Effects of surface porosity on tungsten trioxide(WO3) films' electrochromic performance

In this paper, the correlation between the electrochromic performance and the surface morphology of the tungsten trioxide (WO3) thin films sputtered by dc reactive magnetron sputtering with widely varying target-substrate distances was investigated. It is found that the optical density change (ΔOD) of films is strongly affected by the target-substrate distance. The coloration efficiency (CE) at 633 nm was also found to be sensitive to the target-substrate distance, with 16 cm2/C of film sputtered at 6 cm and 50 cm2/C at 18 cm. X-ray diffraction showed that the crystal structure of films was amorphous. By using atomic force microscope to identify the surface porosity of the sputtered WO3 films, we found that the film at longer target-substrate distance was rough, porous, and having a cone-shaped columns morphology, thus offering a good electrochromic performance for opto-switching applications.