Observation of self-trapped polaron exciton signatures in ligand-assisted lead halide perovskite nanocrystals

We report the spectroscopic observation of self-trapped polaron exciton signatures in colloidally synthesized, ligand-assisted CsPbBr₃Cs₄PbBr₆ nanocrystals. Temperature dependence of photoluminescence spectroscopy show broad shoulder emissions below the exciton signatures in visible wavelength region, which are attributed to the formation of self-trapped polaron excitons corresponding to the relaxation of electrons and holes localized at Pb₂³⁺, and Pb³⁺ sites, respectively. While self-trapped polaron exciton emission peaks shifted to lower energies side, its intensity substantially enhanced as the temperature decreases from 180 to 14 K. The polaron-excitons band splitting is attributed to the changes in self-trapped energy levels driven by the strong exchange field of Pb-6p orbitals during relaxation process, coupled with specific lattice distortions arising from phonon anharmonicity; those self-trapped carriers act like non-radiative centers for deteriorating exciton emission intensity.