Evidence of large polarons in photoemission band mapping of the perovskite semiconductor CsPbBr$_3$

TL;DR

This study provides experimental evidence of large polaron formation in CsPbBr₃, revealing how electron-phonon interactions influence its electronic structure and effective mass, with implications for understanding its optoelectronic properties.

Contribution

The paper demonstrates the presence of large polarons in CsPbBr₃ using photoemission spectroscopy and validates the Feynman polaron model for predicting carrier effective mass ab-initio.

Findings

Valence band dispersion shows increased hole effective mass.

Electron-phonon coupling mainly involves Pb-Br bond distortions.

Feynman polaron model agrees with experimental data.

Abstract

Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic materials with outstanding transport properties which are not yet fully understood. We find signatures of large polaron formation in the electronic structure of the inorganic LHP CsPbBr$_3$ by means of angle-resolved photoelectron spectroscopy. The experimental valence band dispersion shows a hole effective mass $0.26\pm0.02\,\,m_e$, 50% heavier than the bare mass $m_0 =0.17 m_e$ predicted by density functional theory. Calculations of electron-phonon coupling indicate that phonon dressing of the carriers mainly occurs via distortions of the Pb-Br bond with a Fr\"ohlich coupling parameter $\alpha=1.82$. A good agreement with our experimental data is obtained within the Feynmann polaron model, validating a viable theorical method to predict the carrier effective mass of LHPs ab-initio.