Improvement of optical properties and thermal stability in the TiO₂/Si₃N₄/Ag/Si₃N₄/TiO₂ low-emissivity coating

Concentrated photovoltaic/photothermal systems play an increasingly important role in solar energy utilization, and the low-emissivity (low-e) coating is the key to improving the utilization efficiency. However, conventional low-e coatings struggle to achieve a balance between spectral selectivity and thermal stability, hindering the advancement of CPV/T technologies. In this paper, a low-e coating with the film structure of TiO₂/Si₃N₄/Ag/Si₃N₄/TiO₂ is developed by magnetron sputtering, achieving an average transmittance of 0.85 at visible and reflectance of 0.76 at near-infrared. Both the maximum transmittance and reflectance reach 0.92. The optical properties remain unchanged after heat treatment at 200 °C for 72 h in air. The optical profiler, SEM, and XRR are utilized for the microscopic characterization. By regulating the microstructure of the ultra-thin Ag layer, the coating with lower porosity and sheet resistance is developed, thereby the dissipation and absorption of photons are reduced, thereby the optical properties and thermal stability are optimized. The Si₃N₄ layers not only optimize the optical constants matching but also protect the Ag layer from being oxidized by TiO₂ at high temperatures. As a result, the optical properties and thermal stability of the low-e coatings are improved by regulating the coating microstructure and film structure.