Oxadiazole derivatives as bipolar host materials for high-performance blue and green phosphorescent organic light-emitting diodes

摘要

By combining two n-type groups, pyridine and oxadiazole, with one p-type carbazole group, two novel bipolar hosts, namely 2-(3-(9 H -carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-5-(pyridin-2-yl)-1,3,4-oxadiazole ( PyOxd-mCz ) and 2-(4′-(9 H -carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-5-(pyridin-2-yl)-1,3,4-oxadiazole ( PyOxd-pCz ) have been developed as hosts for blue and green phosphorescent organic light-emitting diodes (PhOLEDs). The two compounds exhibit similar HOMO levels of ?5.64 eV for PyOxd-mCz and ?5.63 eV for PyOxd-pCz and the same LUMO level of ?2.60 eV. With a more twisted configuration due to meta connections, PyOxd-mCz possesses a higher triplet energy level ( E _(T) = 2.77 eV) and more balanced carrier transport than PyOxd-pCz ( E _(T) = 2.60 eV). PyOxd-mCz hosted devices achieve a peak current efficiency of 39.7 cd A ~(?1) and a maximum EQE of 20.8% with a low turn-on voltage of 3.5 V for FIrpic and 55.2 cd A ~(?1) and 16.4% for Ir(ppy) _(3) . Apart from the appropriate frontier molecular orbital levels and sufficiently high triplet energy of PyOxd-mCz , the more balanced carrier transport plays a key role for excellent device performance.