Computations of the band structure and linear optical properties of methylammonium bismuth bromide and methylammonium galluim bromide using FHI-aims Code

Abstract

Ab initio calculations for the Linear Macroscopic Dielectric analyses of CH₃NH₃BiBr₃ and CH₃NH₃GaBr₃ (having 8.33% dopant replacement percentage each) as possible replacements for the Lead based perovskite CH₃NH₃PbBr3 were done in this work using Perdew–Burke–Ernzerhof functional of Density Functional Theory as implemented by FHI-aims Code. Optimized lattice constants were calculated for CH3NH3BiBr3 and CH₃NH₃GaBr₃ to be 9.10 Å and 8.27 Å respectively. The Lowest Unoccupied Molecular Orbital (LUMO), Highest Occupied Molecular Orbital (HOMO) and calculated band gap, Band Structure and Density of States (DOS) plots were made to analyse the band structures of these crystals and they were found to be metallic with a band gap of 0.00018636 eV and 0.00022286 eV for CH₃NH₃BiBr₃ and CH₃NH₃GaBr₃ respectively. The imaginary and real parts of the inter-band and intra-band contribution to the linear dielectric tensor within the e imagi Random Phase Approximation (RPA) of the optical properties of these perovskites were calculated and an average dielectric tensor of 3.1209467 and 2.789173 was found for CH₃NH₃BiBr₃ and CH₃NH₃GaBr₃ respectively. From the absorption data and dielectric tensor components calculated, both materials have an average absorption peak at frequency of 16.5373nm at 1.006567 MeV, 16.2040nm at 1.227600 MeV respectively, across all cubic planes.