Dynamical co-existence of excitons and free carriers in perovskite probed by density-resolved fluorescent spectroscopic method

TL;DR

This paper introduces a density-resolved fluorescent spectroscopy technique to study the coexistence of excitons and free carriers in perovskite, revealing their dynamics and enabling accurate estimation of exciton binding energy.

Contribution

The study presents a novel spectroscopic method for directly observing exciton and free carrier dynamics in perovskite, resolving previous conflicts in exciton binding energy measurements.

Findings

First experimental observation of density-dependent coexistence of excitons and free carriers.

Accurate estimation of exciton binding energy and effective mass.

No ionic polarization contribution detected.

Abstract

Using transient fluorescent spectra at time-zero, we develop a density-resolved fluorescent spectroscopic method for investigating photoproducts in CH3NH3PbI3 perovskite and related photophysics. The density dependent dynamical co-existence of excitons and free carriers over a wide density range is experimentally observed for the first time. The exciton binding energy (EB) and the effective mass of electron-hole pair can be estimated based on such co-existence. No ionic polarization is found contributing to photophysical behavior. It also solves the conflict between the large experimentally measured EB and the small predicted values. The spectroscopic method also helps to detect the true free carrier density under continuous illumination without the interference of ionic conductivity. Our methods and results profoundly enrich the study and understanding of the photophysics in perovskite materials for photovoltaic applications.