Unraveling the Intercalation Chemistry of Hexagonal Tungsten Bronze and Its Optical Responses

In an attempt to promote energy saving through the clever control of varying amounts of visible light and solar energy in modern buildings, there has been a surge of interest in the novel design of multifunctional glass windows otherwise known as "smart windows". The use of chromogenic materials (e.g., tungsten oxides and their alloys) is widespread in this cooling energy technology, and for the case of hexagonal tungsten oxide (h-WO₃)-based systems, the overall efficiency is often hindered by the lack of a systematic and fundamental understanding of the interplay of intrinsic charge transfer between the alkali-metal ions and the host h-WO₃. In this work, we present a first-principles hybrid density-functional theory investigation of bulk hexagonal tungsten bronzes (i.e., alkali-metal-intercalated h-WO₃) and examine the influence of the intercalation chemistry on their thermodynamic stability as well as optoelectronic properties. We find that the introduction of the alkali-metal ion induces a persistent n-type electronic conductivity, and dramatically reduces the optical transmittance (down to ∼28%) for infrared wavelengths while maintaining fair optical transparency for next-generation electrochromic devices in very energy efficient chromogenic device technology.