Atomistic Simulations of Methylammonium Lead Halide Layers on PbTiO₃ (001) Surfaces

The substantial increase in the power conversion efficiency of hybrid perovskite solar cells, to date reaching more than 20% in the laboratory, has strongly motivated research on this class of organic-inorganic materials and related devices, particularly based on CH₃NH₃PbI_3-xX_x/TiO₂ heterostructures (X = Cl,Br). Taking under consideration that a ferroelectric substrate may act as an efficient electron transporter, positively influencing charge collection across the interface and allowing the tuning of the halide perovskite (HP) - ferroelectric junction, we performed extensive density functional theory calculations on CH₃NH₃PbI_3-xCl_x layers deposited on tetragonal PbTiO₃ (PTO) (001) surfaces, to study their structural and electronic properties. The main findings of this study are as follows. (i) A ferroelectric polarization pointing from the PTO/HP interface to the PTO is favorable for the photogenerated electrons transfer across the interface and their transport to the collecting electrode. (ii) The PTO internal electric field leads to a position dependent energy levels diagram. (iii) The HP gap may be tuned by chlorine concentration at the interface, as well as the by the surface terminations of PbTiO₃ and hybrid perovskite layers. (iv) The presence of the PTO ferroelectric surface is likely to have just a slight orientational effect on the (CH₃NH₃)⁺ dipoles. (Graph Presented).