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In the light of the ever increasing dangers of global warming, the efforts to reduce energy consumption by radiative cooling techniques have been designed, but are inefficient under strong sunlight during the daytime. With the advent of metamaterials and their selective control over optical properties, radiative cooling under direct sunlight is now possible. The key principles of metamaterial-based radiative cooling are: almost perfect reflection in the visible and near-infrared spectrum (0.3–3 µm) and high thermal emission in the infrared atmospheric window region (8–13 µm). Based on these two basic principles, studies have been conducted using various materials and structures to find the most efficient radiative cooling system. In this review, we analyze the materials and structures being used for radiative cooling, and suggest the future perspectives as a substitute in the current cooling industry.

Radiative cooling is a well-researched area. For many years, surfaces relying on radiative cooling failed to exhibit a sub-ambient surface temperature under the sun because of the limited reflectance in the solar spectrum and the reduced absorptivity in the atmospheric window. The recent impressive developments in photonic nanoscience permitted to produce photonic structures exhibiting surface temperatures much below the ambient temperature. This paper aims to present and analyze the main recent achievements concerning daytime radiative cooling technologies. While the conventional radiative systems are briefly presented, the emphasis is given on the various photonic radiative structures and mainly the planar thin film radiators, metamaterials, 2 and 3D photonic structures, polymeric photonic technologies, and passive radiators under the form of a paint. The composition of each structure, as well as its experimental or simulated thermal performance, is reported in detail. The main limitations and constraints of the photonic radiative systems, the proposed technological solutions, and the prospects are presented and discussed.

Passive daytime radiative cooling technology presents a sustainable solution for combating global warming and accompanying extreme weather, with great potential for diverse applications. The key characteristics of this cooling technology are the ability to reflect most sunlight and radiate heat through the atmospheric transparency window. However, the required high solar reflectance is easily affected by environmental aging, rendering the cooling ineffective. In recent years, significant advancements have been made in understanding the failure mechanisms, design strategies, and manufacturing technologies of daytime radiative cooling. Herein, a critical review on anti‐environmental aging passive daytime radiative cooling with the goal of advancing their commercial applications is presented. It is first introduced the optical mechanisms and optimization principles of radiative cooling, which serve as a basis for further endowing environmental durability. Then the environmental aging conditions of passive daytime radiative cooling, mainly focusing on UV exposure, thermal aging, surface contamination and chemical corrosion are discussed. Furthermore, the developments of anti‐environmental aging passive daytime radiative cooling materials, including design strategies, fabrication techniques, structures, and performances, are reviewed and classified for the first time. Last but not the least, the remaining open challenges and the insights are presented for the further promotion of the commercialization progress. In this paper, the passive daytime radiative cooling against environmental aging is reviewed with the goal of advancing their commercial applications. This paper introduces the optical mechanisms and optimization principles of PDRC, discusses the environmental aging conditions of PDRC, and focuses on developments of anti‐environmental aging passive daytime radiative cooling materials, including design strategies, fabrication techniques, structures and performances.

Highlights This review comprehensively presents recent advancements in spectrally selective daytime radiative cooling (SSDRC) materials, focusing on their fundamental characteristics, primarily concerning their structures and properties. The fabrication principles and corresponding operational mechanisms of several typical SSDRC materials are systematically introduced. Based on the latest research, this review highlights the innovative applications in personal thermal management, outdoor building cooling, and energy harvesting, while also discussing the challenges and prospects for the future development of daytime radiative cooling. Daytime radiative cooling is an eco-friendly and passive cooling technology that operates without external energy input. Materials designed for this purpose are engineered to possess high reflectivity in the solar spectrum and high emissivity within the atmospheric transmission window. Unlike broadband-emissive daytime radiative cooling materials, spectrally selective daytime radiative cooling (SSDRC) materials exhibit predominant mid-infrared emission in the atmospheric transmission window. This selective mid-infrared emission suppresses thermal radiation absorption beyond the atmospheric transmission window range, thereby improving the net cooling power of daytime radiative cooling. This review elucidates the fundamental characteristics of SSDRC materials, including their molecular structures, micro- and nanostructures, optical properties, and thermodynamic principles. It also provides a comprehensive overview of the design and fabrication of SSDRC materials in three typical forms, i.e., fibrous materials, membranes, and particle coatings, highlighting their respective cooling mechanisms and performance. Furthermore, the practical applications of SSDRC in personal thermal management, outdoor building cooling, and energy harvesting are summarized. Finally, the challenges and prospects are discussed to guide researchers in advancing SSDRC materials.

HighlightsA lamellar-structured fluorinated cellulose nanofiber aerogel film is prepared by filtration-induced delaminated gelation and ambient drying.The aerogel film demonstrates exceptional mechanical flexibility and resistance to complex deformations.The aerogel film displays low thermal conductivity, high visible-light transmittance and superior selective infrared emissivity, rendering it high solar-thermal regulatory cooling performance.The demand for highly porous yet transparent aerogels with mechanical flexibility and solar-thermal dual-regulation for energy-saving windows is significant but challenging. Herein, a delaminated aerogel film (DAF) is fabricated through filtration-induced delaminated gelation and ambient drying. The delaminated gelation process involves the assembly of fluorinated cellulose nanofiber (FCNF) at the solid–liquid interface between the filter and the filtrate during filtration, resulting in the formation of lamellar FCNF hydrogels with strong intra-plane and weak interlayer hydrogen bonding. By exchanging the solvents from water to hexane, the hydrogen bonding in the FCNF hydrogel is further enhanced, enabling the formation of the DAF with intra-layer mesopores upon ambient drying. The resulting aerogel film is lightweight and ultra-flexible, which possesses desirable properties of high visible-light transmittance (91.0%), low thermal conductivity (33 mW m−1 K−1), and high atmospheric-window emissivity (90.1%). Furthermore, the DAF exhibits reduced surface energy and exceptional hydrophobicity due to the presence of fluorine-containing groups, enhancing its durability and UV resistance. Consequently, the DAF has demonstrated its potential as solar-thermal regulatory cooling window materials capable of simultaneously providing indoor lighting, thermal insulation, and daytime radiative cooling under direct sunlight. Significantly, the enclosed space protected by the DAF exhibits a temperature reduction of 2.6 °C compared to that shielded by conventional architectural glass.

Passive daytime radiative coolers (PDRCs) with exceptionally high solar reflectance and emissivity in the atmospheric window can provide sub-ambient cooling while reducing buildings’ cooling energy demand. However, glare and esthetic issues limit their application to high-rise buildings while may increase the building’s heating energy needs. Passive colored radiative coolers (PCRCs), based on fluorescent materials, convert part of the absorbed UV and visible solar radiation into emitted light, providing color and reducing the thermal balance of the materials and the potential visual annoyance. This article investigates the state of the art on the PCRC based on fluorescent technologies. Seven articles presenting different combinations of PDRC technologies with fluorescent components to create PCRCs of various colors are presented and analyzed in detail. Quantum dots and phosphors embedded in polymer matrices and combined with reflecting and emitting layers were used as the fluorescent layer of the seven developed green, red, yellow, and yellow–green films. The proposed PCRCs are characterized by very significant differences in cooling performance, although most presented sub-ambient surface temperatures. Their cooling potential is comparatively investigated in terms of the testing climatic conditions and their optical characteristics. The potential increase of their surface temperature, caused by the addition of the fluorescent component, is analyzed through comparisons between the proposed PCRCs and the corresponding white PDRCs without the fluorescent component. The average temperature difference of the green, red, yellow, and yellow–green films against the reference PDRCs is found to be 0.66 °C, 2.6 °C, 1.7 °C and 1.4 °C, respectively. A relevant decreasing trend, but not statistically significant, is observed between the temperature increase caused by the fluorescent additives and the corresponding photoluminescence quantum yield.

Radiative cooling, taking advantage of the coldness of the sky, has a potential to be a sustainable alternative to meet cooling needs. The performance of a radiative cooling device is fundamentally limited by the emissivity of the sky, therefore depends heavily on the regional weather conditions. Although the sky emissivity is known to increase with the dew point temperature, the feasibility of radiative cooling remains elusive in the equatorial tropical climate, where the weather is humid, cloudy, and constantly changing. It is pointed out that a high degree of thermal insulation of the radiative cooling system can be effective under such extreme weather conditions. A new method to characterize dynamic sky conditions is presented, namely to measure the sky window emissivity in the zenith direction. It is shown that a sub‐ambient cooling up to 8 °C is possible during daytime and that the cloud base is not a complete blackbody and can be used as a heat sink for radiative cooling. The feasibility of radiative cooling in hot and humid climates has been elusive, due to the opaqueness of the sky and the high cloud coverage. To overcome these compromising environmental factors, various heat gain channels are meticulously suppressed and 8 degrees temperature reduction from ambient is demonstrated during the daytime under the cloudy sky of Singapore.

For wearable electronics/optoelectronics, thermal management should be provided for accurate signal acquisition as well as thermal comfort. However, outdoor solar energy gain has restricted the efficiency of some wearable devices like oximeters. Herein, wireless/battery‐free and thermally regulated patch‐type tissue oximeter (PTO) with radiative cooling structures are presented, which can measure tissue oxygenation under sunlight in reliable manner and will benefit athlete training. To maximize the radiative cooling performance, a nano/microvoids polymer (NMVP) is introduced by combining two perforated polymers to both reduce sunlight absorption and maximize thermal radiation. The optimized NMVP exhibits sub‐ambient cooling of 6 °C in daytime under various conditions such as scattered/overcast clouds, high humidity, and clear weather. The NMVP‐integrated PTO enables maintaining temperature within ≈1 °C on the skin under sunlight relative to indoor measurement, whereas the normally used, black encapsulated PTO shows over 40 °C owing to solar absorption. The heated PTO exhibits an inaccurate tissue oxygen saturation (StO2) value of ≈67% compared with StO2 in a normal state (i.e., ≈80%). However, the thermally protected PTO presents reliable StO2 of ≈80%. This successful demonstration provides a feasible strategy of thermal management in wearable devices for outdoor applications. This article presents a radiative cooled wireless/battery‐free patch type tissue oximeter with nano/microvoids polymer (NMVP) for eliminating the thermal issue of optoelectronics. The NMVP integrated tissue oximeter serves a temperature within ≈1 °C on the skin under direct sunlight relative to indoor measurement, delivering reliable tissue oxygen saturation, unlike normally black encapsulated devices.

Daytime passive radiative cooling (DPRC) has remained a challenge over the past decades due to the necessity of precisely defined materials with a significantly high emissivity of thermal radiation within the atmospheric transparent window wavelength range (8–13 μm) as well as high reflectivity in the solar spectrum (0.2–3 μm). Fortunately, recent advances and technological improvements in nanoscience and metamaterials are making it possible to create diverse metamaterials. This enables the production of DPRC in direct solar irradiation. The development of a material that is appropriate for effective DPRC is also a noteworthy development in this field of technology. This review gives a thorough introduction and discussion of the fundamental ideas, as well as the state-of-the-art and current trends in passive radiative cooling, and describes the cutting-edge materials and various photonic radiator structures that are useful in enhancing net cooling performance. This work also addresses a novel skylight window that offers passive cooling developed at the Åbo Akademi (ÅA) University, Finland. In conclusion, nanomaterials and nanoparticle-based coatings are preferred over all other approaches for commercialization in the future because of their low cost, the ability for large-scale production, simplicity in fabrication, and great potential for further increasing cooling performance.

Daytime radiative cooling serving as a method to pump heat from objects on Earth to cold outer space is an attractive cooling option that does not require any energy input. Among radiative cooler structures, the multilayer‐ or photonic‐structured radiative cooler, composed of inorganic materials, remains one of the most complicated structures to fabricate. In this study, transparent sapphire‐substrate‐based radiative coolers comprising a simple structure and selective emitter‐like optical characteristics are proposed. Utilizing the intrinsic optical properties of the sapphire substrate and adopting additional IR emissive layers, such as those composed of silicon nitride thin film or aluminum oxide nanoparticles, high‐performance radiative coolers can be fabricated with a low mean absorptivity (3–4%) at 0.3–2.5 µm and a high mean emissivity of over 90% at 8–13 µm. Experiments show that the fabricated radiative coolers reach temperature drops of ≈10 °C in the daytime. From the theoretical calculations of radiative cooling performance, the sapphire‐substrate‐based radiative coolers demonstrate a net cooling power as high as 100 Wm−2. Sapphire substrate‐based radiative coolers are introduced having simple structure and selective emitter‐like optical characteristics. Utilizing the intrinsic optical properties of sapphire substrate and additional IR emissive materials, high‐performance radiative coolers composed of purely inorganic materials are presented without elaborate simulation work previous studies reported.