Dual-Sublattice Entropy via Cation-Anion Co-Doping Enables Broadband Solar Absorption and Efficient Photothermal...

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 Si₃N₄ 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.