A Self‐Assembled 2D Thermofunctional Material for Radiative Cooling

The regulation of temperature is a major energy‐consuming process of humankind. Today, around 15% of the global‐energy consumption is dedicated to refrigeration and this figure is predicted to triple by 2050, thus linking global warming and cooling needs in a worrying negative feedback‐loop. Here, an inexpensive solution is proposed to this challenge based on a single layer of silica microspheres self‐assembled on a soda‐lime glass. This 2D crystal acts as a visibly translucent thermal‐blackbody for above‐ambient radiative cooling and can be used to improve the thermal performance of devices that undergo critical heating during operation. The temperature of a silicon wafer is found to be 14 K lower during daytime when covered with the thermal emitter, reaching an average temperature difference of 19 K when the structure is backed with a silver layer. In comparison, the soda‐lime glass reference used in the measurements lowers the temperature of the silicon by just 5 K. The cooling power of this simple radiative cooler under direct sunlight is found to be 350 W m−2 when applied to hot surfaces with relative temperatures of 50 K above the ambient. This is crucial to radiatively cool down devices, i.e., solar cells, where an increase in temperature has drastic effects on performance. Powerless thermal management with zero emissions is more important than ever in the world's current climate crisis. This work unravels the radiative sky cooling potential of self‐assembled 2D crystals, showing that only a single layer of microspheres is necessary for achieving the maximum infrared emissivity, and thus the best cooling performance, greatly reducing the material costs for future upscaling and applicability.