Unraveling the Stable Phase, High Absorption Coefficient, Optical and Mechanical Properties of Hybrid Perovskite CH3NH3PbxMg1-xI3: Density Functional Approach

The quest for materials with optimum photovoltaic conversion efficiency leads to the discovery of CH3NH3PbI3, which has shown exciting improvement in power conversion efficiency over the silicon-based technology. This research uses PBEsol-KJPAW and PBE-NC functionals as implemented in Density Functional Theory to study the effects of Mg doping in CH3NH3PbxMg1-xI3 solar cell absorber for x = 0, 0.25, 0.50, 0.75, 1. The Absorption spectrum is computed with Time-Dependent Density Functional Perturbation Theory while the optical gap is calculated by fitting the absorption edge with the Tauc direct and indirect allowed transition. The ground state energy and lattice parameters of CH3NH3PbxMg1-xI3 in its cubic, tetragonal and orthorhombic crystal structures were calculated to determine its equilibrium structure, which appears to be simple cubic for all the compounds of interest. Electronic band gaps were calculated to be in the range 1.16-2.12 eV, suggesting an excellent power conversion efficiency according to the Shockley-Queisser efficiency limit, but the optical gaps extrapolated from the absorption spectrum is larger than the calculated DFT band gap, as a result of underestimation of calculated DFT band gap in Mg-doped CH3NH3PbI3. CH3NH3PbxMg1-xI3 is found to be brittle, moderately incompressible with Vickers hardness in the range of 6.776-9.153 GPa. The brittleness, incompressibility, and hardness all increase with Mg concentration. The amount of solar radiation absorbed and the percentage reflectivity depends on the concentration of Pb while transmitted solar irradiation increase with Mg concentration, these suggest that MAPbI(3) may have better power conversion efficiency compared to Mg-doped MAPbI(3), but Mg doping will reduce the toxic effect of Pb and produce much more power conversion efficiency than the silicon-based solar cell.