United States. Advanced Research Projects Agency-Energy (Awards DE-AR0000471 and DE-AR0000181)

Concentrating Solar Power.

This work was supported by National Basic Research Program of China
(Project No. 2013CB933301) and National Natural Science Foundation of
China (Project No. 51272038). L.V.B. was supported by China Postdoctoral
Science Foundation. A.O.G. was supported by the Volkswagen
Foundation (Germany) and via the Chang Jiang (Yangtze River)
Chair Professorship (China). G.P.W. acknowledges support from the
Center for Nanoscale Materials, a U.S. Department of Energy Office of
Science User Facility, and support by the U.S. Department of Energy,
Office of Science, under Contract No. DE-AC02-06CH11357. In addition,
the authors acknowledge financial support obtained from the Virtual
Institute for Theoretical Photonics and Energy. This review is part of the
Advanced Optical Materials Hall of Fame article series, which recognizes
the excellent contributions of leading researchers to the field of optical
materials science.

/pdf/broadband-metamaterial-absorbers-1ivwczjh1w.pdf

Broadband Metamaterial Absorbers

Control of thermal radiation at high temperatures is vital for waste heat recovery and for high-efficiency thermophotovoltaic (TPV) conversion. Previously, structural resonances utilizing gratings, thin film resonances, metasurfaces and photonic crystals were used to spectrally control thermal emission, often requiring lithographic structuring of the surface and causing significant angle dependence. In contrast, here, we demonstrate a refractory W-HfO2 metamaterial, which controls thermal emission through an engineered dielectric response function. The epsilon-near-zero frequency of a metamaterial and the connected optical topological transition (OTT) are adjusted to selectively enhance and suppress the thermal emission in the near-infrared spectrum, crucial for improved TPV efficiency. The near-omnidirectional and spectrally selective emitter is obtained as the emission changes due to material properties and not due to resonances or interference effects, marking a paradigm shift in thermal engineering approaches. We experimentally demonstrate the OTT in a thermally stable metamaterial at high temperatures of 1,000 °C. The ability to control thermal radiation at high temperatures is of interest for thermal photovoltaics. Here, Dyachenko et al. engineer the epsilon-near-zero frequency of a metamaterial and connected optical topological transition to selectively enhance and suppress the thermal emission in the near-infrared spectrum.

/pdf/controlling-thermal-emission-with-refractory-epsilon-near-4fqkzlzg31.pdf

Controlling thermal emission with refractory epsilon-near-zero metamaterials via topological transitions.

Solar thermophotovoltaic devices have the potential to enhance the performance of solar energy harvesting by converting broadband sunlight to narrow-band thermal radiation tuned for a photovoltaic cell A direct comparison of the operation of a photovoltaic with and without a spectral converter is the most critical indicator of the promise of this technology Here, we demonstrate enhanced device performance through the suppression of 80% of unconvertible photons by pairing a one-dimensional photonic crystal selective emitter with a tandem plasma–interference optical filter We measured a solar-to-electrical conversion rate of 68%, exceeding the performance of the photovoltaic cell alone The device operates more efficiently while reducing the heat generation rates in the photovoltaic cell by a factor of two at matching output power densities We determined the theoretical limits, and discuss the implications of surpassing the Shockley–Queisser limit Improving the performance of an unaltered photovoltaic cell provides an important framework for the design of high-efficiency solar energy converters The ability of photovoltaic devices to harvest solar energy can be enhanced by tailoring the spectrum of incident light with thermophotovoltaic devices Bierman et al now show that one such device achieves a solar-to-electrical efficiency of 68%, exceeding that of the solar cell alone

Enhanced photovoltaic energy conversion using thermally based spectral shaping

We demonstrate a broadband, polarization independent, wide-angle absorber based on a metallic metasurface architecture, which accomplishes greater than 90% absorptance in the visible and near-infrared range of the solar spectrum, and exhibits low absorptivity (emissivity) at mid- and far-infrared wavelengths. The complex unit cell of the metasurface solar absorber consists of eight pairs of gold nano-resonators that are separated from a gold ground plane by a thin silicon dioxide spacer. Our experimental measurements reveal high-performance absorption over a wide range of incidence angles for both s- and p-polarizations. We also investigate numerically the frequency-dependent field and current distributions to elucidate how the absorption occurs within the metasurface structure.

Metasurface Broadband Solar Absorber

We demonstrate a high-efficiency solar-thermophotovoltaic system (STPV) using a monolithic, planar, and spectrally selective absorber/emitter. A complete STPV system using gallium antimonide (GaSb) cells was designed and fabricated to conduct power generation tests. To produce a high-efficiency STPV, it is important to match the thermal radiation spectrum with the sensitive region of the GaSb cells. Therefore, to reach high temperatures with low incident power, a planar absorber/emitter is incorporated for controlling the thermal radiation spectrum. This multilayer coating consists of thin-film tungsten sandwiched by yttria-stabilized zirconia. The system efficiency is estimated to be 16% when accounting for the optical properties of the fabricated absorber/emitter. Power generation tests using a high-concentration solar simulator show that the absorber/emitter temperature peaks at 1640 K with an incident power density of 45  W/cm2, which can be easily obtained by low-cost optics such as Fresnel lenses. The conversion efficiency became 23%, exceeding the Shockley–Queisser limit for GaSb, with a bandgap of 0.67 eV. Furthermore, a total system efficiency of 8% was obtained with the view factor between the emitter and the cell assumed to be 1.

High-efficiency solar-thermophotovoltaic system equipped with a monolithic planar selective absorber/emitter

A high-efficiency solar thermophotovoltaic (STPV) system has been demonstrated using spectrally selective planar absorber/emitter systems and a GaSb TPV cell In this study, a novel approach for designing the STPV system based on the efficiency of unidirectional radiative heat transfer has been introduced To achieve high extraction and photovoltaic conversion efficiencies, the spectrally selective absorber/emitter based on a coherent perfect absorber composed of a thin molybdenum layer sandwiched between hafnium layers was applied The extraction efficiency was further investigated with respect to the absorber/emitter area ratio The experimental efficiency of STPV reached 51% with the area ratio of 23

https://iopscience.iop.org/article/10.7567/APEX.9.112302/pdf

Unidirectional radiative heat transfer with a spectrally selective planar absorber/emitter for high-efficiency solar thermophotovoltaic systems

A new technique to determine the k = 0 components in the density of states (DOS) and band widths of excitons in microcrystallites is demonstrated. Applying the technique to anthracene microcrystallites, it is found that DOS shows up as a broad tail which extends towards higher energies, in addition to sharp peaks. The width of the tail, representing the exciton band width, takes its maximum value of 340 cm −1 for microcrystallites of 65 A, but it becomes narrower as the size of microcrystallites reduces. For microcrystallites with a diameter larger than 65 A, the observed band width decreases asymptotically to 300 cm −1 , which represents the band width of a bulk crystal.

Densities of states and band widths of excitons in anthracene microcrystallites embedded in PMMA

The thermal stability of spectrally selective few-layer metallo-dielectric structures is evaluated to analyze their potential as absorber and emitter materials in solar thermophotovoltaic (STPV) systems. High-efficiency (e.g., STPV) systems require materials with spectrally selective properties, especially at high temperatures (>1273 K). Aiming to develop such materials for high-temperature applications, we propose a few-layer structure composed of a refractory metal (i.e., Mo) nanometric film sandwiched between the layers of a dielectric material (i.e., hafnium oxide, HfO2) deposited on a Mo bulk substrate. In vacuum conditions (

Evaluation of thermal stability in spectrally selective few-layer metallo-dielectric structures for solar thermophotovoltaics

Easy-to-fabricate, high-temperature, thermally-stable radiators are critical elements for developing efficient and sustainable thermophotovoltaic energy conversion devices. In this frame, a trilayer-on-substrate structure is selected. It is composed of a refractory metal -molybdenum - constituting the substrate and an intermediate thin film sandwiched between two hafnia transparent layers. An in-depth analysis shows that two spectrally distinct interference regimes take place in the hafnia layer-molybdenum thin film substructure, and that backward and forward thermally-emitted waves by the thin film are selected in two distinct interferential resonating cavities. The interference regimes within and between these cavities are key to the spectral shaping of thermal emission. The radiative performances of the structures are evaluated by introducing a figure of merit. Using the example of a GaSb cell, it is shown that the structure can be optimized for providing the broadband large emission with a steep cutoff required for mitigating photoconversion losses.

Makoto Shimizu

Papers

High-efficiency solar-thermophotovoltaic system equipped with a monolithic planar selective absorber/emitter

Unidirectional radiative heat transfer with a spectrally selective planar absorber/emitter for high-efficiency solar thermophotovoltaic systems

Densities of states and band widths of excitons in anthracene microcrystallites embedded in PMMA

Evaluation of thermal stability in spectrally selective few-layer metallo-dielectric structures for solar thermophotovoltaics

Spectrally shaping high-temperature radiators for thermophotovoltaics using Mo-HfO2 trilayer-on-substrate structures