Structure and properties of electrochromic WO3 produced by sol-gel methods

摘要

Electrochromic tungsten oxide thin films produced by dip-coating from a sol-gel solution are of interest for large area electrochromic window applications. The influence of the sol-gel formulation and the subsequent processing of the film required to produce uniform WO$-3$/ films are discussed together with the effects of the dipping and processing parameters on the structure, optical properties, and electrochemical behavior of the films. The electrochemical behavior of the films has been studied using cyclic voltammetry. Electrolyte solutions, with different cations for insertion into the WO$-3$/ layers, have been used in this work, and the resultant coloration of the films studied using spectrophotometry. Both colored and uncolored films have been studied using Rutherford backscattering spectrometry (RBS) and scanning electron microscopy (SEM). The sol-gel processing steps are shown to have a significant influence on the film microstructure and therefore the electrochemical coloration behavior of the films, as well as the lifetime of the film under repeated cycling. Results are shown illustrating the coloration behavior of the films, and the transmittance of the films over the visible and near-infrared spectra. WO$-3$/ films approximately 0.15 $mu@m thick are highly transparent and color quite uniformly, although the process is not completely reversible. There is evidence from the auger electron spectroscopy (AES) and RBS data that there is residual carbon in the films after conventional processing. Some progress has been made toward examination of the effect of this carbon on both coloration efficiency and long-term switching life of prototype devices.