Smart building block with colored radiative cooling devices and quantum dot light emitting diodes.

In this study, we design a smart building block with quantum-dot light-emitting diode (QLED) and colored radiative cooling devices. A smart light-emitting building block is fabricated using a bottom-inverted QLED that emits green light, an insulating layer, and a top radiative cooling structure that emits mid-infrared light. The heat generated during QLED operation is measured and analyzed to investigate the correlation between heat and QLED degradation. The top cooling part is designed to have no impact on the QLED's performance and utilizes Ag-polydimethylsiloxane as a visible-light reflector and mid-infrared absorber/emitter. For the colored cooling part, white radiative cooling paint is used instead of Ag-polydimethylsiloxane to improve cooling performance, and red and yellow paints are employed to realize vivid red and yellow colors, respectively. We demonstrate a smart imitation house system with a smart light-emitting building block as the roof and analyze the cooling of the heat generated during QLED operation. A maximum cooling effect of up to 9.6 °C is observed compared to the imitation house system without the smart light-emitting building block, effectively dissipating heat generated during QLED operation. The smart light-emitting building block presented in this study opens new avenues in the fields of lighting and cooling systems.

Vivid Colored Cooling Structure Managing Full Solar Spectrum via Near-Infrared Reflection and Photoluminescence.

Colored radiative cooling (CRC) offers an attractive alternative for surface and space cooling, while preserving the aesthetics of an object. However, there has been no study on the CRC using phosphors in regard to vivid coloration, sophisticated performance investigation, retention of properties, functionality, and structural flexibility all at once. Thus, to manage the entire solar spectrum, a colored cooling structure comprising a near-infrared (NIR)-reflective bottom layer and a top colored layer with a phosphor-embedded polymer matrix is proposed. The structure is paintable, vividly colored, hydrophobic, and ultraviolet (UV) and water resistant. In the daytime outdoor measurement, the structure with red, orange, and yellow colors exhibited lower temperature than a control group using commercial white paint by 4.7 °C, 7.2 °C, and 7.4 °C, respectively. After precise theoretical and experimental time-tracing temperature validation, the CRC performance enhancement from NIR reflection and photoluminescence effects was thoroughly analyzed, and a temperature reduction of up to 16.1 °C was achieved for the orange-colored structure. Furthermore, experiments of hydrophobicity infusion and exposure to UV and deionized water verified the durability of the colored cooling structure. In addition, flexible-film-type colored cooling structures were demonstrated using different bottom reflective layers, such as a silver thin film and porous aluminum oxide particle-embedded poly(vinylidene fluoride-co-hexafluoropropylene), suggesting the potential applicability of these colored cooling structures for vivid-colored, functional, and durable CRC.

Designing a self-classifying smart device with sensor, display, and radiative cooling functions via spectrum-selective response.

This paper presents a self-classifying smart device that intelligently differentiates and operates three functions: electroluminescence display, ultraviolet light sensor, and thermal management via radiative cooling. The optical and electrical properties of the materials and structures are designed to achieve a spectrum-selective response, which enables the integration of the aforementioned functions into one device without any noise or interference. Spectrum-selective materials that absorb, emit, and radiate light with ultraviolet to mid-infrared wavelengths and device structures designed to prevent interference are achieved by using thin metal films, dielectric layers, and nanocrystals. The designed self-classifying smart device exhibits bright blue light emission upon current supply (display), green light emission upon exposure to UV light (sensor), and radiative cooling (thermal management). Furthermore, a smart device and house system with a display, UV light sensor, and radiative cooling performance was demonstrated. The findings of this study open new avenues for device integration in next-generation wearable device fabrication.

Spectrally Selective Nanoparticle Mixture Coating for Passive Daytime Radiative Cooling.

Passive daytime radiative cooling, which is a process that removes excess heat to cold space as an infinite heat sink, is an emerging technology for applications that require thermal control. Among the different structures of radiative coolers, multilayer- and photonic-structured radiative coolers that are composed of inorganic layers still need to be simple to fabricate. Herein, we describe the fabrication of a nanoparticle-mixture-based radiative cooler that exhibits highly selective infrared emission and low solar absorption. Al2O3, SiO2, and Si3N4 nanoparticles exhibit intrinsic absorption in parts of the atmospheric transparency window; facile one-step spin coating of a mixture of these nanoparticles generates a surface with selective infrared emission, which can provide a more powerful cooling effect compared to broadband emitters. The nanoparticle-based radiative cooler exhibits an extremely low solar absorption of 4% and a highly selective emissivity of 88.7% within the atmospheric transparency window owing to the synergy of the optical properties of the material. The nanoparticle mixture radiative cooler produces subambient cooling of 2.8 °C for surface cooling and 1.0 °C for space cooling, whereas the Ag film exhibits an above-ambient cooling of 1.1 °C for surface cooling and 3.4 °C for space cooling under direct sunlight.

High-Performance Daytime Radiative Cooler and Near-Ideal Selective Emitter Enabled by Transparent Sapphire Substrate.

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.

Fabrication of high-transmittance and low-reflectance meter-scale moth-eye film via roll-to-roll printing.

Anti-reflection technology is a core technology in the field of optoelectronic devices that is used to increase efficiency by reducing reflectance. In particular, the bio-mimetic moth-eye pattern has the advantage of being independent of wavelength, polarization, and angle of incidence. In this study, we fabricated a 1.1 m wide meter-scale moth-eye film using roll-to-roll printing. A uniform moth-eye pattern with a height of 170 nm was formed, which reduced the average reflectance value by 3.2% and increased the average transmittance value by 3.1%, in a wide wavelength range of 400-700 nm. Additionally, the moth-eye film coated with a self-assembled monolayer (SAM) exhibited a contact angle of 140.3°, almost equal to the superhydrophobic angle of 150°. Furthermore, the contact angle, transmittance, and reflectance of the SAM-coated moth-eye film were maintained after an environmental test, which was conducted for 168 h at 60 °C and 80% humidity.