Multifunctional applications of triazine/carbazole hybrid thermally activated delayed fluorescence emitters in organic light emitting diodes

Abstract

Multifunctional materials that can simultaneously act as doped or non-doped emitters and as host materials in both single-color and white organic light-emitting diodes (OLEDs) are rarely reported and so precious. A novel 1,3,5-triazine/carbazole hybrid thermally activated delayed fluorescence (TADF) emitter, 9-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-carbazole (m-CzTrz), is designed and synthesized by adopting the meta-substitution style to tune the steric conformation. In comparison with its reported ortho-substituted isomer o-CzTrz, m-CzTrz possesses dramatically higher hole and electron mobilities due to its less twisted conformation. m-CzTrz exhibits multiple functions when used in OLEDs. First it is an intrinsically sky-blue TADF emitter and exhibits high external quantum efficiencies (η_ext) of 19.2% and 16.7% in doped and non-doped blue OLEDs. Then m-CzTrz acts as a perfect host for traditional coumarin fluorophores and iridium phosphors. More importantly, m-CzTrz acts as a host emitter in single-emitting-layer TADF/phosphorescence hybrid white OLEDs and a high η_ext of 19.8% was realized. m-CzTrz always exhibits a higher brightness and higher power efficiencies but requires a lower driving voltage than isomer o-CzTrz in all devices, despite their identical HOMO and LUMO levels, verifying the importance of molecular structure engineering and appropriate steric conformation. Characterized by facile synthesis, low cost, and always high performance, m-CzTrz may find wide applications as a multifunctional TADF material in OLEDs.