Defects in Halide Perovskites: Does It Help to Switch from 3D to 2D?

TL;DR

This study uses first-principles calculations to compare defect concentrations in 2D and 3D halide perovskites, finding 2D variants generally have fewer defects but are more sensitive to defect-induced traps affecting performance.

Contribution

It provides a detailed computational analysis showing lower defect levels in 2D perovskites and discusses how defects impact their optoelectronic properties, highlighting the effects of alloying.

Findings

2D perovskites have lower equilibrium defect concentrations than 3D.

Defects cause more bonding disruptions in 2D than in 3D networks.

Deep trap states from defects explain broad sub-bandgap emission.

Abstract

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.