A weak electron transporting material with high triplet energy and thermal stability via a super twisted structure for high efficient blue electrophosphorescent devices

A high triplet energy (ET = 3.2 eV) electron transporting/hole blocking (ET/HB) material, 1,2,4,5-tetra(3-pyrid-3-yl-phenyl)benzene (TemPPB) with a super twisted structure and high thermal stability has been synthesized. An external quantum efficiency (EQE) of 19.6% was achieved by using TemPPB as the ET/HB material in a blue electrophosphorescent device, much higher than the EQE of 12.5% for the device using the conventional ET material, 3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ). In addition, the weak ET property of TemPPB resulting from its super twisted structure can be enhanced via n-type doping with LiF. An EQE of 24.5% was achieved by combining n-type doping and a double-emission layer. This shows an alternative way to design ET/HB materials with high ET and improved thermal stability for blue electrophosphorescent devices.