Thermophotovoltaics (TPVs) have the potential to exhibit higher power conversion efficiencies than traditional photovoltaics (PVs), with a broad range of applicability from waste recovery systems to aerospace solutions. They operate by preferentially radiating above bandgap photons via a high temperature optical emitter, whose spectrum is tuned through choice of materials and geometry. For TPV to be practically implemented, the emitters must be designed with a simple optical structure while remaining thermally stable. Here, we demonstrate coating/substrate bilayer thin films as a solution to these design criteria. With the optical data of 53 high melting point materials, we simulate the bilayer emissivity as a function of coating thickness for each thermochemically stable emitter operating at 1,800 °C. Emitter-cell systems are characterized by the cell power density and TPV conversion efficiency, constituting a TPV performance metric space. For a given bilayer and bandgap these coating thickness parameterized figures of merit form a performance metric curve, with the best points defining a tradeoff zone. We screen the resulting performance metric curves, identifying trends based on optical properties of the system, finding a high degree of tunability through the material selection step. For GaSb cells, >49% efficiency is achieved using AlN/W, a 5.6% increase over bulk W. By calculating the figures of merit for all systems with varying bandgap, we find unique emitter choices per PV cell for achieving the highest potential efficiency. Our materials screening approach uses physical data to identify improvements to emitters for experimental TPV designs.
Semiconductor emitter can possibly achieve sharp cutoff wavelength due to its intrinsic bandgap absorption and almost zero sub-bandgap emission without doping. A germanium wafer based selective emitter with front-side antireflection and backside metal coating is studied here for thermophotovoltaic (TPV) energy conversion. Optical simulation predicts the spectral emittance above 0.9 in the wavelengths from 1 to 1.85 um and below 0.2 in the sub-bandgap range with sharp cutoff around the bandgap, indicating superior spectral selectivity behavior. This is confirmed by excellent agreement with indi…
Graphene---hexagonal-boron-nitride---InSb near-field structures are designed and optimized to enhance the output power and energy efficiency of the thermophotovoltaic systems working in the temperature range of common industrial waste heat, $400~\rm K \sim 800~\rm K$, which is also the working temperature range for conventional thermoelectric devices. We show that the optimal output electric power can reach $3.5\times10^{4} \rm\ W/\rm m^2$ for the system with a graphene---hexagonal-boron-nitride heterostructure emitter and a graphene-covered InSb cell, whereas the best efficiency is achieved b…
Thermophotovoltaic systems in principle enable utilization of heat that is usually regarded as wasted energy. However, the wavelength selectivity of the thermal emitter required for high efficiencies is rather difficult to control with conventional designs. Here, we design a thermophotovoltaic system comprising silicon rods as thermal emitter with a relatively narrow emission spectrum and a photovoltaic cell with a band gap corresponding to 1.76 $μ$m, and verify efficient power generation. By accurately measuring the heat flux that enters the emitter, the emitter temperature, and the electrica…
A standardized method for measuring thermophotovoltaic (TPV) efficiency has not been yet established, which makes the reported results difficult to compare. Besides, most of the TPV efficiencies reported to date have been obtained using small view factors, i.e., large cell-to-emitter distances, so the impact of the series resistance is usually underestimated, and the optical cavity effects, i.e., the multiple reflections taking place between the emitter and the cell, are not accounted for experimentally. In this work, we present an experimental setup able to measure the TPV efficiency under hi…
Near-field thermophotovoltaic systems functioning at 400$\sim$900~K based on graphene-hexagonal-boron-nitride heterostructures and thin-film InSb $p$-$n$ junctions are investigated theoretically. The performances of two near-field systems with different emitters are examined carefully. One near-field system consists of a graphene-hexagonal-boron-nitride-graphene sandwich structure as the emitter, while the other system has an emitter made of the double graphene-hexagonal-boron-nitride heterostructure. It is shown that both systems exhibit higher output power density and energy efficiency than …
Germanium Thermophotovoltaic Devices Achieving 7.3% Efficiency Under High-Temperature Emission by Empirical Calorimetry
Instituto de Energía Solar, Universidad Politécnica de Madrid（马德里理工大学太阳能研究所）；Departament d’Enginyeria Electrònica, Universitat Politècnica de Catalunya（加泰罗尼亚理工大学电子工程系）；GR-MECAMAT, Department of Engineering, Universitat de Vic – Universitat Central de Catalunya（维卡中央大学工程学院 GR-MECAMAT）
Solar energy promises a viable solution to meet the ever-increasing power demand by providing a clean, renewable energy alternative to fossil fuels. For solar thermophotovoltaics (STPV), high-temperature absorbers and emitters with strong spectral selectivity are imperative to efficiently couple solar radiation into photovoltaic cells. Here, we demonstrate refractory metasurfaces for STPV with tailored absorptance and emittance characterized by in-situ high-temperature measurements, featuring thermal stability up to at least 1200 C. Our tungsten-based metasurface absorbers have close-to-unity …
Thermophotovoltaic (TPV) energy conversion efficiency has recently surpassed 30%. The key behind such high efficiency is the inclusion of a highly efficient mirror in the rear of the TPV cell that turns back to the thermal emitter the outband energy photons. Efficiencies over 50% could be theoretically attainable by approaching a mirror reflectance of 100%. However, the very few percent of outband absorption significantly deteriorate the conversion efficiency. Thus, current research focuses on developing advance mirror designs able to reach an extreme high outband reflectance over 95%. In this…
We report the fabrication and measurement of thermophotovoltaic (TPV) cells with efficiencies of >40%, which is a record high TPV efficiency and the first experimental demonstration of the efficiency of high-bandgap tandem TPV cells. TPV efficiency was determined by simultaneous measurement of electric power output and heat dissipation from the device via calorimetry. The TPV cells are two-junction devices comprising high-quality III-V materials with band gaps between 1.0 and 1.4 eV that are optimized for high emitter temperatures of 1900-2400°C. The cells exploit the concept of band-edge spec…
Emitted photons stemming from the radiative recombination of electron-hole pairs carry chemical potential in radiative energy converters. This luminescent effect can substantially alter the local net photogeneration in near-field thermophotovoltaic cells. Several assumptions involving the luminescent effect are commonly made in modeling photovoltaic devices; in particular, the photon chemical potential is assumed to be zero or a constant prescribed by the bias voltage. The significance of photon chemical potential depends upon the emitter temperature, the semiconductor properties, and the inje…
We evaluate near-field thermophotovoltaic (TPV) energy conversion systems focusing in particular on their open-circuit voltage (Voc). Unlike previous analyses based largely on numerical simulations with fluctuational electrodynamics, here, we develop an analytic model that captures the physics of near-field TPV systems and can predict their performance metrics. Using our model, we identify two important opportunities of TPV systems operating in the near-field. First, we show analytically that enhancement of radiative recombination is a natural consequence of operating in the near-field. Second…
It is well known that performance of a thermophotovoltaic (TPV) device can be enhanced if the vacuum gap between the thermal emitter and the TPV cell becomes nanoscale due to the photon tunneling of evanescent waves. Having multiple bandgaps, multi-junction TPV cells have received attention as an alternative way to improve its performance by selectively absorbing the spectral radiation in each subcell. In this work, we comprehensively analyze the optimized near-field tandem TPV system consisting of the thin-ITO-covered tungsten emitter (at 1500 K) and GaInAsSb/InAs monolithic interconnected ta…
Numerous studies have reported performance enhancement of a thermophotovoltaic (TPV) system when an emitter is separated by nanoscale gaps from a TPV cell. Although a $p$-$n$-junction-based TPV cell has been widely used for the near-field TPV system, a Schottky-junction-based near-field TPV system has drawn attention recently with the advantage of the easy fabrication. However, existing studies mostly focused on the generated photocurrent only in the metal side due to the fact that required energy for the metal-side photocurrent (i.e., Schottky barrier height) is smaller than the bandgap energ…
The performance of a near-field thermophotovoltaic system with a tandem-cell structure composed of thin-film p-doped GaSb and n-doped InAs sub-cells on a gold backside reflector is theoretically investigated. The temperatures of the Ga-doped ZnO emitter and the tandem cells are set as 1800 K and 300 K, while the thicknesses of GaSb and InAs sub-cells are considered as 1.5um and 0.5um, respectively. Fluctuational electrodynamics along with the multilayer dyadic Green's function is used to study near-field radiative heat transfer with the consideration of photon chemical potential, whereas radia…
Near-field thermal radiation transfer overcoming the far-field blackbody limit has attracted significant attention in recent years owing to its potential for drastically increasing the output power and conversion efficiency of thermophotovoltaic (TPV) power generation systems. Here, we experimentally demonstrate a one-chip near-field TPV device overcoming the far-field blackbody limit, which integrates a 20-um-thick Si emitter and an InGaAs PV cell with a sub-wavelength gap (<140 nm). The device exhibits a photocurrent density of 1.49 A/cm2 at 1192 K, which is 1.5 times larger than the far-fie…
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 elect…
We propose a concentrated thermionic emission solar cell design, which demonstrates a high solar-to-electricity energy conversion efficiency larger than 10\% under 600 sun, by harnessing the exceptional electrical, thermal and radiative properties of the graphene as a collector electrode. By constructing an analytical model that explicitly takes into account the non-Richardson behavior of the thermionic emission current from graphene, space charge effect in vacuum gap, and the various irreversible energy losses within the subcomponents, we perform a detailed characterization on the conversion …
