Phenylcarbazole-dipyridyl triazole hybrid as bipolar host material for phosphorescent OLEDs

Two bipolar host materials p-cbtz and m-cbtz, with both phenylcarbazole and 3,5-bis(2-pyridyl)-1,2,4-triazole moieties, were synthesized and tested for fabrication of various phosphorescent OLEDs. The sp³-hybridized nitrogen atom of the 1,2,4-triazole fragment disrupted the π-conjugation between the electron donating 9-phenylcarbazole and the electron accepting 3,5-bis(2-pyridyl)-1,2,4-triazole and, thus, exhibited promising properties such as bipolar charge transport feature (10^-5-10^-6 cm² V^-1s^-1) and with relatively high triplet energy gaps (E_T = 2.82-2.75 eV). As a result, they were utilized as universal hosts for various phosphorescent OLEDs, showing high efficiencies and low efficiency roll-off. For example, the devices hosted by m-cbtz achieve maximum external quantum efficiencies (η_ext) of 8.8% for blue, 16.7% for green, 17.5% for yellow and 16.7% for red-emitting OLEDs. In addition, we also fabricated dual-emitter WOLEDs with a co-doped single emissive layer, exhibiting satisfactory device efficiencies (12.4%, 25 cd A^-1, 17 lm W^-1) and with highly stable chromaticity (CIEx = 0.26-0.27 and CIEy = 0.38) at an applied voltage of 8 to 11 V. These results demonstrated that the newly synthesized, phenylcarbazole-dipyridyl triazole host materials are advantageous for fabrication of various highly efficient phosphorescent OLEDs.