The technological potential of poly(methyl methacrylate) (PMMA)-based composite films doped with lanthanide-doped sol–gel derived ionosilicas (IS-Ln) previously proposed for luminescent down-shifting (LDS) and luminescent solar concentrator (LSC) layers connected to photovoltaic (PV) cells is extended here to electrochromic devices (ECDs), targeting the fabrication of single energy harvesting/conversion/management LSC-LDS/PV/ECD systems. These integrated devices have foreseen application in the windows of future zero-energy buildings of smart cities. The proof-of-concept is given with the report of the electro-optical performance of an ECD comprising an optimized electrolyte film composed of PMMA, IS-Nd, and IS-Eu, and the 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid. This amorphous electrolyte is stable below 160 °C, exhibits high ionic conductivity (2.13 × 10 −4 and 8.76 × 10 −4  S cm −1 at room temperature and 44 °C, respectively), and emits in the visible (red color) and near-infrared (NIR) spectral regions. The device demonstrated fast switching speed (50 s) and high transparency in the visible-to-NIR spectral regions (transmittance ( T ) = 79/96/89/77% at 555/1000/1500/1650 nm in the as-prepared state, respectively). Upon application of ±2.5 V for 200 cycles, at the same wavelengths, the T bleached / T colored values were 44/28, 46/26, 39/20, and 27/9%, respectively, and the coloration efficiency (CE) values CE in /CE out values were −302/+181, −381/+228, −446/+267 and −734/+440 cm 2  C −1 , respectively. Highlights Red-emitting ECDs enabling dynamic control of sunlight and offering uninterrupted heat emission were fabricated. The ECD electrolyte composition was based on the formulation used previously for films acting both as LDS and LSC layers. The electrolyte included PMMA, Nd 3+ /Eu 3+ -doped ionosilicas, and 1-butyl-3-methylimidazolium hexafluorophosphate. The electrolyte is amorphous, exhibits good ionic conductivity, and emits in the visible (red color) and NIR regions. The ECD performance proves its applicability in an integrated LSC-LDS/PV/ECD system.