Pb clustering and PbI₂ nanofragmentation during methylammonium lead iodide perovskite degradation

Studying defect formation and evolution in MethylAmmonium lead Iodide (MAPbI₃) perovskite layers has a bottleneck in the softness of the matter and in its consequent sensitivity to external solicitations. Here we report that, in a polycrystalline MAPbI₃ layer, Pb-related defects aggregate into nanoclusters preferentially at the triple grain boundaries as unveiled by Transmission Electron Microscopy (TEM) analyses at low total electron dose. Pb-clusters are killer against MAPbI₃ integrity since they progressively feed up the hosting matrix. This progression is limited by the concomitant but slower transformation of the MAPbI₃ core to fragmented and interconnected nano-grains of 6H-PbI₂ that are structurally linked to the mother grain as in strain-relaxed heteroepitaxial coupling. The phenomenon occurs more frequently under TEM degradation whilst air degradation is more prone to leave uncorrelated [001]-oriented 2H-PbI₂ grains as statistically found by X-Ray Diffraction. This path is kinetically costlier but thermodynamically favoured and is easily activated by catalytic species.