Transparent heat regulating (THR) materials and coatings for energy saving window applications: Impact of materials design, micro-structural, and interface quality on the THR performance

This review highlights the development of energy saving transparent heat regulating (THR) materials and coating for energy saving window applications. Current state-of-the-art technologies including transparent heat reflecting mirror (THM), thermo-chromic (TC), transparent solar cells (TSC), and luminescent based materials have been discussed. The coating performance primarily depends on the selection of materials, surface and structural morphology, dielectric passivation growth process and architecture of the multi-layered structure. The micro-structural properties of thin metal/metal oxide layer, and its impact on the heat reflecting coating have been studied extensively. Growth of high quality continuous thin film with fewer defects is an essential part of the infrared reflecting and/or heat regulating coatings. Henceforth, in this review, detailed analysis of growth of continuous and thin metal layer influence of the seed layer (germanium and nickel) and doping on the growth mechanism of thin metal have been discussed. Surface morphology and electronic properties of metal layer/multi-layered coatings have been studied in detail for THR applications. A wide range of metal oxides and their physical properties have been considered for use as passivation layer in the THR coating structure. Among several THR structures, the architecture comprising of dielectric-metal-dielectric (DMD) stack is known to exhibit the best heat reflecting performances. While the metal component typically comprises of silver (Ag), copper (Cu), and nitride based materials, dielectrics are made from metal oxides such as BaSnO3, TiO2, SnO2, ZnO, HfO2, Cu2O, and ZrO2. Selection of passivation layer and tuning of micro-structural properties are very crucial to enhance the visible transmittance without sacrificing infrared reflectance. Optical properties of the dielectric layer can be controlled with growth mechanism and varying content of impurity dopant. Metal doped dielectrics play a key role to enhance the visible light transmission while maintaining infra-red reflection. Through in-situ materials engineering, crystal quality of the dielectric improves which has significant role on the THR performance. Furthermore, impact of rapid thermal annealing (RTA) technique to improve crystal quality of metal oxides without oxidizing the thin metal layer is also emphasized. In the subsequent sections, synthesis of thin films by using sputtering methods, thermal evaporation and e-beam evaporation methods using inexpensive materials for large scale deployment of coating have been discussed. Neutral colored Cu-based THR smart windows is developed through tuning the structural property of TiO2. The simulated Cu-based THR window shows ≥10 °C temperature reduction when compared to conventional glass based windows. Thermal stability of copper and silver based multilayer enhanced through ultra-thin metal and dielectric interface engineering. Transparent conducting oxides (TCOs) are also an essential candidate for the wide band gap semiconductor based THR application. Recent progress in TCOs material has been briefly discussed in one of the section of the review. Hetero-epitaxy of metal oxides (ITO/ZnO) shows promising characteristics as heat insulating materials. Impact of growth process and surface morphology of the TCO have been studied to evaluate the performance of the TCO as heat insulating materials. In addition, advanced hybrid composite based heat reflector coatings for energy efficient building applications is also highlighted in the later section of the review. The industrial utilization and efficacy of heat reflector metal oxides, when incorporated into polymeric/pigments/fibers and heat reflecting durable paints as advanced hybrid composites coatings has been discussed. The progression of solution based metal nanowire (MNW) and optical properties for the heat regulating applications have been included. Dielectric/metal-NW/dielectric multilayer can be a potential candidate for the development of low cost THR film. The solution based THR methods has potential to be mass customized in economic ways and can be viable at industrial scale. Thermo-chromic materials are also considered as prospective candidate for the transparent coating applications. Recent development of VO2 bilayer, trilayer, micro-pattern and nano-plate films have been discusses to enhance the luminous transmittance and solar modulation ability. Transparent solar cells, based on the infra-red absorption through up-conversion nanoparticles are viable candidates for the development of environmental friendly heat regulating systems. Recent advancement of inorganic, polymer, perovskite, and luminescent based transparent solar cell (TSCs) with heat reflecting mirror have been evaluated for smart windows applications. For the deployment of large scale THR film, low cost materials, roll-to-roll (R2R) sputter and atmospheric pressure chemical vapour deposition (APCVD) have been assessed for the industrial applicability. The progression of THR materials with thermo-chromics, self-cleaning and TSCs materials can enhance the overall performance of the smart/transparent coatings for thermal management and heat regulating functionalities.