Effect of MA Orientation and Thickness on the Bandgap in Quasi-2D Perovskites (PEA)2(MA)n-1Pb n I3n+1

Three-dimensional organic-inorganic perovskite photovoltaic materials such as MAPbI(3) (MA is methylammonium) have rapidly advanced in their performance over the past decade, but their instability remains a problem. One of the methods to improve their stability is by reducing their dimensionality. This dimensional reduction is performed by adding a large organic cation as a spacer to separate the original bulk lattice, forming a new quasi-two-dimensional (quasi-2D) structure. In this study, we have investigated the electronic properties of the quasi-2D structures of (PEA)(2)(MA)(n -1)Pb(n)I3(n+1) (PEA is phenylethylammonium) with various thicknesses (n) and MA orientations using first-principles calculations. Our results show that the bandgap decreased as the number of layers (n) increased for quasi-2D structures. The structures with in-plane MA orientations and mirror symmetry along the z-axis are found to be the most stable and exhibit a converging trend in the bulk bandgap when n > 6. Our calculations reveal how the orientations of the MA ions affect the electronic structure via symmetry breaking and electric dipole formation. These two factors change the electrostatic potential and band energies of the individual PbI3 layers, which lead to enhanced band splitting and bandgap reduction. The results of this work would give some insights into the electronic structures and may provide directions for the crystal growth design of quasi-2D perovskite materials in photovoltaic applications.