## 6 Conclusions  
Presently, there has been a longstanding demand for highly spectral solar selective absorber coatings for augmenting the solar thermal conversion performances of Concentrated Solar Power technologies. This review article primarily examines various innovative structures of solar thermal selective coatings (STSCs) and their deposition processes, aimed at enhancing photothermal conversion efficiency by effectively controlling light transmission and reflection. Furthermore, this paper discusses technological advancements in various types of STSCs, including material synthesis, spectral structure designs, and induced aging mechanisms, to assess their optical properties and thermal stability. Additionally, the challenges in creating cermet and multilayered solar absorbers that maintain long-term  
Figure 10: Diagram of the six-layer selective absorber design [102].  
thermal stability under various operating conditions, aiming to drive the advancement of next-generation STSCs are also discussed. Some of the key findings of the current study are given below as follows:  
1. The primary methods for preparing STSCs include vacuum thermal evaporation, magnetron sputtering, and cathodic arc evaporation. Among these, magnetron sputtering is widely used in industrial applications because it can produce structurally stable STSCs and is suitable for large-scale production.
2. Given their high conversion efficiency and stability at elevated temperatures, some novel double cermet solar selective coatings are promising candidates for solar thermal conversion at 600 \({}^{\circ}\)C. Paradigms include AIN-W, Pt-Al\({}_{2}\)O\({}_{3}\), WTi-Al\({}_{2}\)O\({}_{3}\), and Mo-Al\({}_{2}\)O\({}_{3}\), all of which exhibit excellent long-term optical properties and superior photothermal performance.
3. Utilizing a multilayer surface design such as W/TiAlSiN/TiAlN/TiAlSiON/TiAlSiO can broaden the absorption range within the solar spectrum. Solar absorptance can be greatly enhanced by precisely designing the anti-reflection layer. On the other hand, while multilayered absorbing designs can achieve high solar absorption, maintaining compositional consistency and thermal stability during cyclic heating remains a significant challenge in developing practical STSCs using these types of structures.
4. Multilayered STSCs hold potential as candidates for solar thermal conversion at temperatures up to 550 \({}^{\circ}\)C when deposited on an appropriate substrate. Their exceptional stability and capability to function in the air, rather than requiring vacuum conditions, address the challenges of simplifying manufacturing processes and reducing costs.
5. The diffusion and oxidation aging-induced mechanisms are the most extensively studied for STSCs, with efforts focused on enhancing their long-term stability. Furthermore, it mostly indicated that the oxidation of STSCs at high temperatures includes the oxidation of metals, metal oxides, and alloys. In contrast, these oxides exhibit diverse behaviors at high temperatures.  
The author acknowledges the Interdisciplinary Research Center for Sustainable Energy systems (IRC-SES), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia. The author declares no conflicts of interest. The author declares no conflicts of interest.