The effect of moisture on the structures and properties of lead halide perovskites: a first-principles theoretical investigation

With efficiencies exceeding 20% and low production costs, lead halide perovskite solar cells (PSCs) have become potential candidates for future commercial applications. However, there are serious concerns about their long-term stability and environmental friendliness, heavily related to their commercial viability. Herein, we present a theoretical investigation based on the ab initio molecular dynamics (AIMD) simulations and the first-principles density functional theory (DFT) calculations to investigate the effects of sunlight and moisture on the structures and properties of MAPbI 3 perovskites. AIMD simulations have been performed to simulate the impact of a few water molecules on the structures of MAPbI 3 surfaces terminated in three different ways. The evolution of geometric and electronic structures as well as the absorption spectra has been shown. It is found that the PbI 2 -terminated surface is the most stable while both the MAI-terminated and PbI 2 -defective surfaces undergo structural reconstruction, leading to the formation of hydrated compounds in a humid environment. The moisture-induced weakening of photoabsorption is closely related to the formation of hydrated species, and the hydrated crystals MAPbI 3 ·H 2 O and MA 4 PbI 6 ·2H 2 O scarcely absorb the visible light. The electronic excitation in the bare and water-absorbed MAPbI 3 nanoparticles tends to weaken Pb-I bonds, especially those around water molecules, and the maximal decrease of photoexcitation-induced bond order can reach up to 20% in the excited state in which the water molecules are involved in the electronic excitation, indicating the accelerated decomposition of perovskites in the presence of sunlight and moisture. This work is valuable for understanding the mechanism of chemical or photochemical instability of MAPbI 3 perovskites in the presence of moisture. The degradation mechanism of perovskite materials when exposed to moisture and sunlight has been fully explored.