Transparent radiative cooling cover window for flexible and foldable electronic displays

The study addresses the need for a transparent, foldable cover window for flexible and foldable electronic displays that simultaneously provides high optical clarity, robust mechanical protection, strong moisture barrier properties, and effective thermal management. Conventional approaches to thermal management in high-brightness electronics mainly dissipate heat via the device backside using highly thermally conductive composites or heat spreaders/sinks, which is challenging for compact, outdoor-exposed devices. Radiative cooling via thermal emission through the atmospheric window (8–13 µm) offers passive heat dissipation without energy input, but existing transparent radiative coolers often sacrifice visible transparency or lack the mechanical strength and moisture impermeability required for cover windows. Clear polyimide (c-PI) is a promising cover-window substrate but exhibits low emissivity in the atmospheric window, especially within thickness constraints (~50 µm) necessary for foldability. The research hypothesis is that embedding optically modulating PMMA-infiltrated SiO2 aerogel microstructures into c-PI, leveraging refractive-index matching and phonon-polariton/Mie effects, can yield a thin, transparent radiative cooling cover window that meets optical, mechanical, and barrier requirements while effectively suppressing device temperature rise under both sunlight and dark conditions.

Prior work on radiative cooling materials includes opaque, highly reflective structures achieving sub-ambient cooling by suppressing solar absorption and enhancing mid-IR emission, but these are unsuitable where transparency is needed. Transparent or translucent radiative coolers have been realized using: (i) transparent silica photonic crystals reducing substrate temperature by up to 13 °C outdoors; (ii) silica micro-grating structures with ~90% visible transmittance and ~90% emissivity in 8–13 µm, lowering solar cell temperature by 3.6 °C at 830–990 W/m²; (iii) scalable, flexible films with randomly dispersed silica in polymethylpentene (TPX) achieving ~93% emission (2.5–20 µm); (iv) silica nanospheres in TPX with ~90% transmittance (0.3–1 µm) and 85% emissivity (8–20 µm), lowering solar cell temperature by 5 °C; and (v) transparent bamboo-epoxy composites with ~80% visible transmittance and ~95% mid-IR emissivity. While increasing silica content in polymer matrices (e.g., PDMS) boosts mid-IR emissivity, it typically reduces visible clarity. An approach using silica aerogel microparticles with an optical modulator (n-hexadecane) in PDMS leveraged refractive-index matching to achieve >91% visible transparency and >98% emissivity (8–13 µm) and suppressed silicon temperature rise by up to 8.5 °C at ~920 W/m². However, many transparent radiative coolers lack the high elastic modulus, scratch resistance, and moisture barrier required for foldable cover windows. UTG and c-PI are common cover-window bases; UTG is brittle and poorly foldable, and PDMS-based emitters have insufficient modulus. Materials like TPX, silk fibroin, and cellulose often have poor moisture impermeability. Thus, there is a gap for a thin, transparent radiative cooling cover window combining high MIR emissivity with strong mechanics and barrier properties, motivating c-PI-based composites.

Synthesis: SiO2 aerogel microparticles (4–10 µm) were mixed with 20 wt% PMMA in anisole at a mass ratio of 1:5 using a planetary mixer (2000 rpm, 3 min), then baked at 100 °C for 1 min to remove anisole, fully infiltrating the aerogel pores with PMMA. The resulting PMMA–SiO2 microstructures were dispersed in clear polyimide (c-PI) to target 0–24 wt% (0–16 vol%). The mixture was spin-coated onto an Al substrate (700 rpm, 5 min), then cured at 100 °C for 30 min and 150 °C for 30 min. The ~50 µm thick films were peeled off. Characterization: Optical transmittance/reflectance (300–2500 nm) were measured by UV–Vis–NIR spectrometer with integrating sphere; IR (2.5–25 µm) by FTIR. Visible haze was computed (ASTM D1003) as diffuse/total transmittance. Mechanical properties were measured via tensile tests (ASTM D412 dogbones, 100 N load cell, 0.1 mm/min). Moisture barrier was quantified as WVTR at room temperature and 100% RH (ASTM F1249). Device tests—indoor: Films (c-PI or metamaterial) were attached as cover windows to a plate heater (4×4 cm²) using liquid-state c-PI as a thin adhesive (~0.5 µm after cure). Samples were mounted in an aluminized wood frame covered with LDPE to minimize convection and placed under AM1.5G 1000 W/m² illumination. Heater power densities of 100 W/m² (indoor) or 400 W/m² (outdoor tests) simulated device heat. Temperatures were logged via type-T thermocouples and IR camera; RH via hygrometer. For LEDs, a commercial blue LED with silicone lens was used; cover films were attached with the same adhesive layer or directly spin-coated (~50 µm). Light output power vs current was measured (ELT-1000), and temperature dependencies were assessed by controlling device temperature to steady-state values observed under illumination. Device tests—outdoor: Samples on heaters were placed in an LDPE-covered, aluminized frame tilted 30° south in Seoul, Korea; solar irradiance was measured by pyranometer. Temperatures of the heater front surface (beneath the film) were recorded during varying ambient (23–29 °C) and RH (30–40%). Modeling: Net cooling power Pnet(T)=Prad−Psurr−Psun+Pnon-rad−Pgen was computed using measured spectral emissivity, blackbody radiance, and assumed surrounding emissivity (outdoor: εsur≈0.2 in 8–13 µm, 1.0 elsewhere; indoor: εsur=1.0 across 2.5–25 µm). Psun was derived from ~3% solar absorption (≈21–23 W/m²). Non-radiative heat transfer was modeled via hc(T−Tamb). Transient temperature responses for simulated and model displays used energy balance mc dT/dt = −(Prad − Psurr − Psun + Pnon-rad − Pgen), with Pgen=100 W/m² for simulated devices.

• Optical: 50 µm films with 24 wt% PMMA–SiO2 achieved 85.5% visible transmission (400–800 nm), ≈97.1% of bare c-PI, with tunable haze from ~0.25 to 0.64 by varying microstructure content; mid-IR emission increased from 60.2% (c-PI) to 94.6% (8–13 µm) and 89.2% (2.5–25 µm). Even 6 wt% content raised 8–13 µm emissivity to ~86%.
• Mechanics: With 24 wt% microstructures, elastic modulus rose 2.2× to 2.51 GPa; tensile strength rose 1.6× to 79.8 MPa; elongation at break >3.7%. Films endured 10,000 bending cycles at 1 mm radius without wrinkles or performance loss. At 28 wt%, elongation dropped to 1.8%.
• Moisture barrier: WVTR decreased from ~172 g/m²-day (c-PI) to ~66 g/m²-day (24 wt%), attributed to increased diffusion path tortuosity.
• Thermal management—simulated displays: Indoor AM1.5G, 100 W/m² device heat, ambient 24 °C/RH 30%: 50 µm metamaterial yielded steady-state 54.6 °C vs 61.5 °C for c-PI (ΔT=−6.9 °C). Increasing thickness to 200 µm slightly increased emissivity (94.6%→95.9%) but raised steady-state temperature to 56.7 °C due to added conduction resistance. Outdoor (Seoul), 400 W/m² device heat, solar ~850 W/m², ambient 23–29 °C/RH 30–40%: ΔT=−8.3 °C vs c-PI; with 100 W/m² device heat outdoors, ΔT=−5.1 °C.
• LEDs: In dark, metamaterial reduced maximum LED temperature by 3.0 °C vs bare; under AM1.5G, by 6.7 °C. At 350 mA and controlled low temperature (minimizing thermal effects), light output power increased to 3081 mW vs 2629 mW (bare) and 2684 mW (c-PI). Accounting for temperature-induced degradation, metamaterial delivered 1.19× (dark) and 1.21× (illuminated) higher light output than bare. Interface quality mattered: air voids degraded optical and thermal performance.
• Flexible display panel: Metamaterial cover lowered peak temperature by 4.6 °C (dark) and 7.1 °C (illuminated), while maintaining clear imagery.
• Modeling: Estimated outdoor net cooling power of ~−109 W/m² at ambient matches high-performance transparent emitters; indoors, enclosure-like surroundings imply Pnet<0 W/m² at ambient, yet device cooling is realized via increased temperature differential enhancing net radiation.

Embedding PMMA-infiltrated SiO2 aerogel microstructures into c-PI exploits phonon-polariton resonance near 9.7 µm and Mie scattering from a broad microstructure size distribution to enhance thermal emission within the atmospheric window, while refractive-index matching between PMMA (≈1.49) and c-PI (≈1.50) preserves visible transparency. This addresses the central challenge of achieving high mid-IR emissivity in an ultrathin (~50 µm), flexible cover window without compromising optical clarity. The composite architecture further strengthens mechanical integrity (higher modulus and tensile strength) and improves moisture barrier performance, satisfying cover-window requirements for foldable displays (e.g., bending to 1 mm radius). Experimentally, the metamaterials effectively suppress device temperatures for simulated displays and LEDs under both dark and solar-illuminated conditions, validating front-side radiative cooling as a complementary strategy to conventional backside heat spreading. Enhanced LED light output arises from reduced thermalization and beneficial optical effects (anti-reflection via graded index and light scattering), reinforcing the dual thermal–optical advantages. Modeling confirms strong outdoor cooling potential and explains indoor behavior under high surrounding emissivity. Overall, the findings directly support the hypothesis that an optically matched emissive microstructure composite in c-PI can serve as an effective, multifunctional radiative cooling cover window for flexible and foldable displays.

The work introduces a 50 µm-thick, transparent radiative cooling metamaterial cover window for foldable and flexible displays by dispersing PMMA-infiltrated SiO2 aerogel microstructures in clear polyimide. The films combine high visible transmission (85.5%) with high atmospheric-window emissivity (94.6%), tunable haze (0.25–0.64), improved mechanical strength (2.51 GPa modulus; 79.8 MPa tensile strength; >3.7% elongation), and reduced WVTR (~66 g/m²-day). These properties translate to substantial device temperature suppression indoors and outdoors (up to 8.3 °C), reduced LED operating temperatures, and increased LED light output (up to 1.21× under illumination). The approach advances practical radiative cooling for transparent, foldable cover windows and demonstrates benefits for both display thermal stability and optical performance. Future work could optimize microstructure size distributions and arrangements, tailor emissive chemistries for broader or more selective spectra, refine adhesive/interface engineering, and explore alternative host/matching polymers (e.g., PET, PVDF) while considering touch sensitivity and thermal stability for diverse display platforms.

• Trade-offs with loading and thickness: Higher filler loading (e.g., 28 wt%) reduced elongation at break to 1.8%, potentially compromising foldability; increased film thickness improved emissivity slightly but raised steady-state temperatures due to added conduction resistance.
• Indoor constraints: In enclosure-like indoor settings with high surrounding emissivity, net cooling power at ambient can be limited (Pnet<0 W/m²), so benefits rely on device overheating relative to ambient to drive net radiation.
• Interface sensitivity: Air gaps between the metamaterial and device degrade both optical clarity and thermal performance; robust, void-free adhesion is essential.
• Material specificity: Performance depends on refractive-index matching and the microstructure distribution; translation to other polymers requires careful matching of optical and processing properties.
• Mechanical window: While 24 wt% meets foldability and strength targets, the processing window for even higher emissivity without sacrificing mechanics may be narrow.