Self-stratified photonic radiative cooling composites with asymmetric thermal conductivity

High-power outdoor electronics, such as 5G base stations, need energy-efficient thermal management. Passive daytime radiative cooling (PDRC) represents a promising solution, but faces practical limitations due to low thermal conductivity and performance degradation associated with coloration. Herein, we demonstrate a hierarchically structured asymmetric bilayer composite, fabricated through a scalable and feasible self-stratification process, which integrates a cholesteric photonic lattice of cellulose nanocrystals (CNCs) with a highly thermally conductive framework of boron nitride (BN) nanosheets. The top photonic CNC layer provides vivid structural color and high mid-infrared emissivity (εMIR = 91.5%), while the bottom BN-rich layer delivers high solar reflectance (96.9%) and enhanced through-plane thermal conductivity (8.9 W m^-1 K^-1). The material achieves a temperature drop of up to 17.8 °C under realistic solar and thermal loads, while its asymmetric heat transfer property suppresses parasitic heat gain from the environment. Furthermore, the composite enables scalable structural color patterning via screen printing without compromising the cooling performance, offering both aesthetic customization and environmental durability. This work presents a scalable self-assembly strategy for high-performance, aesthetically versatile radiative cooling materials that address key challenges in next-generation electronic thermal management.