Effect of Ca substitution on crystal structure and band gap of solar cell material BaSi2.

To ameliorate the potential of a promising solar cell material BaSi 2, the effects of substituting Ba with Ca atoms on the crystal structure and band gap E g of BaSi 2 , were investigated both experimentally and computationally. The solid-solution limit of the Ca atoms in BaSi 2 was approximately 2.3 at.%. Single-crystal X-ray diffraction analysis of Ba 1− x Ca x Si 2 (0.025 ≤ x ≤ 0.072) revealed that the unit cell volume decreases with Ca content x , and the Ba atoms at the A1 crystallographic site are preferentially substituted by Ca atoms. Diffuse reflectance measurements indicated that E g decreases with x (1.24 eV at x = 0 and 1.17 eV at x = 0.07). The density functional theory calculations demonstrate that the experimentally observed decrease in E g by Ca substitution can be explained qualitatively by the combination of the substitution of Ca atoms in the unit cell volume of BaSi 2 and the volume reduction. Lattice parameters normalized with respect to those of BaSi 2 and band gaps of Ba 1− x Ca x Si 2 as functions of Ca content x [Display omitted] • Solid solution limit of Ca atoms in BaSi 2 is approximately 2.3 at.%. • Lattice parameters decrease with Ca content. • Ba atoms at the A1 crystallographic site are preferentially substituted by Ca atoms. • Band gap decreases with Ca content. • Volume reduction due to Ca substitution is a main reason for decrease of band gap. [ABSTRACT FROM AUTHOR]