Sustainable Dual-Mode Smart Windows for Energy-Efficient Buildings

Electrochromic devices (ECDs) combining visible/near-infrared (NIR) transparent amorphous indium zinc oxide (a-IZO) external layers with innovative NIR-emitting electrolytes composed of red seaweed-derived κ-carrageenan (κ-Cg) polysaccharide, glycerol (Gly), and erbium triflate (ErTrif 3 · x H 2 O) are proposed as a valuable technological solution for the development of smart windows providing less heating demand, less glare and more indoors human comfort for the new generation of energy-efficient buildings. The electrolyte preparation is cheap, clean, and fast. The optimized sample including 50 wt% Gly/κ-Cg and 40 wt% ErTrif 3 · x H 2 O/κ-Cg exhibits the highest ionic conductivity (1.5 × 10 –4 S cm –1 at 20 °C) and displays ultraviolet (UV)/blue and NIR emissions associated with the κ-Cg-based host and the Er 3+ ions ( 4 I 15/2 → 4 I 13/2 ), respectively. The 5-layer configuration ECD tested demonstrated fast switching time (50 s), high electrochromic contrast (transmittance variations of 46/51% at 550/1000 nm), high optical density change (0.89/0.75 at 550/1000 nm), outstanding coloration efficiency (450th cycle = –15902/–13400 cm 2 C –1 and +3072/+2589 cm 2 C –1 at 550/1000 nm for coloration and bleaching, respectively), excellent electrochemical stability, and self-healing after mechanical damage. The ECD encompasses two voltage-operated modes: semibright warm (+3.0 V, transmittances of 52/61% at 550/1000 nm) and dark cold (−3.0 V, transmittances of 7/11% at 550/1000 nm).