Effects of Characteristic Length Scales on the Exciton Dynamics in Rubrene Single Crystals

TL;DR

This study investigates how characteristic length scales affect exciton behavior in rubrene single crystals, revealing that local environment and confinement significantly influence exciton dynamics, with implications for organic electronic device miniaturization.

Contribution

It provides a systematic analysis of exciton states and their dynamics in rubrene, highlighting the role of local structural environment and confinement effects on exciton lifetime and decay processes.

Findings

Exciton decay in bulk is governed by thermally activated singlet fission.

Microcrystals trap excitons in dark surface states, extending their lifetime.

Local environment significantly impacts excitonic states and dynamics.

Abstract

As for its inorganic counterparts the future developments in organic electronics are driven by an advanced device miniaturization. Therefore, the opto-electronic behavior of up-to-date devices is progressively governed by the local structural environment. However, there is a lack of organic semiconductor materials providing access to the fundamental structure-functionality relation, either due to limitations by their inherent growth or their optical characteristics. In this work we present a systematic investigation of the optical states, so-called excitons, and their temporal evolution in the prototypical organic semiconductor rubrene by means of time and temperature dependent photoluminescence studies. This material offers the unique possibility of preparing well-defined morphologies with adjustable degree of confinement. By this approach we are able to confirm the direct influence on the temperature dependent optical processes with picosecond resolution already for a spatial localization of excitation on the \mu m length scale. While in bulk single crystals the exciton decay dynamics are governed by thermally activated singlet fission, excitons created in microcrystals are trapped by dark states localized on the surface and leading to a pronounced enhancement of their average lifetime. Our results highlight the impact of the local environment on the excitonic states and their dynamics in organic semiconductors. With respect to the spatial dimensions of organic thin film devices, this correlation and the reported effects emerging by the confinement have to be considered upon further miniaturization and in the development of innovative device concepts, such as photovoltaic cells based on triplet-harvesting.