Effects of Moisture-Based Grain Boundary Passivation on Cell Performance and Ionic Migration in Organic-Inorganic Halide Perovskite Solar Cells.

Because of the polycrystalline nature, grain boundaries (GBs) in hybrid perovskite thin films play critical roles in determining the charge collection efficiency of perovskite solar cells (PSCs), material stability, and in particular the ion migration, considering their relatively soft ionic bonds with low formation energy. Different GB passivation methods are being studied, and introducing PbI2-rich phase at GBs in methylammonium lead iodide (MAPbI3) has been found to be useful. In this study, combining macroscale measurements with tip-based microscopic probing that includes scanning Kelvin probe microscopy for surface potential mapping and conductive atomic force microscopy for charge transport mapping, we revealed the effects of PbI2-rich phase at GBs, which was introduced in moisture-assisted synthesis of MAPbI3 thin films. It was found that PbI2 passivation of GBs could change the surface potential and charge carrier screening and significantly retard current conduction at the GB while enhancing conduction through the grain interior. Inhibition of ion migration at GBs, as well as enhanced PSC device performance, is reported.

Ionic and Optical Properties of Methylammonium Lead Iodide Perovskite across the Tetragonal-Cubic Structural Phase Transition.

Practical hybrid perovskite solar cells (PSCs) must endure temperatures above the tetragonal-cubic structural phase transition of methylammonium lead iodide (MAPbI3 ). However, the ionic and optical properties of MAPbI3 in such a temperature range, and particularly, dramatic changes in these properties resulting from a structural phase transition, are not well studied. Herein, we report a striking contrast at approximately 45 °C in the ionic/electrical properties of MAPbI3 owing to a change of the ion activation energy from 0.7 to 0.5 eV, whereas the optical properties exhibit no particular transition except for the steady increase of the bandgap with temperature. These observations can be explained by the "continuous" nature of perovskite phase transition. We speculate that the critical temperature at which the ionic/electrical properties change, although related to crystal symmetry variation, is not necessarily the same temperature as when tetragonal-cubic structural phase transition occurs.