LAUSR

A myriad of studies and strategies have already been devoted to improving the stability of perovskite films; however, the role of the different perovskite crystal facets in stability is still unknown. Here, we reveal the underlying mechanisms of facet-dependent degradation of formamidinium lead iodide (FAPbI3) films. We show that the (100) facet is substantially more vulnerable to moisture-induced degradation than the (111) facet. With combined experimental and theoretical studies, the degradation mechanisms are revealed; a strong water adhesion following an elongated lead-iodine (Pb-I) bond distance is observed, which leads to a δ-phase transition on the (100) facet. Through engineering, a higher surface fraction of the (111) facet can be achieved, and the (111)-dominated crystalline FAPbI3 films show exceptional stability against moisture. Our findings elucidate unknown facet-dependent degradation mechanisms and kinetics. Description Facet-stabilized films The degradation of formamidinium–based lead iodide perovskite has been shown to depend on which crystal facets are exposed to the surface. C. Ma et al. found that water adhesion was stronger on the (100) facet than on the (111) facet and made these materials more prone to moisture-induced degradation. The authors show how ligand-assisted perovskite film growth could be used create (111)-dominated films with high stability against moisture (up to 85% relative humidity) and thermal stress (85°C) without additional surface passivation. —PDS Formamidinium–based lead iodide perovskite films can be stabilized by growing films with a greater fraction of (111) facets.