Thermostable 1D Photonic Multilayer with Si₃N₄-Driven Outcoupling for Broadband Radiative Cooling with Angular Tolerance

Passive daytime radiative cooling (PDRC) reflects sunlight and emits thermal radiation through the 8–13 μm atmospheric window into space, facilitating energy-free heat dissipation. Inorganic materials resist photothermal degradation plaguing organic counterparts; however, achieving durable, high-performance broadband emitters remains elusive. Here, we report broadband 1D photonic crystal-like structure employing alternating SiO2/Al2O3 four-layers with a topmost Si3N4 outcoupling layer deposited onto an AlN-protected Ag mirror over quartz via magnetron sputtering. The refractive index contrast within 1D induces impedance mismatch, while thinning low-index layers broadens mid-IR emissivity via constructive interference at high-index layers. Si3N4 enhances atmospheric emissivity by enabling mid-IR photon escape through intrinsic phonon-polariton absorption in the atmospheric window and subsurface extinction contrast. AlN preserves Ag’s spectral integrity from oxidation. The structure achieves 97.01% solar reflectivity, 90.67% atmospheric emissivity, and a notable 90.59% broadband emissivity over 8–20 μm range, yielding 157.07 W/m2 cooling power with 13.8 K subambient drop─record-setting for all-inorganic matrices. TG-DSC confirms remarkable thermostability and chemical inertness up to 1450 °C. Outdoor tests recorded subambient drop of 10.9 °C (horizontally) and 4 °C (vertically) under 980 W/m2 and 580 W/m2 solar irradiance, respectively, despite localized feedback, confirming practical viability. This scalable design addresses material scarcity, complexity, and spectral trade-offs for terrestrial and exoatmospheric thermal regulation.