Color-enhanced radiative cooling for building envelopes: Lead-free perovskite films for sustainable passive thermal design

By coupling solar reflective and thermally emissive properties, passive daytime radiative cooling (PDRC) offers a sustainable solution to mitigate UHI phenomena and support the development of more sustainable and resilient urban environments. However, the resultant visual appearance, typically whitish or mirror-like, hinders successful integration into the urban fabric. Indeed, introducing color results in unavoidable sunlight absorption that can compromise the cooling performance. Photoluminescence (PL) mechanism can decrease light-to-heat conversion while adding color to PDRC structures by converting part of the absorbed solar energy into light emission at longer wavelengths. In this study, three different lead-free perovskite nanocrystals (NCs) were introduced to provide vivid coloration to PDRC devices while preserving a notable capability to reject incident shortwave radiation through PL emission. Scalable luminescent layers were fabricated by embedding NCs emitting in the blue, green, and red wavelengths into PMMA, identified as the most compatible polymeric matrix, resulting in colored thin films with minimal solar absorbance ( α ( 0 . 3 − 2 . 5 μ m ) = 0 . 02 − 0 . 04 ) and efficient MIR emissivity ( ϵ ( 8 − 13 μ m ) = 0 . 70 ). Results showed that films could be coupled to several color-tunable multilayer emitters, maintaining excellent thermo-optical properties for successful integration within the urban skins. Finally, a scalable colored PDRC structure based on lead-free perovskites showing good solar reflectance ( ρ = 0 . 80 ), tunable color and photoluminescent emission, and high MIR emittance ( ϵ ( 8 − 13 μ m ) =0.90) was proposed. The cooling performance of the fabricated colored PDRC devices was monitored under outdoor conditions over three consecutive summer days, demonstrating that the proposed colored structures achieve sub-ambient temperatures at night (up to 2 ° C ), while exhibiting no performance penalties compared to bare PMMA despite their vivid coloration.