Hybrid Solar Absorber–Emitter by Coherence‐Enhanced Absorption for Improved Solar Thermophotovoltaic Conversion

Abstract

etc. In order to maximize the utilization of solar energy, STPV requires the cooperative spectral manipulation of a broadband solar absorber and a narrowband infrared emitter. An ideal STPV absorber must simultaneously absorb input power and suppress radiation loss, while an STPV emitter should possess high emissivity only at the target infrared wavelength matching well with the base PV cell. [9] Extensive efforts have been devoted to design and fabricate such absorbers and emitters, including intrinsically spectrally selective materials, [10][11][12] multilayer structures, [13][14][15][16][17] cermets, [18] plasmonic metamaterials, [19][20][21][22][23][24][25][26] and photonic crystals. [27][28][29][30][31] Up to now, most of the reported absorbers and emitters are demonstrated by two isolated structures, [1,2] such as the compound metallic structures patterned on both sides of a substrate in a recently developed STPV system. [30] Such an optical design is intuitive and straightforward. However, the double-side lithography process severely increases the fabrication and systematic complexity. A hybrid absorber-emitter (HAE) with the two functions built on the same surface of the structure is beneficial for STPV. Great efforts have been made to integrate these two functionalities of broadband-absorption and narrowband-emission into one metasurface or plasmonic structure. [32][33][34] However, these structures are rather complicated and the well-defined morphologies are unstable at high temperature that the STPV system requires. An integrated HAE with ideal spectral selectivity, high temperature tolerance as well as scalable fabrication process is highly pursued but still elusive.Most specifically, in this work, we present a much simpler HAE design of a layered structure on a metallic surface based on multiple coherence-enhanced absorption, especially the coherent perfect absorption (CPA) effect. Such a structure is lithography-free, scalable, and possesses excellent thermal stability. In addition, the full spectrum responses demonstrate weak angular-dependence and polarization sensitivity, all of which make it beneficial for the STPV applications.The CPA phenomenon was first proposed for the case that two coherent counter-propagating light beams can be totally absorbed in a dielectric Fabry-Pérot cavity, [35][36][37] which was Spectrum selective absorbers and emitters, with advantages of high temperature tolerance, wide angle insensitivity, and scalable fabrication processes, are crucial components for solar thermophotovoltaics (STPV). A bifunctional hybrid absorber-emitter (HAE) for efficient STPV by sequentially depositing HfO 2 /Mo/HfO 2 films on a metallic surface is demonstrated here. Simultaneous broadband solar absorption and narrowband infrared emission are enabled by the same surface of the structure. By fine steering coherent perfect absorption of the ultrathin Mo film and destructive interference of interfacial reflections across the top HfO 2 layer, the HAE exhibits a measured emissivity of 0.97 at 1.8...

Present Efficiencies and Future Opportunities in Thermophotovoltaics

“…For example, a HfO 2 /Mo/HfO 2 emitter leverages its ultrathin Mo absorber layer and a Fabry-Perot cavity created between the top interface and the bottom reflector to achieve coherent perfect absorption at a wavelength near the cell's bandgap. 60 Similar structures, where a refractory metal such as W is sandwiched between dielectrics, have also exhibited comparable spectral efficiencies. 61,62 However, it is unclear if these promising properties will translate to high performance at operating temperatures.…”

High-Temperature Carbonized Ceria Thermophotovoltaic Emitter beyond Tungsten

“…Engineering absorptivity/emissivity spectra at multiscale wavelengths enable thermal emission-harnessed energy devices including solar steamers, , radiative coolers, − high-efficiency incandescent lamps, and thermophotovoltaics (TPVs). − Each device is tailored to a different spectral range that relies on its working temperature and function. For example, radiative coolers working at room temperature, which present the possibility for the development of passive, heat dissipation technologies, are emissive at mid-infrared wavelengths (i.e., 5–30 μm, corresponding to the blackbody spectrum at room temperature).…”

“…In addition, if radiative coolers are designed to cool outdoor objects such as buildings, automobiles, and antennas, they must be reflective in the solar spectrum (0.3–2.5 μm). − In comparison, solar steamers working at approximately 100 °C maintain unity absorptivity in the solar spectrum but must suppress their emissivity within the blackbody spectrum at such temperatures to minimize the radiative cooling effect. , TPVs are a class of electricity-generating systems without moving parts, thus enabling off-grid, portable, and ultralight power generators for drones, recreational vehicles, and spacecraft. Because most TPVs operate at high temperatures (>800 °C), spectral management of the emissions is necessary within the visible and near-infrared regions (typically 0.5–1.7 μm); the upper and lower cutoff wavelengths are determined by the band gap of the photovoltaic cells and the temperature of the emitters, respectively. − An ideal TPV emitter exhibits a stepwise blackbody spectrum that is matched with the effective wavelength range. Therefore, the development of spectrally engineered, thermally stable emitters furthers the economic viability of TPVs.…”

“…Figure a shows a schematic of a TPV system that converts photon energy released from a high-temperature emitter into electric energy through an array of low-band-gap photovoltaic cells. A variety of heat sources, including concentrated solar energy, combustion engines, and radioisotope reactors, have been attempted to attain emitter temperatures of >800 °C. − Tungsten is the most common refractory metal with a melting temperature exceeding 1200 °C, which makes it suitable for use in a TPV emitter. − ,,, In comparison to other metals, tungsten has relatively small extinction coefficients within the visible and near-infrared regions, which leads to augmented in-band emission with reduced out-of-band emissions (Figure b). However, tungsten is susceptible to oxidation at temperatures >1000 °C, which results in temporal changes in the electric power of TPVs, precluding a long-term operation. , Carbon-based materials such as graphite and SiC exhibit near-unity in-band emissions, but their poor wavelength selectivity degrades the performance of integrated photovoltaic cells and decreases the temperature of the emitters .…”

Broadband Visible and Near-Infrared Absorbers Implemented with Planar Nanolayered Stacks

“…Perfect absorbers over a broad spectral range are highly desirable for various applications including solar-energy harvesting, − thermo-photovoltaics, − sensing and imaging, − photodetectors, − and thermal emitters. , Although the perfect absorber should completely eliminate the transmission and reflection of incident light, natural materials tend to partially reflect and absorb it. Therefore, artificial metamaterials (or metasurfaces) have been extensively investigated and developed to perfectly absorb light across a broad spectral bandwidth from microwave to optical frequencies. − However, some drawbacks hinder their practical application.…”