Combining Optical Transparency with Electrical Conductivity: Challenges and Prospects

Transparent conductors are neither 100% optically transparent nor metallically conductive. From the band structure point of view, the combination of the two properties in the same material is contradictory: a transparent material is an insulator which possesses completely filled valence and empty conduction bands; whereas metallic conductivity appears when the Fermi level lies within a band with a large density of states to provide high carrier concentration. Efficient transparent conductors find their niche in a compromise between a sufficient transmission within the visible spectral range and a moderate but useful in practice electrical conductivity [1–6]. This combination is achieved in several commonly used oxides – In2O3, SnO2, ZnO and CdO. In the undoped stoichiometric state, these materials are insulators with optical band gap of about 3 eV. To become a transparent conducting oxide (TCO), these TCO hosts must be degenerately doped to displace the Fermi level up into the conduction band. The key attribute of any conventional n-type TCO host is a highly dispersed single freeelectron-like conduction band [7–13] (Figure 1.1). Degenerate doping then provides both (i) the high mobility of extra carriers (electrons) due to their small effective mass and (ii) low optical absorption due to the low density of states in the conduction band. The high energy dispersion of the conduction band also ensures a pronounced Fermi energy