| Summary: | To address the challenges of the next generation of smart windows for energy-efficient buildings, new electrochromic devices (ECDs) are introduced. These include indium molybdenum oxide (IMO), a conducting oxide transparent in the near-infrared (NIR) region, and a NIR-emitting electrolyte. The novel electrolytes are based on a sol-gel-derived di-urethane cross-linked siloxane-based host structure, including short chains of poly (ε-caprolactone) (PCL(530) (where 530 represents the average molecular weight in g mol<sup>−1</sup>). This hybrid framework was doped with a combination of either, lithium triflate (LiTrif) and erbium triflate (ErTrif<sub>3</sub>), or LiTrif and bisaquatris (thenoyltrifluoroacetonate) erbium (III) ([Er(tta)<sub>3</sub>(H<sub>2</sub>O)<sub>2</sub>]). The ECD@LiTrif-[Er(tta)<sub>3</sub>(H<sub>2</sub>O)<sub>2</sub>] device presents a typical Er<sup>3+</sup> NIR emission around 1550 nm. The figures of merit of these devices are high cycling stability, good reversibility, and unusually high coloration efficiency (CE = ΔOD/ΔQ, where Q is the inserted/de-inserted charge density). CE values of −8824/+6569 cm<sup>2</sup> C<sup>−1</sup> and −8243/+5200 cm<sup>2</sup> C<sup>−1</sup> were achieved at 555 nm on the 400th cycle, for ECD@LiTrif-ErTrif<sub>3</sub> and ECD@LiTrif-[Er(tta)<sub>3</sub>(H<sub>2</sub>O)<sub>2</sub>], respectively.
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