9:45 - 10:10InvitedID: 245
RECENT ADVANCES IN TADF MATERIALS AND DEVICES
Georgia Tech (Georgia Institute of Technology), Atlanta, United States of America
In this talk, we will discuss recent progress in the design, synthesis and characterization of new TADF systems and their use in OLEDs. In particular, we will present results on the TADF structure-property relationships of carbazole/oxadiazole, on acridane/oxadiazole compounds, and on the performance of OLEDs containing a benzofurocarbazole/diphenyltriazine emitter . Our results show that devices with optimized external quantum efficiency of 22.8% at a luminance of 1,000 cd/m2 are obtained at high concentrations of TADF emitters (up to 50%), challenging current prescriptions for device optimization.
10:10 - 10:25ID: 123
DIMERS ARE FOREVER: PERSISTENT DIMER EMISSION IN THERMALLY ACTIVATED DELAYED FLUORESCENCE MATERIALS
1Department of Physics, Durham University, Durham, United Kingdom; 2Department of Chemistry, Durham University, Durham, United Kingdom; 3Merck KGaA, Performance Materials – Display Solutions, Darmstadt, Germany; 4Institute of Physics, Experimental Physics IV, University of Augsburg, Augsburg, Germany; 5Department of Chemistry, Guru Nanak Dev University, Amritsar, India
We expose remarkable changes in emission colour of carbazole-based thermally activated delayed fluorescence (TADF) compounds that arise from persistent dimer states in thin films and organic light-emitting diodes (OLEDs). Direct photoexcitation of this dimer state in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) reveals the important influence of dimer species on the colour purity of its photoluminescence and electroluminescence. The dimer species is sensitive to the sample preparation method and its enduring presence contributes to the widely reported concentration-mediated redshift in the photoluminescence and electroluminescence of evaporated thin films. This discovery has important implications on the usability of these, and similar, molecules for OLEDs and explains disparate electroluminescence spectra presented in the literature for these compounds. The significant dimerization-controlled changes observed in the TADF process and photoluminescence efficiency mean that careful consideration of dimer states is imperative in the design of future TADF emitters and the interpretation of previously reported studies of carbazole-based TADF materials.
10:25 - 10:40ID: 128
DESIGN STRATEGIES TO IMPROVE THE PERFORMANCE OF BLUE TADF-OLEDS
University of St Andrews, St Andrews, United Kingdom
Research in emitters for organic light emitting diode (OLED) has gathered great interest in scientific and industrial communities alike, particularly with its third generation, where devices based on thermally activated delayed fluorescence (TADF) emitters have been extensively produced and optimised. These emitters can surpass the 25% internal quantum efficiency (IQE) in OLEDs imposed by spin statistics while still using purely organic molecules. The TADF mechanism relies on thermal energy to harvest triplet states, which are initially non emissive, into emissive singlet states by reverse intersystem crossing (rISC)1. Therefore, the TADF mechanism is an alternative triplet harvesting mechanism that works with low-cost materials.
Strong blue TADF emission can be achieved using aromatic donor–acceptor (D–A) molecules, which typically are conjugationally separated with the D and A units orthogonal2. These systems emit from a singlet charge transfer state (1CT), which is energetically very close to its 3CT state through minimised electron exchange. A further excited state such as a local excited triplet state (3LE) situated very close in energy to this 1CT is also required3.
By studying different D-A-D emitters with acceptor moieties containing pyrimidine and pyrazine groups we improved the optical and electrical properties of blue emitters. The small changes in the molecular structure resulted in improved photoluminescence quantum yield (PLQY) and smaller singlet-triplet energy gap, leading to OLEDs with higher external quantum efficiency (EQE) values and better roll-off. Our results give better understanding about how the balance between colour purity and high efficiency devices can be obtained, advancing the design strategies for novel blue TADF materials.
1 P. L. dos Santos et al. J. Mater. Chem. C, 2018, 6, 4842–4853.
2 P. Rajamalli et al. J. Am. Chem. Soc., 2017, 139, 10948–10951.
3 J. Gibson, et al. ChemPhysChem, 2016, 17, 2956–2961.