Recent Advances in Fabrication of Flexible, Thermochromic Vanadium Dioxide Films for Smart Windows

Abstract

Monoclinic-phase VO2 (VO2(M)) has been extensively studied for use in energy-saving smart windows owing to its reversible insulator–metal transition property. At the critical temperature (Tc = 68 °C), the insulating VO2(M) (space group P21/c) is transformed into metallic rutile VO2 (VO2(R) space group P42/mnm). VO2(M) exhibits high transmittance in the near-infrared (NIR) wavelength; however, the NIR transmittance decreases significantly after phase transition into VO2(R) at a higher Tc, which obstructs the infrared radiation in the solar spectrum and aids in managing the indoor temperature without requiring an external power supply. Recently, the fabrication of flexible thermochromic VO2(M) thin films has also attracted considerable attention. These flexible films exhibit considerable potential for practical applications because they can be promptly applied to windows in existing buildings and easily integrated into curved surfaces, such as windshields and other automotive windows. Furthermore, flexible VO2(M) thin films fabricated on microscales are potentially applicable in optical actuators and switches. However, most of the existing fabrication methods of phase-pure VO2(M) thin films involve chamber-based deposition, which typically require a high-temperature deposition or calcination process. In this case, flexible polymer substrates cannot be used owing to the low-thermal-resistance condition in the process, which limits the utilization of flexible smart windows in several emerging applications. In this review, we focus on recent advances in the fabrication methods of flexible thermochromic VO2(M) thin films using vacuum deposition methods and solution-based processes and discuss the optical properties of these flexible VO2(M) thin films for potential applications in energy-saving smart windows and several other emerging technologies.

Doping Sodium Tungsten Bronze-Like (Na5W14O44) Near-Infrared Shielding Functional Units in Bulk Borosilicate Glasses for Energy-Saving Window Applications

“…The transmitted NIR light increases room temperature, leading to excessive use of air conditioning for temperature regulation . Hence, the reduction of energy consumption requires the application of energy-saving windows which should have excellent NIR-shielding ability, while keeping their high visible light transmission property …”

“…5 Hence, the reduction of energy consumption requires the application of energysaving windows which should have excellent NIR-shielding ability, while keeping their high visible light transmission property. 6 Traditional bulk-tinted float glass doped with colorant ions (e.g., Co 2+ ) is a type of NIR-shielding glass. 7 Although tinted glasses absorb NIR light, their visible transmittance is very low, and hence their application for room windows is very limited.…”

A vanadium dioxide-based metamaterial with quatrefoil and circle loaded structure on flexible polyamide substrate for terahertz applications

“…Thanks to the technological advancement, it is possible to produce a smooth and uniform surface for the VO 2 films. For instance, RF magnetron sputtering, van der Waal Epitaxy and chamber-based depositions are among the interesting techniques that were used to growth flexible and uniform VO 2 layers for various applications [29][30][31][32].…”

Metal–Insulator Transition Detection of Vanadium Dioxide Thin Films by Visible Light Reflection

“…The transition temperature setting of vanadium dioxide is usually controlled via impurity doping with lower- and higher-valence transition metals, such as W, Nb, Cr, Mo, and Ti. − However, to obtain highly effective thermochromic and electrochromic properties, a thin film of pure-phase VO 2 is required. For this purpose, various synthesis methods, such as strain-induced single-crystal substrates with the most preferred orientation, buffer layers, narrow oxygen partial pressures, and optimal growth temperatures, have already been studied. − More recently, VO 2 has been adopted as a next-generation material with dynamic color adjustment for reflectors or transmitters because of its significantly low phase change, low power consumption, low toxicity, low-effort manufacture, wide range of fabrication techniques, stability, and reversible electrically and optically activated tuning. − In particular, changes in optical properties, if induced in the visible spectral range, offer considerable benefits and ease of detection. Recently, photonic materials that are color-tunable in the visible spectral range, making them extremely attractive for application in temperature indicators, displays, memory, and other switchable optoelectronic devices, have been developed. − For example, Song et al reported, based on their modeling results, that vanadium dioxide-based metamaterial absorbers for active color rendering and multiband photodetection can be realized .…”