Phonon-driven femtosecond dynamics of excitons in crystalline pentacene from first principles

TL;DR

This paper presents a first-principles method to analyze exciton relaxation in pentacene, revealing femtosecond intraband transitions and the influence of crystal anisotropy on exciton dynamics.

Contribution

It introduces a novel first-principles approach to connect exciton-phonon interactions with real-time exciton propagation in molecular crystals.

Findings

Exciton intraband transitions occur on femtosecond timescales.

Dark-state occupation is a dominant nonradiative relaxation channel.

Crystal anisotropy influences exciton and phonon dispersions.

Abstract

Non-radiative exciton relaxation processes are critical for energy transduction efficiencies in optoelectronic materials, but how these processes are connected to the underlying crystal structure and its associated electron, exciton, and phonon band structures is poorly understood. Here, we present a first-principles approach to explore exciton relaxation pathways in pentacene, a paradigmatic molecular crystal and optoelectronic semiconductor. We compute the momentum- and band-resolved exciton-phonon interactions, and use them to analyse key scattering channels. We find that exciton intraband transitions on femtosecond timescales leading to dark-state occupation is a dominant nonradiative relaxation channel in pentacene. We further show how the nature of real-time propagation of the exciton wavepacket is connected with the longitudinal-transverse exciton splitting, stemming from crystal anisotropy, and concomitant anisotropic exciton and phonon dispersions. Our results provide a framework for understanding time-resolved exciton propagation and energy transfer in molecular crystals and beyond.