Electrodeposited tungsten oxide films: annealing effects on structure and electrochromic performance

Abstract

A potentiostatic electrochemical procedure employing an ethanolic solution of peroxotungstic acid yielded tungsten oxide (WO 3) films specifically for transmissive electrochromic devices (ECDs) such as “smart windows”. The structure–property correlation for the as-deposited thin films of WO 3 and the films annealed at different temperatures (60, 100, 250 and 500 °C) is described. While the as-deposited film comprises of a small volume fraction of nanocrystals, the films annealed at 60 and 100 °C are X-ray amorphous, the 250 °C film is triclinic and the 500 °C film is characterized by mixed triclinic and hexagonal crystalline phases. Scanning electron micrographs (SEMs) clearly reveal the presence of nanograins/crystallite aggregates in the heat-treated films. Coloration renders the 250 °C film amorphous and reduces the crystallite size in the as-deposited and 500 °C films. Thermal analysis provides information pertaining to the critical temperatures at which dehydration, peroxo group decomposition and crystallization occur. Fourier transform infrared (FTIR) spectroscopic data show that while the as-deposited WO 3 film is composed of a peroxopolytungstate ion network with water molecules acting as bridging groups, the crystalline film annealed at 500 °C shows absorption bands characteristic of the W–O–W network (polytungstate ions linked through oxygens). Coloration efficiency (CE; η) and transmission modulation ( ΔT) are slightly higher for the amorphous 60 °C film when compared with the nanocrystalline as-deposited film in the 450< λ<850 nm range, and these properties deteriorate at temperatures ≥100 °C. However, switching times between the colored and bleached states and ion storage capacities show a systematic and significant decrease with increasing annealing temperature. High transmission modulation (67.5% at 632.8 nm), fast bleaching kinetics (∼5 s), high ion storage capacity (22 mC cm −2) and a coloration efficiency of 60.5 cm 2 C −1 at 632.8 nm observed for the as-deposited film render it to be a suitable candidate for electrochromic window applications. An operational lifetime of more than 10 4 cycles has been tested successfully for the as-deposited film against a platinum sheet as an auxiliary electrode.