Understanding the Influence of Microstructure on Dominant Ionic Transport Pathways in Lead Halide Perovskites

Ionic Migration is a leading cause of early device failure in Lead Halide Perovskite (LHP) photovoltaics. Existing studies have proven that phase orientation, light, and voltage correlate strongly with ionic diffusion through the crystalline lattice and transformations into photo-inactive phases. However, the dominant modes of ionic migration at the microscale and their relation to microstructure and morphology are a continual source of debate and controversy in the field. In this work, non-destructive atomic force microscopy (AFM) techniques image the fine sub-grain and grain-boundary dependent ionic relaxation behavior to inform better solution processing strategies. We use electrochemical strain microscopy and conductive AFM to qualitatively identify ionic relaxation on the order of seconds in grains and grain boundaries through unsupervised machine learning analysis, decreasing bias when visualizing spatial relaxation behavior. Importantly, we find that by controlling the precursor chemistry via solvent composition, the microstructure and morphology of the film influence the bulk and grain boundary ionic diffusivity. Intensity-modulated photocurrent spectroscopy (IMPS) complements micro-scale insights and informs future device manufacturing protocols to minimize ionic transport at a fixed composition while maximizing carrier extraction. This multi-scale study gives further insight into how precursor chemistry and processing may be leveraged to improve device stability in addition to existing chemical passivation strategies.