Thin Films for Advanced Glazing Applications

Abstract

Functional thin films provide many opportunities for advanced glazing systems. This can be achieved by adding additional functionalities such as self-cleaning or power generation, or alternately by providing energy demand reduction through the management or modulation of solar heat gain or blackbody radiation using spectrally selective films or chromogenic materials. Self-cleaning materials have been generating increasing interest for the past two decades. They may be based on hydrophobic or hydrophilic systems and are often inspired by nature, for example hydrophobic systems based on mimicking the lotus leaf. These materials help to maintain the aesthetic properties of the building, help to maintain a comfortable working environment and in the case of photocatalytic materials, may provide external pollutant remediation. Power generation through window coatings is a relatively new idea and is based around the use of semi-transparent solar cells as windows. In this fashion, energy can be generated whilst also absorbing some solar heat. There is also the possibility, in the case of dye sensitized solar cells, to tune the coloration of the window that provides unheralded external aesthetic possibilities. Materials and coatings for energy demand reduction is highly desirable in an increasingly energy intensive world. We discuss new developments with low emissivity coatings as the need to replace scarce indium becomes more apparent. We go on to discuss thermochromic systems based on vanadium dioxide films. Such systems are dynamic in nature and present a more sophisticated and potentially more beneficial approach to reducing energy demand than static systems such as low emissivity and solar control coatings. The ability to be able to tune some of the material parameters in order to optimize the film performance for a given climate provides exciting opportunities for future technologies. In this article, we review recent progress and challenges in these areas and provide a perspective for future trends and developments.

Thermochromic Oxide-Based Thin Films and Nanoparticle Composites for Energy-Efficient Glazings

“…This type of variation in T(λ,τ) clearly is the desired one, and a glazing with a VO 2 -based TC thin film transmits more energy below τ c than above τ c . Similar properties have been recorded many times [31][32][33][34][35][36][37][38][39][40][52][53][54]. Figure 3 also demonstrates that corresponding data on R(λ) increase monotonically towards long wavelengths for the metallic-like state above τ c , which is the expected behavior.…”

VO2/TiO2bilayer films for energy efficient windows with multifunctional properties

“…3 Thermochromic glazings are designed to intelligently control the amount of light and heat (mainly in the near infra-red region) that can pass through the glazing as a function of the glazing temperature. 4,5 Vanadium dioxide (VO 2 ) exhibits a sharp change in optical properties around a thermochromic transition temperature (T c ) of 68 °C, which is associated with a semiconductor to metal transition, from a low-temperature monoclinic phase to a hightemperature tetragonal rutile-type phase. [6][7][8] For this reason, vanadium dioxide based thin films have been extensively investigated as possible smart-window coatings.…”

“…Thermochromic performance is characterised by ΔT, the percent transmittance modulation between the cold and hot stage: 5 hot cold T T T − = ∆…”

The Effect of Alkali Metal (Na, K) Doping on Thermochromic Properties of VO2 Films

“…The thermochromic performance is characterised by ΔT, the percent transmittance modulation between cold and hot stages (Equation. 1) 8 . Wavelength integrated luminous and solar transmittance values are expressed in equation (3), where  indicates the thickness of VO2 thin film,  is the spectral sensitivity of the light adapted eye and  the wavelength of light.…”

“…Epitaxial growth has also been shown to reduce the Tc through the promotion of strain 12 . It has been suggested that doping with high valence transition metals reduces the Tc through the injection of electrons into the V 3d valence bands, this destabilises the V-V pairs, promoting the tetragonal VO2(R) phase and so lowering the energy required for the phase transition 13 .…”