Electron transport and electrochromic properties of sol-gel WO3 thin films: Effect of crystallinity

The influence of crystallinity on the electron transport and electrochromic properties of sol-gel WO3 thin films has been investigated. It is observed that, at 200 °C, WO3 films exhibits amorphous crystallinity with disk-like nanostructures while sharp crystalline in the monoclinic structure, at higher temperature (400 °C). The optical band gap of the films sees an increase first and then a decrease with increasing annealing temperature (150-400 °C). The diffusion coefficient of electrons is studied by monitoring an electron-diffusion-controlled electrochromic process and found 0.0007 cm2/s for amorphous WO3, which is increased by an order of magnitude to 0.0066 cm2/s, in the crystalline WO3. It is also observed that the crystalline WO3 exhibits larger optical modulation upon electrocoloration. Electrochemical characterizations reveal high electrochromic performance for crystalline WO3 than the amorphous one, when equal amount of charge is injected. The enhanced optical modulation of crystalline WO3 suggests that more electrons participate in the optical transition yielding electrocoloration due to improved electron transport.