[Paper Review] Mapping the Pathways of Photo-induced Ion Migration in Organic-inorganic Hybrid Halide Perovskites

Taeyong Kim, Soyeon Park+8 more|arXiv (Cornell University)|Oct 10, 2022

Perovskite Materials and Applications4 citations

TL;DR

This study maps photo-induced ion migration in organic-inorganic hybrid perovskites (OIHPs) using in situ laser illumination in a scanning electron microscope with multi-modal characterization. It reveals long-range halide ion migration over hundreds of micrometers and identifies unexpected vertical lead ion migration, providing critical insights for mitigating hysteresis and improving perovskite device stability and design.

ABSTRACT

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 extit{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.

Motivation & Objective

• To understand the microscopic mechanisms of photo-induced ion migration in organic-inorganic hybrid perovskites (OIHPs), which underlie performance degradation and hysteresis in solar cells.
• To identify the specific migration pathways of halide and metal ions (e.g., Pb²⁺) under light exposure in MAPbI₃, FAPbI₃, and mixed-halide systems.
• To investigate the role of ion migration in inducing morphological and compositional changes that affect optoelectronic properties and device performance.
• To develop a multi-modal in situ characterization approach combining laser excitation, secondary electron imaging, EDS, and cathodoluminescence for probing ion transport at the nanoscale.

Proposed method

• Conducted in situ laser illumination using a 515 nm fiber laser (150 fs pulses, 5 MHz repetition rate) focused into a scanning electron microscope (SEM) chamber via a transparent viewport.
• Performed secondary electron imaging and energy-dispersive X-ray spectroscopy (EDS) at 5 keV accelerating voltage and 300 pA beam current to map elemental redistribution after laser exposure.
• Acquired EDS line scans every ~7 µm across the laser-illuminated region with 100 averaged scans (100 ms per scan) to enhance signal-to-noise and minimize beam damage.
• Employed cathodoluminescence (CL) with a 532 nm continuous-wave or 495 nm pulsed laser (100 fs, 80 MHz, 1.25 mW) to probe optical response changes during laser exposure.
• Used blind non-negative matrix factorization (NMF) on CL spectrum images to track dynamic changes in emission features as a function of laser exposure time.
• Maintained high vacuum (~1×10⁻⁶ torr) and used active beam stabilization to ensure alignment stability during long-duration in situ measurements.

Experimental results

Research questions

• RQ1 What are the dominant pathways and long-range transport mechanisms of photo-induced ion migration in OIHPs such as MAPbI₃ and FAPbI₃?
• RQ2 How do different ion species (I⁻, Pb²⁺, MA⁺, FA⁺) redistribute under localized laser illumination, and what are the spatial scales of migration?
• RQ3 What is the role of electron-beam and laser excitation in inducing or modulating ion migration, and how can these be decoupled in in situ measurements?
• RQ4 Are there unexpected migration directions, such as vertical transport of Pb²⁺ ions, and what do they imply for ion transport anisotropy in perovskite films?
• RQ5 How do changes in cathodoluminescence spectra correlate with ion migration and compositional changes during in situ laser exposure?

Key findings

• Photo-induced long-range migration of halide ions (I⁻) was observed over distances exceeding 300 µm in MAPbI₃ and FAPbI₃ films under laser illumination.
• A significant redistribution of iodine was detected via EDS line scans, confirming the migration of I⁻ ions from the illuminated region toward the edges of the film.
• Unexpected vertical migration of Pb²⁺ ions was observed, indicating that ion transport is not limited to in-plane diffusion and may involve out-of-plane pathways.
• Cathodoluminescence spectra showed dynamic changes in emission intensity and peak position with laser exposure time, correlating with ion migration and local compositional changes.
• The combination of variable primary electron energy and in situ laser excitation enabled selective probing of ion transport in surface and bulk regions of the perovskite film.
• The study demonstrates that multi-modal in situ microscopy with controlled laser and electron beam excitation is a viable method for mapping ion migration in emerging optoelectronic materials.