Enhancing Ferroelectric Dipole Ordering in the Organic-Inorganic Hybrid Perovskite CH₃NH₃PbI₃: Strain and Doping Engineering

Hybrid halide perovskites have shown astonishing power conversion efficiencies (PCE) due to their superb optoelectronic properties. However, the fundamental reasons for their high efficiencies compared to their inorganic counterparts are unclear. There has been great controversy regarding one of the most possible reasons, the ability of organic cations to form ferroelectric (FE) domains in hybrid perovskites. Here we study FE dipole ordering of methylammonium (CH₃NH₃⁺, i.e., MA) cations in MAPbI₃. The results indicate that FE ordering exists in tetragonal MAPbI₃. We further explored two approaches to enhancing the FE dipole ordering: (1) application of bi-axial compressive strain (strain engineering), and (2) substitutional doping with smaller halogen anions (doping engineering). These two approaches are based on an increase of the unit cell aspect ratio c/a. We propose the strain and doping engineering as practical means to effectively improve PCE of hybrid-perovskite-based solar cells.