58. Transparent Conductive Oxides

Transparent conducting oxides (TCOs) such as doped ZnO, In₂O₃ and SnO₂ play important roles as transparent electrodes in commercial applications such as display and lighting devices. Although transparency and electrical conductivity are inherently conflicting, TCOs possess both properties simultaneously. To understand the fundamentals of TCOs, the essetials of the transparency and electrical conductivity are reviewed simply. Comprehension of the essentials enables us to develop novel TCOs following the principles of the materials design that carrier conduction paths should be constructed in wide-gap oxides. Because the electronic structure of oxides is considerably different between the conduction band and the valence band, procedures to form the conduction paths for electrons are in contrast to those for holes. In n-type TCOs, only isotropically spread ns⁰ orbitals of heavy cations such as Zn²⁺, In³⁺ and Sn⁴⁺ are necessary to form conduction paths for electrons at the bottom of the conduction band. In p-type TCOs, however, hybridization of orbitals between oxygen 2p⁶ orbitals and other orbitals such as Cu 3d¹⁰, Sn 5s² and S 3p⁶ orbitals is essential to shape conduction paths for holes at the top of the valence band.