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 evapora