Solar selective absorbing coatings (SSACs) harvest solar energy in the form of heat. Due to the abundant solar energy on earth, the application of the SSACs is a promising way to protect the environment by reducing the usage of fossil fuels. Traditionally, the captured solar energy was used in households like water warming and water purification under a low operating temperature of the SSACs (Sharma et al., 2017). To meet the demand of increasing energy consumption, in recent years, increased attention has been focused on the industrial applications of the SSACs at high temperature through Concentrating solar power (CSP) plants, such as central receiver (or “power tower”), solar dishes, solar thermoelectric generators (STEGs), or solar thermophotovoltaics (STPVs) (Cao et al., 2014). In these situations, the SSACs are working under high temperature to obtain high efficiency.

Concentrating solar power (CSP) plants are known as high-temperature solar–thermal systems and are widely used in power supplying. For example, in the “power tower” plant, a collector coated with SSACs is heated to be more than 550°C to melt the salt (usually 60% KNO₃ + 40% NaNO₃), and then the molten salt is used to produce superheated steam for power generation by heat exchange, as shown in Figure 1A. Theoretically, the maximum efficiency of the “power tower” can reach 85% when the system operates at ∼2,200°C. In fact, the “power tower” falls short of this efficiency for a number of reasons, such as the limitation of the thermal stability of the SSACs (Kraemer et al., 2016).

Similar to the “power tower” plant, solar dishes consist of dishes with an array of mirrors and a receiver as shown in Figure 1B. The cylinder receiver is located at the focal plane of the parabolic dish system. Capacities of parabolic dish plants are in the range of 0.01–0.4 MW, and the operating temperatures are in the range of 250–700°C. To further improve the power of the solar dishes, the operating temperature of the absorbers should be increased.

With the emergence of high-temperature SSACs, the solar selective absorber plants can also be used in power generation by STEGs (Weinstein et al., 2015). In STEGs, the heat generated by SSACs is converted to electricity utilizing a thermoelectric device, as shown in Figure 1C. Thermoelectric devices generate a voltage difference when subjected to a temperature gradient due to the Seebeck effect. When the thermoelectric device is put in a closed circuit, then the electrical power is generated. The efficiency of the thermoelectric device relies on the hot-side temperature, which is associated with the temperature of the SSACs due to the Carnot efficiency. Thus, solar irradiance of more than 200 kWm⁻² is needed to support a temperature of more than 600°C for the SSACs.

The heat generated by the SSACs could also be transferred to electrical power by the STPV via a single-junction thermophotovoltaic (TPV) cell as shown in Figure 1D. The utilizing of photon spectrum instead of direct incident sunlight enlarge the usage scale of solar energy. Since electrical power relies on conversion of thermal radiation to electricity, high-efficiency STPV devices require SSACs with high efficiency and thermal stability at high temperatures (>1273 K) (Shimizu et al., 2018).

Compared with solar photovoltaic technology, the solar photothermic technology based on SSACs has higher efficiency (up to 85%) in energy conversion due to a wider utilization of the solar spectrum (Li et al., 2018). As mentioned, the operation temperature of the solar photothermic plants may reach beyond 700°C, approaching the melting point of some metals.

Traditional SSACs like cermets [W-Ni-Al₂O₃(Cao et al., 2015), WTi-Al₂O₃(Wang et al., 2017), Ta-SiO₂(Bilokur et al., 2020)], semiconductor–metal tandems [Ti/TaC/Al₂O₃(Kondaiah et al., 2019), Cr₂O₃/Cr/Cr₂O(Khamlich et al., 2020)], multiple metal/ceramic nanofilms [Cu/Si/Al₂O₃ (El-Mahallawy et al., 2018), Ti/SiO₂/Cu (Hu et al., 2018)], and photonic crystals [HfO₂/Al₂O₃(Chou et al., 2014), HfO₂/Ta (Rinnerbauer et al., 2015)] have been extensively investigated in the past. However, large-scale industrial applications of these SSACs are impeded by the relatively low solar-to-heat conversion efficiency and their low thermal stability at high temperature. For example, a typical Cu diffusion emerged and caused the performance degradation of Cu-SiO₂/Cu tandems above 400°C in a vacuum (Cao et al., 2014). Photonic crystals consist of thicker refractory metals (e.g., W, Ta, Ni, and TiN) and usually show better thermal stability but also suffer from a high IR emittance (Chou et al., 2014; Li et al., 2015). For example, Rinnerbauer et al. designed a Ta-based metallic photonic crystal SSACs by the finite-difference-time-domain method (FDTD) as well as the Fourier modal method. The minimum thermal emittance was 0.256 at 1,000 K and 100 suns (Rinnerbauer et al., 2014). Jiang et al. prepared TiN-based SSACs with TiN nanocavity and SiO₂ filled in as dielectric (Jiang and YANG, 2017). The emittance of this encapsulated nanocavity was ∼ 0.25 before annealing and increased to ∼ 0.35 after annealing at 1,273 K for 2 h in Argon. As a common defect, the mismatch in the thermal expansion coefficients of the metal and substrate material, as well as the oxidation of the metal, will result in SSAC fatigue and delamination after many high-temperature thermal cycles (Tian et al., 2020).

Furthermore, cost-effective scaling of the SSACs is another barrier in meeting the large-scale requirements of potential industrial applications. In the process of large-scale production, special attention should be paid to the thermal stability, durability, and production cost. The optophysical properties of coating must remain stable under long-term operation at elevated temperature, repeated thermal cycling, air exposure, UV radiation, etc. Multiple-layer SSACs consisting of all-ceramic intermediate spacer could be a feasible candidate to address these challenges. Thus, in this article, we review the structure designs of all-ceramic SSACs, and the optical properties and thermal stability of the all-ceramic SSACs in the latest literature are also compared.

Near 98% of the solar radiation energy concentrated in the range of ultraviolet (UV), visible (VIS), and near-infrared (NIR) spectrum with a wavelength from 0.25 to 2.5 μm. Meanwhile, the wavelength of spontaneous blackbody irradiation is in the range of 2.5–25 μm. The ideal SSACs should have high solar absorptance in the UV-VIS-NIR spectrum range and low emittance in the infrared (IR) spectrum range, thus, leading to preservation of solar energy. Assuming the only loss is radiation from the absorbing surface, and the convective losses go to zero, the performance of the SSACs can be defined as the conversion efficiency η, given by η = α ¯ s o l − σ S B ( T h 4 − T a m b 4 ) C × I s o l a r ε ¯ t h e r m (1) where α ¯ _sol is the average solar absorptance, ε ¯ _therm is the average thermal emittance, σ _SB is the Stefan–Boltzmann constant, T _h is the absorber temperature, T _amb is the ambient temperature, I _solar is the solar radiation, and C is the solar concentration ratio. In Eq. 1, the term σ S B ( T h 4 − T a m b 4 ) C × I s o l a r can be simplified to a weighting factor ω, which is determined by the incident flux and the temperature of the absorber and ranges from >>1 to near 0. For example, the weighting factor ω is 10 at 1 kWm⁻² and 400°C, and it drops to 0.14 at 60 kWm⁻² and 350°C, and further drops to 0.05 at 1,000 kWm⁻² and 700°C. It can be inferred that in low temperature and high incident flux situations, thermal emittance is not as important as solar absorptance. However, in high temperature and low incident flux situations, thermal emittance should be considered.

The average solar absorptance ( α ¯ _sol) is given by the expression α ¯ s o l ( θ ) = ∫ 0 μ m + ∞ μ m [ 1 − R λ ] G ( λ ) d λ ∫ 0 μ m + ∞ μ m G ( λ ) d λ (2) where θ is the incident angle of sunlight, λ is the wavelength of solar radiation, G(λ) is the incident solar intensity at an atmospheric mass of 1.5 (AM 1.5), and R _λ is the spectral reflectance.

The average thermal emittance ( ε ¯ _therm) is given by the expression ε ¯ t h e r m ( T ) = ∫ 0 μ m + ∞ μ m [ 1 − R λ ] I b ( λ , T ) d λ ∫ 0 μ m + ∞ μ m I b ( λ , T ) d λ (3) I b ( λ , T ) = C 1 λ 5 ( e C 2 / λ T − 1 ) (4) where C ₁ and C ₂ are 3.743 × 10⁻¹⁶ Wm² and 1.4387 × 10⁻² mK, respectively, and I _b(λ, T) is the blackbody intensity given by Plank’s blackbody radiation (Eq. 4).

The “spectral selectivity” of the SSACs guarantees a high solar absorptance together with a low thermal emittance, which warrants an efficient utilization of the solar energy. The “spectral selectivity” of the SSACs benefits their applications in high-temperature circumstances like power generation and thermal energy storage. Conversely, despite high solar absorptance, the ordinary black absorbers usually exhibit high thermal emittance, which leads to energy loss; thus, they are only applicable in low-temperature circumstances like seawater desalination and water heating (Wang et al., 2020a). Some literature revealed that optical properties of the solar absorbers can be tuned by modifying the geometric parameters of the nanostructures (Wang et al., 2020b; Wang et al., 2020c).

To enable the application of SSACs at high temperature, in recent years, research attention has been focused on improving the efficiency and durability of SSACs in vacuum and in air (Xu et al., 2020). Among all the SSACs, all-ceramic SSACs showed a promising solar–thermal conversion efficiency as well as high thermal stability. The main task of this section is to review the structure design and performance of the all-ceramic SSACs in the latest literature.

In this paper, we reviewed the structure designs of all-ceramic SSACs in the latest literature. The optical properties and thermal stability of the all-ceramic SSACs are also compared. To the goal of high conversion efficiency and thermal stability at high temperature over 400°C, currently some novel all-ceramic SSACs can be used, such as the transition metal boride, carbide, oxide, nitride, oxynitride-based materials (TM-B/C/O/N/ON, TM = Ti, Hf, Zr, Cr, etc.), the all-ceramic plasmonic metamaterials, and the high-entropy ceramic-based materials.

We also proposed some suggestions to further improve the solar thermal conversion efficiency and the long-term thermal stability as follows.

1) Transition metals, such as Ti, Hf, Zr, Cr, etc., have ideal forbidden bandwidth, and after combining with oxygen, nitrogen, oxynitrides, etc., they can be promising absorbing materials for the all-ceramic SSACs. However, a multiple-layer structure design is essential to utilize the optical trap and the interference effect since all-ceramic SSACs with single layer are less effective in “spectral selectivity.” Furthermore, TiN and refractory metal boride or nitride, such as TiB₂, ZrN, and ZrB₂, can be used as the reflective base layer to reduce thermal emittance. The thickness of the base layer should be >100 nm to reflect all the IR light.