First-principles investigations of structural stability and electronic band structure of CH3NH2BiI3 for lead-free perovskite solar cell application

In this work, first principles density functional theory (DFT) was used to investigate the structural stability and electronic structures of CH3NH2BiI3 lead-free perovskite. From the results, CH3NH2BiI3 perovskite was predicted to be stable in monoclinic phase (space group P21) with lattice parameters, a = 8.165 A, b = 13.194 A, c = 8.272 A, and fi = 90.03. The formation enthalpy per formula unit (4H) of CH3NH2BiI3 was found to be 0.13 eV lower than the total ZIH of CH3NH2 molecule and bulk BiI3, indicating its stability with respect to CH3NH2 and BiI3. In addition, the chemical potential diagram shows the stable region of CH3NH2BiI3, indicating that CH3NH2BiI3 perovskite can be synthesized. From band structure calculations, CH3NH2BiI3 has an indirect band gap of 1.58 eV which is comparable to 1.60 eV of CH3NH3PbI3. However, the valence band maximum (VBM) was found to be mainly contributed by N 2p and I 5p, instead of the expected Bi 6s. It is relatively flat compared to the VBM of CH3NH3PbI3, and thus has a larger hole effective mass. However, this theoretical prediction on monoclinic CH3NH2BiI3 with enhanced structural stability, synthesizability, and small band gap suggests its capability to be a promising candidate in substituting the leadbased perovskite solar cells.