[Paper Review] Opposites Attract, Muons as Direct Probes for Iodide Diffusion in Methyl Ammonium Lead Iodide

Dominic Ferdani, Andrew L. Johnson+3 more|arXiv (Cornell University)|Jan 11, 2018

Advanced NMR Techniques and Applications3 citations

TL;DR

This study uses muon spin relaxation (μSR) to directly probe iodide ion diffusion in methylammonium lead iodide (MAPbI3), a key perovskite material for solar cells. The technique detects the motion of iodide ions via the interaction of implanted muons with the local magnetic environment, yielding a diffusion coefficient of 1.6 × 10⁻¹⁴ cm²/s at 300 K, providing direct evidence of ionic mobility in the absence of external bias.

ABSTRACT

The volume of research into organo-lead hailde perovskites is increasing rapidly, with perovskite solar cell efficiencies reaching as high as 22 percent. There is considerable evidence that mobile ions in the perovskite strongly influence the properties of the solar cell, with the majority of studies carried out on whole cells under bias. Here we use muon spin relaxation to directly probe iodide diffusion in methyl ammonium lead iodide (MAPI). This is the first time that has been used to detect iodide diffusion in any material and the results provide valuable insight into the movement of ions in lead halide perovskites. The experiment was carried out in the dark with no external biases applied and allowed us to calculate a diffusion coefficient of 1.6 x10-14 cm2/s for iodide in MAPI at 300 K.

Motivation & Objective

• To directly probe iodide ion diffusion in methylammonium lead iodide (MAPbI3) without external electrical bias.
• To overcome limitations of indirect measurements in whole-cell configurations commonly used in perovskite research.
• To provide quantitative insight into ionic mobility in halide perovskites using a novel experimental approach.
• To establish muon spin relaxation as a viable tool for studying ion dynamics in hybrid perovskites.

Proposed method

• Implanted positive muons serve as local magnetic probes in the MAPI crystal lattice.
• Measured muon spin relaxation rates reflect the local magnetic field distribution, sensitive to mobile iodide ions.
• Experiments were conducted in the dark and without external bias to isolate intrinsic ionic diffusion.
• Time-dependent muon spin relaxation spectra were analyzed to extract the ionic diffusion coefficient.
• The method relies on the principle that mobile iodide ions induce dynamic fluctuations in the local magnetic field at muon sites.
• Data were fitted using a model that accounts for the time evolution of the muon spin polarization due to ion motion.

Experimental results

Research questions

• RQ1 What is the intrinsic diffusion coefficient of iodide ions in methylammonium lead iodide under zero-bias conditions?
• RQ2 How does muon spin relaxation respond to the motion of iodide ions in the perovskite lattice?
• RQ3 Can muon spin relaxation detect and quantify ionic diffusion in halide perovskites where conventional techniques are limited?
• RQ4 What is the magnitude of iodide ion mobility in MAPI at room temperature?
• RQ5 How does the ionic diffusion behavior observed here compare to that inferred from device-level measurements?

Key findings

• The study reports a direct measurement of iodide diffusion in MAPI with a diffusion coefficient of 1.6 × 10⁻¹⁴ cm²/s at 300 K.
• This value was obtained under zero-bias, dark conditions, eliminating contributions from electronic or electrochemical effects.
• The results confirm that iodide ions are mobile in MAPI even without external electrical fields.
• The muon spin relaxation technique successfully detected ionic motion, validating its use as a direct probe for ion dynamics.
• The measured diffusion coefficient is consistent with the presence of significant ionic conductivity in perovskite materials.
• This work establishes a new experimental pathway for studying ion transport in hybrid perovskites using muon-based spectroscopy.