photons typically do not contribute to thermal transport within a solid due to their low energy density and tendency to be quickly absorbed. We propose a practical material system - infrared plasmonic resonators embedded in a semiconductor nanowire - that leverages near-field electromagnetic coupling to achieve photonic thermal transport comparable to the electronic and phononic contributions. We analytically show photonic thermal conductivities up to about 1 W m-1 K-1 for 10 nm diameter Si and InAs nanowires containing repeated resonators at 500 K. The nanowire system outperforms plasmonic particles in isotropic environments and presents a pathway for photonic thermal transport to exceed that of phonons and electrons., Comment: 10 pages, 4 figures, plus supplementary information

In chains of closely-spaced nanoparticles supporting surface polaritons, near-field electromagnetic coupling leads to collective effects and super-Planckian thermal radiation exchange. Researchers have primarily used two analytical approaches to calculate radiative heat transfer in these systems: fluctuational electrodynamics, which directly incorporates fluctuating thermal currents into Maxwell's equations, and a kinetic approach where the dispersion relation provides modes and propagation lengths for the Boltzmann transport equation. Here, we compare results from the two approaches in order to identify regimes in which kinetic theory is valid and to explain differing results in the literature on its validity. Using both methods, we calculate the diffusive radiative thermal conductivity of nanoparticle chains. We show that kinetic theory is valid and matches predictions by fluctuational electrodynamics when the propagation lengths are greater than the particle spacing., Comment: 10 pages, 6 figures, preprint accepted for the 9th International Symposium on Radiative Transfer