Electrolyte-Free All-Solid-State Electrochromic Device Based on an Organic/Inorganic Semiconductor Heterojunction

Electrochromic (EC) technologies offer great prospects of applications in smart windows and nonemissive displays due to their low energy consumption and electrically controlled optical modulation. However, electrolytes, as the essential component of traditional electrochemical EC devices, cause challenges such as limited device design strategies and problems in safety, stability, and integration, hindering the full commercialization of EC devices. Herein, an innovative strategy for designing electrochromic devices is proposed using organic/inorganic semiconductor heterojunctions. The heterojunction is constructed by the C-rich carbon nitride-conjugated polymer (CPCN) with the π* electron accumulation effect and TiO₂ nanocrystals with high dielectricity with an architecture of TiO₂/CPCN/TiO₂ to build interfacial energy barriers or electron transfer routes. These interfacial effects facilitate electron injection, transport, and accumulation, causing changes in the electronic structure of materials and thus their light absorption properties, thereby achieving bistable electrochromism. Using the heterojunction, a nonelectrochemical EC device with the symmetric architecture of FTO/TiO₂/CPCN/TiO₂/FTO is fabricated, free of electrolytes, enabling an intrinsic all-solid state. The design concept and underlying mechanism are illustrated and substantiated through electrochemical impedance spectroscopy and Mott-Schottky experiments. The electrochromic performances are evaluated through the optical contrast (ΔT), coloration efficiency, response time, and cyclic and long-term stability. The electrochromic display performance based on the laser-etching strategy is particularly investigated.