This guest editorial introduces the Special Section on Thermal Photonics in Energy.

The influence of the optic axis orientation of hexagonal boron nitride (hBN) on the near-field radiative heat transfer between hBN slabs as well as between graphene/hBN heterostructures is studied. A modified 4  ×  4 transfer matrix method is employed to calculate the near-field radiative heat flux (NFRHF) between the media. The numerical results show that the NFRHF will decrease when the optic axis of hBN is tilted off the direction of the energy flow for bare hBN slabs. The reason is that hyperbolic phonon polaritons excited in the hyperbolic bands of type I are largely suppressed for tilted optic axis, though surface phonon polaritons can be excited in the hyperbolic bands. On the contrary, the NFRHF between two graphene/hBN heterostructures is affected by the coupling of SPPs excited at the vacuum/graphene interface with those at the graphene/hBN interface and the formation of a hybrid mode, by which the NFRHF is maximum when the hBN slabs are arranged with strong in-plane anisotropy of the surface. The results obtained in this work may provide a promising way for manipulating near-field radiative heat transfer between anisotropic materials.

The outstanding thermomechanical and broadband spectral absorptance of silicon carbide (SiC) coatings makes it a promising material for high-performance optoelectronics devices and photothermal applications. We propose sputtered SiC coatings to develop: (i) broadband infrared emitters for passive radiative cooling and (ii) a spectrally selective ultrathin nanocomposite absorber. For radiative cooling, a proof of concept device has been fabricated by topping the as-sputtered 2.5-μm-thick SiC films with a single pair of distributed Bragg’s reflector (DBR). Owing to the infrared surface polariton resonances of SiC, broadband infrared emissivity of 0.8 is achieved in atmospheric transparency window (8 to 14  μm). The higher refractive index contrast (Δn) in DBR is achieved by depositing high refractive index dense Si film on top of low refractive index nanoporous SiO₂, which was fabricated by oblique angle deposition approach. A single pair of DBR on top of SiC considerably reduces the absorption of the overall solar spectrum by 18%. In addition, impact of various materials and number of pairs of DBRs on radiative cooling are also numerically investigated. We also designed and fabricated SiC-based spectrally selective solar absorber coatings. An ultrathin SiC-based nanocomposite absorber with sandwich-like configuration consisting of SiC/Ag-SiC/SiC on reflective substrate has exhibited an outstanding absorptance of 91.24% in the wavelength range of 280 to 2000 nm. The surface plasmon polaritons of metal inclusion within the SiC matrix of an ultrathin nanocomposite (40 nm) layer has contributed to broadband light absorption, whereas the top most layer of SiC (40 nm) acts as antireflection layer. Conclusively, the versatility of sputtered SiC coatings has proven to be promising for radiative cooling and spectrally selective solar absorbers.

Great efforts have been devoted to the investigation of selective emitters and filters for improving the conversion efficiency of thermophotovoltaic (TPV) systems. This work proposes a spectrally selective ultrathin cell with two-dimensional (2-D) metasurface for high-efficiency TPV energy conversion. The cell is composed of Ag substrate, ultrathin gallium antimonide (GaSb) photovoltaic layer, and 2-D metasurface comprising symmetric 2-D metallic grating. The spectral-normal absorptance of the proposed structure is calculated by the finite-element method and the results show significant enhancement in the photon absorption comparing to the free-standing GaSb layer with the same thickness, which is attributed to the excitation of magnetic polariton and Fabry–Perot resonance. The effects of using indium tin oxide and graphene as front electrodes on the absorptance of the proposed ultrathin cell are investigated. The TPV system efficiency is also evaluated using the proposed ultrathin cell. The results show that the TPV efficiency with the proposed ultrathin cell can approach 29.1% with a black emitter at 1500 K and can be further increased to be 35.1% using an ideal selective emitter. This work will facilitate the development of next-generation high-efficiency TPV systems.

We demonstrate that white coatings consisting of silicone embedded with randomly distributed microbubbles provide highly efficient daytime radiative cooling with inexpensive materials and fabrication processes. In our material system, sunlight is strongly scattered with minimal absorption, and heat is effectively removed through mid-infrared (IR) radiation. In our previous study, solid microsphere-based coatings outperformed commercial solar-rejection white paint in cooling efficiency, but their mechanical robustness needed improvement for practical applications. The material system in our work substantially enhances the mechanical robustness while providing superior cooling performance to commercial solar-rejection paint. For ease of processing, we use nonoptimized structures with reduced optical scattering strength. Strong solar rejection is yet achieved by increasing coating thickness. This strategy is desirable for practical rooftop applications where coating thickness is of minor importance in comparison to cooling performance and materials cost. In addition, silicone is stable in extraterrestrial environments and efficiently radiates heat over broad mid-IR spectrum. These material properties of our silicone coatings promise great potential for radiative cooling in space applications.

We analyze a near-field thermoradiative device that consists of an indium arsenide-based photodiode under negative illumination. We analyze a possible enhancement of conversion efficiency by use of hyperbolic metamaterial (HMM) in place of bulk metallic heat sink. A stack of alternating thin-films of metal [zirconium carbide (ZrC)] and dielectric [silicon dioxide (SiO₂)] is chosen to be the HMM under investigation. The presence of hyperbolic modes creates additional channels of near-field radiative transfer. An increased power density is predicted without a compromise in system efficiency.

Nanoscale size effects give rise to near-field thermal considerations when heating nanoparticles under high laser power. We solve Maxwell’s equations in the frequency domain to analyze near-field thermal energy effects for three nanoparticle assemblies with different variances in particle sizes and show that heat dissipation generally decreases as the spread in nanoparticle sizes increases within the nanoparticle packing. For this study, log-normally distributed copper nanoparticle packings with a mean radius of 116 nm and three different standard deviations (12, 48, and 84 nm) were created by using a discrete element model in which a specified number of particles is generated. The nanoparticle packings in the simulation are created by randomly placing each nanoparticle into the packing domain with a random initial velocity and a position. The nanoparticles are then allowed to interact with each other under gravitational and weak van der Waals forces until they settle to form a stable packing configuration. A finite-difference frequency-domain analysis, which yields the electromagnetic field distribution, is then applied to the packing by solving Maxwell’s equations to obtain absorption, scattering, and extinction coefficients. This analysis is used to calculate the surface plasmon effects due to the electromagnetic coupling between the nanoparticles and the dielectric medium under the different distributions and show that different particle distributions can create different plasmonic effects in the packing domain, which results in nonlocal heat transport. Overall, this analysis helps to reveal how sintering quality can be enhanced by creating stronger laser–particle interactions for specific groups of nanoparticles.

Wavelength-selective thermal devices have great applications in concentrating solar power systems, high-temperature thermoelectric systems, and solar thermophotovoltaics (STPVs). Lack of high-temperature stability and spectrally selective emissivity in different wavelength regions limits their efficiency. We propose a one-dimensional HfO₂  /  Al₂O₃-W nanocomposites  /  W  /  Al₂O₃  /  W multilayered photonic structure as potential wavelength selective thermal devices, and theoretically investigate the emission properties of the proposed Mie-resonance metamaterials from visible (VIS) to midinfrared (MIR) region. HfO₂ thin layer is introduced to serve as an antireflection coating film and W layer acts as an IR reflection layer that enhances the absorptivity/emissivity in VIS and near-infrared (NIR) region while reducing the MIR emission simultaneously. Effects of geometric parameters are discussed, such as different radii and volume fractions of W nanoparticles, the thickness of Al₂O₃-W nanocomposites, and HfO₂ thin film. The proposed thermal absorber and emitter exhibit nearly unity absorptance in both VIS and NIR regions, while the emittance approaches zero in the MIR region. The selective absorption/emission window is tunable by varying geometric parameters. The proposed solar thermal devices have great potentials in engineering applications such as STPVs and solar thermoelectric generator due to flexibility of geometric parameters and ease of fabrication.

When two objects made of a material that supports surface modes are brought in close proximity to each other such that the vacuum gap between them is less than the thermal wavelength of radiation, then the coupling between the surface modes provides an important channel for the heat transfer to occur, which is different from that mediated by long range propagating electromagnetic waves. Indeed, the heat transfer then exceeds Planck’s blackbody limit by several orders of magnitude and consequently has been used for several energy applications, such as near-field thermophotovoltaic systems. This near-field radiative heat exchange has been traditionally and successfully described using fluctuational electrodynamics principles. Here, we describe an alternate coupled harmonic oscillator model approach, which can be used to model the coupling between surface modes and hence the resultant near-field heat transfer. We apply this theory to estimate the near-field heat transfer for the configurations of two metallic nanoparticles and two planar metal surfaces and compare the result with predictions from fluctuational electrodynamics theory.

We design and fabricate a planar energy-saving glass with high transmission of visible light and a high resistance to infrared solar heat. The proposed glass is designed by combining a photonic crystal structure made of alternating low- and high-index dielectrics and a dielectric/metal/dielectric multilayers deposited on conventional glass substrates. After delicate optimizations, a high weight-averaged transmission of infrared solar energy (17%) and low transmission of ultraviolet light (14%) is obtained while retaining over 83% transmission of visible light. As a result, we achieve a high energy-saving efficiency of 83%, which is about 30% higher than that of uncoated glass. The proposed nanofabrication-free energy-saving design is expected to have promising applications in reducing energy consumption of buildings, cars, etc.

We review recent theoretical developments on the nanoscale radiative heat transfer in magneto-optical many-particle systems. We discuss in detail the circular heat flux, the giant magnetoresistance effect, the persistent heat current, and the thermal Hall effect for light in such systems within the framework of fluctuational electrodynamics, using the dipolar approximation. We show that the directionality of heat flux in such systems can in principle be understood by analyzing the competing contributions to the heat exchange of the magnetic-field-dependent dipolar resonances of quantum numbers m  =    +  1 and m  =    −  1. Some potential applications of these effects to thermal and magnetic sensing are also briefly discussed.

We investigate the directional control of narrow-band perfect thermal emission using a nanoscale Yagi–Uda antenna. Although Yagi–Uda antennas were demonstrated to achieve directional control in the optical and radio frequency regimes, they have not been applied for thermal infrared emission. Here, by coupling a nanoscale thermal emitter into a Yagi–Uda antenna, we demonstrate strong directional control of thermal emission with a narrow-band spectrum at the nanoscale. By exploring the effects of the reflector and the director of a Yagi–Uda antenna, the forward emission enhancement factor up to 8.3 is achieved through geometry optimization.

Thermophotovoltaic (TPV) systems are promising for harnessing solar energy, waste heat, and heat from radioisotope decay or fuel combustion. TPV systems work by heating an emitter that emits light that is converted to electricity. One of the key challenges is designing an emitter that not only preferentially emits light in certain wavelength ranges but also simultaneously satisfies other engineering constraints. To elucidate these engineering constraints, we first provide an overview of the state of the art, by classifying emitters into three categories based on whether they have been used in prototype system demonstrations, fabricated and measured, or simulated. We then present a systematic approach for assessing emitters. This consists of five metrics: optical performance, ability to scale to large areas, stability at high temperatures, ability to integrate into the system, and cost. Using these metrics, we evaluate and discuss the reported results of emitters used in system demonstrations. Although there are many emitters with good optical performance, more studies on their practical attributes are required, especially for those that are not yet used in prototype systems. This framework can serve as a guide for the development of emitters for long-lasting, high-performance TPV systems.

Ideal thermophotovoltaic (TPV) converters approach Carnot efficiency in the limit of monochromatic radiative transfer, motivating the design of narrowband thermal emitters and absorbers. Here, we investigate whether this trend holds for converters with realistic losses by studying the effects of transmission bandwidth on the performance of far-field and near-field TPV and thermophotonic (TPX) converters. To bridge the near-field and far-field regimes, the analysis relies on an approximation that the near-field spectral energy flux is a scaled version of the far-field flux, which is validated for weakly absorbing materials using a rigorous near-field simulation. We show that the optimal bandwidth depends on the type of converter. For far-field TPVs with realistic heat losses, narrowband transport is typically detrimental to the efficiency because the converter becomes more susceptible to parasitic loss. However, narrowband transport boosts efficiencies in TPX converters and near-field TPVs as long as the excitation barrier is comparable with the thermal energy. Given the same excitation energy barrier as TPVs, TPX converters benefit from a larger available photon density of states due to the applied bias. This study suggests that near-field TPX converters with a large applied bias have the largest ratio of useful energy flux to parasitic loss. Leveraging this mechanism for actual improvement is contingent on large near-field enhancements improving photon extraction.

A full-spectrum solar energy allocation design based on nanopattern-free multilayered coatings (MLCs) is proposed to boost photovoltaic–thermoelectric (PV–TE) energy conversion efficiency. Above- and below-bandgap photons are sent to gallium arsenide (GaAs) cells and absorptive layers lying between GaAs cells and TE devices, respectively. A low-averaged reflectance of ∼7.9  %   for full-spectrum solar energy under one-Sun AM0 conditions is achieved due to multiple interferences and graded refraction index. A high-averaged absorptance of ∼93.62  %   and ∼91.17  %   for below- and above-bandgap photons are obtained with the assistance of impedance matching enabled by MLCs. Performances are shown to be also insensitive to incident angles, polarization states, and each layer’s thickness. We pave the way to design efficient space-based PV–TE energy conversion systems.

Enhancing light absorption in two-dimensional (2-D) materials using simple planar structures is important for the development of large-area atomic-scale photonic devices but also has been a major challenge. We theoretically present a general strategy to create an atomically thin planar metasurface consisting of a semiconductor monolayer (replacing the array of subwavelength elements in classical metasurfaces), a transparent spacer, and a metallic back reflector, to achieve near-perfect, polarization-sensitive absorption in 2-D materials. This strategy is validated by experimentally demonstrating planar metasurfaces based on atomically thin Ge films. Higher than 90% exclusive absorption, 2.5 times greater than prior demonstration, is achieved in the MoS₂ monolayer for TE polarization. Our strategy can be extended to the infrared spectrum and thus has potential for thermal energy applications.

Indium-tin oxide (ITO) is widely used as a transparent electrode for optoelectronic devices given its large transparency and high conductivity. However, the particular properties of this material greatly depend on the overall fabrication process. We report on the effect of four different postfabrication processes on ITO thin films grown by electron beam evaporation. More specifically, the effect on the overall properties of the evaporated ITO thin films of microwave plasma annealing, rapid thermal processing, and the two processes combined were analyzed. In particular, the morphological, chemical, optical, and electrical properties of the annealed ITO thin films were studied. The experimental results show that the ITO thin films can be turned from opaque to transparent, and their conductivity can be improved by one order of magnitude depending on the particular postfabrication process.

In this work, device modeling of inverted-structure perovskite solar cells employing Cu2ZnSn(SxSe1−x)4 (CZTSxSe1−x) as hole transport layer (HTL) is performed by SCAPS. The bandgap of CZTSxSe1−x can be tuned in a wide range from 0.95 to 1.5 eV through the variation of S doping content, which allows us to adjust the valence band offset (VBO) of the CH3NH3PbI3/CZTSxSe1−x interface. Simulation results indicate that a suitable VBO of 0.25 eV is obtained when S/(S + Se) ratio of CZTSxSe1−x is 0.6, which yields a cell efficiency of 16.75%. Further optimization of the absorber and HTL thickness and doping density in HTL is conducted. Simulation results indicate that the optimal HTL and absorber thickness of are 30 and 700 nm, respectively. Doping concentration of HTL higher than 1019  cm−3 is essential for good device performance. After comprehensive optimization

We systematically investigate the light absorption behavior of the Pb-based perovskite active layer in textured perovskite solar cells (PSCs) based on two-dimensional TiO₂ nano-semisphere electron extraction layer by the finite element method (FEM). The simulation results reveal that enhanced light absorption efficiency of a 100-nm-thick textured perovskite layer achieves 64.8% under AM 1.5 G solar irradiation and owns increase of 16.8% in comparison to the control planar cell. To elucidate the origins of absorption enhancement, the detailed field distribution is investigated, displaying different photonic and plasmonic modes; their mutual coupling jointly resulting in a remarkable light absorption enhancement in the 600- to 800-nm-long wavelength range. Our textured PSCs offer an attractive solution to conserve Pb-based perovskite consumption and realize angle-insensitive and high-efficiency ultrathin PSCs.

This erratum corrects an error in the article “Enhancing silicon solar cells with singlet fission: the case for Förster resonant energy transfer using a quantum dot intermediate” by S. W. Tabernig et al.