Opportunities and challenges in science and technology of WO3 for electrochromic and related applications

Since the discovery of the electrochromic (EC) effect in transition metal oxides in the mid-1960s, intense research and development work spanning four decades has led to many exciting developments in the science and technology of this class of materials. Tungsten oxide (WO₃) has emerged as one of the key materials, not only for EC devices, but also for many other related applications. After many years of technology development efforts, WO₃-based EC “smart windows” have finally emerged as a viable commercial product. In spite of enormous progress being made on the structural, electrical, and optical properties of amorphous and crystalline WO₃, a detailed understanding of the EC effect in this material still remains somewhat qualitative. Although theoretical models based on intervalence charge transfer and polaron formation have been widely accepted, these models are still unable to explain some of the experimental results on the coloration phenomena. The coloration in WO₃ is a structure-sensitive phenomenon, and excess electrons can be either localized or delocalized. The presence of structural defects such as oxygen vacancies, impurities, and degree of disorder plays a crucial role in determining the coloration efficiency. Although significant progress has been made in recent years on the calculation of electronic structure and defect properties of both amorphous and crystalline WO₃, the structural complexity of the material presents many challenges and opportunities for theoretical computation. The unique ability to induce bistable optical and electrical properties in WO₃ by a variety of excitation sources has led to many devices of significant technological interest. Some of the applications currently being pursued include the photoelectrochemical cell for solar energy conversion and storage; photoelectrochemical splitting of water to generate hydrogen; chemical and biological sensors based on the gasochromic effect; photo- and electrocatalysts for a variety of chemical reactions; demonstration of high-temperature (91 K) superconductivity in WO₃ doped with H, Na, and K; synthesis of a new class of hybrid organic/inorganic (WO₃) materials; and application in ultra-high-resolution electron beam lithography. The emergence of nanostructured WO₃ in recent years will undoubtedly provide new opportunities and significant impact to many of these technologies. This paper presents a brief overview of some of the key research issues the author believes will impact the science and technology of this exciting material.