  Vacancy-ordered double perovskites K2SnX6 (X = I, Br, Cl) attract significant
research interest due to their potential application as light-absorbing
materials in perovskite solar cells. However, a deep insight into their
material properties at the atomic scale is yet scarce. Here we present a
systematic investigation on their structural, electronic, optical properties
and phase stabilities in cubic, tetragonal, and monoclinic phases based on
density functional theory calculations. Quantitatively reliable prediction of
lattice constants, band gaps, effective masses of charge carriers, exciton
binding energies is provided in comparison with the available experimental
data, revealing the increasing tendency of band gap and exciton binding energy
as lowering the crystallographic symmetry from cubic to monoclinic and going
from I to Cl. We highlight that cubic K2SnBr6 and monoclinic K2SnI6 are
suitable for the application as a light-absorber for solar cell devices due to
their proper band gaps of 1.65 and 1.16 eV and low exciton binding energies of
59.4 and 15.3 meV, respectively. The constant-volume Helmholtz free energies
are determined through phonon calculations, giving a prediction of their phase
transition temperatures as 449, 433 and 281 K for cubic-tetragonal and 345, 301
and 210 K for tetragonal-monoclinic transitions for X = I, Br and Cl. Our
calculations provide an understanding of material properties of vacancy-ordered
double perovskite K2SnX6, helping to devise a low-cost and high performance
perovskite solar cell.
