Singlet exciton optics and phonon-mediated dynamics in oligoacene semiconductor crystals

TL;DR

This study combines experimental and theoretical approaches to explore singlet exciton behavior and phonon interactions in oligoacene crystals, revealing phonon-bottleneck effects, exciton linewidth origins, and polarization-dependent optical features.

Contribution

It provides the first combined theory and experimental analysis of phonon-driven singlet exciton dynamics in oligoacene crystals, highlighting new phonon-bottleneck effects and polarization dependencies.

Findings

Prediction of phonon-bottleneck effects in pentacene crystals.

Identification of dark excitons as relaxation channels.

Observation of polarization- and temperature-dependent spectral features.

Abstract

Organic semiconductor crystals stand out as an efficient, cheap and diverse platform for realising optoelectronic applications. The optical response of these crystals is governed by a rich tapestry of exciton physics. So far, little is known on the phonon-driven singlet exciton dynamics in this class of materials. In this joint theory-experiment work, we combine the fabrication of a high-quality oligoacene semiconductor crystal and characterization via photoluminescence measurements with a sophisticated approach to the microscopic modeling in these crystals. This allows us to investigate singlet exciton optics and dynamics. We predict phonon-bottleneck effects in pentacene crystals, where we find dark excitons acting as crucial phonon-mediated relaxation scattering channels. While the efficient singlet fission in pentacene crystals hampers the experimental observation of this bottleneck effect, we reveal both in theory and experiment a distinct polarisation- and temperature-dependence in absorption and photoluminescence spectra of tetracene crystals, including microscopic origin of exciton linewidths, the activation of the higher Davydov states at large temperatures, and polarisation-dependent quenching of specific exciton resonances. Our joint theory-experiment study represents a significant advance in microscopic understanding of singlet exciton optics and dynamics in oligoacene crystals.