Effect of Al doping on the electrochromic properties of WO3 thin films

Effect of Al doping on the structural, optic, and electrochromic properties of tungsten trioxide (WO3) thin films was investigated through X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy, optic transmittance, spectral ellipsometry, cyclic voltammetry and chronoamperometry measurements. Undoped and 0.7-4.3% Al doped WO3 films were grown by radio frequency magnetron sputtering. All of the WO3:Al films investigated grew in amorphous phase. The thicknesses of the films varied between 113 and 124 nm. X-ray photoelectron spectroscopy analysis indicated that tungsten atoms exist as W6+ in both undoped and Al doped WO3 films. The optical transmittance of WO3 films is not affected by the Al doping. Their transparency in the visible range is around 90% while the transmittance in the near infrared region seems to slightly increase (around 5%) with increasing Al doping. The optical band gap energies increase with Al doping concentrations exceeding 1.5%. Variation of the coloration / bleaching times and coloration efficiency of the WO3 films with Al doping was analyzed. The highest optical modulation of 44% was obtained for 1.5% Al doped WO3 film at 630 nm.