Band-Gap Engineering of Mo- and W-Containing Perovskite Oxides Derived from Barium Titanate

Abstract

Ferroelectric oxide perovskites are promising materials for use in photovoltaic devices, due to their ability to exploit the bulk photovoltaic effect to achieve high power-conversion efficiency. In this work, we use first-principles methods to investigate the ferroelectric perovskite $[{\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_{3}{]}_{x}\text{\ensuremath{-}}[{\mathrm{Na}\mathrm{Ti}}_{1/2}{\mathrm{Mo}}_{1/2}{\mathrm{O}}_{3}{]}_{1\ensuremath{-}x}$ and $[{\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_{3}{]}_{x}\text{\ensuremath{-}}[{\mathrm{Na}\mathrm{Ti}}_{1/2}{\mathrm{W}}_{1/2}{\mathrm{O}}_{3}{]}_{1\ensuremath{-}x}$ solid solutions for potential use in ferroelectric-based photovoltaics. We find that compositional variations change the band gap, shifting it to the edge of the visible range for the 25% ${\mathrm{Na}\mathrm{Ti}}_{1/2}{\mathrm{Mo}}_{1/2}{\mathrm{O}}_{3}$ composition and to the visible range for some Mo-cation and W-cation arrangements for the 50% ${\mathrm{Na}\mathrm{Ti}}_{1/2}{\mathrm{Mo}}_{1/2}{\mathrm{O}}_{3}$ and 50% ${\mathrm{Na}\mathrm{Ti}}_{1/2}{\mathrm{W}}_{1/2}{\mathrm{O}}_{3}$ compositions. Mo and W substitutions both maintain the ferroelectric properties of the parent ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_{3}$. While the A-site cation arrangement has a minor effect on the band gap, the variations in the B-site cation arrangement and the cation displacements affect the band gap by up to 0.8 eV. Analysis of the structures and the calculated band-gap values shows that the band gap is controlled by the identity of the substituent cation, the O-B-O angles, the relative orientations of the Mo and W substituent atoms, and the B-cation displacement. We demonstrate the thermodynamic feasibility of these solid solutions by formation energy analysis. The decrease of the band gap relative to the parent ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_{3}$ to the standard and transparent photovoltaic range combined with the ferroelectricity maintained make this earth-abundant-containing solid solution a promising candidate for use in high-performance ferroelectric-based photovoltaic devices.