Advances on tungsten oxide based photochromic materials: Strategies to improve their photochromic properties

Photochromic materials have been extensively studied because they are quite attractive and promising for many applications. Tungsten oxide (WO₃), a typical photochromic material, has attracted considerable attention. So far, many advances have been achieved in different subfields of WO₃ photochromism, mainly focused on the enhancement of WO₃ photochromic properties, which depends not only on the chemical features but also on the microstructures, interface and hybrid components. This review aims to summarize recent progress in different advanced strategies to improve WO₃ photochromic properties, such as a fast photoresponse (within 1 min), strong and stable photochromism, visible-light coloration and reversible photochromism. However, it is still challenging to achieve these properties for large-scale commercial applications. In addition, many previous studies have been limited to lab research purposes, although some photochromic WO₃-based devices have been fabricated. To address the above-mentioned issues clearly, this review will provide a discussion of the following aspects. First, the fundamental photochromism of WO₃ is described. Secondly, various advances towards improving its photochromic properties will be summarized. Thirdly, the optical applications of WO₃-based photochromic materials will be reviewed and discussed. And finally, a perspective on the development trends in WO₃-based photochromic materials and devices is proposed.