Large Polaron Generation and Dynamics in 3D Metal-Halide Perovskites

TL;DR

This study combines theoretical and experimental methods to characterize large polaron formation and dynamics in 3D metal halide perovskites, revealing their impact on charge transport and optoelectronic properties.

Contribution

It provides the first comprehensive analysis of large polarons in 3D perovskites, linking spectroscopic features with theoretical models to understand their role in charge mobility.

Findings

Identification of intragap electronic transitions linked to large polarons

Observation of phonon mode renormalization due to polaron formation

Quantitative estimates of increased charge carrier masses and mobilities

Abstract

In recent years, metal halide perovskites have generated tremendous interest for optoelectronic applications and their underlying fundamental properties. Due to the large electron-phonon coupling characteristic of soft lattices, self-trapping phenomena are expected to dominate hybrid perovskite photoexcitation dynamics. Yet, while the photogeneration of small polarons was proven in low dimensional perovskites, the nature of polaron excitations in technologically relevant 3D perovskites, and their influence on charge carrier transport, remain elusive. In this study, we used a combination of first principle calculations and advanced spectroscopy techniques spanning the entire optical frequency range to pin down polaron features in 3D metal halide perovskites. Mid-infrared photoinduced absorption shows the photogeneration of states associated to low energy intragap electronic transitions with lifetime up to the ms time scale, and vibrational mode renormalization in both frequency and amplitude. Density functional theory supports the assignment of the spectroscopic features to large polarons leading to new intra gap transitions, hardening of phonon mode frequency, and renormalization of the oscillator strength. Theory provides quantitative estimates of the charge carrier masses and mobilities increase upon polaron formation, confirming experimental results. Overall, this work contributes to complete the scenario of elementary photoexcitations in metal halide perovskites and highlights the importance of polaronic transport in perovskite-based optoelectronic devices.