Impacts of propagating, frustrated and surface modes on radiative, electrical and thermal losses in nanoscale-gap thermophotovoltaic power generators

Multi-dimensional optimization of In0.53Ga0.47As thermophotovoltaic cell using real coded genetic algorithm

The optimization of thermophotovoltaic (TPV) cell efficiency is essential since it leads to a significant increase in the output power. Typically, the optimization of In_0.53Ga_0.47As TPV cell has been limited to single variable such as the emitter thickness, while the effects of the variation in other design variables are assumed to be negligible. The reported efficiencies of In_0.53Ga_0.47As TPV cell mostly remain < 15%. Therefore, this work develops a multi-variable or multi-dimensional optimization of In_0.53Ga_0.47As TPV cell using the real coded genetic algorithm (RCGA) at various radiation temperatures. RCGA was developed using Visual Basic and it was hybridized with Silvaco TCAD for the electrical characteristics simulation. Under radiation temperatures from 800 to 2000 K, the optimized In_0.53Ga_0.47As TPV cell efficiency increases by an average percentage of 11.86% (from 8.5 to 20.35%) as compared to the non-optimized structure. It was found that the incorporation of a thicker base layer with the back-barrier layers enhances the separation of charge carriers and increases the collection of photo-generated carriers near the band-edge, producing an optimum output power of 0.55 W/cm² (cell efficiency of 22.06%, without antireflection coating) at 1400 K radiation spectrum. The results of this work demonstrate the great potential to generate electricity sustainably from industrial waste heat and the multi-dimensional optimization methodology can be adopted to optimize semiconductor devices, such as solar cell, TPV cell and photodetectors.

Near-field thermophotovotaic devices with surrounding non-contact reflectors for efficient photon recycling

Near-field thermophotovoltaic (TPV) power generation has been attracting increasing attention as a promising approach for efficient conversion of heat into electricity with high output power density. Here, we numerically investigate near-field TPV devices with surrounding reflectors for efficient recycling of low-energy photons, which do not contribute to the power generation. We reveal that the conversion efficiency of a near-field TPV system can be drastically increased by introducing a pair of reflectors above and below the system, especially when the two mirrors are not in contact with the emitter and absorber. In addition, we investigate the influence of non-perfect photon recycling on the TPV efficiency and reveal that near-field TPV systems are more robust against the decrease of the reflectivity of the reflectors than the far-field TPV systems.

New insights into the thermal behavior and management of thermophotovoltaic systems

The thermal behavior of a thermophotovoltaic system composed of a metallo-dielectric spectrally selective radiator at high temperature and a GaSb photovoltaic cell in the far field is investigated. Using a coupled radiative, electrical and thermal model, we highlight that, without a large conductive-convective heat transfer coefficient applied to the cell, the rise in temperature of the photovoltaic cell induces dramatic efficiency losses. We then investigate solutions to mitigate thermal effects, such as radiative cooling or the decrease of the emissivity or the temperature of the radiator. Without extending the radiating area beyond that of the cell, gains by radiative cooling are marginal. However, for a given radiator temperature, decreasing its emissivity is beneficial to conversion efficiency and, in cases of limited conductive-convective cooling capacities, even leads to larger electrical power outputs. More importantly, for a realistic radiator structure made of tungsten and hafnium oxide, larger conversion efficiencies are reached with smaller radiator temperatures because thermal losses and thus needs for cooling are less.

Near-field thermal emission by periodic arrays

Near-field thermal emission can be engineered by using periodic arrays of subwavelength emitters. The array thermal emission is dependent on the shape, size, and material properties of the individual elements as well as the period of the array. Designing periodic arrays with desired properties requires models that relate the array geometry and material properties to the near-field thermal emission. In this study, a periodic method is presented for modeling two-dimensional periodic arrays of subwavelength emitters. This technique only requires discretizing one period of the array, and thus is computationally beneficial. In this method, the energy density emitted by the array is expressed in terms of the array's Green's functions. The array Green's functions are found using the discrete dipole approximation in a periodic manner by expressing a single point source as a series of periodic arrays of phase-shifted point sources. The presented method can be employed for modeling periodic arrays made of inhomogeneous and complex-shape emitters with nonuniform temperature distribution. The proposed technique is verified against the nonperiodic thermal discrete-dipole-approximation simulations, and it is demonstrated that this method can serve as a versatile and reliable tool for studying near-field thermal emission by periodic arrays.

One-Chip Near-Field Thermophotovoltaic Device Integrating a Thin-Film Thermal Emitter and Photovoltaic Cell

Thermal radiation transfer between two objects separated by a subwavelength gap (near-field thermal radiation transfer) can be orders of magnitude larger than that in free space, which is attracting increasing attention with respect to both fundamental nanoscience and its potential for high-power-density and high-efficiency conversion of heat to electricity in thermophotovoltaic (TPV) systems. However, the realization of near-field thermal radiation transfer in TPV systems involves significant challenges because it requires a subwavelength gap and large temperature difference between the emitter and the PV cell while minimizing the heat transfer that does not contribute to the photocurrent generation. To overcome these challenges, here we demonstrate a one-chip near-field TPV device consisting of a thin-film Si emitter and InGaAs PV cell with an intermediate Si substrate, which enables the suppression of the heat transfer due to sub-bandgap radiation by free carriers and surface modes. Through the one-chip integration and thermal isolation using Si process technologies, we realize a deep subwavelength gap (<150 nm) between the emitter and the intermediate substrate without using any external positioners while maintaining a large temperature difference (>700 K). Compared to the equivalent device operating in the far-field regime, we achieve 10-fold enhancement of the photocurrent in the PV cell without degrading the open-circuit voltage and fill factor, demonstrating the potential of our one-chip device for the future applications of near-field thermal radiation transfer.

Harvesting the Electromagnetic Energy Confined Close to a Hot Body

In the close vicinity of a hot body, at distances smaller than the thermal wavelength, a high electromagnetic energy density exists due to the presence of evanescent fields radiated by the partial charges in thermal motion around its surface. This energy density can surpass the energy density in vacuum by several orders of magnitude. By approaching a photovoltaic (PV) cell with a band gap in the infrared frequency range, this nonradiative energy can be transferred to it by photon tunnelling and surface mode coupling. Here we review the basic ideas and recent progress in near-field energy harvesting.

Micron-sized liquid nitrogen-cooled indium antimonide photovoltaic cell for near-field thermophotovoltaics

Simulations of near-field thermophotovoltaic devices predict promising performance, but experimental observations remain challenging. Having the lowest bandgap among III-V semiconductors, indium antimonide (InSb) is an attractive choice for the photovoltaic cell, provided it is cooled to a low temperature, typically around 77 K. Here, by taking into account fabrication and operating constraints, radiation transfer and low-injection charge transport simulations are made to find the optimum architecture for the photovoltaic cell. Appropriate optical and electrical properties of indium antimonide are used. In particular, impact of the Moss-Burstein effects on the interband absorption coefficient of n-type degenerate layers, and of parasitic sub-bandgap absorption by the free carriers and phonons are accounted for. Micron-sized cells are required to minimize the huge issue of the lateral series resistance losses. The proposed methodology is presumably relevant for making realistic designs of near-field thermophotovoltaic devices based on low-bandgap III-V semiconductors.

Spectral control of near-field thermal radiation via photonic band engineering of two-dimensional photonic crystal slabs

In this paper, we numerically investigate a method to obtain narrow-bandwidth near-field thermal radiation spectra by using two-dimensional (2D) photonic crystal (PC) slabs. Our examination reveals that near-field thermal radiation spectra can be artificially controlled via the photonic band engineering of 2D-PC slabs, where the radiation is enhanced in a range of frequencies of the flat bands and suppressed inside the photonic bandgap. By designing a thermal emitter with a 2D-PC slab of appropriate thickness, and by adjusting the gap between the emitter and the absorber, we can implement narrowband near-field thermal radiation that overcomes the far-field blackbody limit in the near-infrared range. Further, its linewidth is as small as Δλ = 0.14 µm.

Spectrally shaping high-temperature radiators for thermophotovoltaics using Mo-HfO2 trilayer-on-substrate structures

Easy-to-fabricate, high-temperature, thermally-stable radiators are critical elements for developing efficient and sustainable thermophotovoltaic energy conversion devices. In this frame, a trilayer-on-substrate structure is selected. It is composed of a refractory metal -molybdenum - constituting the substrate and an intermediate thin film sandwiched between two hafnia transparent layers. An in-depth analysis shows that two spectrally distinct interference regimes take place in the hafnia layer-molybdenum thin film substructure, and that backward and forward thermally-emitted waves by the thin film are selected in two distinct interferential resonating cavities. The interference regimes within and between these cavities are key to the spectral shaping of thermal emission. The radiative performances of the structures are evaluated by introducing a figure of merit. Using the example of a GaSb cell, it is shown that the structure can be optimized for providing the broadband large emission with a steep cutoff required for mitigating photoconversion losses.

Near-field thermophotovoltaic energy conversion using an intermediate transparent substrate

We propose a scheme for near-field thermophotovoltaic (TPV) energy conversion, where thermal emission from an emitter is extracted by an intermediate transparent substrate attached to the top of a photovoltaic (PV) cell. The addition of an intermediate transparent substrate suppresses the unwanted heat transfer from the emitter to the PV cell due to the surface modes on the PV cell while maintaining the enhancement in the interband absorption. We confirm that our scheme is applicable for near-field TPV systems using a silicon (Si) or tungsten (W) emitter. As a specific example, we designed a near-field TPV system composed of a one-dimensional Si photonic crystal thermal emitter, an InGaAs PV cell, and an intermediate Si substrate, and displayed that our scheme could realize both high power density (>5 × 10⁴ W/m²) and high power conversion efficiency (>40%) at a 50-nm gap between the emitter and the intermediate substrate.

High-injection effects in near-field thermophotovoltaic devices

In near-field thermophotovoltaics, a substantial enhancement of the electrical power output is expected as a result of the larger photogeneration of electron-hole pairs due to the tunneling of evanescent modes from the thermal radiator to the photovoltaic cell. The common low-injection approximation, which considers that the local carrier density due to photogeneration is moderate in comparison to that due to doping, needs therefore to be assessed. By solving the full drift-diffusion equations, the existence of high-injection effects is studied in the case of a GaSb p-on-n junction cell and a radiator supporting surface polaritons. Depending on doping densities and surface recombination velocity, results reveal that high-injection phenomena can already take place in the far field and become very significant in the near field. Impacts of high injection on maximum electrical power, short-circuit current, open-circuit voltage, recombination rates, and variations of the difference between quasi-Fermi levels are analyzed in detail. By showing that an optimum acceptor doping density can be estimated, this work suggests that a detailed and accurate modeling of the electrical transport is also key for the design of near-field thermophotovoltaic devices.

Near-Field Heat Transfer between Multilayer Hyperbolic Metamaterials

We review the near-field radiative heat flux between hyperbolic materials focusing on multilayer hyperbolic meta-materials. We discuss the formation of the hyperbolic bands, the impact of ordering of the multilayer slabs, as well as the impact of the first single layer on the heat transfer. Furthermore, we compare the contribution of surface modes to that of hyperbolic modes. Finally, we also compare the exact results with predictions from effective medium theory.

Radiative heat transfer exceeding the blackbody limit between macroscale planar surfaces separated by a nanosize vacuum gap

Using Rytov's fluctuational electrodynamics framework, Polder and Van Hove predicted that radiative heat transfer between planar surfaces separated by a vacuum gap smaller than the thermal wavelength exceeds the blackbody limit due to tunnelling of evanescent modes. This finding has led to the conceptualization of systems capitalizing on evanescent modes such as thermophotovoltaic converters and thermal rectifiers. Their development is, however, limited by the lack of devices enabling radiative transfer between macroscale planar surfaces separated by a nanosize vacuum gap. Here we measure radiative heat transfer for large temperature differences (∼120 K) using a custom-fabricated device in which the gap separating two 5 × 5 mm² intrinsic silicon planar surfaces is modulated from 3,500 to 150 nm. A substantial enhancement over the blackbody limit by a factor of 8.4 is reported for a 150-nm-thick gap. Our device paves the way for the establishment of novel evanescent wave-based systems.

Evanescent coupling between surfaces separated by a distance smaller than the thermal wavelength can lead to radiative heat transfer greater than the blackbody limit. Here, the authors demonstrate this between two macroscopic-scale surfaces, paving the way to harnessing the effect in thermal devices.

Hyperbolic metamaterial-based near-field thermophotovoltaic system for hundreds of nanometer vacuum gap

Artificially designed hyperbolic metamaterial (HMM) possesses extraordinary electromagnetic features different from those of naturally existing materials. In particular, the dispersion relation of waves existing inside the HMM is hyperbolic rather than elliptical; thus, waves that are evanescent in isotropic media become propagating in the HMM. This characteristic of HMMs opens a novel way to spectrally control the near-field thermal radiation in which evanescent waves in the vacuum gap play a critical role. In this paper, we theoretically investigate the performance of a near-field thermophotovoltaic (TPV) energy conversion system in which a W/SiO₂-multilayer-based HMM serves as the emitter at 1000 K and InAs works as the TPV cell at 300 K. By carefully designing the thickness of constituent materials of the HMM emitter, the electric power of the near-field TPV devices can be increased by about 6 times at 100-nm vacuum gap as compared to the case of the plain W emitter. Alternatively, in regards to the electric power generation, HMM emitter at experimentally achievable 100-nm vacuum gap performs equivalently to the plain W emitter at 18-nm vacuum gap. We show that the enhancement mechanism of the HMM emitter is due to the coupled surface plasmon modes at multiple metal-dielectric interfaces inside the HMM emitter. With the minority carrier transport model, the optimal p-n junction depth of the TPV cell has also been determined at various vacuum gaps.