Tailoring properties of hybrid perovskites by domain-width engineering with charged walls

Charged ferroelectric domain walls are fascinating electrical topological defects that can exhibit unusual properties.Here,in the search for novel phenomena,we perform and analyze first-principles calculations to investigate the effect of domain width on properties of domains with charged walls in the photovoltaic material consisting of methylammonium lead iodide hybrid perovskite.We report that such domains are stable and have rather low domain wall energy for any investigated width(that is,up to 13 lattice constants).Increasing the domain width first linearly decreases the electronic band gap from≃1.4 eV to about zero(which therefore provides an efficient band-gap engineering),before the system undergoes an insulator-to-metal transition and then remains metallic(with both the tail-to-tail and head-to-head domain walls being conductive)for the largest widths.All these results can be understood in terms of:(i)components of polarization along the normal of the domain walls being small in magnitude;(ii)an internal electric field that is basically independent of the domain width;and(iii)rather negligible charge transfer between walls.These findings deepen the knowledge of charged ferroelectric domain walls and can further broaden their potential for applications,particularly in the context of halide perovskites for photovoltaics.