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Nanophotonic materials offer spectral and directional control over thermal emission, but in high-temperature oxidizing environments, their stability remains low. This limits their applications in technologies such as solid-state energy conversion and thermal barrier coatings. Here we show an epitaxial heterostructure of perovskite BaZr 0.5 Hf 0.5 O 3 (BZHO) and rocksalt MgO that is stable up to 1,100 °C in air. The heterostructure exhibits coherent atomic registry and clearly separated refractive-index-mismatched layers after prolonged exposure to this extreme environment. The immiscibility of the two materials is corroborated by the high formation energy of substitutional defects from density functional theory calculations. The epitaxy of immiscible refractory oxides is, therefore, an effective method to avoid prevalent thermal instabilities in nanophotonic materials, such as grain-growth degradation, interlayer mixing and oxidation. As a functional example, a BZHO/MgO photonic crystal is implemented as a filter to suppress long-wavelength thermal emission from the leading bulk selective emitter and effectively raise its cutoff energy by 20%, which can produce a corresponding gain in the efficiency of mobile thermophotovoltaic systems. Beyond BZHO/MgO, computational screening shows that hundreds of potential cubic oxide pairs fit the design principles of immiscible refractory photonics. Extending the concept to other material systems could enable further breakthroughs in a wide range of photonic and energy conversion applications., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

Thermophotovoltaic cells are similar to solar cells, but instead of converting solar radiation to electricity, they are designed to utilize locally radiated heat. Development of high-efficiency thermophotovoltaic cells has the potential to enable widespread applications in grid-scale thermal energy storage 1,2 , direct solar energy conversion 3-8 , distributed co-generation 9-11 and waste heat scavenging 12 . To reach high efficiencies, thermophotovoltaic cells must utilize the broad spectrum of a radiative thermal source. However, most thermal radiation is in a low-energy wavelength range that cannot be used to excite electronic transitions and generate electricity. One promising way to overcome this challenge is to have low-energy photons reflected and re-absorbed by the thermal emitter, where their energy can have another chance at contributing towards photogeneration in the cell. However, current methods for photon recuperation are limited by insufficient bandwidth or parasitic absorption, resulting in large efficiency losses relative to theoretical limits. Here we demonstrate near-perfect reflection of low-energy photons by embedding a layer of air (an air bridge) within a thin-film In 0.53 Ga 0.47 As cell. This result represents a fourfold reduction in parasitic absorption relative to existing thermophotovoltaic cells. The resulting gain in absolute efficiency exceeds 6 per cent, leading to a very high power conversion efficiency of more than 30 per cent, as measured with an approximately 1,455-kelvin silicon carbide emitter. As the out-of-band reflectance approaches unity, the thermophotovoltaic efficiency becomes nearly insensitive to increasing cell bandgap or decreasing emitter temperature. Accessing this regime may unlock a range of possible materials and heat sources that were previously inaccessible to thermophotovoltaic energy conversion.

Radiative cooling can alleviate urban heat island effects and passively improve personal thermal comfort. Among many emerging approaches, infrared (IR) transparent films and fabrics are promising because they can allow objects to directly radiate heat through bands of atmospheric transparency while blocking solar heating. However, achieving high solar reflectance while maintaining IR transmittance using scalable nanostructured materials requires control over the shape and size distribution of the nanoscale building blocks. Here, we investigate the scattering and transmission properties of electrospun polyacrylonitrile (PAN) nanofibers that feature spherical, ellipsoidal, and cylindrical morphologies. We find that nanofibers that have ellipsoidal beads exhibit the most efficient solar scattering, mainly due to the additive dielectric resonances of the ellipsoidal and cylindrical geometries, as confirmed through electromagnetic simulations. This favorable scattering decreases the amount of material needed to reach above 95% solar reflectance, which, in turn, enables high infrared transmittance (>70%) despite PAN's intrinsic IR absorption. We further show that these PAN nanofibers (nanoPAN) can enable cooling of surfaces with relatively low solar reflectance, which is demonstrated by covering a reference blackbody surface with beaded nanoPAN. During peak solar hours, this configuration lowers the temperature of the black surface by approximately 50 °C and is able to achieve as low as 3 °C below the ambient air temperature. More broadly, our demonstration using PAN, which is not as IR transparent as more commonly used polyethylene, provides a method for utilizing lower purity materials in radiative cooling.