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arXiv · 1603.04038

Influence of the "second gap" on the transparency-conductivity compromise in transparent conducting oxides: an ab initio study

Viet-Anh Ha·David Waroquiers·Gian-Marco Rignanese·Geoffroy Hautier
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Abstract

Transparent conducting oxides (TCOs) are essential to many technologies. These materials are doped (\emph{n}- or \emph{p}-type) oxides with a large enough band gap (ideally $>$3~eV) to ensure transparency. However, the high carrier concentration present in TCOs lead additionally to the possibility for optical transitions from the occupied conduction bands to higher states for \emph{n}-type materials and from lower states to the unoccupied valence bands for \emph{p}-type TCOs. The "second gap" formed by these transitions might limit transparency and a large second gap has been sometimes proposed as a design criteria for high performance TCOs. Here, we study the influence of this second gap on optical absorption using \emph{ab initio} computations for several well-known \emph{n}- and \emph{p}-type TCOs. Our work demonstrates that most known \emph{n}-type TCOs do not suffer from second gap absorption in the visible even at very high carrier concentrations. On the contrary, \emph{p}-type oxides show lowering of their optical transmission for high carrier concentrations due to second gap effects. We link this dissimilarity to the different chemistries involved in \emph{n}- versus typical \emph{p}-type TCOs. Quantitatively, we show that second gap effects lead to only moderate loss of transmission (even in p-type TCOs) and suggest that a wide second gap, while beneficial, should not be considered as a needed criteria for a working TCO.

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Viet-Anh Ha, David Waroquiers, Gian-Marco Rignanese, Geoffroy Hautier. 2016-03-13. Influence of the "second gap" on the transparency-conductivity compromise in transparent conducting oxides: an ab initio study. https://doi.org/10.1063/1.4950803

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