Exciton-carrier coupling in a metal halide perovskite nanocrystal assembly probed by two-dimensional coherent spectroscopy

TL;DR

This study uses two-dimensional coherent spectroscopy to reveal ultrafast coherent coupling between excitons and free carriers in ligand-free formamidinium lead bromide nanocrystals, highlighting their many-body interactions and dynamics.

Contribution

It provides the first direct spectroscopic evidence of coherent exciton-carrier coupling in perovskite nanocrystals at sub-100 femtoseconds.

Findings

Fano-like spectral lineshape indicating exciton-continuum coupling

Symmetric excited state absorption peaks suggest coupled exciton-carrier states

Ultrafast carrier thermalization and exciton formation within 100 fs

Abstract

The surface chemistry and inter-connectivity within perovskite nanocrystals play a critical role in determining the electronic interactions. They manifest in the Coulomb screening of electron-hole correlations and the carrier relaxation dynamics, among other many-body processes. Here, we characterize the coupling between the exciton and free carrier states close to the band-edge in a ligand-free formamidinium lead bromide nanocrystal assembly via two-dimensional coherent spectroscopy. The optical signatures observed in this work show: (i) a nonlinear spectral lineshape reminiscent of Fano-like interference that evidences the coupling between discrete electronic states and a continuum, (ii) symmetric excited state absorption cross-peaks that suggest the existence of a coupled exciton-carrier excited state, and (iii) ultrafast carrier thermalization and exciton formation. Our results highlight the presence of coherent coupling between exciton and free carriers, particularly in the sub-100 femtosecond timescales.