What Controls the Orientation of TADF Emitters?

Bilal A Naqvi,Eli Zysman-Colman,Zhen Zhang,Stefan Bräse,Tassilo Naujoks,Francesco Rodella,Peter Strohriegl,Ettore Crovini,Wolfgang Brütting,Prakhar Sahay,Markus Schmid

doi:10.3389/fchem.2020.00750

Abstract

Thermally-activated delayed fluorescence (TADF) emitters—just like phosphorescent ones—can in principle allow for 100% internal quantum efficiency of organic light-emitting diodes (OLEDs), because the initially formed electron-hole pairs in the non-emissive triplet state can be efficiently converted into emissive singlets by reverse intersystem crossing. However, as compared to phosphorescent emitter complexes with their bulky—often close to spherical—molecular structures, TADF emitters offer the advantage to align them such that their optical transition dipole moments (TDMs) lie preferentially in the film plane. In this report, we address the question which factors control the orientation of TADF emitters. Specifically, we discuss how guest-host interactions may be used to influence this parameter and propose an interplay of different factors being responsible. We infer that emitter orientation is mainly governed by the molecular shape of the TADF molecule itself and by the physical properties of the host—foremost, its glass transition temperature Tg and its tendency for alignment being expressed, e.g., as birefringence or the formation of a giant surface potential of the host. Electrostatic dipole-dipole interactions between host and emitter are not found to play an important role.

Highlights

Organic light-emitting diodes (OLEDs) are thin-film structures where photons are produced from radiative recombination of electron-hole pairs through an excited state of a molecular emitter material that is commonly embedded in a suitable host matrix to avoid aggregation and, luminescence quenching (Tang et al, 1989)
Using the above mentioned semi-classical dipole model, it follows that the external quantum efficiency of a Thermally-activated delayed fluorescence (TADF) OLED can be dramatically enhanced, if instead of an ensemble of randomly oriented emitter molecules, horizontally aligned transition dipole moments (TDMs) prevail in the system (Figure 1B)
ICzTRZ has the highest orientation factor of Θ = 0.12 in mCP and the lowest is Θ = 0.06 in DPEPO, which is among the best values reported for TADF emitters (Mayr et al, 2014; Byeon et al, 2018; Tanaka et al, 2020)

Summary

Introduction

Organic light-emitting diodes (OLEDs) are thin-film structures where photons are produced from radiative recombination of electron-hole pairs through an excited state of a molecular emitter material that is commonly embedded in a suitable host matrix to avoid aggregation and, luminescence quenching (Tang et al, 1989). The external quantum efficiency ηext of an OLED, i.e., the ratio between extracted photons from a device divided by the number of injected charges, is split into an internal factor ηint comprising charge balance γ, spin statistics ηr and radiative exciton decay qeff, and an outcoupling factor ηout for the fraction of light that is emitted from the OLED and is visible to an observer (Tsutsui et al, 1997). Note that this separation is not strictly valid, because the radiative quantum efficiency is influenced by the device stack as well through the so-called Purcell effect, yielding an effective value qeff (Nowy et al, 2008; Brütting et al, 2013). Using the above mentioned semi-classical dipole model, it follows that the external quantum efficiency of a TADF OLED can be dramatically enhanced, if instead of an ensemble of randomly oriented emitter molecules, horizontally aligned TDMs prevail in the system (Figure 1B)

Methods

Results

Conclusion