Performance-limiting metastable intragrain impurity nanoclusters in metal halide perovskites

Impurity control in metal halide perovskite thin films is crucial for advancing the efficiency and long-term stability of perovskite solar cells. While previous efforts have mainly addressed impurities at grain boundaries and heterointerfaces, efforts have rarely been made to reveal and modulate (sub-)nanoscale impurities within grains. In this work, using low-dose scanning transmission electron microscopy, we observed the nontrivial existence of intragrain impurity nanoclusters, identifying a previously unknown, metastable orthorhombic non-perovskite phase with a parallel chain-like structure, in nominally processed Cs-incorporated mixed-cation perovskite films. First-principles calculations and quasi-in situ observations reveal that these intragrain nanoclusters adversely affect the optoelectronic properties and chemical stability of the perovskite. Guided by these insights, we incorporate inner salts during the solution processing, effectively reducing nanocluster density and enhancing device performance. Clarifying unconventional impurities hidden in perovskite crystal grains opens a research avenue in fundamental understanding and nano-engineering for improved perovskite solar cells and optoelectronics.