LAUSR

DOI: 10.1002/adom.201800707 sunlight. The features of an accurate radiative model, strong selective emission in the atmospheric transparency window, and broadband high reflectance to solar irradiation are each formulated individually and then configured collectively to accomplish this outcome. Because all of the successful examples of daytime radiative coolers possess high solar reflectivity, they are white or silver in color and are thus not visually appealing,[5–7,9] thereby restricting the possible installation locations and limiting their net cooling capacity. Although previous efforts have been paid to incorporate colors into the radiative cooler,[26] the research only dealt with theoretical calculations without experimental demonstrations, and structural optimization of colored radiative coolers has not been performed. Here, we present concepts and strategies for daytime radiative cooling systems that involve comparable attention to engineering design but with the goal of achieving systems that offer aesthetically desired colors and patterns and functional purposes, thus enabling more widespread installation. The experimental demonstration exhibits subambient cooling behaviors under a clear sky while preserving its color. The approaches reported here can address application concepts for wearable electronic devices whose operational temperature is lowered by radiative cooling. Figure 1a exhibits a schematic of a decorative colored passive radiative cooler (CPRC) for aesthetic purposes featuring areas with subtractive primary colors (i.e., cyan, magenta, and yellow) on a silvery background, where the latter area represents a conventional daytime radiative cooler. The CPRC consists of a SE comprising a bilayer of SiO2 (650 nm) and Si3N4 (910 nm), whose thicknesses are defined by extensive numerical optimization; and a metal reflector comprising an Ag film (100 nm) deposited on a silicon substrate (Figure 1b, left). Additional photonic nanostructures were inserted below the SE to generate vivid colors at specific desired areas (Figure 1b, right), which comprised a thin-film resonator composed of a metal–insulator–metal (MIM) structure. The MIM structure determined each color via interference in the 1D stacked layers, where the color generation was precisely controlled by tuning the thickness of the insulator layer (i.e., SiO2 cavity) in the MIM. In this study, the MIM structure was chosen as the colorant structure because it provided minimal loss of solar reflectance and a narrow spectral width compared with other additive color filters such as metal gratings and 1D photonic crystals (1D PhC), Recently developed approaches in passive radiative cooling enable daytime cooling via engineered photonic structure layouts. However, the use of these daytime radiative coolers is restricted owing to their nonaesthetic appearance resulted from strong solar reflection. Therefore, this article introduces a colored passive radiative cooler (CPRC) capable of generating potential cooling power, based on a thin-film optical resonator embedded in an efficient thermal emission structure. This CPRC not only selectively emits infrared wave through the atmospheric transparency window but also displays subtractive primary colors to exhibit the desired appearance. Theoretical analysis and systematic experiments prove the possibility of subambient cooling via CPRC by lowering the temperature to 3.9 °C below the ambient air in the daylight. This is the first example of coloring radiative cooler by photonic structures. Successful demonstration of cooling/coloring behavior with wearable electronic devices under solar irradiation represents a major step forward in the field of temperature-sensitive, flexible, wearable electronic/optoelectronic devices.