Role of Ligand-Ligand Interactions in the Stabilization of Thin Layers of Tin Bromide Perovskite: An Ab Initio Study of the Atomic and Electronic Structure, and Optical Properties

We report results of the state-of-the-art ab initio calculations on two-dimensional (2D) hybrid halide perovskites capped with surface ligands to understand their effects on the stability with respect to bulk MASnBr(3) (MA = CH3NH3). Considering the thinnest (one-unit-cell thick) layers R2SnBr4 with surface ligands of different lengths (R = MA, ethyl ammonium (EA), butyl ammonium (BA), and phenylethyl ammonium (PEA)), it is found that van der Waals (vdW) interactions between the long chain molecules play a crucial role in enhancing the stability of the layers. The vdW contribution in ligand-ligand interactions increases with increasing length of the ligands, and interestingly, the stability of BA(2)SnBr(4) and PEA(2)SnBr(4) layers becomes better than bulk MASnBr(3) and comparable to that of inorganic bulk CsSnBr3. Furthermore, our calculations on the 2D-3D BA(2)SnBr(4) system in which the surface ligands connect the neighboring perovskite layers suggest further enhancement in the stability of the layers. The present study shines light on the role of H center dot center dot center dot Br bonding in deciding the structure of the inorganic part of these thinnest layers and the effect of inclusion of vdW interactions on these H center dot center dot center dot Br bond lengths. The band gap of the layers increases slightly on increasing the length of the ligands, and there is a slight blue shift of the absorption spectrum. All the studied perovskite layers are direct band gap semiconductors, and our results show that the environmentally friendly BA2SnBr4 (PEA(2)SnBr(4)) layers are good candidates for green LEDs with a band gap of similar to 2.28 (2.36) eV as obtained by using the HSE06 hybrid exchange-correlation functional with dispersion correction.