Origin of the stability of two-dimensional perovskites: a first-principles study

Abstract

It usually costs energy to create a surface, so to reduce the instability of a material, it is often desirable to reduce the surface area. However, it has been reported recently that the instability of lead halide perovskites, which are promising materials for high efficiency and low cost solar cells, can be improved by reducing the dimensions of the perovskites from the three-dimensional (3D) bulk to that of two-dimensional (2D) films. So far, there is a lack of general understanding about this unusual behavior, i.e., why a 2D slab cut out of the 3D bulk material could be more stable than the 3D material. Here, we carry out a systematic and comparative study on the thermodynamic stability of 2D and 3D perovskites using the first-principles method based on density functional theory. Taking CsPbBr₃ and MAPbI₃ as prototype examples, we show that the thermodynamic stability of the 2D perovskite originates from the asymmetric surface properties of this material: the AX-terminated 2D APbX₃ are more stable than the PbX₂-terminated 2D counterpart due to fewer surface dangling bonds. The MAI-terminated 2D MAPbI₃ becomes even more stable than its 3D counterpart due to relaxation of the hydrogen bonds at the surface. Our results thus provide deep insights into the stability of 2D slabs versus 3D host materials, which shall be useful for improving the stability of perovskites and designing new 2D materials.