Foldable and flexible displays offer significant advantages in form factor compared to conventional flat displays. However, developing a suitable cover window for these displays presents challenges. The ideal cover window must possess optimal optical properties for clear image delivery, robust mechanical properties to protect the display from damage, and moisture impermeability to safeguard internal components from humidity. Ultrathin glass (UTG) and clear polyimide (c-PI) are potential base materials; however, UTG's fragility limits its use, while c-PI requires improvements in its elastic modulus and moisture impermeability. Effective heat dissipation is crucial for the longevity of high-performance electronic devices, particularly in the context of the increasing prevalence of flexible electronics in diverse environments. Conventional thermal management techniques often involve backside heat dissipation or cooling channels, but these methods can be less effective in confined spaces and when dealing with external solar irradiance. Radiative cooling, a passive cooling technology that dissipates heat to outer space through the atmospheric window (8–13 µm), offers a promising alternative. While opaque radiative cooling materials have been demonstrated, their opacity limits their application to transparent devices. Recent research has explored optically transparent radiative cooling materials, showing promising temperature reduction, but these often lack the necessary mechanical properties and moisture resistance for use in foldable displays. This research aims to address these limitations by developing transparent radiative cooling metamaterials suitable for use as cover windows in flexible and foldable displays.

Existing literature highlights the need for foldable and flexible display cover windows with optimal optical, mechanical, and moisture-impermeable properties. Various materials, including UTG and c-PI, have been investigated, each presenting trade-offs. UTG, while optically excellent, suffers from fragility. C-PI, though flexible and optically suitable, requires improvements in its mechanical strength and moisture barrier. Current thermal management strategies for displays often rely on backside cooling or integrated heat sinks, which may not suffice for flexible electronics exposed to both internal heat generation and external solar radiation. Radiative cooling, a passive method of heat dissipation, has shown promise, with several studies demonstrating transparent radiative cooling materials. However, many of these materials have shortcomings regarding mechanical robustness, particularly the flexibility and foldability required for modern displays. The existing transparent radiative cooling materials often utilized silica (SiO2) microstructures within a polymer matrix to enhance emission in the atmospheric window. While these materials successfully reduced temperature rise, issues of visible clarity and mechanical strength often remained due to material choices or the inclusion of sufficient silica to enhance radiative cooling. This paper aims to overcome these limitations by combining high emissivity SiO2 microstructures with a flexible, optically transparent, and mechanically robust polymer matrix.

The researchers synthesized transparent radiative cooling metamaterials by embedding PMMA-infiltrated SiO2 aerogel microparticles (PMMA-SiO2 microstructures) within a c-PI matrix. The SiO2 aerogel microparticles (4–10 µm) contribute to light emission in the atmospheric window through phonon-polariton resonance and Mie scattering. The PMMA infiltration ensures refractive index matching with c-PI, minimizing light reflection and absorption at visible wavelengths, preserving transparency. The concentration of PMMA-SiO2 microstructures was varied (0–24 wt%) to control haze. The mixture of PMMA-SiO2 microstructures and c-PI was spin-coated onto an aluminum substrate, cured, and then peeled off to create films approximately 50 µm thick. Optical properties (transmittance, reflectance, emissivity, and haze) were measured using UV-Vis-NIR and FTIR spectrometers. Mechanical properties (elastic modulus, tensile strength, and elongation at break) were determined using tensile tests. Water vapor transmission rate (WVTR) was measured to assess moisture impermeability. Radiative cooling performance was evaluated by monitoring temperature changes in simulated displays (heat generation of 100 W/m² and 400 W/m²) covered with the metamaterials under both indoor (AM 1.5 G solar illumination) and outdoor conditions (direct sunlight). The light output power and temperature of commercial blue LEDs and a flexible display panel were measured with and without the metamaterial cover to assess the impact of radiative cooling on device performance. A theoretical model was used to estimate the net cooling power and temperature response of the materials.

The synthesized metamaterials demonstrated several key properties: High visible light transmission (85.5% for 24 wt% PMMA-SiO2), tunable haze (0.1–0.64), and significantly enhanced light emission in the atmospheric window (94.6% for 24 wt% PMMA-SiO2, a 1.5x increase over bare c-PI). The addition of PMMA-SiO2 microstructures substantially improved the mechanical properties of c-PI, resulting in a 2.2 times higher elastic modulus (2.51 GPa) and 1.6 times higher tensile strength (79.8 MPa). WVTR was also reduced by 0.6 times compared to bare c-PI. Under indoor illuminated conditions, the 50 µm-thick metamaterials suppressed the temperature rise of a simulated display with 100 W/m² heat generation by 6.9 °C. In outdoor testing (850 W/m² solar irradiance), the temperature rise of a simulated display with 400 W/m² heat generation was suppressed by 8.3 °C. The metamaterials integrated with commercial blue LEDs showed a 1.17–1.21 times increase in light output power compared to bare LEDs and c-PI integrated LEDs, due to enhanced light scattering and reduced temperature-related performance degradation. Similar improvements were observed with a flexible display panel, where temperature reduction was 4.6 °C (dark) and 7.1 °C (illuminated).

The results demonstrate the successful development of a transparent radiative cooling cover window for foldable and flexible displays. The approach uses refractive index matching to minimize visible light scattering while maximizing infrared emission, thus addressing the limitations of previous transparent radiative cooling materials. The significantly improved mechanical properties and moisture impermeability make the material suitable for real-world applications. The substantial temperature reduction observed both indoors and outdoors underscores the effectiveness of the radiative cooling strategy. The enhanced light output performance of LEDs and flexible displays confirms the practical benefits of integrating this technology into optoelectronic devices. This work suggests a viable pathway for improved thermal management in flexible electronics, leading to longer device lifespan and enhanced performance.

This research successfully demonstrated transparent radiative cooling metamaterials suitable as cover windows for flexible and foldable displays. These metamaterials combine high visible light transmission, tunable haze, excellent radiative cooling performance, enhanced mechanical properties, and improved moisture impermeability. The significant temperature reduction and improved light output of integrated LEDs and displays highlight the potential of this technology for enhancing the performance and longevity of flexible electronics. Future work could explore different polymer matrices, microstructure designs, and integration methods to further optimize performance and expand the range of applications.

The study primarily focused on specific types of c-PI and simulated displays. While the results are promising, further testing is needed with a broader range of display technologies and flexible substrates to confirm generalizability. The long-term stability and durability of the metamaterials under various environmental conditions (e.g., extended UV exposure, temperature cycling) need additional investigation. The theoretical model, while useful, may not completely capture all aspects of the complex heat transfer processes involved.