Keywords: transparent conductors, solar energy, energy efficiency, thin films, doped oxide semiconductors, spectral selectivity, thermochromic, electrochromic, solar power, electrical conductivity, low infrared emittance, thermal properties, smart windows, indoor comfort, nanostructures, nanotechnology
Transparent conductors for solar energy and energy efficiency: a broad-brush picture
Transparent conductors (TCs) have numerous and diverse applications for solar energy applications and for energy efficiency. The largest of these applications, in terms of area, are based on the fact that the TCs have low infrared emittance and can be used to improve the thermal properties of modern fenestration. Depending on whether the TCs are reflecting or not in the near infrared pertinent to solar irradiation, the TCs can serve in 'solar control' or 'low-emittance' windows. Other applications rely on the electrical conductivity of the TCs, which render them useful as current collectors in solar cells and for inserting and extracting electrical charge in electrochromic 'smart windows' capable of combining energy efficiency and indoor comfort in buildings. This paper ventures to give a broad-brush picture of TCs and discuss their properties from the perspective of the radiative properties in our ambience. This approach leads naturally to considerations of spectral selectivity, angular selectivity, and temporal variability of TCs. The spectrally selective materials are thin films based on metals (normally silver) or wide band gap semiconductors with heavy doping (normally based on indium, tin, or zinc). The potential importance of carbon-based TCs is emphasised. Angular dependent TCs, especially those for which the properties are governed by inclined columnar nanostructures, are introduced. A discussion of TC-like materials with thermochromic and electrochromic properties is included, especially with regard to vanadium-dioxide-based films and electrochromic multilayer structures embracing tungsten oxide and nickel oxide.