Temperature-dependent electrochromic cycling performance of solution-processed WO₃ films

Electrochromic tungsten oxide (WO₃) is one of the most commonly used materials for energy-efficient smart windows owing to its reversible optical changes driven by cation insertion/extraction. However, optimizing the fabrication process to balance ion mobility, structural stability, and long-term durability remains a critical challenge. In this study, WO₃ thin films were fabricated via spin-coating followed by thermal treatment at various temperatures. The structural, optical, and electrochemical properties of the films were investigated to determine the optimal conditions for electrochromic performance. Characterization techniques such as differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy, focused ion beam scanning electron microscopy, and electrochemical analysis revealed the critical role of thermal treatment in the fabrication of WO₃ films. Amorphous WO₃ formed at an optimal temperature of 300 °C exhibited the best Li⁺ insertion/extraction dynamics, which resulted in the highest coloration efficiency and stable electrochromic performance over 500 cycles. In contrast, crystalline WO₃ fabricated at 350 °C and above showed reduced stability caused by restricted ion mobility in the dense structure, while films treated at insufficiently low temperatures (80 and 190 °C) exhibited significant performance decline attributed to structural weaknesses. This study underscores the importance of optimizing the thermal processing conditions for enhancing the durability and electrochromic performance of WO₃ films, while emphasizing the necessity of assessing stability through long-term cycle testing.