Near-field Thermophotovoltaics Research Articles

Thermionic-enhanced near-field thermophotovoltaics

Solid-state heat-to-electrical power converters are thermodynamic engines that use fundamental particles, such as electrons or photons, as working fluids. Virtually all commercially available devices are thermoelectric generators, in which electrons flow through a solid driven by a temperature difference. Thermophotovoltaics and thermionics are highly efficient alternatives relying on the direct emission of photons and electrons. However, the low energy flux carried by the emitted particles significantly limits their generated electrical power density potential. Creating nanoscale vacuum gaps between the emitter and the receiver in thermionic and thermophotovoltaic devices enables a significant enhancement of the electron and photon energy fluxes, respectively, which in turn results in an increase of the generated electrical power density. Here we propose a thermionic-enhanced near-field thermophotovoltaic device that exploits the simultaneous emission of photons and electrons through nanoscale vacuum gaps. We present the theoretical analysis of a device in which photons and electrons travel from a hot LaB6-coated tungsten emitter to a closely spaced BaF2-coated InGaAs photovoltaic cell. Photon tunnelling and space charge removal across the nanoscale vacuum gap produce a drastic increase in flux of electrons and photons, and subsequently, of the generated electrical power density. We show that conversion efficiencies and electrical power densities of ∼ 30% and ∼ 70 W/cm2 are achievable at 2000 K for a practicable gap distance of 100 nm, and thus greatly enhance the performances of stand-alone near-field thermophotovoltaic devices (∼10% and ∼10 W/cm2). A key practical advantage of this nanoscale energy conversion device is the use of grid-less cell designs, eliminating the issue of series resistance and shadowing losses, which are unavoidable in conventional near-field thermophotovoltaic devices.

Thermionic-enhanced near-field thermophotovoltaics for medium-grade heat sources

Conversion of medium-grade heat (temperature from 500 to 1000 K) into electricity is important in applications such as waste heat recovery or power generation in solar thermal and co-generation systems. At such temperatures, current solid-state devices lack either high conversion efficiency (thermoelectrics) or high-power density capacity (thermophotovoltaics and thermionics). Near-field thermophotovoltaics (nTPV) theoretically enables high-power density and conversion efficiency by exploiting the enhancement of thermal radiation between a hot emitter and a photovoltaic cell separated by nanometric vacuum gaps. However, significant improvements are possible only at very small gap distances (<100 nm) and when ohmic losses in the photovoltaic cell are negligible. Both requirements are very challenging for current device designs. In this work, we present a thermionic-enhanced near-field thermophotovoltaic (nTiPV) converter consisting of a thermionic emitter (graphite) and a narrow bandgap photovoltaic cell (InAs) coated with low-workfunction nanodiamond films. Thermionic emission through the vacuum gap electrically interconnects the emitter with the front side of the photovoltaic cell and generates an additional thermionic voltage. This avoids the use of metal grids at the front of the cell and virtually eliminates the ohmic losses, which are unavoidable in realistic nTPV devices. We show that nTiPV operating at 1000 K and with a realizable vacuum gap distance of 100 nm enables a 10.7-fold enhancement of electrical power (6.73 W/cm2) and a 2.8-fold enhancement of conversion efficiency (18%) in comparison with a realistic nTPV device having a series resistance of 10 mΩ·cm2.

Micron-sized liquid nitrogen-cooled indium antimonide photovoltaic cell for near-field thermophotovoltaics.

Simulations of near-field thermophotovoltaic devices predict promising performance, but experimental observations remain challenging. Having the lowest bandgap among III-V semiconductors, indium antimonide (InSb) is an attractive choice for the photovoltaic cell, provided it is cooled to a low temperature, typically around 77 K. Here, by taking into account fabrication and operating constraints, radiation transfer and low-injection charge transport simulations are made to find the optimum architecture for the photovoltaic cell. Appropriate optical and electrical properties of indium antimonide are used. In particular, impact of the Moss-Burstein effects on the interband absorption coefficient of n-type degenerate layers, and of parasitic sub-bandgap absorption by the free carriers and phonons are accounted for. Micron-sized cells are required to minimize the huge issue of the lateral series resistance losses. The proposed methodology is presumably relevant for making realistic designs of near-field thermophotovoltaic devices based on low-bandgap III-V semiconductors.

Nanogap near-field thermophotovoltaics.

Conversion of heat to electricity via solid-state devices is of great interest and has led to intense research of thermoelectric materials1,2. Alternative approaches for solid-state heat-to-electricity conversion include thermophotovoltaic (TPV) systems where photons from a hot emitter traverse a vacuum gap and are absorbed by a photovoltaic (PV) cell to generate electrical power. In principle, such systems may also achieve higher efficiencies and offer more versatility in use. However, the typical temperature of the hot emitter remains too low (<1,000 K) to achieve a sufficient photon flux to the PV cell, limiting practical applications. Theoretical proposals3-12 suggest that near-field (NF) effects13-18 that arise in nanoscale gaps may be leveraged to increase the photon flux to the PV cell and significantly enhance the power output. Here, we describe functional NFTPV devices consisting of a microfabricated system and a custom-built nanopositioner and demonstrate an ~40-fold enhancement in the power output at nominally 60 nm gaps relative to the far field. We systematically characterize this enhancement over a range of gap sizes and emitter temperatures, and for PV cells with two different bandgap energies. We anticipate that this technology, once optimized, will be viable for waste heat recovery applications.

High-injection effects in near-field thermophotovoltaic devices

In near-field thermophotovoltaics, a substantial enhancement of the electrical power output is expected as a result of the larger photogeneration of electron-hole pairs due to the tunneling of evanescent modes from the thermal radiator to the photovoltaic cell. The common low-injection approximation, which considers that the local carrier density due to photogeneration is moderate in comparison to that due to doping, needs therefore to be assessed. By solving the full drift-diffusion equations, the existence of high-injection effects is studied in the case of a GaSb p-on-n junction cell and a radiator supporting surface polaritons. Depending on doping densities and surface recombination velocity, results reveal that high-injection phenomena can already take place in the far field and become very significant in the near field. Impacts of high injection on maximum electrical power, short-circuit current, open-circuit voltage, recombination rates, and variations of the difference between quasi-Fermi levels are analyzed in detail. By showing that an optimum acceptor doping density can be estimated, this work suggests that a detailed and accurate modeling of the electrical transport is also key for the design of near-field thermophotovoltaic devices.

Transparent and \u2018opaque\u2019 conducting electrodes for ultra-thin highly-efficient near-field thermophotovoltaic cells

Transparent conducting electrodes play a fundamental role in far-field PhotoVoltaic systems, but have never been thoroughly investigated for near-field applications. Here we show, in the context of near-field planar ultra-thin ThermoPhotoVoltaic cells using surface-plasmon-polariton thermal emitters, that the resonant nature of the nanophotonic system significantly alters the design criteria for the necessary conducting front electrode. The traditional ratio of optical-to-DC conductivities is alone not an adequate figure of merit, instead the desired impedance matching between the emitter and absorber modes along with their coupling to the free-carrier resonance of the front electrode are key for optimal device design and performance. Moreover, we demonstrate that conducting electrodes ‘opaque’ to incoming far-field radiation can, in fact, be used in the near field with decent performance by taking advantage of evanescent photon tunneling from the emitter to the absorber. Finally, we identify and compare appropriate tunable-by-doping materials for front electrodes in near-field ThermoPhotoVoltaics, specifically molybdenum-doped indium oxide, dysprosium-doped cadmium oxide, graphene and diffused semiconductors, but also for ‘opaque’ electrodes, tin-doped indium oxide and silver nano-films. Predicted estimated performances include output power density ~10 W/cm2 with >45% efficiency at 2100 °K emitter temperature and 60 Ω electrode square resistance, thus increasing the promise for high-performance practical devices.

High-performance near-field thermophotovoltaics for waste heat recovery

The US industries reject nearly 20–50% of the consumed energy into the environment as waste heat. Harvesting this huge amount of heat can substantially improve the energy usage efficiency. For waste heat in the medium temperature range (~ 500–900K), traditional solid-state waste heat recovery techniques like thermoelectric generators and thermophotovoltaics (TPVs) are still suffering from relatively low efficiency or power density. In this work, we analyze a near-field TPV system consisting of a plasmonic emitter (indium tin oxide) and a narrow-bandgap photovoltaic cell (InAs) that are brought to deep sub-wavelength distances for high-efficiency and high-power-density waste heat recovery. We show that despite the inclusion of realistic nonradiative recombination rates and sub-bandgap heat transfer, such a near-field TPV system can convert heat to electricity with up to nearly 40% efficiency and 11W/cm2 power density at a 900K emitter temperature, because of the spectral reshaping and enhancement by the thermally excited surface plasmons and waveguide modes. Thus, we show that for waste heat recovery, near-field TPV systems can have performances that significantly exceed typical thermoelectric systems. We propose a modified system to further enhance the power density by using a thin metal film on the cell, achieving a counterintuitively “blocking-assisted” heat transfer and power generation in the near-field regime.

External Luminescence and Photon Recycling in Near-Field Thermophotovoltaics

The importance of considering near-field effects on photon recycling and spontaneous emission in a thermophotovoltaic device is investigated. Fluctuational electrodynamics is used to calculate external luminescence from a photovoltaic cell as a function of emitter type, vacuum gap thickness between emitter and cell, and cell thickness. The observed changes in external luminescence suggest strong modifications of photon recycling caused by the presence of the emitter. Photon recycling for propagating modes is affected by reflection at the vacuum-emitter interface and is substantially decreased by the leakage towards the emitter through tunneling of frustrated modes. In addition, spontaneous emission by the cell can be strongly enhanced by the presence of an emitter supporting surface polariton modes. It follows that using a radiative recombination model with a spatially uniform radiative lifetime, even corrected by a photon recycling factor, is inappropriate. Applying the principles of detailed balance, and accounting for non-radiative recombination mechanisms, the impact of external luminescence enhancement in the near field on thermophotovoltaic performance is investigated. It is shown that unlike isolated cells, the external luminescence efficiency is not solely dependent on cell quality, but significantly increases as the vacuum gap thickness decreases below 400 nm for the case of an intrinsic silicon emitter. In turn, the open-circuit voltage and power density benefit from this enhanced external luminescence toward the emitter. This benefit is larger as cell quality, characterized by the contribution of non-radiative recombination, decreases.

Hot Carrier-Based Near-Field Thermophotovoltaic Energy Conversion.

Near-field thermophotovoltaics (NFTPV) is a promising approach for direct conversion of heat to electrical power. This technology relies on the drastic enhancement of radiative heat transfer (compared to conventional blackbody radiation) that occurs when objects at different temperatures are brought to deep subwavelength distances (typically <100 nm) from each other. Achieving such radiative heat transfer between a hot object and a photovoltaic (PV) cell could allow direct conversion of heat to electricity with a greater efficiency than using current solid-state technologies (e.g., thermoelectric generators). One of the main challenges in the development of this technology, however, is its incompatibility with conventional silicon PV cells. Thermal radiation is weak at frequencies larger than the ∼1.1 eV bandgap of silicon, such that PV cells with lower excitation energies (typically 0.4-0.6 eV) are required for NFTPV. Using low bandgap III-V semiconductors to circumvent this limitation, as proposed in most theoretical works, is challenging and therefore has never been achieved experimentally. In this work, we show that hot carrier PV cells based on Schottky junctions between silicon and metallic films could provide an attractive solution for achieving high efficiency NFTPV electricity generation. Hot carrier science is currently an important field of research and several approaches are investigated for increasing the quantum efficiency (QE) of hot carrier generation beyond conventional Fowler model predictions. If the Fowler limit can indeed be overcome, we show that hot carrier-based NFTPV systems-after optimization of their thermal radiation spectrum-could allow electricity generation with up to 10-30% conversion efficiencies and 10-500 W/cm2 generated power densities (at 900-1500 K temperatures). We also discuss how the unique properties of thermal radiation in the extreme near-field are especially well suited for investigating recently proposed approaches for high QE hot carrier junctions. We therefore expect our work to be of interest for the field of hot carrier science and-by relying solely on conventional thin film materials-to provide a path for the experimental demonstration of NFTPV energy conversion.

Radiative heat conductances between dielectric and metallic parallel plates with nanoscale gaps.

Recent experiments have demonstrated that radiative heat transfer between objects separated by nanometre-scale gaps considerably exceeds the predictions of far-field radiation theories. Exploiting this near-field enhancement is of great interest for emerging technologies such as near-field thermophotovoltaics and nano-lithography because of the expected increases in efficiency, power conversion or resolution in these applications. Past measurements, however, were performed using tip-plate or sphere-plate configurations and failed to realize the orders of magnitude increases in radiative heat currents predicted from near-field radiative heat transfer theory. Here, we report 100- to 1,000-fold enhancements (at room temperature) in the radiative conductance between parallel-planar surfaces at gap sizes below 100 nm, in agreement with the predictions of near-field theories. Our measurements were performed in vacuum gaps between prototypical materials (SiO2-SiO2, Au-Au, SiO2-Au and Au-Si) using two microdevices and a custom-built nanopositioning platform, which allows precise control over a broad range of gap sizes (from <100 nm to 10 μm). Our experimental set-up will enable systematic studies of a variety of near-field-based thermal phenomena, with important implications for thermophotovoltaic applications, that have been predicted but have defied experimental verification.

Radiative heat transfer in the extreme near field.

Radiative transfer of energy at the nanometre length scale is of great importance to a variety of technologies including heat-assisted magnetic recording, near-field thermophotovoltaics and lithography. Although experimental advances have enabled elucidation of near-field radiative heat transfer in gaps as small as 20-30 nanometres (refs 4-6), quantitative analysis in the extreme near field (less than 10 nanometres) has been greatly limited by experimental challenges. Moreover, the results of pioneering measurements differed from theoretical predictions by orders of magnitude. Here we use custom-fabricated scanning probes with embedded thermocouples, in conjunction with new microdevices capable of periodic temperature modulation, to measure radiative heat transfer down to gaps as small as two nanometres. For our experiments we deposited suitably chosen metal or dielectric layers on the scanning probes and microdevices, enabling direct study of extreme near-field radiation between silica-silica, silicon nitride-silicon nitride and gold-gold surfaces to reveal marked, gap-size-dependent enhancements of radiative heat transfer. Furthermore, our state-of-the-art calculations of radiative heat transfer, performed within the theoretical framework of fluctuational electrodynamics, are in excellent agreement with our experimental results, providing unambiguous evidence that confirms the validity of this theory for modelling radiative heat transfer in gaps as small as a few nanometres. This work lays the foundations required for the rational design of novel technologies that leverage nanoscale radiative heat transfer.

Temporal coupled-mode theory model for resonant near-field thermophotovoltaics

A temporal Coupled-Mode Theory model is developed to predict performance of resonant near-field ThermoPhotoVoltaic systems, which typically requires numerically intensive calculations. It is formulated for both orthogonal and non-orthogonal (coupled) modes and includes load-voltage dependencies and non-idealities, such as background absorption and radiation losses. Its good accuracy is confirmed by comparing with exact transfer-matrix calculations for two simple planar systems: a plasmonic emitter across a bulk semiconductor absorber and a metal-backed thin-film semiconductor emitter across an identical absorber.

Spatial and spectral distributions of thermal radiation emitted by a semi-infinite body and absorbed by a flat film

We analyze the radiative power emitted by a semi-infinite medium and absorbed by a flat film located in its vicinity. In the near-field regime, if the film is thin enough, the surface waves at the rear interface of the film can contribute to the heat transfer. As a result, the absorbed power can be enhanced farther from the front surface. In the near-to-far field transition regime, temporal coherence of thermal radiation and the associated interferences can be used to shape the spectrum of the transferred radiative heat flux by selecting approriate geometrical parameters. These results highlight possibilities to control both the location where the radiative power is absorbed in the film and the spectral distribution, which are of paramount importance for applications such as near-field thermophotovoltaics.

Thermal excitation of plasmons for near-field thermophotovoltaics

The traditional approaches of exciting plasmons consist of either using electrons (e.g., electron energy loss spectroscopy) or light (Kretchman and Otto geometry) while more recently plasmons have been excited even by single photons. A different approach: thermal excitation of a plasmon resonance at high temperatures using alternate plasmonic media was proposed by S. Molesky et al. [Opt. Express 21, A96–A110 (2013)]. Here, we show how the long-standing search for a high temperature narrowband near-field emitter for thermophotovoltaics can be fulfilled by thermally exciting plasmons. We also describe a method to control Wein's displacement law in the near-field using high temperature epsilon-near-zero metamaterials. Finally, we show that our work opens up an interesting direction of research for the field of slow light: thermal emission control.

Near-field radiative heat transfer with doped-silicon nanostructured metamaterials

The objective of this work is to evaluate different practically achievable doped-silicon (D-Si) nanostructured metamaterials (including nanowires and nanoholes, multilayers, and one-dimensional gratings) in terms of their potential for enhancing near-field radiative heat transfer at ambient temperature. It is found that both doped silicon nanowires and nanoholes may achieve an enhancement over bulk doped silicon by more than one order of magnitude in the deep submicron gap region. The enhancement is attributed to either the broadband hyperbolic modes or low-loss surface modes or a combination of both. On the other hand, polarization coupling, which can occur in the grating configuration, contributes little to the radiative transfer at the nanometer scale. This work will facilitate the application of nanostructures in more efficient non-contact thermal management, thermal imaging, and near-field thermophotovoltaics.

FUNDAMENTALS AND APPLICATIONS OF NEAR-FIELD RADIATIVE ENERGY TRANSFER

This article reviews the recent advances in near-field radiative energy transfer, particularly in its fundamentals and applications. When the geometrical features of radiating objects or their separating distances fall into the sub-wavelength range, near-field phenomena such as photon tunneling and surface polaritons begin to play a key role in energy transfer. The resulting heat transfer rate can greatly exceed the blackbody radiation limit by several orders magnitude. This astonishing feature cannot be conveyed by the conventional theory of thermal radiation, generating strong demands in fundamental research that can address thermal radiation in the near field. Important breakthroughs of near-field thermal radiation are presented here, covering from the essential physics that will help better understand the basics of near-field thermal radiation to the most recent theoretical as well as experimental findings that will further promote the fundamental understanding. Applications of near-field thermal radiation in various fields are also discussed, including the radiative property manipulation, near-field thermophotovoltaics, nanoinstrumentation and nanomanufacturing, and thermal rectification.