  Organic-inorganic hybrid perovskites (OIHPs) exhibiting exceptional
photovoltaic and optoelectronic properties are of fundamental and practical
interest, owing to their tunability and low manufacturing cost. For practical
applications, however, challenges such as material instability and the
photocurrent hysteresis occurring in perovskite solar cells under light
exposure need to be understood and addressed. While extensive investigations
have suggested that ion migration is a plausible origin of these detrimental
effects, detailed understanding of the ion migration pathways remains elusive.
Here, we report the characterization of photo-induced ion migration in OIHPs
using \textit{in situ} laser illumination inside a scanning electron
microscope, coupled with secondary electron imaging, energy-dispersive X-ray
spectroscopy and cathodoluminescence with varying primary electron energies.
Using methylammonium lead iodide (MAPbI$_3$), formamidinium lead iodide
(FAPbI$_3$) and hybrid formamidinium-methylammonium lead iodide as model
systems, we observed photo-induced long-range migration of halide ions over
hundreds of micrometers and elucidated the transport pathways of various ions
both near the surface and inside the bulk of the OIHPs, including a surprising
finding of the vertical migration of lead ions. Our study provides insights
into ion migration processes in OIHPs that can aid OIHP material design and
processing in future applications.
