Nathan Youngblood1 Clément Talagrand2 Peiman Hosseini2 Harish Bhaskaran1

1, University of Oxford, Oxford, , United Kingdom
2, Bodle Technologies Ltd., Begbroke, , United Kingdom

Maintaining the indoor temperature of industrial and residential buildings consumes a large portion of the energy budget in developed countries, ranging from 20% to 40%. This energy consumption is particularly high in regions of the globe that experience large swings in the environmental temperature from the winter to summer months. To improve efficiency, it is highly desirable to harvest solar radiation in the winter and reflect it in the summer—an impossibility for materials with fixed thermal and optical properties. Using switchable materials to maximize the energy efficiency of buildings throughout the year has a significant environmental and economic advantage to this sector.

Windows are a particularly promising architectural component for targeting efficiency gains. For example, a significant amount of heat is lost through windows in the winter season—as much as 25% in the US and 50% in northern China. “Low-E” coatings can be used to reduce heat transfer, but cannot be actively switched to make use of the near-infrared solar spectrum in winter months. Electro-chromatic smart windows are difficult to fabricate on curved surfaces, require a continuous electric field in the “on” state, and often have unwanted colouration, whereas VO2-based windows contain toxic materials. A smart window technology is needed that is inexpensive, switches in a non-volatile manner, and minimizes changes in visible colouration in both states.

Here, we experimentally demonstrate a smart window which uses a thin-film coating containing a chalcogenide-based phase-change material. Our thin-film coating is able to modulate near-infrared reflection while maintaining neutral-colouration of transmission at visible wavelengths. This transition is non-volatile and only requires energy when switching between the two states. We find experimentally that the total modulation of the near-infrared solar energy is more than a factor of two, while the energy in the visible spectrum varies by only 20%. Additionally, we show that in the far-infrared these thin-films serve as low-emissivity coatings, reducing thermal radiation from a building’s interior during the winter seasons. These combined properties result in a smart window that is simple, affordable, and aesthetically pleasing—three aspects which are crucial for successful adoption of green technology.