Inorganic Halide Double Perovskites with Optoelectronic Properties Modulated by Sublattice Mixing

All-inorganic halide double perovskites
have emerged as a promising
class of materials that are potentially more stable and less toxic
than lead-containing hybrid organic–inorganic perovskite optoelectronic
materials. In this work, 311 cesium chloride double perovskites (Cs2BB′Cl6) were selected from
a set of 903 compounds as likely being stable on the basis of a statistically
learned tolerance factor (τ) for perovskite stability. First-principles
calculations on these 311 double perovskites were then performed to
assess their stability and identify candidates with band gaps appropriate
for optoelectronic applications. We predict that 261 of the 311 Cs2BB′Cl6 compounds are likely
synthesizable on the basis of a thermodynamic analysis of their decomposition
to competing compounds (decomposition enthalpy <0.05 eV/atom).
Of these 261 likely synthesizable compounds, 47 contain no toxic elements
and have direct or nearly direct (within 100 meV) band gaps between
1 and 3 eV, as computed with hybrid density functional theory (HSE06).
Within this set, we identify the triple-alkali perovskites Cs2[Alk]+[TM]3+Cl6, where Alk
is a group 1 alkali cation and TM is a transition-metal cation, as
a class of Cs2BB′Cl6 double perovskites with remarkable optical properties, including
large and tunable exciton binding energies as computed by the GW-Bethe–Salpeter equation (GW-BSE)
method. We attribute the unusual electronic structure of these compounds
to the mixing of the Alk-Cl and TM-Cl sublattices, leading to materials
with small band gaps, large exciton binding energies, and absorption
spectra that are strongly influenced by the identity of the transition
metal. The role of the double-perovskite structure in enabling these
unique properties is probed through an analysis of the electronic
structures and chemical bonding of these compounds in comparison with
other transition-metal and alkali transition-metal halides.