Interface modelling for (CH3)3SPbI3 and (NH2)2CHPbI3 perovskite layers

Abstract

One of the main ways to improve the stability of perovskite solar cells is to incorporate hydrophobic layers that protect the perovskite layer from hydrolysis. We report here a theoretical study on the interface between the compounds (CH3)3SPbI3 and (NH2)2CHPbI3, both in the hexagonal 1D configuration with face-sharing [PbI6]4- octahedra. In the case of the planar formamidinium cation, significant freedom of rotation is observed within the inorganic framework; a question is then raised, about the possibility of cooperativity between inorganic framework and organic cations leading to translational motion of the latter. To explore this possibility, theoretical calculations were performed with Molecular Mechanics and Molecular Dynamics methods, based on an empirical force field that was adjusted so that it preserved the experimentally determined densities and geometries. A sample consisting of a (CH3)3SPbI3 phase adjacent to a (NH2)2CHPbI3 one (both are perovskite-related 1D structures) under periodic boundary conditions, was subjected to molecular dynamics simulations extending over several nanoseconds, for the temperature range 80–140 °C, namely exceeding the usual operating conditions of perovskite solar cells, to enhance the likelihood of diffusive motion. Even under such favorable conditions, our results indicate prohibitively slow kinetics of interdiffusion between the two layers of materials.