Modulation of the cutoff wavelength in the spectra for solar selective absorbing coating based on high-entropy films

This paper demonstrates an intrinsic modulation of the cutoff wavelength in the spectra for solar selective absorbing coating based on high-entropy films. The (NiCuCrFeSi)N ((NCCFS)N) films were deposited by a magnetron sputtering system. Rutherford backscattering spectroscopy analysis confirms the uniform composition and good homogeneity of these high-entropy films. The real and imaginary parts of the permittivity for the (NCCFS)N material are calculated on the basis of the reflectance spectral fitting results. A redshift cutoff wavelength of the reflectance spectrum with increasing nitrogen gas flow rate exists because of the different levels of dispersion when changing nitrogen content. To realize significant solar absorption, the film surface was reconstituted to match its impedance with air by designing a pyramid nanostructure metasurface. Compared with the absorptance of the as-deposited films, the designed metasurface obtains a significant improvement in solar absorption with the absorptance increasing from 0.74 to 0.99. The metasurfaces also show low mid-infrared emissions with thermal emittance that can be as low as 0.06. These results demonstrate a new idea in the design of solar selective absorbing surface with controllable absorptance and low infrared emission for high-efficiency photo-thermal conversion.