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

Abstract

Organic semiconductor crystals stand out as an efficient, cheap, and diverse platform for realizing 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, and find that dark excitons act as crucial phonon-mediated relaxation scattering channels in both pentacene and tetracene. 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 polarization 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 polarization-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. Key points: Compare microscopic modeling to spectroscopic measurements of excitons in oligoacene crystals. Track the singlet-exciton population and optical spectra following phonon-mediated relaxation. Predict a phonon-bottleneck effect in pentacene that is absent in tetracene.