Advances in thermophotovoltaics: materials, devices and systems

Resume : Unlike conversion of solar radiation to electricity using single-junction solar cells, which is limited to efficiencies around 30%, the efficiency of converting thermal radiation into electricity using single-junction TPV cells can exceed 50% through a process called photon recycling. Specifically, thermal radiation that is too low in photon energy to excite electronic transitions in the cell is reflected back to the hot radiator, where it has an opportunity to contribute toward emission of higher energy photons that can be converted directly to electricity. To facilitate this process, we recently created a cell architecture that has a thin air layer behind the light-absorbing semiconductor. The resulting cell (a so-called air-bridge cell, or ABC) reflects back almost all of the low-energy photons, which is essential to reaching high efficiency. Here, I will discuss the development of an InGaAs ABC that achieved a record-high peak conversion efficiency of 32% [1] and our recent efforts to improve performance and power density. [1] Fan, D., Burger, T., McSherry, S. et al. Near-perfect photon utilization in an air-bridge thermophotovoltaic cell. Nature 586, 237–241 (2020). https://doi.org/10.1038/s41586-020-2717-7

Resume : Fuel-based thermophotovoltaic (TPV) systems are emerging as a viable power source for small, portable generators for a spectrum of applications such as UAV's, robotic platforms, and sensors. In TPV systems, an emitter heated to above 1000K emits radiation that is then converted to electricity by a low bandgap photovoltaic cell. One promising class of TPV emitters are two-dimensional photonic crystals (PhCs) made of tantalum, which have shown high-temperature stability at 1150-1250K over 100s of hours [1] and have been implemented in a prototype system with 4.4% fuel-to-electricity efficiency [2]. Tantalum PhCs filled and capped with hafnium oxide can enable even higher optical performance with in-band emissivities of 0.8-0.9. However, two key features are difficult to realize simultaneously: a uniformly filled cavity and a thin capping layer of hafnium oxide [3]. Here, we present a process that results in reduced roughness and better thickness control of the capping layer. We use room temperature reflectance measurements and full system simulations to estimate system performance gains. This selective emitter paves the way toward efficient, practical, and portable mesoscale generators. [1] V. Stelmakh. PhD thesis, Massachusetts Institute of Technology, 2017 [2] W.R. Chan et al. Energy Environ. Sci., 10:1367, 2017 [3] V. Stelmakh et al. J. Phys.: Conf. Series, 773, 012037, 2016

Resume : Low-temperature thermophotovoltaics (TPV) demand very low band gap (< 0.5 eV) semiconductors to maximize output power. As the band gap narrows down, so does the number of available materials. Current technology is mostly based on epitaxially-grown alloys of III-V elements, such as InGaAs(Sb), which present some limitations. First, it is not always possible to obtain the desired band gap, due to technological constraints. Second, their fabrication methods are expensive, an issue that becomes more and more important as lower temperature systems are aimed, because the output TPV power density diminishes rapidly with temperature. Colloidal quantum dots (CQD) are an interesting alternative to epitaxial materials for low-temperature TPV. The band gap of these nanocrystals can be tunned precisely during their synthesis by changing their size. Thus, in principle, any desired optimum bad gap for low-temperature TPV could be achieved by choosing the right combination of material and nanocrystal size. In addition, CQDs are fabricated by low-cost, wet chemical methods. These characteristics allow envisaging efficient, low-cost TPV cells. We give an overview of the potential of CQDs as photovoltaic absorbers for low-temperature TPV devices and review the state-of-the-art.

Resume : In near-field thermophotovoltaics, extreme proximity between a thermal radiator and a photovoltaic (PV) cell allows radiation intensities much greater than the classical blackbody radiation limit. Such enhancement could enable TPV systems operating at low temperatures (< 900 K), thus providing an attractive solution for recycling of waste heat into electricity. However, achieving the required sub-100 nm separation between a hot surface and a PV cell, while maintaining a strong temperature gradient and avoiding contact is a significant technical challenge, which so far has prevented development of practical near-field TPV devices. Our vision for overcoming this challenge relies on micromechanically (MEMS) actuated hot surfaces, which can be actively positioned in the near-field of a PV cell. In our most recent work, we have successfully integrated one of these surfaces with a germanium PV cell, while respecting several key requirements of near-field TPV. We demonstrate >500 K thermal gradient between the tungsten radiator and the PV cell, and we achieve active positioning of the radiator within 100 nm of the PV cell using low-power electrostatic actuation. The conversion efficiency (< 1%) and power density (1,25 uW per square cm) of our system are currently limited by the use simple proof-of-concept Germanium PN junctions. Development of optimized PV cells dedicated to Near-Field TPV therefore appears as the next requirement for successful development of this technology.

Resume : Thermophotovoltaics (TPVs) convert thermal radiation into electricity from local heat sources. Huge amount of effort has been directed toward the implementation of high-efficiency and compact thermophotovoltaic cells, which enable the potential of widespread applications in local thermal energy storage, waste heat recovery, aerospace power generation and direct solar energy conversion. Besides engineering a spectral-efficient hot emitter, there are two other promising approaches to improve the performance of a TPV system. One is to enhance the local photogeneration in the active regions of the cell, and the other is to suppress the radiative loss due to sub-bandgap photon absorption and above-bandgap photon absorption outside the active regions of the cell. Backside surface reflectors are frequently utilized as an electrode and as a mirror for long wavelength photon. In this work, a thin TPV system consisting of an optimized Drude emitter and an InAs p-n junction is numerically investigated with an iterative model combining fluctuational electrodynamics and the full-drift-diffusion equations. A thickness-tunable air-gap back reflector is applied at the back surface of an InAs TPV cell, which is compared to a conventional rear metallic mirror to demonstrate photogeneration enhancement and radiative loss reduction for both the far- and near-field regimes. Fabry-Perot cavity like modes are utilized to improve the photogeneration with the variation of the thickness of the air gap. The photogeneration can be further enhanced by adding a platinum front coating on the TPV cell with thickness-optimized air-gap back reflector. To completely analyze the improvement on the performance by the front coating and back reflector, surface recombination is considered with a charge transport model for different surface conditions. The results of current-voltage and efficiency voltage characterization solved by this iterative model are shown to guide the design for a high-efficient TPV system.

Resume : Increasing power demand for applications ranging from remote instrumentation to drones is driving the development of ultra high energy density power sources in the 5-500 W range. By harnessing the high energy density of hydrocarbon fuels using a thermophotovoltaic conversion process, we can offer an order-of-magnitude increase in energy density compared to batteries. We use a 2D tantalum photonic crystal to match the emission spectrum to the convertible range of the low bandgap PV cells. In addition to good optical performance, the photonic crystal is stable at high temperatures, can be fabricated in large areas, and can be integrated with the rest of the system. In this presentation, we discuss how we developed a practical photonic crystal. Then we provide an overview of the challenges in realizing a full benchtop system. Finally, we present the work currently being undertaken to build a commercially viable TPV system.

Resume : Despite, or possibly because of, the complexity involved in designing a thermophotovoltaic-based power generator, researchers traditionally focus on component maturation, whether solely, on photovoltaics, spectral control, or heat sources. Even groups with an expressed systems' focus tend to have a component preference. This leaves a design space of unexplored possibilities and few mechanisms to identify a viable system path. To help navigate the space, we present a numerical tool – and supporting experimental work – that quantifies how individual components and their couplings, in a fuel-fired TPV system, determine overall system performance, from fuel to photovoltaic, including balance of plant elements. Two sub-models are employed in this effort: Emitter-Cavity-TPV (ECT), which incorporates empirical findings to determine TPV conversion efficiency, effective emitter flux, cell thermal management and a boundary condition required to fully define the second model, Reduced Order Chemical Reactor (ROCR), which relates fuel flow requirements to emitter surface temperatures, thereby completing the energy chain of custody. In addition, by including properties of heat exchangers and estimates of parasitic power consumption, the limits of what can be practically achieved in power, efficiency, size, and weight become clear. We illustrate the use of this model in the service of designing a portable power generator by exploring the material performance requirements for emitters and receivers.

Resume : ABSTRACT Both solar cells and batteries generate quiet DC electric power. However, while solar cells are light weight, they only operate when the sun is shining. A fuel fired thermophotovoltaic (TPV) generator can operate day and night. The development of a light weight fuel fired TPV generator will be described here. In TPV, infrared (IR) sensitive GaSb photovoltaic cells convert energy from a combustion heated glowing ceramic IR emitter into electricity. We present here the design and operation of a first stand alone TPV generator complete with a photovoltaic converter array and a burner / emitter recuperator assembly and support components. This first unit has a durable NiO/MgO ceramic IR emitter operating at 1150 C and a 108 cell photovoltaic converter array tested at up to 50 W. The TPV unit also has a novel omega recuperator to preheat the combustion air increasing the system efficiency. With all the assemblies operating together, the complete TPV unit generates 24 W with the emitter operating at 1150 C and the array operating at 60 C. Modeling presents a path to improve the recuperator to allow the emitter to operate at 1300 C thereby increasing the IR emitted power and therefore the GaSb array and TPV generator should produce 50 W. A portable light weight TPV power supply has applications both for soldiers as well as for unmanned aerial vehicles (UAVs).

Resume : A TPV generator consists of two subassemblies, a central cylindrical Burner - IR Emitter - Recuperator (BER) subassembly surrounded by a second air cooled IR PV Converter Array subassembly (PCA). In 2016, JXC integrated GaSb cells into a cylindrical lightweight PCA compatible with the recently demonstrated BER along with an ignition and control system for a first portable stand alone TPV demonstrator. This demonstrator established a baseline performance for a TPV system. We now envision scale up designs all built around an infrared power converter module (PCM) as a building block. The power converter module (PCM) described here consists of a 54 GaSb cell circuit with improved GaSb cell performance. Each PCM is 74 mm (2.9 ?) x 88 mm (3.5?) with an active 6 cell length of 53 mm (2.1?) and has a projected weight of 125 g. Two of these PCMs can be combined into a TPV generator with a projected power output of 50 W (200 W/kg). Three PCMs can be combined into a 100 W TPV generator tube and four 100 W TPV tubes with 12 PCM can be combined into a quiet light weight TPV generator with a projected power output of 400 W. Herein test data are presented for a PCM producing 25 W at a current of 4.5 A. The 4.5 A is the expected current for operation around a BER with an emitter temperature of 1300 C.

Resume : Thermophotovoltaic (TPV) systems are attracting increasing attention for their potential to realize compact and high-efficiency power generation. To boost the conversion efficiency of TPV, it is important to enhance thermal emission above the bandgap energy of the PV cell and simultaneously suppress sub-bandgap emission. Here, we show our recent experimental demonstrations of far-field and near-field TPV systems based on intrinsic silicon thermal emitters and InGaAs PV cells. We employ intrinsic silicon because it exhibits a step-like increase of absorptivity (emissivity) in the near-infrared range owing to the interband absorption when the thickness is properly adjusted. In the far-field experiment, we develop silicon rod-type photonic crystal thermal emitters and demonstrate near-infrared frequency-selective thermal emission with suppressed long-wavelength emission. Through the quantitative measurement of the input heat flux and the electrical output power, we obtain a heat-to-electrical conversion efficiency of 11.2% at an emitter temperature of 1338 K. In the near-field experiment, we develop a one-chip near-field TPV device integrating a thin-film Si emitter and InGaAs PV cell with an intermediate Si substrate. We realize a deep sub-wavelength gap (<150 nm) and a large temperature difference (>700 K) between the emitter and the intermediate substrate, achieving 10-fold enhancement of the photocurrent compared to a larger-gap (>µm) device at the same temperature.

Resume : In nanoscale gaps, the radiative heat transfer between two objects can go beyond the Planck's blackbody limit by several orders of magnitude. This extraordinary phenomenon (called near-field thermal radiation) is due to additional energy transport by photon tunneling in the near field. As a compelling application of near-field thermal radiation, a near-field thermophotovoltaic (TPV) energy conversion system that can directly convert the heat to electricity has widely been investigated. For practical use of the near-field TPV system, the near-field thermal radiation measurement between parallel plates with the large surface area should be preceded. However, maintaining nanoscale gaps between two surfaces with a large area is extremely challenging. To overcome the challenges in achieving the nanogap between large surfaces, focus has now shifted to enhancing the near-field thermal radiation at a given gap distance via coupled surface plasmons instead of further reducing the vacuum gap distances. This presentation will provide an overview of our efforts of controlling the near-field radiative heat transfer at experimentally achievable vacuum gaps (~ 200 nm) as well as developing a Schottky-junction TPV cell for energy conversion experiments.

Resume : Electrical power output of thermophotovoltaic (TPV) cells can be drastically increased when the infrared emitter is brought in the near field (< 4 µm at 700 K), where the radiative heat exchange is enhanced by several orders of magnitude due to the contribution of evanescent waves. Near-field TPV conversion experiments were reported only recently [1–3], unfortunately with very low output power densities (<10 W.m-2) and conversion efficiencies (<1 %). We first characterized experimentally very-low energy band gap (0.23 eV) TPV cells [4] made of indium antimonide, which require operating below room temperature (77 K). Near field performances of the cells were then assessed with a hot graphite spherical emitter (temperature up to 1000 °C). Impacts of emitter material and cell design on near-field radiative heat transfer and TPV conversion were investigated as a function of emitter-cell distance down to a few tens of nanometers. We demonstrated a 6-fold enhancement of the electrical power photogeneration compared to far-field illumination, leading to power densities as high as 0.75 W.cm-2, 3 orders of magnitude higher than [1–3], and near-field cell conversion efficiency larger than 14 % [5]. [1] A. Fiorino et al., Nature Nanotechnology, 13, 806-811 (2018) [2] T. Inoue et al., Nano Letters, 19, 3948‑3952 (2019) [3] G. R. Bhatt et al., Nature Communications, 11, 2545 (2020) [4] D. Cakiroglu, et al. Solar Energy Materials and Solar Cells, 203, 110190 (2019) [5] C. Lucchesi, et al., arXiv:1912.09394 (2019)

Resume : Thermophotovoltaic (TPV) power generators offer great possibilities for thermal energy conversion when thermal sources with temperatures near 1000 K are available. While the power density of conventional TPV systems is generally determined by Planck's law in the far field, their fundamental performance is known to be dramatically affected by near field effects between the thermal emitter and the photovoltaic cell. In addition, it may be possible to greatly enhance the power density by transforming the thermal emitter to exploit electroluminesce. Taking advantage of an electroluminescent emitter as the source of radiation fundamentally alters the thermodynamics of the system. This allows boosting the achievable power densities by orders of magnitude, and also provides access to electroluminescent and thermophotonic (TPX) heat pumps. In theory, the resulting TPX devices can outperform both TPV and thermoelectric heat engines, and potentially compete even with mechanical thermodynamic machines. In more practical terms functional thermophotonic devices are yet to be demonstrated experimentally, due to several material and design bottlenecks. Here we discuss the thermodynamics and ideal performance of the TPX devices and the ongoing efforts aiming to observe the related effects in practice.

Resume : The performance of a thermophotovoltaic (TPV) system can be enhanced when a gap between a high-temperature thermal emitter and a TPV cell is smaller than a thermal characteristic wavelength. In this near-field TPV (NFTPV) system, a large amount of photon energy exceeding the blackbody limitation can be delivered to the TPV cell owing to the additional contribution of evanescent modes. However, unlike the obvious enhancement of the electrical power throughput of NFTPV resulted from increased radiative energy transport, the power conversion efficiency (PCE) of the NFTPV system does not always increase much compared to that of conventional far-field TPV (FFTPV) system. This is mainly due to the thermalization caused by excited electrons over band edges and the sub-bandgap absorption. In order to consistently enhance both electrical power generation and PCE of NFTPV system, the surface of the emitter and TPV cell has been modified using nanostructures to tailor the spectrum of radiative energy transport. Besides this optical tuning means, recent studies in TPV system have also tried to tune the structure of TPV cell. In particular, a multi-junction TPV cell that is able to surpass the Shockley-Queisser limit of a single-junction device has been suggested. Monolithically stacked tandem cells have a broad spectral conversion range by low bandgap sub-cell while having a large open-circuit voltage due to the relatively low dark current of the high bandgap sub-cell. Therefore, the introduction of the multi-junction TPV cell can lead to a reduction of the thermalization and electrical loss, resulting in an increase of PCE. In this work, we establish a theoretical analysis model for the multi-junction-cell-based NFTPV system and analyze the performance of the NFTPV system consisting of the tungsten emitter and GaSb/InAs monolithic interconnected sub-cells. To determine the spectral and spatial photocurrent generated inside the TPV sub-cells considering the surface and bulk recombination, the continuity equation for the diffusion of minority carriers is solved employing a semi-analytic method. An approximate semi-analytical solution can be derived using the spatial distribution of absorbed energy flux attained by separating the forward and backward propagating radiative heat flux in each sub-cell. Accordingly, the time required for the calculation of the photocurrent generated in each sub-cell can be significantly reduced. Adopting the proposed analysis model, we validated that the electrical power throughput and PCE increase as the tungsten emitter gets closer to the GaSb/InAs tandem cells. Furthermore, it was confirmed that the performance of tandem TPV cells can exceed that of GaSb or InAs single-junction TPV cell. Since the reduction of the calculation time is an essential requirement for the optimization, we are convinced that this analysis method will promote the investigation of the optimized configuration of the tandem-cells-based NFTPV system.

Resume : Thermophotonics (TPX) is a technology close to thermophotovoltaics (TPV), where a heated light-emitted diode (LED) is used as the active emitter of the system [1]. With the development of LEDs and the increase of their achievable quantum efficiency, TPX has come out as an attractive concept for both energy harvesting and refrigeration [2]. The many studies on near-field (NF) thermal radiation and their application into efficient NF TPV devices [3] highlight the possibility to extend the concept to near-field thermophotonics [4], where enhanced energy conversion is due to both the electric control and wave tunneling. This contribution explores the theoretical capabilities of NF-TPX systems. Ideal cases are compared with more realistic structures, involving materials such as Si, GaAs and GaN. Based on the local absorption and emission distributions [5], the results include detailed IV characteristics of the LED and PV cell sides around the maximum power point, and highlight in particular the advantages in comparison to far-field TPX and NF TPV. The impact of the temperature difference between the LED and the PV cell, the thickness of the different elements and the quantum efficiency of the LED are amongst the studied parameters. [1] N. P. Harder and M. A. Green, Semicond. Sci. Technol. 18, S270, 2003. [2] T. Sadi et al., Nat. Phot. 14, 205, 2020. [3] C. Lucchesi et al., arxiv: 1912.09394, 2019. [4] B. Zhao et al., Nano Lett. 18, 5224, 2018. [5] M. Francoeur et al., J. Quant. Spectr. Rad. Transf. 110, 2002, 2009. We acknowledge the funding of EU H2020 FET Proactive (EIC) programme through project TPX-Power (GA 951976).

Resume : Germanium (Ge) is a low-cost semiconductor alternative for the development of thermophotovoltaic (TPV) devices. Previous works on Ge-based TPV devices are scarce with a current record efficiency still low (16.5%, which is only ~ 27% of its thermodynamic limit), and requiring high-capital cost techniques for surface passivation and doping. Thus, there is still a need for the development of truly low-cost and high-efficient Ge-based TPV devices. To address simultaneously both challenges, we propose the development of interdigitated back contact (IBC) Ge-TPV cells whose selective contacts are made by pulsed laser melting the passivated Ge surface. The IBC design (with both contacts at the rear side) eliminates the shadowing losses allowing higher conversion efficiencies and power densities. Also, the use of pulsed laser melting substitutes the most common technologies used up to now for Ge-doping, reducing the complexity and cost of Ge-based devices. Finally, the laser approach enables localized doping, which is of particular interest for IBC devices where the two types of contacts must be defined on the same surface. As a first step for the development of IBC Ge-based devices, we have experimentally investigated the Ge surface passivation. The process is based on in-situ H2 plasma cleaning and intrinsic amorphous silicon carbide layer (a-SixC1-x) deposition at different temperatures by plasma-enhanced chemical vapor deposition (PECVD). Effective minority carrier lifetimes higher than 1000 µs and estimated surface recombination velocities as low as 20 cm/s have been obtained for intrinsic Ge. As a second step, we have developed a method for doping Ge based on pulsed laser melting. First, laser melting of a phosphorus Spin-on Dopant (SoD) solution deposited on bare Ge has resulted in electron active concentration higher than 1·1019 cm-3 with high-mobility 350-450 cm2V-1s-1. Phosphorous doping throughout the a-SixC1-x passivating layer has been also investigated. Preliminary experiments show a high incorporation of phosphorus in Ge, as deduced from Secondary Ion Mass Spectrometry. In the case of p-type doping, very low specific contact resistance (~ 4·10-4 Ωcm) and high back surface reflectivity have been obtained by the laser irradiation of an aluminum layer deposited by e-beam evaporation on the a-SixC1-x passivating layer. As a next step p /n and n /p diodes will be done as the last preparatory work towards the fabrication of the Ge-IBC devices, which have a theoretically predicted efficiency in the order of 20% for an emitter temperature of 1500ºC.

Resume : The H2020 FET-Open AMADEUS Project is focused on the development of an innovative solid-state conversion module capable to store and generate power at high temperature (>1000 °C) exploiting the high-concentrating-ratio radiation of parabolic solar concentrators. The related novel conversion module is developed for energy production based on hybrid thermionic-photovoltaic (TIPV) direct converters. The TIPV device produces high electronic and photonic fluxes to convert heat directly and efficiently into electric power. Once demonstrated the advantages of the scientific concept with respect to mere thermionic energy converters, consisting of an additional voltage boost derived from the photovoltaic cell operation (0.5-1.0 V depending on the active semiconductor employed) and of a significantly enhanced output power (one or two orders of magnitude depending on the anode surface engineering), the activity is now focused on the development of robust and low work-function thermionic elements able to manage large power densities. The TIPV cathodes, formed by nanostructured lanthanum boride films produced on refractory metals for the first time via femtosecond Pulsed Laser Deposition at room temperature and high deposition rates (up to 190 nm/min), achieved a work function as low as 2.60 eV. Such a result is extremely significant since it is comparable to that of single-crystal LaB6 but provided by a low-cost and large-area material. The transparent anode coating, formed by sub-nanometer layers of barium fluoride on gallium arsenide cells, allowed achieving a work-function of 2.1 eV. The talk will discuss both the materials’ development strategy and the latest encouraging results of thermal-to-electrical energy conversion.

Resume : Hybrid perovskite materials have been proposed for thermoelectric applications as they prove to have very low thermal conductivity. However, in order to achieve high thermoelectric performances, doping is required to enhance electrical conductivity. Here, we propose a novel synthesis for zero-dimensional methylammonium bismuth iodide (CH3NH3)3Bi2I9 single crystals and thin-films. Bismuth-based hybrid perovskite has advantages over the more famous lead-based perovskite for higher stability and nontoxicity. In order to keep production costs low, our materials are synthesised in the air ad employing common and cheap solvents. The synthesised materials have been characterised by Uv-Vis, micro-Raman, XRD, EDX spectroscopy measurements, and their morphology studied by SEM. Electrical and thermal conductivity are studied varying the composition: methylammonium bismuth iodide has been doped by antimony to speed up the crystallisation process and to obtain compact and pinhole-free thin-films; tin and sulphur have been added to decrease exciton binding energy, and to increase the carrier concentration and mobility.

Resume : To date, the champion thermophotovoltaic (TPV) cells have an efficiency in the range 24-32% [1-4]. They are all made of InGaAs with an energy bandgap of 0.6 or 0.74 eV and convert thermal radiation from emitters at temperatures larger than 1000 °C. For medium-grade heat sources (<700 °C), cells with a lower energy bandgap (e.g. 0.36 eV with InAs [5]) are required in order to efficiently collect the infrared photons. Since it is currently challenging to build large temperature differences between bodies separated by sub-micrometric distances, the requirement of a low-energy bandgap cell is stringent for near-field thermophotovoltaic converters. This communication reports on the specific design [6], fabrication and characterization [7] of micron-sized indium antimonide TPV cells (energy bandgap of 0.23 eV at 77 K) for the purpose of proving that photovoltaic conversion of near-field thermal photons can be efficient [8]. The fabricated cells exhibit excellent performances in the dark, under far-field and near-field illuminations. In the near field, the key parameters are the size of the cell, the doping of the p-layer and the thickness of the substrate. [1] Fan et al., Nature 586, 2020; [2] Omair et al., PNAS 116, 2019; [3] Woolf et al., Optica 5, 2018; [4] Wernsman et al., IEEE TED 51, 2004; [5] Krier et al., J. Electronic Materials 45, 2016; [6] Vaillon et al., Optics Express 27, 2019; [7] Cakiroglu et al., SEMSC 203, 2019; [8] Lucchesi et al., arXiv:1912.09394, 2019.

Resume : Several works have experimentally demonstrated near-field radiative heat transfer (NFRHT) exceeding the far-field blackbody limit between planar surfaces. However, owing to the difficulties associated with maintaining the nanosize vacuum gap spacing required for measuring a substantial near-field enhancement, these demonstrations have been limited to experiments that cannot be implemented in actual engineering devices. In this work, we describe devices bridging laboratory-scale measurements and potential engineering application of NFRHT to near-field thermophotovoltaics. These devices consist of an emitter and a receiver substrate (5 × 5 mm2) made of doped silicon separated by SU-8 micropillars having diameters of either 20 or 30 um. 4.5-um-deep, 215-um-diameter pits are etched into the emitter substrate where the micropillars are manufactured. The pits enable devices with micropillars significantly taller than the vacuum gap spacing (~ 4.5 to 45 times taller), thus minimizing parasitic conduction without sacrificing device structural integrity. The robustness of our devices enables gap spacing visualization via scanning electron microscopy (SEM) prior to performing NFRHT measurements. We successfully fabricated and characterized six NFRHT devices with vacuum gap spacing from ~ 1000 nm down to ~ 110 nm. The measured NFRHT is in good agreement with fluctuational electrodynamics simulations. We measured a maximum NFRHT enhancement of ~ 28.5 with respect to the blackbody limit for the smallest gap device. Extending micropillar length to a few micrometers while keeping the vacuum gap spacing from ~ 110 to 1000 nm substantially increases the thermal resistance by conduction between the emitter and receiver. For the smallest gap device, the contribution of conduction to the total heat rate would increase from ~ 1.9% with pits to 45% without pits. Despite the large enhancement of NFRHT, a pit-free-device would be unusable for near-field thermophotovoltaic energy conversion where heat conduction is detrimental to device performance. Our devices constitute an important step towards realizing near-field thermophotovoltaic devices.

Resume : 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 × 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 × 1 mm2 device is scaled up for a 5 × 5 mm2 device, the maximum power output is only increased by a factor of ~ 1.08 with respect to the 1 × 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.

Resume : The microstructure-based material has extraordinary controllability of spectral properties of thermal radiation for various energy conversion systems such as thermophotovoltaics, concentrating solar power systems, and so on. The microstructure size should be in the order of thermal radiation wavelength of interest and excellent geometric control in fabrication. In addition, the large-scale fabrication methods of periodic microstructure are significant for practical realization. Several large-scale fabrication methods with high-throughput and low-cost are developed to promote the industrialization of practical application However, such methods are afflicted with structural defects such as vacancy or distortion of microstructure which are randomly distributed in the surface. The expected optical performance is suffered from structural defects in fabrication. For example, the radiation intensity is weakened with the vacancy of the microstructure; peak broadening and shift occur when the distortions of the microstructure. The gap between feature sizes of structural defects and the sample sizes has several orders of magnitude, which inducing the challenge to assess the optical performance of material containing structural defects. We propose a novel approach to reveal the microstructure condition using the diffraction imaging system and to assess the optical performance. With the laser beam scan, the information of microstructures and fine defects on samples are mapped on the diffraction pattern. The phase retrieval algorithm is applied to reconstruct the microstructure condition in large-scale materials. The geometrical features of reconstructed images are analyzed quantitatively and the optical performance is calculated. Moreover, this method provides a path to visualize the feature sizes of the micrometer-scale and sample sizes of the meter-scale within high-speed. It significantly reduces the time for large-scale materials using laser scanning and computer calculation. The non-destructive in-line diagnosis and real-time monitoring of the optical performance can realize in industrialized manufacturing. Furthermore, the thermal radiation properties in defects-containing microstructure-based spectrally selective emitters is worth to study from the scientific point of view. This technique builds a bridge between the defective microstructure pattern and optical performance, which contributes to helping our understanding of the thermal radiation mechanisms from defects included microstructure surfaces.

Resume : Boosting the power density of thermophotovoltaics (TPV) is essential to make it competitive at heat source temperatures lower than 1000 ºC. A record output power density of 0.75 W/cm2 has been recently measured at a moderate emitter temperature (~ 460 ºC) by using a near-field TPV (NF-TPV) arrangement, where a nanoscale separation between the (cold) TPV cell and the (hot) emitter enables a ~6-fold enhancement of the far-field power. However, the very high current densities impose serious constrains on the TPV cell design, which must be either very small or comprise a very dense front metal grid to avoid excessive ohmic losses. None of these requirements are readily implemented in space-constrained NF-TPV devices. In this study, we describe a thermionic-enhanced NF-TPV conceptual device in which electrons are thermionically emitted from the emitter to the TPV cell, establishing a wireless electric connection that avoids the use of front metal electrodes in the cell, and thus, results in negligible ohmic losses and higher power densities. Simulations show that high efficiency and power densities are attainable simultaneously at moderate temperatures (< 1000 ºC). Preliminary experimental results also demonstrate the main operational principles of the concept in the far-field. In this work, a review on the concepts and materials that are needed to implement this concept in practice are presented.

Resume : 2D nanomaterial graphene is at the top of the considerable interest due to the giant electron and hole mobility, charge carrier multiplication, flexibility, optical transparency, chemical inertness. Particularly graphene is intensively explored as a material for thermophotovoltaic applications. There were already revealed that effective graphene based thermophotovoltaic (TPV) emitters and absorbers can be fabricated. It should be mentioned that one of the main limitations stopping the wider application of the graphene in semiconductor device technology is a complex graphene transfer procedure. In this case, graphene is synthesized on the catalytic Cu or Ni foils by chemical vapor deposition. Afterward, follows the long process of the graphene transfer onto the targeted semiconductor or dielectric substrates. During that process, graphene can be contaminated by different adsorbents. Transfer causes wrinkles or ripples to form on graphene transferred onto the flat substrate. However, for fabrication of the more effective TPV systems, photonic crystals and other micro/nanostructures are used. In such a case application of the transferred graphene became even more complicated. Recently there were shown that direct synthesis of the graphene on semiconducting or dielectric substrates by plasma enhanced chemical vapor deposition is possible. Due pecularities of the chemical vapor deposition process, direct synthesis of the graphene on complex surfaces is possible. However, the development of the direct graphene synthesis technology is at the very beginning. In the present research graphene layers were directly synthesized by microwave plasma enhanced chemical vapor deposition on the textured semiconducting monocrystalline Si(100) substrates as well as on Si(100) microstructured by combining deep reactive ion etching and lithographic techniques. The structure of the graphene was investigated by multiwavelength Raman scattering spectroscopy and atomic force microscopy. Coverage of the rough substrates was considered. Optical properties of the samples were studied. Acknowledgements. The research project No. 09.3.3-LMT-K-712-01-0183 is funded under the European Social Fund measure „Strengthening the Skills and Capacities of Public Sector Researchers for Engaging in High Level R&D Activities“ administered by the Research Council of Lithuania.

Resume : In many near-field thermophotovoltaic system performance evaluations, a common assumption is to treat the surface of the thermal emitter and thermal thermophotovoltaic cell as a smooth surface. However, the surface of these units is actually rough. For assessing the performance of the near-field thermophotovoltaic system more accurately，we evaluate the performance of random rough surface near-field thermophotovoltaic system. In order to deal with the rough surface units, we use an effective multilayer model. The model applies the effective medium theory (EMT) to get the dielectric function of each layer. As an example, we evaluate the performance of a GZO/GaSb near-field thermophotovoltaic system with a random rough surface less than 5nm. The average distance between thermal emitter and thermal thermophotovoltaic cell is 50nm. The calculation results show that the higher the roughness, the lower the output short-circuit current, which leads to the decrease of the maximum output power. This research can provide guidance for future near-field thermophotovoltaic system design.

Resume : Currently employed materials in Thermoelectric Generators (TEGs) such as bismuth telluride or lead telluride are scarce, expensive, toxic, and environmentally harmful, relegating this technology to specific niches. However, in recent years, the thermoelectrics paradigm has changed mainly due to the introduction of low-dimensional materials. This miniaturization enabled tailoring some properties of materials, such as reducing thermal conductivity by phonon scattering. Consequently, materials previously discarded due to a large bulk thermal conductivity have gained significant interest. Semiconductor nanowires (NWs) have demonstrated fascinating properties with application in a wide range of fields including energy and information technologies. In particular, increasing attention has been focused on Si and SiGe NWs for thermoelectric generation after recent successful implementation in miniaturized devices. Despite this interest, a proper evaluation of the compositional, morphological and thermoelectrical properties in such nanostructures still represents a great challenge. Typically, the study of individual integrated NWs following the bottom-up approach faces two main issues. The first one is the need for catalyst nanoparticle precursor deposition to grow them, which is usually performed by colloidal solution depositions. The randomness of this process greatly hinders the precise allocation of the desired NW into the employed test microdevices. The second challenge relates to the different architectures of test microdevices devoted to measure one nanomaterial property. These challenges often result in the use of different NW for the measurement of each of the studied properties, being one of the primary sources of error in the characterization of NWs. This work presents the conceptual design, simulation, and fabrication process of a micro-machined device for the complete evaluation of a single bottom-up integrated NW. In the exhibited design, the NWs can grow between multiple pairs of breakable cantilevers. This approach allows the user to select the most suitable sample among the available in the chip after the random deposition of catalyst precursor and discard the rest. The design also incorporates though micro-trenches, which added to a diameter of 3 mm- enable the use of Transmission Electron Microscopy for detailed morphology analysis and compositional characterization techniques such as EELS or EDX. In the same architecture, electrical collectors and isolated heaters are available at both ends of the trenches for thermoelectrical measurements of the NW. Moreover, thermal conductivity evaluation though micro-Raman measurements are improved thanks to the lack of substrate below the NW and the electrical and thermal access to the NW for in-operando analysis. Finally, the fabrication process is designed without using anisotropic etching processes, allowing the chip to be fabricated in any crystallographic direction needed. The device presented here show remarkable utility in the challenging thermoelectrical characterization of integrated nanostructures and the development of multiple devices such as thermoelectric generators. References: [1] G. Gadea Díez et al., "Enhanced thermoelectric figure of merit of individual Si nanowires with ultralow contact resistances," Nano Energy, vol. 67, p. 104191, Jan. 2020. [2] I. Donmez Noyan et al., "SiGe nanowire arrays based thermoelectric microgenerator," Nano Energy, vol. 57, no. November 2018, pp. 492–499, Mar. 2019.

Resume : Near-field thermophotovoltaics (nTPV), where photons from a hot emitter traverse a nanoscale vacuum gap and are absorbed by the photovoltaic (PV) cell to generate electrical power, have attracted intense interest of thermoelectric devices in recent years, for their possibility of increasing power output and efficiency. p-n junctions composed of inversely doped semiconductors are the basic blocks of nTPV. In engineering practice, the p- and n- semiconductors are not uniformly doped. Instead, the doping level exhibits a gradient distribution due to diffusion or ion implantation process. To our knowledge, however, this gradient effect is not considered in any literature. In this paper, we investigate the effect of gradient index on the near-field heat absorption characteristic. During the study, Drude model is adopted for it is usually used to describe the dielectric functions of doped semiconductors. Combing fluctuational electrodynamics and the scattering matrix method applied to multilayer system that models the gradient index slab (GIS), we find that, with proper doping profile (gradient), the absorption ability of the inner location can be improved, even higher than that of the surface. This is to some extend abnormal considering that surface polaritons are excited at the vacuum/medium interface. This phenomenon due to gradient index is promising for nTPV, because it provides a way of enhancing the absorption ability of the depletion region of p-n junction, which lies at the interior of the PV cell instead of at the surface.

Resume : III-V solar cells have reached the highest conversion efficiencies among all photovoltaic materials. This is true for monochromatic or full spectrum photons energy conversion, as well as for single or multi-junction architecture. Historically, the III-V solar cells developments where driven by space exploration requirements, however a growing number of alternative applications start to play a role: concentrated PV, Unmanned Aerial Vehicle, mobility/connectivity, laser power converters & thermophotovoltaics. In this review, we will focus on recent III-V solar cells advances unlocking record conversion efficiencies and/or enabling new applications. The major breakthrough in III-V crystal growth, device design and processing routes will be presented, as well as their cost reduction potential and future evolution. Based on those latest III-V solar cells developments, the thermophotovoltaics innovation opportunities will be discussed.

Resume : Hybrid thermionic-photovoltaic (TIPV) converters were recently proposed for harvesting both electrons and photons emitted from high temperature sources to produce electricity. These novel devices show higher flexibility compared to thermophotovoltaics (TPV) or thermionics (TI) to deliver a high power density throughout a broad temperature range, addressing a key challenge of thermal to electric energy conversion. A three-terminal configuration, i.e. with a common terminal between the TPV and TI parts, allows to separately extract the current generated by photons on the TPV and electrons on the TI subcells. Such operation mode allows to discriminate the contribution from each thermal (TPV or TI) process, preventing current limitations and assessing the voltages separately. In this work, device modeling, epitaxy, processing and characterization of interdigitated back contact (IBC) InGaAs cells are carried out aimed at the implementation of three-terminal hybrid TIPV converters. Optimization of the IBC cell metallization pattern, carried out with a quasi-3D distributed model fed with semi-empirical electronic parameters and solved by SPICE, leads to a fingerless (non-interdigitated) configuration, which relies on a continuous metal sheet for lateral transport at the p-type contact and on the InP substrate at the n-type contact. The punctual n-type metal contact can be as small as the manufacturing technology allows to process and encapsulate it. The fingerless back contact architecture enables minimized shadowing and array-packing losses for a maximized power density. Besides, the cleared front surface facilitates their integration in near-field TIPV arrangements using dielectric micro-spacers. Absolute quantum efficiency and current-voltage measurements under concentrated illumination carried out on large (1 cm2) devices have allowed validating this original three-terminal InGaAs IBC hybrid TIPV cell architecture.

Resume : A solution for short-term storage that provides energy on demand is required to achieve the long-sought dream of a 100% Renewable Electricity System. Thermal energy storage at high temperatures (1400-2500ºC) in conjunction with thermophotovoltaic (TPV) cells to convert heat into electricity can potentially achieve high efficiency and rapid response times. However, in order to provide a cost-effective solution (as compared to batteries or pumped storage) the TPV converter needs to reach high efficiencies and be manufacturable in high volumes at moderate costs. In this scenario, Germanium based converters provide a unique advantage. Being the base of the current multijunction solar cell technology –used in space PV and in terrestrial CPV systems– there is a mature infrastructure in place for substrate fabrication, structure epitaxial growth, and device manufacturing which could be leveraged for TPV. In this paper we will revisit the potential and limitations of TPV cell designs based on germanium and review the challenges in TPV cell growth using MOVPE including 1) p/n junction formation; 2) emitter passivation; 3) Ge autodoping; 4) rear passivation; 5) minimization of FCA and 6) tandem configurations.

Resume : Renewable energy sources are rapidly reaching cost parity with fossil-derived sources, but renewables’ intermittence must be managed in a cost-effective manner to enable widespread deployment. Thermal energy storage with direct thermophotovoltaic (TPV) conversion of the heat back to electricity offers a way to cheaply and efficiently store, and rapidly retrieve, energy from renewables in response to grid demand. One embodiment of this type of system employs a high-temperature (1900-2400 °C) pumped storage medium with multijunction PV cells targeting a 50% system efficiency. Here, we demonstrate an inverted metamorphic multijunction (IMM) photovoltaic cell comprising lattice-mismatched 1.2 eV AlGaInAs and 1.0 eV GaInAs junctions optimized for this application. This device structure differs from traditional IMM solar cells because the mismatched junctions are grown at a single lattice constant. This architecture enables removal of the compositionally graded buffer which otherwise filters light from the junctions below and absorbs sub-bandgap light via free-carrier absorption. Sub-bandgap absorption dramatically reduces the efficiency of TPV systems using high reflectivity cells that employ bandedge spectrum filtering. We discuss the development of three unique components relative to the IMM for this device: 1) a lattice-mismatched 1.2 eV AlGaInAs top junction, 2) a metamorphic contact layer grown after the graded buffer to enable its removal, and 3) a more transparent tunnel junction that is less parasitically absorbing of photons intended for the 1.0 eV GaInAs junction. We maximized AlGaInAs cell quality by selecting for growth conditions that limit the incorporation of oxygen defects, enabling a 0.41 V bandgap open circuit voltage offset at a short-circuit current of 22 mA/cm2. We developed a mismatched GaInAs:Se layer to be used as the front contact layer with low contact resisitance. Lastly, we developed a GaAsSb:C/GaInP:Se tunnel junction suitable for high current densities with more transparency than the GaAsSb:C/GaInAs:Se tunnel junction used in past IMM cells. We characterized the reflectivity of the device using Fourier transform infrared spectroscopy, and, combined with high-intensity flash measurements under a spectrum that approximates the emission from a 2150 °C radiator, estimated an ideal TPV efficiency for this device/radiator combination. We project a peak ideal TPV efficiency of 39.9% for this device at ~30% of the full 118 W/cm2 irradiance, and a 36% ideal TPV efficiency under the full irradiance. We discuss how improvements to the reflectivity and series resistance will increase the ideal TPV efficiency well above 40%, and present initial steps towards these goals.

Resume : GaAs thermophotovoltaic (TPV) cells are designed and processed for a grid-level energy storage system in which excess electricity is stored as high-temperature heat in the 1700-2200°C range and extracted when needed. The crucial cell factors affecting the TPV system efficiency are sub-bandgap reflectance (Rsub) and series resistance (Rs). To enhance Rsub in 0.9-10 µm range, a point-contacted GaAs TPV cell is fabricated with a low refractive index, low-loss dielectric spacer layer between the semiconductor and the metal back contact. This architecture boosts Rsub by reducing metal absorption losses. Rsub increases with greater spacing and smaller diameter of the point contacts, leading to a tradeoff with series resistance Rs. We have fabricated GaAs TPV cells with 3-10 µm diameter point contacts spaced 28.8 µm apart, with ~250 nm SU-8, MgF2, and SiO2 spacer layers. The cells with SU-8 spacer have Voc up to 1.074 V and 18.6% efficiency with no anti-reflection coating under the one-sun spectrum. Rs is measured to have a very low value of 5.7 mOhmcm2 for 0.8 cm2 cells. The cell efficiency peaks at 6 A/cm2 and drops less than 0.5% absolute at 10 A/cm2, near the expected operating current for a 2200°C thermal emitter. Rsub measured by diffuse Fourier-transform infrared (FTIR) spectroscopy is 94.1% for SU-8 and 95.74% for SiO2, weighted by the 2200°C blackbody spectrum. Optimization of the dielectric spacer is expected to lead to Rsub ≥ 96% and a TPV system efficiency ≥ 40%.

Resume : Lightly-doped silicon wafers are popular in semiconductor industry. Characteristic lengths can be nanoscale, and the pattern can be generated in a large area with excellent uniformity. However, these wafers were not considered for thermophotovoltaic (TPV) emitters because radiative properties of lightly-doped silicon is not appealing. Its spectral emittance is epsilon < 0.7 at short wavelengths (lambda < 1.1 um) and dramatically decreases to zero at longer wavelengths. The wafer even becomes semi-transparent in the near-infrared region. In this work, one-dimensionally and two-dimensionally periodic patterns are proposed and fabricated on lightly-doped silicon wafers. These patterns are able to enhance the spectral emittance and expand the emittance plateau from lambda = 1.1 um to lambda = 1.2 um. The emittance in the spectra range 0.5 um < lambda < 1.2 um increase to epsilon = 0.8. The tailored emittance shows a promising way to realize an efficient TPV emitter using cost-effective semiconductor fabrication techniques.

Resume : Solar Thermophotovoltaics (STPV) has garnered great attention due to the pronounced energy transfer efficiency through proper spectral transformation with respect to the bandgap of base solar cells. One of the most critical bottlenecks of STPV is the narrowband thermal emitters. In this talk, I will show our recent endeavors on enabling the narrowband thermal emitters based on plasmonic nanostructures, in which the quality factor of the emitters, thermal stability as well as the system efficiency will be discussed. We suggest our designs may pave the way towards the highly efficient and stable STPV systems in the future.

Resume : Abstract: In order to tailor thermophotovoltaic emitters to match specific photovoltaic receivers we design and investigate spectrally selective high temperature stable emitters. We demonstrate selective band edge emitters based on W-HfO2 refractive multilayer metamaterials and based on monolayers of spherical particles from yttria stabilized zirconia (YSZ) on HfO2 coated W-substrates. Both emitter types are stable up to 1400°C. Since the emitted power scales with the fourth power of temperature and for better match with low band gap photovoltaic cells, very high temperatures well above 1000 °C become very important. Degradation mechanisms and conditions for sustainable selectivity and high thermal stability are discussed. The stability of nanoscaled structured materials at very high temperatures is a scientific topic of fundamental importance in various fields of physical and materials sciences.

Resume : High temperature stable spectral selective emitters can increase the efficiency and radiative power in thermophotovoltaic systems significantly. However, most of the emitters suffer from structural degradation at high temperatures due to various mechanisms such as oxidization, grain growth, diffusion, thermal expansion coefficient, etc. Herein, we present thermal stability of 1D hyperbolic metamaterial emitter structure under different vacuum conditions (ranging from 10-2 to 10-6 mbar) and temperatures up to 1450 °C. We clarify the potential degradation mechanisms initiating the structural instability at high temperatures. Our W-HfO2 multilayers-based metamaterial emitter exhibits a step function-like steep spectral cutoff around 1.7 um and low emissivities/absorptivities above the wavelength corresponding to the bandgap of the PV cell. [1,2] We discuss in detail how the residual oxygen partial pressure can affect the stability of the W-based metamaterial at high temperatures. By reducing the partial oxygen pressure, 1D metamaterial structure exhibits unprecedented thermal stability up to 1400 °C. We also show how to achieve thermal stability up to 1400 °C under technical vacuum conditions of 10-2 mbar and with the help of inert gas encapsulation. References: [1] P. N. Dyachenko, et al., Nature Communications. 7, 11809 (2016). [2] M. Chirumamilla, et al., Scientific Reports 9, 7241 (2019).

Resume : Semiconductor nanowires have several distinct advantages as a system for photo-thermoelectric energy conversion: (i) photonic or plasmonic engineering allow to design the location of light absorption and (ii) strain relaxation enables great freedom for heterostructure band engineering for energy filtering. I will report on a series of experiments exploring the possibility of hot-carrier photovoltaic energy conversion in nanowires. One key element is the ability to efficiently harvest electricity from heat stored in electrons. I will report on a recent experiment where we realized a near-ideal quantum-dot heat engine in devices based on single InAs/InP heterostructure nanowires, realizing power production with Curzon-Ahlborn efficiency (> 50% of Carnot efficiency) at maximum power settings, and reaching more than 70% of Carnot efficiency at maximum efficiency settings [3]. In experiments with light as the energy source, we demonstrated hot-carrier photothermoelectric energy conversion with an open-circuit voltage that exceeds the Shockley-Queisser limit, and we demonstrated avenues to increase quantum yield by use of plasmonic elements. [1] Martin Josefsson, Artis Svilans, et al.: Nature Nanotechn. 13, 920?924 (2018) [2] S. Limpert, et al: New J. Phys. 17, 095004 (2015); Nanotechnology 28, 43 (2017) [3] I-Ju Chen et al., submitted (2019)

Resume : In the last years, thermophotovoltaics has become a more and more attractive solution for heat-to-electricity conversion due to its excellent conversion efficiencies. However, further research is needed to reduce the device cost which is typically based on III-V semiconductors. To tackle this limitation, crystalline germanium (c-Ge) has been proposed as an excellent substrate for low-cost devices. One of the key advances behind high efficiencies of III-V devices is an excellent reflectance of the sub-bandgap photons at the rear surface of the photovoltaic device. By doing so, these photons are reflected back to the thermal emitter reducing its thermal losses and improving the system efficiency. The application of this approach to rear surface of c-Ge thermophotovoltaic devices would pave the way to high efficiency systems using this moderate-cost material. In this work, we report on the development of hole selective contacts on c-Ge that show high reflectance to sub-bandgap photons combined with a high carrier selectivity, i.e. low effective surface recombination velocity (Seff) and contact resistivity (Rc). One of the straightforward ways to create a high reflectance contact is the combination of a wide bandgap material capped by a metal. Consequently, our approach to the hole selective contacts is based on transition metal oxides (TMO) contacted by Indium Tin Oxide (ITO)/Ag on p-type c-Ge. In particular, we explore tungsten, molybdenum and vanadium oxide (WOx, MoOx, VOx) which have shown good results on crystalline silicon. We have electrically characterized these contacts by measuring Seff through effective lifetime measurements using Sinton WCT-120. In addition, we also measured contact resistivity using the transfer length method (TLM). Up to now, the best obtained result is Seff = 570 cm/s and Rc= 78 mohms·cm2 for the MoOx sample. Surface passivation is improved to Seff = 255 cm/s by depositing a thin (⁓3 nm) amorphous silicon layer on the c-Ge surface before TMO deposition. However, contact resistivity increases to more than 1 ohms·cm2 indicating a trade-off between these two magnitudes. Further development of these layers will be done in the next months which could improve these results. Moreover, optical characterization of such contacts will be done for wavelengths ranging from visible to NIR and MIR in order to determine their internal reflectance. Finally, optical simulations will be carried out including the optical experimental data to determine the generation inside the c-Ge substrate. With this information and the electrical characterization, maximum potential efficiencies will be calculated. All these results will be reported in the conference. Acknowledgements: This work was funded by Ministerio de Ciencia, Innovación y Universidades from Spanish government under projects TEC2017-82305-R, ENE2017-87671-C3-2-R and PID2019-109215RB-C41.

Resume : Solar-thermophotovoltaics (STPV) can contribute to high-efficiency solar energy conversion by shaping the solar spectrum to match with a photovoltaic (PV) cell’s useful wavelengths via an intermediate thermal radiation emitter, which can be regarded as a photon-to-photon converter. Efficiency of STPV systems has been drastically increased and getting close to 10% in the last decade. one of the reasons for the recent improvement in the overall conversion efficiency is the use of a planar-shape absorber/emitter. It allows increasing the temperature even with low input power by decreasing the surface area. Also, thermal stability of spectrally selective emitter has been progressed, which enables high-temperature operation. However, in the planar absorber/emitter, the distance between the emitter and the cell should be controlled precisely within 1 mm to obtain a high view factor and also it is difficult to fully utilize photons in terms of photon recycling. For effective photon conversion in STPV, we propose an enclosed-space confined emitter system, and demonstrate its power generation potential. By surrounding the cuboid emitter by TPV cells, it is possible to achieve a view factor of 100% between the emitter and the TPV cells mutually, even if the gap between the emitter and the cells is not as small as <1 mm, with the assumption that the reflectance of the reflector and surface electrode of the TPV cell is 100%. This means that photons emitted from the emitter can be used fully by absorbing photons with higher energy and recycling photons with lower energy than the PV cell’s bandgap. In addition, the area ratio (AR) between the absorber and the emitter is easily controlled to increase the relative emissive power from the emitter without any loss from the absorber surroundings. In the experiment, system efficiency reaches 7.0%, which is obtained after multiplying the power measured from one cell by five. A system efficiency more than 20% can be expected by further improvement with a back surface reflecting TPV cells and a perfectly enclosed space within input power density of 200 W/cm2, which is available with practical Fresnel lens optics.

Resume : With the rapid development of space technology, high-efficiency power systems occupy an increasingly important position in deep space exploration missions. Radioisotope thermophotovoltaic (RTPV) generators convert the heat of radioisotope into electrical energy via thermally infrared radiation photons. It have attracted attention in far-reaching space exploration because of the stable energy conversion efficiency, low heat source load, high reliability, and long life. RTPV generators is mainly composed of radioisotope heat source, emitter, and thermophotovoltaic cell. Radioisotope heat sources are usually accompanied by high-energy rays. However, the thermophotovoltaic cell is a key device for energy conversion, and it is also attacked by rays all the time. At the same time, the isotopes have a specific half-life, and heat will gradually be lost during the service. The output characteristics of RTPV generators are greatly affected by these factors. The InGaAs cell has high energy conversion efficiency and is an excellent energy conversion cell for RTPV generators. Its service performance under radiation needs further exploration. We studied the tolerance of the InGaAs cell in space service. Based on Pu-238 heat source conditions, the external neutron fluence rate and deposition dose was calculated. The attenuation of the surface temperature caused by the half-life of the Pu-238 radioisotope was considered. We have equivalent RTPVs 5-50 years of service time through neutron irradiation experiments. Through the comparative analysis of the test data before and after the InGaAs cell experiment, neutron irradiation has caused certain damage to the InGaAs cell, which reduces its performance. However, the performance degradation rate remains at 10% within 10 years, and the maximum output power can still maintain the original 50% within 50 years. Therefore, RTPV has great potential in long-term service in space environment.

Resume : The thermal behavior of a thermophotovoltaic system composed of a metallodielectric 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.

Resume : While thermophotovoltaics have potential for high efficiency, they are ultimately limited by the fraction of useful photons absorbed by the photovoltaic cell. However, the blackbody emission spectrum below 1500 K generally does not have enough useful photons without enhancement. Two competing methods -- selective thermal emission and external photon recycling – can improve the efficiency, but the former has the advantage of also applying to selective solar absorbers. In this work, we report a spectrally-selective thin-film silicon emitter. Its high-temperature emittance shows strong spectral selectivity at 868 K, and thermal stability is proven by measuring its infrared reflection spectrum before and after 24 hours of thermal cycling. Furthermore, it potentially for scalable manufacturing with a base in thin-film crystalline silicon coated by thin films of earth-abundant materials. Finally, it exhibits exceptional mechanical flexibility, for compatibility with a wide range of thermophotovoltaic cells. In summary, these thin-film silicon selective thermal emitters provide a combination of spectral selectivity, thermal stability, manufacturing scalability, and mechanical flexibility that may benefit the future adoption and use of thermophotovoltaics.

Resume : Thermophotovoltaic (TPV) systems are of significant interest because they can generate electricity using any high-temperature heat source including concentrated solar radiation, industrial waste heat, and heat from fuel combustion. This makes TPV a multipurpose technology with many applications including uninterruptible power supplies, micro-combined heat and power systems, self-powered heating devices and industrial waste heat recovery. A critical design objective for TPV systems is to minimize the heat loss from the emitter by decreasing the portion of out-of-band photons it radiates and by increasing the effective view factor between the emitter and the PV cell. Also, the emitter material should be thermally stable and shock resistant to enable reliable operation at high temperatures (> 1300 K). Our research numerically investigates the design and integration of prolate ellipsoidal optical cavities into TPV systems to improve their energy conversion efficiency and output power density. These cavities, referred to herein as Radiant Energy Spectrum Converters (RESC), enable a high degree of spectral and directional control over radiation from the emitter within a TPV system. In this study, we investigate the application of RESC structures in solar TPV systems. The geometrical parameters of the RESC structure can be tailored to control the level of photon recycling, effective view factor, emission loss, distance between the PV cell and the emitter, as well as power density delivered to the PV cell. The combination of these advantages (emitted radiation highly concentrated onto the PV cell (high power density), very high effective view factor, tunable photon recycling for control of the emitter temperature and spectrum, low emission/parasitic losses, and large PV cell-emitter separation distance) are unique to the RESC structures. Comsol Multiphysics and MATLAB software are used for finite element analysis and numerical computation, respectively, to determine the system efficiency and other important parameters. The results show that for a solar irradiance concentration factor of 300X, for different optimized configurations, the effective view factor and degree of photon recycling can reach 89% and 92%, respectively. In conclusion, the ellipsoidal optical cavity can be integrated into the design of advanced TPV systems and bring them closer to the high theoretical efficiencies TPV systems are capable of.

Resume : This paper presents the design, optimization, and fabrication of a high-efficiency planar STPV system comprising of the spectrally selective absorber and emitter surfaces and GaSb PV cells. The selective absorber consists of a micro-textured tungsten (W) surface that provides a light absorptance of more than 90% at visible and near-infrared wavelengths. The selective emitter is a multilayer metal-dielectric structure of W and Si3N4 with its spectral properties tuned to match with the quantum efficiency of the GaSb cells. A comprehensive thermodynamic model was formulated for a detailed analysis of the transport of power at multiple stages of the STPV system and to derive optimal design parameters for various STPV components. The system was fabricated and tested at various operating temperatures using a high-power continuous-wave laser as a simulated source of concentrated solar irradiation. A heat shield was installed on the absorber side to suppress the undesired radiation loss from the absorber end. An electrical output power density of 1.75 W/cm2 with a maximum conversion efficiency of 8.6% was measured at 1670 K for an equivalent incident solar concentration factor of ~2100. This efficiency is higher than those of previously reported experimental STPV systems. Optical and thermal losses that occurred at multiple stages of the energy conversion process are quantified. Combining the simulation and experimental results, guidelines to further improve the performance of the STPV system are also provided.

Resume : Latent heat thermophotovoltaic (LHTPV) batteries combine very high melting point phase change material (PCM) with thermophotovoltaic (TPV) energy conversion. Electricity is employed to produce the solid-liquid phase transition in the PCM. Consequently, electrical energy is stored in the form of latent heat at very high temperatures (> 1000ºC). When needed, stored energy is released as thermal radiation, and converted back to electricity on demand by TPV. In this study we review the PCM and TPV cell options that result in optimal system designs. Some relevant parameters of the system such as self-discharge, round-trip efficiency, charge/discharge time (power-to-energy ratio), and cost per energy and power capacities, are taken into consideration. The selection of the PCM is key, as it determines the energy density and the store temperature, the latter being directly related to the TPV power generation capacity and thermal insulation losses. The relatively low round-trip conversion efficiency (< 50 %) and the low cost of the PCMs (< 10 €/kWh) result in optimal system designs with small power-to-energy ratios, fitting long duration storage application. We show that LHTPV batteries could compete with electrochemical batteries either when the charging electricity price is low or when there is a heat demand that can be satisfied with the waste heat produced in the TPV converter.

Resume : A standardized method for the measurement of the thermophotovoltaic (TPV) conversion efficiency has not been established yet. Different research groups have used different methods, which make the reported results difficult to compare. Measuring the TPV efficiency is complex because it depends on the net radiative heat flux from the emitter to the cells, which is a result of the multiple reflections taking place between them. So, rigorously, the TPV efficiency should be measured when the cells are integrated in a complete TPV optical cavity. Since this is not practical, indirect methods have been used to measure individual cells and incorporate the cavity effects by means of an “effective” emissivity, which is calculated from the spectral reflectivities of the emitter and the cell. The conditions under which these measurements are conducted (angle of incidence, sample temperature and spectral range) may differ significantly with respect to the real ones. Besides, the models used to calculate such effective emissivity might not be accurate, resulting in misleading estimations of the TPV efficiency. Other methods have been proposed that measure the “radiant heat transfer efficiency” of the TPV cell, which is the ratio of the electrical power to the absorbed radiative power in the cell. This efficiency can be readily obtained by measuring the heat dissipated from the cell, but it only approaches the actual TPV efficiency if the view factor (VF) between cell and emitter is close to one, so that leakage losses are negligible and multiple reflections effectively take place between them. Following this last approach, we propose a TPV efficiency setup based on in-situ measurements and a high VF configuration. Because all the power absorbed by the cell is converted into electricity or heat, the TPV efficiency can be measured in terms of the electrical power generated by the cell and the heat flux that must be removed to maintain the cell at a constant temperature. In the proposed setup, both parameters are measured in-situ under operation conditions that include high VF, by keeping the surface of the emitter and the cell close enough. For example, the VF between 1 cm2 flat parallel surfaces (typical size range for commercial TPV cells) can be >0.9 if they are separated 0.5 mm or less. In the proposed setup, such small gap can be successfully obtained because the same structure that holds the emitter over the cell also includes the probes that contact the device. Any other sideway contacting configuration would impede high VFs. As counterpart, the mentioned structure must be carefully designed to avoid overheating of the contact probes by keeping the emitter thermally isolated. This way, the emitter must be heated by wireless power transmission; in our case, using a high-power laser. Such setup configuration is currently under construction and the upcoming results will be presented in the conference.

Resume : Nanoscale thermal radiation can significantly enhance the radiative heat flux and may have important applications for high-performance thermophotovoltaic devices as well as electroluminescent refrigeration. There are different types of radiative thermoelectric energy converters (RTECs) depending whether the device (usually made of a p-n junction) is located on the high-temperature or low-temperature side, and whether the purpose is for power generation or refrigeration. The thermodynamics and entropy analysis have received less attention despite the fundamental importance. A modified Planck distribution considering chemical potential is often used to calculate the radiative energy transfer as well as the current-voltage relations. The spectral entropy can be obtained via statistical thermodynamics and used to understand the effect of chemical potential on the modified Planck distribution. Our recent study has shown that the reverse saturation current and hence the dark current can be affected by near-field thermal radiation. Furthermore, it is important to understand the chemical potential distributions in analyzing RTECs. The charge transport and photogeneration processes are relevant to each other. This presentation will give an overview of our recent theoretical study on analyzing the dark current and chemical potential distribution in near-field thermophotovoltaic devices.

Resume : To continue improving the performance of near- and far-field thermophotovoltaic (TPV) systems, researchers must carefully design the TPV cell structure to minimize losses and maximize efficiency and power density. This highlights a need for accurate modeling methods to predict the cell performance and its spatial charge and radiation characteristics. Existing methods, however, typically either oversimplify the cell as having uniform transport properties in a detailed balance approach (e.g. uniform photogeneration, minority carrier concentration, recombination losses, etc.), or they do not fully consider cross-influences between charge and radiation transport processes by neglecting the effects of charge dynamics on radiation exchange. We present a new modeling method that fully captures the spatial interactions between charge and radiation transport through an iterative approach. The charge carrier dynamics are calculated using an open-source solver of the well-known Poisson-drift-diffusion equations called Sesame, and the radiative transport characteristics are calculated using a scattering matrix formalism for fluctuational electrodynamics. Importantly, these two sets of equations are linked by the local quasi-Fermi levels from charge dynamics (which provides the photon chemical potential for radiation exchange) and the local photon absorption from radiation exchange (which provides the photogeneration rate for charge dynamics). The models are solved iteratively to yield self-consistent performance and spatial transport characteristics for the TPV cell. We compare our model to a previous approach which calculates spatial transport properties but neglects the influence of charge dynamics on radiation exchange for thin and thick GaSb TPV cells operating in the near-field and the far-field from a thermal emitter. We find that an iterative coupled model is necessary to accurately predict device performance characteristics, especially for near-field TPV operation. In particular, our model is able to capture effects such as near-field enhancement of external luminescence from the cell and photon recycling within the cell that can drastically alter minority carrier concentrations, losses, and device performance. Results from this work should help researchers to better design TPV systems for efficient and practical operation.