Mapping the pathways of photo-induced ion migration in organic-inorganic hybrid halide perovskites

Abstract

Organic-inorganic hybrid perovskites 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 perovskites using 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 and formamidinium 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 samples, including a surprising finding of the vertical migration of lead ions. Our study provides insights into ion migration processes in perovskites that can aid perovskite material design and processing in future applications.

Disentangling Electronic Transport and Hysteresis at Individual Grain Boundaries in Hybrid Perovskites via Automated Scanning Probe Microscopy

“…Since MHP is a photovoltaic material, its property alters upon light illumination. Multiple works have indicated that light illumination changes the MHP property via reducing ion migration activation energy, photoinduced strain, phase segregation, etc. − ,,− Therefore, we performed IV measurements at these locations under both dark conditions and light illumination. Here the illuminating light is a built-in LED light in the microscope; its intensity is equivalent to 0.1 sun.…”

“…In addition, it was shown that nanoscale phase impurities act as both traps for photogenerated charge carriers and sites of photochemical degradation . Studies with secondary ion mass spectrometry unveiled the micrometer level chemical redistribution and degradation under light and an electric field, which affects the surface potential and electrical current in the MHP films. − …”

Dynamic Observations on Formation of Coffee-Ring Structures from the Degradation of Cesium Lead Halide Perovskite Nanocrystals

“…has limited the large-scale applications. 11−14 For example, films of organic− inorganic hybrid halide perovskites suffered from both lateral and vertical migration of various ion species driven by light exposure; 15 perovskite solar cells involving a methylammonium lead iodide (MAPbI 3 ) layer experienced severe structural and chemical changes that directly affected their performance. 16 In particular, the rapid structural degradation of halide perovskites under electron beam irradiation has severely impeded the exploration of their structure−property relationships with a range of characterization techniques as well as the practical applications of their devices.…”

Strategies toward Suppression of Sn(II) Oxidation for Stable Sn–Pb Perovskite Solar Cells

“…In Pb based perovskites, the electrical properties are influenced by ion migration (mainly iodide migration) under operational conditions, which can affect the photovoltaic performance of the devices. , However, the incorporation of Sn in the Pb materials retarded the iodide migration and enhanced carrier mobility in the alloyed perovskite materials . It was proposed that the decrease of the reduced effective mass due to the enlargement of the B–I–B bonds in ABI 3 and the decrease of the activation energy for carrier transport upon Sn incorporation are the main reasons for such an improvement of the electrical properties of the Sn–Pb alloyed material.…”