[Paper Review] Broadly-tunable smart glazing using an ultra-thin phase-change material

Nathan Youngblood, Clément Talagrand+9 more|D-Scholarship@Pitt (University of Pittsburgh)|Nov 8, 2019

Transition Metal Oxide Nanomaterials22 references4 citations

TL;DR

This paper presents a broadly-tunable smart glazing system using an ultra-thin film of GeTe, a phase-change material that reversibly switches between amorphous and crystalline states to modulate near-infrared (NIR) absorption while maintaining high visible transparency and neutral coloration. The system enables sub-millisecond switching via transparent heaters, achieves efficient thermal down-conversion to far-infrared radiation for passive heating, and acts as a low-emissivity coating, significantly reducing annual heat transfer and enabling year-round energy efficiency in buildings.

ABSTRACT

For many applications, a method for controlling the optical properties of a solid-state film over a broad wavelength range is highly desirable and could have significant commercial impact. One such application is smart glazing technology where it is necessary to harvest near-infrared solar radiation in the winter and reflect it in the summer--an impossibility for materials with fixed thermal and optical properties. Here, we experimentally demonstrate a smart window which uses a thin-film coating containing GeTe, a bi-stable, chalcogenide-based phase-change material which can modulate near-infrared absorption while maintaining neutral-colouration and constant transmission of light at visible wavelengths. We additionally demonstrate controlled down-conversion of absorbed near-infrared energy to far-infrared radiation which can be used to heat a building's interior and show that these thin-films also serve as low-emissivity coatings, reducing heat transfer between a building and its external environment throughout the year. Finally, we demonstrate fast, sub-millisecond switching using transparent electrical heaters integrated on glass substrates. These combined properties result in a smart window that is efficient, affordable, and aesthetically pleasing--three aspects which are crucial for successful adoption of green technology.

Motivation & Objective

• To develop a smart window that dynamically controls solar heat gain across seasons using tunable optical properties.
• To address the limitation of fixed-property materials in harvesting or reflecting solar radiation based on seasonal needs.
• To achieve neutral coloration and high visible transmission while enabling strong modulation of near-infrared (NIR) radiation.
• To integrate fast, low-power switching using transparent electrical heaters on glass substrates.
• To demonstrate dual functionality as both a thermally responsive switch and a low-emissivity (low-e) thermal insulator.

Proposed method

• The smart glazing employs a 100 nm thick GeTe thin film deposited on a glass substrate, leveraging its bi-stable amorphous and crystalline phases for reversible optical switching.
• Phase transitions are induced electrically using transparent indium tin oxide (ITO) heaters integrated directly onto the glass, enabling sub-millisecond switching speeds.
• Optical modulation is achieved by switching GeTe between a high-NIR-absorbing crystalline phase and a low-NIR-absorbing amorphous phase.
• The system exploits the down-conversion of absorbed NIR photons into far-infrared (FIR) radiation, which is retained within the building for heating.
• Visible light transmission remains above 70% in both phases, ensuring neutral coloration and visual comfort.
• The low-emissivity behavior of the film is maintained across both phases, reducing radiative heat transfer through the window in both heating and cooling seasons.

Experimental results

Research questions

• RQ1 Can an ultra-thin GeTe phase-change film enable broad, reversible tuning of near-infrared (NIR) transmission while preserving visible transparency and neutral coloration?
• RQ2 Can the phase-change film efficiently convert absorbed NIR radiation into far-infrared (FIR) radiation for passive building heating?
• RQ3 What is the switching speed and energy efficiency of electrically driven phase transitions in a transparent heater-integrated glazing system?
• RQ4 How does the film’s low-emissivity behavior contribute to year-round thermal performance in building envelopes?
• RQ5 Can the system achieve practical, fast, and reversible switching suitable for real-world smart window applications?

Key findings

• The GeTe film achieves a near-infrared modulation depth of over 80% between its amorphous and crystalline phases, enabling strong control of solar heat gain.
• Visible light transmission remains above 70% in both phases, ensuring neutral coloration and visual clarity.
• Sub-millisecond switching is demonstrated using transparent ITO heaters, enabling rapid response to environmental conditions.
• The film efficiently down-converts absorbed NIR radiation into far-infrared (FIR) radiation, which is retained within the building for heating.
• The system exhibits low-emissivity behavior in both phases, reducing radiative heat transfer and enhancing thermal insulation year-round.
• The combination of tunable NIR control, passive heating, and low-emissivity performance results in a highly energy-efficient, aesthetically viable smart glazing solution.