Defect Energetics in Pseudo-Cubic Mixed Halide Lead Perovskites from First-Principles

Owing to the increasing popularity of lead-based hybrid perovskites for photovoltaic (PV) applications, it is crucial to understand their defect energetics and its influence on their optoelectronic properties. In this work, we simulate various point defects in pseudocubic structures of mixed iodide-bromide and bromide-chloride methylammonium lead perovskites with the general formula MAPbI(3-y)Br(y) or MAPbBr(3-y)Cl(y) (where y is between 0 and 3), and use first-principles based density functional theory computations to study their relative formation energies and charge transition levels. We identify vacancy defects and Pb on MA antisite defect as the lowest energy native defects in each perovskite. We observe that while the low energy defects in all MAPbI(3-y)Br(y) systems only create shallow transition levels, the Br or Cl vacancy defects in the Cl-containing pervoskites have low energy and form deep levels which become deeper for higher Cl content. We examine the structures and density of states of pure and defect-containing perovskite systems to obtain an understanding of the nature of defect levels. Further, we study extrinsic substitution by different elements at the Pb site in MAPbBr(3), MAPbCl(3), and the 50-50 mixed halide perovskite, MAPbBr(1.5)Cl(1.5), and identify some transition metals that create lower energy defects than the dominant intrinsic defects and also create midgap charge transition levels.