Recent advances of spectrally selective absorbers: Materials, nanostructures, and photothermal power generation

The vast potential of solar energy has established it as a key player in addressing the global energy shortage.¹ Currently, solar energy utilization spans photovoltaic power generation,² solar-thermal conversion,³ photocatalysis,⁴ and photobiological applications.⁵ Among these, solar-thermal conversion, which transforms solar light into thermal energy, is particularly noteworthy for its directness and efficiency. This process involves absorbing solar photons and converting them into heat, which can then be used to generate electricity or chemical energy.⁶ To optimize solar energy utilization efficiency, spectrally selective absorbers (SSAs) are crucial. These SSAs are designed to maximize solar energy absorption while minimizing thermal loss through infrared (IR) radiation.^7–9 Current solar thermal technologies encompass a range, from concentrated solar power (CSP) systems¹⁰ and solar thermoelectric generators (STEGs)¹¹ to applications in water evaporation,¹² solar thermal sterilization,¹³ and personal thermal management.¹⁴ CSP systems and STEGs, leveraging solar-thermal techniques, are pivotal in power generation.^15,16 According to Carnot efficiency, creating high-temperature surfaces significantly enhances power generation efficiency.¹⁷ Notably, next-generation CSP systems are anticipated to operate at temperatures exceeding 700 °C, heightening the demand for SSAs as radiative heat loss escalates exponentially with temperature, as per the Stefan–Boltzmann law.^18–20 However, these high-temperature conditions present significant challenges for SSAs, which must possess both excellent optical properties and thermal stability.

Since the concept of SSA was introduced in the 1950s by Tabor,²¹ extensive research has focused on developing advanced SSAs through nanostructure design and material selection.^22–24 To date, various nanostructures have been effectively established in SSA development, including intrinsic absorbers,^25,26 optical microcavity multilayer structures,^10,27 dual-ceramic structures,^10,22 simple double-layer structures,^28,29 and plasmonic absorbers.^30,31 Concurrently, there has been significant progress in advanced materials such as ceramic composites,^32,33 two-dimensional (2D) MXenes,^34,35 high-entropy materials,^36,37 and graphene.^38,39 These materials have demonstrated remarkable light-trapping abilities, offering new opportunities for SSAs’ development. Notably, the integration of these emerging materials with established nanostructures holds great promise for developing high-performance SSAs. For instance, 2D MXenes, used as intrinsic SSAs, exhibit superior spectral selectivity compared to traditional materials.²⁵ Similarly, incorporating high-entropy materials into dual-ceramic structures enhances both spectral selectivity and thermal stability, outperforming commercial SSAs.⁴⁰ Moreover, combining ceramic composites with microcavity multilayer or simple double-layer structures has increased the thermal stability of SSAs to above 800 °C.^41,42 These advancements in SSAs are revolutionizing traditional performance standards, potentially resolving the trade-off between optical properties and thermal stability.

This review concentrates on these advanced materials currently being explored for the fabrication of SSA. It begins by highlighting critical aspects of SSAs, such as solar absorptivity, thermal emissivity, photothermal conversion efficiency, and lifetime evaluation. It then covers prevalent fabrication techniques, including magnetron sputtering and the sol–gel method. Subsequently, the review introduces promising materials suitable for SSA preparation and emphasizes recent advancements in various nanostructures, along with the performance enhancements achieved with the synergy of these advanced photothermal materials with nanostructures. Finally, we review their applications in photothermal power generation, including CSP systems and STEGs. This review aims to provide insights for future research that synergizes emerging materials with established nanostructures for next-generation SSAs, thus paving the way for more efficient solar energy utilization.

Electroplating, physical vapor deposition (PVD, e.g., magnetron sputtering), and sol–gel methods are widely used methods for preparing SSAs. Electroplating, one of the earliest coating fabrication methods for SSAs, is characterized by low current density, good dispersion, and high current efficiency.⁵³ It is generally used for preparing black nickel and black chrome absorption coatings.^54–56 However, these coatings have inferior optical properties and thermal stability only in the temperature range of 200–400 °C. Currently, magnetron sputtering and sol–gel methods are popular fabrication processes for solar selective absorption coatings.

Magnetron sputtering has become a commercial method for fabricating SSAs.⁵⁷ It provides high film quality, good controllability, a uniform microstructure, and can achieve roll-to-roll production. This method involves ionizing working gases like Ar in a vacuum and bombarding the target material so that neutral target atoms or molecules are deposited on a substrate to form a coating, as shown in Fig. 2(a). In direct current (DC) sputtering, for instance, the cathode is a metal target, and the anode is usually a grounded rotating substrate. A negative bias voltage of several hundred to a thousand volts is applied to the cathode. The gas atoms or molecules are ionized, accelerating with high energy to strike the target surface.⁵⁸ Part of the target material atoms are sputtered off the surface, while others produce secondary electrons, which then accelerate away from the cathode to form plasma. These secondary electrons maintain glow discharge, creating more high-energy particles that bombard the surface and cause material atoms to leave the target and fly toward the substrate.⁵⁹ 

Magnetron sputtering is widely employed to prepare SSAs with excellent performance. Researchers used N₂ and O₂ as reactive gases and TiAl^44,45 and NbAl⁴⁴ as sputtering targets to prepare ceramic absorbers, achieving high absorptance (>0.95). When the AlTiO sputtered with O₂ as reactive gases served as an anti-reflective layer, the coatings remained stable at 450 °C for 1000 h.⁶⁰ Recently, Li et al. used magnetron sputtering to fabricate all-ceramic solar selective absorbers TiN/TiNO/ZrO₂/SiO₂, achieving dense and uniform coatings that raised the working temperature of traditional absorbers by at least 227 °C.⁶¹ Moreover, multi-target sputtering deposition has become a hot research topic, allowing for more flexibility to fabricate SSAs with excellent properties.⁶² For example, Wang and co-workers used Mo, Zr, and Si multi-target reactive sputtering to fabricate dual-ceramic SSA.⁶³ They controlled the Ar/N₂/O₂ gas flow to deposit Mo, ZrSiN, ZrSiON, and SiO₂ layers with adjustable metallic and dielectric properties, resulting in a dense coating that maintained stability at 600 °C in vacuum for 300 h.

The sol–gel method, a type of wet chemical process, gained popularity in the 1960s.⁶⁴ It is pivotal in the field of material science, particularly for the fabrication of SSA. As shown in Fig. 2(b), this process begins with the dissolution of metal compounds (including metal alkoxides, ethanolates, and halides) in organic solvents or water. Subsequent hydrolysis or alcoholysis reactions convert these solutions into a liquid precursor. The addition of a film-forming agent then triggers further hydrolysis and condensation, resulting in the formation of a stable sol. This sol is then uniformly deposited on a substrate and subjected to thermal treatment. During this phase, solvent mobility between sol networks is reduced, leading to the formation of a gel-like structure.⁶⁵ Sintering processes are then employed to solidify the coating, forming a durable and uniform layer. This process offers low fabrication temperatures, good coating uniformity, and mild chemical conditions. This method also allows for controlled hydrolysis and condensation reactions and is particularly adept at producing materials with high porosity.⁶⁶ In contrast to other techniques like magnetron sputtering, the sol–gel method is notably cost-effective and simpler to execute.⁶⁷ 

Over 15 years ago, researchers used the sol–gel technique to fabricate SSAs,⁶⁸ by embedding carbon nanoparticles in ZnO and NiO matrices on Al substrates. Using polyethylene glycol as a solution gelling agent, they mixed and dispersed oxides with carbon nanoparticles, spin-coated them onto Al substrates, and dried them at 550 °C in an N₂ atmosphere to form a dense oxide matrix. The resulting C–NiO coating had a high solar absorptivity of 0.84 and a low emissivity of 0.04. Continuing advancements in this method have seen improvements in coating performance. For instance, researchers have applied thermally treated organically modified SiO₂ colloids as anti-reflective layers on absorber surfaces, effectively raising their thermal stability to 500 °C.⁶⁹ Grosso et al. employed the sol–gel process to create nanocrystalline RuO₂/SiO₂ composite material layers, exhibiting remarkable thermal stability over prolonged durations of 1000 h at 600 °C in air.⁷⁰ However, these coatings have exhibited a relatively high thermal emissivity of ∼0.28 at 600 °C, leading to significant IR heat loss. To ensure coating stability while minimizing energy loss, Li et al. innovatively used the sol–gel technique to fabricate an all-ceramic structure with plasmonic properties.⁷¹ This involved spin-coating a TiN-containing sol onto an infrared-reflective metal layer and subsequently covering it with SiO₂. After thermal curing, they obtained a coating with high absorption and low emission, achieving a high photothermal efficiency of 89%–93%.

With the development of material science and technology, a growing array of advanced materials is being developed, which shows significant promise for the preparation of high-performance SSAs. These include ultra-high temperature ceramic composites,⁷² 2D MXenes,⁷³ graphene,⁷⁴ and high-entropy materials.⁷⁵  Figure 3(a) illustrates the potential elements that these four materials incorporate in the preparation of SSAs. Notably, these materials offer extensive opportunities for the enhancement of optical properties and for improving thermal and chemical stability.

Transition metal ultra-high temperature ceramic materials such as carbides, nitrides, and borides have been extensively used for the fabrication of SSAs, benefiting from their excellent thermal stability and unique d band structures.^77–82 Notably, these ultra-high temperature ceramics, with their exceptional high-temperature resistance, mechanical strength, wear resistance, and chemical stability, have been widely applied as protective coatings on spacecraft surfaces.^83,84 Early research on these materials-based SSAs mainly focused on single component transition metal ceramics. For example, studies on single coatings of NbB₂, TiN, CrN, ZrB₂, and TiC found good promise for the preparation of SSAs.^81,85–88 However, most of these transition metal ceramic-based absorbers suffer from either low spectral selectivity or insufficient thermal robustness. Although building multilayer structures is able to improve the spectral selectivity of binary ceramic SSAs, they fall short in photothermal conversion applications above 600 °C, particularly for the next-generation CSP system. This is because these materials suffer from heat stress at high operating temperatures, which results in component oxidation and element diffusion responsible for the weakened optical properties.

To address this, researchers began exploring the combination of two different types of ceramics to create composite materials, thereby leveraging the synergistic effects to simultaneously boost optical properties and thermal stability. These ceramic composite materials have shown potential for meeting some harsh requirements, offering a new avenue for the design and optimization of high-performance SSAs. For instance, TiC–ZrC ceramic composite materials demonstrated high spectral selectivity (α/ε = 0.92/0.11) and maintained optical stability at 700 °C in a vacuum over 100 h.⁸⁹ Additionally, doping ultra-high temperature ceramics with rare earth elements and transition metal oxides has proven effective in enhancing SSA performance. Examples include TiC–Y and ZrC–ZrO₂ composite ceramics, which have shown notable thermal stability and optical properties.^90,91

MXenes, a new class of 2D materials composed of transition metal carbides and carbonitrides, represent a significant breakthrough in material science.^92–94 These materials are generally represented by the formula M_n+1X_nT_x, where M is a transition metal, “X” can be either carbon or nitrogen, T stands for surface functional groups, and x indicates the number of these groups, as shown in Fig. 3(b). Their diverse elemental composition and structural versatility endow MXenes with remarkable physical and chemical properties. Since their discovery in 2011, MXenes have found applications across a wide range of fields, including catalysis,⁹⁵ battery,⁹⁶ supercapacitors,⁹⁷ environmental science,⁹⁸ photonic diodes,⁹⁹ and electromagnetic wave management.¹⁰⁰ 

Recent advancements have particularly highlighted the potential of MXenes in photothermal conversion applications. This is attributed to their adjustable bandgaps and electronic structures.^101–103 Wang and co-workers have demonstrated that MXenes exhibit superior light absorption compared to carbon nanotubes (CNTs), especially in applications like solar water evaporation, where a Ti₃C₂ film achieved an 84% evaporation efficiency, comparable to the best photothermal evaporation systems.¹⁰³ In another study, Lu et al. enhanced solar absorption by combining MXene with Co₃O₄ nanoparticles, achieving a high solar absorption of 94%.¹⁰⁴ Additionally, Shi et al. have discovered that MXene ceramic nanosheets are highly effective in near-infrared (NIR) regions, making them viable for applications such as photothermal therapy in cancer treatment.¹⁰⁵ The widespread adoption of MXenes in photothermal conversion highlights their potential in the development of high-performance SSAs.

High-entropy alloys, introduced in 2004 as a groundbreaking class of alloys, stand out from conventional alloys by their composition of at least five elements, each in nearly equal molar ratios.^106–108 This unique composition leads to four core effects that imbue HEAs with exceptional thermodynamic, kinetic, performance, and structural advantages.¹⁰⁹ A key characteristic of these alloys is their high configurational entropy, which lowers the Gibbs free energy, thereby promoting the formation of stable single-phase solid solutions. This leads to remarkable thermal stability and oxidation resistance. Building on the foundation of high-entropy alloys, the field has expanded to include high-entropy ceramics. These are created by integrating non-metal elements such as nitrogen, carbon, oxygen, and boron into the alloy matrix, resulting in a diverse array of high-entropy materials like nitrides,¹¹⁰ carbides,¹¹¹ oxides,^112–114 and borides.¹¹⁵ These materials demonstrate superior physical and mechanical properties.¹¹⁶ Crucially, the unique combination of transition metal elements with distinctive d-band structures and the specific atomic arrangements in high-entropy materials enhance their light absorption capabilities.¹¹⁷ 

High-entropy materials offer improved thermal stability over traditional ceramic materials, attributed to the entropy-assisted stabilization effect.¹¹⁸ The present research focuses on designing multilayer structures and surface morphologies to augment solar absorption.^119,120 Additionally, fine-tuning the composition of high-entropy materials has proven effective in enhancing light trapping capabilities and thermal stability. Initially, high-entropy nitrides and oxynitrides were utilized in the fabrication of SSAs using the magnetron sputtering method, showing distinct advantages over traditional SSAs.^121,122 The structural properties of high-entropy nitrides, which typically present amorphous or fcc structures [Fig. 3(c)], vary depending on their elemental composition.¹¹⁶ More recently, high-entropy alloys,¹²³ oxides,¹²⁴ and borides¹²⁵ have started to be explored for SSA preparation, revealing inherent spectral selectivity superior to binary counterparts. These advancements indicate the potential of high-entropy materials in advancing SSAs and their promising prospects in photothermal applications.

Graphene, a prominent member of the carbon-based material family, is renowned for its two-dimensional layered structure. Each layer is composed of carbon atoms arranged in a hexagonal, honeycomb-like lattice, as depicted in Fig. 3(d).¹²⁶ This lattice structure is crucial for its ability to convert light into heat through lattice vibrations. One of the most notable features of graphene is its capability for broad-wavelength absorption, thanks to its abundant conjugated π-bonds. This characteristic allows graphene to absorb sunlight across almost the entire spectrum, categorizing it as a promising absorbing material.^127,128 However, the inherent refractive index of graphene poses a limitation as it causes partial light reflection, thereby limiting absorption. To overcome this, various strategies, such as employing micro and nanostructures, have been developed to enhance light capture and minimize reflection.¹²⁹ 

One effective approach to augment the light absorption of graphene is the construction of textured surface structures. Additionally, leveraging the thickness-dependent bandgap of graphene allows for spectrally selective absorption by manipulating the thickness of graphene. For example, Jia and co-workers fabricated ultrathin graphene on a metallic trench-like structure, achieving excellent spectral selectivity, omnidirectional absorption, and high thermal stability, resulting in a high photothermal conversion efficiency of 90.1%.¹³⁰ Additionally, researchers have found that reduced graphene oxide (rGO) films, which are simply fabricated from graphene, achieve high spectral selectivity and high-temperature tolerance. Qu and co-workers used a low-cost sol–gel method to prepare rGO films.¹²⁵ By controlling the thickness of graphene flakes and the reduction level, they attained adjustable destructive interference with sunlight, achieving a high absorptance of 0.92 and a low thermal emissivity of 0.04 at 100 °C. The absorber also exhibited long-term high-temperature tolerance at 800 °C.

A significant advantage of intrinsic SSAs is their spectral selectivity, which remains consistent regardless of the metallic substrate used. MXenes materials, as mentioned earlier, show promise in the preparation of intrinsic SSAs. For example, Huang and co-workers reported a black 2D structured Ti₃C₂T_x MXenes material, showing good intrinsic spectral selectivity.²⁵ This absorber, prepared using a simple, scalable vacuum filtration technique, exhibited up to 90% solar absorptivity in the solar spectrum and as low as 10% infrared emissivity in the IR spectrum, as shown in Fig. 4(b). The surface temperature of this absorber under one sun illumination reached 89 °C, surpassing black paint and TiN nanoparticle coatings, as depicted in Fig. 4(c). Recently, Vomiero and co-workers developed a CoSb_x (2 < x < 3) semiconductor absorber with intrinsic spectral selectivity.²⁶ Due to its low bandgap, this absorber demonstrated a high absorptivity of 0.98 across the entire solar spectrum and a low emissivity of 0.18 in the IR region [Fig. 4(d)]. Under one sun illumination, the absorber surface reached 101.7 °C, as shown in Fig. 4(e), marking the highest recorded temperature for an intrinsic SSA in such conditions.

Intrinsic SSAs feature simple structures and ease of preparation, making them widely applicable, especially on flexible substrates.³⁴ However, they also have limitations. First, their spectral selectivity heavily relies on the inherent properties of the materials, and a shortage of suitable materials hinders the development of intrinsic SSAs. Second, the solar absorptance of most of these materials rarely exceeds 0.90, significantly limiting absorber performance. Finally, their high-temperature stability is often poor. Intrinsic absorbing coatings exposed to air can easily degrade due to elemental diffusion in mid-to-high temperature environments.

Multilayer structured SSAs are typically characterized by optical microcavity structures composed of a dielectric–metal–dielectric (DMD) configuration. The alternating layers of dielectrics and metals in the optical microcavity structures can effectively absorb solar energy. Incident light, upon striking the dielectric layer and undergoing refraction, partially transmits to the metal layer for absorption. The residual light undergoes further refraction and is either absorbed or reflected by subsequent layers, efficiently harnessing solar energy, as illustrated in Fig. 5(a). This structure, therefore, enhances the light absorption ability while maintaining low emissivity due to the inclusion of a metal infrared reflective layer.

Traditional optical microcavity structures like Al₂O₃/Pt/Al₂O₃,¹³² MgO/Zr/MgO,¹³³ Cr_xO_y/Cr/Cr₂O₃,¹³⁴ and ZrO_x/Zr/ZrO_x/Al_xO_y¹³⁵ have demonstrated good optical performance. However, the thin metal layers in these conventional DMD structures often lead to poor high-temperature thermal stability, primarily due to interlayer diffusion at elevated temperatures. To improve thermal stability, some studies have explored the use of ultra-high temperature ceramic materials as substitutes for the metal layer. For instance, Wang and colleagues recently introduced a novel DMD structured SSA that replaces the traditional metal layer with TiB₂, as shown in Fig. 5(b). This structure achieved high spectral selectivity (α/ε = 0.934/0.069) at 300 °C.¹³⁶ When depositing on a stainless steel substrate, these SSAs exhibited remarkable thermal stability at 800 °C, with little change in optical performance over 200 h [Fig. 5(c)], proving the effectiveness of incorporating ultra-high temperature ceramics into DMD structures.

Furthermore, researchers have been experimenting with combining ceramic composite materials with traditional DMD designs to develop new quasi-optical microcavity (QOM) structures. These absorbers, benefiting from both material and structural advantages, exhibit enhanced optical performance and thermal stability. For example, Cao and co-workers introduced a QOM structure that replaced dielectric layers with ZrB₂–Al₂O₃ ceramic composite materials and metal layers with ZrB₂, forming a multilayer absorber SiO₂/Al₂O₃/ZrB₂–Al₂O₃/ZrB₂/ZrB₂–Al₂O₃/ZrB₂/SS, as depicted in Fig. 5(d).⁴¹ This structure achieved high spectral selectivity (α/ε = 0.965/0.16 at 82 °C) and exceptional thermal stability at 800 °C [Fig. 5(e)], achieving a total energy conversion efficiency of 67% at a working temperature of 800 °C under 1000 suns.

The DMD multilayer structured absorbers stand out for their exceptional effectiveness in both optical properties and thermal stability, making them highly promising for the development of high-performance SSAs. However, a significant drawback of these systems is their complexity and increased fabrication cost, largely due to the multilayered structure. Specifically, the fabrication process demands stringent control over the thickness of each layer, which could pose a challenge for widespread application.

Dual-ceramic structural SSAs are composed of two ceramic layers capable of absorbing light. In this design, high metal volume fraction (HMVF) ceramic serves as the primary absorption layer, while the ceramic layer with a low metal volume fraction (LMVF) serves as a destructive interference layer. Moreover, a metallic bottom layer acts as an IR reflection layer aimed at reducing IR heat loss. To minimize the reflection loss of sunlight on the SSA surface, dielectric materials with a low refractive index are employed as an antireflection layer. This configuration forms an optical gradient from the bottom to the surface, thereby enhancing the efficiency of solar energy harvesting. The structural diagram of dual-ceramic structural SSAs is shown in Fig. 6(a). These SSAs are designed to overcome the limitations of intrinsic SSAs while avoiding the complexity associated with multilayer SSAs. The dual-ceramic structure is typically achieved through the dielectric doping of metallic particles or by employing gradient nitride ceramics.

SSAs based on dielectric doping exhibit robust thermal stability when made with particles doped into a dielectric matrix at varying volume fractions.¹³⁸ Initially, single-element metal particle doping was used to enhance the performance of SSAs. For example, Wang and co-workers revealed that the doping metal volume fraction and dual-absorbing layer thickness notably affect the reflectance spectrum.¹³⁹ By adjusting these parameters, the optimized Al–AlN ceramics-based SSAs enable a high solar absorptance of 0.92 and a low IR emission of 0.10 at 600 °C. However, single-metal doping can lead to particle diffusion and oxidation at high temperatures, which reduce absorption performance. For example, the Mo–Al₂O₃ ceramics-based SSA experienced optical performance degradation at annealing temperatures above 400 °C, which is due to Mo diffusion and oxidation.¹³⁸ Despite the optimization of metal particles and the dielectric matrix enhancing thermal stability, it remains below 600 °C.

Substituting single metals with alloys for dielectric doping has proven to enhance thermal stability. Ren and colleagues developed WNi–Al₂O₃ ceramic-based SSAs with superior thermal stability at 600 °C.¹⁴⁰ This absorber also exhibited a low total hemispherical emissivity of 0.15 at 500 °C. The same group later employed yttria-stabilized zirconia (YSZ) as a dielectric matrix to develop a WNi–YSZ ceramics-based SSA.¹⁴¹ It achieves a high solar absorptivity of 0.91 and a low emissivity of 0.13 at 500 °C, as well as long-term thermal stability at 600 °C. Furthermore, Cao et al. developed a WTi–Al₂O₃ ceramic-based SSA, which exhibits good thermal stability.¹⁴² After annealing at 600 °C in a vacuum for 840 h, it maintained a high absorption of 0.93 and a low thermal emissivity of 0.103 at 500 °C. This achieves an obvious improvement in thermal stability compared to single metal doping, as shown in Figs. 6(b) and 6(c). The enhanced high-temperature stability stems from Ti atom segregation and partial oxidation in the WTi nanoparticles, which inhibit W element diffusion and agglomeration [Fig. 6(d)]. In addition to metal and alloy doping, metallic ceramic doping has also been used to enhance stability. Wang et al. attempted to dope MoSi₂ ceramic particles into an Al₂O₃ matrix, revealing the impact of ceramic particle doping and thickness on optical performance.¹⁴³ Another study involved doping TiN into AlN and developing Ti_xAl_1-x/(TiN–AlN) HMVF/(TiN–AlN) LMVF/AlN dual-ceramic SSAs.¹⁴⁴ This absorber achieved a high solar absorption of 0.947 and a low thermal emissivity of 0.08. After annealing at 600 °C in air, the solar selectivity decreases to α/ε = 0.92/0.16.

Gradient nitride ceramics have gained attention in the fabrication of double-ceramic SSAs.^145–147 However, the optical properties and thermal stability of binary nitride ceramics are typically insufficient to meet practical usages. Early attempts to develop CrN-based double-ceramic structures could only maintain stability at 400 °C.⁸⁷ At higher temperatures, CrN decomposition led to a decline in optical properties. Similarly, TiN-based double-ceramic structures exhibited good spectral selectivity and thermal stability, but only up to 400 °C. Element doping in nitride ceramics has been an effective approach to optimize absorbers. For example, WAlN,¹⁴⁸ CrMoN,¹⁴⁵ NbMoN,¹⁴⁹ HfMoN,¹⁴⁶ and AlHfN¹⁴⁷ have been used to create double-absorbing layer based SSAs, effectively enhancing photothermal performance and thermal stability. In particular, the HfMoN absorber showed long-term stability at 600 °C.¹⁴⁶ However, this strategy is needed to exactly control metal contents. In addition, the thermal stability of most of these SSAs is still less than 600 °C.

In recent years, high-entropy nitrides have emerged as promising materials to enhance the thermal stability of traditional nitride ceramics. Their remarkable high-temperature stability makes them suitable for thermal barrier coatings.¹⁵⁰ High-entropy nitrides composed of transition metals exhibit a narrow bandgap for solar energy absorption,¹⁵¹ making them ideal for the fabrication of gradient nitride ceramic structures. Gao et al. prepared high-entropy nitride AlCrTaTiZrN by sputtering an AlCrTaTiZr target to create dual-ceramic structured SSA, as illustrated in Fig. 7(a).¹⁵² By adjusting the thicknesses, the absorber achieved a high solar absorptivity of 0.965 and a low thermal emissivity of 0.086 at 82 °C [Fig. 7(b)]. The AlCrTaTiZrN absorption layer plays a crucial role in increasing optical properties. After annealing at 600 °C for 10 h, the spectral selectivity showed only minor attenuation [Fig. 7(c)], indicating excellent thermal stability. Furthermore, by careful selection of components in high-entropy nitride, the same group fabricated an AlCrWTaNbTiN-based dual ceramic SSA, using Al₂O₃ as the anti-reflective layer [Fig. 7(d)].⁴⁰ This SSA exhibits a high solar absorptance of 93% and a low emittance of 6.8% at 82 °C [Fig. 7(e)], along with long-term stability at 600 °C in a vacuum for 240 h, as shown in Fig. 7(f). Additionally, a HfNbTaTiZrN-based SSA displays a high solar absorptivity of 96%, reduced IR emissivity of 8.2% at 82 °C, and omnidirectional absorption. The entropy-driven structural stabilization of this absorber ensured long-term thermal stability, maintaining stable spectral selectivity after annealing at 600 °C for 168 h.¹⁵³ Similar advancements have been made with TiVCrAlZrN,¹⁵⁴ NbMoTaWN,^155,156 AlTiZrHfNbN,¹⁵⁷ Al_0.4Hf_0.6NbTaTiZrN,¹⁵⁸ and AlMo_0.5NbTa_0.5TiZrN¹⁵⁹ as components of double-ceramic SSA. Moreover, the co-sputtering method is also highly effective in the fabrication of high-entropy material-based SSAs.^160–162 The vast component space offered by high-entropy materials presents significant potential for developing high-performance SSAs.

Double-ceramic SSAs often encounter issues like interlayer elemental diffusion and oxidation, which detrimentally affect the optical properties of the resultant SSAs. Adopting a simple double-layer SSA configuration, comprising a single absorbing layer paired with an antireflection layer, can mitigate these problems. Compared to intrinsic absorbers, the antireflective layer effectively endures the stability of the absorbing layer at high temperatures. With fewer layers than multilayer or dual-ceramic SSAs, dual-layer structures are simpler to manufacture, presenting a viable solution for high-temperature solar-thermal applications. The single absorption layer needs to possess strong intrinsic spectral selectivity. Suitable materials for this layer include ultra-high temperature ceramics, ceramic composite materials, and high-entropy materials.

Kotilainen et al. employed CrO_xN_y as an absorption layer and SnO_x as an antireflection layer, which are deposited on a Cu substrate, leading to a simple double-layer SSA.¹⁶³ This absorber achieved a high solar absorption of ∼0.93 and a low thermal emissivity of 0.07. However, it failed at 500 °C due to copper atom diffusion, leading to void formation at the substrate-coating interface and increased thermal emittance. Wang et al. reported a double-layer SSA using self-doped W-WO_x as an absorption layer, which led to high solar absorption of 0.91 and low thermal emission of 0.55.¹⁶⁴ In subsequent studies, Mo-MoO_x and Ti-TiO_x self-doped ceramics were identified as suitable for the fabrication of double-layer SSAs.^165,166 However, the thermal stability of these SSAs typically does not exceed 400 °C.

Employing ultra-high temperature ceramics at the absorption layer can effectively enhance the limited high-temperature stability. Gao and co-workers developed a TiN/Al₂O₃ SSA, maintaining good stability at 500 °C with respectable spectral selectivity (0.92/0.11).¹⁶⁷ Subsequently, boride ceramics like ZrB₂⁸² and TiB₂¹⁶⁸ and carbide ceramics like TiC^169,170 have been proven effective in the development of SSAs, enhancing both thermal stability and optical performance, achieving solar absorption of 0.92–0.93, IR emissivity of ∼0.11, and reliable thermal stability at 600 °C. Ceramic composite materials have also been explored for improving the thermal stability of dual-layer absorbers. A TiB₂–ZrB₂ composite ceramic-based dual-layer SSA was developed, showing a stable solar-thermal conversion efficiency of 82.6% after annealing at 600 °C for 100 h.²⁹ Recently, Gao and co-workers prepared a TiB₂-HfB₂ ceramic composite material, SSA, using magnetron co-sputtering technology, as shown in Fig. 8(a).⁴² Enhanced by a novel pre-annealing strategy, the SSA exhibited a high solar absorptance of 0.932 and suppressed thermal emittance of 0.089, along with remarkable thermal stability. Its optical performance remained almost unchanged even after annealing at 800 °C for 240 h [Figs. 8(b) and 8(c)].

High-entropy materials, primarily high-entropy nitrides have gained increasing attention to develop advanced SSAs. The entropy-induced stabilization effect and hysteresis diffusion effect have potential to inhibit oxidation and diffusion of high-entropy absorption layer. Gao et al. used high-entropy nitride AlCrTaTiZrN as absorbing layer and SiO₂ or Si₃N₄ as antireflection layers, as shown in Fig. 8(d).¹⁷¹ The resulting SSA leads to an outstanding solar absorption of 92.8% and low IR emission of 5.1%. This SSA also demonstrated excellent thermal robustness, maintaining its optical performance nearly unchanged after annealing at 650 °C for 300 h [Figs. 8(e) and 8(f)]. In addition, the AlCrMoTaTiN/Al₂O₃ double-layer SSA achieved high spectral selectivity of α/ε = 0.929/0.082.¹²² Similarly, MoTaTiCrN¹⁷² and TiVCrAlZrN¹⁷³ were also used as absorption layers, leading to favorable optical properties and thermal stability. Notably, incorporating Al into MoTaTiCrN ceramics has been shown to further improve thermal stability, as the presence of aluminum can form a passive film enhancing oxidation resistance. The resulting MoTaTiCr–Al–N-based double-layer SSA raised the operational temperature limit to 700 °C.¹⁷⁴ Overall, dual-layer absorbers offer significant advantages in terms of fabrication simplicity and high-temperature thermal stability, even though their solar absorption typically remains below 0.93.

Plasmonic materials are characterized by the local surface plasmon resonance (LSPR) effect of their nanoparticles (plasmons) under illumination. When incident light interacts with these particles, it induces LSPR due to the polarization of free electrons on their surface.³⁰ This phenomenon involves local oscillations of highly polarizable electrons in the conduction band when exposed to light. Resonance occurs when the wavelength of the incident light matches the plasmonic wavelength on the nanoparticle surface, generating a net surface charge difference and enhancing the electric field strength.¹⁷⁵ Therefore, optical properties like extinction (absorption and scattering) around the plasmonic nanoparticle surface are manifested as resonance absorption peaks in the absorption spectrum.

Precious metals like Au and Ag are commonly used as plasmonic nanoparticles in the fabrication of SSAs.^176,177 However, due to their high cost, researchers are exploring more economical materials with effective optical properties, including transition metals,¹⁷⁸ metal ceramics,¹⁷⁹ and carbon materials.¹⁸⁰ A challenge with many plasmonic materials is their narrow absorption bands, which often necessitate the creation of complex nanostructures to widen the absorption range. Additionally, to achieve low IR emission, an IR reflector is often integrated into plasmonic SSAs. Recently, Zheng et al. employed a selective leaching strategy to corrode aluminum from a Cu-doped aluminum alloy in a sodium hydroxide solution, leaving nanoscale Cu grains on the resulting porous structure [Fig. 9(a)].¹⁸¹ LSPR occurs when incident light hits the Cu particles in this porous structure, creating a strong electric field effect [Fig. 9(b)], resulting in high omnidirectional solar absorption of 0.94 for light incident at angles from 0° to 60°. The Al substrate serves as an IR reflector, yielding a very low thermal emissivity of 0.03 at 100 °C. The coating maintains good optical performance after thermal treatment across a range of temperatures from 100 to 400 °C [Fig. 9(c)].

The effectiveness of plasmonic resonance in enhancing the performance of absorbing coatings is influenced by factors such as particle size, shape, and nanostructure. Advanced nanostructure designs, including supermaterials and multilayered structures, have shown promise in furthering the development of plasmonic SSAs. For instance, Lin and co-workers designed a 3D structured carbon material called graphene supermaterial, SSA.¹⁸² Simulation of the electric field strength distribution around the grooved, ultrathin graphene supermaterial coating surface confirmed plasmon resonance on the graphene particles, achieving high light absorption and an exceptional photothermal conversion efficiency of 90.1%. Additionally, Li et al. designed a plasma metamaterial comprising a three-layer all-ceramic structure [Fig. 9(d)].⁷¹ This structure includes a mirror-like TiN reflection layer, a colloidal TiN nanoparticle absorption layer, and a SiO₂ antireflection layer. The combination of in-plane plasmonic resonance absorption and out-of-plane Fabry–Pérot resonance leads to a high absorption of 95% and a low thermal emissivity of 3%, as shown in Fig. 9(e). This absorber also exhibits remarkable long-term stability at 727 °C [Fig. 9(f)], demonstrating the significant potential for high-temperature solar-thermal applications.

Quasi-blackbody materials are celebrated for their exceptional light absorption capabilities, enabling full-spectrum solar absorption. However, they typically exhibit high thermal emissivity, which leads to considerable heat loss. To address this, pseudo-blackbody absorbers often integrate optically transparent thermally insulating (OTTI) materials atop the blackbody surface.¹⁸³ This approach helps to retain heat and minimize IR thermal losses, achieving spectrally selective absorption akin to that of SSAs.¹⁸⁴ A notable advantage of quasi-blackbody materials is their relatively simple preparation method, circumventing the need for precise control over nanostructures and thickness. These low-cost, readily available black materials also demonstrate better thermal stability than SSAs, making them suitable for high-temperature applications. Furthermore, this design obviates the need for high-vacuum environments, which are typically necessary for SSAs in CSP plants.

The development of high-performance OTTI materials is crucial for reducing heat loss. As depicted in Fig. 10(a), Miljkovic and colleagues developed an OTTI aerogel through the chemical synthesis of aerogels.¹⁸⁵ This aerogel achieved a high transmission rate of 88% in the solar spectrum and a thermally averaged emission of ∼99% at 100 °C. Instantaneous solar thermal efficiencies of about 55% at 1 sun and 80 °C were recorded. Additionally, Zhao et al. employed a low-cost, non-selective high-temperature black paint as the absorbing coating, with an absorptance of 0.97 and a thermal emissivity of 0.85–0.9.¹⁸⁶ They applied an OTTI material on top of the black paint surface. This insulating layer effectively inhibits solid conduction and gas-phase convection while allowing 95% sunlight transmission [Fig. 10(b)]. It exhibits very low transmittance in the infrared spectrum, significantly enhancing the efficiency of solar collectors and meeting a wide range of thermal energy needs. Importantly, this structure does not require costly optical or mechanical components, enabling the collection and utilization of non-concentrated solar energy for large-scale thermal collection devices. Overall, the advancements in SSAs have encompassed a diverse range of structures, including intrinsic spectrally selective materials, multilayer structures, dual-ceramic structures, simple double-layer structures, plasmonic materials, and quasi-blackbody materials assisted by OTTI materials. To provide a detailed comparison and analysis of these various SSAs, Table I presents a summary of their key aspects such as the structure, solar absorptance, thermal emissivity, and thermal stability of some of the most recent advancements in SSAs.

SSAs have achieved wide photothermal conversion applications in various fields. Presently, the most widespread low-temperature application (<150 °C) is in residential solar thermal water systems, which have achieved commercialization. Other areas that have become research hotspots in recent years include solar water evaporation,^187,188 personal thermal management,¹⁸⁹ catalysis,¹⁹⁰ deicing,¹⁹¹ and electricity generation.⁸ In terms of electricity generation, it includes CSP plants and solar thermoelectric generators (STEGs). The former represents a more mature application of solar thermal power technology.¹⁹² The latter involves an all-solid state power generation technique. This section focuses on applications of SSAs in high-temperature PTC systems and STGs.

CSP systems are becoming an alternative global electricity supply, aligning with current global calls for energy conservation and emission reduction, and are expected to meet 12% of global electricity demands by 2050.¹⁹³ However, current commercial conversion efficiencies range from 12% to 37%, which is relatively low, posing greater demands on the development of new generations of commercial selective absorption coatings.¹⁹⁴ In CSP systems, parabolic trough concentrators (PTC) are the predominant method. High-temperature SSAs applied to vacuum collector tubes are crucial for converting light to thermal energy, as shown in Fig. 11(a). These collectors operate at temperatures above 300 °C, even reaching 500–600 °C. According to the Carnot cycle, increasing operational temperature is an effective way to enhance thermoelectric conversion efficiency,¹⁹⁵ as illustrated in Fig. 11(b). This imposes high requirements on high-temperature SSAs.

Li and co-workers used ion beam deposition to develop a Zr_0.3Al_0.7N/Zr_0.2Al_0.8N-based multilayered SSA, achieving a high absorption of 0.953 and a low thermal emissivity of 0.079 at 400 °C with long-term thermal stability for 192 h.¹⁹⁷ High-entropy nitride HfNbTaTiZrN dual absorbing layer based SSA shows good thermal stability. When annealed at 550 °C for 168 h, the absorber maintains a high solar absorptance of 0.938 and a low thermal emittance of 0.194 at 550 °C. The annealed SSA exhibits a good photothermal conversion efficiency of 88.8%. The service life of the SSA when used for a PTC system is estimated according to the Arrhenius equation, demonstrating that the absorber could serve 25 years at 529 °C. Valleti et al. reported a CrN/TiAlN-based SSA that shows good thermal cycling stability at 500 °C with no obvious change in optical properties after 150 cycles.¹⁹⁸ The accelerated thermal aging tests obtained an extrapolated service life of 25 years at 411 °C. Cao et al. reported a W–SiO₂ cermet-based SSA, which shows superior thermal stability at 600 °C in vacuum and reaches around 65% at this temperature under 1000 suns.²⁷ Composite ceramic TiB₂-HfB₂-based SSA exhibits a good solar-thermal efficiency of 68.6% at 800 °C, which remains almost unchanged after annealing at 800 °C for 240 h.⁴² Recently, Wang et al. developed a MoAlSiN/Si₃N₄-based SSA, which exhibits a high solar absorption of 93.2% and suppressed thermal emissivity of 10.5% at 400 °C.¹⁹⁹ This absorber retains stable optical performance even after heat treatment at 800 °C for 200 h in vacuum. Furthermore, its practical total energy conversion efficiencies of 600–800 °C are always above 57%. It is estimated that its service life at 600 °C is more than 25 years.

Thermoelectric power generation primarily relies on solar thermoelectric generators (STEGs), which create a significant temperature difference between the two surfaces of a thermoelectric generator. This is achieved using SSAs on the hot side, as depicted in Figs. 12(a) and 12(b). STEGs are solid-state devices that transform thermal energy into electrical energy via the Seebeck effect, which is generated by a temperature difference (ΔT).^200–202 These devices are portable and can serve as power sources for wearable electronic gadgets. The efficacy of STEGs depends on the high figure of merit (ZT) of the thermoelectric materials and the ΔT across the device. Generating a high temperature on the thermal side by harnessing solar energy is a highly sought-after strategy.²⁰³ Nonetheless, creating a substantial temperature difference under standard low solar flux radiation is challenging, and energy loss due to convection and radiation is a common issue.²⁰² Consequently, the high absorptance and low thermal emissivity of SSAs are crucial for converting solar energy into thermal energy, effectively minimizing heat loss due to IR emission and, therefore, enhancing the performance of solar thermoelectric generators.

Enhancing the effectiveness of solar selective absorption coatings is a viable method to improve their thermoelectric properties. Jiang and colleagues developed a TiN-based 2D photonic crystal SSA and integrated it with a STEG device.²⁰⁴ This effectively increased the temperature difference in the STEG by about 32 °C, which in turn raised the open-circuit voltage by 2 V. Gao and co-workers utilized high-entropy nitride SSA to boost STEGs.¹⁸² The CuCrMnCoAlN-based dual-ceramic absorber had a high absorption of 0.952 and a low emissivity of 0.109. The high solar absorption was achieved through a combination of the interference effect and intrinsic absorption characteristics, which raised the surface temperature to 109.6 °C under one sun illumination. When combined with a commercial TEG, the device enhanced the temperature difference between both sides, achieving an open-circuit voltage of 0.174 V and an output power of 1.99 mW, as shown in Figs. 12(c) and 12(d). Employing a thermal concentration strategy to further increase the temperature difference between the hot and cold sides to 12.2 °C significantly enhanced the output power of the STEG.

Furthermore, by adopting an optical concentration strategy to achieve high temperatures on the surface of the absorber, the efficiency of the STEG can be further improved. Kraemer and others designed a STEG using SSA and high-performance nanostructured thermoelectric materials.¹¹ When exposed to sunlight, a high temperature on the surface of the absorber was attained, enhancing the power generation efficiency to a peak of 4.6%. It is anticipated that with a figure of merit ZT of 2, an emissivity of 0.06, and an optical concentration of ten suns, a high efficiency of nearly 14% can be achieved, paving the way for a very promising strategy to enhance the conversion efficiency of STEG. Subsequently, the same group employed the optical concentration strategy to increase the light intensity to 211 kW m⁻². This approach raised the reported peak efficiency from 5.2% to 9.6% and utilized high-temperature SSA to enable the concentrated STEG to operate stably in a 600 °C high-temperature vacuum environment.²⁰⁵ 

To further improve power generation efficiency, a strategy that integrates passive radiative cooling (RC) technology with solar heat collection on TEG equipment has been proposed.²⁰⁶ This integration leverages radiative cooling technology, which allows heat emission from the surface of a sky-facing object in the form of electromagnetic waves through a transparent atmospheric window of 8–13 µm to the outer cold space at a mere 3 K.^207–209 This method provides a cold source for TEG power generation, which can achieve power generation at night in the absence of light.²¹⁰ In daylight, the system employs SSA as the primary mechanism for converting sunlight into thermal energy to create a temperature difference for power generation. At night, it uses the radiative cooler to emit heat toward cold space, obtaining a temperature difference for power generation and thus achieving continuous power generation.^211,212 Zhang et al. have developed an integrated device that can simultaneously harvest energy from the sun and cold space through a TEG system combined with a solar absorber and radiative cooler, achieving uninterrupted power generation throughout the entire day.²¹³ In contrast to conventional STEGs, this integrative approach of combining solar heating with radiative cooling represents a significant advancement in this field. It overcomes the limitations of STEGs, which are traditionally operational only during daylight hours.²¹⁴ This green technology offers a potential solution to regional energy shortages and supplements existing energy infrastructure. However, the exploration of this integrated technology faces challenges, particularly in the stability of its performance. Future research should, therefore, focus on improving the efficiency and stability of continuous power generation.²¹⁵ 

This paper offers a comprehensive review of the latest advancements in high-temperature SSAs. It summarized the underlying principles of SSA, performance evaluation standards, fabrication methods, advanced materials, and nanostructured design of SSAs, as illustrated in Fig. 13. Among the fabrication techniques, magnetron sputtering is highly regarded for producing high-quality films, which can exactly control the thickness of each layer and achieve roll-to-roll large-area production. However, this technique requires costly equipment, and the utilization ratio of the target is relatively low. On the other hand, the sol–gel technique, gaining increasing attention due to its cost-effectiveness and simplicity, has been applied in preparing high-performance SSAs. However, the spectral selectivity of the absorber fabricated by this technique needs to be improved.

The development of materials science and technology provides many advanced materials suitable for the preparation of high-performance SSA. These materials include ceramic composites, 2D MXenes, high-entropy materials, and graphene, which exhibit superior capabilities compared to traditional photothermal materials. For example, ceramic composites and high-entropy materials have enhanced the thermal stability of SSAs to above 700 °C, which has outperformed commercial and state-of-the-art SSAs. These high thermal stability materials are highly sought after for their potential in demanding applications like CSP systems and solar thermoelectric generators. In addition, by taking advantage of carefully designed nanostructures, both spectral selectivity and thermal stability can be advanced. These include intrinsic absorbers, DMD multilayer structures, dual-ceramic absorption layers, dual-layer structures, plasma absorbers, and OTTI assisted quasi-blackbody materials. Each type has its own unique features and limitations. For instance, intrinsic SSAs, while simple in design, are constrained in terms of spectral selectivity, thermal stability, and their scarcity in nature. DMD multilayer structure and plasma absorber have recorded the highest stable temperatures in current research but face practical application challenges due to their complex fabrication process. The OTTI assisted quasi-blackbody materials using transparent heat insulating materials to achieve spectral selectivity have good thermal stability independent of a stringent vacuum environment. In addition, the fabrication of quasi-blackbody materials avoids the careful control of thicknesses, showing great promise for practical applications. However, this absorber required high-performance transparent heat insulating materials.

The next-generation CSP systems are conjectured to need SSAs that could endure high temperatures above 700 °C. This poses a great challenge to advance the stability of SSAs. The combination of these advanced photothermal materials and nanostructures to further improve the performance of SSA is a promising approach to this end. In particular, the studies on the photothermal performance of 2D MXenes and high-entropy materials are still in their initial stages. These materials have a very abundant composition for the optimization of optical properties and thermal stability. So far, high-entropy materials for the fabrication of SSAs are mainly concentrated on high-entropy nitrides. The potential of other high-entropy materials, such as high-entropy carbides and high-entropy borides, for SSAs needs further research.

This work was supported by the Regional Key Projects of Science and Technology Service Network Program of the Chinese Academy of Sciences No. KFJ-STS-QYZD-139, the Key Program of the Lanzhou Institute of Chemical Physics CAS (No. KJZLZD-4), the CAS “Light of West China” Program, and the Major Science and Technology Projects of Gansu Province (No. 20ZD7GF011).

The authors have no conflicts to disclose.

Zhuo-Hao Zhou: Conceptualization (equal); Data curation (lead); Formal analysis (lead); Investigation (lead); Writing – original draft (equal). Cheng-Yu He: Conceptualization (lead); Validation (lead); Writing – original draft (lead); Writing – review & editing (lead). Xiang-Hu Gao: Conceptualization (lead); Funding acquisition (lead); Supervision (lead); Validation (lead); Writing – review & editing (lead).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.