Pb clustering and PbI2 nanofragmentation during methylammonium lead iodide perovskite degradation

Abstract

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.

Methylammonium lead halide perovskites have great potential for optoelectronic applications but are prone to degradation. Here the authors show that degradation can occur through clustering of Pb containing defects at the grain boundaries while concurrent formation of PbI₂ blocks further reactions, suggesting a strategy for passivation.