Bistriazoles with a Biphenyl Core Derivative as an Electron-Favorable Bipolar Host of Efficient Blue Phosphorescent Organic Light-Emitting Diodes

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%.