First-principles Investigation of The Role of Organic Molecules Inside The α-phase of Hybrid Halide Perovskite CH₃NH₃BX₃ (B= Pb, Sn; X= I, Br, Cl)

The hybrid halide perovskite materials, have attracted significant research attention because of their potential application in low cost highly efficient photovoltaics. While the device efficiencies have rapidly improved, the microscopic origin behind the success of these materials is still unclear. Especially, the presence of mobile cations in the CH₃NH₃BX₃, (MABX₃) materials is expected to play an important role in their fundamental properties. Here we performed a van der Waals-corrected density functional theory investigation of the MA⁺ cation structural and electronic properties in the α-phase of CH₃NH₃BX₃ with B=Pb and Sn. Our results reveal that the MA⁺ cation orientation influences the structural properties of the inorganic network and consequently effects the electronic properties of the materials. For example, if the MA⁺ cation is oriented along a (101)-like direction, the BX₆ octahedral cage will be distorted and the band gap becomes indirect.