Identification of Free and Bound Exciton States and Their Phase-Dependent Trapping Behavior in Lead Halide Perovskites

TL;DR

This study investigates sub-gap energy states in lead halide perovskites, revealing phase-dependent exciton trapping behaviors and providing insights into their photophysical properties and charge dynamics.

Contribution

It uncovers the coexistence of free and bound excitons and explains phase-dependent trapping mechanisms using temperature and intensity-dependent optical measurements.

Findings

Orthorhombic phase shows ultrafast exciton trapping.

Tetragonal phase exhibits low recombination velocity.

Charge trapping suppression linked to increased activation energy.

Abstract

In this work we probe the sub-gap energy states within polycrystalline and single crystal lead halide perovskites to better understand their intrinsic photophysics behaviors. Through combined temperature and intensity-dependent optical measurements, we reveal the existence of both free and bound exciton contributions within the sub-gap energy state manifold. The trapping and recombination dynamics of these excitons is shown to be strongly dependent on the structural phase of the perovskite. The orthorhombic phase exhibits ultrafast exciton trapping and distinct trap emission, while the tetragonal phase gives low monomolecular recombination velocity and capture cross-sections (~10-18 cm2). Within the multiphonon transition scenario, this suppression in charge trapping is caused by the increase in the charge capture activation energy due to the reduction in electron-lattice interactions, which can be the origin for the unexpected long carrier lifetime in these material systems.