University of Perugia researchers have developed a colored passive daytime radiative cooling (PDRC) approach based on lead-free perovskite nanocrystals, addressing one of the key limitations of conventional radiative coolers: their typically white or mirror-like appearance.
PDRC systems rely on a dual optical function - high solar reflectivity and strong thermal emission in the mid-infrared (MIR) - to passively dissipate heat to the cold outer space sink (approximately 3-4 K) through atmospheric transparency windows. In practice, this requires minimizing solar absorption (typically in the
0.25-2.5μm range) while maximizing emissivity in the 3-5μm and 8-13μm bands. However, achieving visible coloration generally introduces additional solar absorption, which increases heat gain and degrades cooling performance.
To overcome this trade-off, the researchers leveraged photoluminescence (PL) as a color-generation mechanism. Instead of converting absorbed sunlight into heat, PL materials re-emit part of the absorbed energy at longer wavelengths (Stokes shift), effectively reducing net heat generation. This allows colored surfaces to maintain cooling functionality.
The team incorporated three types of lead-free perovskite nanocrystals emitting in the blue, green, and red spectral regions into a polymethyl methacrylate (PMMA) matrix, identified as the most compatible host polymer. The resulting films exhibited vivid coloration while maintaining low solar absorbance and efficient MIR emissivity. Importantly, the luminescent layers were fabricated using scalable processes, making them suitable for large-area applications.
These colored films were further integrated with color-tunable multilayer emitters, preserving strong thermo-optical performance. The final PDRC structure demonstrated high solar reflectance, tunable photoluminescent emission, and a high mid-infrared emittance of ε=0.90, enabling efficient radiative heat rejection.
Outdoor testing over three consecutive summer days showed that the colored PDRC devices achieved sub-ambient cooling at night, with temperature reductions of up to 2∘C. Notably, this performance was achieved without measurable penalties compared to non-colored (bare PMMA) reference samples, despite the introduction of vivid coloration.
This work highlights how combining solar reflection with photoluminescent emission enables a twofold cooling mechanism: minimizing solar heat gain while actively converting part of the absorbed radiation into emitted light. By using lead-free perovskites, the approach also addresses concerns related to toxicity and environmental stability, while maintaining compatibility with scalable fabrication.
Overall, the study demonstrates a viable pathway toward aesthetically adaptable radiative cooling materials, supporting their integration into urban environments and contributing to the mitigation of urban heat island effects without increasing energy demand.
