## 3 Recent Advances in Different Configurations of STSCs
### Multilayered-Based STSCs  
Multilayer STSCs comprise dielectric layers instead of alternating metals. The metal layers reduce emissivity in the infrared region, while the dielectric layers improve absorption in the visible spectrum [30]. The selectivity of these multilayer STSCs arises from multiple reflections as light traverses layers with differing refractive indices. Optimal selectivity can be achieved in multilayer STSCs by designing a gradient in refractive index and extinction coefficient from the substrate to the surface. This is accomplished by precisely selecting the thickness and material composition of each layer. Ideally, the anti-reflection layer at the front of the solar absorber should have a refractive index of 1 and an extinction index of zero [82]. Many earlier kinds of multilayered STSCs have been developed by different groups, including AlCrSiON/AlCrSiN/AlCrO [83], Cu/TiAlCrN/TiAlN/AlSiN [84], TiN/nano-multilayered AlCrSiO/AlCrSiO [85], Al/NbMoON/NbMoN/SiO\({}_{2}\)[86], and W/SiAlTiOyNx/AlSiTiNx/SiAlOx [53]. Recently, there has been a focus on solar selective absorbers that exhibit high absorption in the spectra region and have substantial cut-off wavelengths in the long-time wave ranges. These characteristics greatly enhance the solar-to-heat conversion efficiency of the absorber. [PERSON] et al. [87] presented the effects of various process parameters on the optical properties of individual layers and the multilayered STSC of W/WAlSiN/SiO\({}_{2}\)/SiON. Factors such as reactivated gas flowrate, deposition time, sputtering power, and layer thicknesses were analyzed to achieve high spectral selectivity. The results showed a very low reflectance of <5% and a very high reflectance of >95% in the IR regime, leading to an absorptance of 0.955 and an emissivity of 0.10 at 82\({}^{\circ}\). In a later attempt, the thermal stability of the multilayered STSC was examined under cyclic heating conditions for extended periods: 700 \({}^{\circ}\)C for 200 h in a vacuum, and 400 \({}^{\circ}\)C for 500 h and 500 \({}^{\circ}\)C for 100 h in the atmosphere [88]. [PERSON] et al. [89] proposed a multilayered STSC of W/TiAlSiN/TiAlN/TiAlSiON/TiAlSiO, created using magnetron sputtering, which demonstrated excellent spectral selectivity with an absorptance of 0.954 and an emissivity of 0.07. In this structure, the W layer acts as an infrared reflector due to its high reflectivity in the IR region. The metal-like TiAlSiN and TiAlN layers serve as the primary absorbers, while TiAlSiON functions as a semi-absorbing layer. [PERSON] et al. [90] described the infrared emissivity of the W/TiAlSiN/TiAlN/TiAlSiO/TiAlSiON multilayered STSC at increased temperatures ranging from 100 to 650 \({}^{\circ}\)C. It yielded superior spectral se lectivity, with high absorptivity in the solar diapason and minimal thermal emissivity in the infrared range (2.5-25 \(\upmu\)m). [PERSON] et al. [91] analyzed the structural complexity and thermal performance of a nanostructured multilayer STSC. The resulting absorber achieved a total solar absorptance of 0.9504. Considering infrared radiation losses, the solar thermal effectiveness of the STSC was measured at 91.5% under 1000 W/m\({}^{2}\), with a total emittance of 0.15. [PERSON] et al. [92] introduced a multilayered STSC that combined nano-arrays and tungsten nanoholes, achieving a mean solar absorptivity of 0.946 and a solar-to-heat conversion efficacy of 91.5%, respectively. [PERSON] et al. [93] developed a metamaterial solar absorber featuring a conical multilayered cylindrical structure, which exhibited excellent absorption capability in short wavelengths and features a cut-off wavelength at 1710 nm to minimize radiation losses. [PERSON] et al. [37] introduced an STSC composed of six layers (SiO\({}_{2}\)/PtAl\({}_{2}\)O\({}_{3}\)/Pt/PtAl\({}_{2}\)O\({}_{3}\)/CuCoMnOx/SiO\({}_{2}\)), fabricated using a cost-effective dip-coating method (see Figure 7).