preview all symposia

2020 Spring Meeting

Energy materials


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
Start atSubject View AllNum.
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 : 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 : 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 : 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.

Start atSubject View AllNum.
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 : 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.

12:25 LUNCH    
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.

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.

Start atSubject View AllNum.
09:45 COFFEE BREAK    
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 : 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.

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