Regensburg 2013 – scientific programme

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DS: Fachverband Dünne Schichten

DS 8: Charge Transfer Effects in Molecular Materials II (jointly with BP, CPP, HL)

DS 8.4: Invited Talk

Monday, March 11, 2013, 16:00–16:30, H40

High efficiency OLEDs based on delayed fluorescence — •Chihaya Adachi — OPERA, Kyushu University, Fukuoka, Japan

We achieved a novel pathway to reach the ultimate EL efficiency by inventing simple aromatic compounds displaying efficient thermally-activated delayed fluorescence (TADF) with high photoluminescence efficiency, namely *hyperfluorescence*. While we had previously assumed that the S1 level should be significantly higher than the T1 level, i.e., 0.5~1.0 eV higher, due to the presence of electron exchange energy, we found that the proper design of organic molecules can lead to a small energy gap (ΔEST) between them. Relatedly, a molecule displaying efficient TADF requires a very small ΔEST between its S1 and T1 excited states, resulting in enhanced T1 * S1 reverse intersystem crossing (ISC). Such excited states are attainable by the intramolecular charge transfer (CT) of a spatially separated donor and acceptor system. The critical point of the molecular design is the compatibility of a small ΔEST ~ 0 eV and a reasonable radiative decay rate of over 106/s that overcomes competitive non-radiative decay paths, leading to highly luminescent TADF materials. Since the two properties conflict with each other, a delicate balance of the overlap of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is required. Furthermore, to enhance the PL efficiency of a TADF material, geometrical change between its S0 and S1 states should be restrained to suppress non-radiative decay processes. In this work, we designed a novel series of highly efficient TADF emitters that resulted in very high electroluminescence efficiency.