Tuning WO3 film properties for electrochromic applications via annealing and oxygen pressure

Abstract

The objective of this investigation was to examine the intricate relationship between annealing temperature and oxygen partial pressure (PaO₂) in regard to morphological, structural, and electrochemical properties of tungsten trioxide (WO₃) films that were produced through sputtering. The films were deposited under two different PaO₂ values, specifically 0.3 mTorr and 0.5 mTorr, and then underwent annealing at various temperatures: room temperature, 100, 200, 300, and 400 degrees Celsius. X-ray diffraction (XRD) analysis revealed a temperature-dependent transition from an amorphous to a crystalline phase. Morphological analyses conducted with scanning electron microscopy (SEM) indicated a trend towards a smoother surface as both the annealing temperature and PaO₂ rose. At 400 °C, the films exhibited a granular surface finish. Significantly, the film fabricated at 0.3 mTorr and subjected to room temperature (RT) annealing showed cracks, indicating inherent stress in the film. Electrochemical evaluations revealed that the WO₃ film deposited at 0.5 mTorr and annealed later at 200 °C demonstrated enhanced redox performance, better diffusion of ions, and remarkable reversibility. Impressive results were demonstrated in optical studies, attaining 83 % optical modulation, colouration efficiency (CE) up to 30.54 cm²/C, and swift switching durations of 1.17 s for colouration and 0.82 s for decolouration. Moreover, cycling tests showed negligible degradation after 100 cycles for the films deposited at 0.5 mTorr PaO₂ and treated at 200 °C, emphasizing their resilience. This study furnishes a comprehensive knowledge of the consequences of annealing temperature and PaO₂ collectively on WO₃ films, highlighting the novel strategy of enhancing electrochromic efficacy by modifying temperature and meticulously balancing the PaO₂, thus contributing to the progress of energy-efficient smart materials.