Effect of phonon scattering on exciton transport in solid pentacene

Exciton decay and diffusion mechanisms are rooted in material structure and linked to the electronic, excitonic,

and phononic bandstructures. These processes underlie energy transfer and the associated functionality

of energy conversion, as they determine state coherence and lifetime in excitonic materials. These properties are

broadly explored by spatiotemporal spectroscopy and are commonly understood within a kinetic rate equation

picture. A theoretical understanding of the underlying mechanisms and structural origins dominating the associated

rates from first principles is still lacking, and of emerging interest. In this study we present a many-body

perturbation theory scheme to describe exciton transport in solid pentacene. Starting from the GW and Bethe

Salpeter equation (GW-BSE) formalism combined with Density Functional Perturbation Theory (DFPT), we

compute exciton-phonon coupling from ab initio, and the associated phonon-mediated exciton scattering. We

apply our approach on the pentacene molecular crystal to study bright-to-dark exciton transitions as a pathway

to increased efficiency of nonradiative decay processes in this system. We derive a detailed kinetic description

of the intra- and inter-state exciton-phonon scattering, and analyze the exciton propagation upon these relaxation

pathways. We use our results to extend our recent work [1] and trace the relation between the exciton

wavepacket propagation in both the ballistic and the diffusive regimes to crystal symmetry and anisotropy.

[1] D. Y. Qiu, G. Cohen, D. Novichkova, and S. Refaely-Abramson, “Signatures of dimensionality and symmetry in exciton band

structure: Consequences for exciton dynamics and transport,” Nano Letters, vol. 21, no. 18, pp. 7644–7650, 2021.