Matching Optical Admittance to Overcome Limitations of Transparent Conducting Oxides in Electrochromic Infrared Applications

Transparent conducting oxides (TCOs) such as indium tin oxide (ITO) exhibit excellent electrical conductivity and optical transmittance in the visible spectrum, enabling their widespread use in solar cells, smart windows, and display technologies. However, optical transmittance deteriorates sharply in the infrared (IR) due to free-carrier absorption beyond the plasma frequency, causing ITO films to behave metallically with high reflectance. This inherent property limits the extension of electrochromic (EC) devices from visible to infrared wavelengths. In this work, we demonstrate a multilayer optical design strategy based on admittance diagram, enabling IR transmittance through otherwise reflective ITO layers. Using a carefully designed Ta₂O₅/ITO/WO₃/Ta₂O₅/NiO/ITO stack on silicon, the effective optical admittance of the multilayer device was tuned to match that of air, minimizing reflection losses and allowing strong infrared transmission. The fabricated EC device exhibits a transmittance of ~58% at 4 µm, which decreases to ~23% under applied bias, achieving broadband modulation across 2–10 µm. This approach effectively transforms the TCO-based EC structure into an IR-compatible optical switch, overcoming the long-standing conductivity–transparency tradeoff in the mid-IR. The device enables dynamic control of radiative heat transfer, offering a scalable pathway for infrared electrochromic systems applicable to adaptive thermal camouflage and radiative thermal management.