First-Principle Study on Lead-Free Perovskite for Optoelectrical Applications

Abstract

The research into new types of light harvesters for solar cells is driven by the need to increase their efficiency and make them more reliable. One promising material for replacing the dye-molecule light harvesters is the Organometallic perovskite; the most popular among them is methyl ammonium lead iodide, CH₃NH₃PbI₃. Although methyl ammonium lead iodide, (CH₃NH₃PbI₃) has proven to be an effective photovoltaic material, there remains a huge concern about the toxicity of lead.  An investigation into the possible replacement of lead (Pb) with Germanium (Ge), Silicon (Si), and Tin (Sn) in CH₃NH₃PbI₃ was carried out. Before this investigation, structure building, parameter optimization, determination of the best exchange functional, k-grid convergence test, and determination of equilibrium lattice constant and geometry relaxation were carried out for the first set of materials. Visualization for Electronic and Structural Analysis (VESTA) and Avogadro software were used for the structure building while FHI-aims code was used to simulate these Perovskites materials. The BLYP (a parameterization of GGA) exchange functional gave the minimum single-point energy at a minimum run time for all the structures. The lattice constants obtained using Phonopy (with zero-point energy) are 5.894, 5.907, 6.248, 5.950, and 6.049 Å for CH₃NH₃GeBr₃, CH₃NH₃GeI₃, CH₃NH₃PbI₃, CH₃NH₃SiI₃ and CH₃NH₃SnI₃ respectively. The energy band gap calculated for the second set of materials: CH₃NH₃GeI₃, CH₃NH₃PbI₃, CH₃NH₃SiI₃, CH₃NH₃SnI₃, and CH₃NH₃GeBr₃ at their respective equilibrium lattice constants are 1.606, 1.513, 1.804, 1.051 and 1.925 eV respectively. These calculated band gap values were compared with reported theoretical and experimental values. There is a close agreement in calculated lattice constants and bandgaps with reported theoretical and experimental values. Dielectric constants, refractive index extinction coefficient, absorption coefficient, reflectivity, and optical conductivity of these materials were also determined. The optical properties obtained show that Sn and Ge are a good choice for the replacement of Pb; also, the optical properties obtained indicate that these materials have other possible applications in areas other than photovoltaic technology.