A universal solution-processable bipolar host based on triphenylamine and pyridine for efficient phosphorescent and thermally activated delayed fluorescence OLEDs

Host materials play an important role in the development of phosphorescent and thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs). Herein, a universal bipolar host material TPA-(PyF)₃ based on triphenylamine and pyridine was designed and synthesized through a simple Friedel-Crafts reaction with a high yield of 83.5%. TPA-(PyF)₃ possesses a high triplet energy level of 2.83 eV, excellent thermal and morphological stability, and good solution processability. Using TPA-(PyF)₃ as a host material, solution-processed OLEDs with a simple single emitting layer were fabricated. Blue phosphorescent OLEDs based on bis((3,5-difluorophenyl)-pyridine) iridium picolinate (FIrpic) show a maximum current efficiency of 18.5 cd A⁻¹ and a maximum brightness of 13,838 cd m⁻², with a low current efficiency roll-off of 9.7% at the practical brightness of 100–1000 cd m⁻². Green TADF OLEDs using TPA-(PyF)₃ as the host and 2,4,5,6-tetrakis(carbazol-9-yl)-1,3-dicyano-benzene (4CzIPN) as the emitter with the same structure of the phosphorescent OLEDs exhibit a maximum current efficiency of 12.7 cd A⁻¹, compared with 10.2 cd A⁻¹ of the control device based on the common host material of 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy). This study demonstrates the effective utilization of TPA-(PyF)₃ as a universal bipolar host for both phosphorescent and TADF OLEDs.