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.