Scalable and Ultrathin High‐Temperature Solar Selective Absorbing Coatings Based on the High‐Entropy Nanoceramic AlCrWTaNbTiN with High Photothermal Conversion Efficiency

High‐entropy ceramics, especially high‐entropy alloy nitrides (HEANs), have attracted increasing attention in recent years due to their unique structural characteristics that result in their potential use in correlated high‐temperature structural materials. However, research on their optical properties, especially the spectral selectivity, is still in its infancy. According to Carnot efficiency, which demands an extremely high working temperature to elevate the performance of a concentrating solar power (CSP) system, excellent thermal stability is valuable to meet the requirements of solar selective absorbing coatings (SSACs). Herein, the high‐entropy ceramic AlCrWTaNbTiN is introduced as effective absorption layers by adjusting the nitrogen content to design and fabricate an ultrathin multilayer solar absorbing coating, which exhibits a high solar absorptance of 93% and a low emittance of 6.8% at 82 °C. Coating design (CODE) software is used to design and optimize the multilayer solar absorbing coating. Moreover, the optical properties, microstructure, thermal stability, and failure mechanism are investigated. Finite‐difference time‐domain (FDTD) calculations are used to understand the nature of the optical absorption consisting of intrinsic absorption and extinction absorption.