Tuning emittance in films of plasmonic metal oxide nanocrystals for daytime radiative cooling

Radiative cooling offers a passive, low-cost option for thermal management without expending energy on intensive active cooling mechanisms. Doped metal oxide nanocrystals are a promising option for maintaining high solar transparency while accessing tunable emission in the primary atmospheric transparency window from their localized surface plasmon resonance. Finite element method simulations of nanocrystal films are used to explore impacts of nanocrystal size, doping concentration, and film thickness on film emissivity. We reveal trade-offs between emission intensity and selectivity: Targeting selective emission results in unwanted reflectance from nanocrystal coupling limiting the maximum emissivity, while maximizing emissivity through increasing film thickness causes unwanted solar absorption from broadened emission. The trade-offs result in temperature-dependent design rules for optimizing radiated power. An optimized radiative cooler could provide 20 degrees C of cooling, close to the ideal emitter. High emissivity is achievable in real nanocrystals which exhibit extinction peaks close to the best simulated case.