First-Principles Study of Anionic Diffusion in Two-Dimensional Lead Halide Perovskite Lateral Heterostructures.

Perovskite heterostructures have attracted wide interest for their photovoltaic and optoelectronic applications. The interdiffusion of halide anions leads to the poor stability and shorter lifetime of the halide perovskite heterostructures. Covering organic cations on the surface of perovskite heterostructures, the diffusion of ions can effectively be suppressed. However, the migration mechanism on two-dimensional lead halide perovskite lateral heterostructures under different organic cations remains inadequately explored. In this work, we performed first-principles calculations on the ion migration in two-dimensional (2D) lead halide perovskite lateral heterostructures with different interface defects and different cations. We found that the migration of iodine atoms across the interface in the heterostructures is more preferable than that of bromine atoms, regardless of the cations. Meanwhile, the migration of iodine atoms from the in-plane to the out-plane direction has the lowest energy barrier compared to other directions. Our calculations also reveal that both the type of cation and the migration path selected affect the energy barrier for anion migration, exhibiting either inhibitory or promoting effects. Specifically, the organic cation 345FAn, an ammonium ligand, showed an excellent promoting effect on the anion migration, while the BA cation exhibited an inhibiting effect. The calculated interdiffusion rate includes the interfacial single bromine vacancy, which is consistent with previous experimental observations. However, the heterostructures with interfacial single iodine defects exhibit a higher interdiffusion rate. Our findings on the ion migration mechanism in lead halide perovskite lateral heterostructures contribute to both experimental discussions and theoretical insights.