Iridium complexes with cyclometalated 2-cycloalkenyl-pyridine ligands as highly efficient emitters for organic light-emitting diodes

Iridium complexes using 2-cycloalkeylpyridine derivatives were synthesized as cyclometalation ligands for organic light emitting diodes (OLEDs). The latest designed iridium complexes were expected to have high efficiency owing to regulated energy levels and the suppressed triplet-triplet (T-T) annihilation in electroluminescence (EL) devices due to the introduction of rigid and bulky cycloalkenylpyridines. OLEDs were fabricated using the complexes as the dopants in CBP, which was selected as the host material due to a good overlap of the absorption of the iridium complexes with the PL spectrum of CBP. The quantum efficiency of the devices was significantly reduced at high current density and asymptotically approached to the same value for the three devices. It was found that devices based on these iridium complexes emit yellowish-green light with a very high external quantum efficiency of 18.7%.