The impact of exciton delocalization on exciton-vibration interactions in organic semiconductors

TL;DR

This paper investigates how exciton delocalization influences exciton-vibration interactions in organic semiconductors, revealing that delocalization degree determines vibrational coupling and affecting optical properties, with experimental validation and a new probing method.

Contribution

It introduces a unified framework linking exciton delocalization with vibrational interactions in organic semiconductors, combining advanced electronic structure calculations with experimental validation.

Findings

Delocalized excitons mainly couple with crystal phonons.

Localized excitons predominantly interact with molecular quantum fluctuations.

Temperature and pressure affect excitonic peaks as predicted by the model.

Abstract

Organic semiconductors exhibit properties of individual molecules and extended crystals simultaneously. The strongly bound excitons they host are typically described in the molecular limit, but excitons can delocalize over many molecules, raising the question of how important the extended crystalline nature is. Using accurate Green's function based methods for the electronic structure and non-perturbative finite difference methods for exciton-vibration coupling, we describe exciton interactions with molecular and crystal degrees of freedom concurrently. We find that the degree of exciton delocalization controls these interactions, with thermally activated crystal phonons predominantly coupling to delocalized states, and molecular quantum fluctuations predominantly coupling to localized states. Based on this picture, we quantitatively predict and interpret the temperature and pressure dependence of excitonic peaks in the acene series of organic semiconductors, which we confirm experimentally, and we develop a simple experimental protocol for probing exciton delocalization. Overall, we provide a unified picture of exciton delocalization and vibrational effects in organic semiconductors, reconciling the complementary views of finite molecular clusters and periodic molecular solids.