Xin You, Ying Lv, Bin Liu, Pan Li, Xiaoxin Liu, Xiaoxin Liu, Xiaodan Guo, Dongzhi Shan, Xiaoyang Guo, Qiang Fu, Xu Guo, Tienan Wang, Guofa Cai, Xingyuan Liu, Xingyuan Liu
ABSTRACT Multi‐band electrochromic (MBEC) smart windows, capable of selectively modulating solar and thermal radiation from visible (VIS) to mid‐/long‐wave infrared (MWIR/LWIR), show significant potential for energy‐efficient buildings and carbon neutrality. Yet this potential remains unrealized due to the inherent limitations of high‐performance electrochromic materials and electrodes. Herein, we pioneer an MBEC smart window by integrating amorphous/nanocrystalline biphasic tungsten trioxide heterojunction (a,c‐WO 3 ) film with a gradient porous indium tin oxide (GP‐ITO) electrode. The nanostructured biphasic heterojunctions effectively enhance the localized surface plasmon resonance (LSPR) and spontaneously create built‐in electric fields at the heterointerface, enabling efficient optothermal modulation with rapid electron/ion transport kinetics. The dense/porous bilayer GP‐ITO electrode overcomes the trilemma of conductivity, VIS/NIR transmittance, and IR emissivity, boosting modulation range and amplitude. Consequently, the GP‐ITO/a,c‐WO 3 film (1.8 × 1.8 cm 2 ) exhibits remarkable MBEC performance including high optical contrast (71.7%@620 nm, 85.5%@1560 nm, 70.5%@2500 nm), rapid switching (12.4/15.4 s for coloration/bleaching), excellent durability (> 8000 cycles), and notable emissivity/temperature regulation (0.61/0.59 at MWIR/LWIR, 13.8°C). A 5 × 5 cm 2 prototype smart window demonstrates a 15.3°C indoor temperature reduction under AM 1.5G solar irradiation during mode switching. This work offers an effective strategy for MBEC materials/devices in zero‐carbon buildings, dynamic optothermal management, and adaptive camouflage.