1 April 2016
Several experts involved in the PHEBE project – Prof. Andy Monkman, Dr. Sebastian Reineke and Paul Kleine – presented their initial project results at the presitigious 2016 MRS Spring Meeting in Phoenix, Arizona.

The experts presented two papers during the Symposium EP1 “Organic Excitonic Systems and Devices” on behalf of research teams from Durham University, TU Dresden and Kaunas University of Technology. The titles, authors and abstracts for these papers are provided below.

A) Rational Design of Thermally Activated Delayed Fluorescence Materials: The Competition between Internal Conversion and Non-Radiative Decay Processes

Paul Kleine1, Florian Wuest1, Eni Dodbiba1, MartinOberlaender1, Ludwig Popp1, Olaf Zeika1, Ramunas Lygaitis1,2, Simone Lenk1, Reinhard Scholz1, Sebastian Reineke1.
1. Institut für Angewandte Photophysik, Dresden, Germany;
2. Department of Polymer Chemistry and Technology, Faculty of Chemical Technology, Kaunas, Lithuania.

Under electroluminescence operation, about 75% of the excitons formed in organic light-emitting diodes (OLEDs) come to life as triplet states. High quantum yields based on phosphorescent materials have been heavily investigated within the last decades resulting in internal quantum efficiencies (IQE) approaching unity. While phosphorescent OLEDs make use of expensive rare metal complexes to increase triplet harvesting from the non-radiative dark states via efficient spin-orbit coupling to overcome the IQE limit of 25% for fluorescent OLEDs, especially the stability and substantial efficiency roll-off at higher current densities of blue phosphorescent materials remain open issues. The concept of thermally activated delayed fluorescence (TADF) opened new pathways for purely organic material designs to face these fundamental problems. TADF allows via effective reverse intersystem crossing (RISC) for triplet states to be redirected to emissive singlet states, allowing for 100% exciton utilization.
Although there has been an amazing progress in TADF, the overall understanding of the TADF unlocking molecular properties is still in its infancy and the fundamental properties to tackle are still under debate. To increase the IQE of TADF materials, the competition between internal conversion, radiative, and non-radiative rates decides over good and bad emitter molecules. Both from computational and experimental point of view, a great effort has to be made to get access to the relevant photophysical properties of the emitting molecules. In principle, TADF materials are designed to break the conjugation between donor and acceptor units to separate HOMO and LUMO states. As a consequence, the exchange energy is minimized and a small energy split (ΔES-T ) between S1 and T1 level allows for both increased intersystem crossing and substantial RISC rates.
In this talk, we will present our recent efforts in the understanding of the basic concepts of TADF mechanism. The first part will deal with the rational design of a new TADF material class based on phenyl-carbazoles. Reasonable isomer structures have been constructed theoretically and slight stoichiometric modifications were performed to understand how the molecular structure and its steric hindrance affects the RISC. By this approach, the most promising candidates were identified using TD-DFT schemes and were synthesized accordingly. Photophysical properties and device parameters based on the most promising emitters are discussed. Our discussion will also focus on the interplay of local and charge transfer excited states, which is of key importance to unlock efficient TADF-based electroluminescence. The energetic ordering and the relative transition moments of internal CT states between donor and acceptor subgroups with respect to local excitations of either donor or acceptor depend sensitively on the specific molecular structure, discussed widely as key influencing parameters.

B) Control of Interfacial Exciplex Emission by Electric Field and Measuring the Charge Separation Distance

Andrew Monkman1, Hameed Al'Attar1.
1. Durham Univ, Durham, United Kingdom.

Abrupt interface donor (D) acceptor (A) OLEDs offer a rather unique way of studying pinned exciplex states in the solid state. Here we present time resolved spectroscopy and electroluminescence data from such abrupt interface devices, effectively hole transport layer abutted to the electron transfer layer with no defined, fixed width emitter layer, as a function of the energy offsets between the ground (HOMO) and first excited (LUMO) states of the D and A. We show how these offsets critically control the reverse electron transfer step which dictates exciplex lifetime and final emitting species and emission energy. Further, we observe very strong E-field dependent electroluminescence from which we can determine the electron hole separation across the interface. We also find, counter intuitively that increasing E-field increases the efficiency of radiative exciplex emission. We discuss these findings in terms of the Onsager model. Such abrupt junction devices may be use in optical computing and communications given that simple E-field tuning of emission energy is afforded without quenching of the emitting states.