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

TL;DR

This study uses ab initio calculations to analyze how the second gap affects optical transparency in n- and p-type transparent conducting oxides, revealing that p-type oxides are more impacted and questioning the necessity of a wide second gap for TCO performance.

Contribution

The paper provides a detailed ab initio analysis of the second gap's impact on transparency in TCOs, challenging the idea that a large second gap is essential for high-performance TCOs.

Findings

Most n-type TCOs are unaffected by second gap absorption at high carrier concentrations.

p-type TCOs experience reduced transparency due to second gap effects at high doping levels.

A wide second gap is not a strict requirement for effective TCOs.

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.