Colour-tuning of multiple resonance thermally activated delayed fluorescence emitters for hyperfluorescence with long-lifetime device characteristics

Thermally activated delayed fluorescence (TADF) materials with multiple resonance (MR) effects have been widely studied owing to their narrow full width at half maximum (FWHM) and high efficiency. In particular, the MR effect is due to the separation of the distribution of the highest occupied molecular orbitals and lowest unoccupied molecular orbitals inside a molecule by the combination of boron and nitrogen, resulting in a low energy gap between the singlet and triplet energies (E-ST; < 0.3 eV). However, although TADF materials with MR effects have a low Delta E-ST, they typically exhibit a high oscillator strength. These features favour their use as the final emitting materials in hyperfluorescence (HF) devices. In this study, we synthesise a green TADF material, tBN-4BF, with an MR structure and evaluate its effectiveness in HF devices using known TADF materials, 2,4,5,6-tetrakis(9H-cabazol-9-yl)isophthalonitrile and 3,3 ',3 ''-(1,3,5-triazine-2,4,6-triyl)tris(4-(9H-carbazol-9-yl)benzonitrile) (TCz-4mCNTrz), as sensitizers. The evaluation shows an FWHM of 35 nm in a single device with an MR dopant in 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a]carbazole-11-yl)-1,3,5-triazine as the host, achieving 29.3 h at LT50@5000 nit. In addition, an HF device utilising TCz-4mCNTrz as a sensitizer achieves 124 h at an LT50@5000 nit. These results demonstrate the potential of green TADF materials and the lifetime of the HF devices in which they are applied.