Hole traps in sodium silicate: First-principles calculations of the mobility edge

The structure and properties of (Na2O)(0.30)(SiO2)(0.70) sodium silicate glass are studied by combined ab-initio and classical molecular dynamics simulations to identify the sources of electronic traps in the band gap. Structures from classical molecular dynamics melt-quench simulations are taken as initial configurations for first-principles density functional theory structural relaxation, from which electronic properties are determined. An ensemble of thirty glass structures, each containing 660 atoms, is prepared in order to perform statistical analyses. The inverse participation ratio is used as a metric to characterize localized states in the band gap and determine the mobility edge. Structures with varying amounts of local disorder (traps) are compared. The most localized and highest energy trap states are due to Si atoms with 2-3 non-bridging oxygen atoms. Control of the electronic properties of amorphous insulators and semiconductors is essential for the advancement of many technologies, such as photo-voltaics and scintillators, for which the present analysis is indispensable. (C) 2015 Elsevier B.V. All rights reserved.