Thermophotovoltaic Devices Research Articles

Large-area 1D selective emitter for thermophotovoltaic applications in the mid-infrared

Two- or three-dimensionally patterned subwavelength structures, also known as metamaterials, have the advantage of arbitrarily engineerable optical properties. In thermophotovoltaic (TPV) applications, metamaterials are commonly used to optimize the emitter’s radiation spectrum for various source temperatures. The output power of a TPV device is proportional to the photon flux, which is proportional to the emitter size. However, using 2D or 3D metamaterials imposes challenges to realizing large emitters since fabricating their subwavelength features typically involves complicated fabrication processes and is highly time-consuming. In this work, we demonstrate a large-area (78 cm2) thermal emitter. This emitter is simply fabricated with one-dimensional layers of silicon (Si) and chromium (Cr), and therefore, it can be easily scaled up to even larger sizes. The emissivity spectrum of the emitter is measured at 802 K, targeting an emission peak in the mid-infrared. The emissivity peak is ∼0.84 at the wavelength of 3.75 μm with a 1.2 μm bandwidth. Moreover, the emission spectrum of our emitter can be tailored for various source temperatures by changing the Si thickness. Therefore, the results of this work can lead to enabling TPV applications with higher output power and lower fabrication cost.

Semitransparent thermophotovoltaics for efficient utilization of moderate temperature thermal radiation

Recent advances in thermophotovoltaic (TPV) power generation have produced notable gains in efficiency, particularly at very high emitter temperatures. However, there remains substantial room for improving TPV conversion of waste, solar, and nuclear heat streams at temperatures below 1,100°C. Here, we demonstrate the concept of transmissive spectral control that enables efficient recuperation of below-bandgap photons by allowing them to transmit through the cell to be absorbed by a secondary emitter. We fabricate a semitransparent TPV cell consisting of a thin InGaAs-InP heterojunction membrane supported by an infrared-transparent heat-conducting substrate. The device absorbs less than 1% of below-bandgap radiation, resulting in a TPV efficiency of 32.5% at an emitter temperature of 1,036°C. To our knowledge, this represents an 8% absolute improvement (~33% relative) in efficiency relative to the best TPV devices at such low temperatures. By enabling near-zero photon loss, the semitransparent architecture facilitates high TPV efficiencies over a wide range of applications.

Material candidates for thermally robust applications of selective thermophotovoltaic emitters

Thermophotovoltaic (TPV) devices generate electric power from heat by using thermal emitters and photovoltaic effects. The radiation spectrum of the emitters impacts the power output and efficiency of the TPV device. Thus, sustainable TPV devices require emitters that are thermally robust and have the optimized radiation spectrum for the targeted wavelengths. Metals, due to their refractory and optical properties, are commonly used to make TPV emitters with wavelength-selective radiation. This work reviews the physical and chemical properties of 15 refractory metals and provides criteria for determining materials for TPV emitters.

Flame and heat transfer characteristics of a micro combustor with multihole baffle-induced inflows for a thermo-photovoltaic application

For an effective thermo-photovoltaic application, the inflow effects on the flame and heat transfer characteristics of micro power generators are numerically investigated in a multihole baffled micro combustor. The flow inside the baffle hole is considered as a circular pipe flow. Various inlet profiles of the micro combustor are obtained by varying the baffle thickness corresponding to the length of the baffle hole. The velocity distribution of the flow past the baffle plate attains an M-shaped (also called saddle-back), a parabolic, or a fully developed profile according to the baffle thickness. For a baffle thickness less than the fuel hole diameter, the inflow of the combustion chamber has an M-shaped velocity profile with two off-center peaks. Vortical motions and lobe-shaped flame are promoted by this type of inlet condition. Combustion characteristics, including flame length, combustion efficiency, heat loss ratio, and preheating effect, are comprehensively examined to evaluate the possibility as an thermo-photovoltaic device. The results indicate that a micro combustor with a thin baffle (bt ≤ 0.1Df ) increases the combustion efficiency and wall heat flux. This suggests that the current baffled combustor can be effectively applied in both micro power generation systems and micro thermal emitters.

Efficient and scalable GaInAs thermophotovoltaic devices

<h2>Summary</h2> 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 cm²), scalable, single-junction 0.75-eV GaInAs thermophotovoltaic devices reaching an efficiency of 38.8% ± 2.0% and an electrical power density of 3.78 W/cm² at an emitter temperature of 1,850°C. Reaching such a high emitter temperature and power density without sacrificing efficiency is a direct result of combining good spectral management with an optimized cell architecture, excellent material quality, and low series resistance. Importantly, fabrication of 12 high-performing devices on a 2-in 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, illustrated by early results for a two-junction 0.84-eV GaInPAs/0.75-eV GaInAs device.

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%.

Directionally Solidified Cobalt-Doped MgO-MgAl2O4 Eutectic Composites for Selective Emitters

Cobalt-doped MgO-MgAl2O4 eutectic composites were explored for their use as selective emitters for thermophotovoltaic devices. Eutectic ceramic rods with different cobalt content were directionally solidified by using the laser floating zone technique at two processing rates to obtain microstructures with different domain sizes. Thermal emission between 1000 °C and 1500 °C and optical properties (reflectance and transmittance) at room temperature were measured in the Co-doped composites and the effect on microstructure and cobalt content was investigated. Thermal emission consisted of an intense broad band at about 1.67 µm matching with the bandgap of the InGaAs cell. The emission was ascribed to the de-excitation from the 4T1(F) multiplet to the 4A2(F) ground state of the thermally excited Co ions located in the tetrahedral sites of the MgAl2O4 phase. The selectivity of the thermal emission showed a decrease with the cobalt content due to the enhancement of other electronic transitions, which leads to keeping the cobalt content in these composites at low levels (&lt;0.15% at Co) for their use as selective emitters.

Parametrical analysis of a novel CHP system based on solar-gas assisted thermo-photovoltaic device

To facilitate the achievement of the carbon neutrality goal, this study proposed a novel technological route for combined heat and power generation (CHP) system based on a solar-gas assisted thermo-photovoltaic (TPV) device to achieve fossil fuel savings and low-grade solar energy efficient utilization. A thermal-balance model is developed for system calculation and investigation. The effects of key parameters such as concentration ratio, tube receiver area, emitter area on the efficiency and electric-to-heat ratio of CHP system are determined, and the effects of the addition of solar energy on fuel and energy savings are analyzed. The results show that the addition of solar energy can not only increase the output power of TPV system by 40.15 kW/m2 and the TPV system electrical efficiency by more than 14 percentage points, but also can save up to 72.60 % of fuel and 39.72 % of energy. Moreover, each parameter should be designed rationally for different demands to control the electric-to-heat ratio to bring out the maximum potential of CHP systems. This study provides a reference for the design and application of TPV technology.

Improving the performance of steam power cycle through thermo-photovoltaic device: A novel combined system and thermodynamic analysis

In this study, a new system consisting of a thermophotovoltaic device and steam Rankine cycle system is proposed to prevent the mismatch between the combustion temperature and the steam maximum temperature to achieve photo-thermal energy cascade utilization and improve the fuel conversion efficiency. The energy balance model of the proposed system is established, and the influence of parameters such as emitter area and heat exchange coefficients on the system’s performance is investigated. Exergy analysis of the system is performed based on the second law of thermodynamics. The results show that the proposed system has 11–13% higher efficiency than the steam Rankine cycle system. The addition of a thermophotovoltaic device mainly reduces the exergy loss of the boiler due to the temperature difference between the combustion flue gas and the steam. Increasing the emitter area improves the system efficiency, and a system with Si cells is more efficient than a system with GaSb cells. Increasing the heat transfer coefficient of the air heat exchanger and the outlet flue gas temperature to generate a higher flame temperature is crucial to improving the performance of the combined system. The addition of a thermophotovoltaic device does not reduce the exergy loss during combustion. Thus, further energy cascade utilization should consider the efficient conversion of high-order chemical energy of a fuel. This study contributes to the theory of photo-thermal energy cascade conversion and provides new insights into improving the performance of the steam Rankine cycle.

GaAs thermophotovoltaic patterned dielectric back contact devices with improved sub-bandgap reflectance

We demonstrate GaAs thermophotovoltaic (TPV) devices with a patterned dielectric back contact (PDBC) architecture, featuring a dielectric spacer between the semiconductor and back metal contact over most of the back surface for high reflectance, and metal point contacts over a smaller area for electrical conduction. In the TPV application, high sub-bandgap reflectance is needed to reflect unused sub-bandgap photons to the thermal emitter to minimize energy losses in this portion of the thermal spectrum. We explore different PDBC fabrication processes with SU-8 and SiO 2 dielectric spacer layers to maximize sub-bandgap reflectance while minimizing series resistance to increase TPV conversion efficiency. We successfully demonstrate GaAs SU-8 PDBC TPV devices with 2200 °C blackbody-weighted sub-bandgap reflectance of 94.9% and 96.5% with and without a front metal grid, respectively. This is 0.7% and 2.3% (absolute) higher than the mean sub-bandgap reflectance of 94.2% for GaAs baseline TPV devices with 100% Au back contact with front metal grid. Lower sub-bandgap reflectance in TPV devices with front grids indicates the front grid induces light scattering leading to additional parasitic absorption in the TPV device. We also show that for higher contact coverage fractions, the PDBC reflectance cannot in general be treated by a linear interpolation using simple 1D transfer matrix method modeling and should be treated instead as a diffraction grating by solving Maxwell's equations in 3D. • We explore different patterned dielectric back contact (PDBC) fabrication processes with SU-8 and SiO 2 for TPV cells. • Sub-bandgap reflectance over 96% demonstrated using dielectric-metal point contact rear mirror structure, a gain of >1%. • PDBC structures should be modeled as a 3-dimensional diffraction grating by solving Maxwell's equations. • Front grids increase diffuse reflectance thus increasing absorption in the device.

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.

Narrow bandgap photovoltaic cells

Research activities and progress in narrow bandgap (<0.5 eV) photovoltaic (PV) cells for applications in thermophotovoltaic (TPV) systems are reviewed and discussed. The device performance and relevant material properties of these narrow bandgap PV cells are summarized and evaluated. Issues and factors that affect narrow bandgap PV device performance are then considered. Narrow bandgap interband cascade PV devices are presented as a promising route to circumvent or alleviate limiting factors in conventional narrow bandgap TPV devices. The operation principle and special features of interband cascade PV devices are described, along with theoretical modeling and experimental results. Finally, a perspective and outlook is provided in conjunction with the review process. • Bulk narrow bandgap materials have inherent limitations such as a low absorption coefficient and a short diffusion length. • A multi-stage interband cascade architecture circumvents the low absorption coefficient and short diffusion length limitations of bulk materials in photovoltaic applications. • Interband cascade photovoltaic device technology would be further improved by incorporation of other innovations.

Broadband and wide-angle solar absorber for the visible and near-infrared frequencies

A polarization-insensitive broadband and wide-angle solar absorber in the visible to near-infrared regions is proposed and studied. The proposed absorber is in the form of an iron square ring array separated from an iron reflector by a dielectric spacer and a phase change material (Ge2Sb2Te5) layer. About 98% averaged absorption in the spectral range of 380–2000 nm is realized. The electromagnetic filed intensity distribution at several selected absorption peaks is studied to provide physical understanding behind the broadband absorption effect. Besides, the high absorption performance remains stable within a large viewing angle and the designed absorber shows certain dimensional tolerance, which decreases the fabrication complexity and cost and should be attractive for practical applications. Moreover, the potential applications of the proposed scheme on the solar energy harvesting are also illustrated in the last. It is believed that the proposed absorbers should find potential applications in solar energy capturing and conversion, thermo-photovoltaic devices and thermal emitters.

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.

Comprehensive analysis of an optimized near-field tandem thermophotovoltaic converter

It is well known that performance of a thermophotovoltaic (TPV) device can be enhanced if the vacuum gap between the thermal emitter and the TPV cell becomes nanoscale due to the photon tunneling of evanescent waves. Having multiple bandgaps, multi-junction TPV cells have received attention as an alternative way to improve its performance by selectively absorbing the spectral radiation in each subcell. In this work, we comprehensively analyze the optimized near-field tandem TPV system consisting of the thin-ITO-covered tungsten emitter (at 1500 K) and GaInAsSb/InAs monolithic interconnected tandem TPV cell (at 300 K). We develop a simulation model by coupling the near-field radiation solved by fluctuational electrodynamics and the diffusion-recombination-based charge transport equations. The optimal configuration of the near-field tandem TPV system obtained by the genetic algorithm achieves the electrical power output of 8.41 W/cm$^2$ and the conversion efficiency of 35.6\% at the vacuum gap of 100 nm. We show that two resonance modes (i.e., surface plasmon polaritons supported by the ITO-vacuum interface and the confined waveguide mode in the tandem TPV cell) greatly contribute to the enhanced performance of the optimized system. We also show that the near-field tandem TPV system is superior to the single-cell-based near-field TPV system in both power output and conversion efficiency through loss analysis. Interestingly, the optimization performed with the objective function of the conversion efficiency leads to the current matching condition for the tandem TPV system regardless of the vacuum gap distances.

Ultra-Broadband, Polarization-Irrelevant Near-Perfect Absorber Based on Composite Structure.

This paper proposes a near-perfect absorption device based on a cross-shaped titanium nanostructure and a multilayered structure. The multilayered bottom structure consists of alternately SiO2 and Ti. The whole device is put on a TiN substrate. The coupling between cross-shaped titanium nanostructures, and that between the cross-shaped titanium nanostructure and bottom multilayer, can further enhance the absorption at some wavelength where most of the energy is reflected or passes through in the device with a single structure. According to the simulation results, the device presents a nearly perfect absorption in a wavelength range from 300 nm to 2000 nm. The average absorptance in the wavelength range from 500 nm to 1400 nm exceeds 96%. This paper also provides a new idea for realizing perfect absorption, which is extensively used in sensing, controllable thermal emission, solar energy harvesting solar thermo-photovoltaic devices, and optoelectronic metrology.

Wide-angle and broadband solar absorber made using highly efficient large-area fabrication strategy

High performance and cost-effective solar absorbers are crucial for various optical applications, such as solar collection and thermophotovoltaic devices. This study designs and experimentally demonstrates a wide-angle and broadband solar absorber. The proposed absorber is composed of tapered polyimide substrate and Al-Cr-SiO2-Cr-SiO2 thin-film based on the optical interference of the multilayer thin film and excited magnetic resonance of light-trapping structures. The composite process of the colloidal lithography method and magnetron sputtering is employed for efficient fabrication in a large area. The average absorbance is more than 93% from 300 nm to 2500 nm and shows an angular tolerance of up to 60°. The high efficiency and large-area fabrication capability demonstrated by the proposed solar absorber presents future application potential in flexible solar collection devices.

Design and Optimization of One-Dimensional TiO2/GO Photonic Crystal Structures for Enhanced Thermophotovoltaics

In this paper, we theoretically explore the spectroscopic features of various one-dimensional photonic crystal (1D-PC)-based spectrally selective filters. The 1D-PC structure is composed of alternating layers of titanium dioxide (TiO2) and graphene oxide (GO). Employing the transfer matrix method (TMM), the impacts of the incidence angle, the number, and thicknesses of TiO2/GO layers in various 1D-PC stacks on the spectroscopic features of the filters are explored in detail. The proposed 1D-PC structures are designed for practical use for thermophotovoltaic (TPV) applications to act as filters that selectively transmit light below 1.78 μm to a GaSb photovoltaic cell, while light with longer wavelengths is reflected back to the source. The optimal design presented here consists of two Bragg quarter-wave 1D-PC filters with different central frequencies stacked to form a single structure. We demonstrate that our optimized 1D-PC filter exhibits a large omnidirectional stop band as well as a broad pass band and weak absorption losses. These features meet the fundamental exigencies to realize high-efficiency TPV devices. Additionally, we show that when integrated in a TPV system, our optimized filter leads to a spectral efficiency of 64%, a device efficiency of 39%, and a power density of 8.2 W/cm2, at a source temperature of 1800 K.

N-type doping of SiC-passivated Ge by pulsed laser melting towards the development of interdigitated back contact thermophotovoltaic devices

In this article, a method for phosphorous (n-type) doping of germanium based on spin-on dopant sources and Pulsed Laser Melting (PLM) throughout an amorphous silicon carbide (a-SixC1-x:H) layer, which provides both surface passivation and electrical isolation, has been demonstrated, paving the way towards the development of Ge-based interdigitated back contact thermophotovoltaic devices. This method offers simultaneous opening of the a-SixC1-x:H layer and creation of a heavily doped region underneath without using photolithographic steps, eventually enabling a low-cost and scalable manufacturing process. This article focuses on the optimization of the n+/p junction formation by studying the effect of different laser energy fluences and number of pulses on the diffusion profiles measured by secondary ion mass spectrometry, and on the electrical performance characterized by Van der Pauw-Hall technique. Additionally, the crystalline quality after PLM has been analyzed by Rutherford backscattering measurements in channeling conditions, high-resolution X-Ray diffraction and transmission electron microscopy. High level of donor activation (up to 1·1019 cm−3), low sheet resistance (≈50 Ω/□), and high mobility (275–700 cm2/V·s) have been obtained, with a weaker dependency of these parameters on the explored laser energy fluence range. A prototype diode has been developed demonstrating a rectifying behavior but with high saturation current densities. Point-like contact formation will be implemented in future works to reduce the laser irradiated area, and thus, improve the surface passivation and device characteristics.

GeSn (0.524 eV) single-junction thermophotovoltaic cells based on the device transport model

Based on the transport equation of the semiconductor device model for 0.524 eV GeSn alloy and the experimental parameters of the material, the thermal–electricity conversion performance governed by a GeSn diode has been systematically studied in its normal and inverted structures. For the normal p+/n (n+/p) structure, it is demonstrated here that an optimal base doping N d(a) = 3 (7) × 1018 cm−3 is observed, and the superior p+/n structure can achieve a higher performance. To reduce material consumption, an economical active layer can comprise a 100 nm–300 nm emitter and a 3 μm–6 μm base to attain comparable performance to that for the optimal configuration. Our results offer many useful guidelines for the fabrication of economical GeSn thermophotovoltaic devices.