Highly efficient electrophosphorescent organic light-emitting diodes based on iridium(III) complexes containing carbazole units

Since the seminal work on electrophosphorescent organic light-emitting diodes (PHOLEDs) based on fac-[Ir(ppy)3] (Hppy = 2-phenylpyridine), considerable effort has been devoted to the molecular design and synthesis of efficient phosphorescent heavy metal complexes in the fabrication of high-efficiency PHOLEDs, owing to their potential to harness the energies of both the singlet and triplet excitons in the electroluminescence process. Although the emission efficiency and the color of iridium-based PHOLEDs can be readily tuned by structural modifications of the ligand chromophores, the performance of PHOLEDs is largely governed by the charge balance between the electrons and holes from opposite electrodes. It is vital to develop electrophosphors that can show improved charge-balancing features and permits a complete energy transfer between the host and the dopant in the device. The integration of hole-transporting carbazole module and phosphorescent iridium(III) cyclometalate core appears to be a good choice for the design of more efficient devices. We describe here the synthesis and device performance of a series of interesting small-molecule cyclometalated iridium(III) complexes functionalized with various carbazole units.