Ab Initio Calculations of Band Gaps and Absolute Band Positions of Polymorphs of RbPbI₃ and CsPbI₃: Implications for Main-Group Halide Perovskite Photovoltaics

Abstract

Lead halide perovskites have attracted great interest because of rapid improvements in the efficiency of photovoltaics based on these materials. To predict new related functional materials, a good understanding of the correlations between crystal chemistry, electronic structure, and optoelectronic properties is required. Describing the electronic structure of these materials using density functional theory provides a choice of exchange-correlation functionals, including hybrid functionals, and inclusion of spin-orbit coupling, which is critical for the correct description of band gap and absolute band positions (ionization energy). Here, various computational schemes that employ different choices of exchange-correlation and hybrid functionals, and include or exclude spin-orbit coupling were implemented to examine these effects. Using PbI2 as an initial structural model, it is found that standard exchange correlation functionals (PBE) in conjunction with spin-orbit coupling suffice to locate ionization energies efficiently through the use of slab calculations. Band gaps require the use of hybrid functionals carried out on single unit cells and spin-orbit coupling. Polymorphs of alkali metal lead halides, APbI3 (A = Rb, Cs) are examined in the cubic perovskite structure and the reduced dimensional NH4CdCl3/Sn2S3 structure with quasi-two-dimensional connectivity. The somewhat elevated Born effective charges computed for these structures suggest that while the Pb2+ 6s lone-pairs are stereochemically inert, the presence of proximal instabilities could have implications for the functional properties of these materials.