  Two-dimensional (2D) halide perovskites are emerging as promising candidates
for nano-electronics and optoelectronics. To realize their full potential, it
is important to understand the role of those defects that can strongly impact
material properties. In contrast to other popular 2D semiconductors (e.g.
transition metal dichalcogenides MX2) for which defects typically induce
harmful traps, we show that the electronic activities of defects in 2D
perovskites are significantly tunable. For example, even with a fixed lattice
orientation, one can change the synthesis conditions to convert a line defect
(edge or grain boundary) from electron acceptor to inactive site without deep
gap states. We show that this difference originates from the enhanced ionic
bonding in these perovskites compared with MX2. The donors tend to have high
formation energies, and the harmful defects are difficult to form at a low
halide chemical potential. Thus we unveil unique properties of defects in 2D
perovskites and suggest practical routes to improve them.
