The characteristics of band structures and crystal binding in all-inorganic perovskite APbBr3 studied by the first principle calculations using the Density Functional Theory (DFT) method

All-inorganic lead halide perovskites have been attracting much attention for various optoelectronic applications such as solar cells and light-emitting diodes. In this paper, we report the results of studies on the calculations of the electronic structures of APbBr(3) (where A is Li, Na, K, Rb, and Cs) by using the Density Functional Theory (DFT) method. These studies were undertaken for understanding the role of the A cation in their band structure and band gap energy characteristics. The calculations were initiated by finding the optimum lattice constants and the lowest total energy of these APbBr(3) crystals. Ultrasoft pseudopotentials with GGA-PBE and LDA-PZ exchange-correlation functions have been implemented in these calculations. The computation results show that the energy gaps are not significantly affected by the variation of APbBr(3), which are in the range of 1.708-1.769 eV for GGA-PBE pseudo-potentials and 1.148-1.237 eV for LDA-PZ pseudo-potentials. In addition, the calculated Density of States (DOS) indicate that the valence band is predominantly constructed by the Br- anions, while the conduction band is predominantly constructed by the Pb2+ and A(+) cations. However, except for LiPbBr3 and NaPbBr3 perovskites, the A(+) cations do not contribute significantly to the valence band and the lowest level of the conduction band. Moreover, the charge density distributions and Bader analysis reveal that the bonding between the A(+) cation and Br- anion has a stronger ionic character than the bonding between Pb2+ cation and Br anion, which shows partial ionic and covalent bonds character. Therefore, while their bandgap energies are merely determined by the PbBr6 octahedral structure as reported elsewhere, the present results may then also imply that the A(+) cations are not involved significantly in the interband photoexcitation, photovoltaic and charge transport processes in these perovskites, with an exception for the LiPbBr3 and NaPbBr3 perovskites.