Extremely efficient sky-blue organic electroluminescence with external quantum efficiency of ≈37% is achieved in a conventional planar device structure, using a highly efficient thermally activated delayed fluorescence emitter based on the spiroacridine-triazine hybrid and simultaneously possessing nearly unitary (100%) photoluminescence quantum yield, excellent thermal stability, and strongly horizontally oriented emitting dipoles (with a horizontal dipole ratio of 83%).

A thermally activated delayed fluorescent (TADF) emitter (DMAC-TRZ) was reported either as the emitting dopant in a host or as the non-doped (neat) emitting layer to achieve high EL EQEs of up to 26.5% and 20% in OLEDs, respectively.

A series of twisted D–π–A type emitters based on the acridine donor unit and CN‐substituted pyridine, pyrimidine, and benzene acceptor units are studied. They not only allow one to systematically probe the influence of different acceptor strengths, but also permit one to intriguingly probe the influence of tunable conformations (twist angles) within the acceptor moieties through controlling the orientation of asymmetric heteroaromatic ring relative to the donor component. Intramolecular charge‐transfer transitions are observed in all these compounds and emission wavelengths are widely tunable from deep blue to yellow not only by the general acceptor strength due to the characters of heteroarene and CN‐substitution pattern but also by the subtle control of in‐acceptor conformation (twist angles). Small triplet‐to‐singlet energy gaps (ΔEST) and significant thermally activated delayed fluorescence (TADF) characteristics are obtained in a series of D–π–A compounds with sufficient acceptor strengths and tunable in‐acceptor conformation, yielding a series of efficient blue‐green to yellow TADF emitters with promisingly high photoluminescence quantum yields of 90%–100%. Highly efficient blue‐green to yellow TADF organic light‐emitting diodes (OLEDs) having external quantum efficiencies of up to 23.1%–31.3% are achieved using these efficient TADF emitters, which are among the most efficient TADF OLEDs ever reported.

A spiro-linked double D–A molecular architecture has been introduced to construct blue thermally activated delayed fluorescence (TADF) emitters with a preferentially horizontal emitting dipole orientation.

Quantitative analyses of high electroluminescence efficiency of thermally activated delayed fluorescence emitters based on acridine-triazine hybrids," J. Photon. Energy 8(3), 032105 (2018), doi: 10.1117/1.JPE.8.032105. Abstract. Quantitative optical analyses were conducted on the mechanisms of impressively high electroluminescence (EL) efficiency (external quantum efficiency of up to 37%) achieved in previously reported blue organic light-emitting devices (OLEDs) using thermally activated delayed fluorescence emitters based on acridine-triazine hybrids. In addition to high photoluminescence quantum yields and preferentially horizontal emitting dipoles, optical simulation shows that the use of both low-index hole-transport layers (HTLs) and electron-transport layers (ETLs) also substantially contribute to enhanced optical outcoupling efficiencies and EL efficiencies of these devices. Further analyses on optical mode distributions and partitions in devices reveal significantly different optical outcoupling enhancement mechanisms for adopting low-index HTLs (i.e., reduced overall waveguided modes and enhanced microcavity effect) or adopting low-index ETLs (i.e., reduced surface plasmon and transverse magnetic waveguided modes), and their effects are combined to give even larger enhancement when reducing refractive indexes of both. Results of this work clearly indicate that optical properties of carrier-transport layers, in addition to their electrical properties, are critical factors and should also be carefully considered for future development of high-efficiency OLEDs. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.