Neutral Gray‐Black to Colorless ZnO/Viologen Based Photo‐Electrochromic Dual‐Responsive Device

ABSTRACT Global warming is one of the most pressing environmental issues today. Smart windows are an effective energy‑saving measure that reduces building energy consumption by adjusting indoor lighting, thereby lowering carbon emissions. In this study, a photo‐electrochromic dual‐responsive device (PECD) based on ZnO/viologen is designed to achieve color changes from transparent to neutral gray‐black, providing efficient light modulation and energy‐saving performance for smart windows. Two viologen derivatives, Ethyl violet bromide (Et‐V) and 1,1'‐Di(6‐Hydroxyhexyl)‐1,2‐Di(4‐pyridyl) Ethylene Bromide (DHH‐V), are obtained and assembled into three devices: ZnO‐PECD‐1 (Et‐V), ZnO‐PECD‐2 (DHH‐V), and ZnO‐PECD‐3 (Et‐V and DHH‐V). The results demonstrate that the introduction of ZnO significantly enhances the devices' photochromic performances. The photo‐generated electrons facilitate rapid coloration through the reduction of the viologen cation (EtV ²⁺ →EtV ⁺ ). ZnO‐PECD‐3 is prepared by mixing green Et‐V and magenta DHH‐V to achieve a neutral gray‐black colored state, which exhibits strong absorption in the whole visible band with a maximum transmittance modulation (ΔT) of 51.4% at 525 nm. The mechanism study reveals that the photogenerated carriers of ZnO and the redox reaction of viologen synergistically drive the photo‐electrochromic dual‐responsive process. This study provides novel insights for the design and advancement of smart windows.