Novel electron-type host material for unilateral homogeneous phosphorescent organic light-emitting diodes with low efficiency roll-off

Abstract

Two novel simple electron-transport (ET) type host materials, 2,6-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)benzene (MDBIP) and 2,6-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)pyridine (MDBIPy) have been designed and synthesized. The two compounds exhibit high decomposition temperatures (T_d: 444 °C for MDBIP and 450 °C for MDBIPy) and a stable amorphous glassy state (T_g: 108 °C for MDBIP and 110 °C for MDBIPy). In the typical device ITO/MoO₃ (10 nm)/4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB, 80 nm)/TCTA (5 nm)/Host: Ir(ppy)₃ (9 wt%, 20 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm), when MDBIPy was used as host material, the device B showed a maximum efficiency of 21.1%, 75.8 cd A⁻¹ and 83.6 lm W⁻¹. When MDBIPy was utilized as both the electron transporting and host layer, the unilateral homogeneous device C exhibited a maximum efficiency of 20.6%, 74.2 cd A⁻¹ and 71.8 lm W⁻¹. More interestingly, device C showed low efficiency roll-off relative to device B. When the brightness of the device is over 1000 cd m⁻², the current efficiency of device C is higher than device B. At a brightness of 5000 cd m⁻², the current efficiency of device C is only roll off 5.6%. These results demonstrate that utilizing electron-transport type host materials to fabricate unilateral homogeneous PhOLEDs is a promising way to simplify the device configuration and optimize the performance of OLED devices.