Combining high conductivity with complete optical transparency: A band structure approach

– A comparison of the structural, optical and electronic properties of the recently discovered transparent conducting oxide (TCO), nanoporous Ca12Al14O33, with those of the conventional TCOs (such as Sc-doped CdO) indicates that this material belongs conceptually to a new class of transparent conductors. For this class of materials, we formulate criteria for the successful combination of high electrical conductivity with complete transparency in the visible range and emphasize the significant correlation between their structural characteristics and electronic and optical properties. Our analysis suggests that this set of requirements can be met for a group of novel materials called electrides which may have desired features such as connected structural cavities, large bandgaps and near-metallic electronic conductivity. Transparent conducting oxides (TCO) have been known for almost a century [1] and employed technologically for decades. Today, the area of practical applications of this special class of materials which can simultaneously act as a window layer and as an electrically conducting contact layer, is very large [2–4]: it includes optoelectronics (invisible circuits), flat-panel displays, energy supply (solar cells) and energy conservation (“smart” windows) devices. The commercial demand for less expensive, more flexible, environmentally friendly materials that exhibit both high optical transmission and electrical conductivity continues to stimulate further research. All well-known and widely used TCOs (such as In, Sn, Zn, Cd, Ga and Cu oxides and their blends) share similar chemical, structural and electronic properties as well as carrier generation mechanisms. These oxides of post-transition (or transition) metals have relatively close-packed structure with fouror six-fold coordinated metal ions. Upon introduction of native or substitutional dopants, they show high transparency in the visible range (∼ 80–90%) and high electrical conductivity (up to ∼ 10 S/cm). Common to all known TCOs, a highly dispersed band at the bottom of the conduction bands is the most important feature of the host electronic band structure. It provides both i) the high mobility of the extra carriers (electrons) due to their small effective masses and ii) low optical absorption due to a pronounced BursteinMoss shift which helps to keep intense interband transitions out of the visible range [5]. To illustrate how doping alters the electronic band structure of host transparent conductors, we calculated the band structure of undoped and 12.5% Sc-doped CdO using the full-potential linearized augmented plane-wave method [6] (FLAPW) within the screenedexchange LDA approach [7]. As one can see from fig. 1, the optical window broadens upon