## 3 Recent Advances in Different Configurations of STSCs
### Semiconductor-Metal STSCs  
Semiconductor STSCs consist of an underlying metal sheet to manage the emission of heated substrates and emphasize the semiconductor and its bandgap properties. Typically, they possess a bandgap ranging from 2.5 to 1.0 mm. These semiconductors can absorb solar irradiation at wavelengths longer than their bandgap energy [46]. It can absorb radiation within the visible and near-infrared spectrum regions. Semiconducting materials are utilized as anti-reflective coatings. However, their high refractive index results in high surface light reflectance and low absorption of solar radiation, thereby impacting photothermal conversion performance [47]. Additionally, the high refractive index of semiconductors leads to considerable reflectance losses, making them unstable at elevated temperatures [48].  
Superior solar discrimination can be attained by employing absorbers in different forms, such as semiconductor-based STSCs. [PERSON] et al. [49] developed a semiconductor STSC with a 0.963 solar absorptance employing a TiAlSiCN/TiAlC/TiAlSiO/TiAlCN/ TiAlSiCO STSC. This coating demonstrated stability in air at temperatures up to 325 \({}^{\circ}\)C for 400 h and withstood cyclic and vacuum heating conditions up to 652 \({}^{\circ}\)C for 99 h. [PERSON] et al. [50] deposited a TiAlNy/TiAlNx STSC with an Al\({}_{2}\)O\({}_{3}\) non-reflective substrate. A spectrophotometric apparatus was employed to measure the reflectance across the UV to IR range (0.50\(-\)25 \(\upmu\)m). The optimal STSC exhibited a solar absorptivity of 0.93 and an emittance of approximately 0.23 at 550 \({}^{\circ}\)C. [PERSON] et al. [51] developed a tandem absorber based on WAlON/WAIN/Al\({}_{2}\)O\({}_{3}\), which demonstrated a high optical absorptance of 0.94 and a low thermal emissivity of 0.13. Moreover, the proposed STSC also exhibited favorable thermal stability in the air at 450 \({}^{\circ}\)C for 150 h. While these coatings exhibit excellent selectivity, scaling them to an industrial level remains challenging due to the increased production costs associated with using multiple cathodes or targets. To address this issue, several researchers have proposed using a single composite HfZr target for depositing HfZrCN/HfZrC/HfZrO/HfZrON coatings. Recent reports on high-temperature stable solar selective coatings include Mo-HfC/Al\({}_{2}\)O\({}_{3}\)[52], W/SiAlTiDyNx/AlSiThNx/SiAlOx [53], Cr-CrN/TiAlN/AlSiO/AlSiN [54], TiC-ZrB\({}_{2}\)/ZrB\({}_{2}\)/Al2O\({}_{3}\)[55], HfZrC/HfZrCN/HfZrON/HfZrO [56], and Mo/ZrSiN/ZrSiON/SiO\({}_{2}\)[57].  
According to the findings, the tandem semiconductor STSCs exhibited exceptional spectral selectivity, featuring high solar absorptance across the broad solar spectrum and low thermal emissivity in the infrared region. This performance is attributed to the boosted absorption owing to the excitation of the bands in the settled semiconductors with changing bandgaps. Even though semiconductor STSC layers exhibit high refractive indices that lead to significant reflectance losses, incorporating an anti-reflective layer could mitigate this issue. Furthermore, the large-scale commercial application of these coatings remains constrained by the costly and complex industrialization processes associated with semiconductor-metal STSCs.