Orthorhombic lead-free hybrid perovskite CH3NH3SnI3 under strain: an ab initio study

Abstract

We report a computational study where we explore the possibility of tuning the electronic properties of orthorhombic methylammonium tin iodide CH₃NH₃SnI₃ using strains. According to our findings, a moderate [001] strain, smaller than 2%, would open the band gap up to 1.25 eV and enhance the exciton binding energy, opening up new possibilities for the use of CH₃NH₃SnI₃ in technological applications. To better understand the impact of strain, we also examined its influence on bonding properties. The results reveal that the directional pnictogen and the hydrogen bonding are not altered by strains and that the tuning of the electronic properties is the result of changes induced in the orbital contributions to states near the Fermi level and the tilting of the SnI₆ octahedral units.

Fig. 1. (Color online) Phonon dispersion of orthorhombic CH₃NH₃SnI₃. The plots (a) and (b) represent a zoom for frequencies from 0 to 200 cm⁻¹ and the full phonon spectra, respectively. Atoms whose vibrations are associated with the different modes are indicated in (b).

Fig. 2. (a) ELF plot (isosurface = 0.85), (b) 3D isosurface of the reduced density gradient of CH₃NH₃SnI₃ (s = 0.3). Strong attractive interactions are depicted as localized blue lentils, repulsive interactions as red isosurfaces, and van der Waals interactions as thin, delocalized green regions. In the iodine-cation bonding, the organic interactions are shown in red, and the non-covalent interactions are circled in blue. Here, the ELF topology gives a partition into localized electronic domains known as basins. They are used to rationalize the bonding schemes. The synaptic order of a valence ELF basin is determined by the number of core basins with which it shares a common boundary. Basins' spatial locations are very close to the valence shell electron pair repulsion domains.

Fig. 3. (Color online) Electronic band structure of CH₃NH₃SnI₃ calculated with (a) at mBJ-GGA and (b) PBE-GGA.

Fig. 4. Partial electronic density of states of un-strained CH₃NH₃SnI₃. The PDOS provides valuable insights into the contributions of specific atoms and orbitals to the electronic structure of the investigated structure.

Fig. 5. The evolution of band gap and binding energy as a function of strain. In this plot, the Wannier-Mott Model is used to calculate the exciton binding energy E_b. The exciton energy (E_exciton) can be approximated by: E_exciton = E_g − E_b.

Fig. 6. Reduced density gradient S(r) vs. sign(λ₂)ρ(r) for the (a) un-strained and (b) strained structure (ε = +2%) of CH₃NH₃SnI₃.