Colloidal Opaline Composites as Throughput-Scalable, Fully Transparent, and Color-Tunable Radiative Cooling Exterior Films for Outdoor Photovoltaics

Abstract

Particulate thermal metamaterials, consisting of randomly dispersed thermally emissive nanoparticles (NPs) within polymeric films, have attracted significant attention due to their coating processibility, making them highly compatible with large-area, cost-effective manufacturing of radiative coolers with high thermal emissivity. However, conventional particulate thermal metamaterials face challenges such as NP aggregation, leading to high haze, and restricting maximum NP loading. Additionally, their opacity and limited colorations make them less suitable for outdoor applications where aesthetics are important. Herein, we present crystallized NP-based opaline thermal metamaterials. This approach enabled us to achieve highly transparent, colorful, and large-areas radiative coolers with a negligible scattering even at relatively high NP volume fractions, thus significantly enhancing daytime radiative cooling efficiency. Also, by introducing a polymeric topcoat, we achieve superior cooling and transparency using only half the NP loading. Finally, we integrate opaline thermal metamaterials into neutral-colored silicon photovoltaics (PVs) to reduce their operating temperature, resulting in increased open-circuit voltage and improved overall power conversion efficiency. Together with sufficient mechanical strength and environmental resistance, our approach not only makes them suitable for use as radiative cooling exterior films of outdoor PVs but also provides a practical method to enhance PV efficiency and durability through effective thermal management.