  Owing to the increasing popularity of lead-based hybrid perovskites for
photovoltaic (PV) applications, it is crucial to understand their defect
physics and its influence on their optoelectronic properties. In this work, we
simulate various point defects in pseudo-cubic 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 anti-site 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. 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 mid-gap charge transition levels.
