thermochemical, and solar thermophotovoltaics. There exist a range of solutions with high absorptivity for low and intermediate temperatures. [1][2][3] However, for many applications, high operating temperatures (>700 K) are advantageous to achieve higher system effi ciencies. Conventional absorbers are unsuitable at these high operating temperatures since there are more considerations to be taken into account. [ 4 ] Firstly, the materials and structures need to be thermally stable and to maintain their optical properties at these high temperatures. Refractory metals are most advantageous due to their high melting point and low vapor pressure. Secondly, it is crucial that the absorber exhibits spectrally selective absorptance; namely high absorptivity in the shorter wavelength range to absorb most of the solar spectrum and low absorptivity (i.e., emissivity) in the longer wavelength range to minimize losses due to re-emission. Furthermore, this selectivity, i.e., the spectral range of high and low absorptivity, has to be tailored for the specifi c system operating conditions to achieve maximum system effi ciency.It is therefore advantageous to use PhCs which offer the possibility to tailor the spectral absorptance [ 5,6 ] and thus optimize system effi ciency. Several absorbers based on 1D multilayer stacks, [ 7,8 ] 2.5D structures such as pyramids, [9][10][11] 3D PhCs in refractory metals, [ 12,13 ] as well as metamaterials [14][15][16] have been proposed. Here, we demonstrate the suitability of a 2D PhC comprising a square lattice of cylindrical cavities etched into a Ta substrate as a highly effi cient and selective absorber at high temperatures, i.e., above 1000 K. While all of the above approaches achieve good spectral selectivity, the 2D PhC design is a compact and thermally robust structure, minimizing the number of interfaces as compared to multilayer or 3D PhC approaches which is crucial for high temperature stability. At the same time, fabrication is simple and scalable and can be achieved by standard semiconductor processes. In this 2D PhC design, the absorptivity of the material is selectively enhanced by the introduction of cavity modes and the spectral range of enhancement, i.e., high absorptivity, can be tuned A high-temperature stable solar absorber based on a metallic 2D photonic crystal (PhC) with high and tunable spectral selectivity is demonstrated and optimized for a range of operating temperatures and irradiances. In particular, a PhC absorber with solar absorptance α α = = 0.86 and thermal emittance ε ε = 0.26 at 1000 K, using high-temperature material properties, is achieved resulting in a thermal transfer effi ciency more than 50% higher than that of a blackbody absorber. Furthermore, an integrated double-sided 2D PhC absorber/ emitter pair is demonstrated for a high-performance solar thermophotovoltaic (STPV) system. The 2D PhC absorber/emitter is fabricated on a double-side polished tantalum substrate, characterized, and tested in an experimental STPV setup along with a fl at Ta absorber...

We present the results of extensive characterization of selective emitters at high temperatures, including thermal emission measurements and thermal stability testing at 1000 °C for 1h and 900 °C for up to 144 h. The selective emitters were fabricated as 2D photonic crystals (PhCs) on polycrystalline tantalum (Ta), targeting large-area applications in solid-state heat-to-electricity conversion. We characterized spectral emission as a function of temperature, observing very good selectivity of the emission as compared to flat Ta, with the emission of the PhC approaching the blackbody limit below the target cut-off wavelength of 2 μm, and a steep cut-off to low emission at longer wavelengths. In addition, we study the use of a thin, conformal layer (20 nm) of HfO(2) deposited by atomic layer deposition (ALD) as a surface protective coating, and confirm experimentally that it acts as a diffusion inhibitor and thermal barrier coating, and prevents the formation of Ta carbide on the surface. Furthermore, we tested the thermal stability of the nanostructured emitters and their optical properties before and after annealing, observing no degradation even after 144 h (6 days) at 900 °C, which demonstrates the suitability of these selective emitters for high-temperature applications.

After decades of intense studies focused on cryogenic and room temperature nanophotonics, scientific interest is also growing in high-temperature nanophotonics aimed at solid-state energy conversion. These latest extensive research efforts are spurred by a renewed interest in high temperature thermal-to-electrical energy conversion schemes including thermophotovoltaics (TPV), solar-thermophotovoltaics, solar-thermal, and solar-thermochemical energy conversion systems. This field is profiting tremendously from the outstanding degree of control over the thermal emission properties that can be achieved with nanoscale photonic materials. The key to obtaining high efficiency in this class of high temperature energy conversion is the spectral and angular matching of the radiation properties of an emitter to those of an absorber. Together with the achievements in the field of highperformance narrow bandgap photovoltaic cells, the ability to tailor the radiation properties of thermal emitters and absorbers using nanophotonics facilitates a route to achieving the impressive efficiencies predicted by theoretical studies. In this review, we will discuss the possibilities of emission tailoring by nanophotonics in the light of high temperature thermal-toelectrical energy conversion applications, and give a brief introduction to the field of TPV. We will show how a class of large area 2D metallic photonic crystals can be designed and employed to efficiently control and tailor the spectral and angular emission properties, paving the way towards new and highly efficient thermophotovoltaic systems and enabling other energy conversion schemes based on high-performance high-temperature nanoscale photonic materials.