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Advances in thermophotovoltaics: materials, devices and systems

An analysis of the scientific literature indicates a revival of research on thermophotovoltaics, boosted by the development of systems for converting waste or stored heat into electrical power. The symposium will provide an interdisciplinary platform for sharing the latest advances in the field.


Thermophotovoltaics (TPV) refers to thermal to electrical power conversion based on the photovoltaic effect. It is suited for thermal sources operating at temperatures near or above 1000 K, such as waste or stored heat recovery, or solar energy conversion involving an intermediate thermal energy storage. Given the huge potentials of these systems, and recent progresses in high-temperature materials science, photonics, growth and processing of III-V semiconductors, a renaissance of research on thermophotovoltaics has taken place over the last decade. The challenges to tackle are indeed multiple, for designing, fabricating and testing new materials, devices and systems for TPV applications. In this context, the symposium will cover recent advances in areas relevant to the field: selective emitters to tailor the spectrum of radiation useful to photovoltaic conversion and their thermal stability; optimum materials and architectures of the photovoltaic cells and their fabrication and characterization; laboratory experiments assessing the performances of devices and systems; assessment of optical, electrical and thermal losses and their mitigation; new concepts for improving efficiency including hybridization with other thermal-to-electrical power converters; solar-TPV, TPV for space, near-field TPV systems; thermophotonic power generation and cooling, scaling-up of research prototypes. It is expected that the symposium will facilitate networking in this field through the establishment of exchanges across multiple disciplines in physics and engineering.

Hot topics to be covered by the symposium:

  • Tailored spectral thermal emission: photonic crystals, resonant emitters, metamaterials, etc.
  • Tailored spectral reflection and transmission: optical filters and reflectors, plasmonics, etc.
  • High temperature emitters: fabrication and characterization
  • Infrared semiconductors: III-V, quantum nanostructures, etc.
  • Thermophotovoltaic devices: design, fabrication and characterization
  • Thermophotovoltaic applications: solar, space, waste heat recovery, energy storage, etc.
  • Novel concepts: near-field thermophotovoltaics, thermo-photonics, hybrid devices, etc.
  • Competing technologies: thermionics and thermoelectrics
  • Market assessment and exploitation

List of invited speakers:

  • P. Bermel (USA): Stable, flexible, and scalable thin-film silicon-based selective thermal emitters
  • R. Cariou (France): Recent advances in III-V materials, process and solar cells devices - opportunities for TPV applications
  • Y-B. Chen (China): Patterned and lightly-doped silicon wafers for thermophotovoltaic emitters
  • M. Eich (Germany): Metamaterial and particle based selective emitters for thermophotovoltaics
  • L. Fraas (JX Crystals Inc, USA): Light-weight fuel fired TPV DC cylindrical generator
  • B. Hubert (MTPV Power Corporation, USA): Commercially-relevant thermophotovoltaic systems in the modern era
  • T. Inoue (Japan): Far-field and near-field thermophotovoltaic systems based on intrinsic silicon thermal emitters
  • B-J. Lee (Republic of Korea): Towards the development of near-field thermophotovoltaic device operating at experimentally achievable gaps
  • A. Lenert (USA): Control of photon-recycling phenomena in radiation-limited thermophotovoltaic cells
  • H. Linke (Sweden): Hot-carrier photovoltaics in heterostructure nanowires
  • Y. Okada (Japan): Thermal up-conversion of sub-bandgap photons in quantum nanostructures for photovoltaics
  • J. Oksanen (Finland): Thermophotonic cooling - thermophotovoltaics on steroids
  • I. Rey-Stolle (Spain): MOVPE growth and device design of TPV converters based on Ge and III-V arsenides and phosphides
  • V. Stelmakh (USA): A photonic crystal enabled practical thermophotovtaic portable power generator
  • R. St-Gelais (Canada): Progress towards MEMS-controlled near-field thermophotovoltaic energy conversion
  • M.C. Gupta (USA): Micro/Nanostructure-based selective absorber and emitter surfaces for high-efficiency solar thermophotovoltaic applications
  • Z. Zhang (USA): Impact of photon entropy and chemical potential on thermophotovoltaic generators

List of scientific committee members:

  • R. Alcubilla (Universidad Politécnica de Cataluña, Spain)
  • C. Algora (IES-UPM, Spain)
  • P. Bermel (Purdue University, USA)
  • W. Chan (MIT, USA)
  • D. Chubb (NASA, USA)
  • Y. Cuminal (IES-U. Montpellier, France)
  • P.-O. Chapuis (CNRS, France)
  • J. Drevillon (Institut Pprime, France)
  • L. Fraas (JX Crystals, USA)
  • M. A. Green (UNSW, Australia)
  • K. Hanamura (Tokyo Inst. of Technology, Japan)
  • B-J. Lee (KAIST, Republic of Korea)
  • A. Martí (IES-UPM, Spain)
  • K. Park (Univ. Utah, USA)
  • P. Reddy (U. Michigan, USA)
  • E. Tournié (IES-U. Montpellier, France)
  • D. Trucchi (CNR - Institute of Structure of Matter, Italy)
  • E. Yablonovitch (University of California, Berkeley, USA)
  • H. Yugami (Tohoku University, Japan)
  • Z. Zhang (Georgia Tech, USA)


Selected papers will be published in a Special Issue of Optics Express (OSA).

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Systems I : Makoto Shimizu
Authors : Brian Hubert
Affiliations : MTPV Power Corporation

Resume : Waste heat is an enormous and largely untapped source of power that has the potential, once appropriately harnessed, to radically improve energy utilization on a global scale. MTPV Power Corporation is presently deploying thermophotovoltaic systems into the world?s most energy-intensive industries, with waste heat recovery applications in oil & gas, chemical processing, and glass, steel, and cement manufacturing. The company?s micron-gap technology is leveraged in its commercial systems to maintain nanometer-scale distances between emitters and photovoltaic receivers over very large areas in excess of 60 square centimeters. A robust chip module design has been proven to deliver more than a million chip lifetime operating hours, and multi-module systems have delivered tens of megawatt-hours of produced energy with greater than 99% uptime. Unique deployment infrastructure and balance-of-system architectures have been implemented to address the rigors of energy capture in challenging settings such as oil & gas flares, glass foundry flues, and steel mills.

09:45 Coffee break    
Authors : L. M. Fraas1, J. E. Avery1, L. Minkin1, Seth Hettinger1, Ben Francis1, L. Ferguson2
Affiliations : 1JX Crystals Inc, Issaquah, WA 98027, USA 2C12 Advanced Technologies LLC, Everett, WA, USA

Resume : 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).

Authors : S.V. Karnani, C. Mike Waits
Affiliations : Sensors and Electron Devices Directorate, U.S. Army Research Laboratory

Resume : Three core enablers are responsible for the resurged interest in fuel-fired thermophotovoltaics (TPV) for portable power sources: Advances in manufacturing, specifically, high resolution patterning and 3D printing of refractory materials; PV maturation, due to military and medical imaging; and advanced modeling tools for reactor and heat exchanger design. Given the complexity of the problem, however, the research focus tends toward the maturation of components, whether solely, on photovoltaics, spectral control, or heat sources. Even groups with a systems' focus tend to have a component preference. This binary distribution leaves a number of possibilities and few mechanisms to identify an optimum system path. To help navigate the space, we present a design 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. The model is composed of two sub-models: the Emitter-Cavity-TPV model, which incorporates empirical findings to, determine TPV conversion efficiency, effective emitter flux, cell thermal management and a boundary condition required to fully define the Reduced Order Reactor model, 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 efficiency, size, weight and power become clear.

Authors : Rajendra Bhatt, Ivan Kravchenko, Mool Gupta
Affiliations : University of Virginia, Charlottesville, VA 22904, USA; Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA; University of Virginia, Charlottesville, VA 22904, USA;

Resume : This paper presents the design, optimization, and fabrication of a high-efficiency planar STPV system comprising of spectrally selective absorber and emitter surfaces, and GaSb PV cells. The selective absorber consists of a micro-textured tungsten (W) surface that provides 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 and optimization of the transport of power at multiple stages of the STPV system. The system was 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 1.71 W/cm2 with a maximum conversion efficiency of 8.4% was measured at 1676 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 occurred at multiple stages of the energy conversion process are quantified. Combining the simulation and experimental results, guidelines to further improve the performance of STPV systems are also provided.

Authors : Ze Wang, Zhiguang Zhou, Peter Bermel
Affiliations : Purdue University; Apple Corporation

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.

Authors : M. Chirumamilla1, G. V. Krishnamurthy4, D. Jalas1, K. Knopp1, Q.Y. Häntsch2, G. Schneider2, M. Finsel6, T. Vossmeyer6, T. Krekeler5, M. Ritter5, A. Yu Petrov1,3, M. Störmer,4 and M. Eich1,4
Affiliations : 1Institute of Optical and Electronic Materials, Hamburg University of Technology, Eissendorfer Strasse 38, 21073 Hamburg, Germany 2Institute of Advanced Ceramics, Hamburg University of Technology, Denickestrasse 15, 21073 Hamburg, Germany 3ITMO University, 49 Kronverkskii Ave., 197101, St. Petersburg, Russia 4Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Strasse 1, 21502 Geesthacht, Germany 5Electron Microscopy Unit, Hamburg University of Technology, Eissendorfer Strasse 42, Hamburg 21073, Germany 6Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, Hamburg 20146, Germany E-Mail:

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. References: 1. Dyachenko, P.N et al., Nature Communications, vol. 7, no. 11809, 1?8 (2016) 2. Lang, S et al., Scientific Reports, vol. 7, no. 1, p. 13916?13916 (2017) 3. Leib, E.W et al., Journal of Materials Chemistry C, vol. 4, no. 1, pp. 62?74 (2016) 4. Biehs, S.-A. et al. ,Physical Review Letters, vol. 115, no. 17, p. 174301?174301 (2015) 5. Dyachenko, P.N. et al., Optics Express, vol. 23, no. 19, pp. A1236 (2015) 6. Chirumamilla, M. et al., Scientific Reports, vol. 9, Article number: 7241 (2019)

12:35 LUNCH    
Authors : Romain Cariou
Affiliations : Univ Grenoble Alpes, CEA, LITEN, DTS, LMPI, INES, 38000 Grenoble, France

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.

Authors : Kevin L. Schulte,1 Ryan M. France,1 Daniel J. Friedman,1 Colin C. Kelsall,2 Caleb Amy,2 Alina LaPotin,2 Asegun Henry,2 and Myles A. Steiner1
Affiliations : 1. National Renewable Energy Laboratory, United States of America 2. Massachusetts Institute of Technology, United States of America

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 thermophotovoltaic (TPV) re-generation 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 pumped ~2100 °C Si storage medium and multijunction PV cells targeting a 50% system efficiency (Henry, this workshop). Here, we describe the development of a novel 1.2 eV AlGaInAs/1.0 eV GaInAs tandem device for this unique application. The tandem is grown with a single graded buffer that is later removed, increasing spectral collection and power output, and removing a source of free carrier absorption. Materials challenges related to Al-containing III-V materials were overcome to develop an AlGaInAs absorber with a bandgap-voltage offset below 0.40 V. The contact and sheet resistances were optimized for high current density, with the device maintaining a fill factor of 87.6% at 4.6 A/cm2 in high-concentration flash measurements. We describe efforts to develop a rear reflector consisting of a dielectric/metal stack to maximize reflection of sub-band-gap photons back to the emitter. The system efficiency under current device metrics will be evaluated, and a roadmap for future performance will be outlined.

Authors : Iñigo Ramiro
Affiliations : Universidad Politécnica de Madrid

Resume : Low-temperature thermophotovoltaics (TPV) demand very low band gap (< 0.5 eV) semiconductors to maximize output power. As the band gap narrows, 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 tuned 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.

Authors : Ignacio Rey-Stolle
Affiliations : Universidad Politécnica de Madrid Solar Energy Institute

Resume : A solution for short term storage that provides energy on demand is required to achieve to long sought-for dream of a 100% Renewable Electricity System. Thermal energy storage at high temperatures (1800-2500ºC) 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 existing infrastructure 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 ; and 4) tandem configurations

Authors : Madhan K. Arulanandam (1,2), Myles A. Steiner (1), Richard R. King (2)
Affiliations : 1. National Renewable Energy Laboratory, Golden, CO, U.S.A. 2. Arizona State University, Tempe, AZ, U.S.A.

Resume : A grid-level energy storage system is envisioned where excess electricity is stored as high temperature heat in the 1700-2200?C range and extracted when needed using GaAs thermophotovoltaic (TPV) cells. The crucial cell factors affecting the TPV system efficiency are the sub-bandgap reflectance (Rsub) and the series resistance (Rs). To enhance Rsub in the required 0.9-10 micron range, a point-contacted GaAs TPV cell is fabricated with a low refractive index, low-loss dielectric spacer layer inserted between the semiconductor back contact layer and the metal back contact. This architecture boosts the recycling of sub-bandgap photons back to the thermal emitter by minimizing the metal absorption losses. Rsub increases with spacing and smaller diameters of the point contacts, at the expense of increased Rs. For a thermal emitter at 2200 °C, the TPV cells operate at a current density of ~10 A/cm2 where high Rs result in significant power loss, leading to a trade-off between Rsub and Rs. We have fabricated GaAs TPV cells with 10 ?m diameter point contacts spaced 50 µm apart, with a ~250 nm SiO2 spacer layer. Standard photovoltaic measurements indicate a Voc of 1.07 V and Jsc of 20 mA/cm2 (without ARC) at 1000 W/m2, and Rs of 10 mohm.cm2 for 0.1 cm2 cells. The cell efficiency peaked at 5 A/cm2 and has less than 3% relative efficiency loss at 11 A/cm2. Rsub as measured by FTIR was ~92%. Optimization of the SiO2 dielectric is expected to lead to Rsub ?96% and a TPV system efficiency ? 45%.

16:00 COFFEE BREAK    
POSTER SESSION : Alejandro Datas
Authors : Oleg S. Vasilyev, Petr V. Borisyuk, Yuri Yu. Lebedinskii
Affiliations : National Research Nuclear University MEPhI (Moscow Engineering Physics Institute); National Research Nuclear University MEPhI (Moscow Engineering Physics Institute); Moscow Institute of Physics and Technology (State University), National Research Nuclear University MEPhI (Moscow Engineering Physics Institute)

Resume : The study of a special thermophotovoltaic material, namely, a thin nanostructured film consisting of close-packed metal nanoscale particles (with diameter about 2-15 nm) with spatial ordering of nanoparticles in size deposited on the surface of a broadband dielectric material is presented. Due to the dimensional dependence of Fermi energy, the presence of spatially inhomogeneous distribution of metal nanoparticles by size leads to the spatial redistribution of the charge in such a system thus the potential difference in the same direction must be found. The appearance in this system of an electron excited by an external photon (even if low-energy with a wavelength greater than a micrometer) leads to the flow of the electron in the direction of the potential gradient caused by the spatial ordering of nanoclusters in size. Since nanoclusters are metal, this provides the ability to detect photons of different wavelengths and, therefore, provides a wide spectrum of radiation absorption of the proposed system. The presence of contact between nanoclusters means the preservation of electronic conductivity in such a system due to electron tunneling between nanoclusters and percolation effects. The obtained preliminary results on the formation and study of the properties of nanoparticle films have shown that the study of such systems can potentially lead to the change of energy efficiency and energy saving of modern thermal power sources to a completely new level.

Authors : Dilek Cakiroglu (1), Jean-Philippe Perez (1), Axel Evirgen (1,*), Christophe Lucchesi (2), Pierre-Olivier Chapuis (2), Thierry Taliercio (1), Eric Tournié (1), Rodolphe Vaillon (1,2)
Affiliations : 1. IES, Univ Montpellier, CNRS, Montpellier, France 2. Univ Lyon, CNRS, INSA-Lyon, Université Claude Bernard Lyon 1, CETHIL UMR5008, F-69621, Villeurbanne, France * Current address: III-V Lab, Thales Research and Technology, Route départementale 128, 91767 Palaiseau, France

Resume : To date, the champion thermophotovoltaic (TPV) cells have an efficiency in the range 24-29% [1-3]. 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 [4]) 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 [5], fabrication and characterization [6] 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 [7]. 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] Z. Omair et al., PNAS 116, 2019; [2] D. N. Woolf et al., Optica 5, 2018; [3] B. Wernsman et al., IEEE TED 51, 2004; [4] A. Krier et al., J. Electronic Materials 45, 2016; [5] Vaillon et al., Optics Express 27, 2019; [6] Cakiroglu et al., SEMSC 203, 2019; [7] Lucchesi et al., arXiv:1912.09394, 2019.

Authors : Ido Frenkel, Avi Niv
Affiliations : Department of Solar Energy and Environmental Physics, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev

Resume : Thermodynamics has been successful in finding the limits of different power conversion schemes and has advanced our understanding of physics and chemistry; thus, it is accepted as a universal truth subjecting all macroscopic objects. The photovoltaic effect, namely the radiative energy exchange of semiconductors, is, however, governed by the law of detailed-balance. To this date, it is not fully understood how thermodynamics pertains to the photovoltaic effect since the addition of detailed-balance forms an inconsistent set of constraints. In this work, we propose a unification of the first and second thermodynamic laws with the detailed-balance by identifying a third independent variable, in the form of sub-bandgap emissivity, in addition to the existing potential and temperature. This addition allows us to reformulate the three laws governing the photovoltaic effect in a consistent solvable manner, thus advancing our fundamental understanding of light-matter interactions. More importantly, this approach should point to the limiting factors of advanced photovoltaic concepts such as thermophotovoltaics, thermoradiative, and thermophotonics solar power conversion, radiative-cooling, and concentrated multi-junction solar cells for space missions, concepts that are instrumental for our ability to progress and mitigate climate change.

Authors : Alba Jiménez (1), Isidro Martín (2), Gema López (2), Alejandro Datas (1) and Carlos del Cañizo (1)
Affiliations : (1) Instituto de Energía Solar, Universidad Politécnica de Madrid, Av. de la Complutense, 30 28040, Madrid, Spain (2) Electronic Engineering Department, Universitat Politècnica de Catalunya, Jordi Girona 1-3, Barcelona 08034, Spain

Resume : Germanium is regarded as an excellent substrate for the development of low-cost thermophotovoltaic devices. However, the poor properties of the germanium oxide (water-soluble and thermodynamically unstable) which is rapidly formed in air, significantly jeopardizes surface passivation. In this work, a study of the influence of an in-situ H2 plasma cleaning before PECVD deposition of an amorphous silicon carbide stack is carried out ultimately demonstrating a very high minority carrier lifetime (> 1000 µs) when germanium oxide is effectively removed from the interface. Besides of the pre-cleaning step, the parameters for the deposition of amorphous silicon carbide has been also optimized, showing a vast influence of the process temperature in the studied range (150-250 °C), with more than 5-fold lifetime enhancement. Finally, the already passivated samples were subjected to a rapid thermal annealing at temperatures between 250 °C and 500 °C, which originates an additional improvement of the minority carrier lifetime, implying a thermally activated reaction for the surface passivation mechanism with an estimated activation energy of about 1.8 eV. Capacitance-voltage measurements will be conducted in order to determine the underlying mechanism of such reaction. This study will propitiate a greater understanding on the key elements of germanium surface passivation, eventually enabling the development of low cost and highly efficient thermophotovoltaic devices.

Authors : Pablo García-Linares, Juan Villa, Elisa Antolín, Simon Svatek, Marius Zehender, Irene Artacho, Esther López, Iván García, Ignacio Tobías, Antonio Martí and Alejandro Datas
Affiliations : Instituto de Energía Solar, Universidad Politécnica de Madrid, Av. de la Complutense, 30 28040, Madrid, Spain

Resume : In this work, modeling, epitaxy, fabrication and characterization of different interdigitated back contact (IBC) InGaAs thermophotovoltaic (TPV) cells are presented. PC1D modeling reveals the importance of an intrinsic base and an extremely thin emitter, both of which must be circumvented attending to practical fabrication risks. Assuming ideal growth conditions, the simulations also show how an emitter directly grown on top of the semiconductor substrate simplifies and reduces the layer structure at a minimum efficiency cost. The IBC TPV cell design is optimized using a quasi-3D distributed model fed with semi-empirical electronic parameters and solved by SPICE, leading to a fingerless (non-interdigitated) configuration that solely relies on the substrate for the lateral conduction. Different structures are grown by molecular beam epitaxy, from which 1 cm2 back contact TPV cells are fabricated according to this design. Characterization of the first prototypes shows promising results, validating the simulations. The fingerless back contact cells enable minimized shadowing and array-packing losses. Besides, the cleared front surface facilitates their integration in near-field and hybrid thermionic-photovoltaic arrangements, where micro-spacers are needed on the front side of the cell to get close enough to the incandescent source. The trade-off between substrate transparency and lateral series resistance is analyzed as a function of the substrate doping and thickness.

Authors : Gnanavel Vaidhyanathan Krishnamurthy* (1), Manohar Chirumamilla (2), Surya Snata Rout (3), Martin Ritter (3), Alexander Yu Petrov (2), Manfred Eich (2) & Michael Störmer (1)
Affiliations : (1) Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Strasse 1, 21502 Geesthacht,Germany; (2) Electron Microscopy Unit, Hamburg University of Technology, Eissendorfer Strasse 42, Hamburg 21073, Germany; (3) Institute of Optical and Electronic Materials, Hamburg University of Technology, Eissendorfer Strasse 38, Hamburg 21073, Germany

Resume : A Thermophotovoltaic system (TPV) converts radiant thermal energy directly into electricity using a photovoltaic cell. One of the key components for the effective functioning of the TPV is the thermal stability of the selective thermal emitters used. A selective thermal emitter is designed precisely to emit radiation that matches the band gap energy of the photovoltaic cell used. In our work, we fabricate a 1D selective emitters comprising of alternative layers of metal and dielectric materials. The intriguing choice of materials for this application is refractory materials because of their high melting points. The materials used in our selective emitters are W/HfO2, prepared by magnetron sputtering. In-situ XRD annealing experiments are carried out at temperatures above 800 °C and in a vacuum below 1e-5 mbar vacuum to validated the temperature stability. We report structural and morphological changes individually in both the layers and establish a failure mechanism at high temperatures that deteriorates the performance of the 1D selective emitter.

Authors : Daniel Milovich (1), Juan Villa (2), Elisa Antolin (2), Alejandro Datas (2), Antonio Marti (2), Rodolphe Vaillon (2,3,4), Mathieu Francoeur (1)
Affiliations : (1) Radiative Energy Transfer Lab, Department of Mechanical Engineering, University of Utah, Salt Lake City, UT 84112, USA; (2) Instituto de Energía Solar, Universidad Politécnica de Madrid, 28040 Madrid, Spain; (3) IES, Univ Montpellier, CNRS, Montpellier, France; (4) Univ Lyon, CNRS, INSA-Lyon, Université Claude Bernard Lyon 1, CETHIL UMR5008, F-69621, Villeurbanne, France

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.

Authors : Zhen Liu, Makoto Shimizu, Hiroo Yugami
Affiliations : Department of Mechanical Systems Engineering, Tohoku University, Sendai, 980-8579, Japan

Resume : The microstructure-based material has extraordinary controllability of spectral properties for various energy conversion systems, especially at high-temperature. With the adjustability of spectral properties and thermal stability, one of the important applications is as spectrally selective emitters to tailor the spectrum for the thermophotovoltaics system. Several large-scale fabrication methods of periodic microstructure offer a simple, high-throughput, and low-cost path to realize the practical application. However, such methods are afflicted with structural defects such as vacancy or distortion of microstructure which are randomly distributed in the surface, inducing the challenge to assess the optical performance of defective emitter. We propose a novel approach to reveal the microstructure condition using the diffraction imaging system and to assess the optical performance of the emitter. The phase retrieval algorithm is applied to the diffraction pattern which obtains from the emitters. The geometrical features of reconstructed surrogate images are analyzed quantitatively and the optical performance is calculated. This assessment method can significantly reduce the time for the large-scale emitter 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, it builds a bridge between the defective microstructure pattern and optical performance, which contribute to our understanding of the optical mechanisms of defects in the microstructure-based spectrally selective emitter.

Authors : Yang Chen and Heiner Linke
Affiliations : Division of Solid State Physics and NanoLund, Lund University, Box 118, 22100 Lund, Sweden

Resume : Heat-assisted light emission makes it possible to reach an energy converting efficiency above unity in light-emitting diodes (LEDs).Theoretical prediction and experiment verification have been reached within bulk system. However, the literature studies usually employ the optical density of the bulk system and it limits the application away from nano-emitters. As it is well-known, nanostructure modifies the optical density of states significantly.Such modification has the potential to increase the entropy of emission and lead to higher energy conversion efficiency. In this work, a self-consistent method is presented for the first time.The entropy of light emission with heat contribution for a nano-sized round sphere and the planar film can be calculated by this method. We conceptually prove the entropy is a powerful tool to understand the light emission at the nanoscale. By considering the entropy of light emission, we find a nanosphere can have much higher energy converting efficiency than bulk. In addition to the efficiency, we find that the light emission intensity per unit volume material goes up approximately inversely proportional to the sphere radius when the radius reduces from bulk to nanosize, which originally comes from the low photon recycling rate for a nanostructure. Similar results of efficiency and cooling power hold true for the thin film with a small thickness. Such findings indicate the nanostructure is a good candidate for high-efficiency LEDs and coolers.

Authors : Avinash Kumar, Monika Agrawal, Amartya Chowdhury
Affiliations : Centre for Energy and Environment, MNIT-Jaipur (Malaviya National Institute of Technology) J.L.N. Marg, Jaipur (India) -302 017

Resume : In indoor condition, the module has been tested at control parameters at STC(ambient temperature 25ºC, wind speed 1ms-1, radiation 1000Wm-2 at AM1.5 spectrum). Whereas, the temperature of the module in outdoor condition is about 20-30 ºC more than the ambient temperature. For the temperature reduction nowadays the radiative cooling concept has been taken into consideration. The implementation of solar radiative cooler is also varying according to its location in the module. Either it is applied on the front glass of the solar cell or applied as ARC. Both the method has its own benefits and restriction. The radiative cooler applied on the top of the solar front glass can be developed as a Bragg reflector which reduces the parasitic absorption. It does not allow to improve the temperature of the module as well as it emits radiation in the atmospheric window to reduce the module temperature further. This method has a limitation with EVA heat transfer coefficient value which is very low due to that the heat transfer through EVA is very low. Another method applied on the solar cell as ARC only which improve the absorption in 300-1100nm as well as improve the cooling effect by the radiative cooling method. However, both methods have able to reduce the temperature in the range of 5-6ºC.

Authors : Rimantas Gudaitis, Andrius Vasiliauskas, Asta Guobienė, Šarūnas Jankauskas, Viktoras Grigaliūnas, Sigitas Tamulevičius, Šarūnas Meškinis*
Affiliations : Institute of Materials Science of Kaunas University of Technology, Baršausko 59, Kaunas, Lithuania

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.

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09:45 COFEE BREAK    
ADVANCED CONCEPTS I : Mathieu Francoeur
Authors : Yoshitaka Okada
Affiliations : University of Tokyo, Research Center for Advanced Science and Technology (RCAST), Japan

Resume : The concept of intermediate-band assisted hot-carrier or up-conversion solar cell (IB-HCSC) is proposed in order to assist the extraction of hot carriers from an absorber that has an IB [1]. We studied a heterostructure based on 10 layers of In(Ga)As quantum dots (QDs) embedded in an AlGaAs single-junction solar cell designed for IBSC. The QD-IBSC was limited by thermal escape of photo-carriers from QDs at room temperature. In such case, fundamental improvement in conversion efficiency is only possible if the carriers in IB are not in thermal equilibrium with valence or conduction bands. The IB-HCSC provides a high-efficiency limit and enables us to work with various relaxation mechanisms such as thermalization, carrier-carrier scattering and thermal radiation. Under high concentrated illuminations, we confirmed the emergence of a hot carrier population or thermal up-conversion in QDs, which assists the IBSC by providing a thermoelectric gain in voltage without hindering the possibility of sequential 2-step photoabsorption (TSPA). Absolute intensity calibrated photoluminescence spectroscopy indicated that the triggering mechanism happens when the QD ensemble is estimated to have a high carrier concentration behaving almost as a metal-like IB. Experimental results suggest that the hot carrier effect is commonly observed in quantum heterostructures [2] and directions for improvements of IB-HCSCs will be proposed. [1] B. Behaghel et al, Semicond. Sci. Technol. 34, 084001 (2029). [2] D. J. Farrell et al, Nature Commun., 8685 (2015).

Authors : Martin Josefsson; Artis Svilans; I-Ju Chen; Steven Limpert; Adam M. Burke; Eric A. Hoffmann; Sofia Fahlvik; Jonatan Fast; Claes Thelander; Martin Leijnse; Heiner Linke
Affiliations : NanoLund and Solid State Physics, Lund University, Sweden

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)

Authors : Hamidreza Esmaielpour*(1), Daniel Suchet(2), Laurent Lombez(1)(3), Amaury Delamarre(4), Soline Boyer-Richard(5), Alain Le Corre(5), Olivier Durand(5), and Jean-François Guillemoles(1)(3)
Affiliations : (1) Institut Photovoltaique d?Ile de France (IPVF), 18 boulevard Thomas Gobert, 91120 Palaiseau, France; (2) Ecole Polytechnique, Institut Photovoltaïque d?Ile-de-France UMR 9006, 18 boulevard Thomas Gobert, 91120 Palaiseau, France; (3) CNRS-Institut Photovoltaique d?Ile de France (IPVF), UMR 9006, 18 boulevard Thomas Gobert, 91120 Palaiseau, France; (4) Centre for Nanoscience and Nanotechnology (C2N), CNRS, University Paris-Sud/Paris-Saclay, 10 boulevard Thomas Gobert, 91120 Palaiseau, France; (5) Univ Rennes, INSA Rennes, CNRS, Institut FOTON ? UMR 6082, Rennes, France

Resume : Absorption of photons with energies above the band gap of solar cells creates electron-hole pairs with excess kinetic energies. In most type of solar cells, this excess kinetic energy is lost via thermalization mechanism, which is one of the major loss processes in photovoltaic solar cells. Hot carrier solar cells (HCSCs) are proposed to convert the excess kinetic energy of (hot) carriers to electricity via inhibiting thermalization loss. The operation of HCSCs therefore combines the photovoltaic and thermoelectric effects. The latter one follows the Seebeck effect, where the gradient in temperature between hot and cold sides of a material creates an electric potential in the system. Determination of Seebeck coefficients of hot carrier absorbers is important to evaluate their applications in HCSCs. It is possible to find Seebeck coefficients of semiconductors via photoluminescence (PL) spectroscopy, which is a contact-less experiment free of electrical artifacts present in classical measurements. Here, we discuss our results in determination of the photo-Seebeck coefficient of an InGaAsP single quantum well structure via continuous wave PL spectroscopy at various excitation powers and lattice temperatures. In addition, using hyperspectral luminescence imaging, we are able to differentiate the longitudinal (out-of-plane) and transverse (in-plane) photo-Seebeck coefficients of the QW structure via fitting emitted PL spectra with the generalized Planck?s radiation law.

Authors : Toufik Sadi, Ivan Radevici, Vilgail? Dagyt?, Jani Oksanen
Affiliations : Engineered Nanosystem Group, Aalto University

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 TPV systems is generally determined by Planck's law in the far field, their fundamental performance can be dramatically affected by near field coupling between the thermal emitter and the photovoltaic cell, and by transforming the thermal emitter exploit electroluminesce. Especially taking advantage of a thermally enhanced electroluminescent emitter as the source of radiation fundamentally alters the pertinent thermodynamics, allowing a boost of the achievable power densities by orders of magnitude as well as access electroluminescent and thermophotonic (TPX) cooling. 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, however, functional thermophotonic devices are yet to be demonstrated experimentally, due to the need to simultaneously overcome 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.

Authors : C. Lucchesi(1), D. Cakiroglu(2), J-P. Perez(2), T. Taliercio(2), E. Tournié(2), P-O. Chapuis(1), R. Vaillon(2,1)
Affiliations : (1) Univ Lyon, CNRS, INSA-Lyon, Université Claude Bernard Lyon 1, CETHIL UMR5008, F-69621, Villeurbanne, France; (2) IES, Univ Montpellier, CNRS, Montpellier, France

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 7-fold enhancement of the electrical power photogeneration compared to far-field illumination, leading to power densities as high as 7500 W.m-2, 3 orders of magnitude higher than in [1–3], and near-field 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., arXiv:1911.11137 (2019) [4] D. Cakiroglu, et al. Solar Energy Materials and Solar Cells, 203, 110190 (2019) [5] C. Lucchesi, et al., arXiv:1912.09394 (2019)

12:25 LUNCH    
SYSTEMS II : Rodolphe Vaillon
Authors : Veronika Stelmakh
Affiliations : Mesodyne

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.

Authors : Alejandro Datas, Esther López, Alba Ramos, Carlos del Cañizo, Antonio Martí
Affiliations : Instituto de Energía Solar - Universidad Politécnica de Madrid

Resume : Latent heat thermophotovoltaic (LHTPV) batteries comprise a very high melting point phase change material (PCM) combined with thermophotovoltaic (TPV) thermal-to-electric 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 into electricity by the TPV converter. In this study we review the PCM and TPV cell options that result in optimal battery designs. Some relevant battery parameters such as self-discharge, round-trip efficiency, charge/discharge time (power-to-energy ratio), and cost, are discussed based on the choice of PCM and TPV cell components. 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 %), the low cost of the PCMs (< 10 ?/kWh), and the lower relative thermal losses at larger store volumes, results in optimal system designs with small power-to-energy ratios, i.e. long discharge times and large storage capacities. We show that LHTPV batteries could compete with electrochemical batteries when the charging electricity price is low, provided that TPV reaches high enough power density and conversion efficiency.

Authors : Asaka Kohiyama, Makoto Shimizu, Kana Konno, Zhen Liu, Hiroo Yugami
Affiliations : Graduate School of Engineering, Tohoku University

Resume : Total efficiency of solar-TPV systems can be described with two types of efficiencies which we call the extraction efficiency, which expresses how effectively solar incident is converted into emitter thermal radiation, and the PV cell conversion efficiency, which express how effectively the thermal radiation is converted into electricity. It means it is essential to achieve unidirectional radiative transfer from incident solar to emitter thermal radiation and spectral matching between emitter thermal radiation and TPV cell useful wavelengths. Here, a monolithic cubic absorber/emitter of which top surface is the absorber and the other surfaces are the emitters is demonstrated to achieve high extraction efficiency. Every surface has spectrally selective property to reduce thermal radiation loss from the absorber and the emitters. It is revealed that almost 70% of extraction efficiency can be expected. Furthermore, the cubic absorber/emitter is set in a GaSb TPV cell basket which can confine emitter thermal radiation. According to photon recycling from the cell to the emitter by highly confined geometry, PV conversion efficiency can be also improved from non-confinement geometry. The highest system efficiency of 5.6% is obtained at emitter temperature of 1436K in experimental STPV systems in which employ cube absorber/emitter systems. The experimental results indicate that the cube STPV systems have high potential system efficiency exceeding 10? at emitter temperature of 1600K.

Authors : Bierman, D.M.(1), Narayan, T.C.(1), Johnson, B.A.(1), Young, A.R.(1), Nizamian, D.P.(1), Arulanandam, M.(2, 3), Kuritzky, L.Y.*(1), Ponec, A.J.(1), Santhanam, P.(1), Luciano, C.(1), Slack, J.L.(4), King, R.R.(2), Steiner, M.A.(3), Briggs, J.A.(1).
Affiliations : (1) Antora Energy, Inc., USA (2) Arizona State University, USA (3) National Renewable Energy Laboratory, USA (4) Lawrence Berkeley National Laboratory, USA * lead presenter

Resume : Thermophotovoltaic (TPV) devices are solid state heat engines that convert thermal radiation into electricity using semiconductor diodes. In general, higher TPV efficiencies can be realized with higher temperature emitters and wider band gap photovoltaics (PV). High temperature emitters are accessible in thermal energy storage applications. We have demonstrated a world record 32% +/- 2% TPV conversion efficiency with a 0.9 cm2 GaAs-based PV device under a 2430 °C thermal emitter, producing an electrical output power of 2.23 W and a current density of 2.35 A/cm2. Critical to the result was the cell?s high reflectance of photon energies below the device band gap (94.6% weighted average reflectance over a 2200 °C blackbody spectrum). Unlike solar PV, for which sub-band gap light is lost, a TPV cell can reflect and recycle sub-band gap light to the thermal emitter to avoid a key efficiency penalty. The demonstration was made on a custom-built measurement platform in which a ~100 cm2 graphite thermal emitter was heated under vacuum and tested between 970 °C and 2440 °C. The TPV efficiency was evaluated by measuring the electrical output of the TPV compared with the net input power, accounting via calorimetry for all thermal losses in the system. The measured TPV efficiency as a function of thermal emitter temperature was corroborated by our full system modeling predictions. As far as the authors are aware, this is the highest TPV conversion efficiency ever measured, and device improvements should yield > 40% efficiency in the near future.

Authors : L. M. Fraas, J. E. Avery, L. Minkin, Seth Hettinger, Ben Francis
Affiliations : JX Crystals Inc, Issaquah, WA 98027, USA

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.

16:00 COFFEE BREAK    
Authors : Takuya Inoue, Masahiro Suemitsu, Takashi Asano, Susumu Noda
Affiliations : Kyoto University

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.

Authors : Rongqian Wang*, Jincheng Lu & Jian-Hua Jiang
Affiliations : School of Physical Science and Technology & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China * presenter

Resume : Near-field Thermophotovoltaic (NTPV) cells have attracted a wide range of research interest due to their enhanced performances compared to far-field counterparts. However, most of the existing studies are focused on the regime where the thermal emitter has very high temperature, e.g., higher than 1000 K, corresponding to solar radiations or heat radiations from a secondary emitter which receives the solar energy. Besides, due to the considerable frequency mismatch between the thermal emitter and the PV cell, the near-field effect becomes ineffective and the performance of the NTPV systems is significantly reduced. In our previous paper [1], We focus on the situation where the temperature of the thermal emitter is in the range of common industrial waste heat, i.e., 400 K ~ 800 K. To convert such mid infrared thermal radiation into electricity and to make the photon tunneling efficient, we propose to use graphene-h-BN heterostructures as the emitter and the graphene-covered InSb p-n junction as the TPV cell. We show that the optimal output electrical power can reach 76000 Watts per square meters and the best efficiency is achieved with 33% of the Carnot efficiency. These results show that the performances of near-field thermophotovoltaic systems can be comparable with or even superior to the state-of-the-art thermoelectric devices. The significant improvement of the thermophotovoltaic performance is understood as due to the resonant coupling between the emitter and the p-n junction, where the surface plasmons in graphene and surface-phonon polaritons in boron nitride play crucial roles. It is remarked that our findings are consistent with the study in Ref. [2], where the synergy between near-field thermal radiation and inelastic thermoelectricity is shown to have considerably improved performance even in the linear-transport regime. Apart from this, our work is also based on the previous studies where the near-field effects are shown to improve the heat radiation flux by orders of magnitude via infrared hyperbolic metamaterials [3-4]. Reference papers: [1] Rongqian Wang, Jincheng Lu and J.-H. Jiang, Enhancing Thermophotovoltaic Performance using Graphene-BN-InSb Near-Field Heterostructures, Phys. Rev. Applied 12, 044038 (2019). [2] Jian-Hua Jiang and Yoseph Imry, Near-field three-terminal thermoelectric heat engine, Phys. Rev. B 97, 125422 (2018). [3] Bo Zhao and Zhuomin Zhang, Enhanced photon tunneling by surface plasmon polaritons in graphene/h-BN heterostructures, J. Heat Transfer 139, 022701 (2015). [4] Bo Zhao, Brahim Guizal, Zhuomin Zhang, Shanhui Fan, and Mauro Antezza, Near-field heat transfer between graphene/hBN multilayers, Phys. Rev. B 95, 245437 (2017).

Authors : Mikyung Lim, Jaeman Song, and Bong Jae Lee
Affiliations : Korea Institute of Machinery and Materials; KAIST; KAIST

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.

Authors : John DeSutter (1), Lei Tang (2), and Mathieu Francoeur (1)
Affiliations : (1) Radiative Energy Transfer Lab, Department of Mechanical Engineering, University of Utah, Salt Lake City, UT 84112, USA; (2) Department of Mechanical Engineering, University of California, Berkeley, Berkeley, CA 94720, USA

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.

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09:45 COFFEE BREAK    
Authors : Yu-Bin Chen, Yu-Fan Chang, Chu-Yang Wang, Jui-Yung Chang
Affiliations : Dr. Yu-Bin Chen Department of Power Mechanical Engineering, National Tsing Hua University No.101, Section 2, Kuang-Fu Rd., Hsinchu City 30013, Taiwan ; Mr. Yu-Fan Chang Department of Mechanical Engineering, National Cheng Kung University No.1, University Rd., Tainan City 70101, Taiwan ; Mr. Chu-Yang Wang Department of Power Mechanical Engineering, National Tsing Hua University No.101, Section 2, Kuang-Fu Rd., Hsinchu City 30013, Taiwan ; Dr. Jui-Yung Chang Department of Mechanical Engineering, National Chiao Tung University No. 1001, Ta Hsueh Road, Hsinchu City 30010, Taiwan

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.

Authors : Sakakibara, R.* (1, 2), Chan, W.R. (1), Stelmakh, V. (1), Geil, R.D. (3), Ghebrebrhan, M. (4), Joannopoulos, J.D. (1, 5), Solja?i?, M. (5) & ?elanovi?, I. (1)
Affiliations : (1) Massachusetts Institute of Technology, Institute for Soldier Nanotechnologies, Cambridge, Massachusetts, United States (2) Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, Cambridge, Massachusetts, United States (3) University of North Carolina, Chapel Hill, Department of Applied Physical Sciences, North Carolina, United States (4) U.S. Army Natick Soldier Research, Development, and Engineering Center, Natick, Massachusetts, United States (5) Massachusetts Institute of Technology, Department of Physics, Cambridge, Massachusetts, United States * lead presenter

Resume : Fuel-based thermophotovoltaic (TPV) systems are emerging as small, portable generators for sensors and robotic platforms. 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 with hafnium oxide can enable even higher optical performance with an in-band emissivity of 0.8-0.9, which in turn enables high power density and spectral efficiency relative to unfilled tantalum PhCs. However, previous demonstrations of the filled PhC [3] have suffered from poor control of surface topography, a side effect of the cavity filling step. Here, we present a fabrication method for improved surface flatness, which is promising for realizing the filled PhC's theoretical performance. We also use room temperature reflectance measurements and full system simulations to assess system performance gains. This selective emitter paves the way toward efficient and portable mesoscale generators for off-the-grid applications. [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

Authors : Manohar Chirumamilla* (1), Gnanavel Vaidhyanathan Krishnamurthy (2), Tobias Krekeler (3), Surya Rout (3), Martin Ritter (3), Michael Störmer (2), Alexander Yu. Petrov (1,4) and Manfred Eich (1,2)
Affiliations : (1) Institute of Optical and Electronic Materials, Hamburg University of Technology, Eissendorfer Strasse 38, Hamburg 21073, Germany. (2) Institute of Materials Research, Helmholtz-Zentrum Geesthacht Centre for Materials and Coastal Research, Max-Planck-Strasse 1, Geesthacht 21502, Germany. (3) Electron Microscopy Unit, Hamburg University of Technology, Eissendorfer Strasse 42, Hamburg 21073, Germany. (4) ITMO University, 49 Kronverskii Avenue, Saint Petersburg 197101, Russia.

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).

Authors : Dr. Mool Gupta, Rajendra Bhatt
Affiliations : Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, Virginia 22904, USA

Resume : Thermophotovoltaics (TPV) is a versatile technology to generate high electrical power density utilizing multiple sources of heat, such as solar irradiation, radioisotope heaters, combustible materials, thermal storage systems, waste industrial heat etc., as input. TPV systems aim to surpass the efficiency beyond the Shockley-Queisser limit for photovoltaic conversion by tailoring the spectrum of the incident solar light to match the spectral response of a PV cell. Spectrally selective absorbers and emitters can greatly enhance the TPV conversion efficiency by maximizing the absorption of the incident sunlight and suppressing the emission of sub-bandgap and excessive energy photons. One approach of achieving spectral selectivity is through the use of micro and nanostructures to control light emission from surfaces. This presentation reviews optical modeling and characterization techniques of various types of novel nanostructures, including random textures, nanocones, nanoholes, and multilayer metal dielectric stack etc., for the design of high-performance selective surfaces needed for efficient TPV systems. In addition, the fabrication of a GaSb-based experimental TPV system comprising a multilayer metal-dielectric (Si3N4-W-Si3N4) coating-based selective emitter is also presented. The performance of the TPV system was evaluated using a high-power laser as a simulated input for concentrated solar power. The overall power conversion efficiency of 8.4% was measured at 1676 K.

Authors : Richard R. King, Eric Y. Chen, Madhan Arulanandam, and Sean Babcock
Affiliations : Arizona State University, Tempe, Arizona, USA 85281

Resume : Thermophotovoltaic (TPV) cells benefit from high reflectance at the back surface to return unused sub-bandgap photons to the thermal radiator. In recent years, high back reflectance near the bandgap energy has also been found to increase photon recycling superlinearly as back reflectance approaches unity, resulting in a high photon gas density in the cell and boosting external radiative efficiency and voltage. In fact, operation in this regime of strong photon recycling is essential for cells to approach their detailed-balance efficiency limit. In this paper we examine the further increase in photon density, carrier density, voltage and efficiency in TPV cells possible when reflectance is increased for near-bandgap photons and off-axis incidence at the front surface as well. We investigate building these wavelength and angle-selective filters on the TPV cell front surface with relatively simple and readily available layered materials. In extreme cases, electronic states in the TPV cell absorber material are filled up to the front reflector cutoff energy, resulting in an absorber material with higher effective bandgap, which can increase efficiency relative to the original bandgap of the cell absorber. High front reflectance near the bandgap can of course also reduce photogeneration from incident light, and we investigate the resulting trade-offs and opportunities for TPV cell and system efficiency, and the sharp differences from the case of solar cells.

12:15 LUNCH    
Authors : Zhuomin Zhang and Dudong Feng
Affiliations : The George W. Woodruff School of Mechanical Engineering Georgia Institute of Technology Atlanta, GA 30332 USA

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 research theoretical study on analyzing the dark current and chemical potential distribution in near-field thermophotovoltaic devices.

Authors : Myrto Zeneli, M.Z. (1),(2), Alessandro Bellucci, A.B. (3), Gianfranco Sabbatella, G.S. (4), Daniele Trucchi, D.T. (3), Alejandro Datas, A.D. (5), Aristeidis Nikolopoulos, A.N. (1) & Nikos Nikolopoulos, N.N.(1)
Affiliations : (1) Chemical Process and Energy Resources Institute, Centre for Research and Technology Hellas, Thermi, 57001Thessaloniki, Greece (2) Laboratory of Steam Boilers and Thermal Plants, National Technical University of Athens (NTUA), 9 Heroon Polytechniou Str., 15780, Zografou (3) Institute of Structure of Matter ISM-CNR ? DiaTHEMA Lab, Via Salaria km 29.300, 00015 Monterotondo (RM), Italy (4) IONVAC - Ionvac Process Srl.: Viale Anchise, 24/25 00040 Pomezia - Roma - Italy (5) Instituto de Energía Solar, Universidad Politécnica de Madrid, 28040 Madrid, Spain

Resume : Dielectric microspacers (DMS) is a novel microtechnology that can be used to achieve a fixed sub-micron gap distance between two separated surfaces, such as the electrodes of a vacuum diode or the emitter and the cell of a near-field thermophotovoltaic converter. This work investigates the possibility of integrating this technology into a hybrid thermionic-photovoltaic converter operating at ultra-high temperatures (>1000 oC). One of the challenges involved in this system is the flow of excess thermal energy from the cathode to the anode through the DMS that might cause the PV cell overheat. A 3D computational fluid dynamics model is developed in Fluent v17.1 solver to assess the thermal behavior of this device, when the two electrodes are separated at a distance of 8-10 ?m. The numerical model simulates the heat transfer through conduction across the system components, and incorporates the net photon/electron flux between the two electrodes through proper user-defined functions. Different cathode temperatures within the range of 1500-2500 K and various DMS shapes (capillary and cylindrical), patterns (e.g. ring shaped) and sizes are studied. Later on, a parametric study on the materials thermal properties is carried out. The main target of this analysis is to assess, by implementing an advanced simulation method, if the DMS technology can be integrated into either thermionic or thermophotovoltaic devices, without causing any risk of the collector/cell overheat or even its mechanical failure.

Affiliations : ETIENNE BLANDRE, Institut Pprime, CNRS, Université de Poitiers, ISAE-ENSMA,F-86962 Futuroscope Chasseneuil, France ; RODOLPHE VAILLON, IES, Université de Montpellier, CNRS, 34095 Montpellier, France; JÉRÉMIE DRÉVILLON, Institut Pprime, CNRS, Université de Poitiers, ISAE-ENSMA,F-86962 Futuroscope Chasseneuil, France

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.

Authors : Tobias Burger, Dejiu Fan, Sean McSherry, Bosun Roy Layinde, Caroline Sempere, Stephen Forrest, Andrej Lenert
Affiliations : University of Michigan

Resume : With advances in manufacturing enabling high-quality thin-film photovoltaic materials, the key barrier to approaching thermodynamic limits has become our inability to control the radiative processes within the TPV cell. Existing cells lack the ability to i) selectively absorb above-bandgap photons (i.e., selectivity) and ii) directionally suppress radiative recombination of electron-holes pairs (i.e., directivity). These shortcomings have resulted in significant voltage and efficiency losses relative to thermodynamic limits. This presentation will highlight our recent work on designing TPV cell architectures that impart both selectivity and directivity in thin-film cells. Our work exploits a recent technique for fabrication of high-quality thin-film InGaAs cells. This technique facilitates the patterning of both sides of the active InGaAs layer. It also offers a clear path to significant cost reductions by allowing for reuse of the growth wafer.

Authors : Panagiotis Stamatopoulos, P.S.*(1), Myrto Zeneli, M.Z. (1), (2), Aristeidis Nikolopoulos, A.N. (1), Alessandro Bellucci, A.B. (3), Daniele Trucchi, D.T. (3) & Nikos Nikolopoulos, N.N. (1).
Affiliations : (1) Chemical Process and Energy Resources Institute, Centre for Research and Technology Hellas, Thermi, 57001Thessaloniki, Greece; (P.S.); (M.Z.); (A.N.); (N.N.) (2) Laboratory of Steam Boilers and Thermal Plants, National Technical University of Athens (NTUA), 9 Heroon Polytechniou Str., 15780, Zografou (3) Institute of Structure of Matter ISM-CNR ? DiaTHEMA Lab, Via Salaria km 29.300, 00015 Monterotondo (RM), Italy

Resume : During the last years, innovative concepts have been introduced into modern electrical devices, such as multi-junction solar cells and thermophotovoltaic converters. The accurate estimation of the conversion efficiency of such devices has been the driving force to build several numerical tools that can assist their design process. This work aims to develop an in-house code to simulate a 1D p-n junction diode under equilibrium and non-equilibrium conditions. For non-equilibrium conditions, two cases are tested: electron/hole excitation under (1) only bias voltage, and (2) bias voltage and device illumination, either from solar radiation or from a thermally heated emitter. The drift-diffusion and Poisson?s equations are solved using a finite element method, which is based on a piecewise nonlinear Petrov-Galerkin method of second-order accuracy. The total current is evaluated in a post-process manner using the Scharfetter-Gummel scheme. Initially, the model is verified against results obtained from freeware SimWindows32. Later on, a parametric analysis is conducted for various temperatures and semiconductor materials, i.e. GaAs, InGaAs. In contrast to other solvers, this one takes into account the model parameters dependence with temperature, whilst it can be extended to incorporate the effects of thermionic emission in a thermionic device and 2D spatial effects. Finally, the developed code can be used as a stand-alone tool or its results can be integrated into a CFD model, in order to evaluate the thermal performance of a solid-state device.

16:00 COFFEE BREAK    
ADVANCED CONCEPT III : Mathieu Francoeur
Authors : St-Gelais, R.(1)*, Bhatt, G.R.(2), Zhao, B.(3), Roberts, S.(2), Datta, I.(2), Mohanty, A.(2), Lin, T.(2), Hartmann, J.-M.(4), Fan, S.(3), Lipson, M.(2)
Affiliations : (1) Department of Mechanical Engineering, University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada (2) Department of Electrical Engineering, Columbia University, New York, New York 10027, USA (3) Department of Electrical Engineering, Ginzton Laboratory, Stanford University, Stanford, California 94305, USA (4) CEA · Laboratoire d'Électronique des Technologies de l'Information (LETI) Minatec Campus, Grenoble, France.

Resume : In near-field TPV, extreme proximity (i.e., sub-100 nm separation) between the thermal radiator and the photovoltaic (PV) cell is required for overcoming classical blackbody radiation predictions. In this regime, enhancement of radiated power may allow TPV systems operating at low temperatures (< 900 K), where it may notably provide an efficient solution for recycling of waste heat into electricity. However, achieving sub-100 nm separation between parallel surfaces while maintaining a temperature difference of several hundred degrees and avoiding contact is a significant challenge, which so far has prevented development of practical near-field TPV. 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. Our technology meets 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. As such, interest from the PV community is, in our opinion, the next important step for the development of near-field TPV.

Authors : A. Bellucci1, M. Girolami1, M. Mastellone1, S. Orlando, R. Polini1,3, V. Serpente1, and D.M. Trucchi1 E. Antolín2, P.G. Linares2, J. Villa2, A. Martí2, and A. Datas2
Affiliations : 1- Institute for Structure of Matter ISM-CNR, Rome, Italy 2- Instituto de Energía Solar – Universidad Politécnica de Madrid, Madrid, Spain 3- Dept. of Chemical Sciences and Technologies – Univ. di Roma “Tor Vergata”, Rome, Italy

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.

Authors : V. Serpente, A. Bellucci, M. Girolami, M. Mastellone, A. Mezzi, S. Kaciulis, R. Polini, V. Valentini, and D. M. Trucchi
Affiliations : V. Serpente; A. Bellucci; M. Girolami; M. Mastellone, V. Valentini; D. M. Trucchi, DiaTHEMA Lab, Istituto di Struttura della Materia, Consiglio Nazionale delle Ricerche (ISM-CNR), Via Salaria km 29.300, Monterotondo Scalo (RM), 00015, Italy M. Mastellone, Dipartimento di Scienze di Base ed Applicate per l'Ingegneria, Sapienza Università di Roma, Via A. Scarpa 14, 00161, Rome, Italy A. Mezzi; S. Kaciulis, Istituto per lo Studio dei Materiali Nanostrutturati, Consiglio Nazionale delle Ricerche (ISMN-CNR), Via Salaria km 29.300, Monterotondo Scalo (RM), 00015, Italy. R. Polini, Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma ?Tor Vergata?, Via della Ricerca Scientifica 1, Rome, 00133, Italy

Resume : The improvement in solar cells performances is leading the researchers to challenging ideas, like the development of hybrid thermionic-thermophotovoltaic (TIPV) converters, where both electrons and photons are exploited, resulting in a more efficient power production than the separated thermophotovoltaic and thermionic devices. Efficient TIPV converters need suitable materials, especially for the TIPV anode: it must have a work function lower than the cathode?s one to ensure the electron collection; meanwhile, it must allow the transfer of photons from the cathode to the photovoltaic (PV) cell. A possible solution is the deposition of a functional coating on the surface of the PV cell and barium fluoride (BaF2) can be considered a good candidate for these purposes: BaF2 is well-known material for its transparency between 0.2 and 5 µm and the influence of a BaF2 coating on the work function reduction was already studied. Here we study the influence of film thickness and chemical composition of barium fluoride thin films on GaAs substrates on the related work function. Electronic spectroscopies (XPS, UPS) reveal a reduction of work function for the heterostructure to 2.1 eV, then confirmed by output characteristics collected from a specifically realized thermionic converter. The low work function, together with a negligible optical absorption, makes feasible the practical application of barium fluoride coatings within hybrid thermionic-thermophotovoltaic devices.

Authors : Alejandro Datas (1), Rodolphe Vaillon (2), Alessandro Bellucci (3), Daniele Trucchi (3), and Antonio Martí (1)
Affiliations : (1) Instituto de Energía Solar, Universidad Politécnica de Madrid, Madrid, Spain (2) IES, Univ Montpellier, CNRS, Montpellier, France (3) Istituto di Struttura della Materia (ISM-CNR), Rome, Italy

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. We will show simulation results of this device based on fluctuation electrodynamics and Langmuir theories, which indicate that high efficiency and power densities are attainable simultaneously at moderate temperatures (< 1000 ºC). A major advantage of this proposed device is that it significantly mitigates the scalability issues of NF-TPV. A review on the materials that are needed to implement this concept in practice will be also presented.

18:30 AWARD CEREMONY followed by SOCIAL EVENT    

No abstract for this day

No abstract for this day

Symposium organizers
Alejandro DATASTechnical University of Madrid

Instituto de Energía Solar, Avda. Complutense, 30, 28040, Madrid, Spain

+34 910672554
Makoto SHIMIZUTohoku University

Aoba 6-6-01, Aramaki, Aoba-ku, Sendai, 980-8579, Japan

+ 81 022 795 6925
Mathieu FRANCOEURUniversity of Utah

1495 E 100 S (1550 MEK) - SLC UT 84112, USA

+1 801 581 5721
Rodolphe VAILLON (Main)Univ. Montpellier, CNRS

Institut d’Electronique et des Systèmes - 860, rue Saint Priest - Bâtiment 5 - CC 05001 - 34095 Montpellier Cedex 5, France

+33 (0)4 67 14 32 27