Abstract
In this study, we demonstrated a blue phosphorescent organic light-emitting diode (BPOLED) based on a host with two carbazole and one trizole (2CbzTAZ) moiety, 9,9′-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)-1,3-phenylene)bis(9H-carbazole), that exhibits bipolar transport characteristics. Compared with the devices with a carbazole host (N,N’-dicarbazolyl-3,5-benzene, (mCP)), triazole host (3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole, (TAZ)), or a physical mixture of mCP:TAZ, which exhibit hole, electron, and bipolar transport characteristics, respectively, the BPOLED with the bipolar 2CbzTAZ host exhibited the lowest driving voltage (6.55 V at 10 mA/cm2), the highest efficiencies (maximum current efficiency of 52.25 cd/A and external quantum efficiency of 23.89%), and the lowest efficiency roll-off, when doped with bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) as blue phosphor. From analyses of light leakage of the emission spectra of electroluminescence, transient electroluminescence, and partially doped OLEDs, it was found that the recombination zone was well confined inside the emitting layer and the recombination rate was most efficient in a 2CbzTAZ-based OLED. For the other cases using mCP, TAZ, and mCP:TAZ as hosts, electrons and holes transported with different routes that resulted in carrier accumulation on different organic molecules and lowered the recombination rate.
Highlights
The mixture was heated under reflux conditions for 24 h
Yellowish solid was precipitated under these conditions
The crude solid was collected by suction filtration
Summary
Tian-You Cheng 1, Jiun-Haw Lee[2], Chia-Hsun Chen[2], Po-Hsun Chen[2], Po-Sheng Wang[2], Chuan-En Lin[2], Bo-Yen Lin[2], Yi-Hsin Lan[2], Yu-Hsuan Hsieh[3], Jau-Jiun Huang[3], Hsiu-Feng Lu4, Ito Chao[4], Man-kit Leung[3], Tien-Lung Chiu1 & Chi-Feng Lin[5]. Compared with the devices with a carbazole host (N,N’-dicarbazolyl-3,5-benzene, (mCP)), triazole host (3-(biphenyl-4yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole, (TAZ)), or a physical mixture of mCP:TAZ, which exhibit hole, electron, and bipolar transport characteristics, respectively, the BPOLED with the bipolar 2CbzTAZ host exhibited the lowest driving voltage (6.55 V at 10 mA/cm2), the highest efficiencies (maximum current efficiency of 52.25 cd/A and external quantum efficiency of 23.89%), and the lowest efficiency roll-off, when doped with bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) as blue phosphor. When examining the electroluminescence (EL) of the OLEDs with three other hosts (mCP, TAZ, and mCP:TAZ), emission at short wavelength (~390 nm) was observed, corresponding to the emission of the exciton/electron-blocking layer (EBL) and hole-transporting layer (HTL)[24,25] This indicates that the carriers were leaking outside the EML without efficient recombination. By applying the reverse bias after the electrical pulses with different periods, it was found that carriers may accumulate inside the device over approximately 700 μs
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