Near-field Thermophotovoltaic Devices Research Articles

Spectral regulation in thermophotovoltaic devices

Thermophotovoltaic (TPV) device converts thermal radiation into electricity output through photovoltaic effect. High-efficiency TPV devices have extensive applications in grid-scale thermal storage, full-spectrum solar utilization, distributed thermal-electricity cogeneration, and waste heat recovery. The key to high-efficiency TPV devices lies in spectral regulation to achieve band-matching between thermal radiation of the emitters and electron transition of the photovoltaic cells. The latest advances in nanophotonics, materials science, and artificial intelligence have made milestone progress in spectral regulation and recording power conversion efficiency of up to 40% of TPV devices. Here we systematically review spectral regulation in TPV devices at the emitter end as well as the photovoltaic cell end. At the emitter end, spectral regulation is realized through thermal metamaterials and rare-earth intrinsic emitters to selectively enhance the in-band radiation and suppress the sub-bandgap radiation. At the photovoltaic cell end, spectral regulation mainly focuses on recycling the sub-bandgap thermal radiation through optical filters and back surface reflectors located at the front and back of the photovoltaic cells, respectively. We emphasize the light-matter interaction mechanisms and material systems of different spectral regulation strategies. We also discuss the spectral regulation strategies in near-field TPV devices. Finally, we look forward to potential development paths and prospects of spectral regulation to achieve scalable deployment of future TPV devices.

Effectiveness of multi-junction cells in near-field thermophotovoltaic devices considering additional losses

Abstract Thermophotovoltaic (TPV) energy converters hold substantial potential in converting thermal radiation from high-temperature emitters into electrical energy through photovoltaic (PV) cells, offering applications ranging from solar energy harvesting to waste heat recovery. Near-field TPV (NF-TPV) devices, focused on enhancing power output density (POD), exhibit unique potential by harnessing photon tunneling. However, this potential can be mitigated by additional losses arising from high photocurrent densities and corresponding scalability issues. This study comprehensively investigates the effectiveness of multi-junction-based NF-TPV devices, accounting for additional losses. We propose two approximative expressions to quantify the impact of additional losses and characterize current density-voltage curves. Verification against rigorously optimized results establishes a criterion for effective performance. Our method provides precise POD estimations even for devices with 10 or more subcells, facilitating performance analysis across parameters like vacuum gap distance, cell width, emitter temperature, and the number of subcells compared to far-field counterparts. This research outlines a roadmap for the scalable design of NF-TPV devices, emphasizing the role of multi-junction PV cells. The analytical framework we developed will provide vital insights for future high-performance TPV devices.

Analysis of Near-Field Thermophotovoltaic Devices Using Graphene–Germanium Schottky Cell

In this work, we investigate the performance of graphene-based Schottky junction thermophotovoltaic (TPV) devices in near-field conditions. Despite the low cost and excellent photoelectric properties of graphene, earlier studies have focused primarily on the contribution of the graphene layer to the photocurrent, assuming an internal quantum efficiency (IQE) of 100%. Our numerical model of a graphene/germanium Schottky junction TPV device reveals that the semiconductor layer predominates in the generation photocurrent, with an IQE of graphene less than 40%. We also evaluate the photocurrent densities generated by the semiconductor and graphene at an emitter temperature of 1000 K and a vacuum gap of 100 nm. Results show that using an indium tin oxide (ITO)-covered tungsten (W) emitter can increase photocurrents by a factor of around 10 and 11 for the semiconductor and graphene, respectively. Additionally, using a hyperbolic metamaterial (HMM) emitter can enhance photocurrents by around 4.7 and 5.2 times for the semiconductor and graphene, respectively. However, this comes at the cost of higher heat flux from the HMM emitter. Our findings will provide valuable insights for the design and optimization of TPV devices to improve their photocurrent and efficiency.

A multi-junction-based near-field solar thermophotovoltaic system with a graphite intermediate structure

A solar thermophotovoltaic (STPV) system can transform incident concentrated solar energy into electrical energy with an efficiency that could be higher than the Shockley–Queisser limit. Near-field thermophotovoltaic (NF-TPV) devices can generate larger electrical power output than traditional far-field TPV devices with the aid of photon tunneling. Moreover, multi-junction PV cells can boost the performance of TPV devices by effectively distributing the absorbed photon energy inside the PV cell. In this work, we design a multi-junction-based near-field STPV system with a practical and high-temperature stable graphite intermediate structure. To optimize the system configuration, we employ a genetic algorithm and a surrogate model based on an artificial neural network, which enables us to suggest a better design approach for the multi-junction-based NF-STPV system between the power output density and power conversion efficiency maximization scenarios. When the concentration factor of the incident solar energy is 5000 and the absorber-to-emitter area ratio is 3, we can achieve a system efficiency of 23%. By introducing a material whose emissivity is as high as a blackbody on the solar absorber, the system efficiency can be further enhanced up to 35%.

Modeling and experiments of near-field thermophotovoltaic conversion: A review

This article provides an in-depth review of charge transport models utilized to analyze the performance of near-field thermophotovoltaic (NF-TPV) devices, and a comprehensive examination of NF-TPV experiments performed to date. Researchers have used several models to theoretically analyze NF-TPV devices; however, each model can result in different performances of NF-TPV devices, particularly due to characteristics of near-field thermal radiation. For clarification, we sort up-to-date analysis models into four categories; the analytical approximation model, detailed balance analysis, minority carrier separation (MCS) model, and Poisson-drift–diffusion (PDD) model. Key assumptions of each model are enumerated, and the calculated results by each model are compared. For the review of experimental works, we first sort NF-TPV experiments according to the manner of gap control into positioner-, spacer-, and MEMS-based approaches. Then, we introduce the primary design configuration of each approach and the compelling experimental results achieved by state-of-the-art studies. In addition, advanced concepts of hybrid devices and multi-junction PV cells, proposed to further improve NF-TPV performance, are briefly introduced. • A review of charge transport models used to analyze the NF-TPV device is provided. • Models are classified into analytical approximation, detailed balance, MCS, and PDD. • A comprehensive review of all NF-TPV experiments performed to date is conducted. • NF-TPV experiments are sorted into positioner-, spacer-, and MEMS-based approaches.

Improved performance of a near-field thermophotovoltaic device by a back gapped reflector

Various spectral control techniques can be applied to improve the performance of a thermophotovoltaic (TPV) system. For example, a back surface reflector (BSR) can improve the performance of TPV systems. A conventional metal BSR structure enhances the photogeneration rate by increasing the absorption probability of photons via back surface reflection, affording a second chance for absorption. However, surface passivation and external luminescence effects introduced by BSR structures have been previously ignored, which potentially decreases the performance of TPV systems. Recently, a back gapped reflector (BGR) structure was proposed to greatly improve the performance of far-field TPV systems by reducing reflection loss at the semiconductor-metal interface. In the present work, the performance improvement on a thin-film, near-field InAs TPV system with a BGR is investigated, comparing its performance to that with a conventional metal BSR. Surface passivation conditions are also investigated to further improve the performance of TPV systems with back reflectors. The output power and efficiency are calculated using an iterative model combining fluctuational electrodynamics and the full drift-diffusion model. For the well-passivated condition, when the BSR is replaced by the BGR, the calculated conversion efficiency was improved from 16.4% to 21% and the output power was increased by 10% for the near-field regime. Finally, the reflection loss and external luminescence loss are analyzed to explain the performance improvement.

Graphene-based autonomous pyroelectric system for near-field energy conversion

In the close vicinity of a hot solid, at distances smaller than the thermal wavelength, a strong electromagnetic energy density exists because of the presence of evanescent field. Here we introduce a many-body conversion principle to harvest this energy using graphene-based pyroelectric conversion devices made with an active layer encapsulated between two graphene field-effect transistors which are deposited on the source and on the cold sink. By tuning the bias voltage applied to the gates of these transistors, the thermal state and the spontaneous polarization of the active layer can be controlled at kHz frequencies. We demonstrate that the power density generated by these conversion systems can reach 130,{rm mW,cm}^{-2} using pyroelectric Ericsson cycles, a value which surpasses the current production capacity of near-field thermophotovoltaic conversion devices by more than three orders of magnitude with low grade heat sources (T<500,{rm K}) and small temperature differences (Delta Tsim 100,K).

Integrated Near-Field Thermophotovoltaic Device Overcoming Blackbody Limit

Near-field thermal radiation transfer overcoming the blackbody limit has attracted significant attention in recent years owing to its potential for drastically increasing the output power and conve...

Near-field thermophotovoltaics for efficient heat to electricity conversion at high power density

Thermophotovoltaic approaches that take advantage of near-field evanescent modes are being actively explored due to their potential for high-power density and high-efficiency energy conversion. However, progress towards functional near-field thermophotovoltaic devices has been limited by challenges in creating thermally robust planar emitters and photovoltaic cells designed for near-field thermal radiation. Here, we demonstrate record power densities of ~5 kW/m2 at an efficiency of 6.8%, where the efficiency of the system is defined as the ratio of the electrical power output of the PV cell to the radiative heat transfer from the emitter to the PV cell. This was accomplished by developing novel emitter devices that can sustain temperatures as high as 1270 K and positioning them into the near-field (<100 nm) of custom-fabricated InGaAs-based thin film photovoltaic cells. In addition to demonstrating efficient heat-to-electricity conversion at high power density, we report the performance of thermophotovoltaic devices across a range of emitter temperatures (~800 K–1270 K) and gap sizes (70 nm–7 µm). The methods and insights achieved in this work represent a critical step towards understanding the fundamental principles of harvesting thermal energy in the near-field.

Spatial profiles of photon chemical potential in near-field thermophotovoltaic cells

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 near-field thermophotovoltaic devices with nanoscale separation distances between the emitter and the cell. 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 with nanoscale vacuum gaps.

Near-field radiative heat transfer enhancement in the thermophotovoltaic system using hyperbolic waveguides

As an energy conversion system, the near-field thermophotovoltaic (NTPV) device has attracted much attention. To improve the NTPV system output power, an intermediate modulator with the same plasmonic properties as thermal emitters can be intercalated into the two-body system. Surface modes are coupled in the vacuum gap between the modulator and the emitter. The modulator absorbs the enormous radiative energy produced by the coupling of surface modes and transfers it to the cell. However, due to the low penetration depth of surface modes and the loss of energy transmission in the intermediate slab, the direct heat exchange between the emitter and the cell in the three-body system is much smaller than that in the corresponding two-body system. In the present study, we design a hyperbolic metamaterial (HMM)-based three-body NTPV system. Ga-doped ZnO (GZO)/HfO2-multilayer-based HMMs serve as the emitter and the modulator, and GaSb works as the cell. The radiative heat flux from the emitter to the cell is enhanced by the weakly dissipating hyperbolic waveguide, which is attributed to the coupling of the vacuum-GZO surface plasmon polariton (SPP) and the SPPs at the GZO-HfO2 interfaces in the HMM modulator. Compared with the GZO slab emitter, the HMM emitter emits higher spectral heat flux located in the hyperbolic region. Different top layers of the HMM affect the coupling of surface modes. These findings provide a new way to enhance the three-body NTPV system output power.

Performance analysis of three-body near-field thermophotovoltaic systems with an intermediate modulator

Near-field thermophotovoltaic devices are attractive energy conversion systems. In the present study, three-body near-field thermophotovoltaic systems are designed by introducing an intermediate modulator between thermal emitters and photovoltaic cells. The slab/grating emitter and modulator are made from Ga-doped ZnO, while the slab/grating cell is made from gallium antimonide. The scattering matrix method is used to calculate the radiative exchange between the emitter and the cell. The performances of three-body systems of slab-slab-slab/slab-slab-grating/ grating-grating-grating are investigated. The numerical results show that the output power of the three-slab system is significantly higher than that of the corresponding two-slab system. The output power improvement is attributed to the introduction of the modulator which enables the plasmon polaritons at two GZO surfaces to be coupled. The one-dimensional grating on the surface of the photovoltaic cell can serve as an anti-reflection pattern and improve the conversion efficiency and output power of the three-body system. Grating emitters and modulators can support hyperbolic modes and enhance the radiative heat flux of the whole system. The results prove that the three-body system can provide the possibility to improve the performance of a near-field thermophotovoltaic device.

Intermediate-band-assisted near-field thermophotovoltaic devices with InAs, GaSb, and Si based absorbers

A new scheme of near-field thermophotovoltaic devices is proposed by introducing the intermediate-band absorber. The two-step excitation via the intermediate band provides a large photogenerated current density and maintains a high voltage output for the thermal-electric conversion. Energy dissipation processes in devices are analyzed by using the detailed balance model. Results show that the powers and efficiencies of thermophotovoltaic devices with intermediate bands in appropriate positions are clearly larger than those of the conventional counterparts. Based on the optical properties observed in experiments, three types of absorber materials are compared, which indicates that InAs with low energy bandgap is more suitable as a high-temperature material for photon absorptions than GaSb and Si. Comparing the performance of our model with experiments, we show that the InAs based thermophotovoltaic device allows the enhancement of efficiency over a range of gap sizes. The proposed model may open a new field in the application of thermophotovoltaic devices.

Design of an indium arsenide cell for near-field thermophotovoltaic devices

An indium arsenide photovoltaic cell with gold front contacts is designed for use in a near-field thermophotovoltaic (NF-TPV) device consisting of millimeter-size surfaces separated by a nanosize vacuum gap. The device operates with a doped silicon radiator maintained at a temperature of 800 K. The architecture of the photovoltaic cell, including the emitter and base thicknesses, the doping level of the base, and the front contact grid parameters, are optimized for maximizing NF-TPV power output. This is accomplished by solving radiation and charge transport in the cell via fluctuational electrodynamics and the minority charge carrier continuity equations, in addition to accounting for the shading losses due to the front contacts and additional series resistance losses introduced by the front contacts and the substrate. The results reveal that these additional loss mechanisms negatively affect NF-TPV performance in a non-negligible manner, and that the maximum power output is a trade-off between shading losses and series resistance losses introduced by the front contacts. For instance, when the cell is optimized for a 1 x 1 mm2 device operating at a vacuum gap of 100 nm, the losses introduced by the front contacts reduce the maximum power output by a factor of ~ 2.5 compared to the idealized case when no front contact grid is present. If the optimized grid for the 1 x 1 mm2 device is scaled up for a 5 x 5 mm2 device, the maximum power output is only increased by a factor of ~ 1.08 with respect to the 1 x 1 mm2 case despite an increase of the surface area by a factor of 25. This work demonstrates that the photovoltaic cell in a NF-TPV device must be designed not only for a specific radiator temperature, but also for specific gap thickness and device surface area.

High-performance near-field thermophotovoltaic device with CaF&lt;sub&gt;2&lt;/sub&gt;/W multilayer hyperbolic metamaterial emitter

Thermophotovoltaic (TPV) device is a thermoelectric conversion method with great application prospects. In the far-field regime, the thermoelectric power is usually small due to the Planck blackbody radiation limit, but can be substantially enhanced in the near-field regime where evanescent waves will participate in the heat transfer by tunneling. In this aspect, the surface optical density of state is a key factor that will determine the transfer conversion of thermal photons. Plasmonic or phononic resonance materials have been discussed in the literature to acquire large heat flux. Besides, metamaterial is another way to pursue the design freedoms for the same purpose. In this work, we propose a [CaF&lt;sub&gt;2&lt;/sub&gt;/W]&lt;i&gt;&lt;sub&gt;n&lt;/sub&gt;&lt;/i&gt; multilayer based infrared hyperbolic metamaterial (HMM) with high surface density of states as an emitter of a high-performance TPV cell made of an InSb p-n junction (energy bandgap = 0.17 eV). The effective medium theory (EMT) is utilized to describe the electromagnetic behavior of the HMM. The near-field heat flux is calculated based on electrodynamic wave theory and Green's function method, and the photocurrent of thermophotovoltaic device is derived using diffusion equation for semiconductor. For comparison, we design three different radiators, i.e. tungsten film (W), [GaF&lt;sub&gt;2&lt;/sub&gt;/W]&lt;i&gt;&lt;sub&gt;n&lt;/sub&gt;&lt;/i&gt; multilayer hyperbolic metamaterial (HMM), and tungsten-grounded HMM (WHMM). Compared with the pure tungsten radiator, the artificial structure exhibits the hyperbolic dispersion characteristic in a wide frequency range, which gives rise to a higher local density of states, in particular in the hyperbolic-to-elliptic spectral transition region. As a result, the radiation power and the energy conversion efficiency are greatly enhanced, which are more easily realized by a matched emission band achieved by the structural design. We find that the thermophotovoltaic device with WHMM radiator has a similar power and conversion efficiency to that with the HMM radiator. The influence of the substrate can be ignored when the hyperbolic metamaterial is thicker than 140 nm, very beneficial to the actual fabrication of the device. By our system, with multilayer hyperbolic metamaterial (HMM) radiator, a high electric power &gt;1 W/m&lt;sup&gt;2&lt;/sup&gt; and a conversion efficiency about 11% can be obtained at a bias temperature of 200 K and a 100 nm vacuum gap. Compared with nanowire arrays or natural hyperbolic material, the multilayer structure proposed in this paper has obvious advantages in bandwidth and manufacturing and may find important applications in near-field thermophotovoltaic device and other relevant areas.

Indium antimonide photovoltaic cells for near-field thermophotovoltaics

Indium antimonide photovoltaic cells are specifically designed and fabricated for use in a near-field thermophotovoltaic device demonstrator. The optimum conditions for growing the p-n junction stack of the cell by means of solid-source molecular beam epitaxy are investigated. Then processing of circular micron-sized mesa structures, including passivation of the side walls, is described. The resulting photovoltaic cells, cooled down to around 77 K in order to operate optimally, exhibit excellent performances in the dark and under far-field illumination by thermal sources in the [600–1000] °C temperature range. A short-circuit current beyond 10 μA, open-circuit voltage reaching almost 85 mV, fill factor of 0.64 and electrical power at the maximum power point larger than 0.5 μW are measured for the cell with the largest mesa diameter under the highest illumination. These results demonstrate that these photovoltaic cells will be suitable for measuring a near-field enhancement of the generated electrical power.

Thermionic-enhanced near-field thermophotovoltaics

Solid-state heat-to-electrical power converters are thermodynamic engines that use fundamental particles, such as electrons or photons, as working fluids. Virtually all commercially available devices are thermoelectric generators, in which electrons flow through a solid driven by a temperature difference. Thermophotovoltaics and thermionics are highly efficient alternatives relying on the direct emission of photons and electrons. However, the low energy flux carried by the emitted particles significantly limits their generated electrical power density potential. Creating nanoscale vacuum gaps between the emitter and the receiver in thermionic and thermophotovoltaic devices enables a significant enhancement of the electron and photon energy fluxes, respectively, which in turn results in an increase of the generated electrical power density. Here we propose a thermionic-enhanced near-field thermophotovoltaic device that exploits the simultaneous emission of photons and electrons through nanoscale vacuum gaps. We present the theoretical analysis of a device in which photons and electrons travel from a hot LaB6-coated tungsten emitter to a closely spaced BaF2-coated InGaAs photovoltaic cell. Photon tunnelling and space charge removal across the nanoscale vacuum gap produce a drastic increase in flux of electrons and photons, and subsequently, of the generated electrical power density. We show that conversion efficiencies and electrical power densities of ∼ 30% and ∼ 70 W/cm2 are achievable at 2000 K for a practicable gap distance of 100 nm, and thus greatly enhance the performances of stand-alone near-field thermophotovoltaic devices (∼10% and ∼10 W/cm2). A key practical advantage of this nanoscale energy conversion device is the use of grid-less cell designs, eliminating the issue of series resistance and shadowing losses, which are unavoidable in conventional near-field thermophotovoltaic devices.

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.

Energy harvesting from lukewarm photons

Evanescent photons of a hot body at a temperature below 500 K can be efficiently converted into electricity in a near-field thermophotovoltaic device.

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.