Grain Boundaries in Methylammonium Lead Halide Perovskites Facilitate Water Diffusion

Organic-inorganic hybrid perovskites such as methylammonium lead iodide (MAPbI(3)) materials have recently attracted great attention due to their potential for photovoltaic applications. The performance and stability of these perovskite solar cells are sensitive to water and moisture in an ambient environment. Thus, an understanding of how water influences MAPbI(3) and particularly the role of grain boundary (GB) defects is important for developing appropriate mitigation strategies. Herein, water molecular diffusion in Sigma 5-(210) GB is investigated and compared with pristine MAPbI(3) using first-principles calculations. Water diffusion along the Sigma 5-(210) GB is found to be facile with a 0.07eV energy barrier while diffusion barrier from GB core to bulk is 0.24eV. In contrast, the diffusion process in bulk MAPbI(3) is relatively large 0.70eV due to water interactions with the Pb-I network. Further, it is shown that water is more stable in the GB region compared with the pristine system. Thus, the strong thermodynamic and kinetic tendencies for water segregation to the GBs in MAPbI(3) suggests that improving the crystallinity of MAPbI(3) is an effective strategy to slow down water degradation processes in agreement with recent experimental results.