Dual‐Sublattice Entropy via Cation‐Anion Co‐Doping Enables Broadband Solar Absorption and Efficient Photothermal Conversion

ABSTRACT Photothermal conversion is a highly efficient pathway for solar energy utilization, requiring coatings with strong absorption in the solar spectrum (0.3–2.5 µm) while maintaining low emissivity in the mid‐infrared range (2.5–25 µm). Here, we report a simple bilayer solar selective absorber coating (SSAC) based on a high‐entropy boride (HEB) with dual cation–anion site doping. Nitrogen incorporation into the HEB lattice forms B─N bonds and interstitial N─N states, effectively tailoring the bandgap structure, inducing band edge shifts, and broadening the absorption bandwidth. Combined with a Si ₃ N ₄ antireflection layer, the bilayer coating achieves a high solar absorptance of 93.6% and an ultralow thermal emittance of 11.3% at 82°C. The SSAC exhibits excellent thermal stability up to 500°C, with a photothermal conversion efficiency of 89.7% under 100 suns, and maintains a temperature rise of over 91°C even at 1 sun irradiation. Outdoor tests further confirm outstanding solar harvesting capability, where the coating surface temperature exceeds 60°C in winter, ∼20°C higher than that of non‐selective absorbers. This work demonstrates that synergistic cation–anion co‐doping in HEB is an effective strategy to enhance solar absorptance, while the facile bilayer design offers practical advantages for scalable fabrication and deployment in solar‐thermal applications.