  Ruddlesden-Popper hybrid iodide 2D perovskites are put forward as stable
alternatives to their 3D counterparts. Using first-principles calculations, we
demonstrate that equilibrium concentrations of point defects in the 2D
perovskites PEA$_2$PbI$_4$, BA$_2$PbI$_4$, and PEA$_2$SnI$_4$ (PEA: phenethyl
ammonium, BA: butylammonium), are much lower than in comparable 3D perovskites.
Bonding disruptions by defects are more detrimental in 2D than in 3D networks,
making defect formation energetically more costly. The stability of 2D Sn
iodide perovskites can be further enhanced by alloying with Pb. Should,
however, point defects emerge in sizable concentrations as a result of
nonequilibrium growth conditions, for instance, then those defects hamper the
optoelectronic performance of the 2D perovskites, as they introduce deep traps.
We suggest that trap levels are responsible for the broad sub-bandgap emission
in 2D perovskites observed in experiments.
