# Unraveling the exciton binding energy and the dielectric constant in single crystal methylammonium lead tri-iodide perovskite

## TL;DR

This study precisely measures the exciton binding energy and dielectric constant in methylammonium lead tri-iodide single crystals using high magnetic field magneto-reflectivity, revealing phase-dependent excitonic properties.

## Contribution

It provides the first accurate determination of exciton binding energy in single crystals, showing it is independent of grain size and varies with phase.

## Key findings

- Exciton binding energy of 16 ± 2 meV at low temperature.
- Upper limit of 12 ± 4 meV at room temperature.
- Single crystal exhibits narrow excitonic transition linewidths.

## Abstract

We have accurately determined the exciton binding energy and reduced mass of single crystals of methylammonium lead tri-iodide using magneto-reflectivity at very high magnetic fields. The single crystal has excellent optical properties with a narrow line width of $\sim 3$meV for the excitonic transitions and a 2s transition which is clearly visible even at zero magnetic field. The exciton binding energy of $16 \pm 2$meV in the low temperature orthorhombic phase is almost identical to the value found in polycrystalline samples, crucially ruling out any possibility that the exciton binding energy depends on the grain size. In the room temperature tetragonal phase, an upper limit for the exciton binding energy of $12 \pm 4$ meV is estimated from the evolution of 1s-2s splitting at high magnetic field.