Effects of Annealing Temperature on Optical Band Gap of Sol-gel Tungsten Trioxide Films.

Tungsten trioxide (WO₃) is a wide band gap semiconductor material that is used as an important electrochromic layer in electrochromic devices. In this work, the effects of the annealing temperature on the optical band gap of sol-gel WO₃ films were investigated. X-ray Diffraction (XRD) showed that WO₃ films were amorphous after being annealed at 100 °C, 200 °C and 300 °C, respectively, but became crystallized at 400 °C and 500 °C. An atomic force microscope (AFM) showed that the crystalline WO₃ films were rougher than the amorphous WO₃ films (annealed at 200 °C and 300 °C). An ultraviolet spectrophotometer showed that the optical band gap of the WO₃ films decreased from 3.62 eV to 3.30 eV with the increase in the annealing temperature. When the Li⁺ was injected into WO₃ film in the electrochromic reaction, the optical band gap of the WO₃ films decreased. The correlation between the optical band gap and the electrical properties of the WO₃ films was found in the electrochromic test by analyzing the change in the response time and the current density. The decrease in the optical band gap demonstrates that the conductivity increases with the corresponding increase in the annealing temperature.