Novel Ultrathin Quasi-Optical Microcavity-Selective Absorber Based on Ti@a-C Cermet for Solar-Thermal Conversion

The growing attention in solar energy has motivated the development of a highly efficient solar absorber. Under a certain light concentration, increasing the solar spectral absorption of solar absorbers can improve the photothermal conversion efficiency. In this paper, we first designed and prepared a single-layered cermet absorber based on the double-gradient absorption model of refractive index and extinction coefficient with the structure of Ti@a-C cermet layer/a-C protective layer/SiO2 antireflective layer via a magnetron sputtering method. The cermet absorber shows a solar radiation absorption of 96.5% in the ultraviolet, visible, and near-infrared bands of sunlight (300–2500 nm). To further improve the solar radiation absorption of the device, a quasi-optical microcavity structure (QOM) based on cermet–metal–cermet (CMC) was proposed. Due to the synergistic effect of the interband transitions in metal and amorphous carbon (a-C), the plasmon resonance of the metal nanoparticles, and the surface propagating plasmon absorption in the microcavity, the designed and prepared CMC quasi-optical microcavity absorber (CMC-QOMA) reaches a remarkable absorption rate of 97.1% in the solar spectrum band of 300–2500 nm. In addition, the two absorbers are highly insensitive to the incident angle and polarization state of the incident light, demonstrating their high efficiency in practical application scenarios. We also investigated the thermal stability of the absorbers in an atmospheric environment by examining the structure and composition changes before and after annealing. These results indicate that a-C matrix cermet material is an outstanding candidate to improve solar heat conversion and prove that Ti@a-C-based ultrabroadband absorption devices have great potential in solar thermal collection, light detection, and imaging. Furthermore, the proposed device can be fabricated by a simple lithography-free film deposition method, which paves the way for a variety of large-area applications for a-C matrix materials.