Structure and Properties of Wo3 Thin Films for Electrochromic Device Application

Among transition metal oxides, tungsten trioxide is one of the most interesting materials exhibiting a wide variety of novel properties particularly in thin film form useful for advanced technological applications. It exhibits structural transformations and substoichiometric phase transitions, which attracted the attention of researchers over the past few years to explore their potential scientific and technological applications in the fields of display systems and microelectronics. WO3 is probably the most widely studied electrochromic material, especially in an amorphous state. Crystalline WO3 displays a change in optical constants over a wide spectral range in the visible and infrared regions when intercalated with Li or H ions. Tungsten trioxide thin films have been deposited by a number of deposition techniques such as thermal evaporation, electron beam evaporation, chemical vapour deposition and laser deposition. Tungsten trioxide exhibits a cubic perovskite like structure based on the corner sharing of regular octahedra with the oxygen atoms at the corner and the tungsten atoms at the centre of each octahedron. The crystal structure of tungsten trioxide is temperature dependent. It is tetragonal at temperatures above 740 °C, orthorhombic from 330 to 740 °C, monoclinic from 17 to 330 °C, and triclinic from -50 to 17 °C. Tungsten oxide is one of the most promising inorganic materials which exhibit excellent electrochromic, photochromic and gasochromic properties and it has been widely investigated to be used in electrochromic, gasochromic, solar energy, optical modulation, writing–reading–erasing optical devices, flat panel displays, gas, humidity and temperature sensors. In this paper the detailed technological aspects of properties, structures and applications of WO3 are presented.