Broadband Solar Selective Absorber with Dallenbach-Type Bilayer Structure Achieved Using Carbon Nanotube Membranes with Tailored Optical Spectra

Spectrally selective absorbers that maximize solar absorption and minimize thermal radiation loss are crucial for efficient solar thermal energy harvesting. However, limitations imposed by the intrinsic properties of conventional materials hinder the fabrication of interference-based absorbers with the desired optical properties. Herein, a high-performance solar absorber with a Dallenbach-type dielectric–metal tandem structure was fabricated using an ultrathin, subquarter-wavelength-thick single-walled carbon nanotube (SWCNT) membrane with tailored optical spectra as the absorbing layer. By mixing multiple SWCNT chiral structures, the optical response of the absorbing layer was tailored to approximate a theoretical complex refractive index spectrum of dielectrics, thereby enabling high solar absorptance, low infrared emittance, and a low angular dependence with a simple bilayer structure. The fabricated proof-of-concept 1 cm2 absorber exhibited excellent spectral selectivity (solar absorptance/infrared emittance of 0.84/0.03) and an equilibrium temperature of ≈190 °C (270 °C) under nonconcentrated (2× concentrated) sunlight, considerably outperforming a blackbody-like absorber. This study presents a high-performance solar absorber with a simple planar structure and proposes a concept of designing dielectrics with the desired optical properties by mixing various types of structure-sorted SWCNTs.