Low-driving-voltage sky-blue phosphorescent organic light-emitting diodes with bicarbazole-bipyridine bipolar host materials

Abstract

Two molecules, namely 6-([3,3′-bipyridin]-5-yl)-9-phenyl-9H-3,9′-bicarbazole (3-BPyBCz) and 9-(2-([3,3′-bipyridin]-5-yl)phenyl)-9H-3,9′-bicarbazole (o-BPyBCz), are developed as host materials for sky-blue phosphorescent organic light-emitting diodes (PhOLEDs). 9H-3,9′-Bicarbazole and bipyridine were selected as hole- and electron-transporting units respectively. 3-BPyBCz is designed by linking bipyridine directly on the carbazole ring, while o-BPyBCz is designed by linking the p- and n-type units on a phenylene bridge at the ortho-site to each other. Both compounds show high triplet energies (E_T) of 2.74 eV (3-BPyBCz) and 2.98 eV (o-BPyBCz). They are used as host materials to fabricate sky-blue PhOLEDs with bis(4,6-difluorophenylpyridinato-N,C²)picolinato iridium (FIrpic) as the doped emitter. These devices are characterized by extremely low driving voltages, with the turn-on voltages of 2.3 and 2.4 V (at a brightness of 1 cd m⁻²), which should be the lowest ones ever reported for FIrpic-based PhOLEDs so far. o-BPyBCz exhibits superior performance in its PhOLED with a maximum external quantum efficiency and power efficiency of 23.7% and 52.6 lm W⁻¹, indicating the advantage of separately linking the p- and n-type units on a certain bridge in terms of molecular design strategy. The low-driving-voltage merit of these PhOLEDs is attributed to the rapid and balanced charge transportation of these two hosts, which essentially benefits from the combination of bicarbazole and bipyridine as the key functional groups.