Low-Temperature Laser Synthesis of Thin Electrochromic WO3 Films

Amorphous dielectric WO3 films with a surface roughness from 4 to 5 nm have been grown by room-temperature pulsed laser deposition on quartz and c-sapphire substrates using metallic targets. We have examined the effect of the nature of the substrates and the oxygen pressure during the growth process on the transmission spectrum of the WO3 films in the range from 400 to 2000 nm. The parameters of the films have been shown to depend on the oxygen pressure during the growth process. Raising the oxygen pressure from 20 to 60 mTorr during the film growth process increases the transmission of the WO3 films from 40 to 75% in the visible and UV spectral regions and from 10 to 70% in the IR. With increasing oxygen pressure during film growth, the band gap of the WO3 films increases from 3.01 to 3.34 eV in the case of sapphire substrates and from 2.95 to 3.42 eV in the case of quartz substrates, being only slightly dependent on the nature of the substrate. Using a WO3 film grown at room temperature, we have fabricated the first thin-film liquid-electrolyte electrochromic cell. Its transmission in the spectral range from 300 to 900 nm drops by 30% at an applied voltage of 2.5 V, with a coloration time on the order of 2 min.