Ab Initio Design of Low Band Gap 2D Tin Organohalide Perovskites

Four
layered hybrid perovskites, based on tin iodide sheets intercalated
by divalent organic cations (ethylenediammonium, 2,2′-biaziridinium,
2,2′-biimidazolium, and 4,4′-bipyridinium), have been
modeled with ab initio techniques. The crystal structures have been
optimized at the DFT level, not including thermal effects, and finding
and characterizing three minima for each cation; with respect to the
analogues with monovalent cations, the structures are more distorted
and mostly in a staggered arrangement. The interlayer distances are
quite small for all of the systems, due to the single layer of strongly
charged cations between the inorganic sheets. The band profiles and
the band gaps, computed with an additive approach including the effects
of spin orbit coupling and post-DFT correlation corrections, show
an unexpected and interesting feature: with two of the cations some
nearly degenerate low energy levels appear at the bottom of the conduction
band. As a consequence, these systems present unusually low band gaps
(the minimum value being 1.34 eV), suggesting the possibility of light
adsorption in the visible or near-IR regions. The existence of these
low-lying levels has been correlated to the charge and the aromatic
nature of the organic ions, and a simple molecular descriptor, based
on the LUMO energy of the isolated cations, is proposed to design
other tin iodide perovskites with this characteristic.