ABSTRACT
High-quality host materials are indispensable for the construction in the emitting layer of efficient organic light-emitting diodes (OLEDs), especially in a guest and host system. The good carrier transport and energy transfer between the host and emitters are out of necessity. In this work, a wide bandgap and bipolar organic compound, 2,2'-bis(4,5-diphenyl-(1,2,4)-triazol-3-yl)biphenyl (BTBP), conjugating two electron-transporting triazole moieties on a hole-transporting biphenyl core, was synthesized and characterized. The wide bandgap of 4.0 eV makes the promise in efficient energy transfer between the host and various color emitters to apply as the universal host, especially for blue emitters. The close electron and hole mobilities perform the same order of 10-5 cm2·V-1·s-1, identified as bipolar behavior and benefited for carrier balance at low bias. Although carrier transportation belongs to bipolar behavior at a low electrical field, the electron mobility is much faster than the hole one at a high electrical field and belongs to electron-transporting behavior. Employing the BTBP as the host matrix mixed with a phosphor dopant, iridium(III)bis[4,6-di-fluorophenyl-pyridinato-N,C2]picolinate, a high-efficiency sky-blue phosphorescent organic light-emitting diode (OLED) was achieved with a maximum current efficiency of 65.9 cd/A, maximum power efficiency of 62.8 lm/W, and maximum external quantum efficiency of 30.2%.
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.
